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Free keywords:
Fischer−Tropsch synthesis; model catalysts; support effects; Lewis acidity; structure−performance relationships; turnover frequency; CO-FTIR spectroscopy
Abstract:
The effect of the chemical nature of the oxide support on the performance of cobalt Fischer-Tropsch catalysts is investigated. A series of supports is synthesized via monolayer coverage of porous gamma-Al2O3 with various oxides representative of a wide range of Lewis acid base character, as quantified by UV vis spectroscopy coupled to alizarin adsorption. Incorporation of cobalt (20 wt 96) results in model catalysts with identical porosities and similar Co particle sizes (>10 nm), allowing the study of support effects without overlap from diffusional or particle size factors. Under realistic reaction conditions, the initial TOF scales with the acidity of the oxide support, whereas the cobalt time yield and selectivity to industrially relevant C13+ hydrocarbons show a volcano dependence, with a maximum at an intermediate acid-base character. As inferred from in situ CO-FTIR, "selective" blockage of a few cobalt sites, though crucial for CO hydrogenation, by atoms from basic oxides and "unselective" site blockage via decoration of Co nanoparticles (strong metal support interaction) with acidic, reducible oxides cause a decrease in reaction rate for supports with pronounced alkaline and acidic character, respectively. The extent of secondary isomerization reactions of alpha-olefin products, of relevance for chain reinsertion processes and product selectivity, also correlates with the support acidity. For a TiOx/Al2O3 as support, a remarkable C13+ productivity exceeding 0.09 molc gCo-1 h-1) is achieved, owing to the combination of optimal activity and selectivity. These results provide a unifying view of the support effects over a considerably broad study space and delineate a blueprint toward advanced Fischer-Tropsch catalysts.
