Reflections on the Fischer-Tropsch synthesis: Mechanistic issues from a surface science perspective

The Impact of Oxygen Surface Coverage and Carbidic Carbon on the Activity and Selectivity of Two-Dimensional Molybdenum Carbide (2D-Mo2C) in Fischer-Tropsch Synthesis

Transformations of oxygenates (CO2, CO, H2O, etc.) via Mo2C-based catalysts are facilitated by the high oxophilicity of the material; however, this can lead to the formation of oxycarbides and complicate the identification of the (most) active catalyst state and active sites. In this context, the two-dimensional (2D) MXene molybdenum carbide Mo2CTx (Tx are passivating surface groups) contains only surface Mo sites and is therefore a highly suitable model catalyst for structure-activity studies. Here, we report that the catalytic activity of Mo2CTx in Fischer-Tropsch (FT) synthesis increases with a decreasing coverage of surface passivating groups (mostly O*). The in situ removal of Tx species and its consequence on CO conversion is highlighted by the observation of a very pronounced activation of Mo2CTx (pretreated in H2 at 400 °C) under FT conditions. This activation process is ascribed to the in situ reductive defunctionalization of Tx groups reaching a catalyst state that is close to 2D-Mo2C (i.e., a material containing no passivating surface groups). Under steady-state FT conditions, 2D-Mo2C yields higher hydrocarbons (C5+ alkanes) with 55% selectivity. Alkanes up to the kerosine range form, with value of α = 0.87, which is ca. twice higher than the α value reported for 3D-Mo2C catalysts. The steady-state productivity of 2D-Mo2C to C5+ hydrocarbons is ca. 2 orders of magnitude higher relative to a reference β-Μo2C catalyst that shows no in situ activation under identical FT conditions. The passivating Tx groups of Mo2CTx can be reductively defunctionalized also by using a higher H2 pretreatment temperature of 500 °C. Yet, this approach leads to a removal of carbidic carbon (as methane), resulting in a 2D-Mo2C1-x catalyst that converts CO to CH4 with 61% selectivity in preference to C5+ hydrocarbons that are formed with only 2% selectivity. Density functional theory (DFT) results attribute the observed selectivity of 2D-Mo2C to C5+ alkanes to a higher energy barrier for the hydrogenation of surface alkyl species relative to the energy barriers for C-C coupling. The removal of O* is the rate-determining step in the FT reaction over 2D-Mo2C, and O* is favorably removed in the form of CO2 relative to H2O, consistent with the observation of a high CO2 selectivity (ca. 50%). The absence of other carbon oxygenates is explained by the energetic favoring of the direct over the hydrogen-assisted dissociative adsorption of CO.

An oscillating reaction to produce clean fuels

Understanding chemical reaction mechanisms could help synthesize sustainable fuels.

The oscillating Fischer-Tropsch reaction

The mechanistic steps that underlie the formation of higher hydrocarbons in catalytic carbon monoxide (CO) hydrogenation at atmospheric pressure over cobalt-based catalysts (Fischer-Tropsch synthesis) have remained poorly understood. We reveal nonisothermal rate-and-selectivity oscillations that are self-sustained over extended periods of time (>24 hours) for a cobalt/cerium oxide catalyst with an atomic ratio of cobalt to cerium of 2:1 (Co2Ce1) at 220°C and equal partial pressures of the reactants. A microkinetic mechanism was used to generate rate-and-selectivity oscillations through forced temperature oscillations. Experimental and theoretical oscillations were in good agreement over an extended range of reactant pressure ratios. Additionally, phase portraits for hydrocarbon production were constructed that support the thermokinetic origin of our rate-and-selectivity oscillations.

