Kinetics of deactivation by carbon of a cobalt Fischer–Tropsch catalyst: Effects of CO and H2 partial pressures

Abstract

Effects of CO and H2 partial pressures on the deactivation by carbon, in the absence of other deactivation forms, of a CoPt/AlSi Fischer–Tropsch catalyst were investigated during six 800–900h runs at 230°C. Sintering was eliminated by using a catalyst with relatively large Co crystallites; cobalt-aluminate formation and oxidation were eliminated by operation at lower conversions and confirmed with TPR and XANES/EXAFS results; and physically blocking of pores by heavy hydrocarbons was avoided using a large pore support. Both (a) decreasing H2/CO ratio at constant PCO and (b) increasing H2/CO ratio at constant PH2 increase deactivation rate, possibly due to (a) formation of less hydrogenated and (b) more hydrogenated polymeric carbon forms, respectively. Deactivation rate was found to increase with increasing polymeric carbon deposition and decreasing CO-uptakes of aged catalyst samples. Methane selectivity increases as the catalyst deactivates only for runs at high CO pressures.