| 1. |
- Becker, Elin, 1981, et al.
(författare)
-
In Situ Spectroscopic Investigation of Low-Temperature Oxidation of Methane over Alumina-Supported Platinum during Periodic Operation
- 2011
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 115:4, s. 944-951
-
Tidskriftsartikel (refereegranskat)abstract
- Methane oxidation over Pt/Al2O3 at transient inlet-gas conditions was studied in situ using synchronous energy dispersive X-ray absorption spectroscopy, Fourier transform infrared spectroscopy, and mass spectrometry. The employed combination of experimental techniques allows for simultaneous analysis of the electronic state of platinum, surface coverage of reaction intermediates/products, and catalytic activity/selectivity, respectively. By cycling of the feed gas composition between net-oxidizing and net-reducing conditions, the activity for methane oxidation can be increased as compared to continuous net-oxidizing conditions. Using the white-line area of time-resolved X-ray absorption near-edge structure spectra, a quantitative estimation of the surface O/Pt ratio indicates the formation of an inhomogeneous surface oxide on the platinum crystallites during reaction. The obtained temporary high activity can be explained through Langmuir-Hinshelwood kinetics and may result either from the formation of a partially oxidized platinum surface that is more effective for methane dissociation or, more likely, from a period with more reactive chemisorbed oxygen prior to oxide formation.
|
|
| 2. |
- Bounechada, Djamela, 1984, et al.
(författare)
-
Mechanisms behind sulfur promoted oxidation of methane
- 2013
-
Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 15:22, s. 8648-8661
-
Tidskriftsartikel (refereegranskat)abstract
- The promoting effect of SO2 on the activity for methane oxidation over platinum supported on silica, alumina and ceria has been studied by flow-reactor, in situ infrared spectroscopy and in situ high-energy x-ray diffraction experiments under transient reaction conditions. The catalytic activity is clearly dependent on the support material and its interaction with the noble metal both in absence and presence of sulfur. On platinum, the competitive reactant adsorption favors oxygen dissociation such that oxygen self-poisoning is observed for Pt/silica and Pt/alumina. Contrarily for Pt/ceria, no oxygen self-poisoning is observed, which seems to be due to additional reaction channels via sites on the platinum-ceria boundary and/or ceria surface considerably far from the Pt crystallites. Addition of sulfur dioxide generally leads to the formation of ad-SOx species on the supports with a concomitant removal and/or blockage/rearrangement of surface hydroxyl groups. Thereby, the methane oxidation is inhibited for Pt/silica, enhanced for Pt/alumina and temporary enhanced followed by inhibition after long-term exposure to sulfur for Pt/ceria. The observations can be explained by competitive oxidation of SO2 and CH4 on Pt/silica, formation of new active sites at the noble metal-support interface promoting dissociative adsorption of methane on Pt/alumina, and in the case of Pt/ceria, formation of promoting interfacial surface sulfates followed by formation of deactivating bulk-like sulfate species. Furthermore, it can be excluded that reduction of detrimental high oxygen coverage and/or oxide formation on the platinum particles through SO2 oxidation is the main cause for the promotional effects observed.
|
|
| 3. |
|
|
| 4. |
|
|
| 5. |
- Happel, Markus, 1985, et al.
(författare)
-
SOx storage and release kinetics for ceria-supported Pt
- 2009
-
Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 91:3-4, s. 679-682
-
Tidskriftsartikel (refereegranskat)abstract
- The SOx storage and release kinetics on CeO2 have been studied by lean SOx adsorption and temperature programmed desorption for different pairwise configurations of individual monolith samples, i.e., Pt/CeO2 + SiO2, Pt/SiO2 + CeO2, CeO2 + Pt/SiO2 and CeO2 + SiO2. In the case of sole ceria, SOx adsorption proceeds both via SO2 and SO3 adsorption although the latter channel is kinetically favored. Hence, the rate of SO2 oxidation is crucial for the overall SOx storage kinetics. It is also found that physical contact between Pt and ceria is important for the storage process. This is attributed to efficient transport routes for SOx (surface diffusion and spill-over processes) and/or specific adsorption sites at the platinum–ceria interface. The main route for SOx release is found to be thermal decomposition where the effect of platinum is minor, although an indirect effect cannot be ruled out. Different mechanistic scenarios for SOx adsorption are discussed, which may serve as a guide for future experiments.
|
|
| 6. |
|
|
| 7. |
- Kylhammar, Lisa, 1978, et al.
