Catalysts For Hydrogenation Reactions Research Articles

Operando Monitoring of Homolytic Cleavage of H2 into Surface Hydrides on Defective Cerium Dioxide Nanoparticles

AbstractThe use of ceria as catalyst in hydrogenation reactions is a recent trend, as they were uncovered to be active on a number of substrates, including alkynes, alkenes, and enones. While some studies focused on the use of well‐defined shapes, others emphasized the interest of defective structures. Overall, the meeting point of these studies is the need to characterize the surface reactivity vs. H2, by considering a number of interesting textures (films, nanopowders with various grain size or porosity), using monitoring techniques that preserve the original features of the materials. A key question is in particular to assess the possibility to form surface hydride through homolytic H2 splitting at cerium sites rather than at the oxygen one, a route recently highlighted as plausible. To address this question on small crystallites of CeO2, we designed a low‐temperature synthetic route for nanorods with an average grain size of 10 nm. Then, we employed near‐ambient‐pressure X‐ray photoelectron spectroscopy as an operando tool to monitor the cerium surface oxidation state during the initial annealing of the nanopowders, followed by exposure to a moderate pressure of H2 (0.52 mbar). We demonstrate that H2 homolytic splitting at cerium sites is the main activation process on this surface at 100 °C, leading to the oxidation of ca. 30 % of the surface cerium atoms from Ce3+ to Ce4+. The surface hydride species were however not stable at 250 °C, H2 was released, and cerium reduced back to Ce3+. A similar mechanism was observed on a defective ceria material obtained by calcination of CeO(OH)2, with comparable intensities. Overall, we report here CeO2 nanorods, exposing predominantly {100} and {110} facets, as showing an interesting surface reactivity for potential application in hydrogenation reactions, and we expose a straight operando methodology to delineate suitable temperatures to be used.

Effect of carbon oxygen functionalization on the activity of Pd/C catalysts in hydrogenation reactions

This paper presents a study on the effects of oxygen functionalities on a mesoporous and graphitized carbon support (GNP, Graphene Nanoplatelets), on the catalytic activity of Pd/GNP catalysts in hydrogenation reactions. A functionalization method in liquid phase has been employed, using different oxidants (HNO3, H2O2, KMnO4), with the aim to tune the amount and the type of introduced oxygen functionalities. Preformed Pd nanoparticles have been used as Pd‐precursor to limit differences in metal particle size and dispersion on differently functionalized carbon. The catalytic behaviour in benzaldehyde hydrogenation/hydrogenolysis to benzyl alcohol and toluene revealed that the introduction of oxygen functionalities has a generally detrimental effect. NMR relaxometry studies highlighted the weaker interaction between the carbonyl group and the functionalized Pd/GNP surface than the non‐functionalized Pd/GNP demonstrating that the origin of the different catalytic activity lies on the first step of the reaction. O‐functionalities also impacted on the Pd0/Pd2+ ratio at the surface which is an established parameter correlated to the reaction rate.

Progress in Cobalt Complexes for Enabling Asymmetric Hydrogenation, H2 Production Catalysis and Alkene/Alkyne Hydro‐functionalization: A Review

AbstractThe use of hydrogen as an alternate clean fuel over conventional fuels, in addition to its role in chemical industries giving rise to useful products like hydrocarbons, has been the subject of research for many years. The need for hydrogen gas production methods has ever since been in demand. This review offers a comprehensive exploration of applications of cobalt complexes in asymmetric hydrogenation, H2 production catalysis and the hydro‐functionalization of alkenes and alkynes. Cobalt metal stands out, particularly due to its distinctive involvement in potential single‐electron processes. The main objective of this review is to provide an insight into the role of cobalt‐based coordination complexes as catalysts in hydrogenation reactions. It delves into models for enantio‐induction and extensively investigates the role of cobalt complexes in hydrogen evolution and associated reactions. Cobalt selection as the central metal is attributed to its beneficial (II)/(III) redox couple state, which proves valuable in hydride transfer reactions. While the review is structured based on substrate types, its core focus lies in the mechanism‐driven exploration of catalytic systems. These catalytic systems exhibit versatility in facilitating the hydrogenation and hydro‐functionalization of various types of double bonds through the utilization of tridentate ligands.

