Oxygen reduction reaction on cobalt-(6)pyrrole cluster: Density functional theory study

We investigate the potential energy surface profile for various water formation reaction schemes on an unsupported cobalt-(6)pyrrole [Co-(6)Ppy] cluster in the vacuum state by density functional theory (DFT) calculations. We find that in the Co-(6)Ppy cluster, the formation of H ₂O ₂ is energetically not favorable. Instead of forming H ₂O _2ad, the HO _2ad + H reaction forms 2OH _ad or O _ad + H ₂O immediately. The adsorption of H ₂O ₂ on the Co-(6)Ppy cluster is possible only if the H ₂O ₂ molecule comes from or forms outside of the cluster. The formation of two OH molecules instead of H ₂O ₂ on the Co-(6)Ppy cluster suggests that the oxygen reduction reaction (ORR) mechanism on the unsupported Co-(6)Ppy cluster in the vacuum state prefers the direct four-electron reduction to water.