Synthesis of Fe-Co Bimetallic Supported on Cotton Residue Biochar: Catalytic Performance and Mechanistic Investigation for CO2 Hydrogenation

Biochar carriers were prepared from cotton residue waste, and Fe-Co bimetallic catalysts with different ratios were constructed using impregnation method. Their catalytic performance in CO2 hydrogenation to hydrocarbons was investigated. The structural evolution and catalytic behavior of the catalyst were systematically studied through various characterization methods such as XRD, XPS, TEM, BET, and DFT calculations. Based on the characterization of fresh and spent catalysts, the spinel phase of CoFe2O4 in the fresh catalyst is proposed to transform substantially into metallic Fe0 and Co0 upon H2 reduction; during subsequent CO2 hydrogenation, these metallic phases are proposed to partially convert to χ-(Fe1−xCox)5C2 carbides, accompanied by partial graphitization of the biochar support. Because no sample was characterized after H2 reduction but before CO2 hydrogenation, this sequence is inferred. Under the same experimental conditions, Fe15Co5-BC exhibits the best CO2 conversion, with CH4 and C2-C4 alkanes as the main products, indicating that the system possesses bifunctional catalytic characteristics combining CO2 methanation and Fischer–Tropsch synthesis. The CO selectivity exhibited a V-shaped trend with respect to Co loading, with the lowest value achieved at the intermediate Fe/Co ratio (Fe15Co5-BC), mainly attributed to the improved overall CO2 conversion and the efficient conversion of carbon intermediates into hydrocarbons. DFT calculations reveal that Fe-Co bimetallic sites synergistically weaken C-O bonds and thermodynamically favor deep hydrogenation pathways. This work provides experimental and theoretical insights for the application of biochar-based bimetallic catalysts in CO2 hydrogenation.