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Applied Catalysis A, General (v.291, #1-2)
A golden age of catalysis: A perspective by Graham J. Hutchings; Masatake Haruta (pp. 2-5).
Catalysis by gold is one of the fastest growing subjects in chemical science today and is also an interesting topic of research in nanotechnology. In this article, we focus on the early landmark work and provide a historical reflection together with future perspectives as an introduction to this special issue on catalysis by gold.
Simulation of pathways for CO oxidation over Au nano-clusters by paired interacting orbitals (PIO) analysis by Akinobu Shiga; Masatake Haruta (pp. 6-12).
CO oxidation pathways over Au nano-clusters, such as face centered cubic Au14, icosahedron Au13, cubo-octahedron Au13 and tetrahedron Au20 were investigated by using PIO analysis proposed by Fujimoto et al. Four types of oxidation pathways were examined: reaction between (a) adsorbed CO and gaseous O2 (E–R mechanism), (b) adsorbed O2 and gaseous CO (E–R mechanism), (c) adsorbed CO and vicinally adsorbed O2 (L–H mechanism), and (d) adsorbed CO and geminally adsorbed O2 (L–H mechanism). Only in the case of L–H mechanism, when CO molecule and O2 molecule are adsorbed on adjacent two surface Au atoms in vicinal manner (c), CO oxidation occurs favorably. In comparison of the vicinal CO and O2 co-adsorption on icosahedron Au13 with that on cubo-octahedron Au13, CO oxidation is favorable in the former, whereas it is unfavorable in the latter because of considerably longer distance between CO and O2. It is also predicted that CO oxidation is also favorable in the case of vicinally co-adsorbed state of CO and O2 on tetrahedron Au20. The present work has shown that the PIO analysis based on extended Hückel MO is an effective way to investigate reaction pathways, especially in large catalytic systems.
Keywords: CO; Oxidation mechanism; Au clusters; Orbital interaction
CO oxidation on gold nanoparticles: Theoretical studies by Ioannis N. Remediakis; Nuria Lopez; Jens K. Nørskov (pp. 13-20).
We present a summary of our theoretical results regarding CO oxidation on both oxide-supported and isolated gold nanoparticles. Using Density Functional Theory we have studied the adsorption of molecules and the oxidation reaction of CO on gold clusters. Low-coordinated sites on the gold nanoparticles can adsorb small inorganic molecules such as O2 and CO, and the presence of these sites is the key factor for the catalytic properties of supported gold nanoclusters. Other contributions, induced by the presence of the support, can provide parallel channels for the reaction and modulate the final efficiency of Au-based catalysts. Finally, our theoretical simulations allow us to discuss the selectivity of supported Au nanoparticles.
Keywords: Density functional calculations; Gold; Oxidation; Titanium oxide; Nanoparticle
Some recent theoretical advances in the understanding of the catalytic activity of Au by L.M. Molina; B. Hammer (pp. 21-31).
We present a small review of recent density-functional-theory (DFT) studies of the reactivity towards CO oxidation of supported Au nano-particles. The possible structure of the periphery of the interface between a Au particle and an oxide support is discussed. A certain structure, in which low coordinated Au atoms are overhanging the support without binding directly to the oxide atoms, is argued to be prototypical of medium-sized Au particles. This structure is shown to be particularly active both at the edges and at the corners of Au particles. Examples from the literature of Au systems supported on MgO(100) and rutile-TiO2(110) are reviewed and new data are given for the reactivity of facet, edge, and corner sites of a Au34 cluster supported on MgO(100). On the non-reducible oxide support, MgO(100), the CO oxidation is found to occur via CO adsorption to the Au particles and subsequent CO-promoted O2 capture and formation of a CO·O2 reaction intermediate complex. On the reducible oxide support, TiO2(110), the O2 is found to adsorb independently of the CO. However, on this support, the reaction still proceeds via CO·O2 formation rather than via O2 dissociation.
Keywords: PACS; 68.47.Jn; 82.65.+r; 68.43.Fg; 68.35.NpDensity-functional-theory; Au; Clusters; Nano-particles; CO oxidation; Metal-oxide interface
Catalytically active gold: The role of cluster morphology by T.V. Choudhary; D.W. Goodman (pp. 32-36).
