Supported Cobalt Fischer Tropsch Catalysts Research Articles

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.

Quantification and qualification by in-situ FTIR of species formed on supported-cobalt catalysts during the Fischer-Tropsch reaction

In-situ FTIR spectroscopy was employed to investigate the location, chain length and quantity of hydrocarbon species adsorbed on supported-cobalt Fischer-Tropsch catalysts. The length of the hydrocarbon units observed was quantified using an appropriately determined absorption coefficient ratio. The individual amounts of CH2 and CH3 groups were calculated with absorption coefficients derived specifically for adsorbed hydrocarbon species, unlike previous studies, which employ absorption coefficients derived from liquid phase hydrocarbons. Results show that it is possible for reaction products to re-adsorb from the gas phase onto the support as well as spillover to the support from the active metal cobalt. Qualification and quantification of the chain length of these re-adsorbed species has shown that the support material (γ-alumina) selectively re-adsorbs shorter chain length species from the gas phase with a different functional group to the majority of observable species on a Co/Al2O3 catalyst. Comparison of Co/Al2O3 with Co/SiO2, which utilises a more inert support relative to γ-alumina, shows that longer chained species are located on the cobalt metal itself during reaction and can be transported to the γ-alumina support via a process of spillover.

English

Temperature programmed reduction (TPR) is one of the techniques for obtaining information about phases or bulk species in heterogeneous catalysts. Information from TPR analysis can give insights about phase-support interaction and extent of reduction of the phases at different temperatures. TPR technique is a common tool in the characterisation of cobalt based Fischer-Tropsch (FT) catalysts. However, interpretation of TPR profiles of γ-alumina supported cobalt FT catalysts had been characterised with different views on the nature of phases and reduction processes involved. In this report, we use reduction behaviour of unsupported Co3O4 to gain insight for more explicit analysis of TPR profiles of γ-alumina supported Co3O4 catalysts. The transition Co3O4 → CoO → Co in γ-alumina supported catalysts prepared with wet impregnation with aqueous cobalt nitrate and calcined at temperatures ≤ 350°C gave reduction peaks at 300 to 350°C. Reduction peaks at 500 to 600°C were due to Co-Al mixed oxide phases; most likely Co2AlO4 and probably routes formation of the mixed oxide were also discussed. Consideration of tendency of dissolution of γ-alumina during the impregnation of metal salt is instructive toward achieving higher reducibility of Co3O4 in the design of cobalt based Fischer-Tropsch catalysts.   Key words: Temperature programmed reduction (TPR) profile, calcinations, Co3O4, γ-Al2O3, Fischer-Tropsch catalyst.

Stacking disorder in silicon carbide supported cobalt crystallites: an X-ray diffraction, electron diffraction and high resolution electron microscopy study.

Supported cobalt Fischer-Tropsch catalysts are characteristically nanoparticulate and the reduced SiC supported catalyst was found to contain both HCP and FCC polymorphs. This is reflected in the powder XRD patterns and generally there is a poor fit between the experimental and calculated diffractograms. This was ascribed to small crystallite sizes and the occurrence of disorder, manifested as peak broadening and peak shifts. Selected area electron diffraction data of suitably oriented cobalt catalyst grains on silicon carbide supports show non-periodic disorder in the zone axis orientations that contain the common (001) (HCP) and (111) (FCC) reciprocal lattice planes. Both FCC and HCP polymorphs are present in the same grains and these show disorder mainly in the HCP component. The disorder is further examined using high angle annular dark field (HAADF) scanning transmission electron microscopy at atomic resolution and the stacking sequences elucidated. Random sequences of mainly FCC are interrupted by HCP sequences and twin surfaces with reverse stacking sequences are also present. This study highlights the presence of significant disorder in cobalt catalyst grains confirmed by HAADF microscopy.

Effect of Carbon Porosity and Cobalt Particle Size on the Catalytic Performance of Carbon Supported Cobalt Fischer–Tropsch Catalysts

Cobalt catalysts supported on activated carbon and on carbon nanotubes (CNTs) with different porosities were prepared by an incipient wetness impregnation method and characterized by a series of methods. The samples were reduced and then evaluated in a fixed-bed reactor for Fischer–Tropsch (FT) synthesis. The porosity of the carbon support greatly influenced the microstructure, the reducibility, the dispersion, and the FT performance of the cobalt catalysts. The carbon structure and the cobalt dispersion determined CO conversion. CNTs with larger pore sizes were more stable at high temperature in a H2 atmosphere. The cobalt particle size impacted the CO turnover frequency (TOF) and the C5+ selectivity. Larger cobalt particles (up to 7 nm) resulted in higher TOF and C5+ selectivity; for cobalt particles larger than 7 nm, no such increase in these parameters was seen. The carbon support influenced the C5+ selectivity and the C5+ hydrocarbon distribution. Interestingly, the olefin/paraffin ratio of C2 was lo...

