Structure and reactivity of zinc–chromium mixed oxides. Part 1.—The role of non-stoichiometry on bulk and surface properties

Abstract

Zinc–chromium mixed oxides, with Zn/Cr atomic percentage ratios between 33:67 and 50:50, have been prepared by a coprecipitation procedure. Their bulk structure has been investigated by the techniques of X-ray diffraction, infrared spectroscopy and electron spin resonance spectroscopy. They all appear monophasic to X.r.d., consisting of a spinel-like phase Zn_xCr_⅔(1-x)O that, for Zn/Cr > 33:67, includes an excess of zinc, probably in octahedral holes, leading to a non-stoichiometric spinel-like phase with a consequent increase of the lattice parameter a. The progressive increase of the breadth of the e.s.r. signal of Cr³⁺ indicates a progressive variation of the Cr³⁺—Cr³⁺ distance, in the same composition range. Non-vibrational absorptions in the i.r. region indicate semiconductivity phenomena. Calcinations at a high temperature (753 K) cause partial segregation of the excess of zinc as ZnO only for the sample with Zn/Cr = 50:50. The surface properties of these samples have been investigated by B.E.T. surface measurements, by Fourier-transform infrared spectroscopy of adsorbed water, pyridine and CO, and by calorimetric and volumetric measurements of CO adsorption. In contrast to the bulk data, a clear change in the surface properties is observed at the very beginning of the non-stoichiometric range.