Recent studies of the CO oxidation activity exhibited by highly dispersed nano-gold (Au) catalysts have reached the following conclusions: (a) bilayer structures of Au are critical; (b) a strong interaction between Au and the support leads to wetting and electron-rich Au; (c) oxidative environments deactivate Au catalyst by re-oxidizing the support, which causes the Au to de-wet and sinter. Recent results have shown that the direct intervention of the support is not necessary to facilitate the CO oxidation reaction; therefore, an Au-only mechanism is sufficient to explain the reaction kinetics.
Keywords: CO oxidation; Gold catalysis; Particle size; CO oxidation; Nano-gold
The interaction of neutral and charged Au clusters with O2, CO and H2 by Mitsutaka Okumura; Yasutaka Kitagawa; Masatake Haruta; Kizashi Yamaguchi (pp. 37-44).
Hybrid density functional calculations have been carried out for AuO2, AuCO, Au13, Au13O2, Au13CO, Au13H2 and Au55 clusters to discuss the catalytic behavior of Au clusters with different sizes and structures for CO oxidation. From these calculations, it was found that O2 and CO could adsorb onto several Au model systems. Especially, icosahedral Au13 cluster has a relatively weak interaction with O2 while both icosahedral and cubooctahedral Au13 clusters have interactions (∼20kcal/mol) with CO. These findings suggest that the surfaces of the Au clusters are the active sites for the catalytic reactions on the supported and unsupported Au catalysts.
Keywords: Au cluster; Hybrid density functional calculation; Charge-transfer interaction
Electronic structures of Au supported on TiO2 by K. Okazaki; S. Ichikawa; Y. Maeda; M. Haruta; M. Kohyama (pp. 45-54).
Electronic structures of gold nanoparticles supported on TiO2 have been investigated using the electron holography method, scanning tunneling microscopy, and first-principles calculations. By the electron holography method, the dependence of the mean inner potential of the gold nanoparticles on TiO2 on the size of particles has been found. Mean inner potential is given as the zero-order Fourier coefficient of the crystal potential. As the size of nanoparticles becomes smaller than 5 nm, their mean inner potentials become larger than the bulk value. By the scanning tunneling microscopy, the local barrier height, which is the apparent tunneling barrier height between the tip and sample, and energy gap were measured on the gold particles supported on the rutile TiO2(1 1 0) surface. When the particles become smaller than 0.4 nm in height, the local barrier height starts to decrease and the particles change from metal to nonmetal. By the first-principles calculations, the electronic structures and the charge transfers of the gold layer adsorbed on the rutile TiO2(1 1 0) surface have been examined and the effects of the surface stoichiometry have been investigated. The gold atoms are adsorbed on the non-stoichiometric, Ti-rich and O-rich, surfaces stronger than on the stoichiometric surface. The electronic structures of the gold nanoparticles depend on the size of particles and the stoichiometry of the TiO2 surface. All these experimental and theoretical results complementarily provide valuable insight into the origin of the catalytic activity of the Au/TiO2 catalysts.
Keywords: Gold; Titanium dioxide; Pseudopotential method; Transmission electron microscopy; Electron holography; Scanning tunneling microscopy
Reactivity of gold thin films grown on iridium: Hydrogen dissociation by Michio Okada; Shouhei Ogura; Wilson Agerico Diño; Markus Wilde; Katsuyuki Fukutani; Toshio Kasai (pp. 55-61).
Dissociative adsorption of H2 (D2) on Au thin films grown on an Ir{111} surface has been studied with temperature-programmed desorption using a quadrupole mass spectrometer and nuclear reaction analysis. Thin Au{111} films are epitaxially grown on Ir{111}, as confirmed by low-energy electron diffraction and scanning tunneling microscopy. H2 (D2) was dissociatively adsorbed on these Au{111} films, although it is well known that Au surfaces are noble enough not to dissociate hydrogen molecules. We propose a model to explain the unexpected high reactivity of the thin Au{111} surface, where narrowing of the s-band is responsible. Moreover, we found that H (D) atoms can be confined into the interface between the Ir surface and the Au thin film. We demonstrate how reactive Au thin films grown on an Ir surface are, and also compare the Au films with Ag films grown on Ir in order to test the validity/generality of our proposed model.
Keywords: PACS; 68.43.-hGold; Iridium; Hydrogen; Nuclear reaction analysis; Dissociative adsorption
Mechanism of deposition of gold precursors onto TiO2 during the preparation by cation adsorption and deposition–precipitation with NaOH and urea by Rodolfo Zanella; Laurent Delannoy; Catherine Louis (pp. 62-72).
