Mechanistic Insight into the Styrene-Selective Oxidation on Subnanometer Gold Clusters (Au₁₆–Au₂₀, Au₂₇, Au₂₈, Au₃₀, and Au₃₂–Au₃₅): A Density Functional Theory Study

Abstract

We performed a comprehensive study of the reaction mechanism of styrene-selective oxidation to benzaldehye and styrene epoxide on subnanometer gold clusters with the cluster size ranges from around 0.4 to 1.0 nm via the density functional theory (DFT) calculation. The major focuses of the current study are the intrinsic catalytic selectivity and size-dependent activities of gold clusters toward styrene oxidations. The reaction selectivity of styrene oxidation over subnanometer gold clusters, e.g., selective formation to benzaldehyde or styrene epoxide, with the presence of dioxygen as the sole oxidant or the H2/O2 mixture as the reactant is discussed. A new reaction channel leading to the formation of a benzaldehyde product involving the formation of a metastable four-membered ring CCOO* intermediate is proposed, which explains the recent experimental observations of a high yield of benzaldehyde on ∼1.4 nm gold clusters. The effect of the charge state of gold clusters on the reaction selectivity and reaction rate is examined. The results indicated that the reaction selectivity is not affected by the charge state of the cluster by using the Au34– cluster as a benchmark model. However, the reaction rate of styrene oxidation is significantly increased on the anionic gold clusters caused by larger O2 adsorption energies, suggesting higher catalytic activity of anionic clusters. The mechanism of dramatic increase of product selectivity to styrene epoxide using H2 as the additive is explored as well. We find that the major role of the H2 additive is facilitating the dissociation of O2 into an active O atom on subnanometer gold clusters, which leads to high selectivity to the epoxide product. This systematic study enables a quantitative assessment of the size-dependent activity and selectivity of subnanometer gold clusters toward styrene-selective oxidation.