XANES study of the susceptibility of nano-sized cobalt crystallites to oxidation during realistic Fischer-Tropsch synthesis

The oxidation of cobalt during Fischer-Tropsch synthesis (FTS) has long been postulated as a major deactivation mechanism. In this study, wax coated samples of a Co/Pt/Al2O3 catalyst were taken from a 100-barrel/day slurry bubble column reactor operated at commercially relevant FTS conditions, i.e. 230 degrees C, 20 bar, (H-2 + CO) conversion between 50 and 70%, feed gas composition of ca. 50 vol.% H-2 and 25 vol.% CO, P-H2O/P-H2 =1- 1.5, P-H2O = 4-6 bar and quantitatively characterised by X-ray absorption near edge spectroscopy (XANES). The cobalt catalyst samples, carefully removed from the reactor during the course of Fischer-Tropsch synthesis' were protected from air by the FTS wax. It is clear from the XANES measurements that during realistic FTS conditions cobalt crystallites of 6 nm supported on alumina were stable against oxidation to CoO/CoAl2O4 and a gradual reduction of residual cobalt oxide (i.e. following activation in pure hydrogen) was observed. This result is in line with recent thermodynamic analysis of the oxidation and re-reduction of nano-sized cobalt crystallites in water/hydrogen mixtures. (C) 2006 Elsevier B.V. All rights reserved.