Showing papers on "Catalysis published in 2005"
TL;DR: The ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction is analyzed and a necessary criterion for high catalytic activity is found: that the binding free energy of atomic hydrogen to the catalyst is close to zero.
Abstract: The electrochemical hydrogen evolution reaction is catalyzed most effectively by the Pt group metals. As H2 is considered as a future energy carrier, the need for these catalysts will increase and alternatives to the scarce and expensive Pt group catalysts will be needed. We analyze the ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction and find a necessary criterion for high catalytic activity. The necessary criterion is that the binding free energy of atomic hydrogen to the catalyst is close to zero. The criterion enables us to search for new catalysts, and inspired by the nitrogenase active site, we find that MoS2 nanoparticles supported on graphite are a promising catalyst. They catalyze electrochemical hydrogen evolution at a moderate overpotential of 0.1−0.2 V.
3,302 citations
TL;DR: The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.
Abstract: Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.
2,790 citations
TL;DR: Liquid alkanes with the number of carbon atoms ranging from C7 to C15 were selectively produced from biomass-derived carbohydrates by acid-catalyzed dehydration, which was followed by aldol condensation over solid base catalysts to form large organic compounds.
Abstract: Liquid alkanes with the number of carbon atoms ranging from C7 to C15 were selectively produced from biomass-derived carbohydrates by acid-catalyzed dehydration, which was followed by aldol condensation over solid base catalysts to form large organic compounds. These molecules were then converted into alkanes by dehydration/hydrogenation over bifunctional catalysts that contained acid and metal sites in a four-phase reactor, in which the aqueous organic reactant becomes more hydrophobic and a hexadecane alkane stream removes hydrophobic species from the catalyst before they go on further to form coke. These liquid alkanes are of the appropriate molecular weight to be used as transportation fuel components, and they contain 90% of the energy of the carbohydrate and H2 feeds.
1,556 citations
TL;DR: A review of the state of the art in the use of alternative reaction media for green, sustainable organic synthesis is presented in this article, where a novel and effective method for the immobilisation of enzymes as cross-linked enzyme aggregates (CLEAs) is discussed and a combi CLEA, containing two enzymes, for the one-pot conversion of benzaldehyde to S-mandelic acid is reported.
1,392 citations
TL;DR: In this article, a short-stack of proton exchange membrane fuel cells PEMFCs operated at open-circuit voltage 0.95 V was shown to have higher surface area loss than another operated under load 0.75 V.
Abstract: Equilibrium concentrations of dissolved platinum species from a Pt/C electrocatalyst sample in 0.5 M H2SO4 at 80°C were found to increase with applied potential from 0.9 to 1.1 V vs reversible hydrogen electrode. In addition, platinum surface area loss for a short-stack of proton exchange membrane fuel cells PEMFCs operated at open-circuit voltage 0.95 V was shown to be higher than another operated under load 0.75 V. Both findings suggest that the formation of soluble platinum species such as Pt 2+ plays an important role in platinum surface loss in PEMFC electrodes. As accelerated platinum surface area loss in the cathode from 63 to 23 m 2 /gPt in 100 h was observed upon potential cycling, a cycled membrane electrode assembly MEA cathode was examined in detail by incidence angle X-ray diffraction and transmission electron microscopy TEM to reveal processes responsible for observed platinum loss. In this study, TEM data and analyses of Pt/C catalyst and cross-sectional MEA cathode samples unambiguously confirmed that coarsening of platinum particles occurred via two different processes: i Ostwald ripening on carbon at the nanometer scale, which is responsible for platinum particle coarsening from 3t o6 nm on carbon, and ii migration of soluble platinum species in the ionomer phase at the micrometer scale, chemical reduction of these species by crossover H2 molecules, and precipitation of platinum particles in the cathode ionomer phase, which reduces the weight of platinum on carbon. It was estimated that each process contributed to 50% of the overall platinum area loss of the potential cycled electrode.
1,322 citations
TL;DR: These studies provide important clues on the mechanism of the reactions the authors studied and also can be very useful in the process of designing better catalysts in the future.
