Simulation of pathways for CO oxidation over Au nano-clusters by paired interacting orbitals (PIO) analysis

CO oxidation pathways over Au nano-clusters, such as face centered cubic Au14, icosahedron Au13, cubo-octahedron Au13 and tetrahedron Au20 were investigated by using PIO analysis proposed by Fujimoto et al. Four types of oxidation pathways were examined: reaction between (a) adsorbed CO and gaseous O2 (E-R mechanism), (b) adsorbed O2 and gaseous CO (E-R mechanism), (c) adsorbed CO and vicinally adsorbed O2 (L-H mechanism), and (d) adsorbed CO and geminally adsorbed O2 (L-H mechanism). Only in the case of L-H mechanism, when CO molecule and O2 molecule are adsorbed on adjacent two surface Au atoms in vicinal manner (c), CO oxidation occurs favorably. In comparison of the vicinal CO and O2 co-adsorption on icosahedron Au13 with that on cubo-octahedron Au13, CO oxidation is favorable in the former, whereas it is unfavorable in the latter because of considerably longer distance between CO and O2. It is also predicted that CO oxidation is also favorable in the case of vicinally co-adsorbed state of CO and O2 on tetrahedron Au20. The present work has shown that the PIO analysis based on extended Hückel MO is an effective way to investigate reaction pathways, especially in large catalytic systems.