Effects of Alumina Phases on the Structural Evolution of Iron Catalysts for the Catalytic Conversion of CO2 to Olefins

Alumina is extensively used as a catalyst support in a wide range of heterogeneous catalyst systems, where its phase structure significantly influences catalytic properties. Herein, Na-promoted Fe catalysts were impregnated on four different phases of alumina (γ-, δ-, θ-, and α-Al₂O₃) and evaluated for CO₂ hydrogenation to produce hydrocarbons. Among all the alumina tested, the α-Al₂O₃ supported Fe catalyst exhibited the best performance, achieving a selectivity of 47.4% for C_2–4 olefins at a CO₂ conversion of 42%, while remaining stable within 200 hour time on stream. As a comparison, the γ-Al₂O₃ supported Fe catalyst produced mostly CH₄ and CO and deactivates rapidly. In situ characterizations, including Raman, XRD, FTIR, and TPD/TPSR were employed to explore the bulk/surface structural transformation of iron species and elucidate the reaction mechanisms. The distinct differences in catalytic properties are attributed to the variations in surface chemical properties and metal-support interactions, which exert significant influence on CO₂ activation, reduction, carburization, and the generation of FeC_X. Notably, γ-Al₂O₃, with its abundant surface hydroxyl groups, showed weak CO₂ adsorption while strong H₂ adsorption capacity, leading to a more pronounced CH₃O* signal than α-Al₂O₃. This observation suggests an enhanced generation of CH₄ intermediates and a higher hydrogen dissociation capacity, which promotes hydrogenation ability. This study clarifies the impact of the crystalline phases of alumina supports on the structure and composition of iron species and CO₂ hydrogenation activity.