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Catalysis with Colloidal Ruthenium Nanoparticles M. Rosa Axet and Karine Philippot*

UPR8241, Universite ́ de Toulouse, UPS, INPT, CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de NarbonneF-31077 Toulouse cedex 4, France

ABSTRACT: This review provides a synthetic overview of the recent research advancements addressing the topic of catalysis with colloidal ruthenium metal nanoparticles through the last five years. The aim is to enlighten the interest of ruthenium metal at the nanoscale for a selection of catalytic reactions performed in solution condition. The recent progress in nanochemistry allowed providing well- controlled ruthenium nanoparticles which served as models and allowed study of how their characteristics influence their catalytic properties. Although this parameter is not enough often taken into consideration the surface chemistry of ruthenium nanoparticles starts to be better understood. This offers thus a strong basis to better apprehend catalytic processes on the metal surface and also explore how these can be affected by the stabilizing molecules as well as the ruthenium crystallographic structure. Ruthenium nanoparticles have been reported for their application as catalysts in solution for diverse reactions. The main ones are reduction, oxidation, Fischer−Tropsch, C−H activation, CO2 transformation, and hydrogen production through amine borane dehydrogenation or water-splitting reactions, which will be reviewed here. Results obtained showed that ruthenium nanoparticles can be highly performant in these reactions, but efforts are still required in order to be able to rationalize the results. Beside their catalytic performance, ruthenium nanocatalysts are very good models in order to investigate key parameters for a better controlled nanocatalysis. This is a challenging but fundamental task in order to develop more efficient catalytic systems, namely more active and more selective catalysts able to work in mild conditions.

CONTENTS

1. Introduction 1086 2. Interests of Ruthenium and Metal Nanoparticles 1087

2.1. Physicochemical Properties and Interests of Ruthenium 1087

2.2. Interests of Metal Nanoparticles in Catalysis 1087 2.3. Present Challenges in Nanocatalysis and

Place of Ruthenium Nanocatalysts 1088 3. Synthesis Methods of Ruthenium Nanoparticles 1088

3.1. Reduction of Ruthenium(III) Chloride Hy- drate 1089

3.2. Polyol Method 1090 3.3. Use of an Organometallic Precursor 1090 3.4. Supported Nanoparticles 1092

4. Ruthenium Nanoparticles As Catalysts 1092 4.1. Reduction Reactions 1092

4.1.1. Reduction of CC and CO Bonds 1096 4.1.2. Reduction of Nitro Compounds 1097 4.1.3. Hydrodeoxygenation 1100 4.1.4. Reductive Amination of Carbonyl Com-

pounds, Amination of Alcohols, and Other Miscellaneous Reduction Reac- tions 1105

4.2. Oxidation Reactions 1106 4.3. Fischer−Tropsch Reaction 1111 4.4. C−H Activation and Other Reactions 1113 4.5. Transformation of CO2 1113

4.5.1. Transformation of CO2 into HCOOH 1113

4.5.2. Transformation of CO2 into CO, CH4, or C2+ Hydrocarbons 1119

4.5.3. Conclusions on CO2 Transformation 1123 4.6. Dehydrogenation of Amine Boranes 1124

4.6.1. Dehydrogenation of Amine Boranes by Dehydrocoupling 1125

4.6.2. Dehydrogenation of Amine Boranes by Methanolysis 1127

4.6.3. Dehydrogenation of Amine Boranes by Hydrolysis 1128

4.6.4. Dehydrogenation of Amine Boranes by Supported Ruthenium Nanocatalysts 1130

4.6.5. Conclusions on Amineborane Dehydro- genation 1130

4.7. Water Splitting 1130 4.7.1. Ru NPs as Electrocatalysts for HER 1131 4.7.2. Ru NPs as (Photo)catalysts for HER 1133 4.7.3. Conclusions on Water Splitting 1133

5. Concluding Remarks and Outlook 1133 Author Information 1135

Corresponding Author 1135 ORCID 1135 Notes 1135 Biographies 1135

Special Issue: Nanoparticles in Catalysis

Received: July 6, 2019 Published: January 3, 2020

Review

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Acknowledgments 1136 References 1136

1. INTRODUCTION

With symbol Ru and the 44th position in the periodic table of elements, ruthenium is part of the transition metals group. It is considered as a scarce metal with limited availability. This may be hindering wider commercial applications involving ruthenium due to its high price (even if still the least expensive precious metal) and wide fluctuations in the market. The applications of ruthenium mainly concern technological devices and catalysis sectors. In 2018, ruthenium consumption has achieved 42 tons for industrial applications concerning electronics (33%), electrochemistry (17%), and chemistry (37%).1 For instance, ruthenium is commonly added at a small quantity in alloys given its ability to harden them. This is the case of super alloys used for the manufacture of turbine blades of jet engines. It reinforces the rhodium, palladium, and platinum-based alloys used for wear-resistant electrical contacts (high-end spark plugs have electrodes coated with a Pt−Ru alloy; pen tips are made with alloys containing ruthenium). Ruthenium dioxide, RuO2, and ruthenates of lead and bismuth are involved in resistive chips. In electronics, ruthenium is used in the manufacture of hard disks as a coating between two magnetic layers. Regarding catalysis, ruthenium is a polyvalent metal because

it can easily adopt formal oxidation states in a wide range (from II to VIII), leading to a multitude of complexes that display interesting and often unique properties. These properties can be tuned by an appropriate choice of the ligands because these latter strongly affect the reactivity as well as stability of ruthenium complexes. A molecular level understanding of structure−activity relationships in complexes is a key parameter for the development of better catalysts. For instance bipyridines- and terpyridine-containing ruthenium complexes are known for their luminescent and photoredox properties. Such properties are at the basis of the photo- dissociation of water into O2 and H2 (water splitting)

2 and of the development of new generation photovoltaic cells.3

Another important application of ruthenium is the catalytic production of added-value chemicals like acetic acid.4

Carbene-based ruthenium complexes are well-known for their central role in olefin metathesis that provides active molecules or functionalized polymers among others. Ru complexes with phosphorus-containing ligands (for example phosphines, diphosphines as the so-called BINAP, or phosphites) are active for hydrogenation reactions such as hydrogenation of CC and CO double bonds among others, including the enantioselective version.5 Ru complexes are also known for their catalytic performance in the synthesis of formic acid and its decomposition into H2 and CO2 or also the dehydrogenation of alcohols, two important reactions regarding hydrogen storage.6 Finally Ru species are also catalysts of oxidation reactions.7 In heterogeneous conditions, ruthenium is the most active catalyst for the production of ammonia.8 It is also active in the hydrogenation of diverse substrates. As ligands in molecular catalysis, supports play a key-role in the properties of supported ruthenium catalysts due to metal−support interactions. The fine understanding of microscopic properties of the heterogeneous catalysts, in particular, the nature of surface active sites and their chemical or sterical environment is of utmost importance in order to

improve catalytic performances. Finally, the oxidized form of ruthenium, RuO2, is known for its performance in heteroge- neous oxidation catalysis and in electrocatalysis. The exaltation of properties at the nanoscale regime can

increase the relevance of ruthenium for catalysis. The recent progress in nanochemistry allowed having at disposal better controlled Ru NPs in terms of size, dispersion, shape, composition, and surface state, etc. All these characteristics may influence strongly their surface properties and con- sequently their catalytic performance (both reactivity and selectivity), and numerous efforts are presently made in this sense. Using a molecular approach, namely studying the interface between surface atoms and stabilizers (ligands) by a combination of techniques from molecular chemistry (like nuclear magnetic resonance) to theoretical studies allows a better understanding of the surface chemistry of ruthenium nanoparticles. As will be seen in the next sections, these findings give thus a strong basis to better apprehend catalytic processes on the metal surface as well as how these can be affected by the presence of stabilizing molecules or by the crystallographic structure of the ruthenium cores, eventually by taking benefit of these parameters. This review will start by summarizing the physicochemical

properties and interests of ruthenium together with those of metal nanoparticles (section 2) and following, the main synthesis methods to produce ruthenium metal nanoparticles in solution (section 3). Then, the purpose is to provide a synthetic overview of the recent advancements in research that address the investigation of ruthenium metal nanoparticles (Ru NPs) in catalysis in solution (or suspension) conditions in the period 2014−2019 (section 4). The aim is to highlight the potential of ruthenium metal when it is divided at the nanoscale in a controlled manner, namely under the form of well-defined Ru NPs, in colloidal catalysis. Ru NPs have been reported for their application as catalysts in diverse reactions. The reactions reviewed here include reduction, oxidation, Fischer−Tropsch, C−H activation and amine borane dehy- drogenation reactions where Ru NPs show to be very performant. Even if at a lesser extent, Ru NPs have been also investigated for the reduction of carbon dioxide and water splitting process. Relevant works involving Ru NPs in these catalytic reactions will be described. Selection of examples was governed by the degree of control of the characteristics of the described Ru NPs that was made possible by solution synthesis methods, thus allowing precise catalytic investigations. Heterogeneous catalysts are not considered due to the fact the metal nanoparticles they contain are generally poorly controlled due to drastic conditions applied for their preparation. However, a few examples of supported Ru NP- based catalysts are presented. This is justified either by their initial preparation method, which enabled to obtain well- controlled nanostructures, thus providing complementary information to the discussed subjects or by the relevant or pioneering character of the contribution to the field of catalysis. Also, a few papers from earlier years are included due to their high input. Finally concluding remarks and perspectives will be given for each type of reaction treated.

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2. INTERESTS OF RUTHENIUM AND METAL NANOPARTICLES

2.1. Physicochemical Properties and Interests of Ruthenium

Identified and isolated by Karl Karlovich Klaus in 1844,9

ruthenium has as its symbol Ru and the 44th position in the periodic table of elements. Ruthenium is the 74th most abundant metal, a rare element, and is part of the precious metals, being the first of the series beside rhodium, palladium, osmium, iridium, and platinum. With a current price of ca. 7000 €/kg,10 ruthenium is still the least expensive precious metal. Ruthenium is a hard, silvery white metal which is unalterable

in the ambiant air and does not tarnish at room temperature (rt). Ruthenium is a transition metal with electronic configuration [Kr]4d75s1 for the isolated atom in ground state. The oxidation states of ruthenium range from II to VIII, the most common ones being II, III, and IV. These different oxidation states provide a large number of stable ruthenium catalysts (at 16 or 18 electrons). Ruthenium is not easily oxidized at atmospheric condition but RuO2, a stable oxide, may be formed under oxygen pressure. Ruthenium tetroxide (RuO4), a volatile compound, is a powerful oxidizing and very toxic.9 The dissolution of ruthenium is not easy and requires use of aqua regia in heating conditions. Crystalline structure of bulk ruthenium is hexagonal closed-packed (hcp) but at the nanoscale, face-centered cubic (fcc) structure is also known.11−13 Ruthenium is the only noble metal that can crystallize in the nanometer scale with the hcp structure or the fcc one. The anisotropy of the hexagonal system is expected to lead more easily to anisotropic crystals, but there are only a few papers reporting anisotropic Ru NPs, and none with a high aspect ratio.14

The applications of ruthenium mainly concern technological devices and catalysis sectors.15 In catalysis, ruthenium is a polyvalent metal which proved to be active in both homogeneous and heterogeneous conditions. RuCl3·3H2O is often the starting point of a rich coordination and organo- metallic chemistry, thus leading to a wide variety of ruthenium complexes of high interest for homogeneous catalysis. Ruthenium complexes are able to activate unique and multiple bonds and make possible selective C−C, C−H, or C- heteroatom bond formation and cleavage.16 Ruthenium catalysts are thus involved in a great variety of organic reactions, such as alkylation, allylation, arylation, cyclization, cyclopropanation, hydrogenation, hydroformylation, hydro- silylation, hydroxylation, isomerization, olefin metathesis, oxidation, transfer hydrogenation, tandem reactions, water splitting, etc. Ru-catalysis is effectively exploited in the synthesis of natural and biologically active organic compounds, to access recognized chemotherapeutic agents, supramolecular assemblies, smart materials, specialty polymers, biopolymers, agrochemicals, and, increasingly, in valorization of renewable resources as platform chemicals for polymers. Presently, intensive research efforts are devoted in C−H and C−X bond activation, olefin metathesis, and newest trends of green chemistry, such as water oxidation and hydrogen production, reduction of CO2 to CO, oleochemistry, and reactions in eco- friendly media.17 Because of their matter state, heterogeneous transition metal catalysts are also of high interest in catalysis and largely exploited at the industrial level. Heterogeneous catalysts are extended inorganic solids where the d orbitals play

a key role in the adsorption and transformation of substrates. The catalytic activity of transition metals shows a strong periodic effect with a maximum of reactivity for group-VIII transition metals among which ruthenium. Ruthenium is able to chemisorb diverse small molecules such as O2, C2H2, CO, H2, N2, and CO2. In heterogeneous and colloidal conditions, ruthenium is reputed to be active in hydrogenation of nitrogen for ammonia synthesis, hydrogenation of diverse substrates like olefins, and carbonylated molecules but also of aromatics for which molecular ruthenium is not known, as well as for dehydrogenation of amine boranes and hydrogen evolution reactions. Interestingly, it is not very known for hydrogenation of CO2 and dehydrogenation of formic acid. RuO2 turned out to be an excellent oxidation catalyst in heterogeneous catalysis (mainly oxidation of CO) and electrocatalysis (oxidation of water).18

2.2. Interests of Metal Nanoparticles in Catalysis

Heterogeneous transition metal catalysts are extended inorganic solids where the d orbitals play a key role in the adsorption of substrates due to their ability to donate and accept electron density to and from the substrates. This is particularly true for the degenerate states in band structures. The electronic flexibility provided by the d electrons of the metal surface has to be such that the bond with the substrate atoms is intermediate between weak and strong. The metal surface must be able to bind the substrate atoms strongly enough to provoke their dissociation in the chemisorption process. But the surface-atom bond created has to be not too strong, for the bonded substrate atom to be able to further react with other surface-bonded atoms and form the products that can rapidly desorb. If the surface-atom bond is too strong, further reaction will be precluded. The catalytic activity of transition metals shows a strong periodic effect with a maximum of reactivity for group-8 transition metals where ruthenium is located.19

Being part of heterogeneous catalysts, metal nanoparticles (MNPs) have been known for a long time, but a renewed interest emerged in the last three decades for the design of better defined systems.20 Numerous research efforts are devoted to the design of well-controlled MNPs and even at an atomic precision level.21,22 This keen interest for MNPs derives from the particular matter state (finely divided metals) and exalted electronic properties, influencing physical and chemical properties that they present in comparison to bulk metals and molecular complexes. Besides fundamental aspects of research, this interest is also governed by the specific properties and the potential applications that MNPs may find in various domains including optoelectronics, sensing, biomedicine, catalysis, energy conversion, and storage, as nonexhaustive examples.23−26 Several books focus specifically on nanocatalysis.27−37 For catalysis, MNPs are attractive species due to the high surface to volume ratio they display. This ratio is even more pronounced when MNPs are at a size as close as one nanometer, or even below, because the number of surface atoms can be >90%, thus providing a vast number of potential active sites. It is thus of prime importance to have synthesis tools that enable obtaining ultrasmall NPs in order to promote high surface area. Besides the size, other key parameters need also to be controlled. The crystalline structure is important because depending on it, different types of crystalline plans can be exposed at the nanoparticle surface, which can lead to different catalytic properties. Controlling the

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shape of MNPs is another way to orientate the crystalline plans exposed.38−40 The last key parameter but not the least is the composition of MNPs. The composition has to be adjusted depending on the catalysis target. Apart from the nature of the metallic core that may govern the reactivity (some metals are well-known for certain catalyzes but not for others), the surrounding stabilizer for colloidal catalysis (ionic liquids (IL), polymers, surfactants, polyols, ligands, etc.) or the support for supported catalysis (metal oxides, metal organic frameworks (MOFs), carbon derivatives, etc.) may also influence or even orient the catalytic performance. If calcination is usually applied in heterogeneous catalysis in order to suppress any organics and liberate the active sites, such treatment on small nanoparticles can be critical because of sintering. Moreover, naked MNPs are not always optimal catalysts. In modern nanocatalysis, the presence of organic ligands at the NP surface is not seen as detrimental but instead is a way to improve or even modify the chemoselectivity.41 Using ligands as stabilizers allows to make a parallel with molecular catalysis; the ligand interaction with surface metal atoms of the nanoparticles can be compared to ligand interactions with the metal centers in homogeneous catalysts, which is of paramount importance for stability and catalytic properties (activity and selectivity). Ligands can be chosen in order to tune the surface properties of MNPs through steric or electronic effects.42,43 The challenge is to find ligands able to stabilize well-defined MNPs while controlling accessibility at the metal surface and reactivity.41,44

Strongly bound capping ligands (like thiols or phosphines) can result in the poisoning of a nanocatalyst at high surface coverage. But a limited amount of ligand can be beneficial. The coordination of a ligand at a metal surface can also be a way to block selectively some active sites in order to orientate the catalysis evolution. Compared to the investigation of facet dependency,40,45 the ligand influence on the catalytic activity has been less intensively studied but recent results illustrate well the interest to do so.46−50 Ligand-stabilized MNPs can be applied to catalysis as stable colloidal suspensions but also in heterogeneous conditions when deposited on the surface or confined in the pores of a solid support.51 Ionic liquids52 are also very efficient to stabilize metal NPs, and colloidal suspensions in ionic liquids can even be deposited onto inorganic supports.53

2.3. Present Challenges in Nanocatalysis and Place of Ruthenium Nanocatalysts

Having at disposal synthesis strategies that allow access, in a reproducible manner, to well-defined MNPs in terms of size, crystalline structure, composition (metal cores and stabilizing agents), chemical order (bimetallic or multimetallic systems), shape, and dispersion is a beneficial condition to investigate finely their catalytic properties and define structure/properties relationships. Taking advantage of recent developments in nanochemistry in solution, and in particular of the use of molecular chemistry tools, nanocatalysis is now well- established as a borderline domain between homogeneous and heterogeneous catalysis. Nanocatalysts can be seen as assemblies of individual active sites where metal−metal and metal−stabilizer bonds will both have influence.54 Precisely designed MNPs are expected to present benefits from both homogeneous and heterogeneous catalysts, namely high reactivity and better selectivity together with high stability.55

The understanding of structure−properties relationships is required for the design of more performant nanocatalysts in

order to develop more efficient and eco-compatible chemical production.56 If a certain progress has been done in the past decade, this topic remains very challenging. Model nano- catalysts are needed in order to better understand the link between the characteristics of MNPs and their catalytic performance and thus bridge the gap between model surfaces and real catalysts. Each progress that contributes to reduce the gap of knowledge between nanocatalysts and homogeneous catalysts constitutes a step forward the development of more efficient and selective catalytic systems. Intensive efforts in this direction are needed in order to one day be able to anticipate the design of suitable catalysts for a given reaction. Various metals are investigated in nanocatalysis toward these

principles, with a huge number of studies dedicated to gold which is highly reputed for CO oxidation and emerges now in hydrogenation catalysis,57,58 or palladium which intervenes in various C−C coupling reactions and also in hydrogenation catalysis.59,60 Other metals like rhodium, platinum, iridium, nickel, cobalt, and iron, among others, are also the object of numerous studies. Compared to all these metals, the number of works focusing on the use of Ru metal NPs in nanocatalysis may appear to be lower. This may be quite surprising given the large and successful application of this metal in homogeneous catalysis but can be explained by the fact it is an expensive metal. However, as it will be seen hereafter, ruthenium proved to be an interesting metal to carry out precise studies in order to establish structure−properties relationships in diverse catalytic reactions, mainly hydrogenation, hydrodeoxygenation, Fischer−Tropsch, C−H activation, amine borane dehydrogen- ation, water splitting, and carbon dioxide reduction.

3. SYNTHESIS METHODS OF RUTHENIUM NANOPARTICLES

Being part of heterogeneous catalysts, metal NPs have been known for a long time, but a renewed interest emerged in the last three decades for the design of better defined systems, studies in which Ru NPs stand at a good place.33 This arises from fundamental hurdles met in scientific research with badly defined NPs such as the common issue of size dispersity (e.g., 5% in even highly monodispersed samples), the unascertained surfaces of NPs, the unknown core−ligand interfaces, the defects and elusive edge structures in 2D materials, and the still missing information on alloy patterns in bi- and multimetallic NPs. Such imprecisions preclude deep understanding of many fundamental aspects of NPs, including the atomic-level mechanism of surface catalysis.22 Developing synthesis strategies that allow preparing, in a reproducible manner, well-defined MNPs in terms of size, crystalline structure, composition (metal cores and stabilizing agents), chemical order, shape, and dispersion is a prerequisite in order to investigate finely their catalytic properties and determine the links between structural features and catalytic properties. For this purpose, bottom-up liquid-phase techniques are very attractive because they are versatile and easy to use, necessitating straightforward equipment than physic routes. Recent developments in nanochemistry offer efficient tools to reach these objectives and make nanocatalysis to be a recognized domain at the frontier between homogeneous and heterogeneous catalyzes, thanks to better-controlled NPs that allow progressively to take benefit of advantages of both types of catalysts.33 Metal NPs stabilized by ligands allow performing fine surface studies as done with homogeneous catalysts. Indeed such NPs display a metal surface with an

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interface close to that of molecular complexes (isolated surface atoms can be seen like metal centers with their coordination sphere) while benefiting from the influence of neighboring metal atoms. It is also worth to mention that recent developments of theoretical tools allow to bring computational chemistry applied to small NPs to the same level of accuracy and relevance as in molecular chemistry.61 All together nanochemistry and computational chemistry enable to have precise mapping of the surface properties of MNPs. At the nanoscale level, ruthenium showed to be of interest in

diverse catalytic reactions and different synthesis tools have been developed to access well-defined Ru NPs. The synthesis of ruthenium NPs62 is often performed by chemical reduction of ruthenium(III) chloride hydrate because of its availability, using various reagents such as amines, carbon monoxide, hydride salts (NaBH4, LiAlH4), hydrazine, alcohols, citrate salts, or hydrogen. The drawback of these methods is the presence of surface contaminants resulting from the reaction conditions, such as water, salts, organic residues, or even an oxide shell, which can alter the NP properties and limit access to their surface. An elegant approach to circumvent these difficulties is the use of organometallic (or metal−organic) complexes as metal sources which are generally decomposed under hydrogen atmosphere in mild conditions (low temper- ature and pressure) in organic solution.63 The main disadvantages of this approach is the access to the metal precursors and the need to handle them in inert conditions and in degassed organic solvents in order to preserve their initial properties. The gain is the high quality of the obtained NPs which display well-controlled characteristics and allow precise surface studies. In between, the polyol method allows the access to MNPs starting from metal complexes, similarly to the organometallic approach, but usually using harsher synthesis conditions.14 Whatever the preparation method followed, the particles are generally stabilized by a polymer, an ionic liquid, a surfactant, or a ligand added to the reaction mixture for preventing undesired metal agglomeration and precipitation. A

large interest is presently devoted to ligand-protected particles due to the intrinsic physicochemical properties of these ligands which can contribute to tune those of the particles.41 Before describing the catalytic applications of Ru NPs, we will summarize in the next subparts the main strategies developed in order to access Ru NPs in colloidal solutions, namely the reduction of ruthenium trichloride, polyol method, and the use of an organometallic precursor. It is important to note that apart from these very often used methods, others are reported in the literature, such as the usage of ultrasounds or microwaves, microemulsion systems, coprecipitation techni- ques, sol−gel method, and hydrothermal/solvothermal pro- cessing. These synthesis approaches will not be here described because they are not applied for the preparation of the Ru nanocatalysts cited in the following parts of this review. 3.1. Reduction of Ruthenium(III) Chloride Hydrate

The reduction of ruthenium(III) chloride hydrate in water is the most used method to prepare Ru NPs because of its low cost, ease of implementation, and scalability. This method (Figure 1) consists in treating an aqueous solution of commercial RuCl3·xH2O (with x = 3 depending on purity; hereafter referred as RuCl3) by a reducing agent in the presence of a stabilizer, at ambient conditions (room temperature; rt) and without taking specific cautions.64 Diverse reductants can be used among which alcohols (EtOH),65

hydrides (NaBH4, KBH4, or other amine boranes, LiAlH4), 66

as well as hydrogen at low pressure (1−3 bar)67 are very common. Concerning the stabilizers whose role is to avoid the agglomeration of Ru NPs and to control their growth (size, shape), they need to be water soluble. It can be an organic polymer like polyvinylpyrrolidone (PVP), a sugar derivative like cyclodextrins or chitosans, a surfactant like quaternary ammoniums, an ionic liquid (like imidazolium salts) or organic ligands (sulfonated phosphines, phosphonates, etc.), among others. By this way, stable aqueous colloidal suspensions of Ru NPs are fastly obtained that can be directly used for in catalysis in neat water or biphasic media without any purification.

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However, if no purification, one drawback can be the presence of byproducts resulting from the reactants which can act as pollutants at the metal surface. Another inconvenient can be the (partial) oxidation of the metal surface, which is often circumvented by treating the colloidal suspension under hydrogen pressure (low pressure: 1−20 bar) before catalysis. Nevertheless, the so-obtained Ru NPs can be isolated and purified, in particular to have a characterization reference before involving them in catalytic reactions for comparison purposes. 3.2. Polyol Method

In a recent review, Fiev́et, Piquemal, and co-workers recently described into detail the polyol process and its interests (Figure 2) to prepare MNPs with tailored sizes, shapes, compositions, and architectures.14

It is also a low cost and facile process, where a polyol (including 1,2-diols and ether glycols) is used as the liquid organic compound, acting as both as a solvent of the metal precursor and reducing agent as well as sometimes as colloidal stabilizer. The high boiling point of the polyols allows working at high temperature that assures the formation of well- crystallized NPs and enlarges the possibilities of syntheses. The polyol coordination ability to metal precursors and to NP surface via −OH groups both facilitates the dissolution of the metal sources and minimizes the NP coalescence. The high viscosity of polyols favors a diffusion-controlled regime for the NP growth resulting in controlled structures and morpholo- gies. Despite the intrinsic properties of polyols, reducing agents (like acetates or hydrogen), and stabilizers (like polymers or surfactants) are often added to improve the characteristics of the NPs. Concerning Ru NPs, only a few papers describe their formation by the polyol process, mostly from RuCl3.

14 But ruthenium complexes like [Ru(acac)3] have been also described. In the presence of a protecting agent (PVP,68,69

thiol,70 or NaOH71) the formation of isotropic NPs in a size range 1−6 nm has been reported. An example of anisotropic Ru NPs69 and others of fcc Ru NPs (active in CO oxidation,72

reduction of nitrophenol and dehydrogenation of amino- boranes,73 nitrogen reduction for ammonia synthesis,74 or oxygen evolution reaction11) prepared in a polyol have also been reported. 3.3. Use of an Organometallic Precursor

First inspired by Bradley and co-workers,75−78 and then mainly developed by Chaudret and collaborators,79 the use of an organometallic complex is nowadays a well-established method to access model nanocatalysts. It allows getting well-defined soluble MNPs and exploring their surface properties. The key point of this strategy is the use of an organometallic complex (and in some extent metal−organic complex) as the source of metal atoms together with adequate stabilizers. It allows building diverse nano-objects with modulable sizes including ultrasmall size (ca. 1−10 nm) and a metallic surface free of contaminants, which can be tuned at will. An advantageous benefit from organometallic or metal−organic complexes is their easy decomposition in mild conditions (1−3 bar H2, rt, or T ≤ 423 K) through reduction or ligand displacement from the metal coordination sphere in an organic solvent and in the presence of a stabilizer.63 When accessible, olefinic complexes are preferred as they provide clean metal surfaces as treatment by H2 releases alkanes that are inert toward the NP surface and easily eliminated. Using this method, monodisperse assemblies of NPs with an efficient control of size, shape and surface state can be synthesized and then isolated and purified for a fine determination of their characteristics before application in catalysis. [Ru(COD)(2-methylallyl)2] and [Ru(COD)(COT)] (where 1,5-cyclooctadiene (COD) and 1,3,5-cyclooctatriene (COT)) are particularly relevant precursors to access well- defined Ru NPs (Figure 3). [Ru(acac)3] and [Ru3(CO)12] can

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also be used but their decomposition requires higher temperatures and in the case of the latter, CO can remain at the metal surface. However, [Ru3(CO)12] complex allowed to access shape-controlled Ru NPs which is uncommon.80

The choice of the stabilizer is also fundamental as it governs the growth, stability, solubility properties, and catalytic performance of the NPs. Besides organic polymers, like PVP, that provide steric stabilization and weak interaction with the metal surface, a plethora of organic ligands coordinating via N, S, Si, P, or C atoms to the metal surface have been used leading to fine-tuned surface properties.82 Ionic liquids can also be used.83,84 The employment of water-soluble stabilizers, namely polymers like PVP,85 ligands like 1,3,5-triaza-7-phosphaada- mantane (PTA),86 or sulfonated phosphines87 and also cyclodextrins88 allowed production of aqueous suspensions of Ru NPs that are stable and active in C−H activation89 or hydrogenation catalysis,90 thus offering other opportunities in catalysis. If a major inconvenient of this synthesis process is the access

to the metal precursors which are costly, in some cases difficult to prepare, and most often need to be handled under inert atmosphere, the quality of the obtained MNPs is a real plus for fundamental studies. Indeed, a good control over the particle

formation process is achieved, due to the mild reaction conditions. Moreover, except the stabilizer voluntary added or traces of solvent, no contaminant, such as halides or other ions, is introduced. This makes this method powerful to have suitable NP models for performing fundamental studies on surface properties and also for following catalytic reactions, and numerous studies have been done with ruthenium (Figure 4).81,91

The use of H2 as reducing agent to synthesize MNPs leads to hydrogen atoms at the metal surface, a clear advantage for reduction catalysis (vide infra). Computational chemistry performed onto ethanoic acid-stabilized Ru NPs indicated that ruthenium atoms present a positive charge density and hydrogen atoms a negative one, thus showing that hydrogen atoms are likely hydrides.92 The presence of hydrides has been experimentally supported by 1H MAS NMR on PTA-stabilized Ru NPs which presented a signal at −14 ppm, a typical value for hydrides on ruthenium complexes.90 The surface hydrides content has been shown to vary depending on the surface state of Ru NPs but is generally high (>1/surface Ru atom) even after Np transfer into water.81 Although this value can be also modulated with the species present on the surface; in Ru NPs stabilized by carboxylates, the number of hydrides per surface

Figure 4. Schematic representation of some surface studies performed with Ru NPs prepared from an organometallic complex. Reproduced with permission from ref 91. Copyright 2018 American Chemical Society.

