Gold catalytic oxidation

Eigure 3.9 shows temperatures for 50% conversion (T o) of CH3OH and its decomposed derivatives over Pt/y-Al203, Pd/y-Al203, and Au/a-Pe203 catalysts [52]. Eor MeOH oxidation, palladium is more active than platinum, while gold lies in between. These three catalysts are similarly active for the oxidation of HCHO and HCOOH. Catalytic oxidation at temperatures below 0°C can proceed over palladium and platinum for H2 oxidation, while it happens over gold for CO oxidation. 

In the catalysis community, there is considerable interest in the catalytic properties of oxide-supported nanocrystalline gold, which has been found to be remarkably active for the oxidation of CO [Hamta, 1997]. In electrochemistry, the ability of gold to oxidize CO, in the absence of an oxide support, has been known for many years [Roberts and Sawyer, 1964],... 

Scire S, Minico S, Crisafulli C, Satriano C, Pistone (2003) Catalytic combustion of volatile organic compounds on gold/cerium oxide catalysts. Appl Catal B Environ 40(1-8) 43 19... 

Oliveira, R.L., Kiyohara, P.K. and Rossi, L.M. (2010) High performance magnetic separation of gold nanoparticles for catalytic oxidation of alcohols. Green Chemistry, 12 (1), 144-149. 

As dehydrogenases (DH) are widely distributed enzymes, a number of studies have been carried out with these biocatalysts. For example, Willner el al. [20] have used a PQQ-monolayer functionalized gold electrode for the catalytic oxidation of NADH in the presence of Ca2+. In this scheme, the pyrrolo-quinoline quinine co-factor, PQQ, was covalently linked, as before for the GOx system [15, 20, 21], to the Au electrode,... 

C.X. Cai, H.X. Ju, and H.Y Chen, Catalytic oxidation of reduced nicotinamide adenine dinucleotide at a microband gold electrode modified with nickel hexacyanoferrate. Anal. Chim. Acta 310, 145-151 (1995). 

W. R Yan, B. Chen, S. M. Mahurin, S. Dai, and S. H. Overbury, Brookite-supported highly stable gold catalytic system for CO oxidation, Chem. Commun. 1918-1919 (2004). 

Figure 3.1 Four important conditions for catalysis by gold (metal-oxide junction, water, OH- and size of the Au particles or tubes). The figure by each circle is the diameter (nm) of the gold particles or tubes. Gold turns out to be catalytically very active, provided that at least two of the four conditions are fulfilled. For example, in CO oxidation at room temperature even unsupported gold is active in the presence of alkaline (OH-) water (H20).

Gold-catalyzed oxidation of styrene was firstly reported by Choudhary and coworkers for Au NPs supported on metal oxides in the presence of an excess amount of radical initiator, t-butyl hydroperoxide (TBHP), to afford styrene oxide, while benzaldehyde and benzoic acid were formed in the presence of supports without Au NPs [199]. Subsequently, Hutchings and coworkers demonstrated the selective oxidation of cyclohexene over Au/C with a catalytic amount of TBHP to yield cyclohexene oxide with a selectivity of 50% and cyclohexenone (26%) as a by-product [2]. Product selectivity was significantly changed by solvents. Cyclohexene oxide was obtained as a major product with a selectivity of 50% in 1,2,3,5-tetramethylbenzene while cyclohexenone and cyclohexenol were formed with selectivities of 35 and 25%, respectively, in toluene. A promoting effect of Bi addition to Au was also reported for the epoxidation of cyclooctene under solvent-free conditions. 

However, the removal of carbon monoxide by water-gas shift to a low level still demands its selective oxidation to the minimum concentration possible. Much research and development has been conducted during the past decades to find a gold catalyst that can do this the target is usually described by the acronym PROX (preferential oxidation), but sometimes as SCO (selective catalytic oxidation). The task is somewhat simplified by the constraints that are externally imposed the preferred feed gas, often termed idealised reformate, has the composition 1.0% CO, 1.0% 02, 75.0% H2, balance nitrogen or other inert gas, and while of course variations to this composition can be made to explore the kinetics and mechanism, and the effects of the products water and carbon dioxide can be added to observe their effects, the successful catalyst must remove almost all the carbon monoxide (to <10 ppm) and less than 0.5% hydrogen. This requirement is expressed as a selectivity based on the percentage of the oxygen consumed that is taken by the carbon monoxide this should exceed 50%, under conditions where the conversion of carbon monoxide is above 99.5%.5... 

A gold-based material has been formulated for use as a three-way catalyst in gasoline and diesel applications.28 This catalyst, developed at Anglo American Research Laboratories in South Africa, consisted of 1% Au supported on zirconia-stabilized-Ce02, ZrC>2 and TiC>2, and contained 1% CoOx, 0.1% Rh, 2% ZnO, and 2% BaO as promoters. The catalytically active gold-cobalt oxide clusters were 40-140 nm in size. This catalyst was tested under conditions that simulated the exhaust gases of gasoline and diesel automobiles and survived 773 K for 157 h, with some deactivation (see Section 11.2.7). 

Bioelectrocatalytic properties were obtained for FDH at carbon and gold and platinum electrodes [111,113]. The catalytic oxidation current of FDH-modified carbon paste electrodes approached a maximum value at 4-0.5 V vs. Ag/AgCl. At this potential and under optimum conditions, i.e., pH 4.5, fi uctose can be measured between 0.2 and 20 mM in fi uit juices. Most importantly the fructose sensor was insensitive to ambient oxygen. 

About the only catalytic use for silver is in the conversion of ethylene to ethylene oxide by the action of oxygen.42 This reaction is very specific. Other alkenes are oxidized to CO2 and H2O with no epoxide formation observed. Gold is generally inactive as a catalyst for most reactions but it has found some use in catalytic oxidations. 43... 

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