Cobalt Fischer Tropsch Catalysts Research Articles

Manganese as a structural promoter in silica-supported cobalt Fischer-Tropsch catalysts under simulated high conversion conditions

Understanding the deactivation mechanism of cobalt-based Fischer-Tropsch catalysts is of significant practical importance. Herein, we explored the role of manganese as a structural promoter on silica-supported cobalt nanoparticles under simulated high CO conversion conditions, i.e., high relative humidity. The structural changes in cobalt dispersion and oxidation state were followed by in situ Mössbauer emission spectroscopy. Adding manganese oxide to silica-supported cobalt enhanced the dispersion of metallic cobalt in the reduced catalysts. This higher cobalt dispersion, however, led to a stronger tendency of cobalt silicate formation under humid conditions. Without manganese, the cobalt particles sintered, and the larger ones were prone to transformation into cobalt carbide under high conversion conditions. As such, silica is not preferred as a support for practical FTS.

Elucidating deactivation of titania-supported cobalt Fischer-Tropsch catalysts under simulated high conversion conditions

The study of titania-supported cobalt nanoparticles is relevant for industrial Fischer-Tropsch synthesis (FTS). Herein, we report about various deactivation pathways of cobalt supported on P25 titania (cobalt loading 2–8 wt%) under simulated high conversion conditions using in situ Mössbauer spectroscopy. A fraction of metallic cobalt was oxidized under humid FTS conditions. The absolute amount of oxidized cobalt was ∼ 1.2 wt% independent of the cobalt loading, indicating that specific cobalt-titanol interactions are involved in the oxidation process. The formation of cobalt-titanate-like compounds was only observed under very high water-to-hydrogen ratios in the absence of carbon monoxide. Steam considerably enhances cobalt sintering under FTS conditions. As such, deactivation under humid FTS conditions is not only caused by cobalt oxidation but also by enhancing sintering of the active phase.

Manganese promotion of a cobalt Fischer-Tropsch catalyst to improve operation at high conversion

The addition of manganese to cobalt-based Fischer-Tropsch catalysts increases activity and significantly improves liquid fuel yield by decreasing CO2 and CH4 selectivity. These effects are particularly noticeable at high CO conversions where the catalyst is exposed to high H2O, and low CO and H2, partial pressures. Under these conditions, manganese-promotion causes a shift in the maximum liquid fuel yield from 61 C-% to 79 C-% (at CO-conversion levels of 78% and 91%, respectively), thus enabling once-through operation. Our CO-TPD and DFT studies show that manganese promotion cannot to be linked to enhanced CO adsorption nor promotion of the C-O bond cleavage (both typically postulated mechanisms). Rather, our DFT-calculations indicate that manganese promotion modelled as a manganese oxide cluster on Co(111) is associated with improved oxygen removal from the catalytically active surface, explaining the reduced selectivity towards CO2 at high conversion and, thus, reduced methane selectivity and improved liquid fuel yield.

Influence of operating conditions on the main characteristics of a cobalt Fischer-Tropsch catalyst

Three stages of extraction of the contents of the porous space of cobalt catalysts, continuously operating for different periods of time (Time on stream (TOS) is from 40 to 800 h) in a Fischer-Tropsch fixed bed reactor, were performed to identify carbon-containing compounds using chromatographic analysis. At each stage of extraction, the surface of the samples under study was investigated by SEM and BET methods. It was determined that during operation, as a result of accumulation of linear alkanes in the pores of the catalyst with an average number of carbon atoms from 20 to 25 and a decrease in the specific surface area, the reaction rate reduces by 25-30 %. With the help of oxidative treatment, it was found that elemental carbon is contained in the porous space of the spent catalysts.

Design of Cobalt Fischer-Tropsch Catalysts for the Combined Production of Liquid Fuels and Olefin Chemicals from Hydrogen-Rich Syngas.

