Fischer–Tropsch synthesis: effect of water on Co/Al2O3 catalysts and XAFS characterization of reoxidation phenomena

Abstract Both low loaded 15% Co/Al 2 O 3 and more highly loaded 25% Co/Al 2 O 3 catalysts are studied, in order to explore the impact of cluster size on the stability of the cobalt cluster to support-influenced reoxidation processes at high H 2 O/CO ratios. XAFS and activity data suggest that there are two regions for the water effect: at lower H 2 O/CO ratios water influences CO conversion by reversible kinetic effects while at higher H 2 O/CO ratios cobalt re-oxidation processes occur. The latter regime where water was added at and above 25% are examined. Synthesis conditions were maintained constant while argon balancing gas was replaced by added water. Catalyst samples were withdrawn from the reactor during synthesis at different partial pressures of added water and cooled in the wax product under inert gas. The EXAFS results suggest that, unlike the smaller clusters on unpromoted and, especially noble-metal promoted, 15% Co/Al 2 O 3 catalysts, the larger crystallites (>10 nm by chemisorption and XRD) on 25% Co/Al 2 O 3 undergo oxidation by H 2 O to CoO, most likely confined to the surface. The clusters are re-reduced when H 2 O was switched off, and the activity displayed an important recovery.