| 1. |
- Boutonnet, Magali, et al.
(författare)
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Recent developments in the application of nanoparticles prepared from w/o microemulsions in heterogeneous catalysis
- 2008
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Ingår i: Current Opinion in Colloid & Interface Science. - : Elsevier BV. - 1359-0294 .- 1879-0399. ; 13:4, s. 270-286
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Tidskriftsartikel (refereegranskat)abstract
- This paper reviews the use of microemulsions, especially the water-in-oil (w/o) microemulsions, for preparation of nanoparticles that are employed as catalyst components in heterogeneous catalytic reactions. The objective is to show the growing interest of using microemulsions in the preparation of different types of materials such as metals, single metal oxides or mixed metal oxides with a broad range of application in heterogeneous catalysis and also in electrocatalysis. In most cases, the catalytic material showed improved catalytic properties as a result of the special synthesis environment created by the microemulsions. Still, research is needed for a better understanding of such beneficial effects. In addition, this method needs improvements in order to produce, in an environmentally friendly way, a suitable amount of material for use in industrial-scale catalytic processes.
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| 2. |
- Gudmundson, Sara, et al.
(författare)
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Stability patterns of spatial food webs in coloured environments
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In times when climate change is expected to cause an increased environmental variability it is important to understand how species respond to disturbances. We explore how the stability of species respond to changes in environmental noise by introducing noise colour to different spatial and multi-trophic model systems: (1) a diamond shaped food web with stable oscillations, (2) a stabilized diamond shaped food web, and (3) a food web with stable dynamics. We conclude that adding space and additional trophic levels makes species response to environmental noise colour consistent. All three food webs and species decreased in stability with increased redness, positive temporal autocorrelation, of the environmental noise. Hence, interactions between noise colour and species responsiveness previously found in single- and multi-species models were not found when comparing more natural food webs differing in stability properties. When adding a spatial dimension, all food webs and species increased in stability. Both the diamond shaped and the stabilized diamond shaped food web were significantly more stable than the food web with more typical stable dynamics when existing in a variable and spatial setting. The major route to explain stability and the existence of a diverse world may then be the variable and spatial complexity of nature.
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| 3. |
- Huber, Florian, et al.
(författare)
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Remarks on the passivation of reduced Cu-, Ni-, Fe-, Co-based catalysts
- 2006
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Ingår i: Catalysis Letters. - : Springer Science and Business Media LLC. - 1011-372X .- 1572-879X. ; 110:3-4, s. 211-220
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Tidskriftsartikel (refereegranskat)abstract
- Catalysts containing metals such as Cu, Ni, Fe, Co in their reduced state are often subjected to passivation procedures prior to characterization. Passivation with N2O or O-2 to create a protective oxide layer also results in a certain degree of sub-surface oxidation. The heat released during oxidation is a critical parameter. The extent of bulk oxidation depends on the type of oxidant as well as on the size of the metal particles, as shown for copper catalysts. The final, meta-stable passivation layer requires a certain thickness to sustain exposure to ambient atmosphere. The encapsulation of metal particles in carbon is an efficient method for preserving the metallic state, as demonstrated for metallic nickel and iron with carbon nanofibers. The use of passivated samples for characterization of the active, i.e., reduced, catalyst has limited value.
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| 4. |
- Lualdi, Matteo, et al.
