Cobalt and iron based Fischer-Tropsch synthesis (FTS) catalysts have been studied at conditions close to industrial conditions using a range of in situ techniques; X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), and Raman scattering. The combination of these techniques allows for probing both electronic and structural perturbations in the catalysts leading to changes in activity. Characterization of the FTS catalysts during activation, reaction and initial deactivation will be discussed. Potential deactivation mechanisms such as re-oxidation, carbonaceous deposits and metal-support interactions are discussed in detail. We have used a combined in situ XAS/XRD/Raman catalyst characterization set-up able to operate at 20 bar pressure and high temperatures while simultaneously monitoring the catalyst activity. A rapid 2D CMOS detector allows for XRD acquisition on the same time-scale as collecting XAS spectra (5-10 sec). Additional measurements using in situ Raman and FT-IR on the same catalysts at similar conditions provide comprehensive information on the state of catalysts during FTS. The in situ set-up has been used for modulation enhanced XAS where the catalysts are being exposed to periodic changes in reaction environment. Such in situ experiments generate large amounts of data which require new strategies for data analysis. The application of multivariate curve resolution (MCR) applied to large data-sets from in situ XAS experiments will be presented. The results show that the information which can be extracted from MCR goes well beyond the information from conventional XAS experiments. XAS and XRD have been used to capture the metal-support interaction in situ. The expansion of the unit cell of the γ-Al2O3 support is an indirect evidence of partial diffusion of Co2+ into the γ-Al2O3 lattice taking place during reduction . We also demonstrate that the overall reduction process is inhibited by in situ generated H2O. It is also shown that cobalt nanoparticles supported on γ-Al2O3 re-oxidizes to Co2+ during the first hours of FT synthesis. The Co2+ species are forming an inactive cobalt aluminate phase. We demonstrate that these phenomena are highly dependent on conversion level and location inside the fixed bed reactor. Further, it is discussed how in situ FT-IR can be used to reveal the nature of refractory species accumulating on the catalyst surface during FTS.

Combining in situ characterization techniques to reveal real-time deactivation mechanisms in Fischer-Tropsch synthesis catalysts

253rd National Meeting (2017)

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