The surface composition of CuOx/ZrO2 catalysts as determined by FTIR, XPS, ESR spectroscopies and volumetric CO adsorption

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 H-2 and O-2. 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 Cu-II species were in a distorted octahedral configuration, and in samples heated in dry O-2 at 773 K, in a square-pyramidal configuration. Water vapor adsorption transformed the latter species into distorted octahedral complexes. In all samples, heating in O-2 at 773 K anchored copper to the zirconia surface. All copper was present as Cu-II. Evacuation of these samples at 773 K caused no copper reduction, whereas heating with H-2 above 450 K reversibly reduced Cu-II 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 Cu-II to Cu-I 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 O-2 at 773 K, CO adsorption at RT yielded Cu-I-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).