Shedding light on the reversible deactivation of carbon-supported single-atom catalysts in hydrogenation reaction

Shedding light on the reversible deactivation of carbon-supported single-atom catalysts in hydrogenation reaction

The structure-activity relationship of copper hydride nanoclusters in hydrogenation and reduction reactions.

Copper hydrides are highly active catalysts in hydrogenation reactions and reduction processes. Three Stryker-type copper hydride nanoclusters (NCs), [(TPP)CuH]6, [(TCP)CuH]6 and [(TOP)CuH]6 (TPP = triphenylphosphine, TCP = tricyclohexylphosphine and TOP = tri-n-octylphosphine), were synthesized in this study. Due to variations in the electron-donating properties of the phosphine ligands, the UV-visible absorption spectra of the three NCs exhibited notable distinctions. The influence of the phosphine ligands on the effectiveness of the NCs as hydride sources in hydrogenation processes, as well as on the applicability as homogeneous catalysts for reduction reactions, was systematically studied. Due to the highest electron-donating properties of the TOP ligand, [(TOP)CuH]6 was found to exhibit superior performance in both hydrogenation reactions and catalytic reduction reactions. Moreover, these hydrophobic NCs worked well as heterogeneous catalysts in the reduction of 4-nitrophenol.

Promoting hydrogenation without side reactions over Pd nanoparticles uniformly dispersed in MFI zeolite

Zeolites have been widely applied as support materials for metal nanoparticles. However, when developing a metal/zeolite catalyst for hydrogenation reactions, the Brønsted acid sites of zeolites can promote side reactions and the zeolite structure can inhibit the access of reactants to the metal, resulting in low activity and selectivity. In this study, these challenges are addressed by preparing a metal/zeolite catalyst via a modified ion-exchange followed by reduction method, and subjecting the catalysts to two test reactions, hydrogenation of vanillin and fatty acid methyl esters (FAMEs). After forming Pd nanoparticles within ZSM-5, it was treated with a NaOH solution in a controlled manner to form mesopores around Pd and replace hydronium ions with sodium ions to remove Brønsted acid sites. Pd/ZSM-5 catalysts prepared with controlled NaOH treatment showed higher hydrogenation reactivity and improved selectivity toward the desired hydrogenation products than those prepared without NaOH treatment. This study presents a simple strategy to prepare metal/zeolite catalysts to promote hydrogenation without side reactions.

Nickel Nanoparticles Immobilized on Pristine Halloysite: An Outstanding Catalyst for Hydrogenation Processes

AbstractNickel nanoparticles (NiNPs) immobilized on halloysite‐based supports were straightforwardly synthesized and fully characterized by different techniques with the purpose of evaluating the catalytic performance of these as‐prepared composite materials as catalysts in hydrogenation reactions. Thus, halloysite bearing amino (HAL‐NH2) and ammonium (HAL‐NEt3) groups led to less active catalytic materials in comparison with unfunctionalized halloysite supports, probably due to relative strong metal‐support interactions with amino and tetraalkylammonium functions, consequently hindering the interaction of unsaturated substrates at the surface of the catalytic material. Despite NiNPs supported on pristine halloysite provided merely agglomerates, NiNPs stabilized by quinidine on unfunctionalized halloysite (NiC) proved to be a highly effective and versatile catalyst operating at relative low temperature (25–100 °C) and metal loading (1–20 mol %Ni), very well‐adapted to reduce a large range of functions (alkenes, alkynes, ketones, aldehydes, nitro…). Moreover, NiC was able to selectively reduce substrates from biomass to yield high value‐added products, such as squalane, saturated fatty acids, furfuryl alcohol and γ‐valerolactone.

