Model Cu/ZnO Catalysts for Water-Gas Shift and Methanol Synthesis Based on Single Crystals

Abstract

Abstract We have prepared model catalysts based on Cu(111) and Cu/ZnO(0001) single crystal surfaces. These have been structurally and kinetically characterized using a combination of ultrahigh vacuum surface analyses (XPS, AES, ISS, LEED, TDS) before and after medium-pressure (<2 atm.) rate measurements of the water-gas shift (WGS) and methanol synthesis reactions. Specific reaction rates for WGS on clean Cu(111) and ZnOx-dosed Cu(111) indicate that the dissociative adsorption of H2O is the rate-determining step under our reaction conditions, and that metallic Cu is the active site. The interaction of H2S with Cu(111) and the consequent poisoning of WGS catalysis has also been studied. Cesium addition to Cu(111) causes a 15-fold increase in the WGS rate at a coverage of one Cs atom per ~8 Cu surface atoms. This is attributed to direct participation of a surface CsOH species in H2O dissociation. Vapor-deposited Cu overlayers on the oxygen-terminated ZnO(0001) surface show no uniquely high activity for methanol synthesis. The interaction of CO2 with Cu(111) indicates that CO2 is unlikely to produce any significant coverage of adsorbed oxygen or Cu oxides on the metallic Cu surface under methanol synthesis conditions. In addition, the electronic structure of Cu in and on ZnO clusters and the bonding mechanisms of small molecules on Cu and ZnO clusters have been studied using quantum chemical (INDO) methods.