Abstract: While the nanocatalysis field has undergone an explosive growth during the past decade, there have been very few studies in the area of shape-dependent catalysis and the effect of the catalytic process on the shape and size of transition metal nanoparticles as well as their recycling potential. Metal nanoparticles of different shapes have different crystallographic facets and have different fraction of surface atoms on their corners and edges, which makes it interesting to study the effect of metal nanoparticle shape on the catalytic activity of various organic and inorganic reactions. Transition metal nanoparticles are attractive to use as catalysts due to their high surface-to-volume ratio compared to bulk catalytic materials, but their surface atoms could be so active that changes in the size and shape of the nanoparticles could occur during the course of their catalytic function, which could also affect their recycling potential. In this Feature Article, we review our work on the effect of the shape of the colloidal nanocatalyst on the catalytic activity as well as the effect of the catalytic process on the shape and size of the colloidal transition metal nanocatalysts and their recycling potential. These studies provide important clues on the mechanism of the reactions we studied and also can be very useful in the process of designing better catalysts in the future.
1,192 citations
1,061 citations
TL;DR: The rate of ammonia synthesis over a nanoparticle ruthenium catalyst can be calculated directly on the basis of a quantum chemical treatment of the problem using density functional theory, and offers hope for computer-based methods in the search for catalysts.
Abstract: The rate of ammonia synthesis over a nanoparticle ruthenium catalyst can be calculated directly on the basis of a quantum chemical treatment of the problem using density functional theory. We compared the results to measured rates over a ruthenium catalyst supported on magnesium aluminum spinel. When the size distribution of ruthenium particles measured by transmission electron microscopy was used as the link between the catalyst material and the theoretical treatment, the calculated rate was within a factor of 3 to 20 of the experimental rate. This offers hope for computer-based methods in the search for catalysts.
1,028 citations
TL;DR: In this paper, a new reaction pathway for converting glycerol to propylene glycol via an intermediate was validated by isolating the acetol intermediate, and the results showed that the yield of glycerols increased with decreasing water content.
Abstract: Hydrogenolysis of glycerol to propylene glycol was performed using nickel, palladium, platinum, copper, and copper-chromite catalysts. The effects of temperature, hydrogen pressure, initial water content, choice of catalyst, catalyst reduction temperature, and the amount of catalyst were evaluated. At temperatures above 200 °C and hydrogen pressure of 200 psi, the selectivity to propylene glycol decreased due to excessive hydrogenolysis of the propylene glycol. At 200 psi and 200 °C the pressures and temperaures were significantly lower than those reported in the literature while maintaining high selectivities and good conversions. The yield of propylene glycol increased with decreasing water content. A new reaction pathway for converting glycerol to propylene glycol via an intermediate was validated by isolating the acetol intermediate.
932 citations
TL;DR: It is shown that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products and significant conversions.
Abstract: Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high-tonnage commodities, high-value fine chemicals, agrochemicals and pharmaceuticals: but oxidations are often inefficient. The introduction of catalytic systems using oxygen from air is preferred for 'green' processing. Gold catalysis is now showing potential in selective redox processes, particularly for alcohol oxidation and the direct synthesis of hydrogen peroxide. However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene. Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process, this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide, hydroperoxides or stoichiometric oxygen donors. Here we show that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products ( approximately 98%) and significant conversions. Our finding significantly extends the discovery by Haruta that nanocrystalline gold can epoxidize alkenes when hydrogen is used to activate the molecular oxygen; in our case, no sacrificial reductant is needed. We anticipate that our finding will initiate attempts to understand more fully the mechanism of oxygen activation at gold surfaces, which might lead to commercial exploitation of the high redox activity of gold nanocrystals.
931 citations
TL;DR: In this article, a single-step, low-temperature process for the catalytic production of fuels, such as hydrogen and/or alkanes, from renewable biomass-derived oxygenated hydrocarbons is presented.
