Origin of the cluster-size effect in the hydrogenation of cinnamaldehyde over supported Au catalysts

Gold nanoclusters supported on γ-Al2O3 were more active and selective than platinum nanoclusters in the high-pressure liquid-phase hydrogenation of cinnamaldehyde to cinnamylalcohol, while in the hydrogenation of cyclohexene gold was less active than platinum. The differences in catalytic performance are ascribed to the weaker interaction of gold with the reactants and products compared to platinum. Gold clusters with a diameter below 2 nm are essential to obtain a high activity and selectivity in the hydrogenation of cinnamaldehyde. The size dependence of the selectivity originates from the stronger dependence of the CO hydrogenation rate on cluster size compared to the CC hydrogenation rate. Small clusters exhibit an enhanced π backbonding, which favors CO adsorption over CC adsorption.