Dissociative Adsorption of H2 on PtRu Bimetallic Surfaces

Abstract

We present a theoretical study of the dissociative adsorption of molecular hydrogen on PtRu bimetallic surfaces based on density functional theory (DFT) calculations. We focus on the reactivity of a pseudomorphic Pt monolayer deposited on Ru(0001), Pt1ML/Ru(0001), for which we have obtained a minimum activation energy barrier for H2 dissociation, Eb = 0.32 eV, i.e., ∼0.3 and 0.26 eV higher than on Ru(0001) and Pt(111), respectively. Accordingly, the initial sticking probability for low energy impinging molecules (Ei ≲ 0.1 eV) derived from classical trajectory calculations is various orders of magnitude smaller than on Ru(0001) in apparent contradiction with available experimental data. However, undercoordinated Pt atoms in the borders of Pt pseudomorphic islands and isolated Ru atoms or small Ru aggregates in a Pt-rich two-dimensional PtxRu1-x surface alloy provide active sites allowing nonactivated H2 dissociative adsorption. These two possible defects in a full pseudomorphic Pt monolayer deposited on Ru(0001) might be responsible for the relatively high initial sticking probabilities obtained experimentally for ∼1.0-1.2 ML of Pt evaporated over Ru(0001), only ∼30-65% smaller than on Ru(0001).