STRUCTURAL CHARACTERIZATION OF MALACHITE-LIKE COPRECIPITATED PRECURSORS OF THE BINARY CuO-ZnO CATALYSTS: BULK AND SURFACE PROPERTIES

Copper and zinc hydroxycarbonates with Cu/Zn atomic ratios of 100/0, 92/8, 85/15, 77/23, 67/33 and 50/50 were prepared by coprecipitation at constant PH. The samples with higher Cu/Zn ratios are monophasic with malachite structure, whereas the richer zinc samples contain, in addition to malachite, the aurichalcite phase, whose concentration is very low in the 77/23 specimen but increases with zinc content. The cell volume of the zincian-malachite phase decreases up to similar to 30% of zinc content. This result is related, as also shown by DTA measurements, to an increasing stability of compounds richer in zinc, to increasing covalent effects and/or to less octahedral site distortion around the metal atoms when zinc substitutes Cu2+ ions in the malachite structure. XPS investigations have shown that both in the monophasic and biphasic samples the surface Cu/Zn atomic ratios are nearly equal to those found by chemical analysis, so indicating that an homogeneous dispersion of zinc in the solid solutions and a good interdispersion of the zincian-malachite and aurichalcite phases are both achieved. A comparative study of the photoelectron and X-ray excited Auger transitions of Cu and Zn gives also support for an increasing covalency of the chemical bonds in zincian-malachites with respect to pure malachite. Moreover, the analysis of the initial state effect on the full width half maximum of the Cu(2p(3/2)) and of the Zn(2p(3/2)) peaks gives evidence that zinc substitutes copper in the less octahedrally distorted (type-II) metal site of the malachite structure.