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1091

Ru atom was found to be significantly lower (ca. 0.4 H/surface Ru atom) by both experimental and theoretical techniques, as the result of the coordination mode of the stabilizer.92 The surface hydrides can be displaced by coordination of CO at the surface. NMR methods, in particular solid-state 2H NMR, evidenced H−D exchange between Ru NPs surface and ligand sites: incorporation of 2H atoms in the alkylchains of HDA used as capping ligand was observed, as the result of a C−H activation phenomenon.93,94 This was further exploited in order to perform the deuteration of different substrates (vide infra). Using 13CO as a probe molecule and IR (Infrared) and MAS NMR (magic angle spinning nuclear magnetic resonance) techniques provided indirect information on location and mobility of ligands at metal surface and helped to understand the surface properties and catalytic reactivity of NPs.95 For instance, it has been demonstrated that the strong coordination of phosphine ligands at a Ru NP surface blocks CO mobility contrarily to the few, weak bonds involved when a polymer is used as stabilizer. Similar strategies allowed localizing carbene96 or betaine adduct of NHC−carbene and carbodiimide95 ligands at Ru NP surface. CO oxidation was used to compare the reactivity of phosphine- and PVP- stabilized Ru NPs by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and wide-angle X-ray scattering (WAXS): CO oxidation proceeds at rt in each case, but a rapid deactivation occurred for PVP-stabilized NPs due to the formation of RuO2, while phosphine effectively protects the NPs against bulk oxidation. Reduction of 13CO2 by H2 was studied on PVP- and phosphine-stabilized Ru NPs by solid-state MAS NMR spectroscopy. Formation of 13CO was observed in mild conditions (3 bar H2, 393 K),

97 which was reduced upon heating into CH4 or hydrocarbons in a Fischer−Tropsch process as observed also when studying reduction of CO at the surface of the same Ru NPs.98

3.4. Supported Nanoparticles

As the main purpose of this review is to discuss on the application of Ru NPs into colloidal (or suspension) conditions, the synthesis of supported Ru NPs is here only briefly discussed. From the synthesis methods described above, it is quite easy to access supported Ru NPs using different types of supports (most often oxide-type and carbon-based supports). The most simple strategy is certainly the immobilization of preformed Ru NPs following an impregna- tion method, meaning mix a chosen support (eventually previously treated by treatment in temperature or vacuum) with a colloidal suspension of Ru NPs. If any, the porosity of the support will enable the NPs to diffuse inside the pores of the matrix and thus to be dispersed. An important point in this approach is the size of the pores which needs to be compatible with the NP size in order to get a high dispersion level. A favorable advantage deals with the presence of anchoring groups at the surface or in the pores of the support. The anchoring groups are generally chosen in order to provide interaction with the metal NPs and thus retain them more firmly than with simple physical adsorption. This interaction can be electrostatic, π−π stacking, or even covalent in nature. For example, immobilization of Ru NPs, previously prepared

from an organometallic precursor, into alumina, silica, or carbon materials99 was carried out by this way in order to improve stability and recovery of the nanomaterials and also take advantage of the support properties during catalysis. Aqueous suspensions of Ru NPs prepared by reduction of

RuCl3 64 as well as by polyol suspensions14 can also be used to

disperse NPs onto a support. The main advantage of this route is that the control of the NP growth is previously performed in solution and is generally kept after their deposition on the support. This makes possible to carry out comparison catalytic studies from NPs displaying similar characteristics in terms of size, shape, and stabilizer nature, either being in suspension or supported conditions, but it is a two-step synthesis process. Another strategy consists in the direct synthesis of NPs in the presence of the chosen support, keeping all the reaction conditions equal otherwise. Functionalized supports bearing chemical groups similar to those present in the stabilizers can improve the grafting of the NPs and their stability. Ionic liquids can be used also as stabilizing layer in the presence of an extra ligand or not.83 If this strategy is a one-step process, the structural characteristics of the growing NPs can be strongly influenced by the support properties which make comparison studies more complicated or even impossible.

4. RUTHENIUM NANOPARTICLES AS CATALYSTS

In the next sections, the use of Ru colloidal NPs as catalysts is described. Reduction reactions are mainly focused on arene hydrogenations, which have been extensively studied using Ru NPs as catalysts. Other reduction reactions like of nitro- benzene and azo compounds with NaBH4 are reported as well. Ru-based catalysts are outstanding for this kind of reductions, but the intensive work in these reactions is also due to the fact that the properties of Ru NPs can be easily evaluated, namely electronic and steric effects of the surface ligands, the crystalline structure, or the addition of a second metal, among others. Similarly, CO oxidation with O2 can be used as a model reaction to evaluate such parameters. Hydro- deoxygenation, a valuable procedure to upgrade biomass, is also studied with Ru NPs catalysts. Remarkably, bimetallic systems such as RuNi and RuFe NPs gave interesting results which pave the way to new applications of hydrodeoxygena- tion. An objective beyond is its application directly to biomass compounds and not only limited to oxygen containing model compounds. More recently, C−H activation has been described with Ru NPs, allowing to selectively deuterate organic compounds in mild conditions. Colloidal Ru NPs have found less application in other types of catalytic reactions such as oxidations or Fischer−Tropsch and a few others, which are also described thereafter. Contrarily to Ru complexes, Ru NPs are not commonly reported for the transformation of CO2, but recent papers provide promising results. In the opposite, Ru NPs are largely studied in the dehydrogenation of amine boranes. If often in supported conditions, but Ru NPs in colloidal suspensions are also highly performant and ruthenium is among the best catalysts for this reaction. Ru-based NPs are presently the object of a renewed interest in water-splitting catalysis, with some catalysts showing a performance approaching that of Pt in the hydrogen evolution reaction.

4.1. Reduction Reactions

Rh, Ir, and Ru compounds are very well-known as effective homogeneous catalysts.100 Similarly, the emerging single atom catalysts for reduction reactions are based in these metals.101 It is not surprising that Ru NPs have found applications as catalysts for a large panel of reduction reactions, mainly CC and CO bonds, in a broad range of reaction conditions. Ru NPs used as catalysts in reduction reactions are synthesized by one of the methodologies described above using a large variety

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1092

T ab le

1. R u N P s as

H yd ro ge na ti on

C at al ys ts

of A re ne s an d C ar bo

ny l C om

po un

st ab ili zi ng

ag en t

sy nt he tic

m et ho do lo gy

ch ar ac te ri za tio

n ca ta ly tic

re ac tio

n co nd iti on s

co m m en ts

re f

te rm

in al an d in te rn al al -

ky ne s

re du ct io n of

R uC

l 3 in

1, 2- pr op an ed io l

T G A ,I C PS

N M R ,I R ,

X PS

,p ho to lu m in es -

ce nc e m ea su re -

m en ts ,D

hy dr og en at io n of

st yr en e by

H 2

fu ll hy dr og en at io n of

st yr en e to

et hy lc yc lo he xa ne

us in g

te rm

in al al ky ne

ca pp ed

R u N Ps ;s el ec tiv e hy dr og en at io n to

et hy lb en ze ne

us in g in te rn al al ky ne

ca pp ed

R u N Ps ; T O Fs

an d qu an tit y of

ca ta ly st no t gi ve n;

re cy cl in g te st s no t

re po rt ed

14 0

R uC

l 3 (0 .2 8 m m ol ), so di um

ac et at e (2

m m ol ), 1, 2-

pr op an ed io l( 10 0 m L) ,4 38

K ,3 0 m in ,a lk yn e (0 .8 4

m m ol ), to lu en e (1 00

m L)

ca ta ly st ,s ty re ne

(1 m L) ,T

H F, H

2 (1 0 ba r) ,2 98

ch ol es te ro l-d

er iv ed

N H C

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,A A S,

N M R

hy dr og en at io n of

ar en es

by H

2 lig an d ba ck bo ne

go ve rn s se le ct iv ity

an d ac tiv ity

bu lk y lig an d

di sp la yi ng

hi gh er

se le ct iv ity ; T EM

an al ys is af te r ca ta ly si s;

re cy cl in g an d le ac hi ng

te st s no t re po rt ed

13 8

[R u( C O D )( C O T )] ,N

H C

(0 .2

eq ui v) ,H

2 (3

ba r) ,

T H F,

29 8 K

ca ta ly st (2

m g, 0. 01

m m ol R u) ,a re ne

(0 .2 m m ol ),

T H F (1

m L) ,H

2 (5

ba r) ,r t, 20

lo ng -c ha in

N H C

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,W A X S,

SS N M R ,I R

hy dr og en at io n of

ar en es

an d al ke ne s by

H 2

se le ct iv ity

m od ul at ed

w ith

su rf ac e lig an d

13 5

[R u( C O D )( C O T )]

(1 00

m g) ,N

H C

(0 .1 − 0. 3

eq ui v) ,H

2 (3

ba r) ,T

H F (5 0 m L) ,2

98 K ,2

0 h

ca ta ly st (1

m g) ,s ub st ra te

(0 .2

m m ol ), so lv en t (1

m L) ,H

2 (3 .5 − 5 ba r) ,2

98 − 30 3 K

ch ir al N H C

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,I C P,

EA ,I R ,

SS N M R

hy dr og en at io n of

ar en es

an d al ke ne s by

H 2

ne gl ig ib le en an tio

m er ic ex ce ss

ob se rv ed ; no

re cy cl in g te st or

ch ar ac te ri za tio

n of

th e sp en t ca ta ly st s

13 4

[R u( C O D )( C O T )]

(3 95 .6

m g) ,N

H C

(0 .2 − 0. 5

eq ui v) ,H

2 (3

ba r) ,p en ta ne

(1 50

m L) ,2 98

K ,2 0 h

ca ta ly st (2

m g) ,s ub st ra te (0 .1 5 m m ol ), so lv en t (2

m L) ,H

2 (5 − 60

ba r) ,2

98 − 35 3 K ,1

5 h

ch ir al N H C

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,W A X S,

IC P,

A E,

IR ,S

N M R

hy dr og en at io n of

se ve ra ls ub st ra te s by

H 2

ne gl ig ib le en an tio

m er ic ex ce ss

ob se rv ed ; no

re cy cl in g te st ;

T EM

of th e sp en t ca ta ly st s

13 3

[R u( C O D )( C O T )]

(3 95 .1 6 m g, 1. 26

m m ol ), N H C

(0 .5 eq ui v) ,H

2 (3

ba r) ,p en ta ne

(1 50

m L) ,2 98

K ,

20 h;

[R u( C O D )( C O T )]

(1 20

m g) ,N

H C

(0 .2

eq ui v) ,H

2 (3

ba r) ,p

en ta ne

(4 5 m L) ,2

98 K ,2

0 h

ca ta ly st (1

m g) ,s ub st ra te

(0 .1

m m ol ), so lv en t (1

m L) ,H

2 (1 0−

25 ba r) ,2

98 − 31 3 K

PP h 3

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,X R D ,X

PS ,E

A ,

W A X S,

T G A

hy dr og en at io n of

se ve ra lp

ol yc yc lic

ar om

at ic

hy dr oc ar bo ns

by H

go od

ac tiv iti es

an d se le ct iv iti es

un de r m ild

re ac tio

n co nd iti on s; ra te an d se le ct iv ity

de pe nd

on nu m be r of

cy cl es

on th e su bs tr at e;

se le ct iv ity

de pe nd

on th e nu m be r an d

na tu re

of su bs tr at e su bs tit ue nt s; no

re cy cl in g te st ; no

ch ar ac te ri za tio

n of

th e sp en t ca ta ly st

11 2

[R u( C O D )( C O T )]

(9 40 0 m g) ,P

Ph 3 (0 .4 eq ui v)

H 2

(3 ba r) ,T

H F (4 00

m L) ,2

98 K

ca ta ly st (3

m g) ,s ub st ra te

(0 .6 2 m m ol ), so lv en t

(1 0 m L) ,H

2 (3 − 20

ba r) ,3

03 − 35 3 K

ph os ph in es

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,X R D ,X

PS ,E

A ,

W A X S,

T G A

hy dr og en at io n of

ar om

at ic ke to ne s by

H 2

re du ct io n of

th e ar en e fa vo re d ag ai ns t ke to ne

gr ou p;

se le ct iv ity

m od ul at ed

by th e su rf ac e lig an d;

ca ta ly tic

re ac tio

n pr of ile

11 6

[R u( C O D )( C O T )]

(4 00

m g) ,P

Ph 3 or

dp pb

(0 .4

eq ui v)

H 2 (3

ba r) ,T

H F (4 00

m L) ,2

98 K

ca ta ly st (2

m ol % ), su bs tr at e (1 .2 4 m m ol ), so lv en t

(1 0 m L) ,H

2 (3 − 20

ba r) ,3

03 K

ch iti n

re du ct io n of

R uC

l 3 w ith

N aB H

4 T EM

,X R D ,I C P

hy dr og en at io n of

be nz yl gl yc id yl et he r an d ot he r

ar en es

by H

no hy dr og en ol ys is si de

pr od uc ts ; no

R u le ac hi ng

as as ce rt ai ne d by

IC P;

T EM

af te r ca ta ly si s sh ow

a sl ig ht ly

in cr ea se

of N P si ze

14 1

R uC

l 3 (7 1. 6 m g) ,c hi tin

(2 .9 7 g) ,N

aB H

4 (3 0. 6 m g) ,

H 2O

(9 m L) ,3

03 K ,3

.5 h

ca ta ly st (0 .8 m ol % R u) ,s ub st ra te (1

m m ol ), H

2O (5

m L) ,H

2 (2 0 ba r) ,3

23 K ,1

.5 h

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1093

T ab le

1. co nt in ue d

st ab ili zi ng

ag en t

sy nt he tic

m et ho do lo gy

ch ar ac te ri za tio

n ca ta ly tic

re ac tio

n co nd iti on s

co m m en ts

re f

fu lle re ne

C 60

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,I R ,N

M R ,

W A X S,

R am

an ,

EX A FS

,X PS

hy dr og en at io n of

tr an s- ci nn am

al de hy de

by H

2 se le ct iv ity

to tr an s- ci nn am

yl al co ho l of

77 %

T O F = 12 8 h−

1 14 3, 15 6

[R u( C O D )( C O T )] ,C

60 (0 .0 3−

1 eq ui v) ,H

2 (3

ba r) ,

C H

2C l 2,

29 8 K

ca ta ly st (5

m g) ,t ra ns -c in na m al de hy de

(4 m m ol ),

IP rO

H (3 0 m L) ,p

yr id in e (4 .5

eq ui v) ,H

2 (2 0

ba r) ,3

43 K ,2

0 h,

10 00

rp m

po ly si lo xa ne

m at ri x

re du ct io n of

[R u( C O D )( 2- m et hy la lly l) 2]

w ith

H 2 in

a fu se d- si lic a co at ed

co lu m n

T EM

hy dr og en at io n of

va ri ou s ca rb on yl co m po un ds

on -

co lu m n re ac tio

n by

H 2 ch ro m at og ra ph y

re cy cl in g te st s

15 7

[R u( C O D )( 2- m et hy la lly l) 2]

(0 .1

m g) ,H

2 (0 .1

ba r) ,

31 3−

46 3 K (0 .5

K /m

in ), 10

h ca ta ly st (0 .3

m ol

% ), su bs tr at e, H

2 (0 .5

ba r) ,3

63 K ,r et en tio

n tim

e (5 .2

ph os ph in e- fu nc tio

na liz ed

IL re du ct io n of

R uO

2 or

[R u( C O D )( 2- m et hy la lly l) 2]

w ith

H 2

T EM

,X R D ,X

PS ,

N M R ,I R

hy dr og en at io n of

va ri ou s su bs tr at es

by H

2 se le ct iv ity

tu ne d w ith

re ac tio

n co nd iti on s; po is on

te st w ith

H g;

re cy cl in g te st an d le ac hi ng

of 9 pp m

of R u in

th e

hy dr og en at io n of

st yr en e

12 2

R uO

2 or

[R u( C O D )( 2- m et hy la lly l) 2]

(0 .0 18

m m ol ),

ph os ph in e- fu nc tio

na liz ed

io ni c liq ui ds

(0 .0 18

m m ol ), [B M IM

]B F 4

(1 m L) ,H

2 (1 0 ba r) ,3

48 K ,

4 h

ca ta ly st (s ub st ra te /R

u = 10 0) ,s ub st ra te

(1 m L

so lu tio

n at

1. 8 M ), H

2 (5 0 ba r) ,3

03 K ,1

5 h

cy cl od ex tr in

po ly m er

re du ct io n of

R uC

l 3 w ith

N aB H

4 T EM

,I R ,X

R D ,T

G A ,

U V − vi s, N M R

hy dr og en at io n of

ce llu lo se -d er iv ed

pl at fo rm

m ol -

ec ul es

by H

re cy cl ed

5 co ns ec ut iv e ru ns ; T EM

af te r ca ta ly si s sh ow

ed a

sl ig ht

R u N P ag gr eg at io n an d a sl ig ht

in cr ea se

of N P si ze

14 4

R uC

l 3 (3 .6 × 10

− 3 m m ol ), cy cl od ex tr in

po ly m er

(0 .5

g) ,N

aB H

4 (0 .5 m L,

0. 1 M ), H

2O (1 .5 m L) ,2 73

K ca ta ly st (3 .6

× 10

− 3 m m ol ), su bs tr at e (5

m m ol ),

H 2O

(1 m L) ,H

2 (4 0 ba r) ,3

53 − 40 3 K ,2

− 4 h

N H C

re du ct io n in

si tu

of R u−

N H C

co m pl ex

du ri ng

hy dr og en at io n re ac tio

n us in g H

T EM

hy dr og en at io n of

le vu lin ic ac id

by H

2 R u N P fo rm

ed du ri ng

R u ho m og en eo us

ca ta ly ze d hy dr o-

ge na tio

n re ac tio

n 13 6

ca ta ly st (0 .1 m ol

% ), su bs tr at e (4 .3 1 m m ol ), H

2O (1 0 m L) ,H

2 (1 2 ba r) ,4

33 K ,1

60 m in

ch ir al N -d on or

lig an ds

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

hy dr og en at io n of

ac et op he no ne

de ri va tiv es

by H

2 lo w en an tio

m er ic ex ce ss es ; no

re cy cl in g te st ; no

ch ar ac te r-

iz at io n of

th e sp en t ca ta ly st

15 8

[R u( C O D )( C O T )]

(3 0 m g, 0. 1 m m ol ), ch ir al lig an d

(0 .0 2 m m ol ), H

2 (3

ba r) ,T

H F (8 0 m L) ,2

98 K

ca ta ly st (0 .0 1 m m ol ), su bs tr at e (1

m m ol ), he pt an e

(2 5 m L) ,H

2 (4 0 ba r) ,3

23 K ,1

6 h

IL th er m al de co m po si tio

n of

[R u( C O D )( 2- m et hy la lly l) 2]

T EM

hy dr og en at io n of

th e al de hy de

in te rm

ed ia te

or ig in at ed

fr om

th e ac id -c at al yz ed

cl ea va ge

of lig ni n β- O -4

m od el by

H 2

R u N Ps

on IL

ar e ac tiv e in

ar en e an d ke to ne

hy dr og en at io n;

bi fu nc tio

na lr ea ct io n m ed ia co nt ai ni ng

bo th

a B rø ns te d ac id

ca ta ly st an d R u N Ps

le ad

to go od

yi el ds

of 2- ph en yl al co ho l

12 0

[R u( C O D )( 2- m et hy la lly l) 2]

(1 0−

20 m g) ,I L

(1 .4 − 2. 8 g) ,3

48 K ,1

8 h

ca ta ly st (5 − 30

m ol

% ), su bs tr at e (0 .0 5 m m ol ,0 .1

M ), H

2 (1 0 ba r) ,4

53 K ,5

− 20

m in

or 37 3 K ,

1− 3 h

R uP

t/ PP

P st ep w is e re du ct io n of

[R u( C O D )( C O T )]

an d [P t

(C H

3) 2( C O D )] ) or

[P t 2 (d ba ) 3 ] w ith

H 2

T EM

,W A X S,

IR hy dr og en at io n of

tr an s- ci nn am

al de hy de

by H

2 co re − sh el l st ru ct ur e, se le ct iv ity

tu ne d by

st ru ct ur e an d

co m po si tio

n of

th e ca ta ly st ; sy ne rg is tic

ef fe ct s ob se rv ed

11 7

[R u( C O D )( C O T )]

(5 7−

14 2 m g) ,[ Pt

(C H

3) 2( C O D )] ) (1 50 − 24 0 m g) ,P

PP (0 .2 4

m m ol ), T H F,

H 2 (3

ba r) ,3 43

K ,1 8 h;

[P t 2 (d ba ) 3 ]

(9 8−

24 6 m g) ,[ R u( C O D )( C O T )]

(1 42 − 22 7 m g) ,

PP P (0 .2 2 m m ol ), T H F,

H 2 (3

ba r) ,r t, 18

ca ta ly st (2 .5

m g) ,t ra ns -c in na m al de hy de

(7 .5

m m ol ), no na ne

(3 .5

m m ol ), 2- Pr O H

(5 0 m L) ,

H 2 (2 0 ba r) ,3

43 K

T EM

af te r ca ta ly si s sh ow

ed th at

sh el lr ic h N Ps

ag gl om

er at ed

an d co al es ce d af te r ca ta ly si s w hi le ri ch

R u or

sh el l-R

u N Ps

w er e st ab le

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1094

T ab le

1. co nt in ue d

st ab ili zi ng

ag en t

sy nt he tic

m et ho do lo gy

ch ar ac te ri za tio

n ca ta ly tic

re ac tio

n co nd iti on s

co m m en ts

re f

R uF

e/ SI LP

re du ct io n of

[F e[ N (S i( C H

3) 3)

2] 2]

2 an d [R u( C O D )

(C O T )]

w ith

H 2

T EM

,X A FS

hy dr og en at io n of

su bs tit ut ed

ar om

at ic su bs tr at es

Fe 25 R u 7

5/ SI LP

hi gh ly se le ct iv e fo r ke to ne

hy dr og en at io n,

w hi le R u/ SI LP

pr oc ee d to

th e fu ll hy dr og en at io n of

th e

fu rf ur al ac et on e m ol ec ul e;

ho t fil tr at io n te st re cy cl ed

tw ic e

w ith

ou t lo ss

of ac tiv ity

13 1

Fe [N

(S i( C H

3) 3)

2] 2]

2 (1 5. 1−

75 .3

m g) ,[ R u( C O D )

(C O T )]

(2 5. 2−

63 .1

m g) ,S

IL P (5 00

m g) ,

m es itl ye ne

(5 m L) ,H

2 (3

ba r) ,4

23 K ,1

8 h

ca ta ly st (0 .0 16

m m ol

of m et al ), fu rf ur al ac et on e

(0 .4

m m ol ), B M I·P

F 6 (1

m L) ,m

es ity le ne

(0 .5

m L) ,H

2 (2 0 ba r) ,3

73 K ,1

8 h

R uF

e/ H D A

re du ct io n of

[F e[ N (S i( C H

3) 3)

2] 2]

2 an d [R u( C O D )

(C O T )]

w ith

H 2

T EM

,I C P,

W A X S,

IR ,m

ag ne tic

m e-

su ar em

en ts

hy dr og en at io n of

st yr en e an d 2- bu ta no ne

se le ct iv ity

tu ne d by

R u/ Fe

ra tio

; no

re cy cl in g te st ; no

ch ar ac te ri za tio

n of

th e sp en t ca ta ly st

11 8

Fe [N

(S i( C H

3) 3)

2] 2]

2 (0 .5

m m ol ,1

88 .3

m g) ,

[R u( C O D )( C O T )]

(0 .5

m m ol ,1

57 .7

m g) ,H

D A

(1 .5

m m ol ,3 62 .2

m g) ,m

es itl ye ne

(1 0 m L) ,H

2 (3

ba r) ,4

23 K ,1

8 h

ca ta ly st (5

m ol

% ), su bs tr at e (2

m m ol ), B M I·P

F 6 (1

m L) ,m

es ity le ne

(0 .5

m L) ,H

2 (3

ba r) ,r t,

24 h

R uS n/ ph os ph in e

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2 fo llo w ed

by re ac tio

n w ith

tr ib ut yl tin

hy dr id e

T EM

,H R T EM

, W A X S,

IR ,N

M R

hy dr og en at io n of

st yr en e by

H 2

sy nt he si s of

tin -d ec or at ed

na no pa rt ic le s; re ac tiv ity

tu ne d by

Sn su rf ac e sp ec ie s

11 3

[R u( C O D )( C O T )]

(1 57

m g, 0. 50

m m ol ), PV

P (1

g) or

dp pb

(2 0. 8 m g, 0. 04 9 m m ol ,0

.1 eq ui v) ,T

H F

(6 0 m L) ,H

2 (3

ba r) ,r t, 68

h; tr i-n -b ut yl tin

hy dr id e

(1 3. 5 μL

,0 .0 5 m m ol ,0 .1 eq ui v) ,T

H F (1 0 m L) ,r t,

18 h

ca ta ly st (0 .0 3 m m ol R u) ,s ty re ne

(1 m L) ,T

H F (5

m L) ,H

2 (3

ba r) ,r t

R u- Pd

C u yo lk − sh el l na no -

cr ys ta ls

st ep w is e re du ct io n of

[P d( ac ac ) 2 ]/ C uC

l 2· 2H

2O an d

R uC

l 3 T EM

,X R D ,I C P

hy dr og en at io n of

st yr en e, di ph en yl ac et yl en e,

4- ni tr oc hl or ob en ze ne

fc c ch ar ac te r of

R u de pe nd s on

% Pd

; th e re du ct io n of

ni tr o

gr ou p w as

m or e pe rf or m an t w he n us in g fc c N Ps

co m pa re d

to hc p N Ps ; th e op po si te

tr en d w as

ob se rv ed

in st yr en e

hy dr og en at io n;

no re cy cl in g te st s; no

ch ar ac te ri za tio

n of th e

sp en t ca ta ly st ; no

re cy cl in g te st

15 4

[P d( ac ac ) 2 ] (7 .5

m g) ,C

uC l 2· 2H

2O (0 − 40

m g) ,

ol ey la m in e (3

m L) ,1 -o ct ad ec yl en e (3

m L) ,E

tO H ,

(1 m L) ,3 93

K ,1 0 m in ;R

uC l 3 (1 5. 6 m g) ,E

tO H

(1 m L) ,4

73 K ,1

2 h

ca ta ly st (0 .0 05

m m ol ), st yr en e (0 .1 7 m m ol ) or

di ph en yl ac et yl en e (0 .0 56

m m ol ), to lu en e (1 .5

m L) ,H

2, 35 3 K ; ca ta ly st (0 .0 05

m m ol ), 4-

ni tr oc hl or ob en ze ne

(6 m g) ,t ol ue ne

(0 .5

m L) ,

D M F (1 .5

m L)

H 2 (b al lo n) ,3

68 K

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1095

of stabilizing agents, such as polymers,73,74,102−111 phos- phines,112−116 N-donor ligands,50,117,118 ILs,83,84,119−131

NHC,96,132−139 alkynes,140 chitin,141 fullerene,50,142,143 cyclo- dextrins,144,145 dendrimers,146−148 and others. The stabilizing agents not only allow to synthesize and maintain the Ru NPs stable but also modulate their surface chemistry in a way which can be beneficial to obtain more efficient catalysts. The modulation of the surface properties is of major interest for catalysis, as the presence, or lack, of surface stabilizers can improve both the activity and selectivity on a given reaction. The noninnocent role of surface compounds in catalysis is nowadays well accepted, and therefore, more efforts are devoted to this topic.91 Because of the accessibility and the surface sensitivity toward the metal surface of reduction reactions, they have also been used as an indirect character- ization method to understand the surface of Ru-based NPs.113

Also, the addition of a second metal has been successfully used to improve the catalytic performances of Ru NPs cata- lysts.26,41,149−152 In this case, not only the nature of the second metal, but also the composition, the crystalline structure, or the chemical order of the associated metals (alloy, core−shell, among others), play an important role in the results. More complex systems, based in the combination of three153,154 or four155 different metals, have been also described as catalysts for reduction reactions. 4.1.1. Reduction of CC and CO Bonds. Reduction

of CC and CO double bonds have been extensively studied using Ru colloidal NPs as catalysts. Tables 1 and 3 summarize Ru catalyzed hydrogenation reactions of substrates containing these double bonds. By far, styrene has been the most studied substrate, but also a plethora of other arene-type compounds, ketones, aldehydes, among others, is also described. Selective hydrogenation reactions can provide useful

information about the surface chemistry of the nanoparticles. For example, in the case of the hydrogenation of styrene, as Ru is very active in the hydrogenation of the arene moiety, the obtention of the partially hydrogenated product (ethyl- benzene) is challenging and can give information about the role of the stabilizing surface compounds, such as their steric hindrance or electronic properties or the potential blockage of active sites. Ru NPs capped with terminal and internal alkynes showed different activity and selectivity in the selective hydrogenation of styrene; NPs capped with internal alkynes were highly selective toward the hydrogenation of the vinyl group.140 The characterization of the Ru NPs combined with theoretical calculations suggested that internal and terminal alkynes coordinate differently to the Ru surface; η2 side-on configuration and RuCCH−, respectively; which could explain the different reactivity of both systems. Likewise, the deposition of Sn atoms onto the surface or Ru/PVP or Ru/ dppb NPs modulated the reactivity of these systems when used as catalysts in the styrene hydrogenation.113 Indeed, the amount of Sn able to be accommodated onto the Ru NPs surface was dependent on the capping agent; Ru/PVP was able to integrate more Sn on the surface, when compared to Ru/ dppb, in which the reaction with tin precursor is limited due to the presence of the bulky ligand. Then, the nature of the stabilizing agent together with the amount of Sn deposited on the ruthenium surface tuned the catalytic activity of the Ru NPs (Table 2). Introducing 0.2 equiv of Sn onto the Ru/PVP catalyst led to a highly selective catalyst, as 95% of styrene was obtained at 100% of conversion. The same selectivity was

reached by only introducing 0.05 equiv of Sn onto the Ru/ dppb surface. Both the presence of a bulky ligand and of a small amount of tin onto the surface led to a highly selective catalyst. The increase of the amount of tin on the NP surface was detrimental to the activity in both catalysts used, Ru/PVP and Ru/dppb, indicating that the control of the selectivity is more likely due to a decrease on the reaction rate, than to a specific reactivity. This later has not being checked for instance by following the reaction over time. Similarly, styrene and 2-butanone hydrogenation selectivity

was modulated by the Fe content in RuFe NPs stabilized with HDA.118 The same synthesis procedure allowed preparation of a series of RuFe NPs displaying several Ru/Fe ratios, in this case using a supported ionic liquid phase (SILP)131 as a stabilizer. Fe25Ru75/SILP was highly selective for ketone hydrogenation in furan-based substrates, while Ru/SILP promoted the full hydrogenation of the substrates. The reduction of furfuralacetone was found highly sensitive to the amount of iron in the catalyst. Best compromise in terms of activity and selectivity was obtained for a Fe25Ru75 composition. Reaction rates for the CO hydrogenation of intermediates in furfuralacetone reduction were calculated to be 0.107 and 0.025 M/h for Fe25Ru75 and Ru100, respectively. These data and also reaction profiles over time supported that by adding a second metal to the ruthenium catalyst the hydrogenation of the heteroarene can be suppressed but also that the hydrogenation of the ketone group can be enhanced, leading to a highly selective catalyst. The crystalline structure of the metal cores has been found

to also influence the reactivity of Ru nanocatalysts in hydrogenation reactions. The crystalline structure of Ru NPs synthesized by epitaxial growth on PdCu alloyed NPs could be controlled in a way to obtain Ru NPs presenting a fcc or a hcp structure.154 The crystal structure of the nanoparticles affected the catalytic activity of the hydrogenation of 4-chloronitro- benzene; fcc Ru NPs had a superior activity when compared to the hcp ones. In opposition, fcc Ru NPs were less efficient in the hydrogenation of styrene. The reported conversion of styrene toward ethylbenzene at 4 h of reaction was over 98% catalyzed by hcp Ru NPs compared with 53% conversion with fcc Ru NPs catalyst. The different reactivity toward the reduction of the two different functional groups was attributed to a different adsorption of the substrates over Ru surface, but no further evidence is reported. Styrene hydrogenation activity and selectivity were also

tuned with Ru NPs bearing two different rigid and bulky NHC ligands derived from cholesterol.138 The different perform- ances observed were related to the flexibility of the NHC backbones; while ligands with higher steric hindrance lower

Table 2. Hydrogenation of Styrene with Ru/PVP/Sn or Ru/ dppb/Sn NPs as Catalystsa

product ratio A:B:C (%)

Sn equiv Ru/PVP/Sn Ru/dppb/Sn

0 0:0:100 0:0:100 0.05 0:1:99 0:95:5 0.1 0:5:95 0:88:12 0.2 0:95:5 0:99:1 0.5 15:85:0 52:47:1

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1096

the amount of ligand on the NP surface, higher quantities of free faces are accessible at the metallic surface, which are needed for the hydrogenation of aromatic rings, and therefore reduces the selectivity toward partially hydrogenated product. Differences on activity were also reported for substrates like acetophenone, biphenyl, and naphthalene. Other NHC ligands displaying different backbones and substituents at the N atoms have been also used as stabilizers for Ru NPs.133−135,138 The reactivity of these species in catalyzed hydrogenation reactions was governed by the bulkiness of the ligand, nevertheless, the use of slightly different synthetic and catalytic reaction conditions make the comparison difficult among them. In Ru NPs stabilized with phosphines, PPh3 or dppb, both

arene and carbonyl group of the acetophenone coordinate to the NPs surface competitively, giving predominantly the fully hydrogenated product. It was pointed out that the steric hindrance of the phosphine ligand governed the selectivity in several reduction reactions.116 The reported TON for Ru/ PPh3 are superior to those for Ru/dppb system in the hydrogenation of acetophenone, but not being a general rule, which indicates that the activity and selectivity depend on the reaction conditions too. In contrast to ruthenium systems, for Rh NPs stabilized by the same phosphine ligands, no ligand effect was observed. Polycyclic aromatic hydrocarbons were also hydrogenated

with Ru/PPh3 NPs under mild reaction conditions. 112 The

selectivity in the hydrogenation reaction of naphthalene, phenanthrene, triphenylene, and pyrene was mainly governed by experimental conditions, and the nature and number of substituents of the substrates (Figure 5).