Adjusting hydrocarbon product distributions in the Fischer–Tropsch (FT) synthesis is of notable significance in the context of so-called X-to-liquids (XTL) technologies. While cobalt catalysts are selective to long-chain paraffin precursors for synthetic jet- and diesel-fuels, lighter (C10–) alkane condensates are less valuable for fuel production. Alternatively, iron carbide-based catalysts are suitable for the coproduction of paraffinic waxes alongside liquid (and gaseous) olefin chemicals; however, their activity for the water–gas-shift reaction (WGSR) is notoriously detrimental when hydrogen-rich syngas feeds, for example, derived from (unconventional) natural gas, are to be converted. Herein the roles of pore architecture and oxide promoters of Lewis basic character on CoRu/Al2O3 FT catalysts are systematically addressed, targeting the development of catalysts with unusually high selectivity to liquid olefins. Both alkali and lanthanide oxides lead to a decrease in turnover frequency. The latter, particularly PrOx, prove effective to boost the selectivity to liquid (C5–10) olefins without undesired WGSR activity. In situ CO-FTIR spectroscopy suggests a dual promotion via both electronic modification of surface Co sites and the inhibition of Lewis acidity on the support, which has direct implications for double-bond isomerization reactivity and thus the regioisomery of liquid olefin products. Density functional theory calculations ascribe oxide promotion to an enhanced competitive adsorption of molecular CO versus hydrogen and olefins on oxide-decorated cobalt surfaces, dampening (secondary) olefin hydrogenation, and suggest an exacerbated metal surface carbophilicity to underlie the undesired induction of WGSR activity by strongly electron-donating alkali oxide promoters. Enhanced pore molecular transport within a multimodal meso-macroporous architecture in combination with PrOx as promoter, at an optimal surface loading of 1 Prat nm–2, results in an unconventional product distribution, reconciling benefits intrinsic to Co- and Fe-based FT catalysts, respectively. A chain-growth probability of 0.75, and thus >70 C% selectivity to C5+ products, is achieved alongside lighter hydrocarbon (C5–10) condensates that are significantly enriched in added-value chemicals (67 C%), predominantly α-olefins but also linear alcohols, remarkably with essentially no CO2 side-production (<1%). Such unusual product distributions, integrating precursors for synthetic fuels and liquid platform chemicals, might be desired to diversify the scope and improve the economics of small-scale gas- and biomass-to-liquid processes.

On Efficiency of Vanadium-Oxide Promoter in Cobalt Fischer-Tropsch Catalysts

On Efficiency of Vanadium-Oxide Promoter in Cobalt Fischer-Tropsch Catalysts

The Vital Role of Step-Edge Sites for Both CO Activation and Chain Growth on Cobalt Fischer–Tropsch Catalysts Revealed through First-Principles-Based Microkinetic Modeling Including Lateral Interactions

Microkinetic modeling is employed to predict catalytic turnover rates, product distributions, preferred mechanistic pathways, and rate- and selectivity-controlling elementary reaction steps for the Fischer-Tropsch (FT) reaction. We considered all relevant elementary reaction steps on Co(112̅1) step-edge and Co(0001) terrace sites as well as such important aspects as coverage-related lateral interactions, different chain-growth mechanisms, and the migration of adsorbed species between the two surfaces in the dual-site model. CHx–CHy coupling pathways relevant to the carbide mechanism have favorable barriers in comparison to the overall barriers for the CO insertion mechanism. A comparison of reaction barriers indicates why cobalt is such a good FT catalyst: CO bond scission and chain growth compete, while termination to olefins has a slightly higher barrier. The predicted kinetic parameters correspond well with experimental kinetic data. The Co(112̅1) model surface is highly active and selective for the FT reaction. Adding terrace Co(0001) sites in a dual-site model approach leads to a substantially higher CH4 selectivity at the expense of the C2+-hydrocarbons selectivity. The chain-growth probability decreases with increasing temperature and H2/CO ratio, caused by faster hydrogenation of the hydrocarbon chains. The elementary reaction steps for O removal and CO dissociation significantly control the overall CO consumption rate. Chain growth occurs almost exclusively at step-edge sites, while additional CH4 stems from CH and CH3 migration from step-edge to terrace sites. Replacing CO by CO2 as the reactant shifts the product distribution nearly completely to CH4, which is related to the much higher H/CO coverage ratio during CO2 hydrogenation in comparison to CO hydrogenation. These findings highlight the importance of a proper balance of CO and H surface species during the FT reaction and pinpoint step-edge sites as the locus of the FT reaction with low-reactive terrace sites near step-edge sites being the origin of unwanted CH4.

Influence of Manganese on the Catalytic Properties and Structure of the Active Surface of a Cobalt Fischer–Tropsch Catalyst

It is shown that the Fischer–Tropsch synthesis in the presence of Co–xMn/SiO2 cobalt catalysts with a manganese content of 0.5–2 wt % is characterized by high activity and C5+ hydrocarbon selectivity. The optimum concentration of manganese as a promoter is 1 wt %. The influence of manganese on the state of the surface of the catalysts in oxide and reduced forms is studied by a number of physicochemical methods. The formation of solid solutions of cobalt and manganese oxides of the xCo3O4–(1−x)Mn3O4 type is confirmed. The relationship between the promoting effect of manganese and the structure of active sites and the character of adsorption of H2 and CO on the surface of the catalysts is ascertained.

