Effects of Pd and Co intimacy in Pd-modified Co/TiO2 catalysts for direct CO2 hydrogenation to fuels: the closer not the better

Abstract

Direct CO₂ hydrogenation to liquid fuels is a sustainable approach to decarbonize the future air transport. This reaction proceeds through a tandem pathway involving the reverse water gas shift reaction (RWGSR) to produce CO and the subsequent traditional CO-Fischer–Tropsch synthesis. On Co-based catalysts, the introduction of dopants can improve CO₂ activation, enhance the RWGSR activity and decrease the methanation side reaction. We reported that alkali-promoted Co/TiO₂ catalysts outperform the unpromoted ones in terms of activity and selectivity towards C₂₊ (C. Scarfiello, K. Soulantica, S. Cayez, A. Durupt, G. Viau, N. Le Breton, A. K. Boudalis, F. Meunier, G. Clet, M. Barreau, D. Salusso, S. Zafeiratos, D. P. Minh and P. Serp, J. Catal., 2023, 428, 115202). To further improve the catalytic performances, we doped an alkali-promoted Co/TiO₂ catalyst with palladium, which is active for the RWGSR and promotes hydrogen spillover. The effect of noble metal location in relation to cobalt, a rarely studied parameter, was investigated by using bimetallic and mixtures of monometallic catalysts. This study demonstrates that whatever the location of Pd and its loading (0.03–0.9 wt%), doping with this metal leads to an improvement in catalytic activity. Furthermore, we show that the proximity between Co and Pd has a pronounced effect on the selectivity of the reaction. The best configuration to achieve higher activity and C₂₊ selectivity is obtained using mixtures of monometallic catalysts.