On the Cobalt Carbide Formation in a Co/TiO2 Fischer-Tropsch Synthesis Catalyst as Studied by High-Pressure, Long-Term Operando X-ray Absorption and Diffraction

Ilse K van Ravenhorst; Adam S Hoffman; Charlotte Vogt; Alexey Boubnov; Nirmalendu Patra; Ramon Oord; Cem Akatay; Florian Meirer; Simon R Bare; Bert M Weckhuysen

doi:10.1021/acscatal.0c04695

On the Cobalt Carbide Formation in a Co/TiO₂ Fischer-Tropsch Synthesis Catalyst as Studied by High-Pressure, Long-Term Operando X-ray Absorption and Diffraction

ACS Catal. 2021 Mar 5;11(5):2956-2967. doi: 10.1021/acscatal.0c04695. Epub 2021 Feb 19.

Authors

Affiliations

¹ Inorganic Chemistry and Catalysis Group, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, Utrecht 3584 CG, The Netherlands.
² SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource (SSRL), Menlo Park, California 94025, United States.
³ Institute of Chemistry and The Center for Nanoscience and Nanotechnology, Hebrew University of Jerusalem, Jerusalem91905, Israel.
⁴ Department of Chemical and Biological Physics, Weizmann Institute of Science, 234 Herzl Street, Rehovot 76100, Israel.
⁵ Honeywell UOP, Des Plaines, Illinois 60016, United States.

Abstract

Operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) were performed on a Co/TiO₂ Fischer-Tropsch synthesis (FTS) catalyst at 16 bar for (at least) 48 h time-on-stream in both a synchrotron facility and a laboratory-based X-ray diffractometer. Cobalt carbide formation was observed earlier during FTS with operando XAS than with XRD. This apparent discrepancy is due to the higher sensitivity of XAS to a short-range order. Interestingly, in both cases, the product formation does not noticeably change when cobalt carbide formation is detected. This suggests that cobalt carbide formation is not a major deactivation mechanism, as is often suggested for FTS. Moreover, no cobalt oxide formation was detected by XAS or XRD. In other words, one of the classical proposals invoked to explain Co/TiO₂ catalyst deactivation could not be supported by our operando X-ray characterization data obtained at close to industrially relevant reaction conditions. Furthermore, a bimodal cobalt particle distribution was observed by high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray analysis, while product formation remained relatively stable. The bimodal distribution is most probably due to the mobility and migration of the cobalt nanoparticles during FTS conditions.