Cobalt-catalyzed Fischer Tropsch Synthesis Research Articles

Atomic-scale understanding of the impact of CoO phase in cobalt-catalyzed Fischer-Tropsch synthesis: A combined computational and experimental study

In cobalt-catalyzed Fischer-Tropsch synthesis (FTS), the active metallic cobalt phase always suffers from oxidation under FTS reaction conditions, leading to the coexistence of metallic cobalt and cobalt oxide (CoO) phases. However, there still exist contradictory views on the impact of CoO phase in FTS. Some researchers proposed that the oxidation of metallic Co to CoO causes the deactivation of cobalt-based FTS catalysts, while some held the view that CoO phase could act as the potential active phase in FTS. In this study, by combining theoretical calculations and experiments, we systematically investigated the fundamental FTS reactions including CO adsorption and activation, C-C coupling, and CxHy hydrogenation on CoO phase, and compared them with those on metallic cobalt phase, aiming at identifying the exact role of CoO, understanding how CoO influences the activity and selectivity, and unraveling the intrinsic mechanism that determines the catalytic behaviors of CoO phase in FTS. The results reveal that the CoO(200) crystal facet is the dominantly exposed surface structure for CoO phase under FTS reaction conditions. The d-band center of CoO(200) surface shifts downward in comparison with that of active metallic Co surface, which causes difficulties in adsorbing and activating CO and gives rise to high energy barriers for the dissociation of C-O bond. This determines the inactive nature of CoO phase in FTS reaction. As for the product selectivity, the CoO(200) surface restricts the C-C chain growth due to its higher barriers for C-C coupling, facile desorption ability for light olefins, and lower energy barrier for CH4 formation compared with metallic cobalt, and consequently leads to the loss of selectivity to C5+ hydrocarbons and enhancement of selectivity to light olefins and undesired methane. Moreover, a strong linear relationship between the experimental observed selectivity change and the DFT-calculated energy difference of CoO and metallic cobalt phases was found, validating the DFT-calculated results. This study provides a comprehensive understanding of the nature and intrinsic catalytic behaviors of CoO phase in cobalt-catalyzed FTS.

Enhancing Kerosene Selectivity of Fischer-Tropsch Synthesis by Periodical Pore Drainage Via Hydrogenolysis

Accumulation of wax inside the catalyst pores during transient cobalt-catalyzed Fischer-Tropsch synthesis (FTS) leads to unfavorable product distribution and low activity by imposing internal mass transfer limitations. The condensation of paraffin severely changes the apparent product stream that actually leaves the reactor before the catalyst pores are filled completely and the steady state is reached. Thus, the product distribution of the transient FTS is less complex than expected in comparison to the steady-state FTS and increasingly consists of hydrocarbons (HCs) with an average chain length in the range of kerosene (C<sub>9</sub>-C<sub>17</sub>). So, in order to prevent FTS from reaching a steady state, the pores are drained periodically by hydrogenolysis (HGL). The alternating HGL is realized by a switch from syngas (H<sub>2</sub>, CO) to pure hydrogen at a reaction temperature in the range of 210°C to 240°C. The alternating process leads to an improvement in kerosene selectivity of 48%, 37%, and 28% at 210°C, 220°C and 240°C, respectively. Furthermore, the influence of temperature on the hydrogenolysis of long-chain HCs was experimentally investigated. It was found that temperature affects methane selectivity severely. A high hydrogenolysis temperature is favorable as this leads to a severely decreased overall methane selectivity and, thus to a higher production rate of alkanes within the carbon number range of 9 to 17.

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.

Understanding the promotional effect of Pt/CeO2 in cobalt-catalyzed Fischer-Tropsch synthesis using operando infrared spectroscopy at moderated pressures

Fischer-Tropsch (FTS) reaction is a well-known catalytic process for the conversion of synthesis gas into liquid fuels. The addition of a water gas shift (WGS) catalyst to the FTS one has been postulate to notably increase the efficiency of the process. In order to investigate this issue, we conducted the FTS reaction over a Co-Re/Al2O3 catalyst combined with an optimal WGS Pt/CeO2 catalyst. We observed a notable increase of CO conversion in presence of the Pt/CeO2 catalyst that a priori could be attributed to the WGS reaction. However, the WGS reaction is unfavourable at pressures higher than 1 bar and CO/CO2 hydrogenation over Pt/CeO2 could be more favoured under FTS reaction conditions. In order to gain insights on this fact and elucidate the role of Pt/CeO2 in the FTS reaction we have performed an operando DRIFTS-MS study under close FTS reaction conditions at 4 bar over the Pt/CeO2 catalyst.

