Decoupling the deactivation mechanisms of a cobalt Fischer–Tropsch catalyst operated at high conversion and ‘simulated’ high conversion
Operating the Fischer–Tropsch synthesis at high conversion would allow a simpler once-through configuration to be used for small-scale biomass and waste to liquid. The effect of high conversion, and consequently high partial pressure of H2O and low partial pressures of CO and H2, on the stability of nano-sized cobalt crystallites in Pt–Co/Al2O3 is investigated. Whilst hydrothermal environments are commonly investigated for deactivation, this is typically done under ‘simulated’ high conversion (low conversion, high partial pressure of water) conditions. Thus, the current study will present the differences between real high conversion and ‘simulated’ high conversion and attempt to decouple the deactivation mechanisms associated with both cases. At conversion levels higher than XCO = 70% for this study (T = 220 °C, p = 20 bar, feed of H2 : CO : N2 = 2 : 1 : 3), sintering, cobalt aluminate formation and cobalt oxidation led to rapid deactivation. ‘Simulated’ high conversion is found to cause less cobalt aluminate formation, but more carbon deposition. At very high conversion (XCO > 97%) enhanced reversible deactivation was exhibited due to oxidation and re-reduction of cobalt (shown via in situ magnetometer). A ‘maximum’ conversion seems to exist for a specific cobalt crystallite size caused by its oxidation.