Study of the effect of water on Fischer–Tropsch synthesis over supported cobalt catalysts

The effect of water on the activity and selectivity of unpromoted and Re-promoted cobalt Fischer–Tropsch catalysts supported on Al2O3, SiO2, and TiO2 has been studied in a fixed-bed reactor at 483 K and 20 bar. Common for all the catalysts was an increase in C5+ selectivity and a decrease in the CH4 selectivity at increased conversion or by external water addition. Promoting with Re increased the reaction rate [gHC/(gcat h)] for all catalysts and the C5+ selectivity for the SiO2- and TiO2-supported catalysts. It was found that the Co/Al2O3 and CoRe/Al2O3 catalysts deactivate when water is added during Fischer–Tropsch synthesis. For the Co/SiO2 and CoRe/SiO2 catalysts the reaction rate increased with increasing conversion or upon water addition. However, at high concentrations of water the catalysts deactivate rapidly. An increased reaction rate was also observed for the Co/TiO2 and CoRe/TiO2 catalysts with increasing conversion or with the addition of small amounts of water. At higher partial pressures of water the reaction rate decreased. Water interacts with the catalysts in different ways, and the effect of water is discussed in terms of Co particle sizes, secondary reactions, adsorbed species on the catalyst surface, and diffusion in liquid-filled pores.