XPS INVESTIGATION OF COBALT-PROMOTED ZINC-MANGANITE SPINELS

In previous works concerning the bulk and catalytic propefties of cobalt-promoted zinc manganites it was found that they were efficient spinel-based catalysts for the reduction of NO and N2O by propane and propene [1,2]. In this work some results of the investigation of the surface properties of this system by XPS are reported. These catalysts, having Col(Co + Zn) ratios = x = 0,0.01,0.05,0.10, were previously prepared by decomposing in air, at 723 and 973 K, monophasic carbonate precursors made by a rhodochrosite-like (MnCOr) phase. Co-Zn-Mn spinel-like solid solutions were formed at both temperatures [1]. Cobalt was detectable by XPS at the surface of the samples with x=0.05 and 0.10, the signal being too low to be revealed in the case of the lowest Co content (x=0.01). The analysis of spectroscopic features of the Co(2p) transition suggests the simultaneous presence of Co3* and Co2* species at the catalysts surface. Manganese is mostly in the trivalent chemical state, as the Ì4n(2p31) photoelectron peaks have their maxima close to 642 eV [3]. The surface chemical composition of the spinel was affected by the Co content, calcination temperature and ageing. In particular, due to the ageing, the precursor carbonate phase reappears on the spinel surface of the Co containing catalysts but only for the solids calcined at 973 K, as evidenced by XRD analysis. This was also supported by the curve-fitting of C(1s) photoelectron peak characterized by a component at higher binding energy (289.3 eV) that can be related to carbonate species. [1] G. Fierro, M. Lo Jacono, M. Inversi, R. Dragone, G. Ferraris, Appl. Cat. 8,30, I73 (2001). [2] G, Ferraris, G. Fierro, M, Lo Jacono, M. Inversi, R. Dragone, Appl. Cat. 8,36,25I (2OO2). [3] H,W. Nesbitt, D. Banerjee, American Mineralogist, 83, 305 (1998).