Nanocatalysis on Supported Oxides for CO Oxidation

Active gold and palladium nanoparticles supported on a variety of oxides (CeO₂, ZrO₂, Al₂O₃, SiO₂, MgO and ZnO) were synthesized using laser vaporization and microwave irradiation methods. The catalytic activities for CO oxidation on the nanoparticle catalysts were evaluated and compared among different oxide supports. The effect of shape on the catalytic activity is demonstrated by comparing the activities of the Au and Pd catalysts deposited on MgO nanocubes and ZnO nanobelts. The Au/CeO₂ nanoparticles deposited on MgO nanocubes exhibit high catalytic activity and stability. The enhanced catalytic activity is attributed to the presence of a significant concentration of the corner and edge sites in MgO nanocubes. The Au- and Pd-doped Mn₂O₃ nanoparticles show promising results for the low temperature CO oxidation. Several approaches for incorporating the Au and Pd nanocatalysts within mesoporous oxide supports are presented and discussed.     

Modeling gold/iron oxide interface system

The effect of gold particle size and Au/FeO _x interface on the electronic properties and catalytic activity using samples of Au/SiO₂/Si(100), Au/FeO _x /SiO₂/Si(100), FeO _x /Au/SiO₂/Si(100) has been modelled. Nanosize gold particles of varying size were fabricated by deposition of a 10 nm thick gold film onto SiO₂/Si(100) substrate by electron beam evaporation followed by modification using low energy Ar⁺ ion bombardment or Ar⁺ ion implantation. These modifications formed Au islands of decreasing size accompanied by the strong redistribution of the Au 5d valence band structure determined by ultraviolet and X-ray photoelectron spectroscopy (UPS, XPS) and increased activity in catalytic CO oxidation. The gold/iron oxide interface was prepared by deposition of iron oxide using pulsed laser deposition (PLD). The structural properties of gold and iron oxide were characterized by XPS, atomic force microscopy (AFM), transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS). Generally, the formation of gold/iron oxide interface increases the catalytic activity in CO oxidation regardless of the sequence of deposition, namely either Au/FeO _x /SiO₂/Si(100) or FeO _x /Au/SiO₂/Si(100) is formed. Furthermore, the interface formed is operative in determining the catalytic activity even if gold is not exposed to the surface, but it is located underneath the iron oxide layer. This is a promoting effect of the Au nanoparticles, which is more efficient than that of the bulk like Au films.     

Deposition of Gold Particles on Mesoporous Catalyst Supports by Sonochemical Method, and their Catalytic Performance for CO Oxidation

Supported gold catalysts on the mesoporous (MSP) metal oxides were prepared by a one-step, ultrasound-assisted reduction method, and characterized by XRD, HRTEM, EDX, BET, and XPS analysis. Their catalytic activities were examined in the oxidation of CO. Compared to the Au/Fe₂O₃(MSP) catalyst, the Au/TiO₂(MSP) and Au/Fe₂O₃-TiO₂(MSP) catalysts exhibited higher catalytic activity in the oxidation of CO at low temperatures. The high catalytic activity of Au/TiO₂(MSP) was attributed to the metallic state of the gold nanoparticles, their small size (2–2.5 nm), and their high dispersion on the catalyst support.     

Gold dispersed on TiO2 and SiO2: adsorption properties and catalytic behavior in hydrogenation reactions

Titania-supported Au catalysts were given both low temperature reduction and high temperature reduction at 473 and 773 K, respectively, and their adsorption and catalytic properties were compared to identically pretreated Pt/TiO₂ catalysts and pure TiO₂ samples as well as Au/SiO₂ catalysts. This was done to determine whether a reaction model proposed for methanol synthesis over metals dispersed on Zn, Sr and Th oxides could also explain the high activities observed in hydrogenation reactions over MSI (Metal-Support Interaction) catalysts such as Pt/TiO₂. This model invokes O vacancies on the oxide support surface, formed by electron transfer from the oxide to the metal across Schottky junctions established at the metal-support interface, as the active sites in this reaction. The similar work functions of Pt and Au should establish similar vacancy concentrations, and O₂ chemisorption indicated their presence. However, these Au catalysts were completely inactive for CO and acetone hydrogenation, and ethylene hydrogenation rates were lower on the supported Au catalysts than on the supports alone. Consequently, this model cannot explain the high rate of the two former reactions over TiO₂-supported Pt although it does not contradict models invoking specialinterfacial sites.     

