Revisiting the Role of Fe3O4 in CO2 Hydrogenation to Olefins via Amorphous Aluminum Hydroxide Modification

Catalytic conversion of CO2 to olefins offers a viable route for carbon utilization. Fe-based catalysts are widely investigated for this process, where iron carbides (FeCx) mainly serve as the active phases; more Fe3O4 formation is associated with inferior catalytic performance. Herein, we revisit this concept by introducing amorphous aluminum hydroxide (Ala) as a structural promoter for K-modified Fe catalysts (KFe). Moderate Ala incorporation (≤5 wt %) decreases CO selectivity, from ∼61% to ∼37% at 280 °C and 1.0 MPa, and from ∼11% to ∼9% at 320 °C and 3.0 MPa, while maintaining high olefin selectivity above 70% and high olefin space–time yields (∼375 mgolefin·gcat–1·h–1) during 500 h of continuous operation at 320 °C and 3.0 MPa. A series of structural characterizations show that Ala incorporation not only suppresses excessive formation of Fe5C2, thereby reducing coke deposition, but also stabilizes a structurally distinct Ala-modified Fe3O4 phase that displays increased surface stability and more favorable intermediate adsorption. Kinetic studies and in situ spectroscopic analyses further reveal that Ala modification stabilizes reaction intermediates and mitigates dynamic surface carbonate fluctuations. These findings challenge the conventional perception of Fe3O4 and demonstrate that targeted structural modulation of iron oxide/carbide equilibria can improve catalyst stability and olefin productivity in the CO2 hydrogenation.