TiO2 Supported Nano—Au Catalysts Prepared via Solvated Metal Atom Impregnation for Low-Temperature CO Oxidation

TiO₂ supported nano-Au catalysts were prepared by solvated metal atom impregnation (SMAI) method. The catalysts were characterized by means of AAS, TPD, H₂ reduction desorption (H₂-RD), XRD, TEM, XPS and tested for low-temperature CO oxidation. XRD and TEM results showed that the pretreatment temperature had an influence on the particle size of Au/TiO₂catalysts. The average particle size increased with the increase in pretreatment temperature. XPS indicated that gold in the catalysts was presented in the form of metallic state clusters. Catalytic studies showed these catalysts were very active and stable in low-temperature CO oxidation. The CO oxidation activity of the catalysts increased as the Au particle size decreased. The measurement results of AAS, TPD and H₂-RD revealed that there were some organic fragments on the surface of Au particles which might be responsible for the high stability of the Au/TiO₂ catalysts.     

Preparation of Supported Gold Catalysts for Low-Temperature CO Oxidation via “Size-Controlled” Gold Colloids

Catalytically active gold model catalysts have been designed via “size-controlled” gold colloids of 2-nm mean particle size. They were prepared by reduction of chloroauric acid with tetrakis(hydroxymethyl)phosphonium chloride in an alkaline solution, followed by adsorption of gold colloids on TiO₂and ZrO₂at a pH lower than the isoelectric point of the metal oxides. Investigation of the size of the gold particles in solution by UV-vis spectrophotometry in combination with HRTEM indicated that the gold colloids are rather stable in alkaline solution, during pH-change and purification with dialysis. Ageing of the solutions showed that the particle size slowly increased over a time scale of 4 months. Analysis of the dried catalysts by XRD and HRTEM corroborated that the particle size was nearly preserved during the immobilization process. Only in the case of high loadings (16.6 wt%, compared to the calculated nominal monolayer coverage of 45–55 wt%), incomplete adsorption occurred, affording more inhomogeneous dispersion and some aggregation. After calcination at 673 K, both zirconia- and titania-based catalysts containing 1.7 wt% Au exhibited high activity in low temperature CO oxidation. Although the particle size on both supports was comparable, the Au/TiO₂catalyst showed significantly higher activity than the Au/ZrO₂catalyst. The uncalcined Au/TiO₂also exhibited high activity, whereas the uncalcined Au/ZrO₂was inactive under the same conditions, corroborating that not only the gold particle size but also the support plays a key role in CO oxidation.     

Reducibility of supported gold (III) precursors: influence of the metal oxide support and consequences for CO oxidation activity

The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO₂, Au/Al₂O₃, Au/TiO₂) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by deposition-precipitation of an aqueous Au(III) complex with urea, and contained the same gold loading (~1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O₂/He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO₂ and Au/CeO₂ developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al₂O₃ remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina ~ 0. The EXAFS of samples pretreated in air at 773 K and in H₂ at 573 K reveals the presence of only metallic particles for Au/TiO₂ and Au/Al₂O₃. Au(III) supported on CeO₂ remains unreduced after calcination, but reduces during the treatment with H₂. CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO₂ is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.     

Quantitative determination of gold active sites by chemisorption and by infrared measurements of adsorbed CO

Quantitative measurements of CO chemisorption in the range 140–180 K, supported by FTIR data on adsorbed CO, were performed on Au/TiO₂, Au/Fe₂O₃, and Au/CeO₂ catalysts. On the first two samples, which had similar particle size distributions, an average Au/CO chemisorption stoichiometry of about 3, referred to step-edge Au atoms, was found. On Au/CeO₂, where very small clusters and quite large particles are present, the CO-chemisorbed volume was much higher than expected, due to the prevailing contribution of very small Au clusters. On the same sample, a change in the IR absorption coefficient was observed and was reasonably explained.     