(författare)
-
Effects of oxidation and redox-properties on the selectivity of heat-treated Ag/Al2O3 catalysts for HC-SCR of NOx
- 2007
-
Ingår i: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1572-9028 .- 1022-5528. ; 42-43:1-4, s. 119-122
-
Tidskriftsartikel (refereegranskat)abstract
- Ag/Al2O3 catalysts (2 wt% Ag) have been prepared and calcined at different temperatures to render catalysts with different silver particle size or silver configuration. The differences in activity and selectivity of these catalysts are related to the activity for oxidation of NO and hydrocarbons, the NOx storage properties, and to the oxidation state of silver.
|
|
| 8. |
- Kylhammar, Lisa, 1978
(författare)
-
Exhaust aftertreatment under lean conditions, regenerable SOx traps and NOx reduction catalysts
- 2007
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this thesis was to investigate i) potential materials for regenerable SOx traps and ii) the influence of silver distribution within Ag/Al2O3 catalysts on the selective reduction of NOx by hydrocarbons (HC-SCR). Both areas are highly relevant for present and future vehicle aftertreatment technologies for lean exhausts.In the first part, CeO2, Al2O3 and Al2O3:MgO samples, with or without Pt, were prepared and evaluated by flow-reactor measurements for SOx storage and release performance. To facilitate the understanding of the SOx adsorption mechanism, combined diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy and mass spectrometry were employed. In the second part, 2wt% Ag/Al2O3 samples were prepared and calcined at 550, 850 and 950°C, respectively, in order to gain catalysts with different Ag distribution. By flow-reactor measurements, the oxidation of NO and hydrocarbons was investigated and correlated to NOx reduction performance. Moreover, X-ray photoelectron spectroscopy (XPS) was used to study dispersion and redox-properties of the silver species.The results show that Pt/CeO2 samples are efficient SOx adsorbents under normal lean exhaust temperatures, 200-500°C, but also possible to regenerate at 600°C under lean conditions. The SOx adsorption and regeneration properties are improved by increased Pt content at 250°C. DRIFTS analysis reveal that at 250°C, SOx is stored both as surface and bulk sulfates on CeO2 samples and that the presence of Pt increases the rate of bulk sulfate formation. For the Pt/CeO2 sample, both the SOx storage capacity and the rate of bulk sulfate formation are higher at 400°C compared to 250°C. The results from this study indicate the presence of different SOx storage sites on Pt/CeO2 samples, both regenerable and non-regenerable, but the effect of the latter seems here to be minor.The Ag/Al2O3 samples calcined at 550 and 850°C reveal similar HC-SCR performance, while the sample calcined at 950°C show lower NOx reduction capacity but higher activity for HC oxidation. Contrary to the samples calcined at lower temperatures, the latter sample shows ability for oxidation of NO into NO2 in the absence of hydrocarbons. XPS measurements reveal that the high temperature treatment at 950°C resulted in samples with low silver dispersion and Ag species showing less redox-properties compared to the Ag/Al2O3 sample calcined at 550°C.
|
|
| 9. |
|
|
| 10. |
- Kylhammar, Lisa, 1978, et al.
(författare)
-
Regenerable ceria-based SOx traps for sulfur removal in lean exhausts
- 2008
-
Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 83:1-2, s. 268-276
-
Tidskriftsartikel (refereegranskat)abstract
- Bare and Pt-containing CeO2, Al2O3:MgO mixed oxide and Al2O3 have been investigated as potentialregenerable sulfur oxides (SOx) traps. The samples were evaluated by lean SOx adsorption andtemperature programmed desorption using synthetic gas compositions. In addition, combined DRIFTspectroscopy and mass spectrometry were employed to obtain mechanistic information on theadsorption of SOx. The results suggest Pt/CeO2 as promising SOx trap material owing to a high storagecapacity at 250 8C in combination with efficient release above 600 8C. The presence of Pt is generallyfound to enhance the lean SOx storage capacity at 250C for CeO2-based samples. Lean SO2 adsorption onCeO2 is found to proceed via the formation of surface and bulk sulfates, where the latter is formed morerapidly for the Pt-containing CeO2 sample. Ceria samples pre-exposed to high amounts of SO2 at 250 and400C show lower SOx storage capacity and higher SOx release as compared to fresh samples. Thisindicates that under the conditions used in this study, a part of the storage sites on CeO2 are nonregenerable.
|
|