(författare)
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Effect of Ti and Al addition via direct synthesis to SBA-15 as support for cobalt based Fischer-Tropsch catalysts
- 2012
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Ingår i: Applied Catalysis A. - : Elsevier BV. - 0926-860X .- 1873-3875. ; 443-444, s. 76-86
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Tidskriftsartikel (refereegranskat)abstract
- Different mesoporous SBA-15 supports doped with Ti and Al at 5 and 10 wt% have been synthesized by means of direct synthesis. The supports have been characterized by N 2-adsorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), NH 3-temperature programmed desorption (NH 3-TPD), H 2-chemisorption, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM). Titanium doped materials showed to have much shorter channels than the pristine SBA-15, while aluminum addition did not have a significant effect on channel length. After impregnation with 12 wt% Co, the catalysts were further characterized and tested in the Fischer-Tropsch synthesis at industrially relevant process conditions (483 K, 20 bar, H 2/CO ratio = 2.1, pellet size: 53-90 μm) with and without external water addition. The S C5 + values of the different SBA-supported catalysts were low, especially at low conversion levels (i.e. low water partial pressure), suggesting that CO diffusion limitations increased the H 2/CO ratio inside the 1-dimensional (1D) porous network. The selectivity data showed a correlation between the channel length and the extent of CO-diffusion limitations at much shorter diffusion distances than those for conventional 3D porous supports. Water partial pressure showed to increase the syngas diffusion rate (i.e. removal of diffusion limitations on reactants’ arrival), to have a positive kinetic effect on the rate and to favor longer chain hydrocarbons.
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| 5. |
- Lualdi, Matteo, et al.
(författare)
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Evidence for diffusion-controlled hydrocarbon selectivities in the fischer-tropsch synthesis over cobalt supported on ordered mesoporous silica
- 2011
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Ingår i: Topics in catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 54:16-18, s. 1175-1184
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Tidskriftsartikel (refereegranskat)abstract
- A series of four cobalt-based catalysts (two of which promoted with ruthenium) supported on SiO2 or SBA-15 were prepared and tested in the Fischer-Tropsch synthesis at industrially relevant process conditions (483 K, 20 bar, H2/CO ratio = 2.1, pellet size: 53-90 μm). The catalysts were characterized by N2-adsorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), H2-chemisorption and transmission electron microscopy (TEM). Ru as promoter enhanced the activity but not the selectivity to long-chain hydrocarbons ({S}-{C}-{ 5+}). The {{S}}-{{C}}-{5+}} values of the SBA-supported catalysts were very low, especially at low conversion levels (i.e. low water partial pressure), suggesting that CO diffusion limitation increased the H2/CO ratio inside the 1-dimensional (1-D) porous network. A superimposition of the selectivity results on the correlations found in our recent study, derived for Co-based catalysts supported on γ-Al2O3, α-Al2O3 and TiO2 free from diffusion limitations, was made. While the SiO2-supported catalysts with a 3-D porous structure followed the correlations, the SBA-catalysts deviated significantly at low conversions, giving a further indication that the selectivity results of these catalysts were affected by CO diffusion limitations. Hence, it may be concluded that the kinetically significant diffusion distances (i.e. those long enough to cause an intrapore H 2/CO ratio higher than that of the bulk gas phase) are probably much shorter for 1-D porous networks than for conventional 3-D supports. This is explained by a significantly lower effective diffusivity in 1-D porous networks. The potential of using the correlations between non-ASF distributed hydrocarbons and C5+, to give insight on the occurrence of diffusion limitations, was confirmed by superimposing data from the literature that were anticipated to be influenced by CO diffusion limitations.
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| 6. |
- Lualdi, Matteo, et al.
(författare)
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Investigation of mixtures of a Co-based catalyst and a Cu-based catalyst for the fischer-tropsch synthesis with Bio-Syngas : The importance of indigenous water
- 2011
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Ingår i: Topics in catalysis. - : Springer-Verlag New York. - 1022-5528 .- 1572-9028. ; 54:13-15, s. 977-985
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Tidskriftsartikel (refereegranskat)abstract
- A series of different mechanical mixtures of a narrow-pore Co/γ-Al2O3 catalyst and a Cu-based WGScatalyst has been investigated in the low-temperature Fischer-Tropsch synthesis (483 K, 20 bar) with a model bio-syngas (H2/CO = 1.0) in a fixed-bed reactor. The higher the fraction of WGS-catalyst in the mixture, the lower is the Co-catalyst-time yield to hydrocarbons. This is ascribed to a strong positive kinetic effect of water on the Fischer-Tropsch rate of the Co-catalyst, showing the importance of the indigenously produced water, especially in fixed-bed reactors where the partial pressure of water is zero at the reactor inlet. A preliminary kinetic modeling suggests that the reaction order in PH2O is 0.3 for the Co/γ-Al2O3 catalyst in the range of the studied reactor-average partial pressures of water (i.e., 0.04-1.2 bar).