Acetylene Hydrogenation on Au-Based Catalysts

Hydrogenation of acetylene has been investigated on Au/TiO₂, Pd/TiO₂ and Au-Pd/TiO₂ catalysts at high acetylene conversion levels. The Au/TiO₂ catalyst (avg. particle size: 4.6 nm) synthesized by the temperature-programmed reduction-oxidation of an Au-phosphine complex on TiO₂ showed a remarkably high selectivity to ethylene formation even at 100% acetylene conversion. Au/TiO₂ prepared by the conventional incipient wet impregnation method (avg. particle size: 30 nm), on the other hand, showed negligible activity for acetylene hydrogenation. Although the Au catalysts showed a high selectivity for ethylene, the acetylene conversion activity and catalyst stability were inferior to the Pd-based catalysts. Au-Pd catalysts prepared by the redox method showed high acetylene conversions as well as high selectivity for ethylene. Interestingly Au-Pd catalysts prepared by depositing Pd via the incipient wetness method on Au/TiO₂ showed very poor selectivity (comparable to mono-metallic Pd catalysts) for ethylene. High-resolution transmission electron microscopy (TEM) studies coupled with energy dispersive X-ray spectroscopy (EDS) showed that while the redox method produced bimetallic Au-Pd catalysts, the latter method produced individual Pd and Au particles on the support.     

Catalytic activity of ethylene oxidation over Au,Ag and Au–Ag catalysts: Support effect

Au, Ag and Au–Ag catalysts on different supports of alumina, titania and ceria were studied for their catalytic activity of ethylene oxidation reactions. An addition of an appropriate amount of Au on Ag/Al₂O₃ catalyst was found to enhance the catalytic activity of the ethylene epoxidation reaction because Au acts as a diluting agent on the Ag surface creating new single silver sites which favor molecular oxygen adsorption. The Ag catalysts on both titania and ceria supports exhibited very poor catalytic activity toward the epoxidation reaction of ethylene, so pure Au catalysts on these two supports were investigated. The Au/TiO₂ catalysts provided the highest selectivity of ethylene oxide with relatively low ethylene conversion whereas, the Au/CeO₂ catalysts was shown to favor the total oxidation reaction over the epoxidation reaction at very low temperatures. In comparisons among the studied catalysts, the bimetallic Au–Ag/Al₂O₃ catalyst is the best candidate for the ethylene epoxidation. The catalytic activity of the gold catalysts was found to depend on the support material and catalyst preparation method which govern the Au particle size and the interaction between the Au particles and the support.     

CO oxidation over Au/TiO2 prepared from metal-organic gold complexes

A series of Au/TiO₂ catalysts has been prepared from precursors of various metal-organic gold complexes (Au _n , n = 2–4) and their catalytic activity for CO oxidation studied. The Au/TiO₂ catalyst synthesized from a tetranuclear gold complex shows the best performance for CO oxidation with transmission electron microscopy of this catalyst indicating an average gold particle size of 3.1 nm.     

Performance of Gold Nanoparticles Supported on Carbon Nanotubes for Selective Oxidation of Cyclooctene with Use of O2 and TBHP

Gold nanoparticles supported on multi-walled carbon nanotubes (Au/CNTs) were developed for the selective epoxidation of cyclooctene with oxygen and small amount of tert-butyl hydroperoxide (O₂-TBHP). We found that the Au/CNTs could provide the best combination of selectivity and conversion in comparison with the supported gold catalysts with several other carriers like active carbon, graphite, TiO₂, SiO₂ and Al₂O₃. The conversion of cyclooctene and the selectivity to epoxide increased with the amount of TBHP, but both reached almost maxima when the TBHP amount was higher than 5.0 mol% of cyclooctene. The CNTs-supported gold nanoparticles with mean sizes ranging from 3.1 to 15.0 nm could be prepared by using sol-immobilization method. The Au/CNTs catalysts with smaller gold particle size were related to higher epoxide yield, indicating a size effect of gold nanoparticles on the catalytic performance. The results suggested that the epoxidation of cyclooctene over the Au/CNTs with use of O₂-TBHP would be structure-sensitive.     

