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,Cem Akatay,Simon R Bare,Ramon Oord,Charlotte Vogt,Nirmalendu Patra,Alexey Boubnov,Florian Meirer,Bert M Weckhuysen

doi:10.1021/acscatal.0c04695

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.

Highlights

Cobalt nanoparticles (NPs) supported on TiO2 comprise one of the most industrially applied Fischer−Tropsch synthesis (FTS) catalysts
The catalytic activity of the 10 wt % cobalt NPs supported on TiO2 (Co/TiO2) FTS catalyst was measured by on-line GC, while measuring
Via the combination of operando X-ray spectroscopy and diffraction methods, we were able to follow the state of Co within a Co/TiO2 catalyst material under realistic FTS conditions and most notably observed cobalt carburization without a decrease in measured catalyst activity and without cobalt oxide formation

Summary

Introduction

Cobalt nanoparticles (NPs) supported on TiO2 comprise one of the most industrially applied Fischer−Tropsch synthesis (FTS) catalysts. The proposed deactivation mechanisms for Co-based FTS catalysts are related to the conversion of the active metallic phase into an inert phase via, for example, Co re-oxidation or carburization,[1,14,21−23] the formation of support-Co phases that form through strong metal−support interactions,[18,24−26] the loss of active surface area due to crystalline growth,[1,12,13,27] and by fouling via, for example, hydrocarbon deposition in the form of coke species.[17,28] In particular, the effect of Received: October 28, 2020 Revised: January 5, 2021 Published: February 19, 2021

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Conclusion