Mechanism of the Mn Promoter via CoMn Spinel for Morphology Control: Formation of Co₂C Nanoprisms for Fischer-Tropsch to Olefins Reaction

The Fischer-Tropsch to olefins (FTO) reaction over Co₂C catalysts is structure-sensitive, as the catalytic performance is strongly influenced by the surface structure of the active phase. The exposed facets determine the surface structure, and it remains a great challenge to precisely control the particle morphology of the FTO active phase. In this study, the controlling effect of the Mn promoter on the final morphology of the Co₂C nanoparticles for the FTO reaction was investigated. The unpromoted catalyst and several promoted catalysts with Ce, La, and Al were also studied for comparison. For the Mn-promoted catalysts, the combination method of the Co and Mn components plays a crucial role in the final morphology of Co₂C and thus the catalytic performance. For the CoMn catalyst prepared by coprecipitation, Co₂C nanoprisms with specifically exposed facets of (101) and (020) can be obtained, which exhibit a promising FTO catalytic performance with high C_2-4⁼ selectivity, low methane selectivity, and high activity under mild reaction conditions. However, for the Mn/Co catalyst prepared via impregnation, Co₂C nanospheres are formed, which exhibit high methane selectivity, low C_2-4⁼ selectivity, and low activity. For the unpromoted catalyst and the catalysts promoted by Ce and La, Co₂C nanospheres are also obtained, with catalytic performance similar to that of the Mn/Co catalyst prepared via impregnation. Due to the high stability of the Co₂AlO_x composite oxide, no Co₂C phase can be formed for the catalyst promoted by Al.