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

Abstract

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 C2–4= 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 C2–4= 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 Co2AlOx composite oxide, no Co2C phase can be formed for the catalyst promoted by Al.