Synthesis of Supported Bimetal Catalysts using Galvanic Deposition Method

Supported bimetallic catalysts have been studied because of their enhanced catalytic properties due to metal‐metal interactions compared with monometallic catalysts. We focused on galvanic deposition (GD) as a bimetallization method, which achieves well‐defined metal‐metal interfaces by exchanging heterogeneous metals with different ionisation tendencies. We have developed Ni@Ag/SiO₂ catalysts for CO oxidation, Co@Ru/Al₂O₃ catalysts for automotive three‐way reactions and Pd−Co/Al₂O₃ catalysts for methane combustion by using the GD method. In all cases, the catalysts prepared by the GD method showed higher catalytic activity than the corresponding monometallic and bimetallic catalysts prepared by the conventional co‐impregnation method. The GD method provides contact between noble and base metals to improve the electronic state, surface structure and reducibility of noble metals.     

Effect of ultra-fine gold particle addition to metal oxides in ethylene oxidation

Oxidation of ethylene was carried out over alumina-supported metal oxide catalysts and highly dispersed gold catalysts, respectively, under atmospheric pressure. The ethylene was completely oxidized to produce carbon dioxide and water with both metal oxide and gold catalysts. The activity of gold catalyst prepared by deposition method was much higher than that of supported metal oxide catalysts. Ultra-fine gold particles on Co₃O₄ were more active than on Al₂O₃. Fe₂O₃/Al₂O₃ and MnO₂/Al₂O₃ catalysts were more active than MoO₃/Al₂O₃ catalyst. The activity of the supported metal oxide catalysts was greatly enhanced by addition of gold particles. It was therefore considered that gold particles promote dissociative adsorption of oxygen and the adsorbed oxygen reacts with adsorbed ethylene on support adjacent to the active site.     

Electronic Metal-Support Interaction-Modified Structures and Catalytic Activity of CeOx Overlayers in CeOx/Ag Inverse Catalysts

Electronic metal-support interactions (EMSIs) of oxide-supported metal catalysts strongly modifies the electronic structures of the supported metal nanoparticles. The strong influence of EMSIs on the electronic structures of oxide overlayers on metal nanoparticles employing cerium oxides/Ag inverse catalysts is reported herein. Ce₂O₃ overlayers were observed to exclusively form on Ag nanocrystals at low cerium loadings and be resistant to oxidation treatments up to 250 °C, whereas CeO₂ overlayers gradually developed as the cerium loading increased. Ag cubes enclosed by {001} facets with a smaller work function exert a stronger EMSI effect on the CeO_x overlayers than Ag cubes enclosed by {111} facets. Only the CeO₂ overlayers with a fully developed bulk CeO₂ electronic structure significantly promote the catalytic activity of Ag nanocrystals in CO oxidation, whereas cerium oxide overlayers with other electronic structures do not. These results successfully extend the concept of EMSIs from oxide-supported metal catalysts to metal-supported oxide catalysts.     

Reactivity of Metal Catalysts in Glucose–Fructose Conversion

A joint experimental and computational study on the glucose–fructose conversion in water is reported. The reactivity of different metal catalysts (CrCl₃, AlCl₃, CuCl₂, FeCl₃, and MgCl₂) was analyzed. Experimentally, CrCl₃ and AlCl₃ achieved the best glucose conversion rates, CuCl₂ and FeCl₃ were only mediocre catalysts, and MgCl₂ was inactive. To explain these differences in reactivity, DFT calculations were performed for various metal complexes. The computed mechanism consists of two proton transfers and a hydrogen‐atom transfer; the latter was the rate‐determining step for all catalysts. The computational results were consistent with the experimental findings and rationalized the observed differences in the behavior of the metal catalysts. To be an efficient catalyst, a metal complex should satisfy the following criteria: moderate Brønsted and Lewis acidity (pK_a=4–6), coordination with either water or weaker σ donors, energetically low‐lying unoccupied orbitals, compact transition‐state structures, and the ability for complexation of glucose. Thus, the reactivity of the metal catalysts in water is governed by many factors, not just the Lewis acidity.     

