| 1. |
- Arnby, Karl, 1974, et al.
(författare)
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Characterization of Pt/γ-Al2O3 catalysts deactivated by hexamethyldisiloxane
- 2004
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 54:1, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- The deactivation of alumina supported platinum and iron catalysts by hexamethyldisiloxane (HMDS) exposure has been investigated. Three catalysts (Pt/Al2O3, Fe/Al2O3 and Fe/Pt/Al2O3) were prepared and the influence of short- and long-term deactivation on the activity for oxidation of ethyl acetate was studied. The catalysts were characterized using BET, ICP-AES, XPS, SEM and CO chemisorption. The deactivation was found to proceed by deposition of silicon-species in form of silicate (SixOy) that block the active sites on the surface of the catalysts. The silicon seems to rather attach to platinum and iron sites than to the alumina surface. The Pt/Al2O3 catalyst was moderately deactivated by HMDS even though the silicate was blocking almost the entire platinum surface. Adding iron to the catalyst increased the tolerance towards HMDS as fewer Pt sites were blocked for the Fe/Pt/Al2O3 sample. The deactivation of the two platinum containing samples was reversible since the silicate could partly be removed from the Pt sites by regeneration whereby most of the activity was restored. However, for the Fe/Al2O3 sample the deactivation was more severe and irreversible.
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| 2. |
- Arnby, Karl, 1974, et al.
(författare)
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Influence of ammonia on CO and methanol oxidation over Pt/γ-Al2O3 catalysts modified by Mg
- 2004
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 49:1, s. 51-59
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the effect of modifying Pt/γ-Al2O3 catalysts with magnesium on the oxidation of methanol and carbon monoxide in presence of ammonia. The purpose for adding magnesium to the catalysts was to enhance the low-temperature activity and to make catalysts more resistant towards ammonia inhibition. The catalysts were prepared by wet-impregnation using platinum nitrate and magnesium nitrate solutions. The platinum content was constant for all catalysts, while the magnesium content ranged from 0 to 5 wt.%. The catalysts were characterized by CO chemisorption, AAS, XRD, XPS and BET. The addition of magnesium resulted in the formation of both MgAl2O4 spinel and MgO, and did slightly improve the low-temperature activity for carbon monoxide oxidation, while the activity for methanol oxidation decreased compared to an unpromoted Pt/γ-Al2O3 catalyst. The inhibition by ammonia for carbon monoxide oxidation was low for all catalysts, while the methanol oxidation was substantially inhibited by ammonia. Adding magnesium to Pt/γ-Al2O3 catalysts did not make the catalysts more resistant to ammonia inhibition.
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| 3. |
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| 4. |
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| 5. |
- Arnby, Karl, 1974, et al.
(författare)
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Investigation of Pt/γ-Al2O3 catalysts with locally high Pt concentrations for oxidation of CO at low temperatures
- 2004
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 221:1, s. 252-261
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Tidskriftsartikel (refereegranskat)abstract
- A new method for preparing supported catalysts, in which Pt was distributed in locally high concentrations on the γ-Al2O3 support, was studied. These catalysts were compared with a conventionally prepared Pt/γ-Al2O3 catalyst in which Pt was deposited evenly on the support. The object was to ascertain whether it is possible to prepare catalysts that retain heat released from exothermic reactions to a higher extent and thereby become more low-temperature active than a conventionally prepared catalyst. A significant improvement of the activity was observed for the catalysts prepared with locally high Pt concentrations when CO (1%, 1000 and 100 ppm) was oxidized at a constant O2 concentration (10%). The improved activity is discussed in terms of heat transfer, mass transfer, and structure sensitivity. Differences in heat transfer appear to be the least probable reason for the enhanced activity for the catalysts with locally higher Pt concentrations, whereas structural effects also seem to be an unlikely explanation. Differences in mass transfer seem, however, to be a more likely reason for the improved activity.