The Effect of Water on the Activity and Selectivity for Carbon Nanofiber Supported Cobalt Fischer–Tropsch Catalysts

The effect of water on the activity and selectivity for a series of carbon nanofiber supported Fischer–Tropsch cobalt catalysts has been investigated. Fischer–Tropsch synthesis was carried out in a fixed bed reactor at industrially relevant conditions (483 K, 20 bar, H2/CO = 2.1). Three catalysts were prepared either by incipient wetness impregnation or wet impregnation and consisted of 12 or 20 wt% cobalt deposited on carbon nanofibers of the platelet and fishbone structures. Addition of 20 and 33 mol% water to the reactor inlet increased the reaction rates, but also the deactivation rates of all the catalysts. As a result of the high water concentrations, the catalysts were irreversibly deactivated. A positive correlation between the amount of water in the reactor and the selectivity to long-chain hydrocarbons (C5+) was found.

Molecular structure and localization of carbon species in alumina supported cobalt Fischer–Tropsch catalysts in a slurry reactor

The paper focusses on the identification of molecular structure of carbon species in 15wt.% Co/Al2O3 supported catalyst exposed to different conditions of Fischer–Tropsch synthesis in a continuously stirred tank reactor (CSTR). Characterization of wax-extracted spent catalyst by a combination of techniques showed significant carbon deposition under the conditions where the catalyst exhibited a noticeable deactivation. Carbon deposition was less important when catalyst activity was relatively stable. TPH-MS suggested the presence of different carbon species on surface. GC–MS analysis indicated that the strongly adsorbed carbon deposited on wax-extracted spent catalyst was mainly constituted of α-olefins, n-paraffins, branched alkanes/alkanes, 2-methyl-branched fatty aldehydes and 2-ketones. 2-Methyl-branched fatty carboxylic acids were mostly detected at high H2O pressure in the reactor. Branched alkenes/alkenes (2-methyl-alkenes/alkanes) were favored at high CO conversion. Alpha-alkyl cinnamic aldehydes were detected under hydrogen deficient conditions. STEM-EELS analysis suggests that carbon is located both on cobalt particles and on alumina support, the carbon species situated on the support being more reluctant to hydrogenation.

Influence of operating conditions in a continuously stirred tank reactor on the formation of carbon species on alumina supported cobalt Fischer–Tropsch catalysts

The paper focuses on the identification of carbon species using a wide range of ex situ characterization techniques (TPH/MS, TGA-DTA, XPS, TOF-SIMS) in spent 15wt% Co/Al2O3 supported catalysts exposed to different conditions of Fischer–Tropsch synthesis in a continuously stirred tank reactor (CSTR) at 20bar of total pressure, different gas-space velocities and H2/CO ratios. Three types of carbonaceous species were uncovered in the used catalysts: (i) hydrcarbobns (probably wax), (ii) strongly adsorbed hydrocarbons and (iii) amorphous polymeric carbon. The amounts of deposed carbon species strongly depend on the operating conditions. In agreement with the observed slow catalyst deactivation, only very small amounts of strongly adsorbed hydrocarbons and polymeric carbon were observed when the catalyst was exposed to a H2/CO=2 syngas ratio and moderate carbon monoxide conversions. Lower gas space velocity and lower H2/CO ratio in syngas lead to larger amounts of deposed carbon species. Strongly adsorbed hydrocarbons and amorphous polymeric carbon seem to contribute to catalyst deactivation. Cobalt carbide and graphitic carbon were not detected in the used catalysts by ex situ methods.

