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Andrei Y Khodakov - One of the best experts on this subject based on the ideXlab platform.
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pore size effects in high temperature fischer tropsch synthesis over supported iron catalysts
Journal of Catalysis, 2015Co-Authors: Kang Cheng, Mirella Virginie, Vitaly V Ordomsky, Catherine Cordier, P A Chernavskii, Mikhail I Ivantsov, Sebastien Paul, Ye Wang, Andrei Y KhodakovAbstract:Abstract This paper addresses the effect of support pore sizes on the structure and performance of iron catalysts supported by mesoporous silicas in high-temperature Fischer–Tropsch synthesis. A combination of characterization techniques showed that the size of supported iron particles was controlled by catalyst pore sizes. The larger iron particles were localized in large-pore supports. Iron carbidization with carbon monoxide resulted in preferential formation of Hagg iron carbide (χ-Fe 5 C 2 ). Larger iron oxide crystallites in large-pore supports were much easier to carbidize than smaller iron oxide counterparts in small-pore supports. The catalytic performance in Fischer–Tropsch synthesis was attributed to iron carbide. Higher Fischer–Tropsch Reaction rates, higher olefin, and C 5+ selectivity were observed over larger pore iron catalysts. High dispersion of iron oxide in small-pore silicas was not favorable for carbon monoxide hydrogenation because of poor iron carbidization.
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fischer tropsch synthesis in milli fixed bed reactor comparison with centimetric fixed bed and slurry stirred tank reactors
Catalysis Today, 2011Co-Authors: Stephane Chambrey, Pascal Fongarland, H Karaca, S Piche, Anne Gribovalconstant, Daniel Schweich, Francis Luck, Sabine Savin, Andrei Y KhodakovAbstract:Abstract This paper presents a comparative study of Fischer–Tropsch synthesis in single channel milli-fixed bed, conventional centimetric fixed bed and slurry stirred tank reactors. In the three reactors, the catalytic measurements were carried out with the same conventional platinum-promoted alumina supported cobalt catalyst at 493 K and 20 bar using a stoichiometric syngas ratio (H 2 /CO = 2). The single channel milli-fixed bed reactor displays a higher initial Fischer–Tropsch Reaction rate than the conventional centimetric fixed bed reactor. This effect was assigned to a better temperature control and less significant catalyst deactivation during the startup of the single channel milli-fixed bed reactor. The slurry stirred tank reactor shows much lower hydrocarbon productivity than the milli- and centimetric fixed bed reactors, which is probably due to incomplete catalyst reduction. A considerable catalyst deactivation due to the uncontrolled temperature hike can occur during the reactor startup in the conventional centimetric fixed bed reactor. The slurry stirred tank and milli-fixed bed reactors show similar apparent deactivation behavior.
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fischer tropsch synthesis relations between structure of cobalt catalysts and their catalytic performance
Catalysis Today, 2009Co-Authors: Andrei Y KhodakovAbstract:Fischer-Tropsch synthesis has been experiencing a strong revival in recent years, due to the resource utilization considerations and environmental concerns. Cobalt supported catalysts represent the optimal choice for the synthesis of long-chained hydrocarbons from syngas with high H2/CO ratio. This paper reviews the state of the art related to the influence of cobalt particle size and cobalt phase composition, catalyst support and support texture, and promotion with noble metals on Fischer-Tropsch Reaction rates, hydrocarbon selectivity and catalyst stability. Possible mechanisms of catalyst deactivation and modification of cobalt active sites during the Reaction are also discussed. Several requirements to the design of cobalt Fischer-Tropsch catalysts have been specified.
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cobalt dispersion reducibility and surface sites in promoted silica supported fischer tropsch catalysts
Journal of Catalysis, 2007Co-Authors: Jeansebastien Girardon, P A Chernavskii, Anne Gribovalconstant, E Quinet, L Gengembre, Andrei Y KhodakovAbstract:Abstract Cobalt particle size, cobalt reducibility, and metal surface sites in a series of ruthenium- and rhenium-promoted cobalt silica-supported Fischer–Tropsch catalysts were studied by X-ray diffraction, UV–vis spectroscopy, in situ X-ray absorption, in situ magnetic method, X-ray photoelectron spectroscopy, DSC-TGA thermal analysis, and propene chemisorption. The catalysts were prepared by co-impregnation; in several catalyst syntheses, sucrose was added to the impregnating solutions. Mononuclear octahedral cobalt complexes were observed in the catalysts after impregnation and drying. Cobalt repartition on silica in the impregnated and dried catalysts depended primarily on the pH of the impregnating solution. Cobalt repartition was uniform on the silica surface if the pH of the impregnating solution was higher than the point of zero charge (PZC) of silica, but was less uniform at pH below that of the PZC of silica. Cobalt dispersion proceeded during catalyst calcination in air. Decomposition of cobalt nitrate and crystallization of cobalt oxide seemed to be the crucial steps in the preparation of highly dispersed cobalt catalysts. Promotion with noble metals resulted in greater cobalt dispersion, probably due to higher concentrations of cobalt oxide crystallization sites. Addition of sucrose modified the structure of supported cobalt complexes and led to higher temperatures of crystallization of cobalt oxide and to catalysts with extremely high cobalt dispersion. In situ magnetization measurements show that promotion with Ru moderated the temperature of reduction of cobalt oxide to metal phases, whereas the effect was less significant for Re-promoted catalysts. The addition of sucrose during impregnation, although significantly enhancing cobalt dispersion, did not diminish cobalt reducibility. Due to a combination of high cobalt dispersion and reducibility, the ruthenium- and rhenium-promoted catalysts prepared using sucrose had the highest number of cobalt metal surface sites. Fischer–Tropsch Reaction rates were determined principally by the number of cobalt surface sites, with high cobalt dispersion and easy reducibility resulting in more active Fischer–Tropsch catalysts.
