First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment

TL;DR

This study uses first-principles calculations to explore how oxygen influences the surface composition and stability of Ag-Cu alloys, providing insights into their catalytic behavior in ethylene epoxidation.

Contribution

It presents a detailed phase diagram of Ag-Cu alloy surfaces under various conditions, revealing the formation of different surface structures relevant to catalysis.

Findings

Copper segregates to the surface in oxygen-rich environments.

Multiple surface structures, including copper oxide layers, are stable under industrial conditions.

Results align with recent experimental observations.

Abstract

In this paper we investigate by means of first-principles density functional theory calculations the (111) surface of the Ag-Cu alloy under varying conditions of pressure of the surrounding oxygen atmosphere and temperature. This alloy has been recently proposed as a catalyst with improved selectivity for ethylene epoxidation with respect to pure silver, the catalyst commonly used in industrial applications. Here we show that the presence of oxygen leads to copper segregation to the surface. Considering the surface free energy as a function of the surface composition, we construct the convex hull to investigate the stability of various surface structures. By including the dependence of the free surface energy on the oxygen chemical potential, we are able compute the phase diagram of the alloy as a function of temperature, pressure and surface composition. We find that, at temperature and pressure typically used in ethylene epoxidation, a number of structures can be present on the surface of the alloy, including clean Ag(111), thin layers of copper oxide and thick oxide-like structures. These results are consistent with, and help explain, recent experimental results.