Relation between hydrocarbon selectivity and cobalt particle size for alumina supported cobalt Fischer–Tropsch catalysts

The relation between hydrocarbon selectivity and cobalt particle size was studied using different alumina supports. Alumina phases, i.e. δ-, θ- and α-Al2O3 were produced from a small and a medium pore γ-Al2O3 by heat treatment. All catalysts were prepared by incipient wetness impregnation resulting in a cobalt loading of 12, 20 and 30wt.% cobalt. The catalysts were tested in a fixed bed reactor at 20bar, 483K and H2/CO=2.1. The relation between methane, olefin and C5+ selectivities was studied for 2–14nm cobalt particles supported on δ- and θ-Al2O3 supports prepared from the two different γ-Al2O3. The C5+ selectivity increases with increasing Co particle size for particles less than 8–9nm where the selectivity goes through a maximum. The C5+ selectivity decreases for larger Co particles and then seems to approach a constant value independent of the alumina phase and pore size. The lowest CH4 selectivity and the highest olefin to paraffin ratio were obtained at maximum C5+ selectivity.

Effects of pore diameter on particle size, phase, and turnover frequency in mesoporous silica supported cobalt Fischer–Tropsch catalysts

A series of mesoporous silica supported cobalt catalysts for the Fischer–Tropsch reaction with pore diameters from 3 to 22 nm was synthesized and characterized. Detailed X-ray diffraction measurements were used to determine the composition and particle diameters of the metal fraction, analyzed as a three-phase system containing Co fcc, Co hcp and CoO. Catalyst properties were determined at three stages of catalyst history: (1) after the initial calcination step to thermally decompose the catalyst precursor into Co 3O 4, (2) after the hydrogen reduction step to activate the catalyst to Co and (3) after the FT reaction. Small Co hcp particles were detected in all reduced catalysts contributing significantly to the Co surface area. The reaction rate increased with the pore diameter of the silica support. The results indicate the importance of careful catalyst characterization in determining the factors that contribute to reactivity. In addition, the identification of significant quantities of two cobalt metal phases suggests that further study of the intrinsic activity of each phase, as well as the structural features of the supports that determine the distribution of cobalt phases and particle sizes is warranted.

Intergranular and intragranular cobalt repartitions in alumina supported Fischer–Tropsch catalysts promoted with platinum

This paper focuses on the intergranular and intragranular cobalt repartitions in alumina supported cobalt Fischer–Tropsch catalysts promoted with platinum (0.05–0.1 wt.%) with a wide range of cobalt contents (10–25 wt.%). The catalysts were characterized at both nano- and micron-scale levels using several characterization techniques. In the calcined catalysts, Co3O4 crystalline phase was the major cobalt phase, no hardly reduced cobalt aluminates were detected. The size of the Co3O4 crystallites varied from 8 to 12 nm. At the micron-scale range, the catalysts were constituted of grains of 90 μm diameter. Scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX) showed the uniform intergranular and intragranular cobalt repartitions in the catalysts. Increase in cobalt content up from 10 to 25 wt.% did not alter the cobalt repartition. Cet article porte sur les répartitions intergranulaire et intragranulaire du cobalt dans les catalyseurs Fischer–Tropsch supportés par l'alumine et promus par du platine (10–25% poids Co et 0.05–0.1% poids Pt). Les catalyseurs ont été caractérisés à l'échelle nano- et micrométrique par une combinaison de techniques physico-chimiques. Dans les catalyseurs calcinés, le Co3O4 est la phase principale du cobalt, la taille des cristallites Co3O4 variant entre 8 et 12 nm. La présence d'aluminate de cobalt n'a pas été détectée. Une étude par microscopie MEB–EDX a montré qu'à l'échelle micrométrique la répartition du cobalt entre les différents grains et à l'intérieur d'un grain des catalyseurs reste homogène même à haute teneur en cobalt.

Effect of promotion with ruthenium on the structure and catalytic performance of mesoporous silica (smaller and larger pore) supported cobalt Fischer–Tropsch catalysts

The effects of promotion with ruthenium on the structure of cobalt catalysts and their performance in Fischer–Tropsch synthesis were studied using MCM-41 and SBA-15 as catalytic supports. The catalysts were characterized by N 2 physisorption, H 2-temperature programmed reduction, in situ magnetic measurements, X-ray diffraction and X-ray photoelectron spectroscopy. It was found that monometallic cobalt catalysts supported by smaller pore mesoporous silicas ( d p = 3–4 nm) had much lower activity in Fischer–Tropsch synthesis than their larger pore counterparts ( d p = 5–6 nm). Promotion with ruthenium of smaller pore cobalt catalysts led to a considerable increase in Fischer–Tropsch reaction rate, while the effect of the promotion with ruthenium was less significant with the catalysts supported by larger pore silicas. Characterizations of smaller pore cobalt catalysts revealed strong impact of ruthenium promotion on the repartition of cobalt between reducible Co 3O 4 phase and barely reducible amorphous cobalt silicate in the calcined catalyst precursors. Smaller pore monometallic cobalt catalysts showed high fraction of barely reducible cobalt silicate. Promotion with ruthenium led to a significant increase in the fraction of reducible Co 3O 4 and in decrease in the amount of cobalt silicate. In both calcined monometallic and Ru-promoted cobalt catalysts supported by larger pore silicas, easy reducible Co 3O 4 was the dominant phase. Promotion with ruthenium of larger pore catalysts had smaller influence on cobalt dispersion, fraction of reducible cobalt phases and thus on catalytic performance.