Abstract: We have recently developed a single-step, low-temperature process for the catalytic production of fuels, such as hydrogen and/or alkanes, from renewable biomass-derived oxygenated hydrocarbons. This paper reviews our work in the development of this aqueous-phase reforming (APR) process to produce hydrogen or alkanes in high yields. First, the thermodynamic and kinetic considerations that form the basis of the process are discussed, after which reaction kinetics results for ethylene glycol APR over different metals and supports are presented. These studies indicate Pt-based catalysts are effective for producing hydrogen via APR. Various reaction pathways may occur, depending on the nature of the catalyst, support, feed and process conditions. The effects of these various factors on the selectivity of the process to make hydrogen versus alkanes are discussed, and it is shown how process conditions can be manipulated to control the molecular weight distribution of the product alkane stream. In addition, process improvements that lead to hydrogen containing low concentrations of CO are discussed, and a dual-reactor strategy for processing high concentrations of glucose feeds is demonstrated. Finally, various strategies are assembled in the form of a composite process that can be used to produce renewable alkanes or fuel-cell grade hydrogen with high selectivity from concentrated feedstocks of oxygenated hydrocarbons. # 2004 Elsevier B.V. All rights reserved.
TL;DR: Acetoxylation of ethylene to vinyl acetate was used to investigate the mechanism of the promotional effect of gold (Au) in a palladium (Pd)-Au alloy catalyst, demonstrating that the critical reaction site for VA synthesis consists of two noncontiguous, suitably spaced, Pd monomers.
Abstract: Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of gold (Au) in a palladium (Pd)-Au alloy catalyst. The enhanced rates of VA formation for low Pd coverages relative to high Pd coverages on Au single-crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two noncontiguous, suitably spaced, Pd monomers. The role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product, while inhibiting the formation of undesirable reaction by-products.
TL;DR: A first-principles study demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules, and shows that a SWNT can strongly adsorb up to 8 wt % hydrogen.
Abstract: We report a first-principles study, which demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules. The first H2 adsorption is dissociative with no energy barrier while the other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a SWNT can strongly adsorb up to 8 wt % hydrogen. These results advance our fundamental understanding of dissociative adsorption of hydrogen in nanostructures and suggest new routes to better storage and catalyst materials.
TL;DR: The data indicate that exposed edge plane defects and nitrogen doping are important factors for influencing adsorption of reactive intermediates and for enhancing electrocatalysis for the ORR at nanostructured carbon electrodes.
Abstract: Nondoped and nitrogen-doped (N-doped) carbon nanofiber (CNF) electrodes were prepared via a floating catalyst chemical vapor deposition (CVD) method using precursors consisting of ferrocene and either xylene or pyridine to control the nitrogen content. Structural and compositional differences between the nondoped and N-doped varieties were assessed using TEM, BET, Raman, TGA, and XPS. Electrochemical methods were used to study the influence of nitrogen doping on the oxygen reduction reaction (ORR). The N-doped CNF electrodes demonstrate significant catalytic activity toward oxygen reduction in aqueous KNO(3) solutions at neutral to basic pH. Electrochemical data are presented which indicate that the ORR proceeds by the peroxide pathway via two successive two-electron reductions. However, for N-doped CNF electrodes, the reduction process can be treated as a catalytic regenerative process where the intermediate hydroperoxide (HO(2)(-)) is chemically decomposed to regenerate oxygen, 2HO(2)(-) O(2) + 2OH(-). The proposed electrocatalysis mechanisms for ORR at both nondoped and N-doped varieties are supported by electrochemical simulations and by measured difference in hydroperoxide decomposition rate constants. Remarkably, approximately 100 fold enhancement for hydroperoxide decomposition is observed for N-doped CNFs, with rates comparable to the best known peroxide decomposition catalysts. Collectively the data indicate that exposed edge plane defects and nitrogen doping are important factors for influencing adsorption of reactive intermediates (i.e., superoxide, hydroperoxide) and for enhancing electrocatalysis for the ORR at nanostructured carbon electrodes.
