화학공학소재연구정보센터
Journal of Catalysis, Vol.160, No.2, 148-154, 1996
Distribution of Cations in Mixed Zn-Mn-Al-O Containing Spinels, Model Catalysts 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 prepared 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 with 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 charge balance the average valence of Mn should then be 2.11. However, chemical analysis of the sample with x = 0.5 shows that there is further oxidation up Mn-2.82+, which implies that cation vacancies phi must be present in the lattice. The above-mentioned chemical analysis, combined with an analysis of the line intensities of the X-ray pattern (Rietveld method, X-ray pattern simulation) and MAS-NMR results (which rule out the presence of Al3+ in tetrahedral positions), leads to the formula (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 tetrahedral 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 oxidation level of Mn exists in all the series, the general formula of the studied solutions should be close to (Zn(1-x)(2+)Mn(0.20x)(2+)Mn(0.63)(3+)phi(0.17x))[Mn(0.17x)(3+)Al(2-0.34)(3+)phi(0.17x)]O-4.