Synthesis of metal–cadmium sulfide nanocomposites using jingle-bell-shaped core-shell photocatalyst particles

Photocatalytic deposition of gold (Au) and silver (Ag) nanoparticles was investigated using jingle-bell-shaped silica (SiO₂)-coated cadmium sulfide (CdS) nanoparticles (SiO₂/CdS), which each had a void space between the CdS core and SiO₂ shell, as a photocatalyst. A size-selective photoetching technique was used to prepare the jingle bell nanostructure of SiO₂/CdS. Nanoparticles of Au and Ag were deposited by irradiation of the photoetched SiO₂/CdS in the presence of the corresponding metal complexes under deaerated conditions. Chemical etching of Au-deposited particles enabled the selective removal of CdS without any influence on the surface-plasmon absorption of Au. TEM analyses of the resulting particles suggested that some particles were encapsulated in hollow SiO₂ particles, while other Au particles were deposited on the outer surface of the SiO₂ shell. Emission spectra of the photoetched SiO₂/CdS showed that the metal deposition developed a broad emission with a peak around 650 nm originating from surface defect sites, the degree being dependent on the kind of metal nanoparticles and their amount of deposition. This fact can be explained by the formation of metal–CdS binary nanoparticles having defect sites at the interface between metal and CdS.     

Liquid‐phase hydrogenation of cinnamaldehyde over Cu‐Au/SiO2 catalysts

The synthesis, characterization, and application of silica‐supported Cu‐Au bimetallic catalysts in selective hydrogenation of cinnamaldehyde are described. The results showed that Cu‐Au/SiO₂ bimetallic catalysts were superior to monometallic Cu/SiO₂ and Au/SiO₂ catalysts under identical conditions. Adding a small amount of gold (6Cu‐1.4Au/SiO₂ catalyst) afforded eightfold higher catalytic reaction rate compared to Cu/SiO₂ along with the high selectivity (53%, at 55% of conversion) toward cinnamyl alcohol. Characterization techniques such as x‐ray diffraction, H₂ temperature‐programmed reduction, ultraviolet‐visible spectroscopy, transmission electron microscopy, Fourier‐transform infrared spectra of chemisorbed CO, and x‐ray photoelectron spectroscopy were employed to understand the origin of the catalytic activity. A key genesis of the high activity of the Cu‐Au/SiO₂ catalyst was ascribed to the synergistic effect of Cu and Au species: the Au sites were responsible for the dissociative activation of H₂ molecules, and Cu⁰ and Cu⁺ sites contributed to the adsorption‐activation of C?C and C?O bond, respectively. A combined tuning of particle dispersion and its surface electronic structure was shown as a consequence of the formation of Au‐Cu alloy nanoparticles, which led to the significantly enhanced synergy. A plausible reaction pathway was proposed based on our results and the literature. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3300–3311, 2014     

Effect of calcination temperature on the catalytic activity of Au colloids mechanically mixed with TiO2 powder for CO oxidation

In order to elucidate the role of the contact structure between gold and metal oxide support in low-temperature CO oxidation, a mechanical mixture of colloidal gold with TiO₂ powder was prepared and calcined at different temperatures. The sample calcined at 473 K, which is composed of spherical gold particles with a mean diameter of 5.1 nm and TiO₂ powder, is poorly active for CO oxidation at temperatures up to 473 K. The catalytic activity appreciably increases with an increase in calcination temperature up to 873 K even though gold particles grow to larger ones, reaching a level with almost the same turnover frequency as that of Au/TiO₂ prepared by a deposition–precipitation method. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rational design of gold catalysts with enhanced thermal stability: post modification of Au/TiO2 by amorphous SiO2 decoration

Au/TiO₂ is highly active for CO oxidation, but it often suffers from sintering in high-temperature environments. In this work, we report on a novel design of gold catalysts, in which pre-formed Au/TiO₂ catalysts were post decorated by amorphous SiO₂ to suppress the agglomeration of gold particles. Even after being aged in O₂–He at 700 °C, the SiO₂-decorated Au/TiO₂ was still active for CO oxidation at ambient temperature.     

