The impact of cobalt aluminate formation on the deactivation of cobalt-based Fischer-Tropsch synthesis catalysts

It has been reported that cobalt aluminate formation is a cause of deactivation during Fischer-Tropsch synthesis (FTS), as it forms at the expense of active cobalt and is irreducible during FTS. To study this quantitatively, wax-coated Co/Pt/Al2O3 catalyst samples were removed periodically from an extended demonstration reactor FTS run operated at commercially relevant conditions and analysed with X-ray Absorption Near Edge Spectroscopy (XANES). With XANES, wax protected spent samples could be analysed in a "pseudo in-situ" mode. Under commercially relevant FTS conditions the catalyst undergoes reduction and minimal amounts of cobalt aluminate were found. It is proposed that the cobalt aluminate is formed from the residual CoO present in the catalyst after reduction. Additionally, the formation of aluminate was investigated with XANES and X-ray photoelectron spectroscopy (XPS) and TPR-MS on catalysts taken from laboratory continuous stirred tank reactor (CSTR) runs with varying water partial pressure (1-10 bar). Even at high water partial pressures (P-H2O = 10 bar, P-H2O/P-H2 = 2.2) only around 10% cobalt aluminate is formed while the metallic fraction of cobalt still increased compared to the fresh catalyst. The work shows that cobalt aluminate formation during FTS at realistic conditions is not a major deactivation mechanism. (C) 2011 Elsevier B. V. All rights reserved.