Gold on TiO2 prepared by cation adsorption (CA) and deposition–precipitation with urea (DP urea) and NaOH (DP NaOH) were characterized by various techniques during the preparation in order to determine the nature of the species deposited and the chemical phenomena occurring during these preparations. In the case of cationic adsorption of the [Au(en)2]3+ complex, we showed that the preparation has to be performed at room temperature to avoid the decomposition of the complex and the reduction of gold. In such a way, small gold particles are obtained after calcination, but the gold loading is low. The methods of deposition–precipitation (DP NaOH and DP urea) involve both the deposition of a gold(III) species on the TiO2 surface, but the nature of these species is different. For the DP NaOH, we propose that [AuCl(OH)3]−, the main species present at the pH 8 of the preparation, reacts with hydroxyl groups of the TiO2 surface, and forms a grafted hydroxy-gold compound. This may explain the limited amount of gold deposited on TiO2 by this method. For the DP urea method, all the gold present in solution is deposited on the TiO2 surface as a gold(III) precipitate, which is not gold(III) hydroxide, but an amorphous compound containing nitrogen, oxygen and carbon. This compound arises from a reaction between the gold precursor and the products of decomposition of urea. The metallic gold particles obtained after calcination exhibit a decreasing size when the time of DP urea increases. We propose that the progressive increase of pH, due to urea decomposition at 80°C, results in changes in the surface charge density of the gold precipitate particles, and leads to a fragmentation of the particles.
Keywords: Gold; TiO; 2; Deposition–precipitation; Cation adsorption; EXAFS
Understanding preparation variables in the synthesis of Au/Al2O3 using EXAFS and electron microscopy by J.H. Yang; J.D. Henao; C. Costello; M.C. Kung; H.H. Kung; J.T. Miller; A.J. Kropf; J.-G. Kim; J.R. Regalbuto; M.T. Bore; H.N. Pham; A.K. Datye; J.D. Laeger; K. Kharas (pp. 73-84).
The catalytic performance of Au/Al2O3 catalyst is highly sensitive to preparation procedure. EXAFS and TEM characterization of key steps in the preparation in conjunction with activity measurements result in deeper insights into precautions needed and the complex manner residual chloride impacts catalytic activity. Chloride affects the morphological (Au particle size), chemical (reducibility) as well as the catalytic (poisoning) properties of Au. Alternate preparation procedures to the conventional calcination of a catalyst prepared with deposition–precipitation at neutral pH were explored to increase Au loadings. It was found that low temperature H2 reduction of a catalyst prepared at low pH but washed with NaOH is an effective preparation method.
Reduction behavior of nanostructured gold catalysts supported on mesoporous titania and zirconia by L. Ilieva; J.W. Sobczak; M. Manzoli; B.L. Su; D. Andreeva (pp. 85-92).
The reduction behavior of gold supported catalysts on mesoporous titania and zirconia was studied. The TPR, ESR, HRTEM and XPS methods were applied for the characterization of the state and structure of the catalysts. It was established that the nature of support plays a decisive role in the reactivity of the catalysts, in particular in their redox properties. The mesoporous nanostructured materials used in this study as supports for gold nanoparticles show very different behavior under hydrogen treatment. The ability of nanostructured zirconia to create F-centers leads to different charging of the supports surface, i.g. the supported gold nanoparticles which is the main reason for the observed differences between the both studied gold supported catalysts on mesoporous titania and zirconia.
Keywords: Nanosized gold; Nanostructured mesoporous zirconia and titania; Reduction behavior; TPR; ESR; XPS; HRTEM
Interaction of small molecules with Au(310): Decomposition of NO by C.P. Vinod; J.W. Niemantsverdriet Hans; B.E. Nieuwenhuys (pp. 93-97).
The interaction of NO, N2O, CO, O2, H2 and N2 with a stepped Au(310) surface has been studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). In the temperature (80–500K) and pressure (up to 5×10−6mbar) range used, no adsorption of O2, H2, CO and N2 was observed. However, NO and N2O adsorb on this stepped surface. Decomposition of NO to N2O was observed at temperatures as low as 80K. A possible mechanism for the formation of N2O from NO has been discussed on the basis of our TP-XPS results.
Keywords: Nitric oxide (NO); Au(3; 1; 0); Steps and kinks; Defects; X-ray photoelectron spectroscopy (XPS); Temperature programmed desorption (TPD); Gold catalysis; Oxygen; Nitrous oxide (N; 2; O)
The effect of calcination temperature on the adsorption of nitric oxide on Au-TiO2: Drifts studies by M.A. Debeila; N.J. Coville; M.S. Scurrell; G.R. Hearne (pp. 98-115).
