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

Copper and zinc hydroxycarbonates with CuZn atomic ratios of 100/0,92/8,85/L5,77/23, 67/33 and 50/50 were prepared by coprecipitation at constant pH. The samples with higher CuZn ratios are monophasic with malachite structure, whereas ttre richer zinc samples contain, in addition to malachite, the aurichalcite phase, whose concentration is very low n the 77/23 specimen but incrcases with zinc content. The cell volume of the zincian-malachite phase decreases up to ca. 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 CuZn atomic ratios are nearly equal to those found by chemical analysis, so indicating that an homogeneous dispenion of ziinc in the solid solutions and a good interdispersion of the zincian-malachite and aurichalcite phases are both achieved. A comparative study ofthe photoelecfron and X-ray excited Auger ransiiions of Cu and Zn gives also support for an increasing covalency of the chemîcal bonds in zincian-malachites with rcspect to pure malachite. Moreover, the analysis of the initial state effect on the full width half maximum of the Cu(2p3/2) and of the Zn(2p32) peaks gives evidence that zinc substitutes copper in the less octahedrally distorted (type-II) metal site of the malachite structure.