Dynamic Metal–Semiconductor Electrical Interfaces in Model Cu|ZnO Hydrogenation Catalyst Structures

Cu/ZnO/Al2O3 catalysts are commonly used for methanol synthesis; yet, the chemical state of the Cu|ZnO interface remains debated. Recent studies have suggested that interfacial fields and junction effects could explain observed shifts in binding energies of intermediates (e.g., CO2δ− and HCOO−) involved in the reduction of CO2 to methanol on Cu|ZnO. In this study, we probe Cu|ZnO interfacial chemistry by measuring junction electrical characteristics under N2, CO2, and H2 gas mixtures from 50 to 250 °C and up to 10 bar. The pristine interface behaves as a nonideal rectifying Schottky diode, and H2 exposure increases n-type donor density in the ZnO and lowers Schottky-barrier height, with full transition to an ohmic junction. These observations imply the electrostatic potential gradient at the putative active-catalyst interface could change under reaction conditions, depending on the precise composition of the oxide phase. H-doping of ZnO is reversed in a N2 or CO2 stream at 200 °C without any O2, inconsistent with proposed Cu–Zn alloying or oxygen-vacancy formation mechanisms suggested in studies of methanol synthesis on Cu/ZnO/Al2O3 catalysts, and instead suggests reversible hydrogen doping of ZnO. The addition of CO2 reduces the rate of H doping into ZnO, while water addition strongly suppresses it. The time scale for the H-doping of Cu|ZnO devices by H2 is much longer than that estimated for H diffusion in bulk ZnO, suggesting that H2 dissociation and/or H insertion into ZnO are the elementary steps governing the rate of H doping of the ZnO. Junction electrical measurements can thus inform how electro-active interfaces invoked in catalysis evolve under reaction conditions.