Gold catalysts, supported experimental

Gold catalysts containing NHC ligands can also promote cycloisomerisation reactions. Bicyclo[3.1.0]hexanes 137-139 can be prepared from the cycloisomerisation of 1,5-enynes bearing a propargyUc acetate (135) in the presence of catalytic amounts of [AuCl(lPr)]/AgBF (Scheme 5.36) [41]. The cycloisomerisation reaction of 135 occurs by a 1,3-OAc shift/aUene-ene cyclisation/l,2-OAc shift sequence. Experimental results with allenyl acetate 136 support this hypothesis as 139 is obtained in higher ratios than 137 and 138 [41b],... 

Tyrpical experimental procedures are as follows for DP method, which is used for the industrial production of gold catalysts. To an aqueous solution of H AuCU, the pH of which is 2-3, an aqueous solution of NaOH is added to adjust the pH at a fix point in the range of 6-10. In neutral or weakly basic solution AuClj" ion is transformed into AuCl (OH)4l ( = 4 0). To this solution support materials are immersed or dispersed. When the pH, concentration, and temperature are carefully adjusted, solid Au(OH)3 precipitate is deposited exclusively on the... 

Gold nanoparticles have received significant attention in recent years because of their unique catalytic activity [37 3]. Supported gold nanoparticles in the range of 2-5 nm are effective as catalysts for a variety of reactions including selective oxidation of propane to propylene oxide [44] and low-temperature CO oxidation [45]. Numerous experimental studies have focused on understanding the effect of particle size in gold catalysts however, the picture is often complicated by the lack of a monodisperse size distribution. 

The experimental results presented above clearly show that there are three important controlling factors which determine the catal3d ic properties of gold supported on metal oxides for oxidation. The first is the selection of suitable metal oxide supports. In the complete oxidation (combustion) of CO and nitrogen-containing organic compounds like trimethylamine gold catalysts are... 

The examples mentioned above require development of new chemical transformation processes. Associated with these processes is the need to develop new catalytic systems. Improved understanding of current catalytic systems would facilitate the scientific discovery of new ones. New experimental and computational techniques are needed. Insufficient understanding of catalysis in general is demonstrated by the system of supported gold catalysts Although these catalysts have a wide variation of reaction selectivity, there is little understanding of the nature of their active sites. 

Recent studies of ceria systems have focused on the development of synthetic approaches to make size-controlled and shape-controlled nanostructures. The thermolysis of an acidified Ce(N03)4 solution was reported as a method for the preparation of nanocrystalline CeOg ( 4 nm, surface area 180 m -g ). The deposition of gold on these nanocrystalline particles provides experimental evidence for the size effect of supports. It was shown that the activity of the resulting gold catalyst in CO oxidation was two orders of magnitude higher than that of catalysts prepared by co-precipitation or by Au deposition on a regular cerium oxide support (surface area 70 m -g ). 

Already in 1929 it was proposed by Schwab and Pietsch that the catalytic reaction on supported metal catalysts often takes place at the metal-oxide interface. This effect is known as adlineation, however, up to the present there is only little direct experimental evidence. In one example, the oxidation of CO on nanoscale gold, it is presently discussed whether the catalytic action takes place at the particle upport interface. Adlineation is strongly related to the effect of reverse spillover, where the effective pressure of the reactants in a catalytic process is enhanced by adsorption on the oxide material within the so-called collection zone and diffusion to the active metal particle (see Fig. 1.55 and also The Reactivity of Deposited Pd Clusters). The area of the collection zone and thus the reverse spillover are dependent on temperature, on the adsorption and diffusion properties of the reactants on the oxide material, as well as on the cluster density. 

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