Catalytic activity and selectivity of Palladium and Nickel catalysts in hydrogenation reactions of nitro- and acetylene compounds

This paper presents the results of a study on the catalytic activity and selectivity of nickel and palladium catalysts in hydrogenation reactions of nitro- and acetylene compounds. It was shown that the activity and selectivity of nickel catalysts in the hydrogenation of phenylacetylene depend on the nature of modifying additives (Cu, Zn, Ti, Cr, Bi, Ti–Cu, Mn, Fe), and the activity and selectivity of palladium catalysts based on a polymer of metal complexes depends on the method of their preparation. It was found that for certain concentrations of the active phase of palladium (0.8 wt.%) and the polymer of potassium humate (1.0 wt.%.) in the catalyst, where palladium and the polymer were deposited on bauxite-094 together, the catalyst exhibits the greatest activity and selectivity when hydrogenating phenylacetylene and potassium orthonitrophenolate.

Hydriding pathway for Ni nanoparticles: Computational characterization provides insights into the nanoparticle size and facet effect on layer-by-layer subsurface hydride formation

Nickel and its alloys are often used in the form of nanostructured materials, e.g., as catalysts for hydrogenation reactions, for hydrogen-storage applications, and as battery materials. This study focuses on subsurface hydride (NiHx) formation in Ni nanoparticles. The pressure at which hydrogen is incorporated within the nanoparticle surface is probed using grand canonical Monte Carlo (GCMC) simulations. An important observation is that the Ni nanoparticle size can have a significant effect on the hydride formation. Unlike bulk Ni, which forms the hydride phase at very high H2 pressures, subsurface NiHx can form at significantly lower pressures with 2.7–9.1 nm particles. The (111) and (100) facets and facet edges of these nanoparticles possess remarkably different H incorporation characteristics compared to the nanoparticle core. The easier H-absorption in Ni nanoparticles is explained in terms of a nucleation-and-growth process, which begins at the facet edges and proceeds towards the interior of the nanoparticle. The NiHx and Ni phases co-exist in nanoparticle systems, which is not observed in bulk Ni. A computational characterization approach is used to gain insights into the layer-by-layer H incorporation, which can be valuable for devising material preparation strategies for adsorption-based hydrogen storage and catalysis.

Ionic Liquids/SiO2 Supporting Pd Nanoparticles: Efficient Catalysts in Hydrogenation Reaction

Palladium nanoparticles (ca. 4.8 nm) were synthesized in presence of 1-n-butyl-3-methylimidazolium tetraflouroborate (BMI.BF4) and 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF6) and PMI.Si.(OMe)3.Cl functionalized ionic liquids using the sol-gel method. The characteristics of the sol-gel method, ionic liquid on the palladium content was studied, as well as the silica morphology and texture of the support and the hydrogenation activity. The palladium content in the resulting xerogels (ca. 0.22 wt% Pd/SiO2) was shown to be independent of the sol-gel process. The xerogels synthesized in acidic conditions formed materials with larger pore diameters, which in turn might be responsible for the higher catalytic activity in hydrogenation of the alkenes and arenes obtained with the heterogeneous catalyst (Pd/ILs/SiO2).

Hydrogenation of naphthalene to decalin catalyzed by Pt supported on WO3 of different crystallinity at low temperature

Two tungsten oxides (WO3-500 and WO3-900) were prepared at different calcination temperatures by using ammonium mettungstate as tungsten source. The physicochemical properties of WO3 before and after Pt loading were systematically characterized by XRD, SEM, TEM, XPS, H2-TPR, and NH3-TPD. The influence of the crystallization degree of WO3 on the hydrogenation of naphthalene was studied under low-temperature reaction conditions. Compared with the WO3-900 support, WO3-500 obtained by calcination at lower temperature exhibited a lower crystallization degree with a large amount of W5+ species, which resulted in strong interactions with Pt. The strong interaction between W species and Pt contributed to the high acidic strength. The Pt/WO3-500 catalyst demonstrated excellent catalytic performance for naphthalene hydrogenation to decalin at low reaction temperature (70°C) with full conversion and 100% decalin selectivity. Under identical conditions, conversion and decalin selectivity were only 26.7% and 1.7% over the Pt/WO3-900 catalyst. Combining the characteristics of the catalysts and their catalytic results, we revealed the promoting role of oxygen defects in WO3-supported Pt catalysts in the hydrogenation of naphthalene to decalin, which will provide theoretical guidance for designing efficient WO3-supported Pt catalysts for hydrogenation reactions.