Ru NPs are able to hydrogenate nonconjugated CC double bonds in very mild reaction conditions. In the case of α-pinene (Figure 6), Ru NPs have proven to be very efficient among other metals, such as Pd or Ni. Also, the reaction is more selective when performed in water.159 This explains than mainly Ru NPs stabilized with water-soluble polymers are described for this application and also that water-soluble Ru

salts are the preferred starting precursors to synthesize them (Table 3).107,109,159−162 Usually high selectivities toward cis- pinane are reported, and the catalytic systems can be recycled several times without significative loss of activity. Interesting enough, Ru NPs synthesized in the presence of a β- cyclodextrin polymer145 were able to selectively convert phenylacethylene to styrene in water under mild conditions (1 bar H2, 323 K).

4.1.2. Reduction of Nitro Compounds. Besides the reduction of CC and CO bonds by molecular hydrogen, Ru NPs are also active in the reduction of nitro derivatives, using H2

50,110,121,142,164 or NaBH4 73,74,103,104,106,148 as reducing

agents (Tables 4 and 5, respectively). Similarly, azo compounds were reduced in related conditions, by using Ru NPs as catalyst and NaBH4

106 or N2H4 111,165 as reductants

(Table 5). The catalytic hydrogenation of nitrobenzene may lead to

aniline, by hydrogenation of the nitro group, and/or to cyclohexylamine, by reduction of both the nitro and arene moieties, but other byproducts can be produced during the hydrogenation reaction, including azoxy, azo, and hydrazo derivatives, among others.166−168 Reactions performed in the liquid phase have used a variety of metal catalysts (Ni, Pt, and Pd), but Ru, due to its excellent ability to hydrogenate aromatic rings, is an interesting alternative to obtain selectively cyclohexylamine, or if modified conveniently, aniline.169,170

Ru/C60 system has demonstrated to be highly selective for the reduction of nitrobenzene, being able to hydrogenate the nitro group in first place and successively after the aromatic ring (Figure 7).164 This behavior is in contrast with that of other Ru-based heterogeneous catalysts.171 Theoretical calculations have shown that the coordination of the arene on Ru/C60 NPs is favored over the nitro group in terms of adsorption energy, but the addition of hydrides onto the Ru NP surface, which are likely to be present on the surface during the hydrogenation reaction, favors the coordination through the nitro group (Figure 7). Ligand effects on the selective hydrogenation of nitro-

benzene to cyclohexylamine were further studied by introducing several stabilizing ligands onto the surface of the Ru NPs.50 Ru/C60, Ru/PVP, and Ru/NHC proceeded in a stepwise manner (Figure 8), producing aniline first and then cyclohexylamine. This agrees with the fact that the reaction selectivity is mainly governed by surface hydrides present onto the Ru NPs surface. Ru/HDA showed a slightly different behavior which can be explained by the lability of the ligand. Even if the selectivity was mainly dominated by the intrinsic nature of the small Ru NPs, a clear influence of the ligands was also noticed. Less donor ligands promoted the hydrogenation of the N-phenylhydroxylamine intermediate, leading to more active and selective catalysts. Reported TOFs at 1 h of reaction were 136.9, 129.2, 82.8, 64.8 h−1 Ru/C60, Ru/HDA, Ru/PVP, and Ru/NHC, respectively. The evaluation of the catalytic properties and the reaction

kinetics in the reduction of nitroarenes or azo dyes with NaBH4 is widely used to obtain information about the performances of a catalyst because it can be easily implemented and conveniently measured by UV−vis spec- trophotometry (Table 5). These reactions have been reported to be sensitive to the size and structure of Ru NPs. Ru nanocages or nanoframes displaying a fcc structure have been synthesized through the chemical etching of a sacrificial seed,73,74 and tested as catalysts in the reduction of 4-

Figure 6. Hydrogenation of α-pinene.

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1097

T ab le

3. R u N P s as

H yd ro ge na ti on

C at al ys ts

of A lk en es

st ab ili zi ng

ag en t

sy nt he tic

m et ho do lo gy

ch ar ac te ri za tio

n ca ta ly tic

re ac tio

n co nd iti on s

co m m en ts

re f

po ly vi ny l al co ho l (P V A )

re du ct io n of

R uC

l 3 w ith

H 2

T EM

,X PS

,I C P,

co nf oc al

la se r sc an ni ng

m ic ro -

sc op e (C

LS M )

hy dr og en at io n of

α -p in en e an d

ot he r al ke ne s by

H 2

re cy cl ed

ei gh t tim

es w ith

ou t lo ss

in th e ca ta ly tic

ac tiv ity

an d

se le ct iv ity

15 9

R uC

l 3 (2 .1 m g, 0. 01

m m ol ), PV

A (M

w :7

8, 00 0, 15

m g) ,H

2O (m

L) ,H

2 (5 0 ba r) ,3

23 K

ca ta ly st /α

-p in en e 10 00 /1 ,α

- pi ne ne

(1 0 m m ol ), w at er ,H

2 (2 0

ba r) ,3

43 K

m et hy l la ur at e- m od ifi ed

ca rb ox ym

et hy lc el lu lo se

(H M -C M C )

re du ct io n of

R uC

l 3 w ith

H 2

T EM

,X R D ,C

LS M ,D

LS ,

IR hy dr og en at io n of

α -p in en e by

H 2

96 .6 %

co nv

w ith

98 .4 %

co nv ; re cy cl ed

20 tim

es w ith

lo ss

of ac tiv ity

du e to

ca ta ly st s ag gl om

er at io n an d R u le ac hi ng

(m ea su re d by

IC P)

16 0

R uC

l 3 (0 .0 08

m m ol ), H M -C M C

(2 m g) ,H

2O (2

m L) ,H

2 (2 0 ba r) ,

33 3 K

ca ta ly st (2

m g) ,α

-p in en e (5

m m ol ), 2 m g N a 2 C O

3, w at er ,H

2 (1 5 ba r) ,3

48 K ,5

T PG

S- 10 00

re du ct io n of

R uC

l 3 w ith

H 2

T EM

,X PS

,X R D ,D

LS ,

IC P

hy dr og en at io n of

α -p in en e by

H 2

re cy cl ed

at 10 0%

co nv er si on

up to

14 tim

es ,t he n ab ru pt

de cr ea se

of co nv er si on ; T EM

of th e sp en t ca ta ly st s

in di ca te s N P ag gl om

er at io n

10 7

R uC

l 3 (2

m g) ,T

PG S- 10 00

(2 m L 0. 5%

in H

2O ), (2

m L) ,H

2 (5

ba r) ,

32 3 K

ca ta ly st (0 .0 1 m m ol ), α -p in en e (2

m m ol ), N a 2 C O

3 (2

m g) ,H

2 (1 5

ba r) ,r t, 32 3 K

tr ib lo ck

co po ly m er

re du ct io n of

R uC

l 3 w ith

H 2

T EM

,X R D ,X

PS ,U

V − vi s

hy dr og en at io n of

α -p in en e by

H 2

re cy cl ed

at 10 0%

co nv er si on

up to

5 tim

es ,t he n ab ru pt

de cr ea se

of co nv er si on ; T EM

of th e sp en t ca ta ly st s

in di ca te s N P ag gl om

er at io n

10 9

R uC

l 3 (2

m g) ,T

PG S- 10 00

(2 m L 0. 5%

in H

2O ), (2

m L) ,H

2 (5

ba r) ,

32 3 K

ca ta ly st (0 .0 1 m m ol ), α -p in en e

(2 73

m g) ,H

20 (2

m L) ,H

2 (3

ba r) ,3

13 K ,2

β- cy cl od ex tr in

po ly m er

re du ct io n of

[R u( N O )( N O

3) 3]

w ith

N aB H

4 T EM

,D LS

,N M R ,I R ,

X PS

,T G A

hy dr og en at io n of

te tr ad ec en e an d

ot he r lo ng -c ha in

al ke ne s by

H 2

R u N Ps

or ga ni ze d in to

sm al l w or m -li ke

m ic ro do m ai ns

of si ze -c on tr ol le d na no pa rt ic le s; ca ta ly st re cy cl ed

an d re us ed

10 tim

es w ith

ou t lo ss

of ac tiv ity

14 5

[R u( N O )( N O

3) 3]

(2 69

m g, 40

μm ol ,) ,( 7. 8 m g, 0. 03

m m ol ), C T A B

(2 35

m g of

th e po ly m er

(0 .4 m m ol

of am

m on iu m

gr ou p) ,N

aB H

4 (4

m L,

0. 1 M ), H

2O (8

m L) ,2

98 K

ca ta ly st (4 0 μm

ol ), su bs tr at e (2

m m ol ), w at er

(1 2 m L) ,H

2 (1 0

ba r) ,3

03 K ,1

.5 h

se m ih yd ro ge na tio

n of

ph en yl ac et yl en e w ith

10 0%

se le ct iv ity

to w ar d st yr en e

hy dr og en at io n of

ph en yl ac et yl en e

ca ta ly st (4 0 μm

ol ), su bs tr at e (2

m m ol ), w at er

(1 2 m L) ,H

2 (1

ba r) ,3

23 K ,2

0 h

m on tm

or ill on ite

cl ay

re du ct io n of

[R u( N H

3) 6] C l 3 w ith

N aB H

4 T EM

,S A X S,

IC P,

B ET

hy dr og en at io n of

al ke ne s by

H 2

re cy cl ed

9 tim

es w ith

a sl ig ht ly lo ss

of ac tiv ity

16 3

[R u( N H

3) 6] C l 3,

m on tm

or ill on ite

cl ay ,N

aB H

4 (4

m L,

0. 1 M ), H

2O (4 0

m L) ,r t

ca ta ly st (0 .1

g) ,s ub st ra te

(2 m L) ,

w at er

(1 2 m L) ,H

2 (5 − 20

ba r) ,

31 3−

37 3 K

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1098

T ab le

4. R u N P s as

H yd ro ge na ti on

C at al ys ts

of N it ro be nz en e D er iv at iv es

st ab ili zi ng

ag en t

sy nt he tic

m et ho do lo gy

ch ar ac te ri za tio

n ca ta ly tic

re ac tio

n co nd iti on s

co m m en ts

re f

fu lle re ne

C 60

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,W A X S,

IC P,

IR ,

R am

an ,

EX A FS

,X PS

, D FT

re du ct io n of

ni tr ob en ze ne

by H

2C at al ys t (5

m g)

ni tr o-

be nz en e (4

m m ol ), do de ca ne

(1 m m ol ), H

2, (3 0 ba r) ,

Et O H

(3 0 m L) ,3

53 K

ch em

os el ec tiv e an d st ep w is e hy dr og en at io n;

D FT

16 4

[R u( C O D )( C O T )]

(3 0−

25 0 m g) ,C

60 (0 .1 0−

0. 16

or 0. 18

m m ol ) H

2 (3

ba r) ,C

H 2C l 2

(5 0−

40 0 m L) ,2

98 K

ca lc ul at io ns

sh ow

th at

th e co or di na tio

n m od e of

ni tr ob en ze ne

ch an ge s

w ith

th e hy dr id e co ve ra ge ; re cy cl in g te st w ith

sl ig ht ly de cr ea se

of ac tiv ity ; T EM

ch ar ac te ri za tio

n of

th e sp en t ca ta ly st

PV P,

H D A ,f ul le r-

en e C

60 ,N

H C

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,I C P,

IR ,

D FT

re du ct io n of

ni tr ob en ze ne

by H

2 ch em

os el ec tiv e an d st ep w is e hy dr og en at io n;

D FT

[R u( C O D )( C O T )]

(9 0−

25 0 m g) ,s ta bi liz er

(0 .0 4 m m ol

C 60 ,o

r 0. 18

m m ol

H D A ,o

r 0. 38

m m ol

N H C ,1 00

m g, or

10 00

m g of

PV P) ) H

2 (3

ba r) ,T

H F,

29 8 K

ca ta ly st (0 .0 25

m m ol

of R u)

ni tr ob en ze ne

(4 m m ol ),

do de ca ne

(1 m m ol ), H

2, (3 0 ba r) ,E

tO H

(3 0 m L) ,3 53

K ca lc ul at io ns

po in t ou t th at

hy dr id e co ve ra ge

is cr uc ia l fo r ad so rp tio

n of

th e ph en yl hi dr ox yl am

in e in te rm

ed ia te ; su rf ac e lig an ds

m od ul at e th e

ac tiv ity

an d se le ct iv ity

C 66 (C

O O H ) 1

2 re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,I C P,

IR ,

SS N M R ,

SA X S,

W A X S,

X PS

,t om

og ra -

ph y

re du ct io n of

ni tr ob en ze ne

by H

2 as se m bl ie s of R u N P;

se le ct iv ity

to w ar d an ili ne

up to

90 % ;n

o si gn ifi ca tiv e

ch an ge

on th e si ze

of N P af te r ca ta ly si s (b y T EM

); no

re cy cl in g te st s

14 2

[R u( C O D )( C O T )]

(0 .1 3−

0. 36

m m ol ),

C 66 (C

O O H ) 1

2 (0 .0 2−

0. 2 eq ui v) ,H

2 (3

ba r) ,

T H F (1 0−

15 0 m L) ,2

98 K

ca ta ly st (5

m g)

ni tr ob en ze ne

(4 m m ol ), do de ca ne

(1 m m ol ), H

2, (3 0 ba r) ,E

tO H

(3 0 m L) ,3

53 K

ph os ph in e- fu nc -

tio na liz ed

[B M M IM

] 3 [t pp t]

re du ct io n of

R uO

2 w ith

H 2

T EM

,X R D ,

X PS

re du ct io n of

ni tr ob en ze ne

by H

2 be tt er

ac tiv ity

th an

co m m er ci al R u/ C ; th e ad di tio

n of

[B M M IM

] 3 [t pp t]

is in

de tr im en t of

th e ac tiv ity ; no

ch ar ac te ri za tio

n of

th e sp en t ca ta ly st ;

no re cy cl in g te st s

12 1

R uO

2 (3

m g, 0. 02 25

m m ol ), [B M M IM

] 3 [t pp t]

(1 6. 3 m g, 0. 02 25

m m ol ), IL

(1 m L) ,H

2 (4

ba r) ,3

43 K

ca ta ly st (1 7. 75 .1 0−

3 m m ol ), IL

(1 m L) ,n

itr ob en ze ne

de ri va tiv e (s ub st ra te /R

u = 20 0) ,d od ec an e (1

m m ol ), H

2, (5 0 ba r) ,E

tO H

(3 0 m L) ,3

33 K

R uR

uO 2/ PV

P st ep w is e re ac tio

n; re du ct io n of

[R u( ac ac ) 3 ] ov er

pr ef or m ed

ir on

ox id e N Ps

T EM

,X R D ,

X PS

,X R F,

D LS

,I C P,

re du ct io n of

ni tr ob en ze ne

by H

2 ca ta ly st s di sp la ys

a R u4

+ / R u0

m ix tu re ;s om

e sy nt he si s le ad

to a m ix tu re

of m on om

et al lic

N Ps ; se le ct iv e hy dr og en at io n to w ar d an ili ne ; re cy cl in g

te st

11 0

ir on

ox id e N Ps

(1 5 m g) ,d

io ct yl et he r (7

m L) ,

1, 2- he xa de ca ne

di ol

(0 .0 5 g) ,O

A (1 0 μL

), [R u( ac ac ) 3 ] (0 .0 25

g) ,5

58 K ,4

5 m in

ca ta ly st (3

g· L−

1 ) ni tr ob en ze ne

(0 .0 6 μM

), H

2, (3 0 ba r) ,

42 3 K

R uC

o/ O A

re du ct io n of

[R u 3 (C

O ) 1

2] an d [C

o( ac ac ) 2 ] in

he pt an ol

T EM

,X A FS

, X R D ,I C P,

X A N ES

, EX

A FS

hy dr og en at io n of

4- ni tr os ty re ne

du m bb el l-s ha pe d C o−

R u na no st ru ct ur e co m po se d of

a C o na no ro d w ith

tw o en ds

ca pp ed

w ith

R u na no pl at es ; tu ni ng

m et al la tt ic e st ra in

11 0

[R u 3 (C

O ) 1

2] (8

m g) ,[ C o( ac ac )2 ] (6 .6

m g) ,

gl uc os e (1 0 m g) ,h

ep ta no l (2

m L) ,O

A m

(4 m L) ,4

23 K ,2

ca ta ly st (0 .3

m ol

% 4- ni tr os ty re ne

(0 .5

m m ol ), C M eO

H (3

m L) ,H

2 (b al lo n) ,2

98 K

R u w ith

3% la tt ic e co m pr es si on

ex hi bi ts hi gh

se le ct iv ity

fo r hy dr og en at io n

of 4- ni tr os ty re ne

to 4- am

in os ty re ne ;r ec yc le d 4 tim

es ;D

FT ca lc ul at io ns

R u−

Pd C u

yo lk − sh el l na no -

cr ys ta ls

st ep w is e re du ct io n of

[P d( ac ac ) 2 ]/ C uC

l 2· 2H

2O an d R uC

l 3 T EM

,X R D ,I C P

hy dr og en at io n of

st yr en e, di ph en yl ac et yl en e,

4- ni tr oc hl or ob en ze ne

fc c ch ar ac te r of

R u de pe nd s on

% Pd

; th e re du ct io n of

ni tr o gr ou p w as

m or e pe rf or m an t w he n us in g fc c N Ps

co m pa re d to

hc p N Ps ; th e

op po si te

tr en d w as

ob se rv ed

in st yr en e hy dr og en at io n;

no re cy cl in g

te st s; no

ch ar ac te ri za tio

n of

th e sp en t ca ta ly st ; no

re cy cl in g te st

15 4

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1099

nitrophenol, in order to demonstrate the higher reactivity of this crystallographic structure. Ru fcc icosahedral nanocages, which are very stable against temperature retaining their structure up to 573 K, were active in this reaction and displayed higher activities than Ru hcp NPs.74 Ru cubic, octahedral, and icosahedral nanocages were tested as catalysts displaying rate constants of 17.62, 20.64, and 41.21 s−1 mg−1, respectively. Likewise, Ru fcc nanoframes, synthesized as well by chemical etching of a nanosized template, performed better in this reaction than Ru nanowires displaying a hcp structure.73

In this case, the rate constants of Ru fcc nanoframes were reported to be 0.022 min−1 in opposition to 0.005 min−1

displayed by the hcp Ru nanowires. Nevertheless, no recycling test or characterization of the spent catalysts are reported. The reaction is also sensitive to the size of the Ru NPs.106 Ru NPs ranging from 2.6 to 51.5 nm were synthesized by a polyol reduction (using RuCl3 as Ru source and PVP as capping agent) where the size of the as-synthesized NPs was controlled mainly by the reaction temperature but also with the pH of the solution. Catalytic activity of the different sized Ru NPs was compared with that of other reported noble metal NPs. Ru- based catalysts were more active for the nitrophenol reduction than other nanosized metals (Ag, Au, Ir, and Pt). The reactivity of Ru NPs was dependent on their size and displayed a volcano trend, where 8 nm sized NPs were observed to be the most performant. The degradation of azo dyes was also successfully achieved using this Ru-based catalytic system. A multidentate bulky ligand with weak interactions with the metal NPs but strong enough to stabilize them has been described.104 The amphiphilic tripodal ligand tris(1,2,3-triazolyl)-polyethylene glycol (tristrz-PEG) (Figure 9), allowed to stabilize several metal NPs (Fe, Co, Ni, Cu, Ru, Ag, Pt, Pd, and Au). Ru NPs displayed a high catalytic activity in the reduction of nitrophenol and was recycled three times. Lattice strain can modify the electronic structure of catalysts

and therefore affect the adsorption of reactants. The reduction of [Ru3(CO)12] and [Co(acac)2] in heptanol using oleylamine as stabilizer allowed preparing dumbbell-shaped CoRu nanostructures, where a Co nanorod is capped with a Ru plate. NPs of several Ru/Co ratios were synthesized, and Co0.23−Ru0.77 catalyst was shown to be highly selective toward −NO2 hydrogenation (99%) in the hydrogenation of 4- nitrostyrene to 4-aminostyrene. The selectivity of RuCo NPs follows a volcano-type curve with increasing the Ru compressive lattice strain.172

4.1.3. Hydrodeoxygenation. To produce basic chemicals and renewable fuels from biomass feedstocks, it is necessary to remove oxygen from these materials due to the high amount of oxygenated moieties present in their structure. Hydrodeoxyge- nation is a metal catalyzed reaction, which allows removal of oxygen from oxygen-containing compounds in the presence of H2.

174−176 Ni, Co, Mo, Pt, Rh, Ru, among other supported metals have been used to upgrade biomass model com- pounds.175 Lignin, one of the components of biomass, requires depolymerization through C−O cleavage followed by hydro- deoxygenation. Likewise, cellulose requires the same procedure to produce polyols. Also, hydrodeoxygenation of vegetable oils can produce long-chain alkanes, a renewable fuel from biomass.174 Unsupported Ru NPs have found applications in hydrodeoxygenation of long-chain fatty acids177 and lignin monomeric and dimeric model substrates,130 including bimetallic RuNi NPs,102,178 eucalyptol,179 and carbonylT

ab le

4. co nt in ue d

st ab ili zi ng

ag en t

sy nt he tic

m et ho do lo gy

ch ar ac te ri za tio

n ca ta ly tic

re ac tio

n co nd iti on s

co m m en ts

re f

[P d( ac ac ) 2 ] (7 .5

m g) ,C

uC l 2· 2H

2O (0 − 40

m g) ,

ol ey la m in e (3

m L) ,1

-o ct ad ec yl en e (3

m L) ,

Et O H ,( 1 m L) ,3

93 K ,1

0 m in ; R uC

l 3 (1 5. 6

m g) ,E

tO H

(1 m L) ,4

73 K ,1

2 h

ca ta ly st (0 .0 05

m m ol ), st yr en e (0 .1 7 m m ol ) or

di ph en yl a-

ce ty le ne

(0 .0 56

m m ol ), to lu en e (1 .5

m L) ,H

2, 35 3 K ;

ca ta ly st (0 .0 05

m m ol ), 4- ni tr oc hl or ob en ze ne

(6 m g) ,

to lu en e (0 .5

m L) ,D

M F (1 .5

m L)

H 2 (b al lo n) ,3

68 K

Chemical Reviews Review

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1100

T ab le

5. R u N P s as

R ed uc ti on

C at al ys ts

of N it ro be nz en e an d A zo

D er iv at iv es

U si ng

N aB

H 4 or

N 2H

4 as

R ed uc in g A ge nt s

st ab ili zi ng

ag en t

sy nt he tic

m et ho do lo gy

ch ar ac te ri za tio

n ca ta ly tic

re ac tio

n co nd iti on s

co m m en ts

re f

PV P

R u fc c ic os ah ed ra ln

an oc ag es ; ch em

ic al et ch in g of

Pd R u co re − sh el l

N Ps .R

uP d N P (0 .1

m g) ,F

eC l 3 (3 0 m g) ,K

B r (3 00

m g) ,P

V P

T EM

,X R D ,t he r-

m al st ab ili ty

fo l-

lo w ed

by us in g

in si tu

X R D

re du ct io n of

4- ni tr op he no l by

N aB H

4 R u fc c st ru ct ur e en ha nc es ca ta ly tic

pr op er tie s; R u cu bi c, oc ta he dr al ,a nd

ic os ah ed ra l na no ca ge s ra te

co ns ta nt s: 17 .6 2,

20 .6 4,

an d 41 .2 1

s− 1 m g−

1 , re sp ec tiv el y

(5 0 m g) ,H

C l (0 .1 8 m L) ,H

2O (4 .8 2 m L)

ca ta ly st (0 .2 m M ,0 .5 m L) ,N

aB H

4 (2 0

m M ,1

m L)

4- ni tr op he no l( 0. 2 m M ,

1 m L) ,H

2O ,2

98 K

PV P

R u fc c na no fr am

es ;c he m ic al et ch in g of

Pd R u co re − sh el lN

Ps .R

uP d

N P,

Fe C l 3 (2 5 m g) ,K

B r (1 50

m g)

T EM

,X R D ,I C P

re du ct io n of

4- ni tr op he no l by

N aB H

4 R u fc c na no fr am

es ac tiv e in

th is re ac tio

n; no

re cy cl ab ili ty

or st ab ili ty

te st s af te r ca ta ly si s

PV P (2 5 m g) ,H

C l (0 .1 5 m L) ,H

2O (2 .8 5 m L)

ca ta ly st (1 0 μL

0. 21 8 m M ), N aB H

4 (5

μL ,2

M ))

4- ni tr op he no l (2 9. 5 μL

, 0. 5 m M ), H

2O (0 .6 9 m L) ,2

98 K

PV P

re du ct io n of

R uC

l 3 in

n- pr op an ol ,

T EM

,X R D ,U

V −

vi s, D LS

,X PS

re du ct io n of

4- ni tr op he no l an d ot he r

ni tr ob en ze ne

de ri va tiv es

by N aB H

R u N Ps

si ze s fr om

2. 6 to

51 .5 nm

by ad ju st in g th e pH

an d te m pe ra tu re ;

si ze

de pe nd en t ca ta ly tic

ac tiv ity ; be tt er

pe rf or m an ce s th an

Pt an d Ir

N Ps ; lo ss

of ac tiv ity

af te r se ve n re cy cl in g cy cl es ; fe w in fo rm

at io n

ab ou t th e sp en t ca ta ly st

10 6

R uC

l 3 (5 00

μL ,1

00 m M ), PV

P (5 0 m M ), n- pr op an ol

(1 0 m L) ,

30 3−

37 1 K ,1

0 h

C at al ys t( 4 μL

,1 0 nM

), N aB H

4 (2

m L

m M ,0

.1 M ), ni tr oa re ne

(2 0 μL

,1 0

m M ), 29 8 K

am ph ip hi lic

tr ip od al

lig an d tr is (1 ,2 ,3 -t ri -

az ol yl )- po ly et hy -

le ne

gl yc ol

re du ct io n of

R uC

l 3 w ith

N aB H

4 T EM

,X PS

,U V −

vi s

re du ct io n of

ni tr ob en ze ne

by N aB H

4 an d tr an sf er

hy dr og en at io n

R u N P ac tiv e in

re du ct io n re ac tio

ns in

w at er ; R u N P re cy cl ed

3 tim

es w ith

ou t si gn ifi ca nt

lo ss

of ac tiv ity ; T EM

of th e sp en t ca ta ly si s

in di ca te s a sl ig ht ly in cr ea se

of th e N P si ze

10 4

ca ta ly st (0 .2 − 2 m ol

% ), N aB H

4 (1 0

eq ui v) ,n

itr oa re ne

(1 eq ui v) ,2

98 K

R uC

l 3 (1

eq ui v) ,s ta bi liz er

(1 eq ui v) ,N

aB H

4 (1 0 eq ui v) ,H

2O (6

m L) ,2

98 K

ca ta ly st (0 .2 − 2 m ol

% ), N aO

H (0 .2

m m ol ), ni tr oa re ne

(0 .1

m m ol ),

H 2O

/i -p ro pa no l (1 /4 ,5

m L) ,3

53 K ,2

4 h

de nd ri m er

re du ct io n of

R uC

l 3 w ith

N aB H

4 T EM

,X R D ,X

PS ,

U V − vi s, IR ,c y-

cl ic vo lta m m o-

gr am

re du ct io n of

p- ni tr op he no l by

N aB H

4 no

re cy cl in g te st ; no

ch ar ac te ri za tio

n of

th e sp en t ca ta ly st

14 7

R uC

l 3· 3H

2O (1 0 m L,

1. 63

× 10

− 3 M ), de nd ri m er

(4 .2

× 10

− 5 M ),

N aB H

4 (5

m L,

1 M ), M eO

H (6 5 m L) ,H

2O (1 00

m L) ,r t, 24

h ca ta ly st (1 00

μL ), N aB H

4 (0 .2 5 m L,

10 0 m M ), p- ni tr op he no l( 0. 25

m L, 1

m M ), 29 8 K

po ro us

po ly m er

re du ct io n of

R uC

l 3 w ith

N aB H

4 or

et hy le ne

gl yc ol

T EM

,D R X ,I C P,

X PS

,B ET

,N M R

re du ct io n of

ni tr oa re ne s

R u N Ps

m or e ef fic ie nt

w he n st ab ili ze d w ith

th e po ly m er

co m pa re d to

ot he r st an da rd

su pp or ts ; be st ca ta ly st re cy cl ed

11 tim

es w ith

lo ss

of ac tiv ity ; T EM

,N M R ,X

PS ,I C P of

th e sp en t ca ta ly st s in di ca te

th at

is st ab le

10 3

R uC

l 3· 3H

2O (1 5 m g) ,p ol ym

er (5 0 m g) ,N

aB H

4 (4

m L,

1. 63

× 10

− 2

M ), M eO

H (2 0 m L) ,r t, 24 ; R uC

l 3· 3H

2O (1 5 m g) ,p

ol ym

er (5 0

m g) ,e th yl en e gl yc ol

(5 0 m L) ,4

53 K ,3

or 4 h

ca ta ly st (5

m g) ,N

aB H

4 (2 .5

m m ol ),

ni tr oa re ne

(0 .5

m m ol ), T H F/ H

20 (1 /3 ,m

L) ,2

98 K

PV P

re du ct io n of

R uC

l 3 in

et hy le ne

gl yc ol

at 44 3 K ,

T EM

,X R D ,U

V −

vi s, X PS

hy dr og en at io n of

or an ge

I (a zo

dy e)

by N

2H 4

de gr ad at io n ki ne tic

cu rv es

m ea su re d by

ab so rb an ce

in te ns iti es

of or an ge

I at 51 2 nm

;R u N Ps

sh ow

ed be tt er

pe rf or m an ce s th an

Pt an d

11 1

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1101

T ab le

5. co nt in ue d

st ab ili zi ng

ag en t

sy nt he tic

m et ho do lo gy

ch ar ac te ri za tio

n ca ta ly tic

re ac tio

n co nd iti on s

co m m en ts

re f

Ir N Ps ; R u N ps

ar e po is on ed

w ith

H 2S ,t hi s pa rt ic ul ar ity

is ex pl oi te d

to pr ep ar e pa pe r st ri ps

fo r H

2S ga s de te ct io n

R uC

l 3 (1 2. 3 m g) ,P

V P (5 5. 5 m g) ,e th yl en e gl yc ol

(1 0 m L) ,4

43 K ,

6 h

C at al ys t (8

nM ), or an ge

I (4

μL ,1

0 m M ), N

2H 4 (2

m L,

0. 8 M )

