Fischer-Tropsch synthesis: An XAS/XRPD combined in situ study from catalyst activation to deactivation

A rhenium promoted (1 wt.%Re/20 wt.%Co/gamma-Al2O3) and an un-promoted (20 wt.%Co/gamma-Al2O3) FischerTropsch catalyst were studied in situ throughout the common steps of laboratory catalyst testing, that is, reduction, pressurization, the initial period before reaching pseudo-steady state, deactivation, and post-mortem analysis. High-resolution X-ray powder diffraction (HR-XRPD) was combined with X-ray absorption spectroscopy (XAS) in order to reveal the changes occurring during the experimental procedure. A mass spectrometer (MS) connected to the reactor outlet allow monitoring of the gas phase and accordingly the catalyst performance with respect to activity. Fischer-Tropsch synthesis was performed at 493 K, 18 bar, H-2/CO = 2.1, and >50% CO conversion. The reduction at 673 K confirmed the transition from Co3O4 to CoO and eventually a mixture of face-centered cubic (fcc) metallic cobalt, hexagonal close packed (hcp) metallic cobalt and a fraction of unreduced cobalt. The promoted catalyst exhibits a transformation during the initial stages of the Fischer-Tropsch synthesis possibly associated with minor re-oxidation of cobalt to Co2+ tetrahedrally coordinated, part of a CoAl2O4 surface structure. Throughout Fischer-Tropsch synthesis, HR-XRPD and XAS signals are unaffected, suggesting the absence of bulk cobalt transformations occurring at the initial Fischer-Tropsch deactivation period. No significant gradients in the concentration of cobalt species or the size of the crystallites have been found as a function of the length of the catalytic bed. (C) 2012 Elsevier Inc. All rights reserved.