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 diflraction, infrared spectroscopy and electron spin resonancg- spectroscopy. They all appear monophasic to X.r.d., consisting of a spinel-like phase Zn,Cri,,-,,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.
Structure and reactivity of Zinc-Chromium mixed oxides: I. The role of non stoichiometry on bulk and surface propertie
FUBINI, Bice;GIAMELLO, Elio;
1988-01-01
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 diflraction, infrared spectroscopy and electron spin resonancg- spectroscopy. They all appear monophasic to X.r.d., consisting of a spinel-like phase Zn,Cri,,-,,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.
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