Microstructure of Supported Cobalt Fischer-Tropsch Catalysts

Microstructure of Supported Cobalt Fischer-Tropsch Catalysts — The structures of metallic cobalt particles supported on silica (with and without Ru promoter) and on alumina have been investigated in-situ by conventional and anomalous X-ray diffraction. Comparison with simulated XRD patterns shows the structure to consist of a defect phase characterised by the presence of stacking faults, the density of which is directly related to the temperature at which the cobalt oxide is reduced. Controlled modification of the cobalt microstructure, while maintaining the same support and promoter has been achieved with limited variation of the particle size, notably via CO - H2 treatment leading to the formation and decomposition of cobalt carbide. Independently of the support or the presence of a promoter, low reduction temperatures increase the tendency to form hexagonal close packed (hcp) structure whilst at high reduction temperatures face centred cubic (fcc) stacking is preferentially formed. The catalytic activity for CO conversion in Fischer-Tropsch conditions is greatly increased for catalysts with a majority of hcp stacking compared to those containing predominantly fcc phase particles.

Effect of calcination atmosphere and temperature on γ-Al2O3 supported cobalt Fischer-Tropsch catalysts

The effect of calcination temperature and atmosphere on the properties of c-Al2O3 supported cobalt Fischer-Tropsch catalysts has been investigated. One common precursor for all the catalysts was prepared by incipient wetness impregnation of c-Al2O3 with an aqueous solution of cobalt nitrate hexahydrate. It was subjected to four different calcination atmospheres (air/50% steam: 30 mL/min, air: 30 mL/min, air: 50 mL/min, N2: 30 mL/min) and eight different calcination temperatures (range: 473–723 K), making the total number of samples 32. Both the post calcination nitrogen content and the cobalt dispersion were measured. The results demonstrated that in order to maximise the cobalt dispersion, it is necessary to use low calcination temperatures and remove the precursor decomposition products (NO, NO2 ,H 2O) efficiently. The Fischer-Tropsch synthesis performance of two catalysts calcined at the same temperature, but at different air flow rates was evaluated. No significant effect of the air flow rate was found on the turnover frequency or C5+ selectivity, but a high flow rate resulted in 30% higher activity per gram catalyst.

Role of surface oxygen in the preparation and deactivation of carbon nanofiber supported cobalt Fischer–Tropsch catalysts

Surface oxygen groups on the carbon nanofiber surfaces are important for the preparation of highly dispersed cobalt catalysts and for the stabilization of the particles against sintering. However, the surface oxygen makes the catalysts easily deactivate during Fischer–Tropsch synthesis by oxidation of metallic cobalt. The activity could be recovered by re-reduction.

The Effect of Water on the Activity and Selectivity for γ-Alumina Supported Cobalt Fischer–Tropsch Catalysts with Different Pore Sizes

The effect of water on the activity and selectivity for a series of γ-Al2O3 supported cobalt Fischer–Tropsch catalysts has been studied in an isothermal fixed-bed reactor at T = 483 K, P = 20 bar, and H2/CO = 2.1. The catalysts were produced applying incipient wetness impregnation and consisted of 20 wt.% cobalt and 0.5 wt.% rhenium deposited on γ-Al2O3 supports with different pore characteristics. For the narrow-pore catalysts, addition of water corresponding to an inlet partial pressure ratio of PH2O/PH2 = 0.4 reduced the reaction rates. In contrast, for a catalyst with larger pores the same water pressure increased the reaction rates. For all catalysts, water amounts equal to PH2O/PH2 = 0.7 at the reactor inlet suppressed the reaction rates and led to permanent deactivation. The addition of water increased the C5+ selectivity and decreased the CH4 selectivity for all catalysts. The pore characteristics seem to determine the effect of water on the rates.