Palladated composite of MOF and cyclodextrin nanosponge: A novel catalyst for hydrogenation reaction

In attempt to develop green protocols for organic transformations, a novel catalyst is prepared by combination of the features of metal-organic frameworks and polymers of cyclic carbohydrates. In detail, cyclodextrin nanosponge was synthesized from β-cyclodextrin that is a cyclic carbohydrate with capability of formation of inclusion complex and then hybridized with the as-prepared iron based metal-organic framework. The composite was then palladated to furnish a catalyst for hydrogenation of nitroarenes in aqueous media at ambient temperature. It was postulated that cyclodextrin nanosponge in the catalyst backbone can act as a nanoreactor to encapsulate and transfer nitroarenes in aqueous media and control the growth of Pd nanoparticles by acting as a capping agent. The results approved high efficiency of the catalyst, superior to the palladated CDNS and metal-organic framework. Using this catalyst, steric substrates with low solubility in water as well as substrates with competing functionalities could efficiently be hydrogenated. Moreover, the catalyst could be recycled several times with low Pd leaching and loss of activity.

Promoting H2 Activation over Molybdenum Carbide by Modulation of Metal‐Support Interaction for Efficient Catalytic Hydrogenation

AbstractTransition metal carbides (TMCs) have been used extensively as catalysts for hydrogenation reactions. However, TMC catalysts suffer from low activity for H2 activation, which is vital to their hydrogenation performance. Herein, we report a feasible strategy to improve the ability of TMCs (e. g., Mo2C) for H2 activation by modulation of the metal‐support interaction (MSI). The MSI can be tuned by introducing N element into carbon supports. Specifically, N can modulate the orbital structure of Mo to promote H2 activation. Experimental and theoretical results reveal that H2 activation ability of the metal sites can be altered by loading them onto carbon supports, and optimized by doping N into the corresponding carbon matrices. Consequently, N‐doped carbon‐supported Mo2C (Mo2C/NC) catalyst, possessing improved H2 activation ability, displays remarkably enhanced catalytic hydrogenation performance for the reduction of nitro compounds. The conversion and selectivity over Mo2C/NC are nearly 100 % under mild conditions.

Immobilization of an Iridium(I)-NHC-Phosphine Catalyst for Hydrogenation Reactions under Batch and Flow Conditions

Na2[Ir(cod)(emim)(mtppts)] (1) with high catalytic activity in various organic- and aqueous-phase hydrogenation reactions was immobilized on several types of commercially available ion-exchange supports. The resulting heterogeneous catalyst was investigated in batch reactions and in an H-Cube flow reactor in the hydrogenation of phenylacetylene, diphenylacetylene, 1-hexyne, and benzylideneacetone. Under proper conditions, the catalyst was highly selective in the hydrogenation of alkynes to alkenes, and demonstrated excellent selectivity in C=C over C=O hydrogenation; furthermore, it displayed remarkable stability. Activity of 1 in hydrogenation of levulinic acid to γ-valerolactone was also assessed.

Synthesis of nickel boride and investigation of availability as an additive in the molten carbonate fuel cell anode material

Molten carbonate fuel cells are notable as clean energy systems and do not need pure hydrogen as fuel, and can use natural gas and hydrocarbons. Nickel boride is used as catalysts for hydrogenation reactions, because of its high micro-hardness, chemical and thermal stability, selectivity and activity for liquid phase reactions. In this study, the positive effect of nickel boride as an additive in the anode material was investigated. Nickel boride was synthesised by chemical reduction with solvo-thermal route. Reduction reaction of nickel chloride hexahydrate salt with sodium borohydride was performed for the nickel boride synthesis. Anode green sheets were prepared using the nickel boride additive with different rates (0%, 1%, 2% and 3%) by tape casting. Green sheets were subjected to sintering operation to get anode sheets. As a result of SEM analysis, increasing the nickel boride additive in the slurry caused to decrease in particle size and more homogeneous dispersion of the particles.

Spot the difference: hydrogen adsorption and dissociation on unsupported platinum and platinum-coated transition metal carbides.