Tuning the Metal–Support Interaction and Enhancing the Stability of Titania-Supported Cobalt Fischer–Tropsch Catalysts via Carbon Nitride Coating

Supported metal catalysts have found numerous applications in many catalytic reactions, including Fischer–Tropsch (FT) synthesis. Metal dispersion, metal reducibility, catalytic performance, and ca...

Decoupling the deactivation mechanisms of a cobalt Fischer–Tropsch catalyst operated at high conversion and ‘simulated’ high conversion

In situ magnetometer study shows that high conversions facilitate sintering, reversible Co(ii)O and irreversible CoAl2O4 formation within cobalt-based Fischer–Tropsch systems.

The effect of silanol groups on the metal-support interactions in silica-supported cobalt Fischer-Tropsch catalysts. A temperature programmed surface reaction

The effect of H bonded and isolated silanol groups on the metal support interactions of SiO2-supported Co catalysts in Fischer-Tropsch (FT) synthesis are reported. SiO2 supports were pre-treated in order to improve the concentration of isolated SiOH on the surface of the SiO2. Isolated SiOH groups act as an aid to achieving relatively good metal dispersions by creating metal anchoring sites, and enhances smaller metal crystallites deposition on the surface of the SiO2. SiO2 supports were pre-treated in two different ways: (a) pre-treatment by thermal method (HT); (b) pre-treatment by ethylene glycol (EG). The incipient wetness impregnation technique was adopted for the preparation of the catalysts, each catalyst loaded with approximately 10 wt% Co on each of the three supports designated as Co/SiO2 – UT, Co/SiO2 – HT, and Co/SiO2 – EG. Co/SiO2 – UT was used as a reference for this study. A number of characterization methods were applied to study the chemical and physical properties of the supports and the catalysts namely BET, XRD, ICP-OES, FTIR, H2 chemisorption, TEM, TPR, and TPSR. The results from H2 chemisorption, TEM and XRD indicated that the Co/SiO2 – EG showed relatively smaller metal crystallites and a better cobalt crystallite dispersion while TPR, CO TPD, and TPSR studies indicated that Co/SiO2 – EG catalyst exhibited stronger metal-support interaction, unlike the other two catalysts. Catalyst evaluation experiments showed that Co/SiO2-HT catalysts showed better FT activity while the Co/SiO2 – EG catalyst favored light olefins. FT catalyst evaluation results were in agreement with the TPSR studies.

Amino-Ended Hyperbranched Polyamide Modified SBA-15 as Support for Highly Efficient Cobalt Fischer-Tropsch Synthesis Catalyst

Among the Fischer-Tropsch synthesis (FTS) catalysts, Cobalt catalysts are currently attracting a lot of research interests due to their high activity and high selectivity. But the dispersion and reducibility of cobalt catalysts with moderate interaction between cobalt and support are still a challenge. A novel amino-ended hyperbranched polyamide (AEHPA) was used to modify SBA-15 and then was applied as support for obtaining cobalt catalyst (15Co/SBA-15-N). The catalysts were characterized by XRD, TEM, XPS and H2-TPR techniques. The results showed that AEHPA doped mesoporous SBA-15 caused the generation of N species in the SBA-15 pore channels. The N-Co bonds resulted in the formation of highly-dispersed cobalt nanoparticles with uniform sizes inside the ordered mesopores of support. AEHPA doping was an effective way to modify the surface properties of the SBA-15 for immobilizing cobalt nanoparticles. Compared with the conventional 15Co/SBA-15 catalyst without doping AEHPA, the AEHPA doped 15Co/SBA-15-N catalyst showed improved cobalt dispersion and stabilized cobalt location, which led to much better reaction stability as well as C5+ selectivity.

Correlation of metal–organic framework structures and catalytic performance in Fischer–Tropsch synthesis process

