Supported gold and silver

The current range of applications for which supported gold catalysts can be used cannot currendy be described as broad, although 

Unfortunately, this requirement for the presence of aqueous base may well represent a strong limitation to the use of supported gold catalysts, as various base-catalysed side reactions such as keto-enol tautomerism or oxidative decarbonylation could diminish selectivities. 

Supported silver catalysts are relatively commonly used in gas phase oxidations of alcohols. Benzyl alcohol can be selectively oxidised to benzaldehyde using a 0.6% Ag/pumice catalyst with 100% selectivity, although its activity is less than a similar Pd material. However, a mixed Pd-Ag/pumice bimetallic increases the activity whilst retaining the 100% selectivity to benzaldehyde. The authors of this study concluded that the role of the Pd was to activate the substrate whereas the highly dispersed silver particles served to activate the oxygen. Hence the mechanism was one of cooperation between the Ag° and Pd sites the alloy phase, detected by EXAFS, was not considered to play an important role. 

In recent years, the unexpected observation of highly active Au as a low temperature CO oxidation catalyst59,60 has initiated extensive research activity into the use of supported gold for liquid phase oxidation reactions. In general, the adsorption characteristics and catalytic properties of Au depend crucially on particle size, which can be controlled by the preparation method and the support.61-66 The crucial question involving gold catalysis, which as yet has not been fully answered, is the concept of why Au nanoparticles exhibit such radically different behaviour than bulk Au.67-70 

The reaction pathway for glycerol oxidation, shown in Fig. 9.7 is complex and a considerable challenge exists if high selectivities to glyceric acid are to be obtained. 

Supported Pt and Pd were shown to yield some selectivity in batch conditions using air as oxidant, but the principle products were the unwanted single carbon species such as CO2, HCHO and HCOOH. Under these conditions supported Au catalysts were totally inactive. Using pure oxygen and 3 bar pressure, Au became active, and the formation of Cl by-products was eliminated when NaOH was added. Using 1 wt% Au supported on either graphite or activated carbon, 100% selectivity to glyceric acid was 

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Sarina S., Waclawik E. R., and Zhu H. Photocatalysis on supported gold and silver nanoparticles under ultraviolet and visible light irradiation. Green Chemistry. 15 no. 7... 

Figure4.17 aAnelectron beam hittingthecatalystsurface sets off several processes the electrons that are transmitted through the sample are used for generating b the TEM image (showing gold and silver nanoparticles supported on y-alumina [98]).

Figure 1.3. (A) Photographs of silver island films (SIFs) deposited onto glass and plastic supports. (B) Normalized absorbance of zinc, copper, gold and silver nanostructured particles on a glass support. Atomic force microscope images of SIFs on (C) glass (D) plastic support.

The fluorine chemistry of gold and silver in anhydrous hydrogen fluoride was carried out with co-workers G. M. Lucier, S.FI. Elder, and J.M. Whalen with the support of the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Science Division of the U.S. Department of Energy under Contract Number DE-AC-03-76SF00098. 

RGO form stable cryogels suitable for unidirectional ice crystallization [173, 221, 223]. As-obtained 3D porous monoliths are good supports for the nanocrystaUine gold and silver catalysts [173]. 

Direct Oxidation of Propylene to Propylene Oxide. Comparison of ethylene (qv) and propylene gas-phase oxidation on supported silver and silver—gold catalysts shows propylene oxide formation to be 17 times slower than ethylene oxide (qv) formation and the CO2 formation in the propylene system to be six times faster, accounting for the lower selectivity to propylene oxide than for ethylene oxide. Increasing gold content in the catalyst results in increasing acrolein selectivity (198). In propylene oxidation a polymer forms on the catalyst surface that is oxidized to CO2 (199—201). Studies of propylene oxide oxidation to CO2 on a silver catalyst showed a rate oscillation, presumably owing to polymerization on the catalyst surface upon subsequent oxidation (202). 

Figure 24. Size of metal particles and support pores in silver and gold catalysts for gas-phase propylene epoxidation.

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