Au/MnO2–TiO2 catalyst for preferential oxidation of carbon monoxide in hydrogen stream

The catalytic activity of nanosize gold catalysts supported on MnO₂–TiO₂ and prepared by deposition–precipitation method has been investigated for preferential oxidation of carbon monoxide in H₂ stream. The catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy, X-ray diffraction, nitrogen sorption, transmission electron microscopy, and X-ray photoelectron spectroscopy. The influence of pH in the preparation process and the amount of MnO₂ loading on the catalytic properties of the Au/MnO₂–TiO₂ catalysts were also studied. Fine dispersion of gold nanoparticles on all the supports was obtained. Especially, Au/MnO₂–TiO₂ with MnO₂/TiO₂ mol ratio of 2:98, showed a mean Au particle size of 2.37 nm. The nanosized support constrained the size of gold. The addition of MnO₂ on Au/TiO₂ catalyst improved the selectivity of CO oxidation without sacrificing CO conversion in hydrogen stream between 50 and 100 °C. This could be attributed to the interactions of gold metal with MnO₂–TiO₂ support and the optimum combination of metallic and electron-deficient gold on the catalyst surface.     

An innovative approach to electro-oxidation of dopamine on titanium dioxide nanotubes electrode modified by gold particles

Au/TiO₂/Ti electrodes were prepared by galvanic deposition of gold particles from an acidic bath containing KAu(CN)₂ in the presence of a citrate buffer onto TiO₂ nanotubes layer on titanium substrates. Titanium oxide nanotubes were fabricated by anodizing titanium foil in a DMSO fluoride-containing electrolyte. The morphology and surface characteristics of Au/TiO₂/Ti electrodes were investigated using scanning electron microscopy and energy-dispersive X-ray, respectively. The results indicated that gold particles were homogeneously deposited on the surface of TiO₂ nanotubes. The nanotubular TiO₂ layers consist of individual tubes of about 40–80 nm diameters. The electro-catalytic behavior of Au/TiO₂/Ti electrodes for the dopamine electro-oxidation was studied by cyclic voltammetry and differential pulse voltammetry. The results showed that Au/TiO₂/Ti electrodes exhibit a considerably higher electro-catalytic activity toward the oxidation of dopamine. The catalytic oxidation peak current showed a linear dependence on dopamine concentration and a linear calibration curve was obtained in the concentration range of 0.5–2.5 mM of dopamine.     

Enhancement of MTBE photocatalytic degradation by modification of TiO2 with gold nanoparticles

The photocatalytic degradation of methyl tert-butyl ether by gold-modified TiO₂ has been studied by a combination of high resolution electron microscopy, X-ray photoelectron spectroscopy and reactor measurements. The optimum gold loading corresponded to a mean Au particle size of ⩽ 3 nm at which point the gold may no longer be metallic. Such catalysts exhibited a threefold rate enhancement compared to unmodified TiO₂, possibly due to injection of photo-excited electrons from semiconducting Au into the TiO₂ conduction band. Since the presence of dissolved oxygen was crucially important to good performance, it is possible that an additional factor was lowering of the local work function, thus promoting electron transfer to dioxygen molecules adsorbed at the Au/TiO₂ interface.     

Particle size effects of gold on the kinetics of the oxygen reduction at chemically prepared Au/C catalysts

Gold nanoparticles with narrow and controlled size distributions have been synthesized chemically and deposited onto a carbon support. Using the resulting gold on carbon (Au/C) catalysts, Au particle size effects on the kinetics of the oxygen reduction reaction (ORR) were analyzed in acidic media (0.5 M H₂SO₄). From rotating ring-disk electrode (RRDE) voltammetric studies, it was found that, for bulk gold, the number of electrons, n, involved in the ORR was nearly constant at potentials above −0.2 V. On the contrary, for the catalysts with diameters less than 10-15 nm, the value of n increased as the potential became more negative, and the highest value of n was obtained when the size of Au particles was less than 3 nm. Those results showed that further reduction of H₂O₂ or direct 4-electron reduction of O₂ proceeded at relatively low overpotential on extremely small gold clusters.     