Hydrogenation reactions are involved in several processes in heterogeneous catalysis. Platinum is the best-known catalyst; however, there are limitations to its practical use. Therefore, it is necessary to explore alternative materials and transition metal carbides (TMCs) have emerged as potential candidates. We explore the possibility of using cheap TMCs as supports for a Pt monolayer, aiming to reduce the amount of the noble metal in the catalyst without a significant loss of its activity towards H2 dissociation. Hence, analyzing H2 dissociation from a fundamental point of view is a necessary step towards a further practical catalyst. By means of periodic DFT calculations, we analyze H2 adsorption and dissociation on Pt/β-Mo2C and Pt/α-WC surfaces, as a function of hydrogen surface coverage (ΘH), resembling a more realistic model of a catalyst. H2 dissociation rates were analyzed as a function of the reaction temperature. The results show that Pt/C-WC and Pt/Mo-Mo2C have a Pt-like behavior for H2 dissociation at ΘH > 1/2 ML. At a particular temperature of 298 K, Pt/C-WC and Pt/Mo-Mo2C have low energy barriers for H2* → 2H* (0.13 and 0.11 eV, respectively), close to the value of Pt (0.06 eV). For the highest coverage, i.e. ΘH = 1, Pt/C-WC has a lower activation energy and a higher reaction rate than Pt. Finally, the H2 dissociation rate is higher in Pt/Mo-Mo2C than in Pt when increasing the temperature above 298 K. Our results put Pt/C-WC and Pt/Mo-Mo2C under the spotlight as potential catalysts for H2 dissociation, with a similar performance to Pt, paving the way for further experimental and/or theoretical studies, addressing the capability of Pt/TMC as practical catalysts in hydrogenation reactions.

Carbon Black as a Support in Palladium Catalysts for Hydrogenation of Organic Compounds

The possibility of using carbon black (CB) as a support of palladium catalysts for the selective hydrogenation of organic compounds is considered. The advantages of Pd/CB catalysts are demonstrated by examples of liquid-phase hydrogenation of aromatic aldehydes and nitro compounds. The combination of high catalytic parameters (activity, selectivity, and stability) achieved by these catalysts in hydrogenation reactions indicates that they are promising for use on an industrial scale, in particular in the syntheses of fine chemicals.

Liquid Metals in Catalysis for Energy Applications

Summary To keep up with the fast-paced transitioning of the global energy sector, which is constantly thriving to enable reliable, economic, and sustainable energy production, catalysis research has been required to continuously evolve in response. The challenges in the existing systems are predominantly due to dependencies on heterogeneous solid catalysts that are susceptible to coking. In this respect, liquid-metal (LM) catalysts have been demonstrated to have a critical advantage over conventional catalysts. Recently, LMs acquired a place in catalysis, with a reputation often synonymous with interesting properties and a remarkable ability to break trade-offs between homogeneous and heterogeneous catalysis. This review bridges the fundamental principles of LM research and the recent advances in LM-based thermal and electrochemical catalysis for energy applications. Moreover, emerging approaches for the improved utilization of LMs are outlined, and the concepts requiring greater research attention that could enable the development of exciting energy solutions are highlighted.

Supported Palladium Nanocatalysts: Recent Findings in Hydrogenation Reactions

Catalysis has witnessed a dramatic increase on the use of metallic nanoparticles in the last decade, opening endless opportunities in a wide range of research areas. As one of the most investigated catalysts in organic synthesis, palladium finds numerous applications being of significant relevance in industrial hydrogenation reactions. The immobilization of Pd nanoparticles in porous solid supports offers great advantages in heterogeneous catalysis, allowing control of the major factors that influence activity and selectivity. The present review deals with recent developments in the preparation and applications of immobilized Pd nanoparticles on solid supports as catalysts for hydrogenation reactions, aiming to give an insight on the key factors that contribute to enhanced activity and selectivity. The application of mesoporous silicas, carbonaceous materials, zeolites, and metal organic frameworks (MOFs) as supports for palladium nanoparticles is addressed.

Vicinal Na+ as structure guardians of atomically dispersed Ru catalysts in hydrogenation reactions

Vicinal Na+ as structure guardians of atomically dispersed Ru catalysts in hydrogenation reactions