Investigation of the dynamic active center for CO 2 hydrogenation to light olefins over Na-modified iron catalysts

Abstract

Alkali-modified iron catalysts often undergo phase transitions during CO2 hydrogenation, leading to mixed products and lower reactivity. Further elucidating their active sites, concurrently suppressing by-products, and maintaining stability remain significant challenges. This work initially explores the reactivity of pure Fe5C2 and Fe3O4 nano-catalysts, followed by the regeneration of deactivated catalysts by syngas-induced carburization to evaluate the phase-dependent catalytic behavior. Subsequently, Na2CO3 dopant was added via solid-phase mixing or wet impregnation to modify the single-phase iron catalysts. The resulting active sites responsible for olefin formation were further investigated, and changes in CO selectivity were explored. The results confirm a particle size-dependent dynamic phase transition between the active Fe5C2 phase and the inactive Fe3O4 phase. This transition leads to an initial decline in catalytic activity, followed by stabilization toward a pseudo-steady state. The physical mixing of Fe5C2 and Na2CO3 significantly enhances selectivity for light olefins among hydrocarbons. However, the Na2CO3 additive also further promotes the oxidation of Fe5C2 to Fe3O4, enhancing watergas shift activity. Additionally, Na2CO3 suppresses surface hydrogen dissociation, thereby preventing further CO conversion into olefins and long-chain products via the Fischer-Tropsch synthesis (FTS) reaction. Together, these effects result in higher CO selectivity during CO₂ hydrogenation over the Na2CO3-modified Fe5C2 catalyst. Operating at a low gas space velocity (GHSV) drastically reduces CO selectivity by promoting the CO-based FTS reactions; however, the associated high CO2 conversion and elevated water partial pressure also promote and accelerate the oxidation of Fe5C2 to Fe3O4. These findings highlight that olefin formation originates at the iron carbide-alkali interface and that catalyst stability is governed by a complex interplay of phase transitions, alkali metal promoter effects, and reaction conditions. Moreover, the observed oxidation acceleration at low GHSV may aid in the rapid screening of stable iron-based catalysts in future studies.