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fischer tropsch synthesis over silica supported cobalt catalysts mesoporous structure versus cobalt surface density
Applied Catalysis A-general, 2003Co-Authors: Andrei Y Khodakov, Rafeh Bechara, Anne GribovalconstantAbstract:Abstract The effect of support mesoporous structure and cobalt content on cobalt dispersion and reducibility was studied using two series of Fischer–Tropsch (FT) silica supported cobalt catalysts. The first series of the catalysts was supported by an SBA-15 periodic mesoporous silica with narrow pore size distribution, the second series was supported by a commercial mesoporous silica with broader pore size distribution. It was shown that in a wide range of cobalt surface densities (0–50 Co/nm 2 ), cobalt dispersion in silica supported catalysts was largely influenced by support texture. Cobalt dispersion was higher in Co catalysts supported by the SBA-15 silica with a pore diameter of 9.1 nm than in the commercial mesoporous silica with an average pore diameter of 33 nm. A more than 10-fold increase in cobalt surface density did not result in any noticeable sintering of Co 3 O 4 particles in SBA-15 periodic mesoporous silicas; the cobalt dispersion seems to be maintained by catalyst mesoporous structure. The effect of support pore diameter on cobalt dispersion was less significant for the catalysts supported by commercial silicas with broader pore size distribution. At the range of cobalt surface densities from 5 to 15 Co/nm 2 , higher Fischer–Tropsch Reaction rates were observed over cobalt catalysts supported by the SBA-15 periodic mesoporous silica. This effect was attributed to higher cobalt dispersion in these catalysts. An increase in cobalt surface densities did lead to any significant changes in hydrocarbon selectivities and in chain growth probabilities for both series of supported catalysts.
Aleix Comasvives - One of the best experts on this subject based on the ideXlab platform.
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facile fischer tropsch chain growth from ch2 monomers enabled by the dynamic co adlayer
ACS Catalysis, 2019Co-Authors: Lucas Foppa, Marcella Iannuzzi, Christophe Coperet, Aleix ComasvivesAbstract:Numerous chain growth mechanisms, namely CO insertion and carbide, and active sites (flat and stepped surfaces) have been proposed to explain how hydrocarbons are formed from syngas during the Fischer–Tropsch Reaction on Ru catalysts, particularly active and selective toward long-chain products. While these Reaction pathways are supported by density functional theory (DFT) calculations, computational models often considered surfaces at rather low adsorbate coverage. A systematic comparison of chain growth mechanisms including the CO adlayer present on the catalyst’s surface under Reaction conditions is therefore not available due to the challenging representation of co-adsorbate interactions in DFT models. Here, we show that the high coverage of chemisorbed CO on the metal surface favors the carbide mechanism on flat surfaces according to ab initio molecular dynamics simulations, which introduce the complex adlayer effects at the Reaction temperature of 200 °C. At the considered CO and H coverages (0.50–0.72 and 0.24 monolayer, respectively) hydrocarbon formation involves CH2 monomers yielding ethylene and propylene as primary products, consistent with the selectivity observed in experiments. Such mechanism is favored by the presence of the CO adlayer. Indeed, in the absence of co-adsorbed CO, methane may be formed on the flat surface and the first C–C bond occurs preferentially on stepped surfaces via CH and CH2 monomers with a higher free-energy barrier (55 kJ mol–1) compared to the coupling of two CH2 species at high CO coverage on the flat surface (20 kJ mol–1). Therefore, the CO adlayer strongly modulates the nature of chain growth monomers and active sites and drives the formation of hydrocarbons during Ru-catalyzed Fischer–Tropsch. Overall, these results show how adsorbate–adsorbate interactions dictate Reaction mechanisms operating in adlayers, ubiquitous in heterogeneous catalysis.
Anders Holmen - One of the best experts on this subject based on the ideXlab platform.
