Fischer–Tropsch synthesis: TPR and XANES analysis of the impact of simulated regeneration cycles on the reducibility of Co/alumina catalysts with different promoters (Pt, Ru, Re, Ag, Au, Rh, Ir)

Abstract

The goal of this work is to explore the ability of the metal-promoted 25%Co/Al2O3 catalyst to maintain good contact between the metal and cobalt and continue facilitating Co oxide reduction after simulated regeneration cycles through oxidation–reduction treatments, an approach designed to simulate the catalyst regeneration process. Unpromoted 25%Co/Al2O3 catalyst was also subjected to treatments and served as a reference. Seven metal promoters were examined in this work, including Pt, Ru, Re, Ag, Au, Rh, and Ir. Fresh and treated catalysts were evaluated by both TPR and XANES spectroscopy, the latter approach utilizing linear combination fittings with appropriate reference compounds. With the unpromoted catalyst, oxidation–reduction cycles tended to have two effects: (1) a fraction of CoO species that lost their interaction with the support emerged and (2) a fraction of more strongly interacting CoO species was formed. A comparison between the freshly calcined sample and samples subjected to simulated regeneration cycles was demonstrated. Pt-, Ru-, Re-, Ag-, and Rh-promoted 25%Co/Al2O3 catalysts maintained their ability to facilitate Co oxide reduction after undergoing oxidation–reduction cycles even up to 3 cycles, while with Ir- and, especially, Au-25%Co/Al2O3 some losses were observed, suggesting some separation between the promoter and cobalt occurred following the treatment cycles. TPR profiles also suggest that some separation of Ru from Co occurs with simulated regeneration cycles, although it does not impact the extent of reduction of Co after three cycles.