Catalytic Combustion of Benzene over Au Supported on Ceria and Vanadia Promoted Ceria

Complete oxidation of benzene over Au/CeO₂ and Au/V₂O₅/CeO₂ catalysts were studied. Gold was supported on CeO₂ from different sources by deposition precipitation method. The catalysts were characterized by XRD, BET, X-ray photoelectron spectroscopy, TEM, and H₂-TPR techniques. The catalytic activity toward the complete oxidation of benzene to CO₂ and water were strongly dependent on the kind of CeO₂ sources, loading amount of gold, modified amount of vanadia, and calcination temperature. High activities were obtained on 1% Au/CeO₂ and 2% vanadia modified Au/CeO₂ catalysts calcined at 300 °C. The nanometer size of Au particle and interaction between Au, V₂O₅, and CeO₂ play important roles in determining the activity of benzene complete oxidation.     

Synthesis and characterization of Au/TiO2 core-shell structure nanoparticles

Au/TiO₂ core-shell structure nanoparticles were synthesized by sol-gel process, and the morphology and crystallinity of TiO₂ shell were investigated by TEM and UV-vis absorption spectrometer. Au/TiO₂ core-shell structure nanoparticles could be prepared by the hydrolysis of TOAA (titanium oxide acethylacetonate) in gold sol ethanol solution with water. The thickness of TiO₂ shell on the surface of gold particles was about 1 nm. To investigate the crystallinity of TiO, shell, UV light with 254 nm and radioactive ray of⁶⁰Co were irradiated on the TiO₂-coated gold sol ethanol solution. The surface plasmon band of gold nanoparticles appeared only when the radioactive ray was irradiated on the TiO₂-coated gold sol ethanol solution. From these results, it was found that the TiO, shell was amorphous and the MUA (mercaptoundecanoic acid) layer on the Au particle for its dispersion in ethanol did not act as an obstacle to disturb the movement of electrons onto the surface of Au particles.     

Physically and chemically mixed TiO2-supported Pd and Au catalysts: unexpected synergistic effects on selective hydrogenation of citral in supercritical CO2

The selective hydrogenation of citral was studied with various TiO₂-supported monometallic and bimetallic Pd and Au catalysts and their physical mixtures in supercritical CO₂ (scCO₂). Significant synergistic effects appeared when active Pd species was chemically or physically mixed with less active Au species. The total rate of conversion was greatly enhanced and the selectivity to citronellal (CAL) was improved. The physical properties of those catalysts were characterized by TEM, HRTEM-EDS, XPS, and UV/Vis and their features of H₂ desorption were examined by TPD. The physical and chemical characterization results were used to discuss the reasons for the unexpected synergistic effects observed. The same selective hydrogenation was also conducted in a conventional non-polar organic solvent of n-hexane to examine the roles of scCO₂. The use of scCO₂ was effective for accelerating the hydrogenation of citral and improving the selectivity to CAL.     

Cyclohexane Oxidation Over Size-Uniform Au Nanoparticles (SBA-15 hosted) in a Continuously Stirred Tank Reactor Under Mild Conditions

Cyclohexane oxidation was operated in a continuously stirred tank reactor at system pressures of 0.6–1.0 MPa under an air-like O₂/N₂ atmosphere (rather than pure O₂). Catalytic performance was investigated over Au nanoparticles (size: 3–8 nm) hosted by SBA-15 as well as Au particles (>60 nm) deposited on MCM-41, and high turnover frequencies of desired products were detected over the former. Based on intrinsic activities of representative catalysts, we derived a size-sensitivity feature of cyclohexane oxidation over Au particles.     

