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

A rhenium promoted (1 wt.%Re/20 wt.%Co/γ-Al2O3) and an un-promoted (20 wt.%Co/γ-Al2O3) Fischer–Tropsch 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, H2/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.