Modified Co/TiO2 catalysts for CO2 hydrogenation to fuels

Abstract

The direct CO2 conversion to liquid fuels by catalytic hydrogenation (CO2-based Fischer-Tropsch synthesis, FTS), is a sustainable approach to reduce CO2 emissions. This challenging reaction proceeds through tandem catalysis involving reverse water gas shift reaction to produce CO and subsequent traditional CO-FTS. Unmodified Co-based catalysts allow performing the reaction at low temperatures (<250 °C), albeit producing mainly methane. In this study, we modified a commercial TiO2-P25 support by NaBH4 reduction so as to introduce controlled amounts of promoters and oxygen vacancies. The modified and unmodified supports were used to prepare Co-based catalysts, which were evaluated for CO2-based FTS at 220–250 °C and 20 bar. The promoted catalysts outperform the one prepared on commercial TiO2 in terms of activity and selectivity towards C5+. Detailed characterizations of the catalysts were performed to decipher the role of promoters. We show that, besides improving CO2 activation and limiting H2 activation, the presence of alkali on the support allows a modulation of hydrogen spillover in the system. The best catalyst in terms of activity and selectivity is the one for which sodium is deposited in sufficient amount to modulate the hydrogen spillover, which allows an optimal surface C/H ratio on cobalt.