Fischer-Tropsch synthesis XAFS XAFS studies of the effect of water on a Pt-promoted Co/Al2O3 catalyst

The impact of water on the deactivation of a 0.5% Pt-promoted 15% Co/Al2O3 catalyst was studied by XAFS. Catalyst samples were withdrawn from the reactor during synthesis at different partial pressures of added water and cooled in the wax product under an inert gas blanket. Synthesis operating conditions were maintained constant while differing amounts of argon were replaced by added water. Below 25% added water (H2O/CO = 1.2; H2O/H-2 = 0.6), the slight negative effect on activity was reversible, and no changes were observed in the EXAFS or XANES spectra. This indicates that the effect of water in this range is most likely kinetic. However, XAFS results strongly suggest that, above 25% water addition, the sudden irreversible loss in activity is due to reaction of the cobalt clusters with the support, forming cobalt aluminate-like species. The XAFS and previously reported activity data indicate that there are two regions for the water effect: at lower H2O/CO ratios water influences CO conversion by reversible kinetic effects while at higher H2O/CO ratios irreversible oxidation of cobalt occurs. (C) 2003 Elsevier Science B.V. All rights reserved.