Effect of catalyst pore size on the catalytic performance of silica supported cobalt Fischer–Tropsch catalysts

A series of cobalt catalysts supported on silica with different pore size were prepared by incipient wetness impregnation method. N 2 physisorption, XRD, H 2-TPR, H 2-TPD, DRIFTS and O 2 pulse reoxidation were used to characterize the catalysts. The results showed that the pore size of the support had a significant influence on the Co 3O 4 crystallite diameter, catalyst reducibility and Fischer–Tropsch activity. The larger pore could cause the Co/SiO 2 to form larger Co 3O 4 crystallite and decreased its dispersion. Catalysts with different pore size showed different CO adsorption property. The catalysts with pore size of 6–10 nm displayed higher Fischer–Tropsch activity and higher C 5+ selectivity, due to the moderate particle size and moderate CO adsorption on the catalysts.

Control of Metal Dispersion and Structure by Changes in the Solid-State Chemistry of Supported Cobalt Fischer–Tropsch Catalysts

Controlling preparation variables in supported cobalt Fischer–Tropsch catalysts has a dramatic effect on the dispersion and distribution of cobalt, and determines how active and selective the resulting catalyst will be. We detail specific examples of catalyst synthesis strategies for modifying interactions between the support and the cobalt precursor, promoting reduction, stabilizing catalysts to high-temperature treatments, minimizing deleterious support metal interactions, and controlling the distribution of cobalt on large support particles. It is important to optimize the support and precursor interaction strength, so that it is strong enough to obtain good dispersion but not too strong to prevent low temperature reduction. We show examples in which formation of surface complexes and epitaxial matching of precursor and support structures improves dispersion dramatically. Reduction promoters can help in those cases where support–precursor interactions are too strong. We show how substitutions of silicon into a titania lattice stabilizes surface area and retards formation at high oxidation temperatures of cobalt ternary oxides that reduce only at very high temperatures—an important consideration if oxidative coke removal is necessary. In addition, surface treatment of TiO2 with an irreducible oxide like ZrO2 can inhibit deleterious support interactions that can block surface cobalt sites. Selectivity can also be dramatically altered by catalyst synthesis. We illustrate a case of large (2 mm) SiO2 particles onto which cobalt can be added either uniformly or in discrete eggshells, with the eggshell catalysts having substantially higher C5+ selectivity. These approaches can lead to optimal Fischer–Tropsch catalysts with high activity and C5+ selectivity, good physical integrity, and a long life.

Fischer—Tropsch synthesis over cobalt catalysts supported on zirconia-modified alumina

The effect of adding zirconia to the alumina support on supported cobalt Fischer-Tropsch catalysts has been studied. At 5 bar and H 2 :CO ratio 9:1 zirconia addition to the support leads to a significant increase in both activity and selectivity to higher hydrocarbons as compared to the unmodified catalysts. Reducibility and cobalt dispersion on the other hand are not improved by the presence of zirconia compared to the unmodified catalysts. SSITKA measurements have been performed in order to determine the intrinsic activity per active site. At constant temperature, zirconia-modified and unmodified catalysts showed basically the same intrinsic activity. Similar results were obtained with a noble metal (Pt) promoted catalyst. The promoting effect appears to be mainly due to coverage effects rather than a change in the intrinsic activity of the active sites. The turnover frequencies were found to be independent of pressure but strongly temperature dependent. However, the increase in turnover frequency did not account for the entire increase in reaction rate with temperature. This indicates that also the coverage of reactive intermediates increases with increasing temperature.

Silica supported cobalt Fischer–Tropsch catalysts: effect of pore diameter of support

The influence of the effect of average pore diameter of silica support on the physical and chemical properties of supported cobalt catalysts and their performance in the Fischer–Tropsch synthesis was investigated. Silicas with different mean pore diameter (20, 40, 60, 100 and 150 Å) were impregnated with cobalt nitrate to produce catalysts containing 20 wt.% cobalt. The metal crystallite size and degree of reduction was found to increase with increasing pore diameter of the support for supports with an average pore diameter larger or equal to 40 Å, and hence the dispersion decreased. In impregnated catalysts, the metal crystallites seems to appear in clusters on the support. With increasing average pore diameter, the size of these clusters increases. In the Fischer–Tropsch synthesis, the 100 Å supported catalyst proved to be the most active and selective catalyst for hydrocarbon formation. The C 5+ and methane selectivity passed through a maximum and minimum at the 100 Å supported catalyst, respectively, which can be explained quantitatively using the reactant transport model proposed by Iglesia et al.