TL;DR: A reaction mechanism for alcohol oxidation catalyzed by Au:PVP NCs is proposed in which a superoxo-like molecular oxygen species adsorbed on the surface of the small Au NCs abstracts a hydrogen atom from the alkoxide.
Abstract: Gold nanoclusters (phi = 1.3 nm) stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP NCs) readily oxidize benzylic alcohols to the corresponding aldehydes and/or carboxylic acids under ambient temperature in water. Kinetic measurement revealed that smaller Au:PVP NCs exhibit higher catalytic activity than larger (9.5 nm) homologues and, more surprisingly, than Pd:PVP NCs of comparable size (1.5 and 2.2 nm). On the basis of the marked difference in the kinetic isotope effect and activation energy between Au:PVP and Pd:PVP NCs, a reaction mechanism for alcohol oxidation catalyzed by Au:PVP NCs is proposed in which a superoxo-like molecular oxygen species adsorbed on the surface of the small Au NCs abstracts a hydrogen atom from the alkoxide.
TL;DR: This Review describes how transfer of oxygen atoms, bond activation, and coupling of fragments can be mediated by atomic or cluster metal ions, and improves the understanding of the intrinsic operation of a practical catalyst at a strictly molecular level.
Abstract: Gas-phase experiments with state-of-the-art techniques of mass spectrometry provide detailed insights into numerous elementary processes. The focus of this Review is on elementary reactions of ions that achieve complete catalytic cycles under thermal conditions. The examples chosen cover aspects of catalysis pertinent to areas as diverse as atmospheric chemistry and surface chemistry. We describe how transfer of oxygen atoms, bond activation, and coupling of fragments can be mediated by atomic or cluster metal ions. In some cases truly unexpected analogies of the idealized gas-phase ion catalysis can be drawn with related chemical transformations in solution or the solid state, and so improve our understanding of the intrinsic operation of a practical catalyst at a strictly molecular level.
TL;DR: This high-performance catalyst, which consists of stable sulphonated amorphous carbon, is recyclable and its activity markedly exceeds that of other solid acid catalysts tested for ‘biodiesel’ production.
Abstract: The production of diesel from vegetable oil calls for an efficient solid catalyst to make the process fully ecologically friendly. Here we describe the preparation of such a catalyst from common, inexpensive sugars. This high-performance catalyst, which consists of stable sulphonated amorphous carbon, is recyclable and its activity markedly exceeds that of other solid acid catalysts tested for 'biodiesel' production.
TL;DR: In this paper, an efficient, environmentally benign method for the preparation of esters from alcohols under mild, neutral conditions without the need for carboxylic acid derivatives and condensing agents was developed.
Abstract: An efficient, environmentally benign method for the preparation of esters from alcohols under mild, neutral conditions without the need for carboxylic acid derivatives and condensing agents was developed. Catalyst design, based on new Ru(II) hydrido carbonyl complexes incorporating electron-rich PNP and PNN ligands has resulted in the novel complex (I) which is an outstanding catalyst for the dehydrogenation of primary alcohols to esters and H2 under neutral conditions.
TL;DR: Enantioselective reactions catalyzed by urea and thiourea derivatives as general acid catalysts as well as diastereoselectives reactions are described.
Abstract: Hydrogen-bonding interaction plays a crucial role in the molecular recognition and activation processes of various biologically important reactions that are mediated by enzymes and antibodies in living organisms. Recently, it has been shown that a hydrogen-bonding donor can be used as a general acid catalyst for various types of reactions in organic chemistry. In this article, we describe enantioselective reactions catalyzed by urea and thiourea derivatives as general acid catalysts as well as diastereoselective reactions. This perspective provides an overview of this rapidly growing field.
TL;DR: The enhanced reactivity observed with these new catalytic systems has been attributed to the formation of unsaturated and reactive [PdL] species which can readily undergo oxidative addition reactions with ArX to yield [Pd(Ar)X(L)].
