Influence of gold particle size on the aqueous-phase oxidation of carbon monoxide and glycerol

Carbon-supported Au particles with mean sizes ranging from 5 to 42 nm and unsupported Au powder were evaluated as catalysts in the aqueous-phase oxidation of CO and glycerol. For the aqueous-phase oxidation of CO at pH 14 and 300 K, the turnover frequency (TOF) for the 5-nm Au particles was 5 s−1, whereas the TOF for large supported Au (42 nm) and bulk Au were only 0.5 and 0.4 s−1, respectively. The observed rate of peroxide formation during CO oxidation also was much higher on the small Au particles. Oxidation of glycerol in the aqueous phase at 333 K and elevated pH over the same catalysts revealed a similar influence of particle size, with the 5-nm Au particles giving a TOF of 17 s−1 at pH 13.8 and the larger particles and bulk Au nearly an order of magnitude less active. However, large Au particles (>20 nm) were more selective to glyceric acid. The lower selectivity of small Au particles is attributed to a higher formation rate of H2O2 during glycerol oxidation, because peroxide promotes CC cleavage reaction.