The role of urea in Cu–Zn–Al catalysts for methanol steam reforming

Abstract  

Impregnated Cu–Zn over Al₂O₃ exhibits high activity with the use of a lower amount of active metal relative to conventional co-precipitation catalysts. The activity of the catalyst could be enhanced by addition of urea to the metal salt solution during impregnation. The H₂ yield from Cu–Zn catalysts with urea is 42%, while the H₂ yield from catalyst without urea is only 28% in a continuous system at 250 °C and 1.2 atm. The H₂ yield of the catalyst with urea in this study could compete with that of commercial catalysts. The role of urea in the Cu–Zn catalysts was investigated. X-ray diffraction (XRD) analysis of the catalysts shows that the crystal size of CuO could be reduced by the addition of urea. The XRD diffractogram of the catalyst prior to calcination also shows the formation of NH₄NO₃, which could aid in dissociation of metal clusters. Scanning electron microscopy (SEM) images of catalysts show the size of Cu–Zn compound clusters and also their dispersion over the Al₂O₃ surface on the impregnated catalysts. The addition of urea could also yield smaller Cu–Zn compound clusters and better dispersion compared with the impregnated catalyst without urea. Such impregnated Cu–Zn catalysts with urea could be alternative novel catalysts for methanol steam reforming.     

Easy-handling and low-leaching heterogeneous palladium and platinum catalysts via coating with a silicone elastomer

We have developed a practical protocol for coating of commercial Pd/Al₂O₃ and Pt/Al₂O₃ catalysts in micro-powders with a silicone elastomer. Compared to original catalysts, the treated catalysts are easier to weight and transfer, and they are easier to recover by simple filtration. More importantly, the metal leaching of treated catalysts was significantly reduced. The treated catalysts worked very well in diverse hydrogenation reactions.     

Alkylation of 2-methylquinoline with alcohols under additive-free conditions by Al2O3-supported Pt catalyst

Supported metal nanoparticle catalysts are studied for alkylation of 2-methylquinoline with benzyl alcohol under additive-free conditions in N₂ atmosphere. Among various metal-loaded Al₂O₃ catalysts and supported Pt catalysts, Pt metal nanocluster loaded-Al₂O₃ pre-reduced in H₂ at 500 °C shows highest yield (82%) of the product (2-phenethyl-quinoline). The catalyst is reusable, shows higher turnover number than a previous homogeneous catalyst, and shows good to moderate yield for alkylation of 2-methylquinoline with various alcohols. The reaction is driven by the borrowing-hydrogen pathway, in which aldehyde formed by dehydrogenation of alcohol undergoes aldol condensation with 2-methylquinoline to give the alkene intermediate which is finally hydrogenated by Pt-H species.     

Preparation of rhodium catalysts on laminar and zeolitic structures by anchoring of organometallic rhodium

Rhodium catalysts supported on six different aluminosilicate structures were prepared by hydrogen reduction of a cationic organometallic rhodium complex anchored to the support. The precursor active phase was incorporated in acetone medium through ion exchange using [Rh(Me₂CO)_x(NBD)]ClO₄ as the metal precursor species, in which NBD is 2,5‐norbornadiene and (Me₂CO)_x is acetone. The effect of the structure and characteristics of the support on metal load and dispersion was studied in the heterogeneous catalysts thus prepared. The supports were characterized by X‐ray diffraction, energy‐dispersive X‐ray analysis, volumetric adsorption and surface acidity. For the precursors and catalysts, the metal load was determined by UV–VIS spectra, the reduction temperature was determined by differential scanning calorimetry, and rhodium dispersion was measured by chemisorption. The structure of the materials used as supports had a great influence on the catalyst prepared. A higher metal content was achieved in the supports with laminar structures, whereas better dispersion was shown by the catalysts supported on zeolitic structures. Copyright © 2001 John Wiley & Sons, Ltd.     

Monolayer oxide catalysts from alkoxide precursors, part IV. Vanadia, titania and stibia on SiO2, SnO2, and TiO2 in 2-propanol oxidation

Vanadia monolayer catalysts supported on SnO₂, ZrO₂, TiO₂, and SiO₂, similarly as titania and stibia monolayers deposited on SiO₂, have been synthesized by reacting the corresponding metal alkoxides with hydroxyls on the carrier surfaces. The metal ions loads in monolayer systems were determined. The catalysts activity was tested in 2-propanol transformations. The nature of carrier has a strong influence on the dehydrating to dehydrogenating activity ratio of vanadia monolayer.     

Metal Oxide Catalysts for Selective Reduction of NOx by Hydrocarbons: Toward Molecular Basis for Catalyst Design

Metal oxides are stable and highly durable catalysts for the selective catalytic reduction (SCR) of NO by hydrocarbons and potential candidates for practical use. This review focuses on the development as well as the fundamental understanding of metal oxide based catalysts for selective reduction of NO by hydrocarbons. Our studies on the SCR-deNOx properties of Ga₂O₃/Al₂O₃, Cu-Al₂O₃, and Ag-Al₂O₃ catalysts are presented and it is attempted to demonstrate the advantages of this type of catalysts. On the basis of several spectroscopic characterizations, the effect of important factors, such as dispersion, coordination, and the electronic states of the metal cation, on the intrinsic catalytic activity are quite well clarified. From the in situ FTIR results, the reaction mechanism is understood in terms of formation and reaction of surface molecules. The structural and kinetic information obtained at the molecular level provides a useful strategy for designing better deNOx catalysts using metal oxides.     