PV P

re du ct io n of

R uC

l 3 in

n- pr op an ol ,

T EM

,X R D ,U

V −

vi s, D LS

,X PS

hy dr og en at io n of

az o dy es

by N aB H

4 R u N Ps

de co m po se s az o dy es

in se co nd s; no

re cy cl ab ili ty

te st ; no

in fo rm

at io n ab ou t th e sp en t ca ta ly st

10 6

R uC

l 3 (5 00

μL ,1

00 m M ), PV

P (5 0 m M ), n- pr op an ol

(1 0 m L) ,

30 3−

37 1 K ,1

0 h

ca ta ly st (4

μL ,1

0 m M ), az o dy e (2 0

μL ,1

0 m M ), N aB H

4 (2

m L m M ,

0. 1 M )

4- su lfo ca lix [4 ]a re ne

re du ct io n of

R uC

l 3 w ith

N aB H

4. T EM

,S EM

,X R D ,

T G A ,I R ,D

LS hy dr og en at io n of

az o dy e by

N 2H

4 re cy cl ed

9 tim

es ;l ea ch in g te st ;s pe nt

ca ta ly st ch ar ac te ri ze d by

SE M ,I R ,

X R D

16 5

R uC

l 3 (0 .4 02

m m ol ), st ab ili ze r (0 .2 01

m m ol ), N aB H

4 (2 .4

m m ol ),

H 2O

(1 00

m L) ,2

98 K ,1

2 h

ca ta ly st (0 .5 m g) ,a zo

dy e (0 .0 5 m M ),

N 2H

4 (1 5 μL

), H

20 (3

m L)

R uP

d na no sh ee ts

st ep w is e re du ct io n of

[P d( ac ac ) 2 ] an d [R u( ac ac ) 3 ]

T EM

,X R D ,X

PS ,

IC P

re du ct io n of

4- ni tr op he no l by

N aB H

4 su bm

on ol ay er ed

R u de po si te d on

ul tr at hi n Pd

na no sh ee ts ; be tt er

pe rf or m an ce s in

te rm

s of

ac tiv ity

th an

m on om

et al lic

R u an d Pd

N Ps

in bo th

re ac tio

17 3

[P d( ac ac ) 2 ] (1 6 m g) ,P

V P (3 0 m g) ,c itr ic ac id

(1 70

m g) ,C

T A B (6 0

m g) ,[ W (C

O ) 6 ] (1 00

m g) ,D

M F,

(1 0 m L) ,3

53 K ,1

h; [R u

(a ca c)

3] (4

m g) ,P

V P (5 0 m g) ,a sc or bi c ac id

(5 0 m g) ,e th yl en e

gl yc ol

(1 0 m L) ,4

33 K ,1

ca ta ly st (P d:

7. 6 m M ; R u:

1. 0 m M ),

N aB H

4 (2 5 μL

,2 M ), 4- ni tr op he no l

(4 .9 5 m L,

0. 15

m M ), H

2O ,2

98 K

re du ct io n of

1- oc ty ne

ca ta ly st (P d:

7. 6 m M ; R u:

1. 0 m M ),

1- oc ty ne

(7 3. 5 μL

,0 .0 5 m m ol ),

n- de ca ne

(1 0 μL

,0 .0 5 m m ol ), Et O H

(6 m L) ,H

2 (1

ba r) ,2

98 K

A uP

dR u

st ep w is e pr oc ed ur e us in g ga lv an ic re pl ac em

en t

T EM

,U V − vi s

re du ct io n of

4- ni tr op he no l an d az o

dy e by

N aB H

no re cy cl in g te st ; no

ch ar ac te ri za tio

n of

th e sp en t ca ta ly st

15 3

C oC

l 2 (1

m L,

0. 4 M ),

ca ta ly st (1 00

μL ,5

pM ), N aB H

4 (1 00

μL ,1 00

m M

M ), 4- ni tr op he no l( 10 0

μL ,1

m M ), bu ffe r (7 00

μL ), 29 8 K

N aB H

4 (1 00

m L,

8 m M ), so di um

ci tr at e (1

m M ), H A uC

l 4 (6 0 m L,

0. 44

m M ), PV

P (1 % ), 32 3 K 2 h;

N aB H

4 (0 .4

m L,

0. 5 M ), 0. 31

m L,

20 m M ), 32 3 K ,2

h; R uC

l 3 (0 .1 66

m M )

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1102

compounds by a bimetallic RuFe bifunctional catalyst119

(Table 6). Lignin monomeric and dimeric model compounds, such as

phenol, guaiacol, diphenyl ether (4-O-5), benzyl phenyl ether (α-O-4), 2-phenylethyl phenyl ether (β-O-4), and benzofuran (β-5), have been hydrodeoxygenated using several metallic NPs (Pt, Rh, Ru, and Pd) stabilized in different ILs.130 In general, Pt/IL systems were more active and selective with all substrates, while Rh and Ru displayed similar behavior, the nature of the IL slightly modifying the selectivity. Ru NPs synthesized over a porous organic network exhibited high catalytic performance in stearic acid hydrogenation reaction with 95.6% conversion of stearic acid.177 The alcohol- hydrogenated product was then hydrodeoxygenated to produce C18 alkane or decarbonylated to C17 alkane. The ratio between C17/C18 could be modulated by the temper-

ature and pressure of the catalytic reaction. The Ru NPs stabilized with the porous organic network were better performing than other Ru-supported heterogeneous catalysts. Bifunctional Ru120,179 or RuFe119 NPs stabilized in IL or

SILP have been used as catalysts in the hydrodeoxygenation of eucalyptol, hydrogenation of the aldehyde intermediate originated from the acid-catalyzed cleavage of lignin β-O-4 model, and the hydrodeoxygenation of carbonyl-substituted aromatic substrates. Hydrodeoxygenation is often carried out with bifunctional catalysts that contain both metal and acid sites and are generally prepared by dispersing the metal NPs in a solid acidic support.174 Ru/SILP NPs were highly active and selective to the formation of p-menthane from eucalyptol, and the reaction selectivity was dependent on the acidity of the SILP.179 Acid cleavage of lignin β-O-4 model in the presence of Ru NPs allowed hydrogenation of the aldehyde intermediate product into 2-phenylalcohol in good yields.120 Bimetallic RuFe/SILP+IL-SO3H

119 was shown to be a very efficient system in the hydrodeoxygenation of carbonyl groups contained in aromatic substrates, the presence of Fe in small amounts (25%), preventing the hydrogenation of the aromatic ring131 and leading to the production of the aromatic dehydrodeoxygenated product in a very selective manner. The catalyst had a large substrate scope and could be easily recycled four times without loss of activity. NixRu100−x catalysts (x = 0, 75, 80, 85, 90, 95, and 100,

where x represents the molar percentage of Ni), were prepared

Figure 7. (a) π-mode coordination of a nitrobenzene molecule on a facet of a naked 2C60−Ru13 molecular complex. (b) NO2-mode coordination of a nitrobenzene molecule on the edge of a naked 2C60−Ru13 molecular complex. (c) Evolution of the energy difference between the two adsorption modes with respect to the ratio of H per Ru surface atoms present on the metallic cluster. Reproduced with permission from ref 164. Copyright 2016 American Chemical Society.

Figure 8.Most stable states after N-phenylhydroxylamine adsorption on (a) Ru13−(C60)2 and (b) Ru13H18−(C60)2. (c) Time−concentration curve for nitrobenzene hydrogenation with Ru−C60. Reproduced with permission from ref 50. Copyright 2018 American Chemical Society.

Figure 9. Amphiphilic tripodal ligand tris(1,2,3-triazolyl)-poly- ethylene glycol (tristrz-PEG).

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T ab le

6. R u N P s as

H yd ro de ox yg en at io n C at al ys ts

st ab ili zi ng

ag en t

m et ho do lo gy

ch ar ac te ri za tio

n re ac tio

n co nd iti on s

co m m en ts

re f

PV P

re du ct io n of

R uC

l 3 in

et ha no l/

H 2O ,

T EM

re hy dr og en at io n of

ce llo bi os e

se le ct iv ity

to w ar d so rb ito

ld ep en ds

on re ac tio

n pH

an d

m et al us ed ; no

re cy cl ab ili ty

te st ; no

in fo rm

at io n

ab ou t th e sp en t ca ta ly st

18 0

R uC

l 3 (0 .1 0 g, 0. 5 m m ol ), PV

P (0 .5 5 g, 5 m m ol ),

et ha no l (1 00

m L) ,H

20 (1 00

m L) ,3

53 ,2

h ca ta ly st (1 .6 7 × 10

− 3 m ol

R u/ L) ,c el lo bi os e (7 .3 1 m m ol ), H

20 (3 0

m L) ,H

2 (4 0 ba r) ,3

93 K ,1

2 h

or ou s or ga ni c ne tw or k

re du ct io n of

R uC

l 3 w ith

N aB H

4 T EM

,X R D ,T

G A ,

N M R ,I R ,X

PS ,

N 2 so rp tio

n, D FT

,I C P

N H

3- T PD

an al -

ys is

de hy dr og en at io n of

lo ng -c ha in

fa tt y ac id s

be tt er

ac tiv ity

an d se le ct iv ity

th an

R u ov er

in or ga ni c

su pp or ts ; re cy cl ed

6 tim

es w ith

ou t lo ss

of ac tiv ity ;

sp en t ca ta ly st an al yz ed

by T EM

,X R D

an d X PS

sh ow

in g no

ch an ge

re sp ec t th e as -s yn th es iz ed

m at er ia l

17 7

R uC

l 3 (6 0 m g) ,p

ol ym

er (2 00

m g) ,M

eO H

(1 30

m L) ,N

aB H

4 (1 0 m L,

1 M ), 29 8 K

ca ta ly st (2 0 m g) ,s ub st ra te

(0 .3 50

m m ol ), w at er

(7 0 m L) ,H

2 (3 0

ba r) ,4

53 K

IL re du ct io n in

si tu

of m et al sa lts

du ri ng

hy dr og en at io n

re ac tio

n us in g H

T EM

,X PS

,X R D

C − O

cl ea va ge

an d hy dr od eo xy ge na tio

n lig ni n m on om

er ic an d

di m er ic m od el co m po un ds

by H

ca ta ly st re cy cl in g fo r di ph en yl et he r us in g Pt

ba se d

ca ta ly st ; lo ss

of ca ta ly tic

ac tiv ity

af te r 3 ru ns

13 0

ca ta ly st (0 .0 1 m m ol m et al ), IL

(2 g, su bs tr at e (1

m m ol ), H

3P O

4 (0 .1 5

g) ,H

2 (5

ba r) ,4

03 K ,1

0 h

SI LP

de co m po si tio

n of

[R u( co d( m et hy la lly l) 2]

by H

2 T EM

,I C P

hy dr od eo xy ge na tio

n of

eu ca ly pt ol

in te gr at io n of

bo th

a m et al an d ac id

ca ta ly st on to

a si ng le su pp or t; se le ct iv e ca ta ly st s fo r th e hy dr o-

de ox yg en at io n of

eu ca ly pt ol

to p- m en th an e;

se le c-

tiv ity

de pe nd s on

th e ac id ity

of th e SI LP

17 9

[R u( co d( m et hy la lly l) 2]

(4 0. 8 m g) ,S

IL P (4

g) ,

C H

2C l 2 (4 0 m L) ,H

2 (1 20

ba r) ,3

73 K ,1

6 h

ba tc h:

ca ta ly st (7 5 m g) ,e uc al yp to l( 2. 4 m m ol ), H

2 (1 20

ba r) ,4 23

flo w : ca ta ly st (5 47

m g) ,e uc al yp to l (0 .0 5 M

eu ca ly pt ol

in he pt an e,

0. 3−

0. 9 m L/

m in ), H

2 (8 0 ba r, flo w = 3−

37 N

m L/

m in ),

38 6−

42 0 K

R uF

e/ SI LP

+I L- SO

3H re du ct io n of

[F e[ N (S i( C H

3) 3)

2] 2]

2 an d

[R u( C O D )( C O T )]

w ith

H 2

T EM

,S EM

,B ET

hy dr od eo xy ge na tio

n of

ca rb on yl -s ub st itu

te d ar om

at ic su bs tr at es

hi gh ly se le ct iv e;

no hy dr og en at io n of

ar om

at ic

m oi et ie s; ca ta ly st re cy cl ed

4 tim

es w ith

ou t lo ss

of ac tiv ity ; no

le ac hi ng

11 9

Fe [N

(S i( C H

3) 3)

2] 2]

2 (1 8. 8 m g) ,R

u( C O D )( C O T )]

(4 7. 0 m g) ,S IL P (5 00

m g) ,m

es itl ye ne

(5 m L) ,H

2 (3

ba r) ,4

23 K ,1

8h ; Fe R u/ SI LP

(3 75 .0

m g, 0. 15

m m ol ), ac et on e (5

m L) ,I L- SO

iH (2 04 .0

m g,

0. 37 5 m m ol ), rt ,1

ca ta ly st (5 8 m g, co nt ai ni ng

0. 01 5 m m ol

m et al an d 0. 03 8 m m ol

(2 .5 0

eq ui v)

IL -S O 3H

), su bs tr at e (0 .3 8 m m ol ), m es ity le ne

(0 .5 m L) ,H

2 (5 0 ba r) ,4

48 K ,1

0 h

R uN

i/ C T A B

re du ct io n of

R uC

l 3 an d N iC l 2 w ith

N aB H

4 T EM

,X R D ,X

PS hy dr og en ol ys is of

th re e lig ni n m od el su bs tr at es

N i re sp on si bl e fo r th e hy dr og en ol ys is ; R u an d R h ar e

pr ed om

in an tly

ac tiv e in

th e hy dr og en at io n of

th e

ar om

at ic ri ng s; hy dr og en at io n re te

de pe nd s on

R H

an d R u lo ad in g; hy dr og en ol ys is of

C (s p3 )−

O bo nd s

is pr ef er re d ov er

C (s p2 )−

O bo nd s

17 8

N iC l 2· 6H

2O (4 0. 4 m g, 0. 17

m m ol ), R uC

l 3· 3H

2O (7 .8

m g, 0. 03

m m ol ), C T A B (1 00

m g, 0. 27 4

m m ol ), N aB H

4 (2 0 m g, 0. 52 9 m m ol ), H

2O (3

m L) ,2

73 K

ca ta ly st (9 .4 5 × 10

− 3 m m ol ), ar om

at ic et he r (0 .1 89

m m ol ), H

2O (1

m L) ,H

2 (1

ba r) ,3

68 K ,1

6 h

R uN

i/ PV

P re du ct io n of

R uC

l 3 an d N iC l 2 w ith

N aB H

4 T EM

,X R D ,X

A S,

X A N ES

,E X A FS

, U V − vi s

hy dr og en ol ys is of

th re e lig ni n m od el su bs tr at es

N iR u (8 5%

N ia nd

15 % R u, N is ur fa ce

en ri ch ed ) be st

ca ta ly st ; bi m et al lic

sy st em

s be tt er

pe rf or m an ce s

10 2

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1104

by reduction of RuCl3 and NiCl2 with NaBH4, 102 and later,

tested as catalysts in the dehydrodeoxygenation of β-O-4 model compound. The yield and selectivity were correlated to the Ru/Ni ratio following a volcano-type curve (Figure 10). Ru NPs were able to hydrogenate the aromatic ring, while the increasing amount of Ni enhanced the C−O cleavage, Ni85Ru15 being the catalyst giving higher amounts of monomeric species. In addition, under the catalytic conditions studied, fully hydrogenated dimeric compounds did not undergo further C− O hydrogenolysis (Figure 11). More recently, it has been reported the application of the

same procedure to synthesize RuNi NPs but in the presence of the surfactant cetyltrimethylammonium bromide (CTAB) instead of PVP.178 Similar results were found, i.e., NixRu100−x catalysts were efficient toward C−O cleavage, while Ru NPs were mainly active in the arene hydrogenation (Figure 12). Cellulose can be converted to polyols through hydro-

deoxygenation reaction catalyzed by Ru-based nanocata- lysts.180−185 In a pioneering work,180 water-soluble Ru NPs were used to conduct hydrogenation and hydrogenolysis reactions of cellobiose into monomeric polyols, thus opening a new route for the valorization of cellulose, the world’s most abundant biopolymer. In this work, 2.4 nm of Ru NPs were obtained by reduction of RuCl3 in the presence of PVP in an ethanol/water mixture at 353 K. The catalytic reduction of cellobiose was conducted at 393 K at 40 bar of H2. Ru over performed other metals such as Pd, Pt, and Rh, in terms of selectivity to produce sorbitol (100% conversion and selectivity). Subsequently, Ru supported catalysts have been used to upgrade cellulose, mainly using carbonaceous supports.181,182,184,185 Interestingly enough, support effects were reported for this reaction by using the transfer hydrogenation methodology instead of molecular H2.

182 Ru over several carbon supports was reported to be active using 2- propanol as reduction agent, but Ru over alumina was not active to produce sugar alcohols from cellulose.

4.1.4. Reductive Amination of Carbonyl Compounds, Amination of Alcohols, and Other Miscellaneous Reduction Reactions. To obtain primary amines several methodologies have been developed, including hydroamino- methylation/hydroamination,186,187 alcohol amination,188,189

and reductive amination of carbonyl compounds.189,190

Colloidal Ru-based catalysts have found applications in these later reactions for the production of primary amines from ammonia.191 This could open new opportunities, for instance, to upgrade biomass-derived oxygen-rich materials.191−193

Heterogeneous catalysts for alcohol amination are scarce191,193−202 but include the use of Ru-based materi- als.191,193,194,199,201 These later are mainly supported catalysts, which often display better performances than other metals for this reaction,193,194 although Ni-based catalysts were also displaying high performances.197,198 Amino acids were obtained from α-hydroxyl acids derived from biomass and ammonia in high yields in the presence of Ru/CNT through the amination reaction.193 Ru/CNT catalyst surpassed other metal-based catalysts, including Pd, Pt, Rh, and Ir over CNT and Ni Raney, in terms of activity, and also other Ru-based catalysts supported in oxides such as SiO2, Al2O3, ZrO2, CeO2, and MgO. As mentioned before, colloidal-based catalysts allow a fine-tuning of their properties, if compared to supported catalysts, which permits access to more detailed information about the impact of certain characteristics in a given reaction. Recently, nonsupported Ru NPs stabilized with CTAB (ca. 2−Ta

bl e 6.

co nt in ue d

st ab ili zi ng

ag en t

m et ho do lo gy

ch ar ac te ri za tio

n re ac tio

n co nd iti on s

co m m en ts

re f

th an

m on om

et al lic

co un te rp ar ts ; lo w H

2 pr es su re

en an hc es

hy dr og en ol ys is ov er

hy dr og en at io n

N iC l 2· 6H

2O (4 .4

m g, 0. 01 87

m m ol ), R uC

l 3· 3H

2O (0 .9

m g, 0. 00 33

m m ol ), C T A B (4 8. 8 m g, 0. 44

m m ol ), N aB H

4 (4

m g, 0. 11

m m ol ), H

2O (3

m L) ,

27 3 K

ca ta ly st (0 .0 22

m m ol

m et al an d 0. 44

m m ol

PV P in

3 m L H

2O ),

su bs tr at e (0 .2 2 m m ol ), H

2O (1

m L) ,H

2 (1 0 ba r) ,3

68 K ,1

6 h

de po ly m er iz at io n of

or ga no so lv lig ni n-

ca ta ly st (0 .0 22

m m ol

m et al

an d 0. 44

m m ol

PV P in

3 m L H

2O ), su bs tr at e (5 0 m g) ,H

2O (1

m L) ,H

2 (1 0 ba r) ,3

68 K ,1

6 h

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9 nm) were investigated in direct amination of octanol and other alcohols into primary amines in the presence of ammonia.191 This work revealed that the amination of alcohol toward octylamine is insensitive to the size of the nano- particles, but the selectivity is not at high conversions. The self- coupling of the amine, leading to less selective systems because of the formation of secondary and tertiary amines, is almost suppressed for small NPs, therefore leading to highly selective catalyst (89% conversion, 90% selectivity). Electronic and steric properties of the NPs and the substrates are claimed to be plausible explanations of the size sensitive of this reactions but without any further evidence.

Ru-based catalysts have found applications in the reductive amination of carbonyl compounds in order to obtain amines selectively.189,192,199,203−208 Special focus is given to primary amines using NH3 and H2. Similarly to the amination of alcohols, the reductive amination of carbonyl compounds allows efficient upgrading of oxygen-rich biomass deriva- tives.189,192,199 Other metal-based heterogeneous catalysts have been successfully used in this catalytic reaction,205,209−217 but Ru seems to be highly efficient to produce primary amines.189,205 Up to now, Ru-based catalysts used in this reaction consist mainly in supported materials. It has been evidenced that a support effect on the performances of supported Ru catalysts.199,204 Ru/Nb2O5, Ru/TiO2 and Ru/ SiO2 catalysts displayed a different behavior in the reductive amination of furfural.199 Ru/Nb2O5 was very efficient for this reaction, and this fact was attributed to the lower electron density of Ru NPs deposited on Nb2O5 when compared to those of Ru/TiO2 and Ru/SiO2, which gave more electron-rich Ru surfaces. Support effects were also evidenced elsewhere,189

but in this case the control of the reactivity was related to the mixture of Ru and RuO2 on the surface. Recently, unsupported Ru NPs displaying a fcc structure proved to be an extremely efficient catalyst for the reductive amination of furfural and other substrates.192 The fcc Ru NPs (TOF = 1850 h−1, at 363 K) outperformed Ru/Nb2O5 (TOF = 520 h

−1, at 363 K) and Rh/Al2O3 (TOF = 990 h

−1, at 353 K) catalysts in terms of activity but displaying similar selectivity toward the primary amine (99%, 99%, and 92%, respectively). This catalyst was reused four times and was highly active and selective for other substrates. Other reduction reactions have been studied using Ru NPs

as catalysts, such as transfer hydrogenation reactions,155,218 or reduction of NOx105 which are summarized in Table 7.

4.2. Oxidation Reactions

Ru NPs have been successfully used as catalysts in oxidation reactions. Thus, the oxidation of several substrates with oxidation agents such as tert-butyl hydroperoxide (TBHP),219 H2O2,

220 or aerobic conditions135 is described in the literature (Table 8). Water-soluble Ru NPs were used in the allylic oxidation of α-pinene by TBHP to produce verbenone with 39% yield.219 Also, Ru NPs catalyzed the

Figure 10. (a) Thirteen products identified after β-O-4 hydrogenolysis. (b) Yields of monomers and dimers over Ni, Ru, and NiRu with varying Ni/Ru ratio. Reaction conditions: 0.22 mmol β-O-4, 3 mL of freshly prepared aqueous solution containing 0.022 mmol of metal and 0.44 mmol of PVP, 10 bar H2, 403 K, 1 h. Adapted with permission from ref 102. Copyright 2014 American Chemical Society.

Figure 11. Kinetic study on hydrogenolysis of β-O-4 over (a) Ni85Ru15 and (b) Ru. Reaction conditions: 0.22 mmol of β-O-4, 3 mL of freshly prepared aqueous solution containing 0.022 mmol of metal and 0.44 mmol of PVP, 10 bar H2, 403 K. Adapted with permission from ref 102. Copyright 2014 American Chemical Society.

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1106

oxidation of substrates such as 3,3,5,5-tetramethylbenzidine, o- phenylenediamine, and dopamine hydrochloride by H2O2. Ru/ PVP NPs converted ethanol to acetaldehyde with molecular O2 (30 bar).

221 Milder conditions (1 bar O2) were applied in the oxidation of alcohol and amine derivatives, using aerobic conditions by Ru/NHC NPs.135 The oxidation with Ru/NHC NPs proceeded smoothly, and it was also possible to perform consecutive oxidation/hydrogenation reactions. WAXS anal- yses of the Ru/NHC catalysts exposed to air showed that amorphous ruthenium oxide was formed only at the surface of the nanoparticles providing an unoxidazed Ru core, thus indicating the stability of the Ru nanosystem in the applied conditions. Because of the importance of CO removal from car exhaust

or fuel-cell systems, CO oxidation has been studied thoughtfully, both theoretically and experimentally.222 CO oxidation can be seen also as a model reaction, similar to the case of styrene hydrogenation as previously mentioned, which can bring further information about metal NPs nature and characteristics.223 Mono- and bimetallic Ru-based catalysts synthesized by wet procedures have been investigated for CO oxidation (Table 9). The influence of parameters such as Ru crystal structure, size, and in bimetallic systems, the ratio of the two metals, on the activity of the reaction has been underlined. Ru NPs displaying fcc or hcp crystalline structures were prepared selectively from [Ru(acac)3] and RuCl3, respectively, with controllable sizes ranging from 2 to 5.5 nm.13,224 The crystalline structure was controlled by the choice of the Ru source and the solvent, ethylene glycol or triethylene glycol, and the size was adjusted by varying the concentration of reagents and the stabilizer (PVP). TEM and XRD analyses pointed out the fcc character of the Ru NPs. In situ XRD probed the high thermal stability of the Ru fcc NPs, which were stable up to 723 K. The CO oxidation was dependent on both crystalline phase and size; small Ru fcc NPs outperformed hcp ones when displaying small sizes, while hcp Ru NPs were more performant at larger sizes (Figure 13). Ru nanochains

were synthesized in water from Ru seeds with cetyl trimethylammonium bromide as capping agent. The self- assembled nanochains were more efficient as CO oxidation catalysts than Ru nanoseeds (3.5 nm) and Ru spheres (6 nm).225

Bimetallic RuPd,226 RuCu,227,228 and RuCo3O4 229 catalysts

have been described as well. Ru deposited onto Co-rods and further thermally treated gave RuCo3O4 species, which were active toward the CO-oxidation reaction and outperformed the corresponding monometallic NPs (Figure 14).229 DFT calculations attributed the enhancement of the catalytic activity of RuCo3O4 species to the charge transfer from ruthenium to Co3O4, which activated more efficiently O2 and lowered the activation energy. A series of RuPd NPs have been synthesized from RuCl3 and

K2[PdCl4] by tunning the Ru/Pd ratio. 226 The crystallographic

structure of the bimetallic NPs changed from fcc to hcp when increasing the Ru content. Surface characterization was performed using solid-state 2H NMR; 2H NMR spectra after 2H adsorption showed that the chemical shift of the hydrides on the surface of the NPs depends on their composition (Figure 15). Ru0.5Pd0.5 was the most active catalyst, performing better than other RuPd mixtures and also than monometallic Ru, Pd, and Rh based catalysts (Figure 15). Following a similar procedure, nanosized RuxCu1−x alloys

were synthesized, which is remarkable because Ru and Cu are completely immiscible in bulk phase.227,228 XRD, TEM, and EDX suggest that Cu and Ru atoms are randomly mixed to form alloy structures. As observed with the close RuPd NPs system described above, the catalytic activity of RuxCu1−x alloys in the CO oxidation reaction depends on the Ru/Cu ratio; Cu0.2Ru0.8 nanoparticles demonstrated the best catalytic activity. IR studies provided better insights on the catalytic system. CO adsorbed onto the NPs surface was observed by IR; pure Ru NPs, displayed a CO band at 1986 cm−1 along with those of free CO gas at 2200−2050 cm−1. A blue-shift was observed when increasing the Cu content in the samples. After

Figure 12. Hydrogenolysis/hydrogenation of (left) 1-phenoxy-2-phenylethane (β-O-4 linkage), (middle) benzyl phenyl ether (α-O-4 linkage), (right) diphenyl ether (4-O-5 linkage), and product yield for selected metal combinations catalyzed by Ru100−xNix NCs. The black arrows refer to the M15Ni85 NCs, and the corresponding yields are in black. The blue arrows refer to the M60Ni40 NCs, and the corresponding yields are in blue in parentheses. The fractions comprise partially/fully hydrogenated dimers (orange), nonhydrogenated monomers (darker green), and hydrogenated monomers (lighter green). Adapted with permission from ref 178. Copyright 2018 The Royal Society of Chemistry.

