Fischer-Tropsch Synthesis to Olefins: Catalytic Performance and Structure Evolution of Co₂C-Based Catalysts under a CO₂ Environment

Cobalt carbide (Co₂C) nanoprisms derived from CoMn composite oxides exhibit promising catalytic performance for Fischer-Tropsch to olefins (FTO) synthesis via H₂-lean syngas conversion, but with nearly 45 C% of CO₂ selectivity. The work herein was aimed to investigate the effect of CO₂ in the feed on the structure-performance relationship of Co₂C-based catalysts during a realistic FTO process. An obvious negative effect of CO₂ was observed on the catalytic performance, and the presence of CO₂ greatly decreased the catalytic activity and olefin formation rate, while it facilitated methane formation. In addition, the product distribution shifted toward light components at increasing CO₂ content, and a typical methanation regime with low selectivity to olefins was observed for CO₂ hydrogenation. A structural characterization suggested that the Na-promoted Co₂C nanoprisms remained stable under FTO working conditions, and weak linearly and bridge adsorbed CO molecules were observed when the temperature reached 250 °C in a flow of CO-containing gas. However, the CO₂ environment hindered CO adsorption, and the strong CO₂ adsorption ability led to decreased CO coverage and a high local H₂/CO ratio on the catalyst surface. The as-obtained CO-lean and H-rich surface microenvironment gradually changed the morphology of Co₂C nanostructures from nanoprisms to nanospheres. Some of the Co₂C was even transformed into metallic Co. The change of the catalyst structure and the surrounding environment inhibited the adsorption of surface intermediates and the subsequent chain growth. This work provides important insights for further catalyst optimization and suggests that CO₂ removal is necessary for recycling the tail gas or using CO₂-containing feedstocks for industrial FTO processes over Co₂C-based catalysts.