(96d) Selective Oxidation Reactions Catalyzed by Au In the Aqueous Phase
A variety of Au and Au-Pd catalysts were prepared and characterized to examine the role of support composition, gold metal particle size, water, Pd and OH- on oxidation reactions. X-ray absorption spectroscopy at the Au LIII edge revealed that as-prepared samples contained cationic Au that was reduced to a predominately metallic state following treatment in He at 623 K. Scanning transmission electron microscopy confirmed the Au particle sizes estimated by X-ray absorption spectroscopy. Interestingly, carbon-supported gold catalysts that were inactive for CO oxidation in the gas phase were very active in liquid water, especially at high pH. Likewise, selective oxidation of glycerol over Au/carbon in the aqueous phase was influenced substantially by hydroxyl concentration. However, peroxide was also detected in the aqueous phase oxidation reactions of both CO and glycerol. The presence of peroxide was correlated to C-C cleavage reactions during glycerol oxidation. The selectivity of glycerol oxidation to glyceric acid was higher over an Au-Pd catalyst compared to monometallic Au, presumably because of rapid decomposition of peroxide over Pd. The influence of gas-liquid-solid contacting was evaluated by comparing the product distribution from glycerol oxidation over Au/TiO2 in a batch autoclave reactor and a fixed bed upflow reactor. The unique hydrodynamics of the fixed bed system allowed for secondary oxidation products such as tartronic acid and oxalic acid to form in substantial amounts, which contrasts the product distribution observed in a batch system.