Microscopic Investigation of H₂ Reduced CuO_<i>x</i>/Cu(111) and ZnO/CuO_<i>x</i>/Cu(111) Inverse Catalysts: STM, AP-XPS, and DFT Studies

Abstract

Understanding the reduction mechanism of ZnO/CuOx interfaces by hydrogen is of great importance in advancing the performance of industrial catalysts used for CO and CO2 hydrogenation to oxygenates, the water-gas shift, and the reforming of methanol. Here, the reduction of pristine and ZnO-modified CuOx/Cu(111) by H2 was investigated using ambient-pressure scanning tunneling microscopy (AP-STM), ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and density functional theory (DFT). The morphological changes and reaction rates seen for the reduction of CuOx/Cu(111) and ZnO/CuOx/Cu(111) are very different. On CuOx/Cu(111), perfect “44” and “29” structures displayed a very low reactivity toward H2 at room temperature. A long induction period associated with an autocatalytic process was observed to enable the reduction by the removal of chemisorbed nonlattice oxygen initially and lattice oxygen sequentially at the CuOx–Cu interface, which led to the formation of oxygen-deficient “5–7” hex and honeycomb structures. In the final stages of the reduction process, regions of residual oxygen species and metallic Cu were seen. The addition of ZnO particles to CuOx/Cu(111) opened additional reaction channels. On the ZnO sites, the dissociation of H2 was fast and H adatoms easily migrated to adjacent regions of copper oxide. This hydrogen spillover substantially enhanced the rate of oxygen removal, resulting in the rapid reduction of the copper oxide located in the periphery of the zinc oxide islands with no signs of the reduction of ZnO. The deposited ZnO completely modified the dynamics for H2 dissociation and hydrogen migration, providing an excellent source for CO2 hydrogenation processes on the inverse oxide/metal system