Cobalt Ferrite Nanoparticles to Form a Catalytic Co–Fe Alloy Carbide Phase for Selective CO2 Hydrogenation to Light Olefins

Monodisperse nanoparticles (NPs) of CoFe2O4 were synthesized as efficient catalyst precursors for CO2 hydrogenation to produce high value-added C2–C4 olefin products, which are important building blocks for the chemical industry. The resulting Na-promoted CoFe2O4 catalysts supported on carbon nanotubes (Na–CoFe2O4/CNT) exhibited high CO2 conversion (∼34%) and light olefin selectivity (∼39%), outperforming other reported Fe-based catalysts under similar reaction conditions. Their performance was superior to that of single-metal NP catalysts (Na–Fe3O4/CNT and Na–Co/CNT) and a physically mixed (Na–Fe3O4 + Co)/CNT catalyst. The superior performance of the Na–CoFe2O4/CNT catalyst can be attributed to the facile formation of a unique bimetallic alloy carbide (Fe1–xCox)5C2, which results in higher CO2 conversion and better selectivity toward light olefins in comparison with conventional χ-Fe5C2 active sites derived from Fe-only catalysts and significantly improved heavy hydrocarbon (C2+) formation in comparison with the Co2C sites of Co-only catalysts. The single-source precursor CoFe2O4 exclusively forms a single-phase alloy carbide promoted by the Na promoter, whereas the mixed (Na–Fe3O4 + Co) precursor forms an isolated Co phase with the alloy carbide phase, promoting undesirable CH4 formation. An optimal value of x ≤ 0.2 for (Fe1–xCox)5C2 was predicted using the cluster expansion method and density functional theory, resulting in a stable bimetallic alloy structure.