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| 7. |
- Lualdi, Matteo, et al.
(författare)
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On the effect of water on the Fischer-Tropsch rate over a Co-based catalyst : The influence of the H2/CO ratio
- 2013
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861 .- 1873-4308. ; 214, s. 25-29
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Tidskriftsartikel (refereegranskat)abstract
- The effect of water partial pressure on the Fischer-Tropsch (FT) rate of a cobalt catalyst supported on narrow-pore γ-Al2O3 was investigated at industrially relevant process conditions (483 K, 30 bar, pellet size: 53-90 μm). Inlet water partial pressure was varied up to 9 bar by external water vapour addition at different H2/CO molar ratios ranging from 1 to 3. The effect of water was found to be positive on FT-rate independently of the H2/CO ratio, but more significantly at H 2-poor condition. Temperature-programmed hydrogenation (TPH) was used to verify the presence of unreactive carbon species on the catalyst after 22 h on stream at the different conditions with and without exposure to about 9 bar water. A higher temperature feature that could be associated to amorphous polymeric carbon was detected at H2-poor conditions but remained unchanged upon 2 h of water exposure which did not result in a change in the amount of amorphous polymeric carbon detectable by TPH.
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| 8. |
- Lögdberg, Sara
(författare)
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Development of Fischer-Tropsch catalysis for gasified biomass
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In order to secure the energy supply to an increasing population and at the same time limit the damage to Earth, i.e. avoiding a fatal climate change as a result of anthropogenic emissions of greenhouse gases (primarily CO2), immediate action is necessary. This includes reducing the energy consumption, increasing the energy conversion efficiency, and using renewable energies. The transport sector is the one most dependent on fossil energy and it stands for a significant part of the energy consumption in the world. For instance, in EU-25 transportation stands for 30 % of the total final energy consumption and relies to 98 % on oil. Being the only renewable energy possible to convert into liquid fuels biomass, as a means for reducing the CO2 emissions from the transport sector, will play an important role in the near future. The conversion of biomass into transportation fuels is preferentially done via gasification followed by the fuel synthesis. The whole production chain from biomass to final fuel is very dependent on R&D, in order to become competitive with the fossil fuels. Fischer-Tropsch (FT) diesel made from biomass is a viable option for reducing the CO2 emissions from transportation since it may be blended with conventional diesel in any concentration. Furthermore, since its composition is almost the same as of petroleum-based diesel (although cleaner) the same distribution system and engines may be used, which facilitates its introduction on the market. This thesis presents the results of the laboratory work performed in 2003 – 2007 at the Department of Chemical Technology, KTH, and at the Department of Chemical Engineering, NTNU (the Norwegian University of Science and Technology) in Trondheim. Part of the work has been performed in close cooperation with the Department of Chemical and Biological Engineering at Chalmers University of Technology. All FT experiments were performed in a fixed-bed reactor at 210 ºC and 20 bar. Pure mixtures of H2, CO and N2 were used as feed to the reactor. Steam was also occasionally introduced. Selectivity to C5+ was used as a measure of the catalysts’ ability to grow long-chain hydrocarbons, which is desirable when diesel is the product aimed for. The first part of the thesis deals with the direct conversion of a H2-poor syngas, which is obtained upon gasification of biomass, into FT hydrocarbons. “H2-poor” means that the H2/CO ratio is lower than what is required by the stoichiometry (~ 2.1) of the FT synthesis (reaction 1). In order to increase the H2/CO ratio to the required one, internal water-gas-shift (WGS) is needed (reaction 2). FT: CO + 2H2 “-CH2-“ + H2O (1) WGS: CO + H2O CO2 + H2 (2) The H2/CO usage ratio has been used as a measure for the internal WGS activity, it is defined as