Revealing the Effect of Sodium on Iron-Based Catalysts for CO2 Hydrogenation: Insights from Calculation and Experiment

The promotional effects of sodium on Fe5C2 catalysts for CO2 hydrogenation were systematically investigated by a synergistic combination of density functional theory (DFT) calculations and experimental methods. Na was found to markedly alter the electronic structure of Fe5C2 surface, leading to a reduction of the CO2 dissociation barrier from 0.45 to 0.08 eV and a decrease of the hydrogen binding energy. Moreover, Na lowers methane's selectivity by hindering further hydrogenation of CH2 and enhances C–C coupling probability by promoting the chain growth of CH2. Additionally, Na strengthens alkenes' selectivity by facilitating dehydrogenation of alkyls and boosting the desorption of alkenes. The theoretical findings were confirmed by experimental results. Adding Na to Fe5C2 catalyst was found to facilitate CO2 conversion and alkenes' selectivity. Especially, the O/P (olefin/paraffin) ratio of C2–C4 hydrocarbons increases from 2.0 to 9.7 and the C5+ hydrocarbons' selectivity increases from 12.6% to 51.8%. This study further deepens the understanding of the promotional effect of sodium on Fe-based catalysts for CO2 hydrogenation and enlightens the rational design of highly selective catalysts.