Abstract: Palladium-mediated cross-coupling reactions are attractive organometallic transformations for the generation of C--C, C--N, C--O, and C--S bonds. Despite being widely employed in small-scale syntheses, cross-coupling reactions have not found important industrial applications because until recently, only reactive aryl bromides and iodides could be used as substrates. These substrates are generally more expensive and less widely available than their chloride counterparts. Over the past few years, new catalytic systems with the ability to activate unreactive and sterically hindered aryl chlorides have been developed. The new catalysts are based on palladium complexes that contain electron-rich and bulky phosphine or carbene ligands. The enhanced reactivity observed with these new systems has been attributed to the formation of unsaturated and reactive [PdL] species which can readily undergo oxidative addition reactions with ArX to yield [Pd(Ar)X(L)].
TL;DR: It is found that the presence of surfactants depresses the catalytic activity of the silver particles to some extent by inhibiting the adsorption of reactants onto the surface of the particles.
Abstract: In this work, we investigate the catalytic properties of silver nanoparticles supported on silica spheres. The technique to support silver particles on silica spheres effectively avoids flocculation of nanosized colloidal metal particles during a catalytic process in the solution, which allows one to carry out the successful catalytic reduction of dyes. The effects of electrolytes and surfactants on the catalytic properties of silver particles on silica have been investigated. It is found that the presence of surfactants depresses the catalytic activity of the silver particles to some extent by inhibiting the adsorption of reactants onto the surface of the particles. Electrolytes either increase the migration rate of reactants in the solution resulting in an increase in the catalytic reaction rate or inhibit the adsorption of reactants onto the surface of the silver particles leading to a loss in the activity of the metal particles.
TL;DR: These new cascade catalysis protocols allow the invention of enantioselective transformations that were previously unknown, including the asymmetric catalytic addition of the elements of HF across a trisubstituted olefin.
Abstract: A new strategy for organocatalysis based on the biochemical blueprints of biosynthesis has enabled a new laboratory approach to cascade catalysis. Imidazolidinone-based catalytic cycles, involving iminium and enamine activation, have been successfully combined to allow a large diversity of nucleophiles (furans, thiophenes, indoles, butenolides, hydride sources, tertiary amino lactone equivalents) and electrophiles (fluorinating and chlorinating reagents) to undergo sequential addition with a wide array of α,β-unsaturated aldehydes. These new cascade catalysis protocols allow the invention of enantioselective transformations that were previously unknown, including the asymmetric catalytic addition of the elements of HF across a trisubstituted olefin. Importantly, these domino catalysis protocols can be mediated by a single imidazolidinone catalyst or using cycle-specific amine catalysts. In the latter case, cascade catalysis pathways can be readily modulated to provide a required diastereo- and enantiosele...
TL;DR: Orthogonal nature of the present catalyst immobilization approach should allow the design of other superparamagnetic nanoparticle-supported asymmetric catalysts for a wide range of organic transformations.
Abstract: Novel heterogenized asymmetric catalysts were synthesized by immobilizing preformed Ru catalysts on magnetite nanoparticles via the phosphonate functionality and were characterized by a variety of techniques, including TEM, magnetization, and XRD. These nanoparticle-supported chiral catalysts were used for enantioselective heterogeneous asymmetric hydrogenation of aromatic ketones with very high enantiomeric excess values of up to 98.0%. The immobilized catalysts were easily recycled by magnetic decantation and reused for up to 14 times without loss of activity and enantioselectivity. Orthogonal nature of the present catalyst immobilization approach should allow the design of other superparamagnetic nanoparticle-supported asymmetric catalysts for a wide range of organic transformations.
TL;DR: Using the SECM technique, combinations of metals with enhanced electrocatalytic activities when compared with the constituent, pure metals are identified and addition of Co to Pd, Au, and Ag clearly decreases the ORR overpotential, in agreement with the proposed model.
