In situ investigation on Co-phase evolution and its performance for Fischer–Tropsch synthesis over Nb-promoted cobalt catalysts

Abstract

Nb-promoted cobalt catalysts were prepared by the incipient wetness impregnation method and characterized by ex/in situ XRD, in situ DRIFTS, N₂ physisorption, TEM, EDS-mapping, H₂-TPR, CO/H₂-TPD, and XPS. Their performance was investigated in a fixed bed reactor for Fischer–Tropsch synthesis (FTS). As regards the sample texture, niobium oxide (NbO_x) can obviously reduce the BET surface area and pore volume, and promote the formation of uniform cobalt particles with a smaller size. The presence of NbO_x complexes can weaken the metal–support interaction, while it also makes Co₃O₄ species easily reducible to metallic cobalt and favours the formation of a hexagonal close-packed cobalt phase, as confirmed by the results of H₂-TPR and ex/in situ XRD. What's more, NbO_x can enhance the adsorption stability and the amount of CO adsorbed under approximately actual FTS reaction conditions, as verified by the results of CO-TPD. In addition, NbO_x complexes can be considered as weak electron donors, as confirmed by the results of XPS, leading to a higher concentration of surface carbon species. According to the performance results, the selectivity to C₅⁺ products increased to 89.3% from 85.6% and the CO conversion was significantly increased to 94.1% from 70.7% with the increase of Nb loading to 1.57 wt% from zero. It is found that highly selective production of heavy hydrocarbons can be achieved at a higher activity level over Nb-promoted cobalt catalysts.