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 C2–4 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 FeCX. 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.