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| 2. |
- Bounechada, Djamela, 1984, et al.
(författare)
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Effect of periodic lean/rich switch on methane conversion over a Pd/Rh-based three way catalyst in the exhausts of natural gas vehicles
- 2012
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 119-120, s. 91-99
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Tidskriftsartikel (refereegranskat)abstract
- The behavior of a commercial Ce–Zr promoted Pd-Rh/Al2O3 catalyst for the abatement of methane from the exhausts of natural gas vehicles (NGVs) is studied in presence of large amounts of water under both stationary conditions and by periodically switching from lean to rich feed. Under stationary conditions with both stoichiometric (λ = 1.00) and lean (λ = 1.02) feed catalyst deactivation is observed after prolonged exposure to the reaction mixture. Periodic rich pulses in a constant lean feed gas result in the stabilization of catalytic performances.A higher methane conversion than those obtained with stoichiometric and lean feed mixtures is observed under rich conditions, during an experiment carried out by performing lean pulses (λ = 1.02) in a constant rich feed gas (λ = 0.98). The analysis of reactants conversion and products distribution suggests that different chemistries are involved under lean and rich conditions. Only reactions of complete oxidation of H2, CO, CH4 and NO occur under excess of oxygen, whereas under rich conditions NO reduction, CH4 steam reforming and water gas shift also occur.The effect of symmetric oscillation of the exhausts composition around stoichiometry is also addressed by periodically switching from slightly rich to slightly lean composition with different oscillation amplitudes (Δλ = ±0.01, ±0.02 and ±0.03). Higher and more stable methane conversion performances are obtained than those observed under constant λ operations. The presence of a more active PdO/Pd0 state is suggested to explain the enhancement of catalytic performances.
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| 4. |
- Bounechada, Djamela, 1984, et al.
(författare)
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Enhanced methane conversion under periodic operation over a Pd/Rh based TWC in the exhausts from NGVs
- 2013
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Ingår i: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1572-9028 .- 1022-5528. ; 56:1-8, s. 372-377
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Tidskriftsartikel (refereegranskat)abstract
- The enhancement of methane oxidation performances under periodic operation over a commercial Pd–Rh based three way catalyst (TWC) is investigated at different temperatures. Results confirm that under conditions with periodic oscillating feed around stoichiometry (λ = 1 ± 0.02), higher and more stable CH4 conversion are obtained than under conditions with constant stoichiometric feed. In particular higher CH4 conversion is obtained in the rich part of the cycle than in the lean one, the difference being more pronounced at high temperature. A narrow turning point for the TWC activity is finally observed under slightly rich conditions, which is characterised by a marked increase of CH4 conversion, paralleled by total consumption of O2 and NO and formation of small amounts of CO, H2 and NH3. Results suggest that the oxidation state of palladium plays a key role in the observed enhancement of catalyst performances.
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| 5. |
- Bounechada, Djamela, 1984, et al.
(författare)
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Mechanisms behind sulfur promoted oxidation of methane
- 2013
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 15:22, s. 8648-8661
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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.
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| 6. |
- Bounechada, Djamela, 1984, et al.
(författare)
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SO2 adsorption on silica supported iridium
- 2017
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 146:8, s. 084701-
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of SO2 with Ir/SiO2 was studied by simultaneous in situ diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, exposing the sample to different SO2 concentrations ranging from 10 to 50 ppm in the temperature interval 200–400 ◦C. Evidences of adsorptionof sulfur species in both absence and presence of oxygen are found. For a pre-reduced sample in the absence of oxygen, SO2 disproportionates such that the iridium surface is rapidly saturated with adsorbed S while minor amounts of formed SO3 may adsorb on SiO2. Adding oxygen to the feed leads to the oxidation of sulfide species that either (i) desorb as SO2 and/or SO3, (ii) remain at metal sites in the form of adsorbed SO2, or (iii) spillover to the oxide support and form sulfates (SO42−). Notably, significant formation of sulfates on silica is possible only in the presence of both SO2 and O2, suggesting that SO2 oxidation to SO3 is a necessary first step in the mechanism of formation of sulfates on silica. During the formation of sulfates, a concomitant removal/rearrangement of surface silanol groups is observed. Finally, the interaction of SO2 with Ir/SiO2 depends primarily on the temperature and type of gas components but only to a minor extent on the inlet SO2 concentration.
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| 7. |
- Bounechada, Djamela, et al.
(författare)
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Vibrational analysis of SO2 on Pt / SiO2 system
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In situ diffuse reflectance infrared Fourier transformed spectroscopy was used to study the interactions of SOx species with Pt/SiO2 between 200 and 400°C, and for SO2 concentrations between 10 and 50 ppm, which represents a concentration range where MISFET sensors exhibit good responses. In parallel, first-principles calculations have been carried out to support the experimental interpretations. It was found that sulfate species were formed on the silica surface, accompanied with removal/rearrangement of silanol groups upon exposure to SO2. Both experimental and theoretical calculations also suggest that the surface species were only formed after SO2 oxidation to SO3 on the metal surface. These evidences support the idea of SO2 oxidation to SO3 as the first step in the process of sulfate formation, followed by spillover of SO3 to the oxide, and finally the formation of sulfate species on the hydroxyl positions on the oxide. The results also indicate that the sulfate formation on silica depends both on the temperature and the SO2 concentration. Furthermore, hydrogen exposure was shown to be efficient for sulfur removal from the silica surface.
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| 8. |
- Bounechada, Djamela, 1984, et al.
(författare)
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Vibrational Study of SOx Adsorption on Pt/SiO2
- 2014
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:51, s. 29713-29723
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Tidskriftsartikel (refereegranskat)abstract
- The formation of ad-SOx species on Pt/SiO2 upon exposure to SO2 in concentrations rang- ing from 10 to 50 ppm at between 200 and 400◦C has been studied by in situ diffuse reflectance infrared Fourier transformed spectroscopy. In parallel, first-principles calculations have been carried out to consolidate the experimental interpretations. It was found that sulfate species form on the silica surface with a concomitant removal/ rearrangement of silanol groups. For- mation of ad-SOx species occurs only after SO2 oxidation to SO3 on the platinum surface. Thus SO2 oxidation to SO3 is the first step in the SOx adsorption process, followed by spillover of SO3 to the oxide and, finally, the formation of sulfate species on the hydroxyl positions on the oxide. The sulfate formation is influenced by both temperature and SO2 concentration. Furthermore, exposure to hydrogen is shown to be sufficiently efficient as to remove ad-SOx species from the silica surface.
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| 9. |
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| 10. |
- Darmastuti, Zhafira, et al.
(författare)
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Detection mechanism studies of SO2 on Pt / SiO2 system
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Experiment was performed with Pt-gate SiC-FET sensors to study the detection mechanism of the sensors. The sensing measurement showed that oxygen influenced the response quite strongly. The sensor response became larger in the presence of oxygen. Experiment with mass spectroscopy indicated the formation of SO3 during the sensing measurement. Further experiment with DRIFT spectroscopy showed the formation of sulfate species on the oxide surface, accompanied by the disappearance of the silanol groups. An explanatory model was built based on quantum-chemical calculations. The results strengthened the experimental results by showing that it was more energetically favorable for SO2 to oxidize into SO3 before being adsorbed on the oxide surface. It was also observed that the overall adsorption reaction was exothermic, the activation energy for the SO2 oxidation was 48,75 kJ/mol, and the rate limiting step was the desorption of SO3 from the Pt surface.
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