Adsorption of NO on TiO2 and Au-TiO2 catalysts calcined at different temperatures has been studied using DRIFTS as a monitoring tool. It was found that NO adsorption on TiO2 is initially dominated by unidentate nitrite (1479, 1464cm−1), a small concentration of nitro species (1440, 1385, 1370cm−1) which later were replaced by nitrate (1611, 1589 and 1480cm−1) and which dominated the spectra after extended exposure of TiO2 to NO. Decomposition of NO3− into NO2− at elevated temperatures is observed and nitrite bound to the surface to form nitrite (1407cm−1), which dominates the spectra after cooing the system to 298K. Coordinated NO on different Ti4+ sites (1906 and 1447cm−1) detected in the early stages of NO adsorption and (1934 and 2005cm−1) were also found after extended exposure of TiO2 to NO. For Au-TiO2, early spectra were also dominated by unidentate nitrite (1476cm−1) for uncalcined Au-TiO2 (designated Dry-Au-T) and (1477cm−1) for Au-TiO2 calcined at 973K (designated Au-T973). In addition, bridging nitrite (1540cm−1) is one of the dominant species seen on Dry-Au-TiO2.The nitrate surface species, which dominate the spectra after extended exposure, are virtually the same and behave in the same way under in situ thermal desorption experiments. However, nitrate formation on Au-TiO2 is relatively fast compared with the situation found for TiO2. Mechanisms for the formation of these species are discussed and compared with previously reported data. For Au-TiO2, two characteristic bands at 1685 and 1644cm−1 for Dry-Au-T and at 1682 and 1643cm−1 for Au-T973 were detected at room temperature and dominated the initial spectra. The sites containing these NO molecules were populated first. For Dry-Au-T, the higher wavenumber mode (1685cm−1), assigned to bridging NO adsorbate, decreased in intensity during NO adsorption to produce a very weak feature and is assumed to contribute to the dissociation of NO on this catalyst. However, the lower wavenumber mode (1644cm−1), assigned to NO adsorbed at the gold-oxide interface, shifts to lower wavenumber and acts as a precursor state for the activated NO states detected at 1750 and 1714cm−1 on raising the temperature to 473K. In contrast, for Au-T973, the lower wavenumber mode (1643cm−1) is associated with a unit which undergoes dissociation, while the high wavenumber mode (1682cm−1) is assigned to a precursor state for the thermally activated NO states detected at 1744 and 1714cm−1 at 473K. This trend is explained in terms of the changes in the degree of interaction between gold and the underlying TiO2 support brought about upon calcination. The results are discussed and correlated with previous observations on gold catalysts in an attempt to assess the impact of these thermally activated NO states on the decomposition and selective catalytic reduction of NO over gold catalysts.
Keywords: Catalyst; Spectral; Decomposition; Nitric oxide; Titania
Iron oxide overlayers on Au/SiO2/Si(100): Promoting effect of Au on the catalytic activity of iron oxide in CO oxidation by László Guczi; Krisztina Frey; Andrea Beck; Gábor Petõ; Csaba S. Daróczi; Norbert Kruse; Sergey Chenakin (pp. 116-125).
Iron oxide layers of 5–10nm thickness were deposited by pulsed laser techniques (PLD) onto either Au films or nano-sized Au particles supported by SiO2/Si(100). Samples were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF SIMS) before and after measurements of the CO oxidation activity. Comparison was made with reference samples either free of iron oxide and/or free of Au particles/films. The initial activity of iron oxide/Au nano-particles/SiO2/Si(100) turned out to be the highest followed by the sample containing a sandwiched Au film. While some reaction-induced changes in the chemical composition of the iron oxide overlayer (“FeO x�, which can be regarded as a mixture of Fe2O3, FeO and FeOOH according to the XPS analysis of the Fe 2p and O 1s core levels) were seen, no Au segregation at the surface was detected by TOF SIMS. The XPS Au 4f spectra indicated, however, that Au atoms might be injected and trapped in this layer. The catalytic activity of the FeO x/Au/SiO2/Si(100) samples must be attributed to active sites located on the iron oxide overlayer promoted by gold underneath. Since Au nano-particles and Au films caused promotion we infer that an electronic effect is in operation due to the occurrence of an FeO x/Au interface in both cases. Since the promotion is stronger for Au nano-particles the hypothesis of a particle size dependent electronic effect may be advanced. For thicker FeO x of about 40–80nm no promotion by Au was found.