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deactivation of cobalt based fischer tropsch catalysts a review
Catalysis Today, 2010Co-Authors: Nikolaos E Tsakoumis, Øyvind Borg, Erling Rytter, Magnus Rønning, Anders HolmenAbstract:Abstract To trace the origin of catalyst deactivation is in many cases difficult. It is usually a complex problem where several mechanisms contribute to the loss of activity/selectivity. Low temperature Fischer–Tropsch synthesis (FTS) is a three phase system having a wide range of products and intermediates. Additionally, high partial pressures of steam will arise during Reaction. Thus, the chemical environment in the Fischer–Tropsch synthesis reactor encompasses a large number of interacting species which may negatively affect catalytic activity. Furthermore, it is an exothermic Reaction and local overheating might occur. Utilization of the produced heat is crucial and the choice of the reactor should be done with respect to the catalyst stability properties. Catalyst deactivation in the Fischer–Tropsch Reaction has been a topic of industrial as well as academic interest for many years. The main causes of catalyst deactivation in cobalt based FTS as they appear in the literature are poisoning, re-oxidation of cobalt active sites, formation of surface carbon species, carbidization, surface reconstruction, sintering of cobalt crystallites, metal–support solid state Reactions and attrition. The present study focuses on cobalt catalyzed Fischer–Tropsch synthesis. The various deactivation routes are reviewed, categorized and presented with respect to the most recent literature.
Erling Rytter - One of the best experts on this subject based on the ideXlab platform.
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deactivation of cobalt based fischer tropsch catalysts a review
Catalysis Today, 2010Co-Authors: Nikolaos E Tsakoumis, Øyvind Borg, Erling Rytter, Magnus Rønning, Anders HolmenAbstract:Abstract To trace the origin of catalyst deactivation is in many cases difficult. It is usually a complex problem where several mechanisms contribute to the loss of activity/selectivity. Low temperature Fischer–Tropsch synthesis (FTS) is a three phase system having a wide range of products and intermediates. Additionally, high partial pressures of steam will arise during Reaction. Thus, the chemical environment in the Fischer–Tropsch synthesis reactor encompasses a large number of interacting species which may negatively affect catalytic activity. Furthermore, it is an exothermic Reaction and local overheating might occur. Utilization of the produced heat is crucial and the choice of the reactor should be done with respect to the catalyst stability properties. Catalyst deactivation in the Fischer–Tropsch Reaction has been a topic of industrial as well as academic interest for many years. The main causes of catalyst deactivation in cobalt based FTS as they appear in the literature are poisoning, re-oxidation of cobalt active sites, formation of surface carbon species, carbidization, surface reconstruction, sintering of cobalt crystallites, metal–support solid state Reactions and attrition. The present study focuses on cobalt catalyzed Fischer–Tropsch synthesis. The various deactivation routes are reviewed, categorized and presented with respect to the most recent literature.
Heiko Oosterbeek - One of the best experts on this subject based on the ideXlab platform.
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bridging the pressure and material gap in heterogeneous catalysis cobalt fischer tropsch catalysts from surface science to industrial application
Physical Chemistry Chemical Physics, 2007Co-Authors: Heiko OosterbeekAbstract:The Fischer–Tropsch (FT) process is the heart of many natural gas conversion processes as it enables the conversion of a mixture of CO and H2 into valuable long-chain hydrocarbons. Here we report on the use of state-of-the-art surface science techniques to obtain information on the relationship between the surface atomic structure of model catalysts and their performance in the Fischer–Tropsch Reaction. Cobalt single crystals and polycrystals were modified with non-reducible oxides as to resemble industrial catalysts. Reflection absorption infrared spectroscopy was used for examining the CO adsorption behaviour at room temperature as well as at 493 K at CO pressures spanning 10−7 to 300 mbar on both (modified) Co single/polycrystals and an industrial catalyst. Polarization modulation was applied to cancel the CO gas phase absorption. Subsequently, they were subjected to Reaction tests in the same apparatus at 1 bar and 493 K. This allowed us to close the material, pressure and instrument gap in the field of Fischer–Tropsch synthesis on cobalt-based catalysts.
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cobalt particle size effects in the fischer tropsch Reaction studied with carbon nanofiber supported catalysts
Journal of the American Chemical Society, 2006Co-Authors: Leendert G Bezemer, Freek Kapteijn, J H Bitter, Herman P C E Kuipers, Heiko Oosterbeek, Johannes E Holewijn, Jos A Van Dillen, Krijn P De JongAbstract:The influence of cobalt particle size in the range of 2.6−27 nm on the performance in Fischer−Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 × 10-3 to 1.4 × 10-3 s-1, while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer−Tropsch catalysis is larger (6−8 n...
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cobalt particle size effects in the fischer tropsch Reaction studied with carbon nanofiber supported catalysts
Journal of the American Chemical Society, 2006Co-Authors: Leendert G Bezemer, Freek Kapteijn, J H Bitter, Herman P C E Kuipers, Heiko Oosterbeek, Johannes E Holewijn, Jos A Van Dillen, Krijn P De JongAbstract:The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under Reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.