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| 6. |
- Arnby, Karl, 1974
(författare)
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Low-Temperature Oxidation of CO and Volatile Organic Compounds over Supported Platinum Catalysts
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The catalytic oxidation of carbon monoxide (CO) and volatile organic compounds (VOC) is important for the reduction of harmful emissions from both vehicles and from stationary sources. Both these applications suffer from poor catalytic efficiency at lower temperatures. The objective of this work was to gain a deeper understanding of the low-temperature oxidation of CO and VOC over alumina supported platinum catalysts. The work has been focused on the preparation of platinum-based model catalysts, extensive flow reactor studies and thorough sample characterisation. A new preparation method was used to prepare supported catalysts with platinum distributed locally in high concentrations on the alumina support. The purpose was to prepare catalysts that would retain the evolved reaction heat to a higher extent and thereby become more active at low temperatures than conventionally prepared catalysts. The catalysts prepared using this new method showed an improved low-temperature activity for the oxidation of CO, however, not for the oxidation of propene or propane. When considering heat transfer, calculations of both temporary effects and heat accumulation showed that the enhanced activity for CO oxidation can not be explained by a heat effect. Instead, mass transfer limitations probably improved the activity to a certain extent, and structural effects can also be of importance. The oxidation of propane over Pt/γ-Al2O3 was shown to be highly influenced by the oxygen concentration. Maximum activity was observed close to stoichiometric conditions. At net-oxidising conditions the catalysts were most likely passivated due to formation of platinum oxide. In a deactivation study, the influence of hexamethyldisiloxane (HMDS) on the oxidation of ethyl acetate over iron-promoted Pt/γ-Al2O3 samples was investigated. The catalytic deactivation was found to proceed as HMDS decomposed into silicate (SixOy), which blocks the active sites on the catalyst surface. The poisoning was found to be selective as silicon rather attached to platinum and iron sites than to alumina. Moreover, depending on the catalyst composition the deactivation was either reversible or irreversible. Promoting the Pt/γ-Al2O3 catalyst with iron increased the tolerance towards HMDS as fewer platinum sites were blocked by silicate.
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| 7. |
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| 8. |
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| 9. |
- Arnby, Karl, 1974, et al.
(författare)
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The effect of platinum distribution in monolithic catalysts on the oxidation of CO and hydrocarbons
- 2005
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 233:1, s. 176-185
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Tidskriftsartikel (refereegranskat)abstract
- Monolithic Pt/gamma-Al2O3 catalysts were prepd. by depositing of the platinum phase, either homogeneously or heterogeneously (i.e., local high platinum concn.), in the washcoat. The influence of the platinum distribution on the ignition and extinction processes for oxidn. of CO, propene, and propane, resp., was investigated by both temp.-programmed and oxygen step-response flow-reactor expts. In addn., in situ XANES spectroscopy was used to follow changes in the chem. state of platinum during propane oxidn. For samples with heterogeneous platinum distribution, the results show an improved low-temp. activity for CO oxidn., whereas no clear improvement is obsd. for oxidn. of propene or propane. Comparison of the results for CO and hydrocarbon oxidn. shows that the improved activity cannot be explained by thermal effects. Moreover, calcns. indicate that the enhanced activity for CO oxidn. can be due to mass-transfer phenomena. This could not be confirmed by our expts., however. Instead, the improved activity for CO oxidn. can be due to varying platinum particle size between the samples. Furthermore, the oxidn. of propane is shown to be highly influenced by the oxygen concn., showing an activity max. for gas compositions close to the stoichiometric. High oxygen levels result most probably in a predominantly oxygen-covered surface, which inhibits the reaction and, as supported by the in situ XANES experiments, passivates the surface by forming platinum oxide.
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| 10. |
- Carlsson, Per-Anders, 1972, et al.
(författare)
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Effect of periodic operation on the low-temperature activity for propane oxidation over Pt/Al2O3 catalysts
- 2004
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 59:20, s. 4313-4323
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Tidskriftsartikel (refereegranskat)abstract
- We report experimental results for the oxidation of propane over alumina supported platinum catalysts with varying Pt dispersions. Flow-reactor experiments introducing 0.15 vol% C3H8 while changing the oxygen concentration step-wise (step-response experiments) from rich to lean composition or periodically (pulse-response experiments) switching between rich and lean O2 concentrations, at a constant inlet gas temperature of 250 °C have been performed. Complementary in situ FTIR spectroscopy experiments for surface analysis have also been carried out. The results reveal a strong correlation between the reactant composition and the catalytic activity, showing an optimum reaction rate for compositions close to the stoichiometric value for complete oxidation of propane. For lean gas compositions the activity is low probably due to a high O-coverage (O self-poisoning) which in turn is likely to also cause platinum oxide formation. However, by periodically introducing rich periods (periodic operation) the activity can temporarily be restored and the surface composition can be kept close to the optimum, leading to an overall improvement of the oxidation rate. Generally, the introduction of short rich periods to a slightly lean mixture (lean-rich cycling) seems to be more efficient than the corresponding rich-lean cycling since the latter suffers from oxygen deficiency limiting the reaction during the long periods.
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