Remediation of Saline Wastewater Producing a Fuel Gas Containing Alkanes and Hydrogen Using Zero Valent Iron (Fe0)

Zero valent iron (Fe0) water remediation studies, over the last 40 years, have periodically reported the discovery of CnH2n+2 in the product water or product gas, where n = 1 to 20. Various theories have been proposed for the presence of these hydrocarbons. These include: (i) reductive transformation of a more complex organic chemical; (ii) hydrogenation of an organic chemical, as part of a degradation process; (iii) catalytic hydrogenation and polymerisation of carbonic acid; and (iv) redox transformation. This study uses wastewater (pyroligneous acid, (pH = 0.5 to 4.5)) from a carbonization reactor processing municipal waste to define the controls for the formation of CnH2n+2 (where n = 3 to 9), C3H4, and C3H6. A sealed, static diffusion, batch flow reactor, containing zero-valent metals [181 g m-Fe0 + 29 g m-Al0 + 27 g m-Cu0 + 40 g NaCl] L−1, was operated at two temperatures, 273–298 K and 348 K, respectively. The reactions, reactant quotients, and rate constants for the catalytic formation of H2(g), CO2(g), C3H4(g), C3H6(g), C3H8(g), C4H10(g), C5H12(g), C6H14(g,l), and C7H16(g,l), are defined as function of zero valent metal concentration (g L−1), reactor pressure (MPa), and reactor temperature (K). The produced fuel gas (422–1050 kJ mole−1) contained hydrogen + CnHy(gas), where n = 3 to 7. The gas production rate was: [1058 moles CnHy + 132 moles H2] m−3 liquid d−1 (operating pressure = 0.1 MPa; temperature = 348 K). Increasing the operating pressure to 1 MPa increased the fuel gas production rate to [2208 moles CnHy + 1071 moles H2] m−3 liquid d−1. In order to achieve these results, the Fe0, operated as a “Smart Material”, simultaneously multi-tasking to create self-assembly, auto-activated catalysts for hydrogen production, hydrocarbon formation, and organic chemical degradation (degrading carboxylic acids and phenolic species to CO2 and CO).

Mechanistic Insights into the Effect of Sulfur on the Selectivity of Cobalt-Catalyzed Fischer–Tropsch Synthesis: A DFT Study

Sulfur is a common poison for cobalt-catalyzed Fischer–Tropsch Synthesis (FTS). Although its effects on catalytic activity are well documented, its effects on selectivity are controversial. Here, we investigated the effects of sulfur-covered cobalt surfaces on the selectivity of FTS using density functional theory (DFT) calculations. Our results indicated that sulfur on the surface of Co(111) resulted in a significant decrease in the adsorption energies of CO, HCO and acetylene, while the binding of H and CH species were not significantly affected. These findings indicate that sulfur increased the surface H/CO coverage ratio while inhibiting the adsorption of carbon chains. The elementary reactions of H-assisted CO dissociation, carbon and oxygen hydrogenation and CH coupling were also investigated on both clean and sulfur-covered Co(111). The results indicated that sulfur decreased the activation barriers for carbon and oxygen hydrogenation, while increasing the barriers for CO dissociation and CH coupling. Combining the results on elementary reactions with the modification of adsorption energies, we concluded that the intrinsic effect of sulfur on the selectivity of cobalt-catalyzed FTS is to increase the selectivity to methane and saturated short-chain hydrocarbons, while decreasing the selectivity to olefins and long-chain hydrocarbons.

Short-channel mesoporous SBA-15 silica modified by aluminum grafting as a support for CoRu Fischer–Tropsch synthesis catalysts

Highly ordered short-channel mesoporous silica SBA-15 with large pores (11.2 nm) was synthesized from tetramethyl orthosilicate, using the block copolymer Pluronic PE-10400 as structure-directing agent, and triisopropylbenzene as a swelling agent.

