Deactivation of a Ru promoted Co/Al_2O_3 Catalyst for FT Synthesis

摘要

Fresh and used unpromoted and promoted 15%Co/Al_2O_3 catalysts were analyzed by XANES in order to provide insight into the catalyst deactivation. XANES analysis of used unpromoted and Ru promoted 15% loaded Co/Al_2O_3 catalysts gave evidence of oxidation of a fraction of the catalyst from water produced during the reaction. From analysis of the preedge feature and XANES derivative spectra, the oxidized clusters showed evidence of a tetrahedral environment, indicating that either Co_3O_4 or Co aluminate was formed. Because bulk oxidation of Co by water is not permitted thermodynamically under FTS conditions, the conclusion was drawn that only the very small clusters interacting with the support and deviating from bulk-like behavior oxidized in the presence of water produced during reaction. Promotion with Ru allowed for the reduction of smaller clusters interacting with the support, yielding more active sites and a higher initial rate of activity. Since these clusters deviated more from the bulk, the rate of deactivation was faster than for the unpromoted catalyst in the continuously stirred tank reactor (CSTR).