Fuel-Rich Combustion Synthesized Co/Al2O3 Catalysts for Wax and Liquid Fuel Production via Fischer-Tropsch Reaction
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Alumina supported cobalt catalysts were synthesized for Fischer Tropsch (FT) reaction using combustion synthesis (CS) method. Catalyst synthesis utilized the redox reaction of hexamethylenetetramine and cobalt nitrate as fuel and oxidizer respectively with an equivalence ratio (φ) of 1, 1.2 and 1.5. Higher equivalence ratios (φ>1.0) were selected to increase the synthesis temperature and thereby decrease the formation of cobalt aluminates. While the calcined catalysts still showed signatures of Co3O4 as the predominant phase at φ = 1.2 & 1.5, these catalysts were characterized by high degree of reduction (DOR = 75 & 77 %, respectively) and higher metal dispersion (D = 12.8 & 13.2%, respectively), compared to the CS catalysts synthesized with unity equivalence ratio (DOR = 69% & D = 11%). The cobalt crystallite size decreased in the following order: CS(φ=1)>CS(φ=1.2)>CS(φ=1.5). The chemisorption studies and the XPS spectral analysis show decreased formation of cobalt aluminates at higher equivalence ratios corroborating reduced metal support interaction and higher dispersion. Higher FT activity was observed for catalysts synthesized with equivalence ratio of 1.2 and 1.5 compared to the catalysts synthesized with φ=1. However, the CO conversion rates reduced from 3.75 molCO/(gcat·s) for CS(φ=1.2) to 2.87 molCO/(gcat·s) for CS(φ=1.5) catalysts. Simultaneously, the hydrocarbon product spectrum shifted from predominantly waxes (C24+) for CS(φ=1) catalysts, to mixed fractions of liquid fuel (C6-C24) and waxes for CS(φ=1.2) & CS(φ=1.5) catalysts. This work investigates the impact of combustion synthesis stoichiometry on the catalyst properties, in particular the formation of cobalt aluminates and its outcome on the FT activity and selectivity.