The Valency Effect on Reaction Pathways in Heterogeneous Catalysis: Insight from Density Functional Theory Calculations

Abstract

Density functional theory (DFT) has been used to determine reaction pathways for several reactions taking place on Pt and Cu surfaces. On Pt(111) all the elementary steps in the hydrogenation of carbon to methane, nitrogen to ammonia, and oxygen to water were investigated. Also on Pt(111) the reactions of C with O and N were studied, and on Cu(111) the reaction of C with H was examined. It is found that reaction pathways are related to the valency of the reactants involved. As the valency of a reactant decreases so too does the coordination number of the site at which the transition state is located. In the hydrogenation reactions on Pt(111) it was found that monovalent adsorbates (for example, CH₃, NH₂ and OH) tend to make transition states close to top sites; divalent adsorbates (CH₂, NH and O) near bridge sites; and tri- and tetra-valent adsorbates (CH, N and C) near three-fold hollow sites. This relationship between the transition state and the valency applies equally to the C+O and C+N reactions on Pt(111), the C+H reaction on Cu(111), and many others from the literature. On this basis, we have developed rules to facilitate the prediction of catalytic reaction pathways without recourse to experimental or theoretical investigations.