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 °C, 20 bar, (H2 + CO) conversion between 50 and 70%, feed gas composition of ca. 50 vol.% H2 and 25 vol.% CO, PH2O/PH2=1–1.5PH2O/PH2=1–1.5, PH2O=4–6 barPH2O=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.