Activity, Selectivity, and Long-Term Stability of Different Metal Oxide Supported Gold Catalysts for the Preferential CO Oxidation in H2-Rich Gas

A comparative study of the catalytic performance and long-term stability of various metal oxide supported gold catalysts during preferential CO oxidation at 80°C in a H₂-containing atmosphere (PROX) reveals significant support effects. Compared to Au/-Al₂O₃, where the support is believed to behave neutrally in the reaction process, catalysts supported on reducible transition metal oxides, such as Fe₂O₃, CeO₂, or TiO₂, exhibit a CO oxidation activity of up to one magnitude higher at comparable gold particle sizes. The selectivity is also found to strongly depend on the employed metal oxide, amounting, e.g., up to 75% for Au/Co₃O₄ and down to 35% over Au/SnO₂. The deactivation, which is observed for all samples with increasing time on stream, except for Au/-Al₂O₃, is related to the build-up of surface carbonate species. The long-term stability of the investigated catalysts in simulated methanol reformate depends crucially on the ability to form such by-products, with magnesia and Co₃O₄ supported catalysts being most negatively affected. Overall, Au/CeO₂ and, in particular, Au/-Fe₂O₃ represent the best compromise under the applied reaction conditions, especially due to the superior activity and the easily reversible deactivation of the latter catalyst.     

Gold catalysts supported on ceria-modified mesoporous zirconia for low-temperature water–gas shift reaction

New gold catalytic system prepared on ceria-modified mesoporous zirconia used as water–gas shift (WGS) catalyst is reported. Mesoporous zirconia was synthesized using surfactant templating method through a neutral [C₁₃(EO)₆-Zr(OC₃H₇)₄] assembly pathway. Ceria modifying additive was deposited on mesoporous zirconia by deposition–precipitation method. Gold-based catalysts with different gold content (1–3 wt. %) were synthesized by deposition–precipitation of gold hydroxide on mixed metal oxide support. The supports and the catalysts were characterized by powder X-ray diffraction, high-resolution transmission electron microscopy, N₂ adsorption analysis and temperature programmed reduction. The catalytic behavior of the gold-based catalysts was evaluated in WGS reaction in a wide temperature range (140–300 °C) and at different space velocities and H₂O/CO ratios. The influence of gold content and particle size on the catalytic performance was investigated. The WGS activity of the new Au/ceria-modified mesoporous zirconia catalysts was compared with that of gold catalysts supported on simple oxides CeO₂ and mesoporous ZrO₂, revealing significantly higher catalytic activity of Au/ceria-modified mesoporous zirconia. A high degree of synergistic interaction between ceria and mesoporous zirconia and a positive modification of structural and catalytic properties by ceria have been achieved. It is clearly revealed that the ceria-modified mesoporous zirconia is of much interest as potential support for gold-based catalyst. The Au/ceria-modified mesoporous zirconia catalytic system is found to be effective catalyst for WGS reaction.     

Nanodispersed Au/Al2O3 catalysts for low-temperature CO oxidation: Results of research activity at the Boreskov Institute of Catalysis