Mechanistic insight into carbon-carbon bond formation on cobalt under simulated Fischer-Tropsch synthesis conditions

Facile C-C bond formation is essential to the formation of long hydrocarbon chains in Fischer-Tropsch synthesis. Various chain growth mechanisms have been proposed previously, but spectroscopic identification of surface intermediates involved in C-C bond formation is scarce. We here show that the high CO coverage typical of Fischer-Tropsch synthesis affects the reaction pathways of C₂H_x adsorbates on a Co(0001) model catalyst and promote C-C bond formation. In-situ high resolution x-ray photoelectron spectroscopy shows that a high CO coverage promotes transformation of C₂H_x adsorbates into the ethylidyne form, which subsequently dimerizes to 2-butyne. The observed reaction sequence provides a mechanistic explanation for CO-induced ethylene dimerization on supported cobalt catalysts. For Fischer-Tropsch synthesis we propose that C-C bond formation on the close-packed terraces of a cobalt nanoparticle occurs via methylidyne (CH) insertion into long chain alkylidyne intermediates, the latter being stabilized by the high surface coverage under reaction conditions.

The mechanism by which C-C bonds form during Fischer-Tropsch synthesis remains debated while spectroscopic identification of reaction intermediates remains scarce. Here, the authors identify alkylidynes as reactive intermediates for C-C bond formation on cobalt terrace sites and moreover show that these intermediates are stabilized by the high surface coverage typical for Fischer-Tropsch synthesis.

Understanding FTS selectivity: the crucial role of surface hydrogen

Monomeric forms of carbon play a central role in the synthesis of long chain hydrocarbons via the Fischer-Tropsch synthesis (FTS). We explored the chemistry of C₁H_xad species on the close-packed surface of cobalt. Our findings on this simple model catalyst highlight the important role of surface hydrogen and vacant sites for product selectivity. We furthermore find that CO_ad affects hydrogen in multiple ways. It limits the adsorption capacity for H_ad, lowers its adsorption energy and inhibits dissociative H₂ adsorption. We discuss how these findings, extrapolated to pressures and temperatures used in applied FTS, can provide insights into the correlation between partial pressure of reactants and product selectivity. By combining the C₁H_x stability differences found in the present work with literature reports of the reactivity of C₁H_x species measured by steady state isotope transient kinetic analysis, we aim to shed light on the nature of the atomic carbon reservoir found in these studies.

Computational investigation of the kinetics and mechanism of the initial steps of the Fischer-Tropsch synthesis on cobalt

A multi-site microkinetic model for the Fischer-Tropsch synthesis (FTS) reaction up to C₂ products on a FCC cobalt catalyst surface is presented. This model utilizes a multi-faceted cobalt nanoparticle model for the catalyst, consisting of the two dominant cobalt surface facets Co(111) and Co(100), and a step site represented by the Co(211) surface. The kinetic parameters for the intermediates and transition states on these sites were obtained using plane-wave, periodic boundary condition density functional theory. Using direct DFT data as is, the microkinetic results disagree with the expected experimental results. Employing an exploratory approach, a small number of microkinetic model modifications were tested, which significantly improved correspondence to the expected experimental results. Using network flux and sensitivity analysis, an in-depth discussion is given on the relative reactivity of the various sites, CO activation mechanisms, the nature of the reactive chain growth monomer, the probable C₂ formation mechanism, the active site ensemble interplay and the very important role of CO* surface coverage. The findings from the model scenarios are discussed with the aim of guiding future work in understanding the FTS mechanism and subsequent controlling kinetic parameters.

Self-Anticoking of a Cobalt Surface by Subsurface Oxygen in the Fischer-Tropsch Synthesis

Understanding the fundamental processes taking place on Co surfaces during the Fischer-Tropsch (FT) synthesis is of great interest and importance. We herein report a self-anticoking mechanism of a cobalt surface by subsurface oxygen. The active carbidic carbon species for FT synthesis tends to transform into the inactive graphitic carbon species on clean Co(0001) and poisons the Co surface. Subsurface atomic oxygen on Co(0001) can stabilize the active carbidic carbon species and quench the transformation process. These results reveal, to the best of our knowledge, for the first time the reactivity of various surface species on Co surfaces that dynamically maintain a delicate balance to enhance the long-term stability of Co catalysts during FT synthesis.

Cobalt and cobalt carbide on alumina/NiAl(110) as model catalysts

Homogeneously dispersed Co nanoparticles on alumina/NiAl(110) exhibit good thermal stability and serve as suitable model catalysts for Fischer–Tropsch synthesis.