Gold Catalysts Supported on Cerium–Gallium Mixed Oxide for the Carbon Monoxide Oxidation and Water Gas Shift Reaction

The synthesis, characterization and catalytic properties of gold supported on ceria, gallia and a cerium–gallium mixed oxide were investigated. The nanostructural characterization of the cerium–gallium support (nominal atomic composition Ce80Ga20) showed that gallium(III) cations are homogenously distributed into the ceria matrix by substituting cerium(IV) cations of the fluorite-type structure of ceria. Au was added to the supports by the deposition–precipitation method using urea. High Au dispersions were achieved for all the fresh materials (D > 60%). The CO oxidation and the water gas shift (WGS) reaction were tested on the whole set of catalysts. All the supported-gold catalysts showed high activity for the CO oxidation reaction. However, those containing gallium in their formulation deactivated due to gold particle sinterization. Au(2%)/CeO₂ was the most active material for the WGS reaction, and the Au(2%)/Ce80Ga20 was as active as a Au(3%)/Ce68Zr32 catalyst for CO oxidation, and even more active than the reference catalyst of the World Gold Council, Au(2%)/TiO₂.     

Mesoporous silica supports for improved thermal stability in supported Au catalysts

In this work, we present a comprehensive review of our research on the role of mesoporous silica pore architecture, composition of the pore walls (addition of Co or Al), and silica surface chemistry (surface modification by TiO₂) to improve the hydrothermal stability of Au particles. We have found that mesoporous silica architecture plays an important role in improving Au stability, with three dimensional mesoporous architectures being less effective than one dimensional (1-D) pores. The tortuous 1-D pores in aerosol silica were found to be most effective at controlling Au particle size. Since Au particles continue to grow larger than the pore diameter, we conclude that Ostwald ripening must be the dominant sintering pathway for these Au catalysts. These catalysts are active for CO oxidation even after the Au particles have grown large enough to block the pores, suggesting that the thin walls of mesoporous silica provide easy access to gas phase molecules. Further improvements in Au stability and reactivity were obtained by surface modification of the aerosol and MCM-41 silica with TiO₂. After TiO₂ modification of the silica, the Au particles remained smaller than the pore size (< 3 nm) even after three cycles of CO oxidation at temperatures up to 400 °C.     

Selective Catalytic Reduction of NO x Over Nano-Sized Gold Catalysts Supported on Alumina and Titania and Over Bimetallic Gold–Silver Catalysts Supported on Alumina

The reduction of NO with octane under lean conditions was examined over gold supported on alumina and titania and over alumina supported bimetallic gold–silver catalysts. The silver loading was either 1.2 or 1.9 wt% whereas 0.3, 1 or 5 wt% gold was used. The catalysts were characterized by means of EDXS, N₂-adsortion, UV–Vis and TEM to correlate recorded results with different preparation methods. UV–Vis measurements indicated that gold was present in the form of fine Au particles, single Au ions and small (Au)_n ^δ+ clusters on the catalysts and silver was mainly present in the form of single Ag ions. The highest NO to N₂ reduction activity was recorded over the 0.3Au–Al₂O₃ catalyst. The Au–TiO₂ catalysts did not result in significant NO to N₂ reduction.     

A comparative study of the selective oxidation of NH3 to N2 over gold, silver and copper catalysts and the effect of addition of Li2O and CeOx

This paper describes the selective oxidation of ammonia into nitrogen over copper, silver and gold catalysts between room temperature and 400 °C using different NH₃/O₂ ratios. The effect of addition of CeO_x and Li₂O on the activity and selectivity is also discussed. The results show that copper and silver are very active and selective toward N₂. However the multicomponent catalysts: M/Li₂O/CeO_x/Al₂O₃ (M: Au, Ag, Cu) perform the best. On all three metal containing catalysts the activity and selectivity is influenced by the particle size and the interaction between metal particles and support.