Magnesium-Tuned Phase Evolution of Fe-Zn-K Catalysts for Stable CO2 Hydrogenation to α-Olefins

Abstract

Iron catalysts are promising for CO2 hydrogenation to α-olefins but suffer from deactivation due to the oxidation of active carbide phases. To address this, we synthesized a series of Mg-promoted FeZnK catalysts via co-precipitation to investigate the impact of alkaline earth metals on catalytic stability and structural evolution. The 3.6MgFeZnK catalyst maintained a CO2 conversion of approximately 40% and an α-olefin selectivity of 49.3% during a 400 h time-on-stream test with negligible degradation. Comprehensive in situ/ex situ characterizations, including XRD, XAFS, MES, XPS, FTIR, and TPD/TPO, were employed to explore the bulk/surface structural evolution of iron species and elucidate the structure-performance relationship. Mg incorporation suppresses Fe reduction and carburization during activation, resulting in a prolonged induction period under CO2 hydrogenation conditions. After gradual in situ carburization, the spent Mg-containing catalysts exhibit an Fe5C2-dominated carbide composition, whereas the Mg-free FeZnK catalyst contains mixed Fe7C3/Fe5C2 phases. Structurally incorporated Mg reinforces the interaction between Fe and Zn species, preserving abundant Fe5C2–ZnO interfacial structures. These retained interfacial structures mitigate the re-oxidation of surface FeCx species by CO2 and H2O, thereby contributing to the long-term catalytic stability. These findings provide a complementary strategy via alkaline earth metal promotion for regulating and stabilizing iron carbide phases in industrial hydrogenation applications.