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 H2 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.