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 CC 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.