Keywords: Au; Iron oxide; CO oxidation; AFM; XPS; SIMS
Low-content gold-ceria catalysts for the water–gas shift and preferential CO oxidation reactions by Weiling Deng; Janice De Jesus; Howard Saltsburg; Maria Flytzani-Stephanopoulos (pp. 126-135).
Low-content (<0.6at.%) gold-ceria samples were prepared by one-pot synthesis by the urea gelation/co-precipitation method, and by sodium cyanide leaching of high-content (5at.%) gold-ceria materials prepared by deposition–precipitation. These catalysts, containing cationic gold in ceria, are active for both the low-temperature water–gas shift (WGS) reaction and the preferential oxidation of CO (PROX). The surface oxygen of ceria, as estimated by H2-TPR, was used to normalize the WGS reaction rates. Cyclic temperature-programmed reduction with intermittent reoxidation showed that the surface structures of gold-ceria catalysts are highly reversible. Considerable reoxidation by oxygen or H2O can occur even at ambient conditions. The stability of low-content gold-ceria catalysts for the PROX reaction in a realistic fuel gas mixture containing 1% CO–0.5% O2–50% H2–10% H2O–15% CO2–He was excellent. No drop in activity or selectivity was found in cyclic operation up to 150°C.
Keywords: Gold; Cerium oxide; Water–gas shift; Selective CO oxidation; Temperature-programmed reduction; Redox; Fuel cells; Preferential oxidation of carbon monoxide
Low temperature CO oxidation on supported and unsupported gold compounds by Jorge M.C. Soares; Michael Bowker (pp. 136-144).
Several simple gold compounds and their physical mixtures with TiO2 were tested for low temperature CO oxidation. No true catalytic activity was found for gold precursors on their own, although both Au2O3 and Au(OH)3 react well with CO even at room temperature in a non-catalytic manner. Despite that catalytic activity was obtained by physically mixing Au(OH)3 or Au2O3 with TiO2 and the results further emphasise the importance of a good contact between the gold and the support for good CO oxidation activity.
Activation of CO, O2 and H2 on gold-based catalysts by Andreea C. Gluhoi; Huib S. Vreeburg; Johan W. Bakker; Bernard E. Nieuwenhuys (pp. 145-150).
Au/CeO x/Al2O3 is highly active for CO oxidation at low temperatures and full conversion is already achieved around 60°C. From experimental results, it is concluded that ceria can act as oxygen supplier probably via Mars and van Krevelen mechanism.The H2–D2 isotopic exchange reaction is also efficiently catalyzed by Au/Al2O3. The gold particle size is of crucial importance for high activity. Most likely, a gold surface rich in defects activates the H2 (D2) molecules.
Keywords: Gold; Alumina; Ceria; CO oxidation; Hydrogen; Deuterium; Isotopic exchange reaction
Au/Fe2O3 nanocatalysts for CO oxidation: A comparative study of deposition–precipitation and coprecipitation techniques by Mikhail Khoudiakov; Mool C. Gupta; Sarojini Deevi (pp. 151-161).
Two synthetic approaches to the preparation of Au/Fe2O3 nanocatalysts were compared: a novel deposition–precipitation technique utilizing thermal decomposition of urea and the conventional coprecipitation method. The products were characterized by X-ray powder diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction. The catalysts prepared by both methods demonstrated very high activity toward CO oxidation in the “as prepared� state. Starting with the same amount of gold precursor, the deposition–precipitation technique produces a catalyst with higher final gold content than the coprecipitation method because it achieves more complete precipitation of the gold from solution. Heat treatment of the samples prepared by both methods resulted in a decrease of the catalytic activity by about an order of magnitude. However, we conclude that heat-treated Au/Fe2O3 catalysts are more suitable for practical applications because of stability issues with the “as prepared� samples.
Keywords: Nanocatalysts; Gold; Iron oxide; Carbon monoxide; CO oxidation; Deposition–precipitation
Preparation of nano-gold in zeolites for CO oxidation: Effects of structures and number of ion exchange sites of zeolites by Jen-Ho Chen; Jiunn-Nan Lin; Yih-Ming Kang; Wen-Yueh Yu; Chien-Nan Kuo; Ben-Zu Wan (pp. 162-169).