This paper presents some important results of the studies on preparation and catalytic properties of nanodispersed Au/Al₂O₃ catalysts for low-temperature CO oxidation, which are carried out at the Boreskov Institute of Catalysis (BIC) starting from 2001. The catalysts with a gold loading of 1–2 wt.% were prepared via deposition of Au complexes onto different aluminas by means of various techniques (“deposition-precipitation” (DP), incipient wetness, “chemical liquid-phase grafting” (CLPG), chemical vapor deposition (CVD)). These catalysts have been characterized comparatively by a number of physical methods (XRD, TEM, diffuse reflectance UV/vis and XPS) and catalytically tested for combustion of CO impurity (1%) in wet air stream at near-ambient temperature. Using the hydroxide or chloride gold complexes capable of chemical interaction with the surface groups of alumina as the catalyst precursors (DP and incipient wetness techniques, respectively) produces the catalysts that contain metallic Au particles mainly of 2–4 nm in diameter, uniformly distributed between the external and internal surfaces of the support granules together with the surface “ionic” Au oxide species. Application of organogold precursors gives the supported Au catalysts of egg shell type which are either close by mean Au particle size to what we obtain by DP and incipient wetness techniques (CVD of (CH₃)₂Au(acac) vapor on highly dehydrated Al₂O₃ in a rotating reactor under static conditions) or contain Au crystallites of no less than 7 nm in size (CLPG method). Regardless of deposition technique, only the Cl-free Au/Al₂O₃ catalysts containing the small Au particles (d_i ≤ 5 nm) reveal the high catalytic activity toward CO oxidation under near-ambient conditions, the catalyst stability being provided by adding the water vapor into the reaction feed. The results of testing of the nanodispersed Au/Al₂O₃ catalysts under conditions which simulate in part removal of CO from ambient air or diesel exhaust are discussed in comparison with the data obtained for the commercial Pd and Pt catalysts under the same conditions.     

Supported Nano-Gold Catalysts for Epoxidation of Styrene and Oxidation of Benzyl Alcohol to Benzaldehyde

Nano-gold particles supported on different alkaline earth oxides (viz. MgO, CaO, BaO and SrO), Gr. IIIa metal oxides (viz. Al₂O_3, Ga₂O_3, In₂O₃ and Tl₂O₃), transition metal oxides (viz. TiO_2, Cr₂O_3, MnO_2, Fe₂O_3, CoO_x, NiO, CuO, ZnO, Y₂O₃ and ZrO₂), rare earth metal oxides (viz. La₂O_3, Ce₂O_3, Nd₂O_3, Sm₂O_3, Eu₂O_3, Tb₂O_3, Er₂O₃ and Yb₂O₃) and U₃O₈ [all prepared by depositing gold on corresponding metal oxide support by deposition precipitation (DP) and/or homogeneous deposition precipitation (HDP) method] were evaluated for their catalytic performance in the liquid phase epoxidation of styrene by tert-butyl hydroperoxide (TBHP) to styrene oxide and also in the solvent-free benzyl alcohol-to-benzaldehyde oxidation (by molecular oxygen or TBHP) reactions. For the epoxidation, the catalytic performance (styrene oxide yield) of the most promising nano-gold catalysts prepared by the HDP method was in the following order: Au/MgO > Au/Tl₂O₃ > Au/Yb₂O₃ > Au/Tb₂O₃ > Au/CaO (or TiO₂). However, for the oxidation of benzyl alcohol to benzaldehyde by molecular oxygen, the order of choice for the most promising catalysts (based on benzaldehyde yield) was Au/U₃O₈ > Au/Al₂O₃ > Au/ZrO₂ > Au/MgO. Whereas, when TBHP was used as an oxidizing agent for the benzyl alcohol oxidation, the order of choice for the most promising catalysts was Au/U₃O₈ > Au/MgO > Au/TiO₂ > Au/ZrO₂ > Au/Al₂O₃. The catalytic performance of a particular supported nano-gold catalyst was thus found to depend on the reaction catalysed by them. Moreover, it is strongly influenced by a number of catalyst parameters, such as the metal oxide support, the method of gold depositon on the support, the gold loading and also on the catalyst calcination temperature. Nano-gold particles-support interactions seem to play an important role in controlling the deposition of gold (amount of gold deposited and size and morphology of gold particles), formation of different surface gold species (Au⁰, Au¹⁺ and Au³⁺) and electronic properties of gold particles and, consequently, control the catalytic performance (both the activity and selectivity) of the supported nano-gold catalysts in the reactions. The nano-gold catalysts prepared by the HDP method showed much better catalytic performance than those prepared by the DP, coprecipitation or impregnation method; in general, the HDP method provided supported gold catalysts with much higher gold loading and/or smaller size gold particles than that achieved by the DP and other methods.     