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1107

T ab le

7. R u N P s as

C at al ys ts

in M is ce lla ne ou

s R ed uc ti on

R ea ct io ns

st ab ili zi ng

ag en t

m et ho do lo gy

ch ar ac te ri za tio

n re ac tio

n co nd iti on s

co m m en ts

re f

C T A B

re du ct io n of

R uC

l 3 w ith

N aB H

4 T EM

,O 2 tit ra tio

n, X R D ,

X PS

am in at io n of

oc ta no l

N Ps

si ze

eff ec t; th re e re cy cl in g te st s; no

le ac hi ng

19 1

R uC

l 3 (0 .2 2 g) ,C

T A B (2 .9 − 5 eq ui v) ,N

aB H

4 (0 .1 3

g) ,h ex an ol (2 .6 − 4. 5 eq ui v) ,H

2O (0 .5 − 4. 5 eq ui v) ,

27 3 K

ca ta ly st (1 0−

20 0 m g) ,s ub st ra te

(1 m L) ,d ec an e (1

m m ol ), N H

3 ga s, H

2 (2

ba r) ,4

53 K ,1

− 24

no ne

ac id ic tr ea tm

en t of

R u/ C a( N H

2) 2

T EM

,X R D ,N

2 ad so rp -

tio n−

de so rp tio

n is o-

th er m s, C O

ch em

is or p-

tio n,

X PS

,I R

re du ct iv e am

in at io n

re cy cl in g te st ,n

o fu rt he r ch ar ac te ri za tio

n of

th e sp en t ca ta ly st

19 2

R u/ C a( N H

2) 2 (2

g) ,2 -p ro pa no l( 15

m L) ,H

N O

3 (2

M un til

pH = 4) ,H

2O (2 0 m L) ,3

33 K ,2

− 4 h

ca ta ly st (0 .2

m g) ,s ub st ra te

(0 .5

m m ol ), N H

3- m et ha no l (4

m L,

8 m m ol ), H

2 (2 0 ba r) ,3

63 K ,0

− 6 h

R uF

e st ep w is e re ac tio

n; Fe SO

4 re du ct io n w ith

N aB H

4 fo llo w ed

by ga lv an ic re du ct io n

T EM

,I C P,

X PS

tr an sf er

hy dr og en at io n

ho t fi ltr at io n te st ;m

et al le ac hi ng

(R u (1 2 pp m ), Fe

(4 pp m ); re cy cl ed

5 tim

es w ith

a sl ig ht ly lo ss

of ac tiv ity

21 8

Fe SO

4 (4 .5

g) ,N

aB H

4 (0 .8

g) ,M

eO H

(6 0 m L) ,

H 2O

(3 60

m L) ;

ca ta ly st (5 0 m g, 1. 3 m ol

% ), su bs tr at e

(1 m m ol ), de ca ne

(1 m m ol ), K O H

(1 5 m ol

% ), 2- Pr O H

(5 m L) ,3 73

R uC

l 3 (1 0 m g) ,F

e N Ps

(1 00

m g) ,M

eO H

N i/ R u/ Pt /A

u re du ct io n of

m et al pr ec ur so rs

w ith

lit hi um

tr ie th yl -

bo ro hy dr id e

T EM

,I C P

tr an sf er

hy dr og en at io n

te tr am

et al lic

ca ta ly st di sp la ye d hi gh er

co nv er si on

to th e de si re d pr od uc t th an

m on o- ,

bi -, or

tr im et al lic

co un te rp ar ts ; no

re cy cl in g te st s; no

ch ar ac te ri za tio

n of

th e sp en t

ca ta ly st s

15 5

N iC l 2,

ca ta ly st (0 .3 − 0. 7 m ol

% ),

4- ph en yl -1 -b ut en e (1

m m ol ),

H 2O

/2 -P rO

H (3 /1 0,

3. 3 m L) ,3

73 K ,2

4 h

R uC

l 3, K A uC

l 4) H

2P tC l 6,

(0 .5 0 m m ol

in to ta l) ,

tr io ct yl ph os ph in e ox id e (0 .5 0 m m ol ), T H F (1 0

m L)

lit hi um

tr ie th yl bo ro hy dr id e (7 .5 m L, 1 M ), rt ,

2 h

R uP

d/ PV

P re du ct io n of

K 2[ Pd

C l 4]

an d R uC

l 3 in

tr ie th yl en e

gl yc ol

T EM

,X R D ,X

PS ,S

SN M R

re du ct io n of

N O x

R uP

d N P di sp la ys

be tt er

N O x re du ct io n ac tiv ity

th an

R h;

th eo re tic al ca lc ul at io ns

sh ow

th at

th e el ec tr on ic st ru ct ur e of

Pd 0. 5R u 0

.5 is si m ila r to

th at

of R h in ve rs e

vo lc an o- ty pe

be ha vi or

in re du ct io n ac tiv ity

w ith

re sp ec t th e at om

ic ra tio

of Pd

an d

R u

10 5

K 2[ Pd

C l 4]

(1 63 .4

m g) ,R

uC l 3 (1 31 .1 ), PV

P (4 44

m g) ,t ri et hy le ne

gl yc ol

(1 00

m L) ,H

20 (4 0 m L) ,

47 3 K

tu bu la r qu ar tz

re ac to r w ith

ca ta ly st ,

m ix tu re

si m ul at in g au to m ot iv e ex -

ha us t, 29 3−

87 3 K

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1108

T ab le

8. R u N P s as

O xi da ti on

C at al ys ts

st ab ili zi ng

ag en t

m et ho do lo gy

ch ar ac te ri za tio

n re ac tio

n co nd iti on s

co m m en ts

re f

am m on iu m

su rf ac ta nt s

(H EA

16 C l, H EA

16 B r,

H EA

16 B F 4 ,T

H E-

A 16 C l)

re du ct io n of

R uC

l 3 an d N iC l 2 w ith

N aB H

4 T EM

,D LS

ox id at io n of

α -p in en e

39 %

yi el d of

ve rb en on e fr om

α -p in en e;

R u N Ps

w ith

am m on iu m

su rf ac ta nt s

H EA

pe rf or m

be tt er

th an

ot he r R u N Ps ;c ou nt er io n (X

= C l, B r, B F 4 ) sl ig ht ly

in fl ue nc es

th e ke to ne

se le ct iv ity ; re cy cl in g te st ; T EM

af te r ca ta ly si s

21 9

R uC

l 3· 3H

2O (1 0 m g, 3. 8 × 10

− 5 m ol ,1

eq ui v) ,

am m on iu m

su rf ac ta nt

(7 .6

× 10

− 5 m ol ,2

eq ui v) ,

N aB H

4 (3 .6

m g, 2. 5 eq ui v) ,H

2O (1 0 m L) ,2

73 K

ca ta ly st (1 .9

× 10

− 5 m ol ), α -p in en e

(1 .9

× 10

− 3 m ol ), t- B H P (5 .7

× 10

− 3

m ol ), w at er

(5 m L) ,3

h, 29 3 K

− co m m er ci al

T EM

,S EM

, D LS

,z et a

po te nt ia l,

U V − vi s

ox id at io n of

se ve ra l

so m e te st us in g O

2 as ox id iz in g ag en t; no

re cy cl in g te st or

ch ar ac te ri za tio

n of th e

ca ta ly st s af te r re ac tio

n 22 0

3, 3, 5, 5- te tr am

et hy lb en zi di ne ,

o- ph en yl en ed ia m in e, an d do pa m in e hy dr o-

ch lo ri de )

ca ta ly st (2 .5 − 20

μg /m

L) ,s ub st ra te (0 .1 m M ),

H 20

2 (0 .1

m M )

lo ng -c ha in

N H C

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,W A X S,

SS N M R ,I R

ox id at io n of

se ve ra ls ub st ra te s w ith

O 2

se le ct iv ity

m od ul at ed

w ith

su rf ac e lig an d;

ox id iz ed

N P ch ar ac te ri ze d by

T EM

an d W A X S;

no re cy cl in g te st

13 5

[R u( C O D )( C O T )]

(1 00

m g) ,N

H C

(0 .1 − 0. 3

eq ui v) ,H

2 (3

ba r) ,T

H F (5 0 m L) ,2

98 K ,2

0 h

ca ta ly st (1

m g) ,s ub st ra te

(0 .2

m m ol ),

tr ifl uo ro to lu en e (1

m L) ,O

2 (1

ba r) ,2 98

K ,

16 h

ox id at io n/ hy dr og en at io n of

se ve ra l su bs tr at es

w ith

O 2, th en

H 2

ca ta ly st (1 − 1. 5 m g) ,s ub st ra te

(0 .2

m m ol ),

tr ifl uo ro to lu en e (1

m L) ,O

2 (1

ba r) ,2 98

K ,

16 h;

H 2 (5

ba r) ,r t or

31 8 K ,4

or 16

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T ab le

9. R u N P s as

O xi da ti on

C at al ys ts

of C O

st ab ili zi ng

ag en t

m et ho do lo gy

ch ar ac te ri za tio

n re ac tio

n co nd iti on s

co m m en ts

re f

PV P

re du ct io n of

[R u( ac ac ) 3 ] or

R uC

l 3 in

et hy le ne

gl yc ol

or tr ie th yl en e gl yc ol

T EM

,X R D

C O

ox id at io n

sy nt he si s of

R u fc c ([ R u( ac ac ) 3 ])

or hc p (R

uC l 3)

de pe nd in g on

th e m et al pr ec ur so r

us ed ; C O

ox id at io n si ze - an d st ru ct ur e- de pe nd en t; hi gh er

C O

ox id at io n ac tiv ity

of fc c R u N P co m pa re d w ith

th at

of hc p R u N Ps ,f or

si ze s la rg er

th an

3 nm

13 ,2 24

[R u( ac ac ) 3 ] or

R uC

l 3 (2 .1

m m ol ), PV

P (1 − 10

m m ol ), so lv en t (2 5−

50 0 m L) ,4 73

K ,3

tu bu la r qu ar tz

re ac to r w ith

qu ar tz

w oo l,

ca ta ly st (1 50

m g) ,g as

m ix tu re

of C O /O

2/ H e (C

O /O

2/ H e:

0. 5/ 0. 5/ 49

m L· m in

− 1 ) ,3

73 K

C T A B

re du ct io n of

[R u( N O )( N O ) 3 ] w ith

N aB H

4 T EM

,X R D ,U

V − vi s, D LS

C O

ox id at io n

R u na no ch ai ns

sy nt he si ze d in

a tw o st es

pr oc ed ur e ar e m or e pe rf or m an t in

C O

ox id at io n th an

R u sp he ri ca lN

PS ;c at al yt ic ac tiv ity

de pe nd s al so

on th e su pp or tu

se d

R u na no ch ai ns

ca n be

re cy cl ed

w hi le R u N Ps

te nd

to in cr ea se

th e si ze

du ri ng

tim e

an d lo ss

so m e ac tiv ity

22 5

[R u( N O )( N O ) 3 ] (1 25

μL ,1

.5 w t % ),

N aB H

4 (5 0 μL

,0 .2 5 M )

fi xe d be d re ac to r, ca ta ly st (1 00

m g) ,g as

m ix tu re

of C O /O

2/ N

2 (C

O /O

2/ N

2: 1/ 5/ 19 ,5

0 m L· m in

− 1 ) ,3

23 − 57 3 K

C T A B (4

m L of

22 m M ), as co rb ic ac id (3 00

m L,

0. 1M

) an d [R u( N O )( N O ) 3 ] (5 0 m L,

1. 5 w t % w ), 34 3 K ,0

.5 h,

rt ,1

2 h

R u−

C o 3 O

4 an ne al in g of

R u

T EM

,X R D ,s pe ci fi c su rf ac e ar ea

an d po re

vo lu m e, T G A ,X

PS ,

D FT

C O

ox id at io n

ca ta ly st st ab le an d ac tiv e af te r 30

h of

us e;

T EM

an d X R D

an al ys es

af te r ca ta ly si s

sh ow

in g no

ap pr ec ia bl e ch an ge

22 9

in co rp or at ed

C o- M O Fs

in N

2 (8 73

K ) an d

th en

in ai r (5 23

K )

fi xe d- be d fl ow

re ac to r; ca ta ly st (5 0 m g) ,

fe ed

ga s (1 % C O ,9 9%

ai r, fl ow

ra te

30 m L/

m in ), 32 3 K

R u x C u 1

− x

po ly ol

sy nt he si s

T EM

,X R D ,X

R F,

in si tu

IR ,

th er m al st ab ili ty

in ve st ig at ed

by in

si tu

sy nc hr ot ro n X R D

m ea su re m en ts

C O

ox id at io n

R u 0

.8 C u 0

.2 di sp la ye d hi gh er

ca ta ly tic

ac tiv ity

th an

ot he r bi m et al lic

m ix tu re s an d

m on om

et al lic

R u an d C u N Ps

22 8

[R u( ac ac ) 3 ], (3 18 .7

m g, 0. 8 m m ol )

[C u( O A c)

2·H 2O

], (2 39 .6

m g, 1. 2 m m ol ),

di et hy le ne

gl yc ol (2 00

m L) ,P V P (4 40

m g,

4 m m ol ), 49 3 K

tu bu la r qu ar tz

re ac to r w ith

qu ar tz

w oo l,

ca ta ly st (1 50

m g) ,g as

m ix tu re

of C O /O

2/ N

2 (C

O /O

2/ N

2: 0. 5/ 0. 5/ 49

m L· m in

− 1 ) ,4

33 K

C u 0

.5 R u 0

.5 po ly ol

sy nt he si s

T EM

,X R D ,X

R F,

in si tu

IR ,

th er m al st ab ili ty

in ve st ig at ed

by in

si tu

sy nc hr ot ro n X R D

m ea su re m en ts

C O

ox id at io n

fc c st ru ct ur e, al lo y N P C u 0

.5 R u 0

.5 be tt er

ca ta ly tic

pe rf or m an ce s in

C O

ox id at io n th at

fc c R u N P

22 7

[R u( ac ac ) 3 ], (7 96 .8

m g, 2. 0 m m ol ))

[C u( O A c)

2·H 2O

], (3 99 .4 m g, 2. 0 m m ol )) ,

di et hy le ne

gl yc ol (3 30

m L) ,P V P (8 80

m g,

4 m m ol ), 49 3 K

tu bu la r qu ar tz

re ac to r w ith

qu ar tz

w oo l,

ca ta ly st (1 50

m g) ,g as

m ix tu re

of C O /O

2/ H e (C

O /O

2/ H e:

0. 5/ 0. 5/ 49

m L· m in

− 1 ) ,4

33 K

R u x Pd

1− x

po ly ol

sy nt he si s

T EM

,X R D ,h

yd ro ge n ab so rp -

tio n by

pr es su re − co m po si tio

n is ot he rm

s, SS N M R ,X

C O

ox id at io n

in cr ea si ng

th e R u co nt en t ch an ge s th e cr ys ta llo gr ap hi c st ru ct ur e fr om

fc c to

hc p;

R u 0

.5 Pd

0. 5 be st ca ta ly st

22 6

R uC

l 3, (2 5. 9−

23 5. 6 m g)

K 2[ Pd

C l 4] ,

(3 2. 6−

29 3. 8 m g) ,t ri et hy le ne

gl yc ol

(1 00

m L) ,H

2O (4 0 m L) ,P

V P (4 44

m g, 4

m m ol ), 47 3 K

tu bu la r qu ar tz

re ac to r w ith

qu ar tz

w oo l,

ca ta ly st (1 50

m g) ,g as

m ix tu re

of C O /O

2/ H e (C

O /O

2/ H e:

0. 5/ 0. 5/ 49

m L· m in

− 1 )

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further exposure to O2, only CO bands corresponding to the CO adsorbed onto Ru atoms remained, indicating that probably CO is activated on this metal. DFT calculations suggested that the Ru atoms are responsible for the CO activation as well and that the presence of Cu lowers the CO adsorption energy. The decrease of the CO adsorption energy

was originated by a site exchange from Ru hollow sites to Ru top sites.228

4.3. Fischer−Tropsch Reaction

Ru-based compounds are very active catalysts for Fischer− Tropsch reaction, but the limitation of their use in industry probably arises from their price even if they are active under milder temperatures and are less sensitive to H2O in comparison to Fe and Co based catalysts, which are greatly exploited.230 This reaction is largely studied in gas phase, but it can be achieved in liquid phase by using Ru NPs.231,232

Fischer−Tropsch reaction with Ru catalysts is a size233−235 and structure236 sensitive reaction (Table 11). Fischer−Tropsch reaction catalyzed by fcc and hcp Ru NPs was studied experimentally and theoretically.236 The main conclusion of the DFT study points out that fcc Ru displays some open facets with low CO dissociation barriers, which is in contrast with the fact that only few edges with low CO dissociation barriers are available in hcp Ru catalyst. Experimentally, synthesized Ru NPS with fcc structure and a size of 6.8 nm showed a high mass specificity toward the reaction, as predicted, and superior to hcp Ru NPs (Figure 16). To obtain better insights of the size effect in Ru NPs-

catalyzed Fischer−Tropsch catalysis, theoretical calculations on the electronic structure of CO adsorbed in Ru step-edge

Figure 14. (a) FESEM and (b) TEM images of the as-prepared Co-MOF precursor. (c) TEM, (d) HRTEM, and (e) SAED images of the Ru− Co3O4 interfacial structure. (f−i) EDS mapping images of the Ru−Co3O4 interfacial structure. Adapted with permission from ref 229. Copyright 2018 The Royal Society of Chemistry.

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1111

sites have been carried out (Figure 17).234 It has been demonstrated that step-edge sites are more reactive toward CO activation than flat surfaces by using theoretical Ru NPs models of 1 and 2 nm diameter in size. The CO cleavage is easier in step-edge sites in larger NPs; this is due to the smaller extent of the Ru−O interaction in the η2 adsorption mode on smaller NPs, which destabilizes the transition state for direct CO cleavage. Experimentally, the size effect was investigated by using Ru

NPs synthesized from RuCl3 and [Ru(acac)3], which allowed the obtaining of Ru NPs ranging from 1.2 to 5.2 nm. Ru NPs catalysts showed a maximum of activity around 2.3 nm for nanoparticles between 1.2 and 3.7 nm. With a further increase of the Ru NPs size, the conversion rate increased strongly.

Also, it was observed that the nanoparticle size affected the selectivity; by increasing the size a decrease on the oxygenate products, selectivity was observed.237 Later on, the study was extended in order to understand the size effect observed235 by combining high-energy XRD with theoretical calculations. By using The high-energy XRD technique, the core and surface atomic-scale structure of real Ru NPs smaller than 6 nm was determined in good detail, allowing identification and quantification of step-edge and terrace sites on the surface of Ru NPs. DFT calculations confirmed that CO dissociation proceeds easily on these surface atoms, and it has been observed that CO hydrogenation correlates with Ru surface atoms with coordination numbers of 10−11. In previous studies by the same authors,238−240 stepped Ru (1121) surfaces, which display low barrier for CO activation and bind reaction intermediates strongly, were compared to Ru (0001) dense surfaces, with a high barrier for CO activation and a high selectivity for methane production. It was pointed out that the sites with low barrier for CO dissociation were responsible for the Fischer−Tropsch reaction with low production of methane; on the other hand, the dense surfaces were the preferred sites for CO hydrogenation to produce methane. Size and surface ligands effects on the Fischer−Tropsch

reaction were also investigated.98 Ru/PVP (1.3 nm size) and

Figure 15. (a) The solid-state 2H NMR spectra for PdxRu1−x nanoparticles and 2H2 gas. All of the samples were measured under 101.3 kPa of

2H2 gas at 303 K. (b) The chemical shift position of the broad absorption lines in PdxRu1−x. (c) Temperature dependence of CO conversion in PdxRu1−x nanoparticles supported on γ-Al2O3; x = 0 (red downward focusing triangles), 0.1 (orange open squares), 0.3 (yellow open triangles), 0.5 (green solid circles), 0.7 (blue-green solid triangles), 0.9 (light-blue solid squares), and 1.0 (blue solid downward facing triangles). Inset: metal composition dependence of T50. Adapted with permission from ref 226. Copyright 2014 American Chemical Society.

Figure 16. Reaction performance of Ru catalysts. (A) Activity of fcc NCs (6.8 nm) and hcp NCs (6.8 and 1.9 nm) at 413 and 433 K. (B) The Arrhenius plot and the extracted apparent FTS barriers are indicated. The reaction was conducted at 3.0 MPa syngas (CO/H2 = 1:2 mol ratio), 0.2 mmol catalyst, 800 rpm stirring. Adapted with permission from ref 236. Copyright 2017 American Chemical Society.

Figure 17. (a) Blyholder model for CO adsorption on Ru surface sites. (b) Different types of terrace and step-edge sites on metal NPs (marked in yellow) of different sizes and experimental NP size effect on reactivity. Adapted with permission from ref 234. Copyright 2016 American Chemical Society.

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Ru/dppb (1.9 and 3.1 nm size) were synthesized from [Ru(COD)(COT)] in the presence of the respective stabilizing agents. This study did not evidence a clear effect of the Ru NPs size on catalysis performance in terms of activity or selectivity. Nevertheless, the dppb ligands on the surface were shown to play a key role on the activity. Ru/PVP NPs were not active at 393 K and slightly active at 423 K, while the Ru/dppb NPs were active in both cases, with high selectivity toward alkenes and alkanes (Table 10).

4.4. C−H Activation and Other Reactions H/D (or T) exchange through C−H activation has been achieved with Ru NPs for several compounds in mild reaction conditions. Nitrogen,85,89,242,243 phosphorus,163,244 and sul- fur245 containing compounds, or alkanes,163,246 have been selectively deuterated using Ru NPs as catalysts, stabilized with PVP, phosphines or NHC ligands, and in some cases by supported Ru catalysts (Table 12). The first study by Chaudret and co-workers on deuteration247 demonstrated that Ru/PVP NPs were able to deuterate pyridines, quinolones, indoles, and alkyl amines with D2 with high chemo- and regioselectivity; this methodology was also successful for the enantiospecific C−H activation/deuteration of amino acids and peptides. Experimental evidence and theoretical calculations showed that the labeling is governed by the coordination of the substrate to the ruthenium surface and that the surface ligands modulate the efficiency of the labeling procedure. Unsupported Ru NPs have been applied as catalysts to other

reactions such as Wittig olefination,163,249 selenylation,245 or isomerization.250 The synthetic procedure and the catalytic reaction conditions, together with the main features of the catalytic system, are summarized in Table 13.

4.5. Transformation of CO2 Because it is a cheap, nontoxic, abundant, renewable feedstock, CO2 appears as an attractive building block in order to produce fuels and value-added products that are currently issued from nonrenewable resources (see Figure 18 for chemicals that may be obtained from CO2),

252,253 but intensive efforts are still required in order to develop technologies for its valorization as a “raw material”.254

Chemical production based on CO2 is not a facile task due to several technical challenges. It requires major scientific breakthroughs because only highly efficient technologies can make it economically viable while aiming at more sustainable

chemical production. The main difficulty to transform CO2 derives from its high thermodynamic stability. Large-scale CO2 transformation requires to develop very effective and selective catalytic systems,255 which present a good balance between the energy needed and the gain obtained (Figure 19). Chemical transformation of CO2 has been largely inves-

tigated with homogeneous catalysts.6,257,258 Heterogeneous (bulk) catalysts are also explored,259 with good performance toward the formation of formic acid, methanol, and dimethyl ether260 or methane.261 More recently, encouraging results were achieved with metal catalysts at the nanoscale prepared by a molecular approach, thus evidencing the relevance of this class of materials for this catalysis.260 As it will be seen hereafter, to our best knowledge, only a few papers describe ruthenium catalysts based on well-defined Ru NPs or bimetallic RuM NPs for the challenging chemical trans- formation of CO2. Products obtained are mainly HCOOH, CO, and CH4 but also C2+ hydrocarbons.

4.5.1. Transformation of CO2 into HCOOH. Formic acid (FA; HCOOH) is a valuable basic chemical with different uses (preservative agent, antibacterial, insecticide, or deicing) and plays also a major role in synthetic chemistry (as an acid, reductant, and precursor) for syntheses.262 Despite a relatively small hydrogen content (4.4 wt %; 53 g·L−1 hydrogen at rt and ambient pressure), FA also provides an alternative for chemical energy storage, being one of the best among liquid storage and transport media for H2.

263 If the chemical reduction of CO2 by using hydrogen is a highly attractive route to produce FA, it remains a significant challenge. This process is thermodynami- cally unfavorable, due to the strong entropic contribution (ΔG0298 = 32.9 kJ mol−1) and thus necessitates appropriate catalysts. Direct hydrogenation of CO2 into FA has been extensively

studied using homogeneous catalysts (mainly based on Ru, Rh, and Ir but also on non-noble metals like Fe, Co, Ni, and Cu) using various conditions and temperatures in the range rt to 393 K).6,257,262,264,265 Efficient complexes display electron-rich metal centers by using electron-donating ligands and are able to activate H2 under the form of hydrides and to transfer these hydrides to CO2 for some of them under mild conditions, but despite excellent catalytic performances (both in terms of activity and selectivity) and heterogenization (mainly on silica- and polymer-based materials or porous organic polymers) to

Table 10. Fischer−Tropsch Activitiesa and Selectivitiesb of Ru NPs as a Function of the Stabilizer, Size, and Reaction Temperaturec

aActivity evaluated from the consumption of H2. TOFs normalized per number of Ru surface atoms. bSelectivity calculated only for methane,

alkanes, and alkenes as products (water and remaining H2 and CO omitted for the sake of clarity). cAdapted with permission from ref 98.

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T ab le

11 . R u N P s as

Fi sc he r−

T ro ps ch

C at al ys ts

st ab ili zi ng

ag en t

m et ho do lo gy

ch ar ac te ri za tio

n re ac tio

n co nd iti on s

co m m en ts

re f

ol ei c ac id

th er m al de co m po si tio

n (5 08

K ) of

[C o 2 (C

O ) 8 ] an d

[R u 3 (C

O ) 1

2] in

di ph en yl et he r

T EM

,A P- X PS

,X A S

(u nd er

ox id iz in g, re -

du ci ng ,a nd

re ac tiv e ga s

en vi ro nm

en ts )

ca ta ly st (7 0 m g)

fe ed

ga s

m ix tu re

of H

2/ C O /A

r (2 0 ba r, H

2/ C O /A

r: 2/ 1/ 0. 08 )

sy nt he si s of

a va ri et y of

C o−

M bi m et al lic

ca ta ly st s; sl ig ht

di ff er en ce s to

th at

of pu re

C o

24 1

PV P

hy dr ot he rm

al sy nt he si s K 2P tC l 4 (0 .0 24

m m ol ), R uC

l 3· xH

2O (0 .2 16

m m ol ), PV

P (1 00

m g) ,H

C H O

(0 .1

m L) ,H

C l

(0 .0 62

m L,

1M ), H

2O (1 5 m L) ,f or m al de hy de

(0 .1 m L,

40 w t % ), 43 3 K ,8

T EM

,I C P,

X R D ,

X A N ES

,E X A FS

ca ta ly st (0 .2

m m ol ), sy n-

ga s (C

O :H

2 = 1: 2 30

ba r) ,4

23 K

R u fc c hi gh er

ac tiv ity

in FT

S hi gh er

se le ct iv ity

to w ar d C 5+

co m po un ds

th an

hc p N P;

re cy cl in g ex pe ri m en ts at

42 3 K sh ow

sl ig ht ly de cr ea se

of ac tiv ity

in fi rs t ru ns

an d

re m ai ne d co nt an t af te r 10

cy cl es ; D FT

ca lc ul at io n po in ts ou t th at

C O

di ss oc ia tio

n is

m or e fa vo ra bl e is fc c R u N P

23 6

PV P

re du ct io n of

[R u( ac ac ) 3 ] in

1, 4- bu ta ne di ol

T EM

,I C P,

IR ,E

X A FS

R u (5 0 μm

ol ), H

2O (3

m L) ,C

O /H

2 (3 0 ba r,

H 2/ C O

= 2) ,

40 3−

50 3 K ,3

− 24

R u N Ps

ra ng in g fr om

1. 2 to

5. 2 nm

; se le ct iv ity

an d ac tiv ity

de pe nd

on R u N Ps

si ze

23 5, 23 7

[R u( ac ac ) 3 ] (3 0 m g) ,P

V P (1 70

m g) ,T

H F (2

m L) ,1

,4 -

bu ta ne di ol

(3 0 m L) ,( 25 − 50 0 m L) ,4

98 K ,2

re du ct io n of

R uC

l 3 w ith

H 2

R uC

l 3 (4 0 m g) ,P

V P (2 20

m g) ,H

2O (1

m L) ,H

2 (2 0 ba r) ,

42 3 K ,2

PV P; dp pb

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,S SN

M R ,i n si tu

am bi en t- pr es su re

X PS

re ac tio

n do ne

on a qu ic k

pr es su re

va lv e N M R

tu be

no si ze

eff ec t; lig an d eff ec t on

th e ac tiv ity

of th e re ac tio

n 98

[R u( C O D )( C O T )] ,P

V P or

dp pb ,H

2 (3

ba r) ,2

98 K

R u (0 .0 2−

0. 05

m m ol

R u) ,

13 C O /H

2 (3

ba r,

13 C O /

H 2 1/ 1) ,3

93 − 42 3 K

1− 5 da ys

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1114

T ab le

12 . R u N P s as

C − H

A ct iv at io n C at al ys ts

fo r La be lli ng

A pp

lic at io ns

st ab ili zi ng

ag en t

sy nt he tic

m et ho do lo gy

ch ar ac te ri za tio

n ca ta ly tic

re ac tio

n co nd iti on s

co m m en ts

re f

R u/ dp pb ,

R uP

t/ dp pb ,

Pt /d pp b

re du ct io n of

[R u( C O D )( C O T )] ; [P t( C H

3) 2( C O D )] ; [P t( db a)

2] w ith

H 2

de ut er at io n of

al ka ne s D

2 (6

ba r) ,3

33 K ,

24 h

is ot op e ex ch an ge

an d P−

C bo nd

cl ea va ge

PV P;

N H C

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

de ut er iu m

an d tr iti um

la be lin g of

pu ri ne

de ri va tiv es

an d ph ar m ac eu tic al s D

2 (2

ba r) ,3

28 − 35 3 K ,3

6 h

hy dr og en -is ot op e la be lin g of

nu cl eo ba se

de ri va tiv es

in m ild

co nd iti on s; br oa d

sc op e;

m od ifi ca tio

n of

th e su rf ac e st ab ili ze r co ul d in cr ea se

th e effi

ci en cy

of th e la be lin g

24 2

[R u( C O D )( C O T )] ,P

V P or

N H C ,H

2 (3

ba r) ,T

H F,

29 8 K

dp pb

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

de ut er at io n of

al ka ne s R u/ dp pb ,s ub st ra te

(1 m L) ,T

H F (1

m L) ,D

2 (6

ba r) ,3 33

K ,

24 h

C − H

ac tiv at io n of

al ka ne s w as

st ru ct ur e de pe nd en t; on ly cy cl op en ta ne

w as

sm oo th ly de ut er at ed

24 4

[R u( C O D )( C O T )] ,d

pp b,

H 2 (3

ba r) ,T

H F,

29 8 K

R u/ C

co m m er ci al ca ta ly st

de ut er iu m

an d tr iti um

la be lin g of

th io et he r

su bs tr uc tu re s in

co m pl ex

m ol ec ul es

C (s p3 )−

H ac tiv at io n di re ct ed

24 7

R u/ C

(5 w t % ,1

21 .2

m g, 30

m ol

% ),

su bs tr at e (0 .2 m m ol ), D

2 (2

ba r) ,s ol ve nt

(2 m L) ,3

33 K ,2

or 72

by a su lfu r at om

; la be lin g of

co m pl ex

st ru ct ur es

in m ild

co nd iti on s

su lfo na te d

N H C

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,W A X S,

IR ,T

G A ,

N M R

de ut er at io n of

L- ly si ne

en an tio

sp ec ifi c H /D

ex ch an ge

of th e am

in o ac id

L- ly si ne ; in fl ue nc e of

pH on

th e ac tiv ity

an d se le ct iv ity : lo w pH

H /D

is re du ce d or

ne gl ig ib le ; hi gh

pH in cr ea se s ac tiv ity

an d ch an ge s se le ct iv ity

[R u( C O D )( C O T )]

(2 50

m g, 0. 8 m m ol ), su lfo na te d N H C

(0 .2

eq ui v) ,K

O tB u (1 9. 7 m g, 0. 17 6 m m ol ,0

.2 2 eq ui v) ,H

2 (3

ba r) ,

T H F (3 0 m L) ,2

98 K ,2

0 h

[R u( C O D )( C O T )] ,P

V P,

H 2 (3

ba r) ,T

H F,

29 8 K

ca ta ly st (2

m g, 8%

), L- ly si ne

(2 1. 92

m g,

0. 15

m m ol ), D 2 (2

ba r) ,D

2O (2

m L) ,

32 8 K ,4

2 h

PV P

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

de ut er at io n of

ch ir al am

in es

de ut er iu m

in co rp or at io n at

st er eo ge ni c ce nt er s; hi gh

se le ct iv ity

to w ar d

he te ro at om

α -p os iti on ;m

ec ha ni st ic st ud ie s su gg es t th at

a di m et al la cy cl e is

th e ke y in te rm

ed ia te

[R u( C O D )( C O T )] ,P

V P,

H 2 (3

ba r) ,T

H F,

29 8 K

ca ta ly st (8

m g, 3. 3%

), su bs tr at e (0 .1 5

m m ol ), D 2 (2

ba r) ,T

H F or

D 2O

(2 m L) ,3

28 K ,3

6 h

PV P

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

de ut er at io n of

ph os ph in e, ph os ph in e ox id e

an d ph os ph ite

ph en yl ri ng s in ph en yl -o

r ph en yl -a lk yl ph os ph in es

ar e se le ct iv el y de ut er at ed

at th e or th o po si tio

n; in di ca tio

n of

lig an d co or di na tio

n tr ho ug th

th e P at om

; no

de ut er at io n of

tr ip he ny lp ho sp hi te

24 8

[R u( C O D )( C O T )] ,P

V P,

H 2 (3

ba r) ,T

H F,

29 8 K

ca ta ly st (8

m g, 3. 3%

), su bs tr at e (0 .1 5

m m ol ), D 2 (2

ba r) ,T

H F (1

m L)

