In Situ X-ray Raman Scattering Spectroscopy of the Formation of Cobalt Carbides in a Co/TiO₂ Fischer–Tropsch Synthesis Catalyst

Abstract

We present in-situ experiments to study the possible formation of cobalt carbides during Fischer-Tropsch synthesis (FTS) in a Co/TiO2 catalyst at relevant conditions of pressure and temperature.The experiments were performed using a combination of X-ray Raman scattering (XRS) spectroscopy and X-ray diffraction (XRD). Two different experiments were performed: (1) a Fischer-Tropsch Synthesis (FTS) reaction of a ~ 14 wt.% Co/TiO2 catalyst at 523 K and 5 bar under H2-lean conditions (i.e., a H2:CO ratio of 0.5) and ( 2) carburization of pure cobalt (as reference experiment). In both experiments, the Co L3-edge XRS spectra reveal a change in the oxidation state of the cobalt nanoparticles, which we assign to the formation of cobalt carbide (Co2C). The C K-edge XRS spectra were used to quantify the formation of different carbon species in both experiments.

Theoretical Perspectives on the Modulation of Carbon on Transition-Metal Catalysts for Conversion of Carbon-Containing Resources

“…An in situ synchrotron XRD study by the Claeys group confirmed the formation of a small amount of Co 2 C in a Co-catalyzed FTS process . This was recently further verified by de Groot and co-workers using in situ X-ray Raman scattering spectroscopy . Our facet-dependent C permeation studies provide an additional explanation for this phenomenon: under FTS conditions, the Co particles expose mostly the close-packed hcp (0001) or fcc (111) facets, where the C atoms only deposit on these facets to form graphene-like structures without permeation.…”

“…Li and co-workers 100 found that the energy change associated with the formation of Co 2 C from Co and syngas (2Co 94 This was recently further verified by de Groot and co-workers using in situ X-ray Raman scattering spectroscopy. 107 Our facet-dependent C permeation studies provide an additional explanation for this phenomenon: under FTS conditions, the Co particles expose mostly the close-packed hcp (0001) or fcc (111) facets, where the C atoms only deposit on these facets to form graphene-like structures without permeation. The formation of bulk Co 2 C is only possible on the other facets, which make up only a small fraction of the Co catalyst.…”

“…Li et al attributed this phenomenon to the formation of graphitic carbon (2Co + CO + H 2 → Co + C graphite + H 2 O), which is energetically more favorable (−1.18 eV). This, however, could not explain the observation of fractional Co 2 C components in experiments94,107 and the deactivation of the Co catalyst in FTS due to carburization after long-term use of the catalyst. An in situ synchrotron XRD study by the Claeys group confirmed the formation of a small amount of Co 2 C in a Co-catalyzed FTS process.…”

X-ray Raman Scattering: A Hard X-ray Probe of Complex Organic Systems

“…Comprehensive reviews on high-pressure XRS studies were provided by Rueff and Shukla, Lee et al, Sternemann and Wilke . XRS has been employed for in situ time-resolved investigation of chemical reactions under well-defined thermodynamic conditions aiming to understand the reaction pathways. ,,− More recently, XRS studies have focused on the study of lithium compounds linked with the emerging interest in lithium-based batteries and/or energy materials in general. ,− …”

Photoelectron velocity-map imaging spectroscopy of nickel carbide: examination of the low-lying electronic states

“…Since the pioneering discovery at the end of the 19 th century, 1 transition-metal (TM) carbides have been of continuous academic and industrial interest to their application in heterogeneous catalysis [2][3][4] of the hydrodenitrogenation reaction, 5,6 CO 2 hydrogenation, [7][8][9] and Fischer-Tropsch synthesis, [10][11][12][13] besides their great astrochemical relevance. [14][15][16] For example, nickel carbides, such as NiC and Ni x C, are considered as essential intermediates in nickel-based catalytic processes.…”