The behavior of inverse oxide/metal catalysts: CO oxidation and water-gas shift reactions over ZnO/Cu(111) surfaces

There is a general desire to improve the configuration of industrial catalysts by taking advantage of the intrinsic properties of metal oxides. On an inverse oxide/metal catalyst, the reactants can interact with defect sites of the oxide nanoparticles, metal sites, and the metal-oxide interface. In this study, we have investigated the oxidation of carbon monoxide (CO + 0.5O₂ → CO₂) and the water-gas shift (WGS, CO + H₂O → H₂ + CO₂) reaction on a series of ZnO/Cu(111) surfaces prepared in different ways. Oxidation of a Zn-Cu(111) alloys with O₂ at 460 K produces ZnO/CuO_x/Cu(111) systems where the size of the ZnO islands is in the range of 5-20 nm. These systems are highly active for the oxidation of CO at moderate temperatures (400-500 K) but have problems of stability when performing the water-gas shift at temperatures above 550 K. ZnO/CuO_x/Cu(111) surfaces prepared by vapor deposition of Zn at 600 K in O₂ exhibit islands of ZnO which are extremely large (400-500 nm) and contain a minimum of 3-4 layers of the oxide. These large islands of ZnO are not as chemically active as the small ZnO clusters yet they have high stability and produce ZnOsbnd Cu interfaces which are efficient as catalysts for the water-gas shift reaction at 550-625 K.