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- Lögdberg, Sara, et al.
(författare)
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Further insights into methane and higher hydrocarbons formation over cobalt-based catalysts with γ-Al2O3, α-Al2O3 and TiO2 as support materials
- 2017
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Ingår i: Journal of Catalysis. - : Academic Press. - 0021-9517 .- 1090-2694. ; 352, s. 515-531
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Tidskriftsartikel (refereegranskat)abstract
- A range of cobalt-based catalysts varying in Co loading and prepared by incipient wetness impregnation of traditional support materials (γ-Al2O3, α-Al2O3 and TiO2), have been studied in the Fischer-Tropsch reaction at industrially relevant process conditions (483 K, 20 bar, H2/CO = 2.1). A high selectivity to C5+ hydrocarbons (SC5+) is to a great extent connected with a high site activity, but not exclusively. We propose that the ratio of monomer-production rate to C–C coupling rate of a catalyst determines chain-growth probability by means of governing the coverage of the monomer on the cobalt surface. We speculate that this ratio depends on e.g. shape, strain and size of the Co crystallites and, therefore, is highly dependent on the choice of support material. No general relationship between Co particle size and SC5+ is found, but individual correlations exist for each support material. Within each support material, there are indications of negative correlations between the chain-growth probability of the C1 ∗ surface intermediate (αC1) and the higher αCn values. This can be rationalized by assuming that the majority of methane is formed by a different mechanism, separate from chain growth, but connected with chain growth through a common carbon pool. We propose that the monomers and the majority of methane are produced at sites different from the ones involved in chain growth. There is no general correlation between αC1 and SC5+ for catalysts with different support materials, possibly due to small differences in cobalt surface coverage of hydrogen. For the TiO2-supported catalysts, a dramatically increased αC1 value observed for catalysts with Co particles smaller than approx. 15 nm, is probably associated with strong metal-support interactions (SMSI). This phenomenon apparently limits the effect (reduction) on the SC5+ when moving toward smaller Co particles.
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- 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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- Rytter, Erling, et al.
(författare)
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Hydrophobic catalyst support surfaces by silylation of gamma-alumina for Co/Re Fischer-Tropsch synthesis
- 2018
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Ingår i: Catalysis Today. - : Elsevier. - 0920-5861 .- 1873-4308. ; 299, s. 20-27
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Tidskriftsartikel (refereegranskat)abstract
- Eight different silanes with chloro or methoxy active ligands have been examined to modify the surface of a standard.-alumina support to investigate the effect of surface acidity and hydrophobicity in Fischer-Tropsch synthesis (FTS) at common operating conditions (210 degrees C; 20 bar; H-2/CO = 2.1). Silanes were impregnated prior to cobalt (Pre) or after cobalt impregnation and calcination (Post). Samples were characterized by standard techniques (FTIR, XRD, TPD, TPR, TGA, BET). In the Post series, cobalt crystallite size is maintained, while it is reduced when cobalt impregnation is performed on a partly hydrophobic surface. Both series of catalysts have FTS performances that deviate significantly from a reference catalyst and, in combination with characterization data, the silanes have modified all the 15 examined catalysts. The Pre series offer options for improved selectivity to higher hydrocarbons in FTS. The effects of silylation on FT performance mechanisms are discussed in terms of gas diffusion limitations, strain in cobalt particles and partial blocking of CO activation sites.
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