Nano-gold was prepared in different types of zeolite (Y, β and mordenite) in pH-adjusted chloroauric acid solution. The effects of the pore structures and the aluminum content on the loading of gold and on the reaction activity for CO oxidation at 0°C were investigated. The gold loading in each zeolite was strongly related to the aluminum content; it was also affected by the structure of the zeolite. HRTEM results revealed that a few particles of gold around 1nm in diameter were formed in the cage of Y-type zeolite with less aluminum loading (1.8wt.%), and there was aggregation of gold species on the exterior surface of the zeolite; in contrast, more particles of gold around 1nm were formed in Y-type zeolite with more aluminum loading (12.4wt.%), no aggregation was observed. The results from the temperature-programmed reduction in hydrogen indicated that the gold species were reduced in the smaller pores (in β and mordenite) only at lower temperatures. Nevertheless, gold was easily sintered in the channels of β and mordenite during CO oxidation, causing severely reducing the activity. The cage-like pore in Y-type zeolite with 12.4wt.% aluminum loading prevented the sintering of gold, and its hydrophilic surface favored the activation of gold species; therefore, this Au/Y catalyst exhibited high catalytic activity and stability for CO oxidation.
Keywords: Gold; Nano-gold; Zeolites; Y-type zeolite; Mordenite; β-type zeolite; CO oxidation
Size and composition dependence in CO oxidation reaction on small free gold, silver, and binary silver–gold cluster anions by Thorsten M. Bernhardt; Liana D. Socaciu-Siebert; Jan Hagen; Ludger Wöste (pp. 170-178).
Gas phase kinetics experiments with mass-selected gold, silver, and binary silver–gold cluster ions in a radio frequency (rf)-ion trap reactor are presented. For the catalytic CO combustion reaction, the adsorption of molecular oxygen onto the free clusters is identified as the first reaction step. A comparison of the measured O2 adsorption reaction rate constants for the investigated Ag nAu m− ( n+ m=1, 2, 3) clusters reveals a pronounced size and composition dependence. Favorable activation of the oxygen molecular bond is only expected in the case of Au2−. In the reaction of the gold dimer with O2 and CO, a coadsorption complex is identified at cryogenic temperatures as decisive reaction intermediate in the observed CO oxidation reaction. Through detailed reaction kinetics measurements in combination with first-principles calculations, a comprehensive understanding of the molecular details of the catalytic reaction cycle emerges. The obtained data also permit the determination of the CO2 formation rate and the catalytic turn-over-frequency in the rf-ion trap reactor. In contrast to gold, odd size silver cluster anions are found to adsorb two oxygen molecules. These Ag nO4− complexes are proposed to be key intermediates in oxidation reactions with silver clusters involved, and first experimental indications for catalytic activity of selected cluster sizes are presented.
Keywords: Gold catalysts; Silver; CO oxidation; Metal clusters; Gas phase kinetics; Mass spectrometry
Low-temperature activity of Au/CeO2 for water gas shift reaction, and characterization by ADF-STEM, temperature-programmed reaction, and pulse reaction by H. Sakurai; T. Akita; S. Tsubota; M. Kiuchi; M. Haruta (pp. 179-187).
In Au/CeO2 catalysts prepared by the deposition–precipitation method, many Au particles smaller than 3nm in diameter were clearly observed by annular dark field scanning transmission electron microscopy (ADF-STEM). The water gas shift activity of Au/CeO2 was compared with that of Pt/CeO2, Au/TiO2, and Cu/ZnO/Al2O3 by using temperature-programmed reaction (TPRe) measurement in a stream of a reaction gas mixture composed of CO, CO2, H2, H2O, and He. Au/CeO2 was found to be the most active at temperatures between 373 and 523K without producing methane below 623K. Based on transient experiments consisting of injecting CO and H2O pulses into H2 and He streams, it is deduced that the water gas shift reaction proceeds over the perimeter interfaces of small gold particles on a reduced cerium oxide surface. The reaction proceeds by formation of a reaction intermediate (possibly a formate) from CO and OH groups, followed by decomposition of the intermediate by H2O into CO2 and H2. Hydrogen in the reactant gas facilitates cerium oxide to form more active reduced surfaces.
Keywords: Gold; Ceria; Annular dark field scanning transmission electron microscopy; Water gas shift; Temperature-programmed reaction; Associated mechanism
Gas-phase epoxidation of propylene over small gold ensembles on TS-1 by B. Taylor; J. Lauterbach; W.N. Delgass (pp. 188-198).
