Tailoring Surface and Penetrating Carbon in Fe-Based Catalysts to Balance the Activity and Stability of Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis (FTS) has attracted intensive attention as a nonpetroleum route for producing bulk chemicals and fuels. The controllable synthesis of high-performance Fe-based catalysts is of significance in industry. It is challenging to achieve higher activity and great stability at the same time due to the complex transformation of iron phases and numerous side reactions. Herein, two carbon species, surface carbon and penetrating carbon, were tuned by adjusting the carburization conditions (CO/H2 ratio and temperature), aiming to control the structure of active phases. The content of penetrating carbon determines the type of iron carbides (Fe2C and/or Fe5C2), among which Fe2C exhibits higher initial activity but poor stability because of severe carbon deposits on the surface. In contrast, Fe5C2, with lower activity, is more stable during the reaction. Moreover, excess surface carbon covering active sites is undesired, whereas moderate preformed surface carbon (especially sp2-type carbon) plays an important role in hindering carbon further diffusion into the bulk phase and enhancing the stability of Fe5C2 to some extent. The combined action of these two carbon species acts as a regulator of the Fe2C-Fe5C2 mixture, balancing activity and stability. This work underlines the two sides of penetrating carbon and surface carbon, emphasizing the importance of carburization manipulation. This inspires optimization of the pretreatment and reaction processes of industrial FTS catalysts.