Ag-Cu alloy surfaces in an oxidizing environment: a first-principles study

TL;DR

This study uses first-principles calculations to analyze the stability and surface structures of Ag-Cu alloys in oxygen environments, explaining experimental observations and identifying stable configurations relevant for catalysis.

Contribution

It provides a comprehensive first-principles analysis of Ag-Cu alloy surfaces in oxygen, including phase diagrams and stability of copper oxide layers, extending previous experimental insights.

Findings

Copper segregates to the surface in oxygen environments.

A stable CuO-like layer can form on silver surfaces.

Surface structures depend on temperature, pressure, and orientation.

Abstract

Recent experiments on model catalysts have shown that Ag-Cu alloys have improved selectivity with respect to pure silver for ethylene epoxidation. In this paper we review our first-principles investigations on the (111) surface of this alloy and present new findings on other low index surfaces. We find that, for every surface orientation, the presence of oxygen leads to copper segregation to the surface. Considering the alloy to be in equilibrium with an oxygen atmosphere and accounting for the effect of temperature and pressure, we compute the surface free energy and study the stability of several surface structures. Investigating the dependence of the surface free energy on the surface composition, we construct the phase diagram of the alloy for every surface orientation. Around the temperature, pressure and composition of interest for practical applications, we find that a limited number of structures can be present, including a thin layer of copper oxide on top of the silver surface and copper-free structures. Different surface orientations show a very similar behavior and in particular a single layer with CuO stoichiometry, significantly distorted when compared to a layer of bulk CuO, has a wide range of stability for all orientations. Our results are consistent with, and help explain, recent experimental measurements.