Alkali-promoted iron-based catalysts for efficient and selective CO₂ hydrogenation to light olefin: A combined mechanistic and DFT insights

The hydrogenation of CO₂ to light olefins over alkali-promoted iron catalysts offers a sustainable pathway for CO₂ utilization and production of valuable chemicals. This study investigates the effect of alkali (Na, K, Cs) promotion on Fe-based catalysts for CO₂ hydrogenation to light olefins. Characterization techniques revealed that alkali promoters enhanced CO₂ adsorption, activation, and catalyst basicity, while weakening H₂ binding and facilitating desorption, shifting selectivity toward light olefins (C₂-C₄=). The K-promoted catalyst achieved 38.3% olefin selectivity at 44.7% CO₂ conversion, attributed to stabilization of χ-Fe₅C₂ and suppression of alkene hydrogenation via moderate electron donation. In contrast, Na delayed Fe reduction and promoted heavier hydrocarbons, while Cs showed weak CO₂ uptake. XPS analysis revealed that K promotion enhances iron carburization, stabilizing the active χ-Fe₅C₂ phase and increasing the C-Fe fraction, which favors light olefin formation. In contrast, Na promotion leads to more oxidized Fe2+/Fe3+ species and oxygenated carbon deposition, promoting heavier hydrocarbon growth. The higher surface carbide content and lower oxidation state in KFe catalysts correlate directly with higher olefin selectivity. In situ DRIFT revealed that CO₂ hydrogenation proceeds via a combined associative-dissociative pathway involving formate, carbonate (CO-C*), and CO intermediates, which transform into Fe-carbide species to drive C-C coupling. DFT simulations determined that alkali doping significantly alters the electronic structure and CO₂ adsorption behavior of Fe₂O₃ surfaces, with the (110) facet being the most active. K doping provided optimal adsorption energy (-0.80 eV) and balanced charge transfer, enhancing CO₂ activation and light olefin selectivity. In contrast, Na led to over-strong adsorption, resulting in heavier hydrocarbons, while Cs showed weak CO₂ binding and low activity.