The Effect of Water on the Activity and Selectivity for γ-Alumina Supported Cobalt Fischer–Tropsch Catalysts with Different Pore Sizes

The effect of water on the activity and selectivity for a series of γ-Al₂O₃ supported cobalt Fischer–Tropsch catalysts has been studied in an isothermal fixed-bed reactor at T = 483 K, P = 20 bar, and H₂/CO = 2.1. The catalysts were produced applying incipient wetness impregnation and consisted of 20 wt.% cobalt and 0.5 wt.% rhenium deposited on γ-Al₂O₃ supports with different pore characteristics. For the narrow-pore catalysts, addition of water corresponding to an inlet partial pressure ratio of P_H2O/P_H2 = 0.4 reduced the reaction rates. In contrast, for a catalyst with larger pores the same water pressure increased the reaction rates. For all catalysts, water amounts equal to P_H2O/P_H2 = 0.7 at the reactor inlet suppressed the reaction rates and led to permanent deactivation. The addition of water increased the C₅₊ selectivity and decreased the CH₄ selectivity for all catalysts. The pore characteristics seem to determine the effect of water on the rates.