Early Main Group Metal Catalysis: How Important is the Metal?

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal–alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca²⁺. The role of the metal was evaluated by a study using the metal‐free catalysts: [Ph₂N⁻][Me₄N⁺] and [Ph₃C⁻][Me₄N⁺]. These “naked” amides and carbanions can act as catalysts in the conversion of activated double bonds (CO and CN) in the hydroamination of Ar NCO and R NCN R (R=alkyl) by Ph₂NH. For the intramolecular hydroamination of unactivated CC bonds in H₂CCHCH₂CPh₂CH₂NH₂ the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.     

Early Main Group Metal Catalysis: How Important is the Metal?

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal–alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca²⁺. The role of the metal was evaluated by a study using the metal‐free catalysts: [Ph₂N^?][Me₄N⁺] and [Ph₃C^?][Me₄N⁺]. These “naked” amides and carbanions can act as catalysts in the conversion of activated double bonds (C?O and C?N) in the hydroamination of Ar? N?C?O and R? N?C?N? R (R=alkyl) by Ph₂NH. For the intramolecular hydroamination of unactivated C?C bonds in H₂C?CHCH₂CPh₂CH₂NH₂ the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.     

Metal Oxide Catalysts for Selective Reduction of NOx by Hydrocarbons: Toward Molecular Basis for Catalyst Design

Metal oxides are stable and highly durable catalysts for the selective catalytic reduction (SCR) of NO by hydrocarbons and potential candidates for practical use. This review focuses on the development as well as the fundamental understanding of metal oxide based catalysts for selective reduction of NO by hydrocarbons. Our studies on the SCR-deNOx properties of Ga₂O₃/Al₂O₃, Cu-Al₂O₃, and Ag-Al₂O₃ catalysts are presented and it is attempted to demonstrate the advantages of this type of catalysts. On the basis of several spectroscopic characterizations, the effect of important factors, such as dispersion, coordination, and the electronic states of the metal cation, on the intrinsic catalytic activity are quite well clarified. From the in situ FTIR results, the reaction mechanism is understood in terms of formation and reaction of surface molecules. The structural and kinetic information obtained at the molecular level provides a useful strategy for designing better deNOx catalysts using metal oxides.     

Effect of chelating agents on catalytic performance of Cr/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> dehydrogenation catalysts

Chromia/alumina (Cr₂O₃/γ-Al₂O₃) catalysts with addition of chelating agents (citric acid or oxalic acid) were prepared by the incipient impregnation method. The resulting catalysts with different citric acid (CA) or oxalic acid (OA) contents were applied to the dehydrogenation of isobutane to isobutene. The influence of chelating agents on the catalysts was investigated by means of BET, SEM, H₂-TPR, NH₃-TPD, and TG-DTG. The results showed that the Cr₂O₃/γ-Al₂O₃ catalysts with addition of CA or OA exerted slightly increase on specific surface area. The addition of the chelating agents as expected, determined a general decrease in the surface acidity. The catalysts with CA or OA have a better anti-coking ability by inhibiting the side reaction of cracking and carbon formation. The addition of CA or OA for preparing these catalysts resulted in a beneficial effect on the reducibility of the Cr species to diminish the reduction temperature. The appropriate content of chelating agents could improve dispersion of metal species in the γ-Al₂O₃ support. The catalytic activity showed an important enhancement when the metal species was impregnated in the presence of CA or OA.     

由M0.02Cu0.4Mg5.6Al1.98(OH)16CO3 (M=Ru,Re)水滑石为前驱体制备的双金属固体碱催化甘油氢解

采用共沉淀法合成了M_0.02Cu_0.4Mg_5.6Al_1.98(OH)₁₆CO₃ (M = Ru,Re)水滑石前驱体,然后经焙烧和还原制备了铜分散度较高的双功能M-Cu/固体碱催化剂.这些双功能催化剂在粗甘油氢解制备丙二醇反应中表现出了很好的催化活性.表征结果证明,M的加入增强了催化剂表面氢的吸附和活化,进而促进了甘油的转化.     