PV P

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

de ut er at io n of

az a co m po un ds

m ild

re ac tio

n co nd iti on s; go od

la be lin g yi el ds

w ith

hi gh

ch em

o- an d

re gi os el ec tiv iti es

24 3

[R u( C O D )( C O T )] ,P

V P,

H 2 (3

ba r) ,T

H F,

29 8 K

ca ta ly st (3 % ), D 2 (1

or 2 ba r) ,T

H F,

rt or

32 8 K ,3

6 h

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1115

T ab le

13 . R u N P s as

C at al ys ts

in O th er

R ea ct io ns

st ab ili zi ng

ag en t

m et ho do lo gy

ch ar ac te ri za tio

n re ac tio

n co nd iti on s

co m m en ts

re f

PV P

hy dr ot he rm

al sy nt he si s

T EM

,I C P,

X PS

,E X A FS

, EP

ae ro bi c cr os s- de hy dr og en at iv e co up lin g (C

− H ) ac tiv a-

tio n

R u na no ca ta ly st w ith

a di ff er en t ox id at io n le ve l; lo ss

of ac tiv ity

af te r 6

ca ta ly tic

cy cl es

24 6

R uC

l 3· xH

2O (0 .2 4 m m ol ), PV

P (1 00

m g) ,

N a 2 C

3H 2O

4·H 2O

(8 0 m g) ,H

C l( 0. 06 2 m L,

1M ),

H 2O

(2 5 m L) ,f or m al de hy de

(0 .1

m L,

40 w t % ),

43 3 K ,8

h, 1 h,

or 24

ca ta ly st (8

m ol

% R u) ,t et ra hy dr oi so qu in ol in e de ri va -

tiv es

(0 .1

m m ol ), in do le s (4

eq ui v) ,H

2O /M

eO H

(1 /1 ), A cO

H (1 0−

48 m L) ,2

98 K

IL re du ct io n of

se ve ra l R u co m pl ex es

w ith

H 2; R u

co m pl ex

(1 .1 6 w t %

R u) ,I L (0 .8 5 m L) ,H

2 (4

ba r) ,3

h, 32 3 K

T EM

,I C P,

X R D ,X

PS W itt ig

ol efi na tio

n go od

yi el ds

in st ilb en e pr od uc ts ,b

ut lo w E/ Z se le ct iv ity ; [R uC

l 2( C

6H 6) ] 2

pr ec ur so r pr od uc ed

th e m os t ac tiv e ca ta ly st ; re cy cl ed

5 tim

es w ith

ou t

ap pr ec ia bl e lo ss

of ac tiv ity

24 9

ca ta ly st (2 50

m g) ,a lc oh ol

(0 .1

m ol ), ph os ph or us

yl id e

(0 .1 1 m ol ), w at er

(5 m L) ,1

h, 34 3 K

m on tm

or ill on ite

cl ay

re du ct io n of

[R u( N H

3) 6] C l 3 w ith

N aB H

4 T EM

,S A X S,

IC P,

B ET

W itt ig -t yp e re ac tio

n of

be nz yl al co ho ls an d ph os ph or us

yl id es

m od er at e yi el d an d lo w di as te re os el ec tiv ity ; no

re cy cl in g te st ; no

ch ar ac te ri za tio

n of

th e sp en t ca ta ly st

16 3

[R u( N H

3) 6] C l 3,

m on tm

or ill on ite

cl ay ,N

aB H

4 (4

m L,

0. 1 M ), H

2O (4 0 m L) ,r t

ca ta ly st (0 .1

g) ,( 3, 4, 5- tr im et ho xy ph en yl ) m et ha no l (1

m m ol ), m et ho xy la te d be nz yl tr ip he ny lp ho sp ho ni um

ha lid e (1 .5 m m ol ), n- B uL

i( 6. 25

m L,

1. 0 m m ol ), T H F

(2 m L) ,3

53 K ,1

R u/ R uO

x/ PV

P hy dr ot he rm

al sy nt he si s R uC

l 3· xH

2O (0 .2 4 m m ol ),

PV P (1 00

m g, N a 2 C

3H 2O

4·H 2O

(1 40

m g) ,H

2O (2 5 m L) ,f or m al de hy de

(4 00

μL ,4 0 w t%

), 43 3 K ,

8 h

T EM

,I C P,

X R D ,X

PS ,

X A FS

)

se le ny la tio

n of

he te ro cy cl es

ac tiv ity

re la te d to

th e ra tio

R u/ R uO

x vo lc an o- sh ap ed

re la tio

ns hi p

24 5

on e se t po st re du ce d by

H 2 (1 ,4

,1 2 h)

ca ta ly st (8

m ol

% R u) ,i nd ol e/ he te ro cy cl e (0 .1

m m ol ),

Ph Se Se Ph

(2 eq ui v) ,2

98 K

on e se t po st ox id iz ed

by O

2 (1 ,4

h) D FT

ph os ph in es

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,T G A

sy nt he si s of

py ra zi ne s fr om

α -d ik et on es

R u N P ac t as

hy dr og en

bo rr ow

in g an d as

de hy dr og en at io n ca ta ly st s;

X an tp ho s st ab ili ze d R u N P pe rf or m s be tt er

th an

ot he r ph os ph in e

st ab ili ze d R u N P;

no re cy cl in g

25 1

[R u( C O D )( C O T )]

(6 0. 0 m g, 0. 19

m m ol ), ph os -

ph in e (0 .1

eq ui v) ,H

2 (3

ba r) ,T

H F (6 0 m L) ,

29 8 K

ca ta ly st (1

m ol

% ), α -d ik et on e (1 .0 m m ol ), am

m on iu m

fo rm

at e (5 .0

m m ol ), D M F (3 .0

m L) ,3

58 K ,1

− 12

N H C

re du ct io n of

[R u( C O D )( C O T )]

w ith

H 2

T EM

,S EM

, IC P,

X PS

is om

er iz at io n of

es tr ag ol e

R u N P fo rm

ed du ri ng

R u ho m og en eo us

ca ta ly ze d ol efi n m et ha te si s ar e

ac tiv e in

al ke ne

is om

er iz at io n re ac tio

n; is ol at ed

R u N ps

be ar in g th e sa m e

N H C lig an d ar e ve ry

ac tiv e fo r is om

er iz at io n of

es tr ag ol ;p

oi so n te st us in g

H g or

ph os ph or us

lig an ds

25 0

[R u( C O D )( C O T )] ,N

H C ,H

2 (1 0 ba r) ,p

en ta ne ,

29 8 K

ca ta ly st (1

m ol % ), es tr ag ol (2

m m ol ), to lu en e (9 .3 m L) ,

H 2 (1 2 ba r) ,4

33 K ,1

60 m in

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solve separation and recovery concerns, homogeneous catalysts are still far from the industrial expectation.260,266

In the opposite, despite the early works involving Pd black267 and Ni-Raney268 and their advantages for continuous operation and product separation, the development of heterogeneous catalysts for this reaction lags signifi- cantly,260,266 but presently, the number of supported nano- particulate metal catalysts tends to increase, mainly based on Pd or Au.253 Very surprisingly, only a few examples of Ru- based heterogeneous catalysts or nanocatalysts are reported although Ru complexes (including heterogenized and isolated single-atomic systems269) are known to be efficient for the synthesis of FA.270 If low to moderate catalytic performances are observed in comparison to the TON or TOF values achieved by ruthenium molecular catalysts encouraging results are reported, as it will be described hereafter. An interesting bridge between homogeneous Ru catalysts

and nanocatalysts has been made by Dupont and co-workers who reported excellent results in the hydrogenation of CO2 using a ruthenium cluster. It is worth to mention that “nanocluster” is usually used for metal NPs that are very small and well-controlled. They studied the behavior of [Ru3(CO)12] dispersed in ionic liquids (ILs).

271 They observed remarkable activity and selectivity for the formation of HCOOH with high TON (17000) and TOF values at mild pressure (total pressure 40 bar; H2/CO2 = 1/1) and temperature (333 K). Among the ILs tested, they observed that the imidazolium-based IL associated with the acetate

anion acts as a precursor for the formation of the catalytically active Ru−H species, as a catalyst stabilizer, and as an acid buffer, shifting the equilibrium toward free formic acid. Moreover, the immobilization of this catalytic system onto a solid support facilitated the separation of FA. What is important to note here is the multiple role of the IL that enhances the catalytic activity of the [Ru3(CO)12] cluster. Second, even if it contains only three ruthenium atoms, the catalytic performance of this Ru cluster strongly encourages studying of more Ru NPs because higher activity can be expected due to the multiple active sites they expose. As a first example of Ru NPs, Kojima and co-workers

reported on the use of metallic RuNPs (primary particles of ca. 3−5 nm and agregates of ca. 200−240 nm) prepared by reduction of RuCl3 in a methyl alcohol solution under solvothermal conditions for the hydrogenation of supercritical CO2 to formic acid in the presence of triethylamine as a base (total pressure 13 MPa ; H2/CO2 = 5/8 ; T = 353 K).

272 The activity was drastically improved by using a prereduction procedure and adding an appropriate quantity of water to the colloidal suspension in methyl alcohol. The most active nanocatalyst was obtained with 4 mL of water, providing a TON (expressed as the number of moles of FA produced per mole of Ru) of 6351 in 3 h. When adding PPh3, a negligible activity was observed, indicating the presence of a negligible amount of Ru ions in solution and discarding the role of molecular species in the catalytic act. Describing the first performance of pure ruthenium colloidal catalyst, this work opened the door toward the use of solution Ru NPs for the hydrogenation of CO2. Srivastava and co-workers published a comparative study on

the reactivity of nanocatalysts made of Ru NPs (ca. 6−22 nm from TEM analysis depending on the Ru loading in the range 1−6 wt %) dispersed onto TiO2 as a support for the hydrogenation of CO2 to FA in the presence or not of an ionic liquid (IL).273 Ru-TiO2 nanocatalysts were prepared by a microemulsion protocol from a suspension of TiO2 (ca. 30 nm) and a suspension of RuCl3 and citric acid followed by a

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1117

reduction treatment of the final solids at 573 K for 2 h. The effects of pressure (total pressure of 30−60 bar with H2/CO2 = 1/1), temperature (313−353 K), reaction time, and presence of water in the absence of IL were first studied. This allowed to determine the best Ru-TiO2 nanocatalyst to be that with the smallest size of Ru NPs (ca. 6.0 nm as determined by TEM for a Ru loading of 3 wt %) with a TOF (expressed as the number of moles of FA produced per mole of Ru per hour) of ca. 28 h−1 at 353 K and a total pressure of 40 bar (H2/CO2 = 1/1). Then, the influence of the addition of an IL on the catalytic conversion was studied from the most promising Ru-TiO2 system just cited (Figure 20). ILs are known to absorb gases and can be expected to improve catalysis involving gaseous reactants.274 Catalytic experiments were performed in 1,3- di(N,N-dimethylaminoethyl)-2-methylimidazolium bis- ( t r i f u o r o m e t h y l s u l f o n y l ) i m i d e ( [ D AM I ] - [CF3CF2CF2CF2SO3]) at different pressures, temperatures, and water contents. TOF values up to ca. 47 h−1 evidenced the IL positive effect on the CO2 hydrogenation into FA. Recyclability studies led to a slight loss of catalytic activity after 10 runs attributed to a Ru leaching into the product phase (ICP analysis of the filtrates). Thus, the use of an IL was clearly beneficial to the catalytic transformation of CO2 into FA by small Ru NPs deposited onto TiO2, this being attributed to the fact IL can act as both as a solvent for the reaction and enabled to capture CO2. But, ILs are also known to be suitable media to stabilize Ru NPs,83,129 being excellent alternatives to surfactants or solid supports. Thus, the IL probably increased the stability of the Ru NPs while favoring exchange at the metal surface. Then, the same group reported data on the solubility of CO2

into various ILs.275 The previously cited IL, [DAMI]- [CF3CF2CF2CF2SO3], provided the best solubility thus

confirming the high potential of this compound. For comparison purpose, three other ILs ([DAMI][TfO] where T fO = t r i f uo rome th ane su l f on a t e ; [mammim] - [CF3CF2CF2CF2SO3] with mammim = 1-(N,N-dimethylami- noethyl)-2,3-dimethylimidazolium and [DAMI][TfO]) were employed to prepare nanocatalysts from four different ruthenium precursors ([RuCl2(C6H6)]2, [Ru(COD)(2-meth- ylallyl)2], [trans-RuCl2(DMSO)4], [Ru(COD)Cl2], [Ru- (COD)(COT)]) by decomposing them under H2 (5 bar) at 323 K, which led to Ru NPs in a size range of 7−14 nm. XPS data (from samples introduced under argon atmosphere) evidenced no RuO2 contamination. Small-angle-X-ray scatter- ing (SAXS) and TEM data revealed that ionic interaction between cations and anions of the ILs plays an important role in the structural features of Ru NPs (stability, size, dispersion, a n d a g g l ome r a t i o n ) . L e s s c o o r d i n a t i n g i o n s [CF3CF2CF2CF2SO3

−] prevent the separation of Ru NPs from IL better than [TfO−], and this effect was dropped while lowering the carbon chain ([mammim][CF3CF2CF2CF2SO3]). These IL-immobilized Ru NPs were then investigated in CO2 hydrogenation in different reaction conditions (temperature: 303−373 K; CO2/H2 total pressure: 20−50 bar, absence or presence of water, nature of IL, etc.). Although their results are not very clear, the authors claimed that the highest activity was observed with the Ru NPs immobilized into [DAMI][TfO]. They also claimed higher catalytic efficiency when using in situ formed [DAMI][TfO]-Ru NPs with TOF up to 3300 h−1 of FA obtained at 323 K and 50 bar in 8 h. Finally a slow decrease in stability was observed after successive recycling. Dupont and co-workers reported on the selective hydro-

genation of CO2 either to FA or to hydrocarbons catalyzed by a colloidal catalytic system prepared by a single-step organo- metallic approach (hydrogen codecomposition of [Fe(CO)5]

Figure 20. TEM images of the Ru-TiO2 nanocatalysts for different Ru contents, catalytic scheme, and recycling studies. Reproduced with permission from ref 273. Copyright 2016 Royal Society of Chemistry.

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and [Ru(COD)(2-methylallyl)2] into small RuFe NPs (ca. 1.7 nm) dispersed in ILs (1-butyl-3-methyl-1H-imidazol-3-ium acetate, BMi·OAc, or 1-butyl-3-methyl-1H-imidazol-3-ium bis((trifluoromethyl)sulfonyl)amide, BMi·NTf2) under mild reaction conditions (DMSO/H2O; 333 K; 30 bar H2/CO2 = 2/1). The selectivity was observed to depend on the nature of the IL anion (Figure 21).256

FA was more produced with ILs containing basic anions (BMi·OAc) with a TOF value of 23.5 h−1, whereas heavy hydrocarbons (up to C21) were more produced with nonbasic anions (BMi·NTf2). The composition of the metal alloy and the basicity/hydrophobicity of the IL ion pair (mainly imposed by the anion) appeared to be the key points for the selective transformation of CO2. First, the IL forms a cage around the NPs that controls the diffusion/residence time of the substrates, intermediates, and products. Second, compared to Ru and Fe monometallic NPs, the presence of Fe in RuFe NPs showed a dual effect: a positive metal dilution effect toward the formation of FA through the formation of bicarbonate species (Figure 22, route (I)) and a synergetic one for the formation of hydrocarbons through the conversion of CO2 to CO followed by chain propagation via FTS pathway (Figure 22, route (II)). This work clearly evidences that the precise design of a

nanocatalyst (here a combination between metal alloy as active phase and IL as stabilizer) can lead to chemoselectivity in CO2 hydrogenation. Not only the ILs act as stabilizers for the NPs,

but also their chemical properties lead to a different interface between the metallic phase, the reactants, the intermediates, and products that orient the catalytic selectivity.

4.5.2. Transformation of CO2 into CO, CH4, or C2+ Hydrocarbons. Catalytic transformation of CO2 into hydro- carbons (like methane and superior alkanes (C2+) or carbon monoxide) is a very attractive alternative to fossil fuels. The hydrogenation of CO2 to methane (CO2 + 4 H2 → CH4 + H2O; −114 kJ mol −1) is well-known as CO2 methanation reaction or Sabatier’s process. This reaction is usually performed at temperature 423−773 K and pressure 1−100 bar.264 Methane is more advantageous because it can be injected directly into already existing natural gas pipelines and it can be used as a fuel or raw material for the production of other chemicals. In addition, CO2 methanation is a more simple reaction which can generate CH4 under atmospheric pressure (production of methanol and dimethyl ether from CO2 requires high pressures ∼5 MPa and conversion is low in the case of MeOH). Thus, the thermochemical conversion of CO2 to CH4 at low temperature has become an important breakthrough in the use of CO2 despite a low conversion. CO2 methanation remains an advantageous reaction with respect to thermodynamics because it is faster than reactions leading to hydrocarbons or alcohols. Both homogeneous and heterogeneous catalysts have been

investigated to hydrogenate CO2 to methane. 259 In heteroge-

Figure 21. (left) Schematic representation of the chemoselectivity observed in CO2 hydrogenation depending on the nature of the IL. (right) (a,b) TEM image of RuFe NPs and size distibution, (c) EDS map, overlay of Ru-L and Fe-K of RuFe NPs in BMI-NTf2. Adapted with permission from ref 256. Copyright 2018 American Chemical Society.

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neous conditions, metals such as Ru, Rh, Ni, Co, Fe, and so forth on various supports are recognized to be effective catalysts for this reaction. Noble metals proved to be efficient catalysts as the result of their high ability to dissociate H2, a required step in CO2 methanation. Note that for most catalysts in use, CO2 methanation is considered to be a linear combination of the reverse water−gas shift reaction (rWGS ; CO2 + H2 → CO + H2O), after which CO can lead to hydrocarbons via FTs pathways and the direct hydrogenation of CO2 into methane (CO2 + 3 H2 → CH4 + H2O). Given that, the choice of the catalyst is essential to get high conversion and selectivity, both varying with the active metal species, support, promoters, and synthesis strategies. For the most significant catalysts, the trends of activity and selectivity can be summarized as follows: activity, Ru > Fe > Ni > Co > Mo; selectivity Ni > Co > Fe > Ru. Ruthenium is renowned as being the most active metal for the methanation of both CO and CO2 and to be quite stable when operating in a wide temperature range. However, Ru is less selective while being more costly in comparison to non-noble metals.264 The catalytic activity can be greatly promoted at the metal/support interface due to synergistic interactions, which can tune the reaction mechanism and in turn the selectivity of CO2 hydrogenation. Thus, when deposited onto oxide supports (such as MgO, SiO2, TiO2, Al2O3, ZrO2, and CeO2), particles of Ni or Ru were reported to promote the formation of CH4.

276 CO2 methanation via better defined heterogeneous Ru-based catalysts received more attention in recent years.277−279 The main objective is to obtain the best catalytic performance in terms of stability, selectivity, CO2 conversion, and CH4 production, especially aimed at mild reaction conditions (i.e., low reaction temperature). In these works, the structure−performance relationships appeared to be a key for the development of highly performant catalysts. A relevant example by Zeng and co-workers280 provides an

elegant alternative to pure heterogeneous catalysts, by combining a solution synthesis approach and a sol−gel approach in order to get a nanomaterial of Ru into a silica matrix. Selective hydrogenation of CO2 into CO was catalyzed by small Ru NPs (ca. 1−3 nm) encapsulated into silica nanowires (denoted as Ru/mSiO2).

239 Combining colloidal and heterogeneous approaches made this catalytic system closer to a nanocatalyst than to pure heterogeneous ones given the presence of better controlled Ru NPs. As shown in Figure 23, a colloidal suspension of Ru NPs was first prepared by following a polyol-assisted method (decomposition of RuCl3 into ethylene glycol at 353 K in the presence of NaOH), and then silica was grown around the Ru NPs by hydrolysis/ condensation of TEOS (tetraethylorthosilicate) using ethylene glycol as a solvent instead of usual ethanol and an organic template (hexadecyltrimethylammonium chloride; CTACl). A calcination step at 573 K allowed elimination of the organic template. Calcination in N2 led to SiO2-encapsulated Ru NPs of almost unchanged size (1−3 nm depending on the Ru content introduced), while calcination in air conditions led to Ru NPs of larger sizes (5−30 nm) due to sintering. Comparatives studies in flow conditions inside a fix bed reactor (temperature: 473−673 K; 25 mL·min−1 of H2/CO2 at ratio 4:1) revealed a selective transformation of CO2 into CO with Ru/mSiO2 calcined in inert conditions and that contained small Ru NPs while the catalyst obtained in air condition and displaying large Ru NPs led preferentially to CH4. Fine surface studies (including temperature-programmed reduction (TPR)

and temperature-programmed desorption (TPD), XPS, and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)), performed on the two catalysts (1−3 nm Ru/ mSiO2 and 5−20 nm Ru/mSiO2) after adsorption of H2 and CO2 revealed the formation of different reaction intermediates on catalyst surface: CO-Run+ on 1−3 nm Ru/mSiO2 and formate species on 5−20 nm Ru/mSiO2, thus explaining the different selectivity observed as the result of different reaction pathways. The high selectivity of CO over CH4 is attributed to low affinity and hence coverage of atomic hydrogen on the surface of the 1−3 nm Ru NPs. DRIFTS, TPR, and TPD experiments supported a surface redox mechanism for CO2 hydrogenation on 1−3 nm Ru/mSiO2, where carbonyl species formed by dissociative adsorption of CO2 and desorbed directly to generate CO. A formate route is established for 5− 20 nm Ru/mSiO2 catalysts, where adsorbed atomic hydrogen associates with adsorbed CO2 to form formate species, which are further hydrogenated to CH4 with sufficient supply of surface hydrogen atoms due to the large metal surface. In addition, 1−3 nm Ru/mSiO2 nanocatalyst demonstrated to be stable in terms of activity and selectivity in extended reaction time up to 50 h. This work provides an elegant way to maintain the advantage of small-sized Ru NPs while having them encapsulated into the pores of a silica support for a selective catalytic transformation of CO2 into CO. Another relevant example by Chaudret and co-workers

describes the use of nickel-coated iron carbide nanoparticles

Figure 23. Schematic representation of the synthesis and TEM/ HREM images of Ru/mSiO2nanocatalysts for selective reduction of CO2 to either CO (top) or CH4 (bottom). Adapted with permission from ref 280. Copyright 2017 Elsevier.

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(ICNPs) prepared by the organometallic approach for the catalytic transformation of CO2 into CO and CH4 in a continuous-flow reactor under atmospheric pressure.281

Interestingly, with this ICNP-based catalytic system, the heating arises from the magnetic properties of the iron cores that are induced after applying a magnetic field. This catalytic system was optimized by deposition onto an inorganic support previously impregnated with 1 wt % Ru (also from an organometallic precursor). CO2 methanation with total selectivity and 93% yield was achieved in a model flow reactor. The presence of small Ru NPs in the alumina support (1 wt %) greatly enhanced the catalytic performance of the system and allowed a highly efficient conversion of CO2 to CH4 in continuous flow (Figure 24). If not a pure Ru catalytic system,

however, this work has the merit to show the synergy afforded by the proximity of Ru NPs onto the catalytic performance of a Ni-based nanocatalyst. Apart from these supported catalysts, Ru NPs dispersed into

ILs also allowed the formation of CO, CH4, or C2x. A previously cited work by Dupont and co-workers,256 described the influence of the nature of the IL used as a stabilizer on the catalytic properties of bimetallic RuFe NPs during hydro- genation of CO2, more precisely on the selectivty (HCOOH vs C2+). In a very recent paper, the same group reported on the conversion of CO2 into CO or light hydrocarbons (C2−C6) under very mild conditions (H2/CO2 = 4:1, 8.5 bar, 423 K) by using bimetallic RuNi NPs deposited into ionic liquids.241 This nanocatalyst was easily prepared by codecomposition of

Figure 24.Magnetically induced Sabatier reaction in continuous-flow reactor using ICNPs-RuSiRAlOx catalyst (ratio H2/CO2 = 4/1, 25 mL min@ 1, 18.3 Lh@1 g(Fe+Ru)@1, or 214.3 L h@1 gRu@1, residence time t = 0.00067 h, P atm). (a) Schematic representation of the reactor, (b) TEM of ICNPs and Ru NPs supported on a SiRAlOx particle, (c) Zoom on small Ru NPs, scale bar = 100 nm, (d) schematic representation of the catalytic system, (e) gas chromatogram obtained for m0Hrms = 28 mT, and (f) catalytic results as a function of m0Hrms. Because the selectivity is total, X (CO2) and Y (CH4) are overlapping. Reproduced with permission from ref 281. Copyright 2016 Wiley.

Table 14. Catalytic Systems for the Hydrogenation of CO2 to Hydrocarbons in ILs a

selectivity (%)b

entry NPs time (h) conv (%) CO CH4 C2−C4 C5−C6 olefins (C2−C4) 1 Ru1Ni2 20 20 26 1 65 8 2 Ru1Ni2 60 25 0 31 55 3 11 3 Ru4Ni3 60 24 0 14 59 19 8 4 Ru3Ni2 60 30 0 5 76 3 16 5 Ru3Ni2

c 60 22 47 7 7 35 4 6 Ru NPs 20 17 0 18 59 23 7 Ni NPs 20 5 2 4 57 37 8 Ru1Ni2

d 20 2 100 9 20

aReactions conditions: Catalyst 20 mg, IL (0.5 mL), CO2/H2 gas (1:4,8.5 bar), 60 h and 423 K. Reprinted with permission from ref 282. Copyright 2019 Elsevier. bSelectivity of the products was calculated as equivalent amount of desired hydrocarbon with respect to the total number of hydrocarbons produced. cReaction was performed in BMI-BF4 hydrophilic IL. dWithout IL

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organometallic precursors [Ni(COD)] and [Ru(COD)(2- methylallyl)2] under H2 atmosphere in an ionic liquid acting both as solvent and stabilizer. The so-obtained RuNi NPs presented a size of ca. 2−3 nm and a Ni-rich core with a Ru- rich shell whatever the synthetic conditions studied, but after the catalytic reactions, an enrichment of Ni in the shell was observed as the result of migration of Ni atoms toward the NP surface under catalytic conditions. In terms of catalytic performance, among the different RuNi compositions tested, Ru3Ni2 NPs dispersed into an hydrophobic IL (BMI·NTf 2 (l- butyl-3-methyl-lH-imidazol-3-ium bis((tri-fluoromethyl) sulfonyl)amide)) offered the highest conversion (up to 30%) and promoted the direct hydrogenation of CO2 into light hydrocarbons. The same Ru3Ni2 NPs gave rise to 22% conversion into hydrophilic IL (1-n-butyl-3-methy l-lH- imidazol-3-ium tetrafluoborate) with CO as the main product (see Table 14). Given the bimetallic RuNi NPs afforded higher efficiencies

(up to 30% of conversion) than their monometallic counter- parts (17% and 5% of conversion with Ru and Ni NPs, respectively), there is a strong synergy effect between Ru and

Ni in this catalytic system. The presence of Ni yielded a more active rWGS catalyst, while Ru increased the FTS toward the heavier hydrocarbons. In addition, as in their previous study with RuFe NPs,256 the obtained results showed that the nature of the IL (mainly, the choice of IL cations and anions) may orient the selectivity of the reaction due to different geometric and electronic properties of the IL-supported metal NPs. Diffusion of reactants, intermediates, or products across the interface between ILs and the catalyst surface plays an important role in the final chemoselectivity. The hydrophobic IL (BMI·NTf2) influenced the hydrogenation of CO2 to heavier hydrocarbons by repelling the formed water from the active catalytic phase of RuNi NPs, hence increasing the water gas shift reaction and increasing the FTS reaction pathways. In the opposite, the dominance of CO pathway into hydrophilic IL (BMI·BF4) results from a higher solubility of the formed water which causes the reduction of FT catalytic active surface species (Figure 25). As a last example, Branco and co-workers described the

hydrogenation of CO2 into methane using in situ formed IL- supported Ru NPs (Figure 26).283 The nanocatalyst was

Figure 25. (top) Schematic representation of the chemoselectivity observed in CO2 hydrogenation by RuNi NPs depending on the nature of the IL. (bottom) (a) Surface composition of Ni in RuNi NPs vs methane selectivity and (b) STEM-HAADF analysis of Ru1Ni2 NPs after catalysis. Adapted with permission from ref 282. Copyright 2019 Elsevier.

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prepared in situ by mixing in an autoclave the [Ru(COD)(2- methylallyl)2] complex and an IL (1-butyl-3-methylimidazo- lium bistrifluoromethanesulfonylimide, [bmim][NTf2], or 1- octyl-3-methylimidazolium bistrifluoromethanesulfonylimide, [omim][NTf2]), followed by the application of hydrogen pressure and temperature (313 K) before introduction of CO2 (up to a total pressure of 80 bar) and increasing temperature (up to 423 K) to perform the catalysis. TEM analysis of the black colloidal solution obtained after catalysis in [omim]- [NTf2] revealed the presence of Ru NPs of ca. 2.5 nm. It is worth mentioning that the presence of PPh3 in the

reaction medium led to no substantial results, whereas methane and water were formed in its absence. Several reaction conditions were first tested using [bmim][NTf2] (amount of catalyst, hydrogen, and CO2 pressures and ratio, reaction time, and temperature). No methane was produced at 20 bar H2, whereas 40 or 60 bar led to the same quantity of methane (up to 4.7% yield, with TON (expressed as mol CH4/mol cat) of 12, after 24 h at 413 K with 60 bar H2). The change of the IL to [omim][NTf2], which is reported to better stabilize NPs, led in general to better performance for CH4 production. The best yield of methane (69%) was achieved with 0.24 mol % ruthenium catalyst, at 40 bar of H2 plus 40 bar of CO2 and at 423 K (see Table 15, entry 8). This work highlights that CO2 can be selectively hydro-

genated to CH4 using a simply prepared nanocatalyst made of Ru NPs dispersed into an IL in reasonable reaction conditions. It also shows that the choice of the IL may change the catalytic performance. The better conversions reached with [omim]- [NTf2] compared to those observed in [bmim][NTf2] are attributed to a better solubility of CO2 (which contributes to a reduced viscosity of the IL and increases both miscibility of reagents in the IL) and also to a better stability of the Ru NPs given the longer alkylchain (C8 against C4) beared by the of [omim][NTf2] IL. Catalysis investigation performed with preformed and isolated Ru NPs led to a reduction in CH4 production of (5% of yield and 25% of TON), thus these comparative results point out that the conditions applied are the key point to achieve higher methane production perform- ance. Moreover this catalytic system is very simple to implement.