Two typical metal–organic frameworks (MOFs), i.e. tris(pyridine-2-carboxylato)-cobalt(III) monohydrate (MOF-1) and (μ2-pyridine 2,6-dicarboxylato)(pyridine 2,6-dicarboxylato) pentaaqua dicobalt(II)dihydrate (MOF-2) were employed for preparation of cobalt Fischer–Tropsch catalysts. Both MOF-derived catalysts were obtained by direct pyrolysis in N2 atmosphere at 500 °C. The pyrolysis of desired MOFs resulted nanoparticles embedded in the porous carbon matrix. Such catalysts can serve as useful catalysts for FT synthesis. Co-MOF-1 derived catalyst exhibited carbon monoxide conversion of 74.8% and selectivity towards long-chain hydrocarbons (C5+) of 49.2%. Also, it showed selectivity for short-chain hydrocarbons (C2–C4) of 36.19% for 50 h on steam while Co-MOF-2 derived catalyst displayed CO conversion of 81.6% and selectivity for long-chain hydrocarbons (C5+) and short-chain hydrocarbons of 56.8% and 28.2%. The superb activity and catalytic efficiency can be ascribed to the MOF precursors structures. This study investigated the relationship between MOF structure and catalytic performance and presented a new approach to design novel super active catalysts with preferable selectivity for Fischer–Tropsch synthesis by opting the suitable MOF precursors.

A robust and precious metal-free high performance cobalt Fischer–Tropsch catalyst

Slurry-phase Fischer–Tropsch catalysis is widely used in the production of synthetic transportation fuels, but places severe stresses on the catalyst due to the demanding hydrothermal and mechanical reaction conditions. In this Article we report the synthesis, characterization and catalytic performance of a robust cobalt-based Fischer–Tropsch catalyst. Combining an inert alpha alumina support with an appropriate cobalt addition method produces a material that is easy to reduce without precious metal additives and is mechanically and hydrothermally stable. Unlike many Fischer–Tropsch catalysts, this material is resistant to sintering and shows excellent selectivity and good activity in slurry-phase testing over 1,000 hours. The Fischer–Tropsch reaction is one of the key means of producing synthetic fuels. Here a deposition method to disperse cobalt nanoparticles across an alpha alumina support is shown to produce a highly stable system capable of withstanding demanding conditions while providing excellent activity.

Activation Energies of the Formation of Methane and Carbon Dioxide over Cobalt Fischer–Tropsch Catalysts Supported on Carbon Nanotubes

The effective activation energies of CO methanation and water gas shift reactions in the Fischer–Tropsch process in the presence of cobalt catalysts supported on oxidized and unoxidized carbon nanotubes have been determined. It has been found that the energy of CO methanation does not depend on the oxidation degree of the support and varies in a narrow range from 135 to 153 kJ/mol, while the values of this parameter for the water gas shift reaction increase in the case of the oxidized support from 40 to 201 kJ/mol due to the change in the mechanism of the process.

Distribution Features of Products of Long-Chain Alkane Hydrogenolysis over Unpromoted Cobalt Catalysts

The experimental data on the hydrogenolysis of long-chain alkanes over suspended cobalt Fischer–Tropsch catalysts are presented which demonstrate the specific character of the molecular-mass distribution of its products (the combination of excess methane, local minimum at С3–С7 carbon chain lengths, and local maximum at С12–С16 carbon chain lengths). These results provide the experimental verification for the model of the mentioned distribution previously advanced by the authors which includes the hypothesis that the probability of С–С bond cleavage in an alkane molecule increases from the chain center to its ends. These distribution features are observed in the conversion of both alkanes of the suspension medium and individual n-alkanes (n-С16Н34) at different temperatures for in situ synthesized nanosized catalysts and commercial oxide samples and for n-С16Н34 after distillation of the hydrocarbon medium of the preliminarily suspended nanosized catalyst.

Cover Feature: Manipulation of Fischer‐Tropsch Synthesis for Production of Higher Alcohols Using Manganese Promoters (ChemCatChem 22/2018)

The Cover Feature illustrates the shifts in selectivity induced by the introduction of manganese to Fischer-Tropsch cobalt catalysts. In their Full Paper, J. Paterson et al. use temperature-programmed reduction, transmission electron microscopy elemental mapping and in situ x-ray diffraction show the formation of CoxMnyO4 species on titania during synthesis, with Co-Mn alloys after reduction, while 2D gas chromatography and NMR were used to analyze the FT wax product with over 50% alcohols at long carbon numbers. More information can be found in the Full Paper by J. Paterson et al. on page 5154 in Issue 22, 2018 (DOI: 10.1002/cctc.201800883).

Controlled synthesis of cobalt nanocrystals on the carbon spheres for enhancing Fischer–Tropsch synthesis performance

Abstract Non-porous carbon sphere was used as support to synthesize supported cobalt Fischer-Tropsch catalysts with high activity and durability. Strong metal-support interaction was avoided and intrinsic activity of pristine cobalt nano-particles was studied. Thermal decomposition synthesis method was applied to obtain cobalt catalysts with high dispersion and narrow particle size distribution. Furthermore the cobalt size can be controlled by the molar ratio of o-dichlorobenzene/ benzylamine. Compared with supported cobalt catalysts prepared by incipient wetness impregnation method and ultrasonic impregnation method, the catalyst prepared by thermal decomposition method showed higher catalytic activity, higher long chain hydrocarbons selectivity and lower methane selectivity.

