DISTRIBUTION OF CATIONS IN MIXED ZN-MN-AL-O CONTAINING SPINELS, MODELCATALYSTS FOR THE REDUCTION OF NITROBENZENE TO NITROSOBENZENE

Nominal Zn1-xMnxAl2O4 (0 less than or equal to x < 1) solid solutions have been prepared from nitrates hi air at 1423 K. MnAl2O4 was prepare d in a flow of 5% H-2/N-2 at 1273 K. All the oxides exhibit the spinel structure with an almost linear variation of the lattice constant wit h x (a = 0.80946 + 0.01182x + 0.00497x(2) [nm]). With the exception of ZnAl2O4, upon preparation the samples eliminate 0.17x of ct-alumina, which is compensated by oxidation of some manganese to Mn3+. For charg e balance the average valence of Mn should then be 2.11. However, chem ical analysis of the sample with x = 0.5 shows that there is further o xidation up Mn-2.82+, which implies that cation vacancies phi must be present in the lattice. The above-mentioned chemical analysis, combine d with an analysis of the line intensities of the X-ray pattern (Rietv eld method, X-ray pattern simulation) and MAS-NMR results (which rule out the presence of Al3+ in tetrahedral positions), leads to the formu la (Zn(0.53)(2+)Mn(0.09)(2+)Mn(0.31)(3+)phi 0.07)[Mn(0.10)(3+)Al(1.84) (3+)phi(0.05)]O-4, where parentheses and square brackets denote tetrah edral and octahedral sites, respectively. Intensity analysis combined with X-ray pattern simulations (without chemical analysis) proves that in all the samples [Mn3+] = 0.17x +/- 0.01. Assuming that the same ox idation level of Mn exists in all the series, the general formula of t he studied solutions should be close to 0.17x))[Mn(0.17x)(3+)Al(2-0.34 )(3+)phi(0.17x)]O-4. (C) 1996 Academic Press, Inc.