follows: where S is the selectivity (of total C-containing products), and the factor F indicates the number of H2 moles required for one CO mole to form the product (e.g. for producing high molecular weight n-paraffins, 2 moles of H2 per mole of CO are required). For the fraction C2 – C4, F will have different values depending on the selectivity to C2, C3 and C4, and it also depends on the olefin/paraffin ratios for those hydrocarbons. In order to reach the highest once-through conversion of the syngas, the H2/CO usage ratio should be equal to the inlet H2/CO ratio. The lower the usage ratio, the higher the relative WGS activity. The combined FT and WGS reactions with H2-poor syngas have been tested for 12 wt% Co and 12 wt% Co – 0.5 wt% Re catalysts supported on γ-Al2O3. It was found that with lower H2/CO ratios in the feed, the syngas conversion and the CH4 selectivity decreased, while the C5+ selectivity and olefin/paraffin ratio for C2-C4 increased slightly. The WGS activity was low for all catalysts, implying a H2/CO usage ratio close to the stoichiometric one (2.1), even for inlet H2/CO ratios of 1.5 and 1.0. By incorporating significant amounts of Fe (20 % of total metal) in the Co catalyst referred to above (by co-impregnation) in order to achieve a 12 wt% bimetal loading on γ-Al2O3, a slightly lower usage ratio was obtainable (1.92) for an inlet ratio of 1.0 for dry conditions. Different Fe:Co ratios ranging from 100 % Fe to 100 % Co (12 wt% bimetal) were tested for an inlet H2/CO ratio of 1.0. The characterisation results indicated that Fe was enriched at the surface, hence covering the more FT-active Co sites, even at low percentage Fe. Not even upon significant replacement of Co by Fe (≥ 20 %) were the usage ratios for dry conditions lowered to any significant extent. The WGS reaction at the low temperature used could be boosted by addition of external water. However, since the catalysts with the highest WGS activity had surface enrichment of Fe, high water partial pressures negatively affect the FT rate and also lead to rapid deactivation by re-oxidation of the FT-active iron phases (iron carbides) or by sintering. Surprisingly, also the WGS activity rapidly deactivated at high partial pressures of water, although Fe3O4 is believed to be the WGS-active phase. Possibly, surface oxidation of Fe3O4 into γ-Fe2O3 may take place during these water-rich conditions. The WGS reaction (reaction 2) is thermodynamically favoured at low temperatures. However, the WGS kinetics over the tested catalysts is far too slow at the FT reaction temperature used in typical low-temperature FT (LTFT) applications, with diesel as the desired product. The second part of this thesis deals with the hydrocarbon selectivity over Co-based catalysts in the FT synthesis with stoichiometric H2/CO feed. An alternative catalyst preparation technique, the microemulsion (ME) technique, was used as a complement to the conventional incipient wetness (IW) impregnation. A great deal of effort was put into the development of the synthesis of Co particles of monodisperse size in a ME and the subsequent deposition of these onto a porous support material. By using the ME technique it was possible to prepare relatively small Co particles in a large-pore support such as TiO2, which is not an easy task using the conventional impregnation technique. With the prepared ME catalyst on TiO2 it was possible to study the effect of Co particle size on FT selectivity irrespective of the pore size of the support. It was found that, at least for Co particles above 10 nm, it is not the Co particle size that is the principal parameter determining the selectivity of a catalyst, but rather the physical and/or chemical properties of the support. Due to the smaller Co particle size (~ 12 nm) of the ME-TiO2 catalyst as compared to a corresponding IW catalyst (~ 26 nm) supported on TiO2, the FT activity of the former was 100 % higher as fresh catalyst. After 120 h on stream the ME catalyst still gave 60 % higher rate to C5+ (expressed as gC5+/gcat,h) compared to both an IW-TiO2 and an IW-γ-Al2O3 catalyst, all catalysts having the same composition (12 wt% Co, 0.5 wt% Re).
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| 9. |
- 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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| 10. |
- 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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