Delivery of Highly Active Noble‐Metal Nanoparticles into Microspherical Supports by an Aerosol‐Spray Method

Noble metal nanoparticles (NPs) with 1–5 nm diameter obtained from NaHB₄ reduction possess high catalytic activity. However, they are rarely used directly. This work presents a facile, versatile, and efficient aerosol‐spray approach to deliver noble‐metal NPs into metal oxide supports, while maintaining the size of the NPs and the ability to easily adjust the loading amount. In comparison with the conventional spray approach, the size of the loaded noble‐metal nanoparticles can be significantly decreased. An investigation of the 4‐nitrophenol hydrogenation reaction catalyzed by these materials suggests that the NPs/oxides catalysts have high activity and good endurance. For 1 % Au/CeO₂ and Pd/Al₂O₃ catalysts, the rate constants reach 2.03 and 1.46 min^?1, which is much higher than many other reports with the same noble‐metal loading scale. Besides, the thermal stability of catalysts can be significantly enhanced by modifying the supports. Therefore, this work contributes an efficient method as well as some guidance on how to produce highly active and stable supported noble‐metal catalysts.     

Identification of Catalytic Sites for Oxygen Reduction in Metal/Nitrogen‐Doped Carbons with Encapsulated Metal Nanoparticles

The development of metal‐N‐C materials as efficient non‐precious metal (NPM) catalysts for catalysing the oxygen reduction reaction (ORR) as alternatives to platinum is important for the practical use of proton exchange membrane fuel cells (PEMFCs). However, metal‐N‐C materials have high structural heterogeneity. As a result of their high‐temperature synthesis they often consist of metal‐N_x sites and graphene‐encapsulated metal nanoparticles. Thus it is hard to identify the active structure of metal‐N‐C catalysts. Herein, we report a low‐temperature NH₄Cl‐treatment to etch out graphene‐encapsulated nanoparticles from metal‐N‐C catalysts without destruction of co‐existing atomically dispersed metal‐N_x sites. Catalytic activity is much enhanced by this selective removal of metallic nanoparticles. Accordingly, we can confirm the spectator role of graphene‐encapsulated nanoparticles and the pivotal role of metal‐N_x sites in the metal‐N‐C materials for ORR in the acidic medium.     

Support Modification of Supported Nickel Catalysts for Hydrogen Production by Auto-thermal Reforming of Ethanol

Recent progress on support modification of supported nickel catalysts for hydrogen production by auto-thermal reforming of ethanol was reported in this review. Nickel catalysts supported on various materials, including metal oxides and metal oxide-stabilized mesoporous zirconias, were prepared by an incipient wetness impregnation method for use in hydrogen production by auto-thermal reforming of ethanol. Various experimental measurements such as NH₃-TPD (temperature-programmed desorption) and TPR (temperature-programmed reduction) were carried out to elucidate the different catalytic performance of supported nickel catalysts. It was revealed that acid property of supporting materials served as one of the important factors determining the catalytic performance. Hydrogen yield over nickel catalysts supported on metal oxides showed a volcano-shaped curve with respect to acidity of the supports. Among the catalysts tested, Ni/ZrO₂ catalyst with an intermediate acidity exhibited a superior catalytic performance. It was also observed that reducibility of nickel catalysts supported on metal oxide-stabilized mesoporous zirconias played a key role in determining the catalytic performance in the auto-thermal reforming of ethanol for hydrogen production. Hydrogen yield over nickel catalysts supported on metal oxide-stabilized zirconias increased with increasing reducibility of the catalysts (with decreasing TPR peak temperature of the catalysts).     

Metal-Support Interaction and Catalysis of the Catalysts Prepared Using Microemulsion

We have developed a new preparation method (ME method) of supported metal catalysts by using microemulsion. The metal particles in the catalyst prepared by this method were interacted strongly with support and were considered to be positively charged, and the SiO₂-supported Rh and Fe catalysts prepared by this method exhibited a unique activity and a good selectivity to oxygenates in the hydrogenation of CO. The Al₂O₃-supported Ni catalyst also exhibited an excellent activity and a strong resistance to carbon deposition in the methane-steam reforming. In this review, these interesting catalytic behaviors of the catalysts prepared by ME method were elucidated from the view-point of the electronic state of metals.     

Changes in the activity of Ru/Fe2O3 catalysts modified with alkali metal salts in the water-gas shift reaction

A study was undertaken to determine the activity of ruthenium catalysts, obtained by deposition of Ru₃(CO)₁₂ on products of iron oxide-hydroxide calcination modified with alkali metals, in the water-gas shift reaction. The activity depends on the kind of starting iron support, the ruthenium precursor and the amount of alkali metal salts. The most active were the catalysts obtained by deposition of Ru₃(CO)₁₂ on calcination products of σ-FeOOH, both modified and unmodified with alkali metal salts, and of α-FeOOH modified with alkali metal salts.