Fischer-Tropsch to olefins (FTO) is recognized as a surface-catalyzed structure-sensitive reaction, and the catalytic performance is strongly influenced by the morphology and exposed facets of the active phase. Here we report the effect of the reduction process on the morphology of the active phase and the catalytic performance for FTO over the CoMn catalyst. For the catalysts reduced by 10% CO-300 degrees C, 10% H-2-300 degrees C and 10% H2-250 degrees C, Co2C nanoprisms were formed after reaching the steady state. However, for the catalysts reduced by CO-300 degrees C and 10% H-2-400 degrees C, Co2C nanospheres were found instead. Both Co2C nanoprisms and nanospheres were present for the spent sample reduced by 10% H-2-350 degrees C. Kinetic study found Co2C nanospheres to possess higher activation energy, and are more sensitive to hydrogen than Co2C nanoprisms. Density functional theory (DFT) calculations were also performed to clarify the structure-performance relationship of Co2C nanostructures for syngas conversion. (C) 2018 Elsevier Inc. All rights reserved.