Insights into the function of silver as an oxidation catalyst byab initioatomistic thermodynamics

Abstract

To help understand the high activity of silver as an oxidation catalyst, e.g., for the oxidation of ethylene to epoxide and the dehydrogenation of methanol to formaldehyde, the interaction and stability of many different oxygen species at the Ag(111) surface has been studied for a wide range of coverages. Through calculation of the free energy, as obtained from density-functional theory and taking into account the temperature and pressure via the oxygen chemical potential, we obtain the phase diagram of O/Ag(111). Our results reveal that a thin surface-oxide structure is most stable for the temperature and pressure range of ethylene epoxidation and we propose it (and possibly other similar structures) contains the species actuating the catalysis. For higher temperatures, low coverages of chemisorbed oxygen are most stable, which could also play a role in oxidation reactions. For temperatures greater than about 775 K there are no stable oxygen species, except for the possibility of O atoms adsorbed at undercoordinated surface sites (i.e., imperfections, defects). At low temperatures $(\ensuremath{\lesssim}400\mathrm{K}$ at atmospheric pressure), provided kinetic limitations can be overcome, thicker oxidelike structures are predicted. Due to their low thermal stability, however, they can be ruled out as playing an important role in the heterogeneous reactions under technical conditions. Bulk dissolved oxygen and a molecular ozonelike species adsorbed at a surface vacancy, as have been proposed in the literature, are found to be energetically unfavorable. The employed theoretical approach for calculating free energies and predicting the lowest energy structures in contact with species in the environment (``ab initio, atomistic thermodynamics''), affords investigation of a system that seamlessly connects standard $(T=0\mathrm{K})$ density-functional theory results, characteristic of ``typical'' theoretical surface science studies, through to those valid for the conditions of catalysis. Though the error bar of the noted theoretical temperatures is noticeable $(\ifmmode\pm\else\textpm\fi{}\ensuremath{\approx}55\mathrm{K}),$ the identified trends and physical descriptions are useful.