4.5.3. Conclusions on CO2 Transformation. The literature provides only a few works showing the potential of Ru-based nanocatalysts (both as monometallic and bimetallic systems) for the thermochemical hydrogenation of CO2. This probably derives from the present (and necessary) trend to use more abundant and less expensive metals for application in catalysis of industrial interest, which is not the case of ruthenium. Even if quite low values have been achieved in terms of activity compared to those reported for homogeneous ruthenium complexes, and even if not numerous today, the obtained results evidenced that the control of size and the nature of chemical environment around the particles are key factors. These findings thus open some ways that merit being more deeply explored in order to get more active Ru-based nanocatalysts, but apart from ILs that were shown to orient the catalytic results by providing adequate chemical environment, to our knowledge, effects of capping ligands have not been studied. Moreover, when considering the needs in terms of mechanistic studies in order to better understand what occurs at the surface of metal NPs, ruthenium may provide nice opportunities because it allows the access to NMR spectroscopic techniques, tools that are usually applied for mechanistic studies with homogeneous catalysts. This possibility needs to be better exploited. Indeed, such an

Table 15. Hydrogenation of CO2 into Methane with Ru NPs Stabilized into [omim][NTf2]

entry precursor [μmol] pH2 [bar] t [h] T [K] yield [%] TON b

1 24 60 24 373 2 25 60 24 413 4 30 3 24 40 24 393 4 22 4 28 40 24 413 10 47 5 25 40 24 423 17 95 6 77 40 48 413 30 51 7 123 40 24 413 51 55 8 125 40 24 423 69 72 9 223 40 48 423 61 36

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approach is expected to provide insights at the atomic level on the surface state of metal nanoparticles as well as on intermediates formed, and thus it could greatly complement usual surface studies coming from heterogeneous catalysis. We believe it is a necessary step in order to define structure− activity relationships to, in turn, design better appropriate nanocatalysts for more efficient and more selective CO2 hydrogenation. Dupont and co-workers reported some data in this direction using ionic liquids. They observed by high- pressure NMR (40 bar H2/CO2 at ratio 1/1) the presence of HCO3

− species on the surface of Ru NPs dispersed in ILs.271

4.6. Dehydrogenation of Amine Boranes

Hydrogen is considered as a clean energy carrier because it can be produced in a renewable way from various nonfossil feedstocks. Hydrogen has a much higher gravimetric energy density than petroleum (120 vs 44 kJ g−1 for hydrogen and petroleum, respectively) and can be readily used to operate high-energy efficient fuel cells that produce water as the only waste, which makes it an ideal alternative energy vector.284

However, a main challenge relies with its storage in secure conditions while having an easy and fast release for an “on demand” usage. When employed as an energy carrier in portable electronic devices and vehicles, hydrogen fuel cells should have the highest possible energy content combined with the smallest possible volume and mass. As a consequence, numerous works focus on the development of strategies for efficient hydrogen storage with the objective to fulfill this criterion. Physical (compressed hydrogen gas, cryocompressed hydrogen storage, and hydrogen adsorbents) and chemical storage systems are studied (e.g., sorbent materials, metal hydrides, organic hydrides, borane−nitrogen (B−N) com- pounds, and aqueous solution of hydrazine), but no hydrogen storage methods are mature enough for industrial applications under mild conditions.284

Covalently bound hydrogen-containing materials, in either liquid or solid form, are very attractive for chemical hydrogen storage because of their generally high gravimetric hydrogen densities. Among them, amine boranes (B−N), with protic N− H and hydridic B−H, have attracted much attention due to their high hydrogen contents and favorable kinetics of

hydrogen release.285 Ammonia borane (NH3−BH3; AB), which is the simplest B−N compound represents a leading material given its high hydrogen density (19.6 wt %), low molecular weight (30.7 g mol−1), solubility in polar solvents like water and methanol (vide infra), high stability under ambient conditions, and environmental friendliness.286 Methyl- amine borane (CH3NH2-BH3; hydrogen content of 17.86 wt %) and dimethylamine borane ((CH3)2NH-BH3; hydrogen content of 17.1 wt %) or also hydrazine borane (N2H4-BH3; hydrogen content of 15.28 wt %) are other substrates of interest but they are less widely investigated, probably due to their lower hydrogen content and necessary conditions for the release. Hydrogen formation is generally quantified by volumetry and the reaction monitored by 11B NMR to analyze the byproducts formed A convenient method to release hydrogen from ammonia

borane consists in its solvolysis using a protic solvent like water or methanol, namely hydrolysis (eq 1) and methanolysis (eq 2), respectively.

· + → · +NH BH (aq) 2H O(l) NH BO (aq) 3H (g)3 3 2 4 2 2 (1)

· +

→ · +

NH BH (sol) 4CH OH(l)

NH B(OCH ) (sol) 3H (g) 3 3 3

4 3 4 2 (2)

The use of a catalyst (homogeneous, heterogeneous, or nanoparticulate) allows to make the solvolysis to occur at rt, leading to the stoichiometric production of 3 equiv of H2. Dehydrocoupling (eqs 3 and 4) is another way to liberate

hydrogen from ammonia borane, using this time a nonprotic solvent like tetrahydrofuran (THF) or toluene. Catalytic activation allows to drive this reaction at rt, mainly using homogeneous species but heterogeneous species and nano- particles are also developed.

· → +n nNH BH (sol) (NH BH ) (s or sol) H (g)n3 3 2 2 2 (3)

· → +n nNH BH (sol) (NHBH) (s or sol) 2 H (g)n3 3 2 (4)

As it will be seen hereafter, kinetic studies allow to quantify catalyst performance (in terms of turnover frequency (TOF),

Figure 27. (a) Dehydrogenation of dimethylamine−borane catalyzed by Ru/APTS NPs in THF at rt, (b) mol H2/mol DMAB vs time graph ([Ru] = 2.24 mM; [DMAB] = 54 mM, 240 equiv of Hg(0) after ∼50% conversion of DMAB), and (c) TEM image of Ru/APTS NPs (∼2.9 nm) after the third catalytic run. Adapted with permission from ref 288. Copyright 2012 Royal Society of Chemistry.

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activation energy (Ea), activation enthalpy (ΔH*), and entropy (ΔS*) values). Other important parameters are the stability and the reusability/recycling of the catalysts, both being key parameters for cost decrease and technology transfer. Numerous metals are used for the dehydrogenation of amine

boranes like noble metals and non-noble ones, among which ruthenium is in the top list, either under the form of molecular complexes, heterogeneous catalysts, or nanoparticles. The next parts of this section will provide recent results in the use of well-defined Ru NPs (mono-, bi-, or trimetallic systems) for dehydrocoupling or solvolysis of amine boranes. Among the reported Ru-based catalytic systems many involve supported nanocatalysts, while only a few articles describe Ru NPs in solution. The examples here presented correspond to catalysts made of Ru NPs prepared in mild conditions of wet chemistry, allowing thus a good control of their characteristics. 4.6.1. Dehydrogenation of Amine Boranes by

Dehydrocoupling. Not a lot of examples describe the use of Ru nanocatalysts for the dehydrogenation of amine boranes in a nonprotic solvent. It mainly concerns dimethylamine borane (DMAB) and THF as solvent as well as supported nanocatalysts. Nevertheless, a few unsupported systems have been reported, as follows. The catalytic performance of 3-aminopropyltriethoxysilane-

stabilized Ru nanoclusters (Ru/APTS) synthesized from the organometallic [Ru(COD)(COT)] complex (Figure 27) has been evaluated in the dehydrogenation of DMAB.287,288 A size control operated by varying the Ru/ligand ratio allowed studying of the influence of this parameter in catalysis (THF, 298 K). Hydrogen evolution started immediately with an initial turnover frequency (TOF) of 53 h−1 for the best system (ca. 2.9 nm) and continued until completion (1 equiv H2 per mol DMAB released). Adding Hg(0) in the catalytic mixture led to suppression of the activity, thus evidencing heterogeneous catalysis (Figure 28). The initial TOF value of 53 h−1 attained with this system was comparable to that of the best heterogeneous rhodium catalyst known at that time (TOF = 60 h−1). Moreover, it was the first example of an isolable, bottleable, and reusable transition metal nanocatalyst for the dehydrogenation of DMAB. APTS concentration increase

significantly decreased the catalytic activity as a result of a higher coverage of metallic surface. This evidenced the necessary compromise between the NP mean size and the surface accessibility to get efficient catalytic behavior. The in situ generation of Ru NPs was also studied taking

benefit of the catalysis reaction conditions for their formation, without adding extra stabilizer.289 [Ru(COD(COT)] easily decomposed during the dehydrogenation of DMAB in THF at 298 K, forming Ru NPs as seen by TEM. NMR studies on the obtained Ru NPs showed their stabilization by B−N polymers resulting from DMAB decomposition. It was the first example of Ru nanocatalyst prepared in situ, displaying a TOF value of 35 h−1 together with a H2 production superior than 1.0 equiv at the complete conversion of DMAB. Oleylamine-stabilized Ru NPs were also used in the

dehydrocoupling of DMAB by S. Özkar’s group.290 The nanocatalyst was generated in situ by reduction of RuCl3 at rt in toluene and in the presence of oleylamine as stabilizer and of DMAB as both reducing agent and catalysis substrate. This led to Ru/oleylamine NPs of ca. 1.8 nm that were reproducibly isolated and fully characterized. These Ru/oleylamine NPs proved to be a highly active catalyst in the dehydrogenation of DMAB, providing a release of 1 mol H2 per mole of DMAB and an initial TOF value of 137 h−1 at 298 K and Ea value of 29 ± 2 kJ mol−1. The optimum ligand/Ru ratio to have active and stable NPs was found to be 3. At this ratio, Ru/oleylamine NPs were shown stable and reusable, giving rise to 20,660 total turnovers and preserving 75% of their initial activity after the fifth catalytic run with the complete conversion of DMAB and the release of 1 equiv of H2. Although these oleylamine- stabilized Ru NPs have a mean size similar to that of APTS- stabilized Ru NPs previously described, their activity is almost the double. This can be explained by a difference in terms of available of active sites: oleylamine being less voluminous than APTS, it probably leads to less-crowded metal surface and consequently to more accessible ruthenium atoms compared to APTS. In the very recent years, the group of F. Ṣen studied several

ruthenium-containing catalytic systems in the dehydrocoupling of DMAB, including colloidal NPs. For example, well-dispersed

Figure 28. (a) Rate of dehydrogenation of 55 mM DMAB vs [APTS]/[Ru] ratio, using Ru/APTS NPs. (b) Plot of mol H2/mol DMAB vs time for the dehydrogenation rxn (55 mM DMAB; 0.25 mM Ru/APTS 3). Adapted with permission from ref 288. Copyright 2012 Royal Society of Chemistry.

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PVP-stabilized RuNi NPs were prepared by a facile sodium hydroxide-assisted aqueous reduction method under inert atmosphere that consists in treating an aqueous solution of RuCl3 and NiCl2 by NaBH4 in the presence of NaOH and PVP as stabilizing agent.291 Optimum conditions in terms of Ru/Ni and PVP/metal ratios were found to be 1/1 and 5/1, respectively. In these conditions, ca. 3.5 ± 0.4 nm in size RuNi NPs, well-dispersed in the polymer matrix, stable, easily isolable, and redispersible have been obtained and charac- terized as RuNi alloy. These NPs were investigated in the dehydrogenation of DMAB (eq 5), an easy catalytic reaction to implement, just consisting in adding the DMAB substrate into a THF colloidal suspension of RuNi/PVP NPs.

· → · +2(CH ) NH BH ((CH ) N BH ) 2H3 2 3 3 2 2 2 2 (5)

RuNi/PVP nanocatalyst allowed a complete release of H2 (1 mol H2 per mol of DMAB) at 298 K in a short time with no induction period. A comparative study performed with Ru/ PVP NPs, Ni/PVP NPs, and a physical mixture of both evidenced the superior performance of the RuNi/PVP nanocatalyst, attributed to its alloy character that provided a synergistic effect. A TOF value of 458.57 h−1 makes it be among the best catalysts reported in the literature for dehydrocoupling of DMAB. This catalytic system also showed a low Ea value of 36.52 ± 3 kJ mol−1, an activation enthalpy (ΔH* = 34.02 ± 2 kJ mol−1), and activation entropy (ΔS*) = −84.47 J·mol−1). High negative values of activation entropy and small activation enthalpy value refer to an associate mechanism in the dehydrocoupling of DMAB. These RuNi/ PVP NPs also appeared to be a reusable catalyst with 78% of initial activity preserved after four catalytic runs and no leaching observed. The same group also published the catalytic performances of

alloyed PdRu/PVP NPs (ca. 3.8 ± 1 nm) in the dehydrocoupling of DMAB (THF, 298 K).292 The synthesis of the NPs was performed by an ultrasonic double reduction technique (reduction of aqueous solution of RuCl3 and K2PdCl4 under ultrasounds at 363 K in the presence of PVP). Their catalytic behavior was compared to those of Pd/ PVP NPs, Ru/PVP NPs, and a physical mixture of both in similar conditions. No induction time and complete DMAB conversion were observed. Kinetic parameters were found tobe

TOF = 803.03 h−1, Ea = 60.49 ± 2 kJ mol−1, ΔH* = 57.99 ± 2 kJ mol−1, and ΔS* = −53.17 J·mol−1. Reusability tests indicated ca. 80% of initial activity kept after four runs. Theoretical calculations by DFT using Pd/PVP, Ru/PVP, and PdRu/PVP model clusters in optimized geometries were performed in order to determine adsorption energy of DMAB. The obtained theoretical results supported well the exper- imental results. The PdRu/PVP cluster presented a markedly lower chemical potential, adsorption energy, and enthalpy values than those of Pd/PVP and Ru/PVP clusters. Also, higher chemical hardness and electronegativity values were observed for PdRu/PVP cluster compared to those of monometallic counterparts. All these differences may explain the outstanding efficiency of the PdRu/PVP NPs. A summary of the catalytic properties of the previously

described soluble Ru nanocatalysts is given in Table 16. The obtained results clearly evidence that colloidal ruthenium is a good metal for the dehydrogenation of DMAB. Interestingly, even if only a few ligands were tested, variation of capping ligand led to a variation in reactivity. Also, these results show the progress attained in terms of performance when associating a second metal like Ni or Pd to Ru. Similar studies with ligand- stabilized alloys could be of interest to perform. F. Ṣen and co-workers also reported on the application of

monometallic, bimetallic, and even trimetallic Ru-based supported NPs in dehydrocoupling of DMAB (THF, 298 K).293−298 If these data are here cited, it is because these catalysts were prepared in mild reaction conditions by reduction of the metal source(s) in the presence of the chosen support sometimes together with a polymer (PVP) or ligand (oleylamine), thus leading to controlled NPs. Graphite,293

graphene,298 functionnalized multiwalled carbon nanotubes (f- MWCNT),294,256 or graphene oxide (GO) were used as a support.295−297 The kinetic parameters measured for these catalysts are summarized in Table 17. It can be seen that different values are obtained depending on the composition of the nanocatalyst both in terms of metal composition and nature of metal−support interaction. Also, in Ṣen’s group’s papers, comparisons with other catalysts reported in the literature are described, highlighting the interest of Ru-based nanocatalysts for DMAB dehydrocoupling. However, it is difficult to rationalize the observed effects because several

Table 16. Comparison of Kinetic Data in Dehydrocoupling of DMAB by Soluble Ru NPs (298 K; THF Except for Ru/ Oleylamine, Toluene; Total Conversion)

nanocatalyst NP mean size (nm) TOF (h−1) Ea (kJ·mol−1) ΔH* (kJ·mol−1) ΔS* (±2 J·mol−1) ref Ru/APTS 2.9 ± 0.9 53.1 288 Ru/B-N polymers 2.9 ± 0.9 35.1 289 Ru/oleylamine 1.8 ± 0.23 137 29 290 RuNi/PVP 3.5 ± 0.4 458.57 36.52 34.02 −84.47 291 PdRu/PVP NPs 3.8 ± 1 803.03 60.49 57.99 −53.17 292

Table 17. Comparison of Kinetic Data in Dehydrocoupling of DMAB (THF, 298 K, Total Conversion) for Diverse Supported Nanocatalysts Studied by F. Ṣen and Coworkers

nanocatalyst NP mean size (nm) TOF (h−1) Ea (kJ·mol−1) ΔH* (kJ·mol−1) ΔS* (±2 J·mol−1) ref Ru/oleylamine-graphite 3.75 ± 0.73 281.5 13.82 11.33 −220.68 293 Ru/PVP-GO 2.09 ± 0.23 896.54 11.45 8.96 −194.02 298 RuCo/f-MWCNT 3.72 ± 0.37 775.28 13.72 11.2 −173.53 294 RuCu/r-GO 3.86 ± 0.47 256.70 16.68 19.18 −205.73 295 RuPtNi/GO 3.40 ± 0.32 727 49.43 296 PdRuNi/GO 3.78 ± 0.43 737.05 55.47 53.36 −33.76 297

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parameters are different. In fact the works performed correspond more to trial−error works than real systematic comparison. More rationalization is thus required in order to define precisely the important parameters to master in order to get the best performance. 4.6.2. Dehydrogenation of Amine Boranes by

Methanolysis. Only a few papers report on the dehydrogen- ation of amine boranes with nanoscale ruthenium using methanol as a solvent. Compared to hydrolysis (vide infra), the methanolysis presents a few merits such as a single gaseous product (H2), recoverable byproducts, and the possibility of handling at low temperatures (<273 K).299

H.-C. Zhou and co-workers described the synthesis of ultrasmall fcc Ru NPs confined into the pores of a soluble and negatively charged porous coordination cage (PCC) of 4.2 nm in size that presents three different cavity diameters (ca. 2.5 and 1.4 nm for inner and intermolecular cavities, respec- tively).299 The preparation of this nanocatalyst consisted in the addition of RuCl3 to a DMF solution of PCC-2 (Na24(Et3NH)6[[Co4(μ4-OH)V]6L8]30

−5·MeOH·10H2O with V = phenolate groups and L = carboxylates) using a Ru/ PCC-2 molar ratio of 2/1, followed by addition of NaBH4 also in DMF solution (Figure 29). This protocol led to a homogeneous colloidal dispersion containing PCC-2-stabilized Ru NPs with a narrow size distribution and a mean size of ca. 2.5 nm that corresponds well to the mean diameter of the inner cavities of the host. HREM analysis clearly showed that Ru/ PCC-2 NPs have a truncated octahedral fcc structure, not usual for Ru NPs, which was also confirmed by powder XRD. XPS analysis indicated a major content of metallic Ru. No

precipitation from the colloidal suspension was observed up to 6 months in ambient air. The isolation of the NPs could be performed by addition of acetonitrile to the DMF suspension, which allowed the precipitation of a black solid redissolvable in DMF. These Ru/PCC-2 NPs were investigated in the dehydrogenation of ammonia borane by simply adding the DMF/MeOH colloidal suspension to solid AB. The catalysis was carried out at 298 K. Reaction was completed after 4.5 min, showing a TOF value of 304.4 mol H2 per mol Ru per min, which appeared to be higher than the TOF value of 205 min−1 reported by Xu and co-workers using a PCC-stabilized Rh nanocatalyst for catalyzing the same reaction.300 The catalytic performance of the Ru/PCC-2 nanocatalyst, being the best catalytic activity ever reported for the methanolysis of AB, was attributed to the small size and the fcc structure of the particles. Furthermore, the anionic and soluble PCC-2 played a critical role in encapsulating, stabilizing, homogenizing, and distributing the metal nanoclusters by regulating the size and the atomic arrangement of the encapsulated NPs. The soluble catalyst Ru/PCC-2 was also five times without a significant loss of activity. The results of F. Wang and co-workers are also among the

best ones in methanolysis of AB. Their nanocatalyst was prepared by direct deposition of ultrafine Ru NPs onto tetrabutylammonium hydroxide-intercalated graphene as a support by the reduction of RuCl3 in water with KBH4 at 303 K.301 The obtained Ru/graphene nanomaterial displayed well-dispersed Ru NPs onto the support with a mean size of ca. 1.6 nm. This nanomaterial was investigated in the methanolysis of AB. Up to 35,600 total turnovers over a period of 300 h and

Figure 29. (left) Representation of the PCC-2 cage. (right) Scheme of the synthesis of Ru/PCC-2 NPs and their investigation as catalyst in dehydrogenation of AB. Adapted with permission from ref 299. Copyright 2018 Elsevier.

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a TOF value of 99.4 min−1 were obtained at 293 K before deactivation and a Ea value of 54.1 ± 2 2 kJ mol−1. Additionally this nanocatalyst showed a satisfactory stability and retained 73.2% of its initial activity at the 15th run. 4.6.3. Dehydrogenation of Amine Boranes by

Hydrolysis. Regarding hydrolytic dehydrogenation of AB, numerous studies are conducted on diverse monometallic and heterometallic nanocatalysts (mainly Pt, Ru, Rh, Ag, Pd) that display high catalytic activity, among which numerous ones are Ru-based catalysts. Various stabilizers and supports are used in order to control the size, morphology, and stability of the NPs. The addition of a second metal to ruthenium appeared to be positive to boost the catalysis. As first example of soluble Ru NPs, one can cite the

metastable Ru NPs reported by O.A. Scherman and co- workers.302 This work relates on a very facile catalytic system made of highly stable Ru NPs (up to 8 months) despite the absence of any extra stabilizer. In fact, the authors simply decomposed RuCl3 by NaBH4 in a H2O/EtOH mixture (1/1) at rt (Figure 30). The presence of monodisperse Ru NPs of ca. 2.2 nm was

evidenced by HREM. The initial concentration of RuCl3 played a crucial role in the control of the NP size. A fcc structure was determined by HAADF-STEM and XPS showed a signal corresponding to Ru(0) for ca. 19.4% together with a signal attributed to remaining RuCl3, which probably acts as stabilizer for the Ru NPs. The % of Ru(0) species increased to over 75% after a complete catalytic cycle. These Ru NPs allowed the hydrolysis of AB yielding hydrogen gas with a TON of 21.8 min−1 at 298 K. The high surface area available at Ru surface translated an Ea value of 27.5 kJ mol−1, which was notably lower than that of other Ru NPs based catalysts. As another example of AB hydrolysis with monometallic Ru

NPs, by S. Özkar, M. Zahmakiran, and co-workers reported on the use of dihydrogenophosphate-stabilized Ru NPs.303 This catalytic system was prepared by reduction of an aqueous solution of RuCl3 and ((C4H9)4N[OP(OH)2O] with DMAB at rt, leading to a stable colloidal dispersion of Ru NPs (ca. 2.9 nm) with no precipitation after 2 days of storage. When investigated in the hydrolysis of AB at rt, this catalytic system presented an initial TOF value of 80 min−1. Moreover, the high stability of these Ru NPs made them long-lived and reusable nanocatalysts for the hydrolysis of AB, providing 56,800 total turnovers over 36 h before deactivation, an initial TOF value of 31.6 min−1 (at 283 K), an Ea value of 69 ± 2 kJ mol−1 and retaining 80% of their initial activity at the fifth catalytic run. These authors also published on the hydrolytic dehydrogen- ation of DMAB catalyzed by similar ((C4H9)4N[OP(OH)2O]- stabilized Ru NPs but synthesized in situ, i.e., in the catalysis medium (Ru/stabilizer ratio = 1/20).304 RuCl3 was reduced by addition of DMAB, being also the catalysis substrate, leading to the formation of Ru NPs of ca. 2.9 nm mean size. Kinetic studies revealed an initial TOF value of 500 h−1 at 298 K and

stability studies an exceptional catalytic lifetime with 11600 total turnovers. M. Rakap published the use of PVP-protected PtRu NPs for

the hydrolysis of AB.305 This catalyst was synthesized by alcoholic reduction of RuCl3 and PtCl6 in the presence of PVP under mild conditions (EtOH; 363 K; 2 h). The obtained colloidal suspension was found stable for months at rt. Isolation of the particles was performed by simple solvent evaporation. Characterization techniques (TEM-EDX, ICP, XRD, XPS) revealed ca. 3.2 nm in size alloyed PtRu NPs with a Pt/Ru composition of 1/1 as well as the presence of Pt(0) and Ru(0) species but no higher oxidation states. The catalytic activity of PtRu/PVP NPs in the hydrolytic dehydrogenation of AB (at 298 K) was much higher than that reached with a physical mixture of monometallic Ru/PVP NPs (ca. 4.6 nm) and Pt/PVP NPs (ca. 4.2 nm) prepared in the same conditions, thus indicating a synergistic effect attributed to Pt−Ru interaction in the alloy although the reduced mean size of the PtRu/PVP NPs may also have an effect. It is worth noting that PtRu/PVP nanocatalyst led to complete hydrogen release (3 mol H2·mol AB

−1) for the hydrolysis of 0.100 M AB solutions in 195 s, corresponding to a record average TOF of 308 min−1 with a low Ea value of 53.3 kJ mol−1. Recyclability tests showed a remaining activity of 72% after the fifth catalytic cycle. The same author also reported on the hydrolysis of AB using RuRh/PVP NPs.306 The nanocatalyst was prepared following the same synthesis method as described for RuPt/ PVP one and also the catalysis performed in the same conditions. This catalyst was shown to be efficient and durable providing an average TOF value of 386 min−1 and Ea value of 47.4 kJ mol−1, thus reflecting a higher efficiency than the previous PtRu system just by changing Pt by Rh in the alloy. Similarly, F. Ṣen’s group reported on the use of RuRh/PVP

nanocatalyst for the hydrolysis of methylamineborane (MAB) at rt307 The NPs were also prepared by alcoholic reduction of a mixture of RuCl3 and RhCl3 in mild conditions (H2O/EtOH; 363 K; 2 h) in the presence of PVP as stabilizer. HREM, XRD, and EELS data indicated alloyed RuRh NPs of ca. 3.4 mean size and XPS data the presence of Ru(0) and Rh(0) species. Then this nanocatalyst was evaluated in hydrolysis of MAB at rt, showing a high efficiency with an initial TOF value of 206.2 min−1, EA value of 43.5 kJ mol−1, as well as ΔH* = 41.18 kJ mol−1 and ΔS* = −104.25 ± 2 J·mol−1. Reusability tests indicated a 67% retention of the initial catalytic activity after five cycles. All together, these characteristics place this nanocatalyst among the best for hydrolysis of MAB, a storage material which may lead to less volatile byproducts than AB. Indeed, the decomposition of AB results in a distinct contamination of released H2 by NH3 and borazine, which is a major problem for application in fuel cells. All the results described above are summarized in Table 18.

Here again it is difficult to rationalize these results given the different parameters used. Nevertheless, they prove the leader

Table 18. Comparison of Kinetic Data in Hydrolysis of Amineboranes at 298 K

nanocatalyst (substrate) NP mean size (nm) TOF (min−1) Ea (kJ·mol−1) ΔH* (kJ·mol−1) ΔS* (±2 J·mol−1) ref

Ru metastable (AB) 2.2 ± 0.5 21.8 27.5 302 Ru/((C4H9)4N[OP(OH)2O] (AB) 2.1 ± 0.9 31.6 69 303 Ru/((C4H9)4N[OP(OH)2O] (DMAB) 2.9 ± 0.9 500 304 PtRu/PVP (AB) 3.2 ± 1.2 308 53.3 305 RuRh/PVP (AB) 3.4 ± 0.4 386 47.4 307 RuRh/PVP (MAB) 3.4 ± 0.3 206.2 43.5 41.18 −104.25 307

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position of ruthenium for the hydrolysis of amine borane and the positive effect of the addition of a second metal like Rh or Pt. As it will be described hereafter, a few papers describe the

use of supported nanocatalysts that were preformed in solution in mild conditions. The preparation of these catalysts generally consists in two steps: (1) reduction of the Ru source to get the colloidal suspension and (2) impregnation of a given support from the colloidal suspension in order to deposit the NPs at the surface or in the pores of the material, followed by evaporation of the solvent. Control of Ru NPs is thus operated in solution before deposition on the support, and the influence of the support can be studied independently. U. B. Demirci and co-workers studied the catalytic

performance of RuCo NPs and RuCu NPs with metal ratio 1/1 prepared by the polyol process ([Ru(acac)3], [Co(acac)2], and [Cu(acac)2] with acetylacetonate (acac); ethylene glycol; 458 K) in the absence of added stabilizer and then deposited onto γ-Al2O3 as a support in the hydrolysis of AB (323−338 K). A higher activity was observed for RuCo than for RuCu NPs, with activation energies of 47 and 52 kJ mol−1, respectively. Moreover, the RuCu NPs presented a similar activity as Ru NPs prepared in the same conditions. The addition of Co thus had a positive effect on the catalytic behavior of Ru that may result from synergistic interactions between Ru and Co atoms in the RuCo NPs. G. Chen, D. Ma, and co-workers prepared a series of NiRu/

ligands alloy NPs at different metal ratios and deposited them onto a carbon black support for their evaluation in the hydrolysis of AB.308 The NPs were prepared by reduction of a diphenylether solution of [Ru(acac)3] and [Ni(acac)2] complexes by triethyborohydride (LiBEt3H) in the presence of oleic acid and oleylamine as stabilizers at 523 K. As confirmed by full characterization (HREM, XRD, XPS), such reaction conditions (strong reducing agent, high temperature) allowed alloying NiRu NPs of ca. 9 nm mean size while Ru and Ni are immiscible in bulk form. The NPs were purified by precipitation with ethanol and redispersed in hexane for their further deposition onto the carbon support followed by solvent evaporation in order to get the final nanocatalysts. Catalysis was done in water at ca. 303 K. A comparison with monometallic Ru NPs and Ni NPs as well as core−shell Ni/ Ru309 stabilized by the same ligand evidenced the superior catalytic activity of the NiRu alloy nanocatalysts (Figure 31). With a complete dehydrogenation of AB in 12 min, the best

activity was obtained with the Ni richest nanocatalyst, namely Ni0.74Ru0.26, while Ni NPs were almost inactive and Ru NPs showed an intermediate activity. Moreover, the Ni@Ru NPs needed almost 3 times as long for a total conversion, thus showing the strong influence of the Ru−Ni interaction in the alloy. The determination of the activation energies, revealed a lower value for NiRu alloy nanocatalyst than for Ni/Ru one. Thus, alloying Ru with Ni decreased the reaction activation energy and significantly enhanced the catalytic activity of Ru. A reusability test showed that the Ni0.74Ru0.26 still exhibited high catalytic activity after five catalytic cycles. Recently, G. Chen and co-workers studied the effect of the

size and of Ru crystal phase on the catalytic activity of Ru/ PVP/γ-Al2O3 nanocatalyst in hydrolysis of AB.

