Au/γ-Al2O3 catalysts for glycerol oxidation: the effect of support acidity and gold particle size

In this study, the effect of gold particle size and surface acidity were investigated in the partial oxidation of glycerol, particularly towards the formation of lactic acid over gold catalysts supported on acidified γ-Al2O3. Alumina-supported gold catalysts were prepared at an optimal pH of approximately 7.0 and tested for the partial oxidation of glycerol. To acidify the catalyst samples, MoO3 or WO3 monolayer was introduced onto the alumina support before gold was deposited. It was observed that under the reaction conditions employed, small gold nanoparticles (ca 4 nm) showed a tendency towards zeroth order kinetics for the oxidation of glycerol. However, bigger gold nanoparticles (ca 20 nm) were inclined towards first order kinetics. It was also established that the nature of the reducing agent affected the size and morphology of the supported gold nanoparticles on γ-alumina and consequently the product spectrum. Catalysts with smaller gold nanoparticles formed predominantly C-3 compounds (glyceric, tartronic and lactic acids) while those with bigger gold nanoparticles formed predominantly C-1 compound (formic acid). Increased Lewis acidity was also found to play a positive role in the formation of lactic acid over γ-alumina-supported gold catalyst.