Detailed microkinetic modelling of syngas to hydrocarbons via Fischer Tropsch synthesis over cobalt catalyst

Fuels production from syngas via Fischer Tropsch synthesis (FTs) is an alternative technology for clean energy production. The microkinetic model is a promising approach for gaining insight into FTs activity. In this study, a systematic microkinetic model was proposed to develop a process for cobalt-catalyzed FTs. All possible elementary reactions based on the carbide mechanism and characteristics of catalyst sites were considered in the kinetic model. The effects of the reaction rate constant, reaction pathways, and H2 to CO ratio were represented by a kinetic parameter, reaction path, and operating parameter, respectively. The model could accurately predict product distribution trends, with an R2 value and mean absolute relative residuals percentage of 0.91–0.93 and 5–43%, respectively, in comparison with experimental data. Hydrogen utilization was predicted and analyzed. High model accuracy was achieved, with a 10−10–10−3% error in the material balance.

Insight into the Intrinsic Active Site for Selective Production of Light Olefins in Cobalt-Catalyzed Fischer–Tropsch Synthesis

Selective production of light olefins remains a great challenge in cobalt-catalyzed Fischer–Tropsch synthesis (FTS). Understanding the nature of the active site and the structure–performance relati...

Investigation of C1 + C1 Coupling Reactions in Cobalt-Catalyzed Fischer-Tropsch Synthesis by a Combined DFT and Kinetic Isotope Study

Understanding the chain growth mechanism is of vital importance for the development of catalysts with enhanced selectivity towards long-chain products in cobalt-catalyzed Fischer-Tropsch synthesis. Herein, we discriminate various C1 + C1 coupling reactions by theoretical calculations and kinetic isotope experiments. CHx(x=0−3), CO, HCO, COH, and HCOH are considered as the chain growth monomer respectively, and 24 possible coupling reactions are first investigated by theoretical calculations. Eight possible C1 + C1 coupling reactions are suggested to be energetically favorable because of the relative low reaction barriers. Moreover, five pathways are excluded where the C1 monomers show low thermodynamic stability. Effective chain propagation rates are calculated by deconvoluting from reaction rates of products, and an inverse kinetic isotope effect of the C1 + C1 coupling reaction is observed. The theoretical kinetic isotope effect of CO + CH2 is inverse, which is consistent with the experimental observation. Thus, the CO + CH2 pathway, owing to the relatively lower barrier, the high thermodynamic stability, and the inverse kinetic isotope effect, is suggested to be a favorable pathway.

Accumulation of liquid hydrocarbons during cobalt-catalyzed Fischer–Tropsch synthesis - influence of activity and chain growth probability

It may take over one year in order to fill FT catalyst pores, depending on activity and chain growth probability.

Insight into the structure and morphology of Run clusters on Co(111) and Co(311) surfaces

The adsorption configurations, growth modes and morphology of a Ru promoter under the approximate conditions of cobalt catalyzed Fischer–Tropsch synthesis (FTS) were investigated by density functional theory (DFT) calculations.

Ga and In modified ceria as supports for cobalt-catalyzed Fischer-Tropsch synthesis

Ga- and In-modified ceria (Ce0.8Ga0.2O2, Ce0.8In0.2O2) materials were used as supports for cobalt-catalyzed Fischer-Tropsch synthesis (FTS). The addition of Ga to ceria was found to improve CO conversion for cobalt-catalyzed FTS, while the addition of In tended to decrease it. A similar trend was observed with the Ag-promoted cobalt/ceria catalysts. Doping of ceria with Ga or In decreased methane and increased the selectivity to olefins and alcohols for Ag-promoted cobalt/ceria. The sum of the products of olefins and alcohols for various catalysts exhibited a decreasing trend as follows: Ag-Co/Ce-Ga>Ag-Co/Ce-In>Ag-Co/Ce. Results of H2-TPR-XANES showed that adding of Ga or In to ceria increases the fraction of Ce3+ in the surface shell for both unpromoted and Ag-promoted catalysts in the range of temperature typical of catalyst activation. This partially reduced ceria plays an important role in controlling the product selectivity of cobalt-catalyzed FT synthesis.

Adsorption energy-driven carbon number-dependent olefin to paraffin ratio in cobalt-catalyzed Fischer-Tropsch synthesis

The factor dominating olefin to paraffin ratio in cobalt-catalyzed Fischer-Tropsch synthesis has been identified via experimental kinetic studies and theoretical calculations including the van der Waals interaction between the adsorbates and Co surfaces. The olefin to paraffin ratio for each carbon number is expressed as a function of adsorption energies of olefin and activation energies of hydrogenation steps. Density functional theory calculations on Co (0001) surface were performed to provide a comprehensive fundamental insight for the chemistry governing the olefin to paraffin ratio. The calculated olefin to paraffin ratio decreases with chain length except for ethylene, which agrees well with the experimental results. The differences introduced to the adsorption energies of olefin by van der Waals functional are correlated with d-band center changes of the Co surface. The carbon number dependent adsorption energies of olefin, rather than the activation energies of their hydrogenation reactions, are the driven force for the carbon number dependent olefin to paraffin ratio. The insights may delineate a new blueprint for the rational design of catalysts with enhanced olefin to paraffin ratio.