Clay-based SiO2 as active support of gold nanoparticles for CO oxidation catalyst: Pivotal role of residual Al

SiO₂ is generally regarded as an “inert” support of gold catalysts for CO oxidation. We find that silica obtained from a natural clay mineral can be used as excellent gold-support for efficient CO oxidation at relatively low temperature. Fine Au particles are found to locate around the Al sites, rather than on pure SiO₂ area. Accordingly, high Au loading and catalytic activity for the clay-based SiO₂ supports are ascribed to the residual aluminum originally from the clay.     

Characterization and catalytic activity of unpromoted and alkali (earth)-promoted Au/Al2O3 catalysts for low-temperature CO oxidation

Various unpromoted and alkali (earth) promoted gold catalysts were characterized by means of XRD, HRTEM, DR/UV–Vis and TPR. Based on the results we conclude that metallic Au is the active species in CO oxidation and that the reduction of Au³⁺ to Au⁰ proceeds below 200 °C. Pretreatment at mild temperatures, viz. 200 °C, results in the highest catalytic performance of Au/Al₂O₃ in low-temperature CO oxidation. Alkali (earth) metal oxide additives are most probably structural promoters. The best promoting effect is found for BaO.     

Influence of modifying additives on the catalytic activity and stability of Au/Fe2O3–MO x catalysts for the WGS reaction

The activity and stability of Au/Fe₂O₃–MO_x catalysts (M = Zr, Mg, Ca, Ni, La, Cu, Zn, Al, Ba, Cr, Co, Ce, Mo, Bi, Ti, Mn) in water-gas shift reaction were investigated extensively. The WGS activity and stability of Au/Fe₂O₃ is improved significantly upon addition of ZrO₂ and to a lesser extend MgO, CaO, NiO, La₂O₃, Cr₂O₃, CuO. In contrast, Bi, Ti and Mn oxides seriously decrease the catalytic activity while additions of Zn, Al, Ba, Co, Ce and Mo oxides do not influence evidently the catalytic activity and its stability. Based on the characterization using the methods of BET-surface area and pore structure XRF, XRD, and H₂–TPR for some of as-prepared and spent samples, it could be concluded that the catalytic activity of gold catalysts supported on composite oxide of Fe₂O₃–MO_x depends not only on the dispersion of the gold particles but also on the reduction property of composite oxide supports, regardless of the fluctuation of gold loading and some change of specific surface area and pore structure due to introduction of the modifying metal oxides. The improvement of catalytic stability may be attributed to the comparative stabilization of high dispersion of gold particles and uneasily sintering of Fe₃O₄ crystallites during the catalytic operation. However, the chemical (electronic) effects exerted by the modifying addition of metal oxides on the catalytic performance of gold catalyst may not be ruled out.     

EPR investigation, before and after adsorption of naphtalene, of mordenite containing Fe3+ and Cr5+ ions as impurities

The Au/SnO₂ catalysts were prepared by a co-precipitation method. The structure of the sample treated at different temperatures was investigated by means of HRTEM and XPS. The structure of the samples after various treatments and their activity in the CO oxidation were compared. The results showed that the catalytic behavior was related to the particle size of gold and surface oxygen species on the support. Highly dispersed gold particles and adsorbed surface oxygens and hydroxyl groups on the support were responsible for the high catalytic activity of the Au/SnO₂ catalyst.     

