Mechanism of Cobalt-Catalyzed CO Hydrogenation: 2. Fischer–Tropsch Synthesis

Chen, W Wei; Filot, Ivo A. W.; Pestman, Robert; Hensen, Emiel J. M.

doi:10.1021/acscatal.7b02758

Cited by 97 publications

(106 citation statements)

References 68 publications

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“…The higher coverage at the step-edge sites also suppresses cleavage of the growing hydrocarbon chains. 55 This view is also consistent with the C 3 H 6 /H 2 reaction experiments (Figure 10), in which C–C coupling is facilitated by the presence of graphitic carbon on terraces, since (i) CH x migration to terraces is suppressed and (ii) higher CH x coverage on step-edge sites suppresses C–C cleavage. Of equal importance is then the observation that the presence of graphitic carbon during C 3 H 6 /H 2 conversion decreases the CH 4 selectivity.…”

Section: Discussionsupporting

confidence: 86%

“…55 Here, we did not use CO as a reactant in order to exclude any influence of CO coverage. 55 The cobalt catalyst containing graphitic carbon was prepared by 13 CO exposure at 260 °C for 30 min followed by H 2 exposure at 260 °C for 30 min. By using labeled 13 CO for deposition, we can track the origin of the carbon atoms in the hydrocarbon products in subsequent C 3 H 6 /H 2 reaction experiments.…”

Section: Resultsmentioning

confidence: 99%

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Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer–Tropsch Reaction: Evidence of a Two-Site Reaction Model

et al. 2018

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One of the well-known observations in the Fischer–Tropsch (FT) reaction is that the CH4 selectivity for cobalt catalysts is always higher than the value expected on the basis of the Anderson–Schulz–Flory (ASF) distribution. Depositing graphitic carbon on a cobalt catalyst strongly suppresses this non-ASF CH4, while the formation of higher hydrocarbons is much less affected. Carbon was laid down on the cobalt catalyst via the Boudouard reaction. We provide evidence that the amorphous carbon does not influence the FT reaction, as it can be easily hydrogenated under reaction conditions. Graphitic carbon is rapidly formed and cannot be removed. This unreactive form of carbon is located on terrace sites and mainly decreases the CO conversion by limiting CH4 formation. Despite nearly unchanged higher hydrocarbon yield, the presence of graphitic carbon enhances the chain-growth probability and strongly suppresses olefin hydrogenation. We demonstrate that graphitic carbon will slowly deposit on the cobalt catalysts during CO hydrogenation, thereby influencing CO conversion and the FT product distribution in a way similar to that for predeposited graphitic carbon. We also demonstrate that the buildup of graphitic carbon by 13CO increases the rate of C–C coupling during the 12C3H6 hydrogenation reaction, whose products follow an ASF-type product distribution of the FT reaction. We explain these results by a two-site model on the basis of insights into structure sensitivity of the underlying reaction steps in the FT mechanism: carbon formed on step-edge sites is involved in chain growth or can migrate to terrace sites, where it is rapidly hydrogenated to CH4. The primary olefinic FT products are predominantly hydrogenated on terrace sites. Covering the terraces by graphitic carbon increases the residence time of CHx intermediates, in line with decreased CH4 selectivity and increased chain-growth rate.

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Section: Discussionsupporting

confidence: 86%

Section: Resultsmentioning

confidence: 99%

Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer–Tropsch Reaction: Evidence of a Two-Site Reaction Model

et al. 2018

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“…The complexity of the FTS process is also captured in the myriad of proposed deactivation mechanisms, which are generally related to the conversion of the active phase, considered as metallic cobalt, into an inert phase. For example, cobalt reoxidation or carburization, the formation of support oxide–cobalt species occurring through strong metal–support interactions (SMSI), the loss of active cobalt surface area arising from crystalline growth (i.e., metal sintering), and finally fouling for example by hydrocarbon deposition in the form of various carbon species formed at the cobalt surface …”

