Tsakoumis, Nikolaos E.; Voronov, Alexey; Ronning, Magnus; van Beek, Wouter; Borg, Oyvind; Rytter, Erling; Holmen, Anders

Journal of Catalysis (2012), 291, 138-148 CODEN: JCTLA5; ISSN: 0021-9517. English.

A rhenium promoted (1 weight%Re/20 weight %Co/.gamma.-Al2O3) and an un-promoted (20 weight%Co/.gamma.-Al2O3) Fischer-Tropsch catalyst were studied in situ throughout the common steps of laboratory catalyst testing, i.e., reduction, pressurization, the initial period before reaching pseudo-steady state, deactivation, and post-mortem anal. High-resolution x-ray powder diffraction (HR-XRPD) was combined with x-ray absorption spectroscopy (XAS) to reveal the changes occurring during the exptl. 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, hcp. (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 were found as a function of the length of the catalytic bed.