Structure, Redox Chemistry, and Interfacial Alloy Formation in Monolayer and Multilayer Cu/Au(111) Model Catalysts for CO2 Electroreduction

High-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD XAS) has been used to probe the geometric and electronic structure of a Cu monolayer model electrocatalyst as a function of applied potential in situ in alkaline electrolyte. In 0.01 M NaOH, a Cu monolayer deposited on an Au(111) single crystal exhibits markedly different redox behavior from Cu multilayers on the same substrate: the Cu monolayer is more stable against oxide formation and, at high potentials (E > 0.5 V vs RHE), undergoes a direct phase transition from Cu-0 to CuO rather than forming an intermediate Cu2O phase. The Cu monolayer in its metallic state at low potentials is expanded by 12.5% to match the substrate lattice constant, which can be expected to influence significantly the interaction of Cu surface atoms with intermediates of the electrochemical CO2 reduction. However, we also find that both strained Cu monolayer and thick, structurally relaxed Cu islands are unstable with respect to place-exchange with subsurface Au atoms. Such segregation phenomena need to be carefully considered and can limit the applicability of lattice strain as independent catalyst design parameter.