A series of Au/TS-1 catalysts with varying gold and titanium contents was prepared by deposition–precipitation (DP) and examined at 140–200°C in a 10/10/10/70 vol.% mixture of hydrogen, oxygen, propylene and helium at a space velocity of 7000mL/h/gcat. Gold loading was found to be closely related to titanium loading, implying that low gold loadings using deposition–precipitation results in an inherently small number of very active sites. Forcing the gold loading to higher values resulted in poor activity and stability. A catalyst prepared with a Si/Ti=36 and a gold loading of 0.05wt% produced 116gPO/h/kgcat at 200°C, which is the highest rate thus reported for a TS-1-based catalyst, with no evidence of deactivation during the 40h temperature program. Catalysts prepared with lower titanium and gold contents resulted in very active catalysts when rates were normalized to the total gold content, 350gPO/h/gAu at 200°C for 0.01wt% Au/TS-1 (Si/Ti=500), indicative of the more efficient use of gold and titanium for the epoxidation reaction. The low gold loadings coupled with the absence of gold particles in TEM micrographs make it likely that, in these materials, significant activity is attributable to gold entities much smaller than 2nm.
Keywords: Epoxidation; Gold; TS-1; Propylene oxide; Propylene
Oxidation of alcohols and sugars using Au/C catalysts by Laura Prati; Francesca Porta (pp. 199-203).
The activity of Au/C catalysts has been reviewed in the liquid phase oxidation of alcohols as a function of preparation method, particle size and dispersion, surface exposure and nature of the support. A strong influence of substrate structure has also been observed. Monometallic Au on carbon catalysts show a unique behaviour in the liquid phase oxidation of alcohols compared with Pt- and Pd-based systems, characterised by a higher resistance to poisoning from both oxygen and (by)products normally affecting other catalysts.
Keywords: Gold on carbon; Oxidation; Alcohol
Oxidation of alcohols and sugars using Au/C catalysts by Massimiliano Comotti; Cristina Della Pina; Roberto Matarrese; Michele Rossi; Attilio Siani (pp. 204-209).
Gold on carbon catalysts, designed for liquid phase oxidation, have been prepared on 80–500g scale and evaluated for alcohols and carbohydrates oxidation using glucose and ethane-1,2-diol as model molecules. Physico-chemical properties of 0.8% gold on carbon X40S and 1% gold on carbon XC72R catalysts are discussed along with scaling up effect.
Keywords: Gold; Carbon; Alcohols; Aldehydes; Scaling up; Catalytic oxidation
Au, Pd (mono and bimetallic) catalysts supported on graphite using the immobilisation method by Nikolaos Dimitratos; Francesca Porta; Laura Prati (pp. 210-214).
The selective oxidation of glycerol was performed using mono and bimetallic catalysts based on Au and Pd and supported on graphite. Moreover, the preparation of bimetallic systems (Au–Pd) was investigated by using different procedures based on sol immobilisation method. The results showed that the use of a bimetallic system significantly improved the activity with respect to the monometallic system; whereas, the selectivity to glyceric acid showed to be dependent upon the reaction temperature and the preparation method used.
Keywords: Glycerol oxidation; Gold; Palladium; Bimetallic catalyst; Glyceric acid
Synthesis of hydrogen peroxide by direct oxidation of H2 with O2 on Au/SiO2 catalyst by Tatsumi Ishihara; Yoshimasa Ohura; Satoshi Yoshida; Yuiko Hata; Hiroyasu Nishiguchi; Yusaku Takita (pp. 215-221).
Direct oxidation of hydrogen by gaseous oxygen was studied on Au/SiO2 catalyst without addition of a halogen compound. It was found that Au supported on SiO2 exhibits the high activity to H2O2 formation by direct oxidation of H2 by O2. Although the same Au catalyst is used, the H2O2 formation rate is strongly dependent on the support oxide. When the acidic oxides such as SiO2 or zeolite are used for support, reasonably high formation rates of H2O2 were achieved. In particular, the H2O2 formation rate is at a maximum on the SiO2 support. The high H2O2 formation rate observed with the SiO2 supported catalyst can be explained by the low activity for H2O2 decomposition. H2O2 formation rate on Au/SiO2 catalyst increased with increasing hydrogen partial pressure withPH21.05, however, it decreased with oxygen partial pressure withPO2−0.505. Therefore, activation of hydrogen on the catalyst seems to be the key step for H2O2 formation by direct oxidation of H2 by O2. Effects of additives on Au/SiO2 catalyst were further studied and it was found that the addition of a small amount of Pd is effective for increasing the H2O2 formation rate. The formation rate and the selectivity for H2O2 at initial 2h were achieved to a value of 3.2μmol/h and 30% under the optimized condition of Au–Pd/SiO2 catalyst.
