Size and structure effects of PtN (N = 12 - 13) clusters for the oxygen reduction reaction: First-principles calculations.

Size and structure effects on the oxygen reduction reaction on PtN clusters with N = 12-13 atoms have been investigated using periodic density functional theory calculations with the generalized gradient approximation. To describe the catalytic activity, we calculated the O and OH adsorption energies on the cluster surface. The oxygen binding on the 3-fold hollow sites on stable Pt12-13 cluster models resulted more favorable for the reaction with O, compared with the Pt13(Ih) and Pt55(Ih) icosahedral particles, in which O binds strongly. However, the rate-limiting step resulted in the removal of the OH species due to strong adsorptions on the vertex sites, reducing the utility of the catalyst surface. On the other hand, the active sites of Pt12-13 clusters have been localized on the edge sites. In particular, the OH adsorption on a bilayer Pt12 cluster is the closest to the optimal target; with 0.0-0.2 eV weaker than the Pt(111) surface. However, more progress is necessary to activate the vertex sites of the clusters. The d-band center of PtN clusters shows that the structural dependence plays a decisive factor in the cluster reactivity.