| 1. |
- Gavrilovic, Lubisa, et al.
(författare)
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The effect of aerosol-deposited ash components on a cobalt-based Fischer–Tropsch catalyst
- 2019
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Ingår i: Reaction Kinetics, Mechanisms and Catalysis. - : Springer. - 1878-5190 .- 1878-5204. ; 127:1, s. 231-240
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Tidskriftsartikel (refereegranskat)abstract
- The effect of ash salts on Co-based Fisher–Tropsch catalysts was studied using an aerosol deposition technique. The major elements in the ash were found to be K, S and Cl. The ash was deposited on a calcined catalyst as dry particles with an average diameter of approx. 350 nm. The loading of ash particles was varied by varying the time of exposure to the particles in a gas stream. Catalyst characterization did not reveal significant differences in cobalt dispersion, reducibility, surface area, pore size, or pore volume between the reference and the catalysts with ash particles deposited. Activity measurements showed that following a short exposure to the mixed ash salts (30 min), there were no significant loss of activity, but a minor change in selectivity of the catalyst . Extended exposure (60 min) led to some activity loss and changes in selectivity. However, extending the exposure time and thus the amount deposited as evidenced by elemental analysis did not lead to a further drop in activity. This behavior is different from that observed with pure potassium salts, and is suggested to be related to the larger size of the aerosol particles deposited. The large aerosol particles used here were probably not penetrating the catalyst bed, and to some extent formed an external layer on the catalyst bed. The ash salts are therefore not able to penetrate to the pore structure and reach the Co active centers, but are mixed with the catalyst and detected in the elemental analysis.
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| 2. |
- Larsson, Mikael, 1967, et al.
(författare)
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The effect of reaction conditions and time on stream on the coke formed during propane dehydrogenation
- 1996
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517. ; 164:1, s. 44-53
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Tidskriftsartikel (refereegranskat)abstract
- Pt/Al2O3and Pt–Sn/Al2O3catalysts, coked during propane dehydrogenation, have been studied using temperature-programmed oxidation (TPO). Time on stream, temperature, and reaction gas composition have been varied. Three different peaks were identified from the TPO profiles on the Pt–Sn catalyst and attributed to different types of coke; coke on and in the vicinity of the metal, coke on the carrier, and graphitic coke on the carrier. The amounts of these types were related to reaction conditions. The formation of the coke belonging to the first two peaks in the TPO profiles increases with temperature and partial pressure of propene. Hydrogen, on the other hand, suppresses the formation. The amount of coke that can be attributed to the third peak increases with temperature and propane partial pressure. A model is discussed where a minor part of the coke deactivates the catalyst. This coke is formed in parallel with the coke that is seen in the first two peaks in the TPO experiments. The graphitic coke formed on the carrier is not formed through this route. The experiments with different time on stream revealed that the first peak reached a constant level after about 15 h, while the second one still increased. Hydrogen was very efficient in preventing coke formation and deactivation but could not remove coke already formed on the catalyst. The hydrogenolysis and cracking mechanisms during the propane dehydrogenation are also discussed.
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| 3. |
- Lögdberg, Sara, et al.
(författare)
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Effect of water on the space-time yield of different supported cobalt catalysts during Fischer-Tropsch synthesis
- 2011
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Ingår i: Applied Catalysis A. - : Elsevier BV. - 0926-860X .- 1873-3875. ; 393:1-2, s. 109-121
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Tidskriftsartikel (refereegranskat)abstract
- The effect of partial pressure of water on the Fischer-Tropsch (FT) rate of six cobalt-based catalysts supported on three different carrier materials (gamma-Al2O3, alpha-Al2O3, TiO2) with varying Co particle sizes was investigated in a fixed-bed reactor by changing space velocity and by external water vapour addition. A typical catalyst pellet size (<100 mu m) for industrial slurry-bed FT reactors was used. Water was found to have a positive kinetic effect, at least up to moderate amounts, on the FT rate of all catalysts in the present study, including the gamma-Al2O3-supported catalyst with pores smaller than similar to 10 nm. The reason for the apparent negative effect on the space-time yield at a direct exposure of Co supported on narrowpore gamma-Al2O3 to high partial pressures of water is due to a rapid and extensive deactivation. This could be ascribed to formation of hard-to-reduce oxidized cobalt species. The choice of support material was found to have a major effect on the response to changes in partial pressure of water, both with respect to deactivation behaviour and kinetics. However, there is a minor Co-particle size effect on the magnitude of the kinetic effect of water, larger Co particles showing a more positive response. Different extents of mass transfer limitations and/or differences in fugacities of H-2, CO and water among the six catalysts could be ruled out as causes for the observed differences.
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| 4. |
- Lögdberg, Sara, et al.
(författare)
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On the selectivity of cobalt-based Fischer-Tropsch catalysts : Evidence for a common precursor for methane and long-chain hydrocarbons
- 2010
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 274:1, s. 84-98
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Tidskriftsartikel (refereegranskat)abstract
- A total of 36 cobalt-based supported catalysts were investigated in the Fischer-Tropsch reaction at industrially relevant process conditions: 483 K, 20 bar, molar H-2/CO ratio = 2.1, pellet size: 53-90 mu m. The effect of adding water vapour to the feed was investigated for 20 of the catalysts, and a H-2/CO ratio of 1.0 was used for a few catalysts. The catalysts differed in support material, Co loading, promoters, Cl content, Co particle size (larger than similar to 6 nm), morphology, degree of reduction and preparation technique and showed a large variation in selectivity. For each set of process conditions, a linear relationship seems to exist between the selectivity to methane (and other light products) and C5+ indicating a common precursor, i.e. a common monomer pool, for all hydrocarbon products. A high selectivity to C5+ is mainly an effect of a high intrinsic chain-growth probability and unlikely to be a result of an enhanced cc-olefin readsorption. The universal effect of external water addition on the hydrocarbon selectivities is limited to a decrease in the methane selectivity. A small proportion of the catalysts developed "pure methanation" sites upon exposure to high partial pressures of water.
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| 5. |
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| 6. |
- Tsakoumis, Nikolaos E., et al.
(författare)
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Structure-Performance Relationships on Co-Based Fischer-Tropsch Synthesis Catalysts : The More Defect-Free, the Better
- 2019
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Ingår i: ACS Catalysis. - : AMER CHEMICAL SOC. - 2155-5435. ; 9:1, s. 511-520
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Tidskriftsartikel (refereegranskat)abstract
- Understanding and utilizing structure-performance relationships in catalytic nanomaterials is the epitome of catalysis science. Knowledge at the atomic level can potentially allow rational design of more selective and energy-efficient catalytic materials. Fischer-Tropsch synthesis on cobalt is an example of a complicated system that operates in a narrow process regime, and the nature of the reaction product is governed by numerous parameters. On an industrial model catalyst, we have simplified the structure of the active, metallic nanoparticles into a predominantly hexagonal close-packed structure via the use of a Co2C precursor. By varying the final reduction temperature, we could mildly modify catalyst microstructural properties at the nanoparticle (NP) level. Catalytic materials, although with minimal structural differences, showed significantly different performance. Evidently there is a narrow window for complete utilization of the hexagonal close-packed Co crystallites that lies between removal of lattice carbon, that remains from the Co2C precursor, and the initiation of stacking disorder, because of a transition to the face-centered cubic Co structure. Fischer-Tropsch synthesis performance indicators show that Co NPs with minimum number of crystal defects outperform catalysts with lattice defects, because of the existence of either lattice carbon or stacking faults. Therefore, catalyst preparation and activation procedures probably should be designed targeting defect-free Co crystallites.
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