Silver addition to a cobalt Fischer–Tropsch catalyst

Silver is known to promote F–T catalysts as a reduction promoter, although it has also been observed to affect catalytic performance of, for instance, cobalt nanoparticles. In this work, a series of silica-supported cobalt catalysts with a constant platinum content (0.04 wt%) and varying silver content were characterized and tested. Characterization shows that mainly coordinatively unsaturated cobalt atoms are substituted at low silver content (probed by N2 IR). Adding more silver results in replacement of both coordinatively unsaturated sites and planar sites (probed by H2-chemisorption and TEM-EDS). The presence of silver on the cobalt surface affects both structure-sensitive CO dissociation and structure-insensitive hydrogenation reactions. The Boudouard reaction shows that the CO dissociation rate decreases with increasing silver content. Hampered CO dissociation is also apparent from temperature-programmed CO IR spectroscopy. However, even at high silver content the surface still contains step-edge sites that can dissociate CO and a small amount of carbon is deposited (quantified by TPH). During the CO hydrogenation at 260 °C, CO conversion is reduced by less than 30% when 8.8 wt% silver is added. This does not correlate well with the loss in cobalt surface area measured by chemisorption (decreases by 93%) or CO dissociation rate probed by CO disproportionation (decreases by 90%). This mismatch is mainly because that the CO consumption rate under methanation condition is largely controlled by carbon and oxygen hydrogenation, rather than CO dissociation. On the other hand, transient experiments show that silver accelerates oxygen-removal steps. It turns out that CO conversion is only moderately affected by silver. The coverage of hydrogenation sites – mostly terrace sites – by silver causes slower CH4 formation. Longer residence of CHx intermediates (determined by SSITKA) accordingly leads to an increase in chain-growth probability and therefore higher C2+ selectivity. SSITKA also confirms that the decrease in steady-state CO/CHx coverage qualitatively correlates to the loss in cobalt surface area. Another consequence of slower hydrogenation is that the olefins-to-paraffins ratios are higher when silver is added to the catalyst. Importantly, covering the cobalt surface with silver affects the F–T product distribution in a similar manner as graphitic carbon deposited on terrace sites as we reported earlier.

Effects of Ag promotion and preparation method on cobalt Fischer-Tropsch catalysts supported on silica-modified alumina

A series of silver-promoted, 20 wt% cobalt Fischer-Tropsch synthesis (FTS) catalysts supported on an alumina modified with 5 wt% silica were prepared using two methods: traditional incipient wetness impregnation (IWI) and a new solvent deficient precipitation (SDP) technique. Catalysts containing silver promoter concentrations of 0.3, 0.6, 1.2, and 2.5 wt% were prepared using each of the two methods. Silver improved the reducibility of the cobalt significantly, lowering reduction temperatures by up to 100 °C, and increasing the extent of reduction by up to 35%. Further, in both preparation methods, changing the silver loading altered the cobalt dispersion. The smallest Co crystallite size was achieved with 1.2 wt% Ag loading, which produced average cobalt crystallite sizes of about 7 nm. The Fischer-Tropsch CO consumption rate increased with decreasing crystallite size and thus was highest for 1.2 wt% Ag. Intrinsic CO consumption rates per Co site (CO turnover frequency) were also measured for each catalyst. A clear increase in the intrinsic turnover frequency (TOF) was observed as Ag loading was increased from 0.6% to 1.2 wt% for both preparation methods. Both 1.2 wt% Ag catalysts produced TOF’s of ∼0.050 s−1 (equivalent to a CO consumption rate of ∼65 mmol gcat−1 h−1), which are 20–30% higher than the catalysts containing 0.3, 0.6, and 2.5 wt% Ag and are comparable to reported rates for commercial FTS catalysts. Higher loadings of Ag (2.5 wt%) resulted in higher extents of reduction, but led to lower TOFs and larger Co crystallite diameters, which is assumed to be due to blockage of active sites and changes in the Ag-Ag and Ag-Co coordination ratio. Thus, silver promotion appears to improve catalytic performance by enhancing cobalt reduction, dispersion, and electronic properties. The SDP and IWI methods produced catalysts with essentially the same properties, but the SDP method is a simpler, one-pot technique that offers many potential advantages.