Synergistic effects of promoters in Fe/YZrO_x catalysts for CO₂ hydrogenation to higher hydrocarbons

The search for suitable support materials and optimal promoters is an attractive approach to control the product selectivity in Fe-catalyzed CO₂ hydrogenation to C₂₊-hydrocarbons (CO₂-modified Fischer–Tropsch synthesis (CO₂-FTS)). This study systematically investigates the effects of metal oxide dopants for ZrO₂ as well as a K promoter with or without a transition metal co-promoter for supported FeO_x species in Fe/ZrO₂-based catalysts on their performance in CO₂-FTS. By combining K with a Mn promoter (Mn/K ratio = 0.4) on a Fe/YZrO_x catalyst, CH₄ selectivity was hindered without decreasing C₂-C₄ olefin selectivity at 300 °C and 15 bar. The low CH₄ selectivity is worth mentioning in comparison with state-of-the-art Fe-based catalysts. The reaction-induced catalyst restructuring was analyzed, revealing a positive influence of the transition metal promoters on the in-situ formation of catalytically active Fe carbides. Temperature-programmed experiments provided insight into the catalyst ability to interact with CO₂ and H₂, and thus into the nature of surface iron carbides formed in-situ. Furthermore, the influence of K and Mn on the formation of surface intermediates under realistic conditions was studied using operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), complementing our understanding of the role of the promoters in improving the formation of the desired products.