Preparation, characterization and catalytic activity of Co-Zn-based manganites obtained from carbonate precursors

Cobalt-zinc manganites at rather low Co content (x = Co/(Co + 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 X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA-DTA) and magnetic susceptibility measurements. For all samples, a single rhodochrosite like phase, CoxZnyMn(1-x-y)CO3, was formed, as detected by XRD. Magnetic analysis confirmed that cobalt and manganese were present as Co2+ and Mn2+ in the rhodochrosite crystal lattice. The carbonate monophasic precursors were decomposed at 723 and 973 K in air and Co-Zn-Mn based spinel-like solid solutions were obtained at both temperatures. The DRS and magnetic susceptibility data suggested that after calcination cobalt was mainly present as Co3+ ions in the octahedral sites of the spinel together with Mn3+ ions. The catalyst reducibility in H-2 was studied by temperature-programmed reduction (TPR). The cobalt containing catalysts reduced almost as the pure ZnMn2O4 suggesting that cobalt does not show a detectable influence on the spinel reducibility. Preliminary results of the catalytic activity for the reduction of NO by hydrocarbons in absence of oxygen are presented. (C) 2001 Elsevier Science B.V. All rights reserved.