Cobalt Ferrite Nanoparticles to Form a Catalytic Co–Fe Alloy Carbide Phase for Selective CO<sub>2</sub> Hydrogenation to Light Olefins
Kwang Young Kim, Hojeong Lee, Woo Yeong Noh, Jungho Shin, Seung Ju Han, Seok Ki Kim, Kwangjin An, Jae Sung Lee
Abstract
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.