Synthesis-Dependent Phase Evolution and Catalytic Behavior of Unsupported Iron Catalysts for CO2 Hydrogenation to C2-C4=

Abstract

Iron-based catalysts are widely investigated for CO2 hydrogenation to light olefins due to their low cost, abundance, and ability to form oxidic and carbidic phases involved in the RWGS-FTS pathway. However, supports and promoters can obscure the intrinsic role of iron phase evolution. Here, unsupported Fe nanoparticles with distinct morphological, compositional and structural characteristics were synthesized, CO-pretreated, and evaluated for CO2 hydrogenation toward C2-C4=. Their textural, structural, redox, and surface properties were examined to relate synthesis-induced differences to phase evolution and catalytic performance. CO pretreatment formed iron carbide phases in all samples, yet marked differences in catalytic behavior showed that the active state cannot be described solely by bulk phase composition. Iron oxide nanopolyhedra (Fe-NP) achieved 41.6% CO2 conversion, the highest C2-C4= selectivity of 27.7%, and the highest light-olefin yield of 11.5%. Surface analysis revealed notable differences in the relative contributions of carbidic and oxidic Fe species, suggesting that performance depends on the surface phase distribution established during activation and reaction. The coexistence of Fe3O4 and FexCy species appears to facilitate RWGS and subsequent Fischer-Tropsch pathways, as proposed in the literature. CO2-TPD further showed that maximum CO2 uptake did not correspond to superior performance, highlighting the combined influence of synthesis-induced structure, surface composition, and reduction-carburization behavior.