Reaction of Au(111) with Sulfur and Oxygen: Scanning Tunneling Microscopic Study

The reaction of sulfur and oxygen with the gold surface is important in many technological applications, including heterogeneous catalysis, corrosion, and chemical sensors. We have studied reactions on Au(111) using scanning tunneling microscopy (STM) in order to better understand the surface structure and the origin of gold’s catalytic activity. We find that the Au(111) surface dynamically restructures during deposition of sulfur and oxygen and that these changes in structure promote the reactivity of Au with respect to SO₂ and O₂ dissociation. Specifically, the Au(111) herringbone reconstruction lifts when either S or O is deposited on the surface. We attribute this structural change to the reduction of tensile surface stress via charge redistribution by these electronegative adsorbates. This lifting of the reconstruction was accompanied by the release of gold atoms from the herringbone structure. At high coverage, clusters of gold sulfides or gold oxides form by abstraction of gold atoms from regular terrace sites of the surface. Concomitant with the restructuring is the release of gold atoms from the herringbone structure to produce a higher density of low-coordinated Au sites by forming serrated step edges or small gold islands. These undercoordinated Au atoms may play an essential role in the enhancement of catalytic activity of gold in reactions such as oxygen dissociation or SO₂ decomposition. Our results further elucidate the interaction between sulfur and oxygen and the Au(111) surface and indicate that the reactivity of Au nanoclusters on reducible metal oxides is probably related to the facile release of Au from the edges of these small islands. Our results provide insight into the sintering mechanism which leads to deactivation of Au nanoclusters and into the fundamental limitation in the edge definition in soft lithography using thiol-based self-assembled monolayers (SAMs) on Au. Furthermore, the enhanced reactivity of Au after release of undercoordinated atoms from the surface indicate a relatively insignificant role of an oxide support for high reactivity.     

Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts

Gold can be compared favorably with Pd and Pt in the catalytic combustion of CH₃OH, HCHO and HCOOH when it is deposited on some reducible metal oxides (-Fe₂O₃, TiO₂, etc.). While the supported gold catalysts are less active in H₂ oxidation, they exhibit much higher activities in CO oxidation. For Au/TiO₂, the effect of catalyst preparation was further investigated. Since the activity for CO oxidation of the gold catalysts is not depressed but enhanced by moisture, they are practically applicable to CO removal from air at room temperature. Gold supported on manganese oxide is especially effective in the selective CO removal from hydrogen, indicating its potential applicability to polymer electrolyte fuel cells using the reformed gas of methanol.     

Hydroformylation of olefins by Au/Co3O4 catalysts

The hydroformylation of olefins over supported gold catalysts in an autoclave reactor under mild conditions (100–140 °C, 3–5 MPa) has been studied. Over Au/AC (activated carbon), Au/PVP (polyvinylpyrrolidone), Au/Al₂O₃, Au/TiO₂, Au/Fe₂O₃, Au/ZnO, Au/CeO₂ and Co₃O₄, 1-olefin mainly remained unchanged and the major products were isomerized olefins or hydrogenated paraffin. In contrast, Au nanoparticles deposited on Co₃O₄ led to remarkably high catalytic activities in hydroformylation reaction with selectivities above 85% to desired aldehydes. The hydroformylation of olefins proceeds preferentially at temperatures below 140 °C, above which the reactions of olefins gradually shifted to isomerization and then to hydrogenation. It appeared that the activity and selectivity of hydroformylation reaction strongly depend on the molecular structure of olefins, which could be ascribed to steric constraints as internal olefins are relatively inappropriate to form alkyl group and subsequent acyl group by insertion of CO. The Au/Co₃O₄ catalyst can be recycled by simple decantation with slight decrease in catalytic activity along with an increase in recycle times, which is a great advantage over homogeneous catalysts. The role of gold nanoparticles can be assumed to dissociate hydrogen molecule into atomic species which reduce Co₃O₄ to Co metal under mild reaction conditions.     

Complete oxidation of 2-propanol over gold-based catalysts supported on metal oxides

This paper concerns the preparation of metal oxide-supported gold catalysts and their application to 2-propanol abatement in order to lower the light off temperature. Catalytic oxidation of 2-propanol was investigated on Au/CeO₂, Au/Fe₂O₃, Au/TiO₂ and Au/Al₂O₃ catalysts prepared from the deposition–precipitation (DP) method. The catalysts are characterized by XRD (X-ray diffraction), BET (Brunner–Emmett–Teller), TEM (transmission electron microscopy), NH₃-TPD (NH₃-temperature programmed desorption), H₂-TPR (H₂-temperature programmed reduction), ICP-AES (inductively coupled plasma-atomic emission spectroscopy) and XPS (X-ray photoelectron spectroscopy) techniques. The catalytic activity of Au/metal oxide samples towards the deep oxidation of 2-propanol to CO₂ and water has been found to be strongly dependent on the kind of supports, the amount of gold loading, the calcination temperature and the moisture content in the feed.