Effect of cobalt source on the reduction properties of silica-supported cobalt catalysts

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/SiO 2 catalysts prepared from nitrate, chloride, and acetate precursors. TPR profiles in hydrogen of the uncalcined catalysts reveal that reduction of Co(NO 3) 2/SiO 2 occurs via an initial reductive decomposition of the nitrate ions, producing CoO x SiO 2 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 CoCl 2/SiO 2 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 500°C prior to reduction leads to the formation of Co 3O 4 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 400°C 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 H 2/CO reaction mixture at 250°C results in further reduction of Co 2+ to Co 0 for the nitrate- and acetate-derived catalysts, which had been only slightly reduced by the prior hydrogen treatment, and partial re-oxidation of Co 0 to Co 2+ for the chloride-derived material, which had been largely reduced by the hydrogen treatment.