Insights into the Electronic Effects of the Zn Promoter on Co₂C Nanoprisms for Fischer-Trospch to Olefin Reaction

Co2C nanoprisms were developed for the Fischer–Tropsch to olefins (FTO) reaction a few years ago, while to the best of our knowledge, the study of the electronic promoters were hardly reported due to the instability of the Co2C nanostructures with promoters. The absence of the promoters limited the performance modulation and industrial application of the Co2C-based FTO catalysts. The work reported here was aimed at developing a new strategy to synthesize the Zn-promoted Co2C nanoprisms and understanding the electronic effects of the Zn promoters on the Co2C facets without confusion over the geometric effects. Detailed characterizations of the carbonized and spent samples suggested that the Zn promoters lowered the stability of the Co2C nanostructures and led to the formation of the surface Co0 species in the spent samples with high Zn contents, which was responsible for the decrease of the O/P ratios and the increase of the CH4 selectivity. The XPS results showed that the electrons transferring from the Zn promoters to the Co2C facets enriched the density of the electron clouds of the Co species on the Co2C facets. Further evidence for the enhanced CO adsorption and dissociation routes were obtained from the measurements of the CO temperature-programmed desorption. The Zn promoters increased the strong adsorption sites, which were not active for the Boudouard reaction and the increase of the CO2 selectivity originated from the Water–Gas Shift reaction. The activation route of the adsorbed CO species on the strong adsorption sites was H-assisted dissociation rather than direct dissociation, resulting in the inhibition of the carbene-mediated routes and the shifting of product selectivity toward light hydrocarbons.