Fe-Zn manganite spinels and their carbonate precursors:preparation,characterization and catalytic activity

摘要

Iron-zinc manganites at different Fe content [x = Fe/(Fe + Zn)= 0,0.01,0.05,0.10] were prepared by thermal decomposition of carbonate precursors obtained by coprecipitation at constant pH.Precursors were characterized by diffuse reflectance spectroscopy9DRS),thermo-gravimetric analysis9TGA-DTA)and magnetic susceptibility measurements.All samples are made by a single rhodochrosite-like phase,Fe_xZn_yMn_(1-x-y)CO_3,in which besides Zn~(2+)and Mn~(2+),only 5% of the total iron was Fe~(2+),the remaining 95% surprisingly being Fe~(3+)ions.The carbonate monophasic precursors were decomposed at 723 and 973 K in air and solid solutions of iron in the zinc manganite spinel-like phase were obtained at both temperatures.Zinc manganite,ZnMn_2O_4,is a 'normal' spinel in which the Zn~(2+)ions occupy the tetrahedral sites and Mn~(3+)ions the octahedral sites of the crystal lattice.When iron is loaded,it is present as Fe~(3+)ions in the octahedral sites of the spinel together with Mn~(3+)ions.The catalyst reducibility in H_2 was studied by temperature-programmed reduction9TPR).The iron-containing catalysts resulted to be slightly more resistant towards reduction than pure ZnMn_2O_4,suggesting that Fe~(3+)ions play a stabilizing effect on the spinel structure.Results of a Mossbauer investigation on the iron chemical state and its coordination symmetry are extensively reported.Some preliminary results of the catalytic behaviour for the reduction of NO by hydrocarbons are presented.