Abstract: We propose guidelines for the design of improved bimetallic (and related) electrocatalysts for the oxygen reduction reaction (ORR) in acidic media. This guide is based on simple thermodynamic principles assuming a simple mechanism where one metal breaks the oxygen-oxygen bond of molecular O(2) and the other metal acts to reduce the resulting adsorbed atomic oxygen. Analysis of the Gibbs free energies of these two reactions guides the selection of combinations of metals that can produce alloy surfaces with enhanced activity for the ORR when compared to the constituent metals. Selected systems have been tested by fabricating arrays of metallic catalysts consisting of various binary and ternary combinations of Pd, Au, Ag, and Co deposited on glassy carbon (GC) substrates. The electrocatalytic activity of these materials for the ORR in acidic medium was examined using scanning electrochemical microscopy (SECM) in a new rapid-imaging mode. This was used to rapidly screen arrays covering a wide range of catalyst compositions for their activity for the ORR in 0.5 M H(2)SO(4). Using the SECM technique, we have identified combinations of metals with enhanced electrocatalytic activities when compared with the constituent, pure metals. Addition of Co to Pd, Au, and Ag clearly decreases the ORR overpotential, in agreement with the proposed model. Catalyst spots that exhibited enhanced electrocatalytic activity in the SECM screening technique were then examined using classical rotating disk electrode (RDE) experiments. The activity of carbon black supported catalyst mixtures on a GC RDE and the electrocatalytic activity determined using the SECM screening technique showed excellent agreement. C/Pd-Co electrodes (10-30% Co) exhibited remarkable activity for ORR catalysis, close to that of carbon-supported Pt.
TL;DR: In this article, the authors investigated the kinetics and selectivities of different solid catalysts for the transesterification of triacetin (a model compound for larger triglycerides as found in vegetable oils and fats) with methanol.
Abstract: Biodiesel is a particularly attractive renewable fuel as it can be used in existing engines, is environmentally friendly, and is readily synthesized from animal fats and vegetable oils. Heterogeneous catalysts offer exciting possibilities for improving the economics of biodiesel synthesis; however, few published investigations have addressed the use of such catalysts to date. The purpose of this research was to investigate the kinetics and selectivities of different solid catalysts for the transesterification of triacetin (a model compound for larger triglycerides as found in vegetable oils and fats) with methanol. Reaction was carried out at 60 °C in a batch reactor with a variety of solid and liquid, acid and base catalysts. The homogeneous phase (i.e., liquid) catalysts (NaOH and H2SO4) were studied for comparison. Amberlyst-15, Nafion NR50, sulfated zirconia, and ETS-10 (Na, K) showed reasonable activities, suggesting that they could be suitable alternatives to liquid catalysts. While on a wt.% basis (of reaction mixture) the homogeneous phase catalysts gave higher rates of reaction, on a rate-per-site basis the solid acids were similar to H2SO4. Sulfated zirconia and tungstated zirconia had comparable turnover frequencies as H2SO4. The deactivation characteristics of some of these catalysts were also studied.
TL;DR: The reaction was shown to be structure insensitive over a series of Pt/SBA-15 materials with particle sizes between 1.7 and 3.6 nm, suggesting that coordinatively unsaturated metal atoms present in small particles are more active for C2H6 hydrogenolysis than the low index planes that dominate in large particles.
Abstract: Platinum nanoparticles in the size range of 1.7-7.1 nm were produced by alcohol reduction methods. A polymer (poly(vinylpyrrolidone), PVP) was used to stabilize the particles by capping them in aqueous solution. The particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM investigations demonstrate that the particles have a narrow size distribution. Mesoporous SBA-15 silica with 9-nm pores was synthesized by a hydrothermal process and used as a catalyst support. After incorporation into mesoporous SBA-15 silica using low-power sonication, the catalysts were calcined to remove the stabilizing polymer from the nanoparticle surface and reduced by H2. Pt particle sizes determined from selective gas adsorption measurements are larger than those determined by bulk techniques such as XRD and TEM. Roomtemperature ethylene hydrogenation was chosen as a model reaction to probe the activity of the Pt/SBA-15 materials. The reaction was shown to be structure insensitive over a series of Pt/SBA-15 materials with particle sizes between 1.7 and 3.6 nm. The hydrogenolysis of ethane on Pt particles from 1.7 to 7.1 nm was weakly structure sensitive with smaller particles demonstrating higher specific activity. Turnover rates for ethane hydrogenolysis increased monotonically with increasing metal dispersion, suggesting that coordinatively unsaturated metal atoms present in small particles are more active for C 2H6 hydrogenolysis than the low index planes that dominate in large particles. An explanation for the structure sensitivity is suggested, and the potential applications of these novel supported nanocatalysts for further studies of structure -activity and structure-selectivity relationships are discussed.