Keywords: Hydrogen peroxide; Direct synthesis; H; 2; oxidation; Au/SiO; 2
Heterogeneously catalysed hydrogenation using gold catalysts by Peter Claus (pp. 222-229).
A comprehensive overview in the field of heterogeneously catalysed hydrogenations over gold surfaces and supported gold catalysts is given. It is also highlighted where the latter are superior to other hydrogenation catalysts. Basic principles including the adsorption and activation of hydrogen, structure-sensitivity, real structure of supported gold catalysts and attempts to identify the active sites are discussed. In reactions exhibiting a selectivity problem, e.g. hydrogenation of two CC bonds or CC versus CO groups, the characteristic feature of gold catalysts is the preferred hydrogenation of one of these groups leading to monoenes, unsaturated alcohols and unsaturated ketones as reaction products important to the chemical industry.
Keywords: Hydrogenation; Gold catalysts; Active sites; Acrolein; α,β-Unsaturated aldehydes; HRTEM; Hydrogen adsorption; Au–In
The use of titania- and iron oxide-supported gold catalysts for the hydrogenation of propyne by Jose Antonio Lopez-Sanchez; David Lennon (pp. 230-237).
Propyne hydrogenation over titania-supported and iron oxide-supported gold catalysts has been investigated under pulse-flow conditions. The two catalysts exhibit different catalytic profiles. The Au/TiO2 catalyst displayed complete selectivity to propene and progressive deactivation, whereas Au/Fe2O3 exhibited selectivities and deactivation patterns dependent on ageing, catalyst pre-treatment and reaction temperature. These results suggest significant differences in the active surfaces of these two catalysts, due to the interaction of the gold metal with the support. The activity of gold catalysts can be modified by the selection of the oxide support and pre-treatment to produce catalysts completely selective to the partially hydrogenated product.
Homogeneous catalysis by gold: The current status of C,H activation by A. Stephen K. Hashmi; Ralph Salathé; Tanja M. Frost; Lothar Schwarz; Ji-Hyun Choi (pp. 238-246).
In the context of own results on the hydroarylation reaction of α,β-unsaturated ketones with heteroaromatic furans, pyrroles and indoles, the current knowledge on the C,H activation by homogeneous gold catalysts is summarized.
Keywords: Alkenes; Alkynes; C,H activation; Friedel–Crafts reaction; Gold; Homogeneous catalysis; Hydroarylation; Michael acceptors
Stabilization of Au(III) on heterogeneous catalysts and their catalytic similarities with homogeneous Au(III) metal organic complexes by S. Carrettin; A. Corma; Marta Iglesias; F. Sánchez (pp. 247-252).
We have found that nanocrystalline CeO2 and Y2O3 are able to stabilize surface Au(III) species, with the final catalyst presenting a direct correlation between the concentration of these species and the catalytic activity for CO oxidation. The same catalysts are also active and extremely selective in performing the homocoupling of arylboronic acids, being the activity directly correlated with Au(III). By preparing Schiff base, Au(III) complexes we have confirmed that Au(III) is very active and selective for the homocoupling reaction, while the analogous Pd(II) metal complex catalyzes the Suzuki cross-coupling reaction. These Au(III) complexes are also very active catalysts, for olefin hydrogenation, giving similar activities than the “isoelectronic� Pd(II) metal complex.
Keywords: Au(III) species; Isoelectronic systems; Gold complexes; Nanocrystalline supports; Coupling reactions; Hydrogenation reactions
Commercial aspects of gold catalysis by Christopher W. Corti; Richard J. Holliday; David T. Thompson (pp. 253-261).
There is the potential to apply catalysis by gold in numerous commercial applications. These practical uses include catalysts for pollution and emission control, chemical processing of a range of bulk and speciality chemicals, the emerging ‘hydrogen economy’ for clean hydrogen production and fuel cell systems, as well as for sensors to detect poisonous or flammable gases or substances in solution. The purpose of this paper is to briefly review the major commercial opportunities to apply heterogeneous gold catalysis and to highlight those areas that are considered to merit particular attention. It is considered that all involved in this exciting field need to carefully consider both the durability of catalysts under representative operating conditions and viable methods of catalyst preparation, in order to commercially apply the new science that has developed in recent years.
Keywords: Au; Industrial application; Commercial; Pollution control; Chemical processing; Fuel cell; Low temperature