Spectroscopic insights into cobalt-catalyzed Fischer-Tropsch synthesis: A review of the carbon monoxide interaction with single crystalline surfaces of cobalt

The present article summarizes experimental findings of the interaction of CO with single crystal surfaces of cobalt. We first provide a quantitative study of non-dissociative CO adsorption on Co(0001) and establish a quantitative correlation between θCO and adsorption site occupation. In light of these findings we revisit the structure of previously reported ordered CO/Co(0001) adsorbate layers. Measurements of the CO coverage at equilibrium conditions are used to derive a phase diagram for CO on Co(0001). For low temperature Fischer-Tropsch synthesis conditions the CO coverage is predicted to be ≈0.5ML, a value that hardly changes with pCO. The CO desorption temperature found in temperature programmed desorption is practically structure-independent, despite structure-dependent heats of adsorption reported in the literature. This mismatch is attributed to a structure-dependent pre-exponential factor for desorption. IR spectra reported throughout this study provide a reference point for IR studies on cobalt catalysts. Results for CO adsorbed on flat and defect-rich Co surfaces as well as particular, CO adsorbed on top sites, and in addition affect the distribution of COad over the various possible adsorption sites.

Accumulation of liquid hydrocarbons in catalyst pores during cobalt-catalyzed Fischer–Tropsch synthesis

Formation, vaporization and accumulation of liquid hydrocarbon products during FTS in a single catalyst pore.

Interplay of reaction and pore diffusion during cobalt-catalyzed Fischer–Tropsch synthesis with CO2-rich syngas

For cobalt-catalyzed Fischer–Tropsch synthesis (FTS), a model was developed to analyze numerically the reaction–diffusion performance in catalyst particles. The model is based on a mesoporous particle and is valid for CO2-free and -rich syngas. The kinetic parameters of all three relevant reactions, CO hydrogenation to C2+ hydrocarbons as well as CH4 and CO2 hydrogenation (mainly) to CH4, were derived at intrinsic conditions (dp=150μm). Thereafter, the effective kinetics considering pore diffusion limitations were derived by using a homemade catalyst in 5mm diameter. From the data measured, a kinetic approach for conversion of CO2 inside the catalyst particle was developed. The experimentally derived Langmuir–Hinshelwood type rate expressions and a variable chain growth parameter α, dependent on temperature and syngas ratio, were implemented into the model. Furthermore, the change of the local reaction rates and selectivities, as a consequence of changing syngas ratio due to diffusion limitation is taken into account. The simulated data of effective kinetics and selectivities are in agreement with the measured data. The simulation predicts that CO2 is only converted in the CO-free core region of large catalyst particles at high temperatures and strong pore diffusion limitations. For CO2 converts mainly to CH4 (selectivity 95%C), a slightly increased overall methane selectivity is expected indicating consumption of CO2. However, this effect was not measurable even with 5mm particles at high temperatures as methanation of CO2 occurs only to a minor extent even at pronounced diffusion limited conditions and is negligible at industrial conditions.

In Situ Scanning Tunneling Microscopy of the Dissociation of CO on Co(0001)

The dissociation of CO, a crucial step in the cobalt-catalyzed Fischer–Tropsch synthesis, was investigated by in situ scanning tunneling microscopy on Co(0001) at CO pressures up to 0.25 mbar and temperatures up to 493 K. A new type of surface reconstruction was observed that is formed by carbidic carbon, as shown by X-ray photoelectron spectroscopy. Using 13CO and titrating 13C with O2, the surface carbon is shown to originate from CO. The results are in contrast to predictions by theoretical work. However, the dissociation rates are 3 orders of magnitude lower than the previously measured methanation rates under equivalent conditions.

Cobalt-Catalyzed Fischer–Tropsch Synthesis: Chemical Nature of the Oxide Support as a Performance Descriptor

The effect of the chemical nature of the oxide support on the performance of cobalt Fischer–Tropsch catalysts is investigated. A series of supports is synthesized via monolayer coverage of porous γ-Al2O3 with various oxides representative of a wide range of Lewis acid–base character, as quantified by UV–vis spectroscopy coupled to alizarin adsorption. Incorporation of cobalt (20 wt %) results in model catalysts with identical porosities and similar Co particle sizes (>10 nm), allowing the study of support effects without overlap from diffusional or particle size factors. Under realistic reaction conditions, the initial TOF scales with the acidity of the oxide support, whereas the cobalt time yield and selectivity to industrially relevant C13+ hydrocarbons show a volcano dependence, with a maximum at an intermediate acid–base character. As inferred from in situ CO-FTIR, “selective” blockage of a few cobalt sites, though crucial for CO hydrogenation, by atoms from basic oxides and “unselective” site blockag...