TL;DR: In this article, the state-of-the-art in both kinetic and molecular modelling used in chemoselective hydrogenations was presented, some special features, like conversion-selectivity relationship and the performance in gas and liquid phase hydrogenations were discussed.
Abstract: Chemoselective hydrogenation of unsaturated aldehydes and ketones over heterogeneous catalysts is a demanding task. The achieved selectivity levels depend both on the electronic and geometric structures of reactants and metal surfaces. Recent development in catalyst preparation, selection of new solvents, catalyst and reactor structures have been reviewed in this work. Additionally, catalyst characterization technique, especially alloy formation and in situ characterizations have been discussed in detail. The deep understanding of the catalyst structure is necessary for development of tailor-made catalysts for chemoselective hydrogenations. The state of art in both kinetic and molecular modelling used in chemoselective hydrogenations was presented. Some special features, like conversion–selectivity relationship and the performance in gas and liquid phase hydrogenations were discussed.
TL;DR: A series of [Mg(1−x)Alx(OH)2]x+(CO3)x/n2− hydrotalcite materials with compositions over the range x = 0.25 − 0.55 have been synthesised using an alkali-free coprecipitation route as mentioned in this paper.
Abstract: A series of [Mg(1−x)Alx(OH)2]x+(CO3)x/n2− hydrotalcite materials with compositions over the range x = 0.25–0.55 have been synthesised using an alkali-free coprecipitation route. All materials exhibit XRD patterns characteristic of the hydrotalcite phase with a steady lattice expansion observed with increasing Mg content. XPS measurements reveal a decrease in both the Al and Mg photoelectron binding energies with Mg incorporation which correlates with the increased intra-layer electron density. All materials are effective catalysts for the liquid phase transesterification of glyceryl tributyrate with methanol for biodiesel production. The rate increases steadily with Mg content, with the Mg rich Mg2.93Al catalyst an order of magnitude more active than MgO, with pure Al2O3 being completely inert. The rate of reaction also correlates with intralayer electron density which can be associated with increased basicity.
TL;DR: The EDX micrographs corresponding to Mg and Ni elemental profiles indicated that nanoparticle Ni metals as catalyst homogeneously dispersed on the surface of MgH(2) revealed good reversible hydriding/dehydriding cycles even at 150 degrees C.
Abstract: We examined the catalytic effect of nanoparticle 3d-transition metals on hydrogen desorption (HD) properties of MgH(2) prepared by mechanical ball milling method. All the MgH(2) composites prepared by adding a small amount of nanoparticle Fe(nano), Co(nano), Ni(nano), and Cu(nano) metals and by ball milling for 2 h showed much better HD properties than the pure ball-milled MgH(2) itself. In particular, the 2 mol % Ni(nano)-doped MgH(2) composite prepared by soft milling for a short milling time of 15 min under a slow milling revolution speed of 200 rpm shows the most superior hydrogen storage properties: A large amount of hydrogen ( approximately 6.5 wt %) is desorbed in the temperature range from 150 to 250 degrees C at a heating rate of 5 degrees C/min under He gas flow with no partial pressure of hydrogen. The EDX micrographs corresponding to Mg and Ni elemental profiles indicated that nanoparticle Ni metals as catalyst homogeneously dispersed on the surface of MgH(2). In addition, it was confirmed that the product revealed good reversible hydriding/dehydriding cycles even at 150 degrees C. The hydrogen desorption kinetics of catalyzed and noncatalyzed MgH(2) could be understood by a modified first-order reaction model, in which the surface condition was taken into account.