CHARACTERIZATION OF COPPER-ZINC MIXED-OXIDE SYSTEM IN RELATION TO DIFFERENT PRECURSOR STRUCTURE AND MORPHOLOGY

Hydroxycarbonate and hydroxynitrate precursors of CuO-ZnO catalysts (Cu/Zn atomic ratio = 67/33) have been prepared by two different methods; the precursor obtained by precipitation at 333 K and constant pH = 8 from mixed nitrate solution with excess of sodium bicarbonate consisted of zincian malachite and aurichalcite, while that obtained by addition of sodium carbonate solution to Cu-Zn nitrate solution is essentially copper hydroxynitrate plus some amount of aurichalcite. By thermal decomposition at 623 K both types of precursor gave a mixture of CuO and ZnO. The mixed oxides were then treated at 873, 1073 and 1273 K in air. X-ray diffraction, diffuse reflectance spectroscopy, scanning electron microscopy and surface area determination were used to characterize the mixed oxide systems. The precursor containing zincian malachite plus aurichalcite, after calcination at 623 K gave rise to well dispersed and much smaller particles of CuO and ZnO than the precursor containing copper hydroxy nitrate plus aurichalcite. No Cu2+ in solid solution in the ZnO zincite structure or Zn2+ in the CuO tenorite lattice were detected by reflectance spectroscopy up to 873 K; the presence of tetrahedral copper (Cu(x)Zn(1-x)O solid solution formation at least at the surface) was evident only in samples calcined at temperatures higher than 1073 K. X-ray diffraction analysis for lattice parameter determination showed that only for samples treated at 1273 K both Cu(x)Zn(1-x)O and Zn(y)Cu(1-y)O solid solution formations are detectable. An unexpected volume decrease of Zn(y)Cu(1-y)O with respect to pure CuO was revealed; the introduction of zinc in the tenorite structure probably changes the local metal symmetry from nearly square planar towards octahedral, producing on overall less distorted and more compact structure.