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 H-2-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 degrees 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 H-2/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.