The Surface Composition of CuO<i>_x</i>/ZrO₂ Catalysts as Determined by FTIR, XPS, ESR Spectroscopies and Volumetric CO Adsorption

Abstract

CuOx/ZrO2 samples prepared by adsorption from copper solutions or by impregnation were characterized by means of FTIR, XPS, ESR, DRS, volumetric CO adsorption, and redox cycles with H2 and O2. In samples prepared by adsorption, the maximum copper uptake corresponded to an extended plateau at 2.5 atoms nm-2. In as-prepared samples, isolated CuII species were in a distorted octahedral configuration, and in samples heated in dry O2 at 773 K, in a square-pyramidal configuration. Water vapor adsorption transformed the latter species into distorted octahedral complexes. In all samples, heating in O2 at 773 K anchored copper to the zirconia surface. All copper was present as CuII. Evacuation of these samples at 773 K caused no copper reduction, whereas heating with H2 above 450 K reversibly reduced CuII to metal copper. Evacuation of as-prepared samples differed according to how samples were prepared. In particular, evacuation of samples prepared from Cu−acetylacetonate or Cu−acetate reduced CuII to CuI at 473 K and to copper metal at higher temperature. Evacuation reduced copper because acetylacetonates and acetates underwent oxidation during desorption. Evacuation up to 773 K of samples prepared from Cu−nitrate caused no copper reduction. In samples heated in O2 at 773 K, CO adsorption at RT yielded CuI−CO and carbonates. Volumetric CO adsorption combined with FTIR showed that copper was highly dispersed on the ZrO2 surface up to 2.5 atoms nm-2.