Optimum Particle Size for Gold-Catalyzed CO Oxidation

The structure sensitivity of gold-catalyzed CO oxidation is presented by analyzing in detail the dependence of CO oxidation rate on particle size. Clusters with less than 14 gold atoms adopt a planar structure, whereas larger ones adopt a three-dimensional structure. The CO and O-2 adsorption properties depend strongly on particle structure and size. All of the reaction barriers relevant to CO oxidation display linear scaling relationships with CO and O-2 binding strengths as main reactivity descriptors. Planar and three-dimensional gold clusters exhibit different linear scaling relationship due to different surface topologies and different coordination numbers of the surface atoms. On the basis of these linear scaling relationships, first-principles microkinetics simulations were conducted to determine CO oxidation rates and possible rate-determining step of Au particles. Planar Au-9 and three-dimensional Au-79 clusters present the highest CO oxidation rates for planar and three-dimensional clusters, respectively. The planar Au-9 cluster is much more active than the optimum Au-79 cluster. A common feature of optimum CO oxidation performance is the intermediate binding strengths of CO and O-2, resulting in intermediate coverages of CO, O-2, and O. Both these optimum particles present lower performance than maximum Sabatier performance, indicating that there is sufficient room for improvement of gold catalysts for CO oxidation.