Junctions between CuOx and ZnOy in sensors for CO and catalystsfor CO hydrogenation

Abstract

Addition of CuOxto defective ZnOy(0.92<y<1.00) yielded a dispersed catalyst that produced methanol at 523 K and 2 MPa with a selectivity (relative to CH4) which was higher than that for a unit area of ZnOyor CuOxalone. Copper oxide–zinc oxide polycrystalline films (prepared by ion beam sputter deposition) were as stoichiometric as the dispersed samples with junctions best represented as CuOx/ZnOy(withx close to 0.5 and y close to 1.0). Photo-STM reveals for the first time the width and nature of the n/p junction, although the gradient of the depletion zone is shallower on the ZnO side of the junction (which is not expected from recent postulates). Responses to CO and H2 were more representative of CuOxthan ZnOy. The properties of such junctions and their role in CO adsorption, catalysis and sensing is discussed, together with the relationship between catalytic and sensor science. Active sites at such junctions may have been detected as those binding CO2– with an IR band at 1557 cm–1 which decompose to release CO2 or convert to formyl species (and then methanol) depending upon the prevailing conditions.