310 For this purpose, they prepared hcp Ru NPs and fcc Ru NPs exhibiting narrow size distributions and similar sizes (ca. 2.4 nm). These Ru NPs were synthesized by decomposing [Ru(acac)3] or RuCl3 at 473 K in triethylene glycol (TEG) in the presence of PVP as a stabilizing agent. Ru NPs of different size/crystal phase were synthesized by adjusting the amount/nature of metal precursors, type of solvents, and the amount of PVP. As demonstrated by characterization results, [Ru(acac)3] led to fcc Ru NPs, while RuCl3 provided hcp Ru NPs. The so- obtained Ru NPs were further deposited onto γ-Al2O3 by wet impregnation method before evaluating their catalytic perform- ance in the hydrolysis of AB at rt The hcp Ru NPs exhibited a higher activity than fcc Ru NPs at similar sizes. Also, with the size increase, the gap of activity became narrow. More interestingly, with the particle size change, an opposite variation of the activity trend for fcc and hcp structured Ru/ γ-Al2O3 was observed (Figure 32). With the size increase, fcc Ru NPs presented an increased

catalytic performance while hcp Ru NPs displayed a converse trend with a decreased performance at higher sizes. A reusability test showed that the fcc Ru NPs still exhibited high catalytic activity after four runs, although fcc Ru has a thermodynamically unstable structure. DFT calculations evidenced that fcc Ru NPs were easier to oxidize than hcp ones (values of adsorption energy of O2 onto (001) crystal plane of fcc and hcp Ru were found to be −2.17 and −1.81 eV, respectively). This difference in oxidation state could explain why hcp Ru NPs were more performant than fcc Ru NPs, without taking into account other parameters. Considering that the surface-to-volume ratio increases with the size decrease (so-called “size effect”) and that smaller NPs are more

Figure 31. Comparison of catalytic activities (left) of activation energies (right) of monometallic Ni, monometallic Ru, Ni/Ru core−shell, and NiRu alloy NPs for AB hydrolysis at 30 ± 1 °C. Adapted with permission from ref 308. Copyright 2012 Wiley.

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subjected to a higher oxidability than larger ones, the surface oxidation may play a dominant role on the fcc Ru NPs catalytic activity while the size effect may be responsible for the activity trend for hcp Ru NPs. Nevertheless other factors like the difference in step edges/step density between fcc and hcp Ru cannot be ruled out, but this requires more mechanism investigations. The results of this work are of particular interest because the influence of Ru crystal structure in Ru NPs is only recently studied, while this parameter could have an important effect in the various possible catalytic applications of Ru NPs. 4.6.4. Dehydrogenation of Amine Boranes by

Supported Ruthenium Nanocatalysts. Despite the objec- tive of this review is to highlight the interests of solution Ru NPs in catalysis, the high number of papers describing the use of supported-Ru NPs (mono- or heterometallic) for the dehydrogenation of amine boranes in the past decade makes that we cannot not mention it.284,311

The preparation of the nanocatalysts is generally done by decomposition of the Ru source (most often RuCl3 or [Ru(acac)3]) in the presence of a reducing agent (NaBH4, polyol) and a chosen support (oxides, Al2O3, SiO2, CeO2, TiO2; carbon derivatives, CNTs, GO; MOFs, etc.) Among the recent papers, one can mention different works by S. Özkar and co-workers who used nanohafnia,312 nanozirconia,313 and silica coated Fe3O4

314 as novel supports of Ru NPs and also that of L. Zhou and co-workers315 with a MOF support for the dehydrolytic dehydrogenation of AB at rt. A second common preparation method is an in situ

synthesis of the Ru NPs directly in the catalytic medium. The synthesis of the Ru NPs is carried out in the presence of a given support and using an amine borane as both reducing agent and catalysis substrate. In these conditions, the NP growth happens in parallel of the dehydrogenation of the amine borane and the catalysis is then pursued. For example G. Fan and co-workers investigated Ru NPs supported onto TiO2 nanotubes316 as well as RuNi317 and RuCo318 NPs deposited onto a graphene-like transition metal carbide (Ti3C2X2; with X = OH and/or F). With Ti3C2X2 supporting material (hydro- philic surface), they observed a very good size control and dispersion of the NPs all over the support, and a good dispersion of the catalyst in the reaction medium, probably

enhancing the contact between the metal surface and the substrate (AB). The so-obtained RuNi and RuCo nano- catalysts provided close catalytic performances, namely TOF/ Ea values of 824.7 mol H2·(mol metal·min

−1)/25.7 kJ mol−1

and 896.0 mol H2·(mol metal·min −1)/31.1 kJ mol−1,

respectively. Moreover, these two catalysts showed a good stability reaching 100% conversion of AB after four catalytic cycles even if a decrease of velocity was observed. These catalytic performances are among the best ones claimed today for Ru-based nanocatalysts as the result of enhanced contact between the metal surface and the substrate.

4.6.5. Conclusions on Amineborane Dehydrogen- ation. Ru is one of the most attractive catalysts in the dehydrogenation of amine boranes and most particularly of ammonia borane due to its high efficiency in accelerating the release of hydrogen from these substrates (either by dehydrocoupling or solvolysis). A high number of papers concern the hydrolytic dehydrogenation of ammonia borane because of its simplicity and green approach given it avoids the use of organic solvents as well as of its high efficiency. The preparation of better defined Ru NPs for this reaction has been extensively investigated using different stabilizers (mainly PVP as polymer and amines as ligands) to get stable colloidal solutions, which were proven to be very active in this catalysis. But the influence of the stabilizing ligand is not studied yet in a systematic way, thus limiting the understanding of the ligand− activity relationships. Also a large panel of supports were tested to deposit the Ru NPs (either by wet impregnation or by direct synthesis of the nanoparticles in the presence of the support) and thus increase the stability of the catalysts as well as getting easier their separation from the reaction media for recycling concerns. Here also the support−activity relationships are not well-studied. For economic purposes, some works provided promising results for the improvement of Ru activity and simultaneously minimize its use/cost by forming Ru-based bimetallic structures (RuCo, RuNi, PtRu, RuRh). If com- petitive results have been already obtained compared to those reached with nanocatalysts of other metals (in particular Rh ones), further research is still needed to improve synthesis methodologies to access more performant catalyst in terms of activity, lifetime, and reusability. More rationalization works are also needed because up to now it is very difficult to compare the numerous results described. 4.7. Water Splitting

Fitting the green chemistry principles and known as the water splitting process, the production of hydrogen from water (eq 6) is a very attractive route toward a clean energy vector and even more if envisaging its activation by sunlight. Besides the requirement in active, stable, and if possible low-cost catalysts, the photoactivated water splitting needs to associate a light- harvester, also called photosensitizer (PS) (organic, molecular complex or inorganic semiconductor material), for allowing the electron transfers. The splitting of water is a redox process consisting in two

successive half reactions, namely oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). It starts by the oxidation of water to molecular oxygen at the anode (eq 7a and (7b) at neutral/acidic and basic pH, respectively). Then the released electrons and protons produce molecular hydrogen at the cathode (eq 8a and (8b) at neutral/acidic and basic pH, respectively).

→ +2H O O 2H2 2 2 (6)

Figure 32. Schematic representation of the effect of Ru crystal structure (fcc vs hcp) on the hydrolysis of AB by Ru/PVP/Al2O3 nanocatalysts. Reproduced with permission from ref 310. Copyright 2018 Elsevier.

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→ + ++ −2H O O 4H 4e2 2 (7a)

→ + +− −4OH O 2H O 4e2 2 (7b)

+ →+ −4H 4e 2H2 (8a)

+ → +− −4H O 4e 2H 4OH2 2 (8b)

These two key steps are generally conducted into two different compartments separated by a proton exchange membrane of a (photo)electrochemical cell. They are kineti- cally slow because of their mechanistic complexity, especially for the oxidation half reaction, and the difficulty of evolving gases from a liquid phase. It is therefore of upmost importance to find suitable catalysts able to accelerate them. A main difficulty is having efficient catalysts with compatible kinetics in order to enable the complete splitting process to occur and so the total conversion of H2O into O2 and H2. Another issue is the stability of the catalysts given the harsh necessary conditions (acidic or basic pH). For these reasons, numerous studies aim at evaluating the catalyst performances by studying only one part of the splitting process (either OER or HER). Intensive research activity has been devoted to the use of

molecular catalysts, among which polypyridyl ruthenium complexes showed to be very active for OER.2,319 Among heterogeneous catalysts, iridium oxide (IrO2) anodes display excellent electroactivity for the OER.320 However, heteroge- neous RuO2 also showed significant activity in the OER.

321

Concerning the HER, in the solid phase, the most active metal in reducing protons and especially in acidic conditions is platinum. Nanomaterials have also received high attention among which Ru-based nanocatalysts emerged as true

potential substitutes for the state-of-the-art platinum and iridium oxide catalysts for OER and HER, under the form of oxide Ru or metal Ru species, respectively. As the application of RuO2 NPs and Ru NPs as (photo)electrocatalysts for the water-splitting process has been reviewed very recently,322 we will not provide here a complete description of these nanocatalysts. Interestingly, among the Ru-based nanocatalysts evaluated in water splitting, only scarce examples describe controlled Ru NPs synthesized in mild conditions of wet chemistry for the HER. Because they correspond well to the objectives of the present review, these works will be hereafter briefly presented.

4.7.1. Ru NPs as Electrocatalysts for HER. The use of Ru-based nanocatalysts for the HER is recent but fast evolving (most of the relevant literature was published in the period 2016−2019). This derives from advantageous characteristics of Ru compared to Pt, the state-of-the-art metal for this reaction. First, in HER the M−H bond energy strongly affects proton reduction catalysis given that a high M−H binding energy favors the adsorption of protons (but hardens the H2 desorption), while a low M−H binding energy results in a contrary effect. With an optimum M−H binding energy (neither too low nor too high), platinum stands at the center of the volcano plot for proton reduction catalysts.323,324 In comparison to Pt, Ru displays a slightly weaker M−H bond which hardly decreases the HER catalytic efficiency, both according to experimental results and DFT calculations.12

Furthermore, Ru showed to be stable both under acidic and basic conditions while Pt is not optimally stable at basic pH. Finally, the Ru cost is lower than that of Pt. All together these

Figure 33. (left) TEM/HREM images and powder-XRD diagram of MeOH/THF stabilized Ru NPs. (right) (a) LSV curves of the Ru/MeOH/ THF nanomaterial (red), Ru powder (blue), and Pt/C (green) in 0.5 M H2SO4 solution at 10 mV·s−1. The LSV curve of a bare GC electrode (orange). (b) Galvanostatic experiment of the Ru/MeOH/THF nanomaterial at a current density of 10 mA·cm−2 in 0.5 M H2SO4, without ohmic drop compensation. (c) LSV curves of the initial Ru/MeOH/THF nanomaterial (red) and after 12 h of galvanostatic experiment (blue) in 0.5 M H2SO4 solution at 10 mV·s

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characteristics have boosted the attractivity of Ru metal as HER electrocatalyst in the last three years. Even if some photocatalytic examples exist, most of the

described systems consist in Ru NPs deposited or supported/ embedded onto conductive C-based (or even metallic) materials that are electrochemically triggered. However, a few papers report on nonsupported systems prepared ex situ through various methods (thermal decomposition/calcination of anhydrous RuO2, Ru salt, or a Ru complex; electroreduction of a Ru salt, Ru perovskite-type precursor, or Ru complex) and then deposited onto the electrode for catalytic evaluation, but the tailored synthesis and rational catalytic fine-tuning of nonsupported Ru-based NPs for water splitting is not a simple matter. First, the use of a stabilizer, typically a coordinating solvent, ligand, or the surface of a material, is mandatory to maintain nanoscale systems, preventing the formation of thermodynamically favored bulk species. Also, the metal oxidation state at the NP surface may evolve and even reversibly switch (typically between metallic Ru and Ru (IV) in RuO2) in contact with air and/or under (electro)catalytic turnover conditions. So, as for all catalysis, disposing of an effective way to have model Ru-based NPs (with controlled size, shape, oxidation state, and surface composition) for the splitting of water is of utmost interest for performing fundamental studies in order to develop more efficient catalysts. In this regard, the use of an organometallic complex as precursor recently allowed getting interesting results. The decomposition of the [Ru(COD)COT)] complex under hydrogen, in a MeOH/THF mixture without any stabilizer, allowed obtaining significantly active Ru NPs when deposited onto glassy carbon (GC) electrodes (Figure 33).325 Thus, the 21.4 nm porous Ru NPs in 0.5 M H2SO4 led to values of η0 ≈ 0 mV, η10 = 83 mV, b = 46 mVdec

−1, TOF100 mV = 0.87 s −1, a

Faradaic efficiency of 97%, and excellent durability for up to 12 h (Figure 33). Also, the electrochemical analysis of 4-phenylpyridine (PP)-

capped Ru NPs (mean size ca. 1.5 nm) synthesized from the same complex and then drop-casted onto a GC electrode (PP- Ru-GC) together with their thoroughly characterization in air

and under HER turnover conditions (in both acidic and basic electrolytes), evidenced the influence of the coordinated PP ligand on the catalytic performance. The surface of these Ru NPs spontaneously oxidized to RuO2 upon exposure to air, yielding a mixed Ru/RuO2 system in which the PP ligand was still present. Although this mixed Ru/RuO2 system was less active toward the HER compared with that of pure Ru NPs, it was converted into the metallic Ru form under reductive conditions (20 min bulk electrolysis at −10 mA·cm−2) at acidic pH (Figure 34).326,327 Thus, the recovered PP-Ru-GC system exhibited values of η0 ≈ 0 mV, η10 = 20 mV, b = 29 mV dec−1, and a TOF as high as 17.4 s−1 at η = 100 mV in 1 M H2SO4, with complete stability after 12 h of continuous operation. In contrast, in 1 M NaOH, the only stable form of the PP-Ru-GC system was a Ru/RuO2 mixture, yielding a slightly less active and stable catalytic system, although still outperforming the performance and stability of commercial Pt/C. The presence of the PP capping agent is believed to induce a good mechanical stability, thus allowing the nanostructured character of the material to be maintained, even after a long run. This hypothesis is supported by DFT calculations, which showed the coordination of 11 PP molecules onto the surface of a Ru55H53 NP both through N-σ and π-coordination modes; the latter was more stable and preferentially took place on the edges of the NP. Furthermore, the d-band energy levels of the surface Ru atoms were significantly modified by the presence of hydride ligands, which have a stabilizing effect, whereas these energy levels were not significantly altered by the PP capping ligands, thus indicating a moderate adsorption strength of the latter onto the NP surface. As a consequence, a larger number of hydride ligands were present on the NP surface compared with those of PP (53 vs 11), thus accounting for the enhanced H2 evolution behavior. These results clearly show that a capping ligand like a phenylpyridine can tune the properties of a Ru nanocatalyst for the HER. Nevertheless, the real effect of the ligand needs to be deeper studied and comparative studies with other ligands need to be performed. Supported Ru-based nanomaterials prepared in more drastic

conditions have been also reported as active species for the

Figure 34. (left) TEM images of Ru-PP NPs at low (a) and high (b) magnification and size histogram. DFT model of PP-protected 1 nm RuNP (Ru55H53σPP9πPP2). (right) polarization curves in 1 M H2SO4 at 10 mVs1 and XPS data for metallic PP-Ru NPs and their Ru/RuO2 surface- passivated counterpart, which formed upon exposure to air. Adapted with permission from refs 326 and 322. Copyright 2018 American Chemical Society and Copyright 2019 Wiley.

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HER showing high influence of the nature of the support on the catalytic performance. The effect of the crystal structure of the Ru phase has also been demonstrated. Moreover, the interaction of metallic Ru with other metal/semimetal-based nanostructures in mixed catalysts was shown to increase the HER catalytic activity compared with that of the respective separated systems, as a result of the synergistic effect between metals, which improves the electron conductivity and lowers the H adsorption energy. 4.7.2. Ru NPs as (Photo)catalysts for HER. Concerning

the inclusion of Ru NPs in HER photocatalytic systems, it is not an easy task given the inherent difficulties in properly transferring electrons from a photosensitive molecule or material to the nanocatalyst, while avoiding undesired back- electron transfer processes. Indeed, the electron-transfer process between Ru NPs and the widely employed molecular PS [Ru(bpy)3]

2+ is generally not optimal.328 Thus, together with a sacrificial electron-donor (SED; e.g., reduced nicotinamide adenine dinucleotide (NADH)) supplying the necessary electrons in half-cell systems, the use of an electron mediator (e.g., methyl viologen) is generally required. Only PSs with sufficient and long-lived charge-separated states after photoexcitation are able to inject electrons into the HER electrocatalyst without the need to use an electron mediator, thus making the systems less complex and more efficient. In this regard, Fukuzumi and co-workers reported on the use of a molecular dyad that acts both as a PS and as an efficient electron supplier for Ru NPs, namely the 2-phenyl-4-(1- naphthyl)quinolinium ion (QuPh+-NA; Figure 35).329 Using PVP-stabilized Ru NPs with QuPh+-NA PS in alkaline solution, they found optimal conditions for the photocatalytic HER. No increase in the photocatalytic activity above a certain optimal catalyst concentration (presumably due to light dispersion and opacity if more nanomaterial present in the reaction medium), and an activity-size dependency of the tested NPs were observed. Small NPs displayed a higher negative charge density, which eased the proton reduction

process but hindered the hydrogen-atom association step because of low density of hydrogen atoms on a single particle. Larger NPs eased the hydrogen-atom association step due to the presence of more hydrogen atoms on their surface but hindered the previous proton reduction process because the negative charge density of the surface was initially lower. As a consequence, the best results were obtained with NPs of intermediate size, namely 4.1 nm.329 Finally, the deposition of the Ru NPs and QuPh+-NA onto oxide-based materials (SiO2, TiO2, CeO2, etc.) led to less agglomeration under HER turnover conditions and enhanced photocatalytic stability with regard to the corresponding nonsupported systems.328 Apart from the QuPh+-NA ion, only the dye Eosin Y330 and the combination of [Ru(bpy)3]

2+ with 9-phenyl-10-methylacridi- nium derivatives as electron mediators331 have led to relative success in the photocatalytic HER with Ru-based NPs.

4.7.3. Conclusions on Water Splitting. Very recently, Ru NPs have received a renewed interest for their application as catalysts in the water splitting process. Available data on nonsupported systems indicate amorphous RuO2-based NPs and highly crystalline Ru NPs as the species of choice for attaining high-performance HER NP electrocatalysts. Remark- ably, the mild conditions of solution chemistry provided interesting catalytic systems to conduct fundamental studies where an effect of capping ligand was observed. The catalytic performances achieved evidenced that Ru NPs may be a potential substitute of Pt which is still the most active metal for this reaction. Concerning the photoactivated version of the HER, even if still in their infancy in terms of development, tandem particle-based photocatalysts proved to be promising candidates.

5. CONCLUDING REMARKS AND OUTLOOK

In this review, we gathered main recent advances in the use of Ru-based NPs as catalysts in relevant catalytic reactions such as reduction, oxidation, Fischer−Tropsch, C−H activation, CO2

Figure 35. Electron-transfer processes involved in photocatalytic HER promoted by Ru NPs in the presence of the QuPh+-NA organic donor− acceptor photoabsorber described by Fukuzumi and co-workers. Adapted with permission from ref 322. Copyright 2019 Wiley.

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transformation, dehydrogenation of amine boranes, and water splitting. All together, the research data here assembled clearly evidence the significance of Ru metal at the nanoscale for these reactions. If from the point of view of industrial applications and thus at large scale and for a long-term, the use of noble metals like Ru in catalytic conversions is certainly not realistic due to economic reasons, Ru systems can allow developing fundamental researches in order to better apprehend the prerequisites for rendering a given catalysis more effective. Recent progress in solution nanochemistry allowed having at

disposal better controlled Ru NPs (in terms of size, dispersion, shape, composition, and surface state, etc.), all these characteristics influencing strongly their surface properties. Even if not always satisfying, this led to progress in the understanding of the relationships between their structure and their potential in catalysis (in terms of both reactivity and selectivity). Most particularly, the surface chemistry of Ru NPs starts to be better understood, which gives a strong basis to better apprehend catalytic processes on the metal surface as well as how these can be affected by the presence of stabilizing molecules or by the crystallographic structure of the ruthenium cores, eventually by taking benefit of these parameters. However, this is only in its infancy and numerous studies are trial−error or screening works and the rationalization of the catalysis findings with the NP structural features is not often done. Such a rationalization is not possible from published works given synthesis conditions and parameters change from one study to another one. More efforts are thus required in order to bridge this gap. This is fundamental if one want to be able one day to anticipate about the needed Ru NP structure for making a target catalysis highly performant and also highly selective, but this is not true only for ruthenium because such studies are generally missing in nanocatalysis whatever the metals used. For instance, regarding the influence of ligand, this is not an easy task because this requires having preformed NPs that enable a complete ligand exchange or have a synthesis method that provides always the same size of particles whatever the stabilizing ligand added in the reaction mixture. To our best knowledge, such means are not accessible yet. Concluding remarks and perspectives are hereafter given more specifically for the catalytic reactions described above. Ru NPs are very versatile catalysts for reduction reactions.

As reviewed above, this versatility is illustrated with the large range of reduction reactions, including the hydrogenation of CC, CO, and −NO2 motives using several reducing agents. Because of the straightforward implementation of some of these reactions, for instance, reduction of styrene by H2 or reduction of −NO2 groups by NaBH4, and the facility to compare the obtained results to other reported works, these reactions are often used as an indirect characterization way to get information on the surface properties of the nanocatalysts. Ru-based nanocatalysts for reduction reactions underwent important evolution in the last years. If first they were only stabilized with simple molecules, ruthenium nanocatalysts are now more complex because their design has strongly benefited from the development of nanochemistry tools. Such evolution is visible either by the use of new and sometimes sophisticated ligands that have been deliberately designed to obtain a desired property or by introducing a second (or more) metal or by using a more reactive fcc structure. Water-soluble ligands or polymers, stabilizers containing long carbon chains, and ligands with a specific electronic property are among examples that have been successfully explored. It is important to note that Ru

NPs systems able to induce chirality are only elusive, even if some efforts have been done in this topic. All the knowledge obtained in these model reactions is currently been used to explore the applicability of Ru NPs in challenging reduction reactions such as hydrodeoxygenation together with C−O cleavage of biomass derivatives. Bimetallic Ru-based systems proved to be very efficient catalysts as the result of the subtle balance of the properties of the metals used, their combination leading to synergistic effects. In contrast, unsupported Ru NPs as catalysts for oxidation reactions are scarce and are essentially devoted to the oxidation of CO. The catalysts of this reaction are principally bimetallic systems with a specific tuning of the NPs, or the metal ratio, or the Ru structure, or both. Ru NPs with a fcc structure have proven to be highly reactive for this reaction. Fischer−Tropsch reaction was demonstrated to be also sensitive to the crystalline structure of the Ru NPs, giving highly active catalysts when adopting the fcc structure. Also, the reaction is sensitive to the size of the Ru NPs which can be related to the CO energy adsorption in different surface positions. The ability of Ru NPs to activate C−H bonds reported in the past has been recently exploited to produce labeled organic compounds in a highly selective manner. A general weakness of the Ru NPs colloidal-based catalysis is the lack of knowledge on the catalytic active species that is operating. Characterization of the spent catalyst, recycling test, hot filtration, among other procedures, are far to be systematically performed, and when carried out they are not always done in the appropriate manner (a typical example is to carry out recycling tests at 100% conversion). In situ or in operando characterization techniques are, by now, scarce for these catalysts. The chemical transformation of CO2 has not been

investigated into detail over well-defined heterogeneous catalysts including nanoparticle-containing ones. This topic remains a challenging but of high interest task given the advantages provided by heterogeneous catalysts compared to homogeneous ones for industrial applications. State-of-the-art data revealed substantial limitations, but no clear insights at the molecular level have been reported, hindering concrete progress. In particular, there is a clear lack of understanding of structure−reactivity correlations and of catalyst designing principles for this catalysis. As described above, recent results involving Ru-based nanocatalysts have shown that efforts performed for the precise design of solution and supported nanocatalysts can lead to the chemoselective CO2 hydro- genation into HCOOH, CO, CH4, or other hydrocarbons. Such studies make a parallel with those reported on Ru molecular complexes. Recent knowledge and know-how in nanotechnology and nanocatalysis should lead to novel strategies in the design of efficient and more stable nano- catalysts for CO2 transformation. Prospective studies with a molecular approach may allow tuning more finely the catalytic properties of nanocatalysts. Mechanistic details being critical to the development of improved nanocatalysts, more investiga- tions in this direction are also required in order to achieve higher catalytic performances. Even if catalytic activities are not elevated with this metal, Ru-based nanocatalysts may offer the possibility to access spectroscopic NMR studies which can be very complementary to infrared studies in order to get mechanistic insights. Associate another metal (Pd, Ni, or Fe) to Ru is certainly a strategy to explore more in order to increase the catalytic performance (both reactivity and selectivity). Also separately optimizing the metal active sites

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and the support may provide benefit. Finally, it is also needed to keep in mind that harmonization is necessary to ensure a constant and dynamic balance of all things to be considered, for a sustainable and green chemistry. The use of H2 issued from green sources like water also appears as a great solution to reach a closed nature’s carbon cycle. Concerning the H2 production by dehydrogenation of amine

boranes, Ru-based nanocatalysts are highly efficient and stand at the top list. If extensive research efforts focused on the dehydrogenation of ammonia borane by hydrolysis (due to its simplicity and green character as well as efficiency), interesting results were also obtained by methanolysis or dehydrocou- pling. These last approaches merit more efforts, at least at the fundamental level, in order to get mechanism insights, enable the development of more performant catalytic systems and improve hydrogen productivity. If numerous kinetics param- eters are available and allow comparing the efficiency of the Ru nanocatalysts reported for the dehydrogenation of amine boranes in water, there is no clear insight explaining the high activity generally observed. What about the real effect of particle size, Ru crystal structure, surface area, stabilizer, and/ or support nature on the catalytic performances? Answers to these questions remain to be found in most cases. Moreover, the catalytic lifetime parameter has received a quite low attention until now, whereas NPs are not thermodynamically stable entities and can be readily deactivated, which may harm their long-term performance. If AB solvolytic dehydrogenation is a promising hydrogen generation system (in particular, for cases that require a convenient and reliable hydrogen source), the decomposition of AB results in a contamination of released H2 by NH3 and borazine, which is a major problem for application in fuel cells. Thus, further efforts are required in order to solve pending issues like breaking the strong B−O bonds in byproducts of AB solvolysis and reducing NH3 release. Other important issues are the storage irreversibility and cost factor. Regeneration of AB from byproducts of solvolysis, especially hydrolysis, is cost-ineffective, as undesired byproducts of the recycling process cannot be converted to the main reactants.243 So, other hydrogen storage materials need to be studied in order to have less volatile byproducts than with AB. Only a few papers deal with AMB and DMAB that are alternative substrates, thus showing that more efforts have to be done in this direction. Ru-based NPs have clearly emerged as promising (electro)-

catalytic systems for the two half-cell reactions in water splitting and potential substitutes of standard Pt and IrOx species used for catalyzing the HER and OER, respectively, in commercial electrolyzers. Most particularly, the development of Ru-based NPs as catalysts for the HER was highly dynamic in the last three years. Reports on nonsupported catalytic systems showed that the active sites of the Ru NPs can be tuned with ease and the surface chemistry resembles that of molecular complexes. In this regard, the organometallic synthesis of nanostructures opens up numerous possibilities through the inexhaustible ligand pool of NP stabilizers. The combination of electrochemical analysis, detailed structural and surface characterization, and DFT modeling of the reaction pathways involved can lead to structure−activity/ stability relationships, thus allowing the subsequent rational improvement of the electrocatalytic HER systems. To conclude, even if less expensive than other noble metals,

the high price and limited abundance of Ru probably hinder the practical applications of Ru NPs-based catalysts for

industrial purposes, but studied systems are of high interest at the fundamental level because they allow doing nice breakthroughs and getting precious insights on the catalytic properties of Ru NPs. As a nonexhaustive example, Ru is a 4d transition metal that in the bulk adopts an hcp structure at all temperatures, but thanks to the development of effective tools, Ru NPs with a crystallographic fcc structure could be prepared although they are thermodynamically unstable, thus high- lighting the interest of modern nanochemistry. In this way, the crystal phase effect of Ru could be explored toward a few catalytic reactions (like CO oxidation, nitrophenol reduction, hydrolysis of ammoniaborane, oxygen evolution reaction), allowing observation of differences compared to hcp Ru NPs. These advances underline that not only the size of the NPs is of paramount importance if one wants to tune finely their catalytic performance but also how important is the control of their other characteristics such as their crystalline structure and their composition/surface state. Indeed, catalytic properties are closely correlated with the catalyst surface geometric and electronic structures and an optimal compromise among reactant adsorption rate, adsorbate−surface interaction, and product desorption is necessary to promote catalytic activity. This is true whatever the target catalytic reaction. It thus requires development of effective synthesis tools in order to have at disposal model NPs with an atomic precision level to be able to conduct precise comparative studies. Besides the synthesis aspects, in operando techniques could bring very useful information on the surface state of the NPs in catalysis conditions (IRFT, NMR, XPS, environmental-HREM, EXAFS, etc.). Such approaches are still rare in the papers describing the interests of well-defined Ru NPs in catalysis. Interestingly, Ru is a metal which permits to take benefit of NMR techniques to access a fine mapping of the surface state of the NPs, as it is generally done for metal active centers in molecular catalysts. Moreover, in parallel of experimental techniques, theoretical studies can afford a better understanding of the influencing parameters of a given catalysis within the aim to develop more performant nanocatalysts in terms of activity and selectivity. Efficient theoretical tools are now accessible that allow obtaining an overview of a nanoscale surface with a resolution close to that usually got for molecular catalysts or extended metal surfaces. As a final message, we thus do believe that future developments crossing experimentally well-defined model metal nanoparticles together with theoretically close simulated nanoclusters will enable nice breakthroughs for the development of more performant nanocatalysts and that Ru is a highly interesting metal to do so.

AUTHOR INFORMATION Corresponding Author

*Phone: +33 (0) 5 33 32 30. E-mail: karine.philippot@lcc- toulouse.fr. ORCID

M. Rosa Axet: 0000-0002-2483-1533 Karine Philippot: 0000-0002-8965-825X Notes

The authors declare no competing financial interest.

Biographies

M. Rosa Axet did her Ph.D. in Tarragona with a thesis on chiral catalysis and nanocatalysis (Prof. Claver and Prof. Castilloń). After a

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postdoctoral fellowship in Trieste (Prof. Milani), Toulouse (Dr. Chaudret and Dr. Philippot), and Paris (Dr. Amouri), she joined CNRSFrance as an associate researcher at the Laboratoire de Chimie de Coordination in Toulouse, where she started her research activities focusing on nanocatalysis. Her current research activities include organometallic and nanomaterials chemistry areas, mainly for applications in catalysis. She is interested in the study of the structure−properties relationships in several nanomaterials including bimetallic, supported, or shape-controlled nano-objects, with special attention to the effects of the stabilizing ligands of the nanoparticles on their properties.

Karine Philippot is research director at CNRS, at the Laboratory of Coordination Chemistry of Toulouse, where she is the head of the team “Engineering of Metal Nanoparticles”. Being involved in different projects, her current research interests cover the design of metal nanoparticles and composite nanomaterials by using molecular chemistry concepts and their applications, mainly in colloidal or supported catalysis and for energy production (CO2 valorization, water-splitting, fuel cells). She is coauthor of 175 peer reviewed papers (including 7 reviews, 9 book chapters, and 6 patents) and over 200 presentations at national and international conferences. She also coedited a special issue devoted to “Catalysis in Solution by Defined Nanoparticles” (Topics in Catalysis, 2013) and the book “Nanoma- terials in Catalysis” (Wiley, 2013).

ACKNOWLEDGMENTS

We acknowledge the Laboratory of Coordination Chemistry (LCC-UPR8241), the Centre National de la Recherche (CNRS), and the University de ToulouseUniversite ́ Paul Sabatier for financial support.

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