Cu/ZnO(0001̄) and ZnOx/Cu(111): Model catalysts for methanol synthesis

In an attempt to understand the relative importance of the various constituents of copper-zinc oxide catalysts for methanol synthesis (2H₂ + CO → H₃COH), we have prepared and characterized a number of single-crystal surface structures of Cu-ZnO. The model catalysts have also been tested in terms of their activity for methanol synthesis. The growth of vapor-deposited Cu overlayers on a ZnO(0001̄) (O face) single crystal has been investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), He⁺ ion scattering spectroscopy (ISS) and low-energy electron diffraction (LEED). The results are consistent with a growth model in which, at room temperature, the first monolayer spreads uniformly across the surface in a p(1 × 1) structure. As more Cu is added, thick Cu(111) islands grow and these are separated by large regions of the p(1 × 1)-Cu monolayer. The Cu(111) islands are rotationally aligned with the ZnO substrate, and at high enough coverages grow together to cover the ZnO. Increasing temperature favors more agglomeration. A clean Cu(111) crystal and one containing a ZnO_x (x ⋍ 3) monolayer were also studied. None of these model catalysts gave rates of methanol production which were measurable in our present experimental limits (TOF < 2 × 10⁻³ molecules site⁻¹ s⁻¹) at 500–600 K and CO + H₂ pressures up to 1500 Torr. Under these “reaction” conditions, the Cu in direct contact with ZnO may be slightly oxidized; all the other Cu is completely metallic. The Cu does not change its character between deposition and reaction conditions, even if heavily oxidized to CuO in between. The addition of CO₂ at very high levels under reaction conditions does not change the character of a Cu(111) model catalyst surface, and no surface oxygen is ever observable after treatment under reaction conditions.