Theoretical and experimental insights into CO2 formation on Co2C catalysts in syngas conversion to Value-Added chemicals

Cobalt carbide (Co2C) has been discovered as the promising active phase for Fischer-Tropsch to olefins (FTO) process and higher alcohols synthesis (HAS) from syngas, in the form of nanoprisms and nanospheres, respectively. However, CO2 formation is inevitable on Co2C catalysts, especially in FTO process. So far, the mechanism of CO2 formation on Co2C surfaces is less understood. This work provides fundamental insights into CO2 formation on Co2C nanospheres and nanoprisms through computational and experimental study. DFT calculations demonstrate that: Co2C (101) and (020) surfaces exposed on Co2C nanoprisms are basically not active for water gas shift (WGS) reactions but the introduction of Na greatly promotes CO2 formation. CO2 is less favored on Co2C (1 1 1) surface which is dominant on Co2C nanospheres, however, the reverse WGS activities can be promoted with Na addition. The experimental results confirmed removing Na from Co2C can efficiently suppress WGS activity, thus reducing CO2 selectivity while the selectivity of light olefins retained. Though previous studies focused on Na addition to promote the yield of light olefins, our work indicates that Na also promotes undesired CO2 formation. Our results suggest that Na is crucial for active phase formation but not necessarily beneficial during the CO hydrogenation reactions.