Is Au55 or Au38 Cluster a Threshold Catalyst for Styrene Epoxidation?

Abstract

The styrene epoxidation on Au38 and Au55 clusters, which could be a benchmark to probe high catalytic activity of gold clusters, has been studied by using density functional theory. Results suggest that epoxidation proceeds via a surface oxametallacycle intermediate (OME structure). Two parallel reaction pathways coexist on the Au38 cluster: O2 dissociates before epoxidation and O2 directly reacts with styrene, whereas only the latter pathway is found on the Au55 cluster, which is induced by different geometries of the Au38 and Au55 clusters. The mechanism of O2 directly reacting is essentially determined by the electronic resonance between electronic states of adsorbed intermediates and Au atoms at reaction sites. Moreover, Au atoms correlated with the reaction on the Au38 cluster are more electropositive than those on the Au55 cluster, which leads to a higher catalytic activity of the former. Thus, the Au38 cluster should be the size threshold for epoxidation catalysis, being consistent with the obtained barrier values.