RECARBONATION PROCESS IN AMBIENT CONDITIONS OVER MANGANESE OXIDE-BASED SPINELS PROMOTED BY COBALT

A combined investigation of surface and bulk properties of Co-promoted zinc manganese oxide (ZnMn2O4) catalysts, carried out by means of X-ray Photoelectron Spectroscopy (XPS) and X-ray diffraction (XRD) analysis, respectively, led to discover an intriguing recarbonation chemistry. These solids were prepared by thermal decomposition of rhodochrosite-like (MnCO3) carbonate precursors at 723 and 973 K and resulted to be monophasic [1]. Indeed at both temperatures only a ZnMn2O4 bulk spinel phase was obtained in which Zn and Co ions occupy the tetrahedral and octahedral sites of the spinel crystal structure, respectively. However, a striking difference appears between the two sets of calcined samples after storage for a long time under ambient conditions. Unlike the homologous samples prepared at 723 K and characterized by a much higher specific surface area, in the catalysts containing cobalt calcined at 973 K a recarbonation process occurred, as evidenced by the components belonging to the carbonate group observed in the XPS spectra of carbon (C 1s) and oxygen (O 1s) core level transitions. This surprising finding was also supported by XRD analysis which for the higher loaded cobalt samples showed even a bulk carbonate compound, identified as the manganese carbonate phase (rhodochrosite) from which the spinels originated by thermal decomposition. Our investigation reveals for the first time that the original carbonate phase can be reformed only on the spinel surfaces of the sintered cobalt-containing catalysts obtained at higher temperature after storage at room temperature and ambient air. [1] G. Fierro, M. Lo Jacono, M. Inversi, R. Dragone, G. Ferraris, Appl. Cat. B, 30, 173 (2001).