Differences in intermediate structures and electronic states associated with oxygen adsorption onto Pt, Cu, and Au clusters as oxygen reduction catalysts

We used ab initio molecular orbital (MO) calculations to study the differences in the intermediate structures and the electronic states involved in the adsorption of O₂ onto 13-atom metal clusters of Pt, Cu, and Au. Additionally, the conditions required for the electrocatalytic oxygen reduction reaction (ORR) on the Pt, Cu, and Au clusters were investigated and discussed. The intermediates involved in O₂ adsorption onto Pt, Cu, and Au were found to be (Pt–O)–(Pt–O), Cu–O, and Au–O₂, respectively. The differences in the O₂ adsorption intermediates is explained on the basis of our analysis of the projected density of state (PDOS) area of the new MOs produced from a mixture of the 2 pπ ^* orbitals of O₂ and the d orbitals of the metal clusters. The formation of the (Pt–O)–(Pt–O) intermediate after the adsorption of O₂ onto the Pt cluster is attributed to the emergence of an antibonding orbital above the Fermi level. Thus, this electronic state can lead to the decomposition and desorption of O₂ molecules, thereby promoting the high-activity level of ORR. For the Cu cluster, a new antibonding orbital was observed below the Fermi level. Moreover, the Cu cluster surface can only promote O₂ decomposition and not O₂ desorption due to the formation of copper oxides. For the Au cluster, no new MOs related to 2 pπ ^* orbitals of O₂ appeared because O₂ was molecularly adsorbed, implying that the Au cluster is an inefficient ORR catalyst. (paper)