(616c) Fischer-Tropsch Synthesis: Characterization of Interactions between Reduction Promoters and Co for Co/Al2O3–Based GTL Catalysts
A direct relationship exists between the FTS rate and the number of surface cobalt atoms available for reaction. For Co/Al2O3 catalysts, the site density depends on two primary factors: (1) the average size of the cobalt clusters on the support and (2) the degree of reduction of cobalt oxide species. The addition of small amounts of reduction promoters such as Pt and Re was studied using a variety of characterization methods including hydrogen chemisorption/pulse reoxidation, TPR, XPS, and EXAFS/XANES. Commercial GTL catalysts are heavily loaded with Co (e.g., 30 g Co/ 100 g Al2O3), which is necessary in part to overcome support interactions with the cobalt species during reduction, and on a related note, to stabilize the clusters against deactivation phenomena. The aim of the current work was to determine the feasibility of utilizing lower Co loadings and to promote reduction of the smaller, more interacting, Co clusters by incorporating small amounts of Pt, Re, and Ru. Results suggest the promoter facilitates reduction of small cobalt oxide species in interaction with the support - increasing active site density. EXAFS measurements indicate direct contact between the metal promoter and cobalt. For promoted 15%Co/Al2O3 catalysts, the cluster size was <10 nm, unstable under FTS conditions. Unpromoted 25%Co/Al2O3 catalysts, with an average cluster size > 10 nm, were more robust. A strategy involving oxidation-reduction cycles to improve the cobalt catalysts of lower loadings was investigated.
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