Section: Figurementioning

confidence: 99%

Capturing the Genesis of an Active Fischer–Tropsch Synthesis Catalyst with Operando X‐ray Nanospectroscopy

et al. 2018

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Astate-of-the-art operando spectroscopic technique is applied to Co/TiO 2 catalysts,which account for nearly half of the worldst ransportation fuels produced by Fischer-Tropsch catalysis.This allows determination of,ataspatial resolution of approximately 50 nm, the interdependence of formed hydrocarbon species in the inorganic catalyst. Observed trends show intra-and interparticular heterogeneities previously believed not to occur in particles under 200 mm. These heterogeneities are strongly dependent on changes in H 2 /CO ratio,but also on changes therebyi nduced on the Co and Ti valence states.W e have captured the genesis of an active FTS particle over its propagation to steady-state operation, in which microgradients lead to the gradual saturation of the Co/TiO 2 catalyst surface with long chain hydrocarbons (i.e., organic film formation).Heterogeneous catalytic reactions such as the Fischer-Tr opsch synthesis (FTS) of long-chain hydrocarbons are dynamic and complex, often comprised of inorganic (catalyst) and organic parts (reactants and products). Conventional and widely applied spectroscopic techniques often focus on either the organic part (e.g., vibrational spectroscopy) or on the inorganic part (e.g., X-ray spectroscopic techniques). The combination of both inorganic and organic information shows great promise to answer long-standing questions about complex catalytic reactions.T he FTS propagates through ac omplex surface polymerization process of adsorbed C 1 reaction intermediates derived from synthesis gas (a mixture of CO and H 2 ). [1][2][3][4][5][6][7] Cobalt-based FTS catalysts are an integral part of this gas-to-liquid (GTL) process because of their high wax selectivity and relatively high stability. [8,9] Theactivation and deactivation of these cobalt nanoparticles supported on an inorganic oxide,such as Al 2 O 3 or TiO 2 ,isbelieved to occur through am ultitude of mechanisms,h owever consensus in literature is often still lacking. While the literature is imbued with proposed deactivation mechanisms, [5,10,11] the equally interesting catalyst activation period is often overlooked. [12,13] During the day(s)-long activation or induction period (which is highly dependent on reaction conditions and the catalyst), FTS catalyst particles are believed to be gradually saturated by afilm of long-chain hydrocarbons,followed by pore filling through which further reactants must diffuse. [12,14,15] The complexity of the FTS process is also captured in the myriad of proposed deactivation mechanisms,w hich are generally related to the conversion of the active phase, considered as metallic cobalt, into an inert phase.F or example,cobalt reoxidation or carburization, [16,17] the formation of support oxide-cobalt species occurring through strong metal-support interactions (SMSI), [8,18,19] the loss of active cobalt surface area arising from crystalline growth (i.e., metal sintering), [11,[20][21][22] and finally fouling for example by hydrocarbon deposition in the form of various carbon species formed at the cob...

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“…[12] Definitive proofs for the role of cobalt carbides in FTS has yet not been provided, partly because of the difficulty to perform in-situ experiments that can correlate the formation of cobalt carbide with the performance of the reaction. [10][11][12][13][14][15] Standard experiments fail in unambiguously identifying the carbide species; X-ray Diffraction (XRD) for example is not very sensitive to the (possibly amorphous) cobalt carbide phases, while all ex-situ experiments may be unreliable because of the chemical instability of cobalt carbide in the absence of reaction conditions. X-ray absorption spectroscopy techniques, on the other hand, are element-specific and able to probe the local structure around and the valence state of Co in the catalyst.…”

Section: Introductionmentioning

confidence: 99%

Elucidating the K‐Edge X‐Ray Absorption Near‐Edge Structure of Cobalt Carbide

et al. 2019

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The cobalt K‐edge X‐ray absorption near‐edge structure of a cobalt carbide compound, synthetized under in‐situ conditions, has been determined by using laboratory‐based and synchrotron‐based X‐ray absorption spectroscopy. We have carburized pure cobalt metal in‐situ, avoiding all adverse effects of metal‐support interactions as well as any degradation of the cobalt carbide formed. A non‐negative matrix factorization was applied to determine the features of the spectrum of cobalt carbide, highly needed to identify this phase in an unambiguous manner in cobalt‐based Fischer‐Tropsch catalysts.

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Mechanism of Cobalt-Catalyzed CO Hydrogenation: 2. Fischer–Tropsch Synthesis

Cited by 97 publications

References 68 publications

Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer–Tropsch Reaction: Evidence of a Two-Site Reaction Model

Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer–Tropsch Reaction: Evidence of a Two-Site Reaction Model

Capturing the Genesis of an Active Fischer–Tropsch Synthesis Catalyst with Operando X‐ray Nanospectroscopy

Elucidating the K‐Edge X‐Ray Absorption Near‐Edge Structure of Cobalt Carbide

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