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

Nb-promoted cobalt catalysts were prepared by the incipient wetness impregnation method and characterized by ex / in situ XRD, in situ DRIFTS, N 2 physisorption, TEM, EDS-mapping, H 2 -TPR, CO/H 2 -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 Co 3 O 4 species easily reducible to metallic cobalt and favours the formation of a hexagonal close-packed cobalt phase, as confirmed by the results of H 2 -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 C 5 + 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.