EFFECT OF COBALT SOURCE ON THE REDUCTION PROPERTIES OF SILICA-SUPPORTED COBALT CATALYSTS
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abstract
The bulk and surface reduction properties of silica-supported cobalt catalysts are influenced by the identity of the cobalt salt employed in catalyst preparation. Temperature-programmed reduction (TPR), X-ray powder diffraction (XRPD), and X-ray photoelectron spectroscopy (XPS) have been used to characterize the reduction, calcination, and catalytic behaviors of a series of 6 wt.% Co/SiO2 catalysts prepared from nitrate, chloride, and acetate precursors. TPR profiles in hydrogen of the uncalcined catalysts reveal that reduction of Co(NO3)2/SiO2 occurs via an initial reductive decomposition of the nitrate ions, producing CoOxSiO2 surface species that are much more difficult to reduce to metallic cobalt than is the unsupported nitrate salt. Complete reduction of the silica-supported acetate is also markedly inhibited compared to that of the unsupported salt. By contrast, reduction of CoCl2/SiO2 occurs in a single step that is virtually unaffected by the presence of the silica support. XRPD analysis confirms that precalcination of the three catalysts at 500C prior to reduction leads to the formation of Co3O4 with the nitrate- and chloride-derived catalysts, but not with the acetate-derived material. TPR profiles and XPS spectra indicate that isothermal reduction in hydrogen at 400C is much less complete for the uncalcined catalysts than for the calcined materials, particularly for the nitrate and acetate precursors. Exposure of the uncalcined, hydrogen treated catalysts to a H2/CO reaction mixture at 250C results in further reduction of Co2+ to Co0 for the nitrate- and acetate-derived catalysts, which had been only slightly reduced by the prior hydrogen treatment, and partial re-oxidation of Co0 to Co2+ for the chloride-derived material, which had been largely reduced by the hydrogen treatment. 1991.
