Insights into the function of silver as an oxidation catalyst by ab initio, atomistic thermodynamics

TL;DR

This study uses ab initio calculations to analyze oxygen species on Ag(111) surfaces, revealing that thin surface-oxide structures are most stable during ethylene epoxidation, providing insights into silver's catalytic activity.

Contribution

The paper introduces a phase diagram of oxygen on Ag(111) surfaces based on density-functional theory, identifying the most stable oxygen species relevant to catalysis.

Findings

Surface-oxide structures are most stable during ethylene epoxidation.

Chemisorbed oxygen dominates at higher temperatures, below 775 K.

Thicker oxide or dissolved oxygen are unlikely to be involved in catalysis.

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 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 under-coordinated surface sites Our calculations rule out thicker oxide-like structures, as well as bulk dissolved oxygen and molecular ozone-like species, as playing a role in the oxidation reactions.