Methanol Synthesis and Reverse Water–Gas Shift Kinetics over Cu(110) Model Catalysts: Structural Sensitivity

The kinetics of simultaneous methanol synthesis and reverse water–gas shift from CO2/H2and CO2/CO/H2mixtures have been measured at low conversions over a clean Cu(110) single-crystal surface at pressures of 5.1 bar. Without CO, ∼8 × 10−3methanol molecules per second per Cu surface atom were produced at 530 K, with an activation energy of 67 ± 17 kJ/mol; and ∼5 CO molecules per second per Cu surface atom were produced, with an activation energy of 78 ± 14 kJ/mol. The rates, compared to previous rates on Cu(100) and polycrystalline copper foil, were higher in both methanol synthesis and CO production, indicating structural sensitivity. The activation energy for methanol synthesis was similar on all these planes, but smaller for reverse water–gas shift on Cu(110) than on polycrystalline Cu. The surface after reaction was covered by almost a full monolayer of adsorbed formate, but no other species like carbon or oxygen in measurable amounts. The addition of CO to the feed caused the rate to increase, and no buildup of tenacious carbon was observed following reaction. Postreaction TPD shows an interesting influence of CO on the nature of the adlayer. These results support a model where the active site for methanol synthesis on real Cu/ZnO catalysts is metallic Cu and suggest that the role of ZnO may be to maintain more of the metallic Cu in ultrathin islands that have (110)-like behavior.