Supported gold

Supported Gold. - When dispersed on a support, gold develops chemical reactivity for hydrogen and oxygen transfer reactions under relatively mild conditions. For example, oxygen transfer between CO and CO2 was observed 

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M. Haruta, A. Ueda, S. Tsubota, and R.M.T. Sanchez, Low-temperature catalytic combustion of methanol and its decomposed derivative over supported gold catalysts, Catalysis Today 29, 443-447 (1996). 

Homogeneous deposition of ultrafine metal particles on the surfaces of fine powder is not easy using PVD. A device for stirring the powder support in a vacuum chamber is needed to avoid heterogeneous deposition. Sputter deposition units equipped with stirring powder supports have already been adapted for the industrial production of Ti02 and carbon-supported gold catalysts by 3M [35]. 

Grinding conditions are modified by the nature of supports. Gold clusters with a diameter of 1.5 nm were obtained on an aluminum-containing porous coordination polymer by grinding in an agate mortar in air for 20 min, followed by the reduction in a stream of H2 in N2 at 120°C for 2 h ]50]. In contrast, ball milling is favorable... 

Supported gold catalysts are, in general, less active than platinum group metal catalysts in the complete oxidation of hydrocarbons however, by choosing... 

CO conversions over Au/Ce02 catalyst were measured in the dry and wet condition as shown in Fig. 1. Similar to other supported gold catalysts, Au/Ce02 catalyst showed higher CO conversions in the presence of water vapor than in the absence of it at the same temperature. Catalytic activities for CO oxidation over Au/Ce02 catalysts prepared at different calcinations temperature were compared in the dry and wet condition as shown in Fig. 2. Au/Ce02 catalyst calcined at 473 K showed the highest initial CO conversion in the absence of water vapor. However, the CO conversion decreased steadily and reached a steady-state value over this catalyst. 

Titanium dioxide supported gold catalysts exhibit excellent activity for CO oxidation even at temperatures as low as 90 K [1]. The key is the high dispersion of the nanostructured gold particles over the semiconducting Ti02 support. The potential applications of ambient temperature CO oxidation catalysts include air purifier, gas sensor and fuel cell [2]. This work investigates the effects of ozone pretreatment on the performance of Au/Ti02 for CO oxidation. 

The intense recent interest in supported gold catalysts has focused on small, nonuniform clusters (e.g., those supported on Ti02), which have been... 

In spite of the low affinity for binding to oxygen, gold(III) alkoxo, hydroxo and even 0X0 complexes have been obtained [6, 7]. These are valuable models for Au-O(H) species which are likely to be involved in oxidation reactions catalyzed by metal-oxide-supported gold [8]. All these complexes have displayed interesting chemical reactivity and, in some cases, remarkable catalytic activity. 

Density Functional Theory (DFT) has shown that 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 catal5dic 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. Also these calculations extended for the adsorption of O and CO on flat and... 

Correlation has been suggested between the morphology, electronic structure and catalytic properties of supported gold nanoparticles with special attention to the role of the substrate/interface behaviour. 

Figure 1. TEM image of a titania supported gold catalyst (1.7wt.% Au) prepared by deposition-precipitation (gold particle size = 5.3+ 0.3 nm, dispersion = 36%). (Reprinted from Reference [84], 2000, with permission from American Chemical Society).

Table 1. Changes of nanostructure and catalytic properties during acrolein hydrogenation in the gas phase after pretreatment of a Zr02 supported gold catalysF with H2 (taken from [85]).

Since Haruta s works in the 1980s [110] CO oxidation is one of the major applications of supported gold catalysts. Interestingly, this metal/substrate combination exhibits much more pronounced structure sensitivity than the Pd, Pt, and Ir catalyzed analogues [9]. 

Figure 9. CO conversion as a function of temperature for supported gold catalysts (a) Au/Zr02, (b) Au/Ti02 (both PVA protected) (Reprinted from Reference [24], 2006, with permission from American Chemical Society).

The liquid-phase oxidation of glycerol was carried out by using carbon-supported gold particles of different sizes (2.7 2 nm) which were prepared by a colloidal route [120]. Indeed, a particle-size effect was observed because the selectivity to glyceric acid was increased to 75% with smaller particle sizes (4)ptmimn = 3.7 nm). 

Size Control of Metal Nanoclusters in Supported Gold Catalysts... 

Figure 14. Reaction pathways for CO oxidation over supported gold catalysts [39].

Bio-ethanol is attracting growing interests in relation to the shift of raw materials from petroleum to biomass. A pioneering work by Christensen is that over MgAl203 support gold is much more selective to acetic acid than palladium and platinum in the aerobic oxidation of ethanol in water in a batch reactor. Figure 32 shows that selectivity to acetic acid exceeds 80% [99]. In contrast, Au/Si02 catalysts prepared by deposition reduction... 

Guerin S, Hayden BE, Pletcher D, Rendall ME, Suchsland J-P, WiUiams LJ. 2006b. Combinatorial approach to the study of particle size effects in electrocatalysis synthesis of supported gold nanoparticles. J Comb Chem 8 791-798. 

Hayden BE, Pletcher D, Suchsland J-P. 2007. Enhanced activity for electrocatal)4ic oxidation of carbon monoxide on titania-supported gold nanoparticles. Angew Chem Int Ed 46 3530-3532. 

Choudhary TV, Goodman DW. 2002. Oxidation catalysis by supported gold nano-clusters. Top... 

Date M, Okumura M, Tsubota S, Haruta M. 2004. Vital role of moisture in the catalytic activity of supported gold nanopaiticles. Angew Chem Int Ed 43 2129-2132. 

Guzman J, Gates BC. 2004. Catalysis by supported gold Correlation between catalytic activity for CO oxidation and oxidation states of gold. J Am Chem Soc 126 2672-2673. 

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