Structural characterization of malachite-like coprecipitated precursors of binary CuO-ZnO catalysts

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 the coprecipitation method. The samples with the ratios 100 0, 92 8 and 85 15 were monophasic with the monoclinic P21 a malachite structure, whereas the richer zinc samples contained in addition to the malachite phase the orthorhombic B2212 aurichalcite, whose concentration was very low in the 77 23 specimen but increased continuously with zinc content. The amount of aurichalcite in the biphasic powders has been estimated by means of quantitative X-ray diffraction and differential thermal analysis. With only those X-ray reflections belonging to the malachite structure taken into account, the unit cell constants for all the solid solution hydroxycarbonates have been determined, showing that as a whole the cell volume of malachite decreases continuously with increasing zinc content. This result is related, as shown also by DTA measurements and thermochemical literature data, 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 for Cu2+ ions in the malachite lattice. Magnetic susceptibility and thermogravimetric measurements together with diffuse reflectance spectra are also presented and discussed.