Insight into the role of Fe₅C₂ in CO₂ catalytic hydrogenation to hydrocarbons

Iron-based catalysts have been widely used to synthesize value-added hydrocarbons through CO₂ hydrogenation. Fe carbide, Fe₅C₂, was proposed as the active phase responsible for C–C coupling during the chain growth. However, the reaction mechanism over iron-based catalysts is still under debate due to the presence of multiple phases of Fe species. In this work, Fe₅C₂ and K-modified Fe₅C₂ were prepared and used to study the relationship between the Fe phase, catalytic performance and reaction pathways. CH₄ and CO selectivities of 46.1 C-mol% and 2.8 C-mol% were obtained on Fe₅C₂ at a CO₂ conversion of 49.8 %, with the main hydrocarbon products being alkanes. With the addition of the promoter K, the C₂-C₄⁼ selectivity increased to above 38.0 C-mol%, and the selectivity to C₅₊ hydrocarbons increased to 23.9 C-mol% at a similar CO₂ conversion. CH₄ and C₂₊ hydrocarbons formed through the CO₂→CO→hydrocarbons (indirect) route over K-modified Fe₅C₂, whereas the direct hydrogenation route occurred on unmodified Fe₅C₂ in addition to the indirect route of CO₂ to CO to hydrocarbons through Fischer-Tropsch synthesis (FTS) processes. This work offers insights in direct CO₂ hydrogenation over Fe₅C₂ and Fe₃O₄-Fe₅C₂ catalysts, which advances the understanding of the functions of active phases and reaction pathways of CO₂ hydrogenation over iron-based catalysts.