Hydrogen adsorption and diffusion on Pt {111} and PtSn {111}

Abstract

PtSn {111} forms two ordered surface alloys namely p(2 × 2) and (√3 × √3R30°) and such bimetallic catalysts are often used in industrially catalysed reactions, such as hydrogenations, to improve performance although the reasons behind the improvements are generally not well understood. In this study density functional theory calculations have been performed for hydrogen adsorption at various sites on Pt {111} and the ordered PtSn {111} surfaces in order to characterise the differences between the pure and doped surfaces. The structural, bonding, electronic and vibrational characteristics of each system are presented indicating that there are no significant bonding differences when adsorption occurs at platinum sites on the surface. Where adsorption occurs at a tin site on the doped surface the adsorption energy is negative or the adsorption site changes to one involving only platinum atoms. Nudged elastic band calculations were used to determine the barrier to diffusion of the hydrogen atoms on the surface. The diffusion calculations show significant differences in the mobility of hydrogen providing a possible explanation for the changes in catalytic action on tin doping of platinum.