Preparation and Structural Characterization of ZrO2/CuOx/Cu(111) Inverse Model Catalysts

CO2 hydrogenation to methanol is regarded as a promising reaction to catalytically convert a major greenhouse gas (CO2) into a value-added product (methanol). In the current study, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were applied to investigate the growth mode of low coverages (<0.2 ML) of ZrO2 in an inverse ZrO2/CuOx/Cu(111) system, which has the potential to achieve high selectivity for a direct CO2-to-methanol transformation. It was found that the morphology of ZrO2 was strongly affected by the preparation method. The ZrO2/CuOx/Cu(111) model catalyst prepared by the oxidation at 600 K of Zr predeposited on Cu(111) exhibited substantial mixing of ZrO2 and CuOx. In contrast, the direct deposition of Zr under an O-2 ambient over CuOx/Cu(111) at 600 K produced small ZrO2 islands (10-12 nm in size) with a two-dimensional structure (i.e., only one layer of ZrO2). XPS studies indicate that both preparation methods lead to ZrO2/CuOx/Cu(111) surfaces. The model catalyst prepared by the direct deposition of Zr in an O-2 ambient was annealed up to 700 K in an ultrahigh vacuum. Both STM and XPS results suggest no apparent change in ZrO2, while CuOx was reduced at such annealing conditions. The island size of 10-12 nm observed for ZrO2 on Cu(111) is much smaller than those seen for CeO2 (30-50 nm) and ZnO (300-500 nm) on the same substrate, opening the possibility for unique chemical properties.