A Comparative Study of Size Effects in the Au-Catalyzed Oxidative and Non-Oxidative Dehydrogenation of Benzyl Alcohol

A hydrotalcite-supported gold catalyst has been found to be efficient for both oxidative and non-oxidative conversions of benzyl alcohol into benzaldehyde. In both cases, the reaction rates were dependent on the size of the Au particles, but the size dependence for the non-oxidative reaction was more pronounced. Our analyses on the intrinsic rates of different sites suggested that all of the atoms on the Au surfaces participated in the oxidative reaction, whereas the edge and corner Au atoms predominantly contributed to the non-oxidative reaction, and that the terrace atoms were at least two orders of magnitude less active than the edge or corner atoms. In both cases, the rate-determining step was C-H bond cleavage. The presence of oxygen significantly enhanced the reaction rate. Herein, we propose that the non-oxidative reaction proceeds through a beta-H elimination step by the low-coordination-number edge and corner Au atoms, whereas the active oxygen species, which are even generated on the terrace Au atoms, might assist C-H bond cleavage under oxidative conditions.