Cobalt Catalyzed Fischer–Tropsch Synthesis: Perspectives Opened by First Principles Calculations

Starting from an overview of general concepts in Fischer–Tropsch synthesis, this perspective article focuses on relevant theoretical works based on density functional theory (DFT) dealing with cobalt based Fischer–Tropsch (FT) catalysis. Our bibliographic analysis highlights key scientific questions in a challenging field for research and industry. Starting from the historical and empirical concepts, we discuss how recent DFT studies give insights into the catalyst structure, the FT reaction mechanisms and the deactivation pathways. There is still no clear consensus within the scientific community about the FT reaction mechanism. Some authors propose that the first elementary step in the FT synthesis consists on CO dissociation taking place at the steps of the cobalt active phase (carbide mechanism) while some others suggest that this molecule is hydrogenated prior to its dissociation no matter the structure of the active site (insertion mechanism). As for the crucial deactivation problem, according to the computed thermodynamic properties presented by some publications, the catalyst oxidation does not seem to play a major role in its deactivation process. By contrast, the interaction of C addatoms with the Co active phase can have a strong influence on the catalyst reactivity and selectivity, through a series of structural rearrangements: the formation of various more or less unsaturated hydrocarbon molecules (such as polyaromatic rings) and the formation of a surface carbide phase. However, the impact of these phenomena on the reactivity and selectivity of the catalyst remain to be investigated in the future.

Effects of Reaction Conditions on Cobalt-Catalyzed Fischer-Tropsch Synthesis: Interactions between Operating Factors

In Fischer-Tropsch Synthesis, because of few reactants (<TEX>$H_2$</TEX>, CO), scarce operating parameters affected on efficiency especially the selectivity of products. In this research, effect of operating parameters on the selectivity of Co-Mn-<TEX>$TiO_2$</TEX> Fischer-Tropsch synthesis catalyst were studied by design of experimental procedure and Taguchi method. According to this research, interactions between operating factors have a crucial effect on light products selectivity. Among these interactions, (temperature<TEX>${\times}$</TEX>feed ratio) has the main influence on light hydrocarbons selectivity. It was concluded that temperature and feed ratio (<TEX>$H_2$</TEX>/CO) were the most integral operating parameters for much greater selectivity of light hydrocarbons.

Deuterium tracer studies on cobalt catalyzed Fischer–Tropsch synthesis: Addition of alcohols

Unlike the iron catalyzed Fischer–Tropsch (FT) synthesis, the additions of ethanol-d 6 and 2-propanol-d 8 during the Co/SiO 2 catalyzed FT reactions did not produce detectable amount of deuterated isomer in any alkane. However, when the FT reactions were conducted over a mixture of FT catalyst Co/SiO 2 and dehydration catalyst Al 2O 3, the d 1, d 2 and d 3 isomers of alkanes from C 7 to C 15 were obtained with a nearly constant D/molecule value of 1.4 when ethanol-d 6 was used as the tracer while d 1–d 6 isotopic isomers of alkanes were observed with a nearly constant D/molecule value of 2.3 when 2-propanol-d 8 was used. The similar results were obtained when 1-propanol was used as the tracer. These results lead us to conclude that the C 2 or C 3 species that initiates the chain growth in the FT reaction resembles ethene or propene. The pathways for the formations of ethylidene and propylidene were proposed.

Inverse kinetic isotope effects and deuterium enrichment as a function of carbon number during formation of C–C bonds in cobalt catalyzed Fischer–Tropsch synthesis

How the deuterium is enriched in hydrocarbons produced by Fischer–Tropsch (FT) synthesis is of broad scientific interest, particularly to those studying the mechanism of the FT reaction and the formation of hydrocarbons in the Earth's crust or in the nebula. In order to determine whether there is an inverse isotope effect and whether there exists the deuterium enrichment in hydrocarbons during the FT reaction, we performed the H 2/D 2 switching experiments in cobalt catalyzed FT reactions and found that the values of α H/ α D are in a range of 0.93–0.98 under five different reaction conditions, a clear indication of the inverse kinetic isotope effect. We also conducted the H 2/D 2 competition experiments using an equal molar ratio of D 2 and H 2 as the reagents and found that the H/D ratios in alkanes from C 6 to C 24 obtained are less than 1 and decrease with increasing molecular size. To account for these results, we proposed a modified alkylidene mechanism for the FT reaction. By combining the modified alkylidene mechanism with the Anderson–Schulz–Flory equation, the results obtained in this study can be quantitatively explained and the isotope fractionation patterns and deuterium enrichment observed in abiogenic hydrocarbons produced by the FT-type reactions can be predicted.