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

Abstract

Monodisperse nanoparticles (NPs) of CoFe2O4 were synthesized as efficient catalyst precursors for CO2 hydrogenation to produce high value-added C-2-C-4 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 (similar to 34%) and light olefin selectivity (similar to 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-Fe3O(4) + 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)(5)C-2, which results in higher CO2 conversion and better selectivity toward light olefins in comparison with conventional chi-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)(5)C-2 was predicted using the cluster expansion method and density functional theory, resulting in a stable bimetallic alloy structure.