| 1. |
- Auvray, Xavier, 1986, et al.
(författare)
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Aging of Cu/SSZ-13 for NH3 SCR in mixed lean condition
- 2018
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Two catalytic methods have been developed to remove the NOx emissions from diesel vehicles and comply with the emission regulations: the NOx storage and reduction process (NSR) and the reduction of NOx with NH3 (SCR). Advanced systems combining these two methods for an optimum efficiency are now studied. SCR catalysts are subjected alternating lean and rich conditions as well as to high temperatures in rich conditions during deSOx of the LNT material. The purpose of the present work is therefore to determine the response of a Cu/SSZ-13 to several aging treatments at 800°C involving rich and/or lean exposure. Deactivation was observed after all types of aging. However strong differences in deactivation degree exists between aging procedures. Lean aging, for 8h at 800°C, was the most moderate aging tested although it brought about significant activity loss below 250°C. Alternating between long lean periods and short rich periods caused slightly stronger deactivation, which was most significant at 200°C. However, the switching frequency between lean and rich had negligible influence on subsequent activity. As shown in Figure 1, SCR activity after 8h exposure in H2 was outstandingly low. It increased monotonically with temperature to reach a maximum of 58% NO conversion. Further investigation pertaining to the rich treatment was performed. The aging time was reduced to 2h and, for direct comparison, a 32h cycling aging was performed, implying a total rich time of 2h. Both samples were severely impacted over the whole temperature range. It revealed that 2h in rich conditions led to stronger deactivation than 8h in lean and lean/rich conditions, emphasizing the deterioration power of H2. Aging caused by rich conditions was so dominant that additional 30h in lean has negligible influence on SCR below 300°C. However, the long additional exposure to lean conditions lowered significantly the SCR activity between 400 and 500°C.
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| 2. |
- Auvray, Xavier, 1986, et al.
(författare)
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Deactivation of Cu/SSZ-13 NH3-SCR catalyst by exposure to CO, H2, and C3H6
- 2019
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Ingår i: Catalysts. - : MDPI AG. - 2073-4344. ; 9:11
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Tidskriftsartikel (refereegranskat)abstract
- Lean nitric oxide (NOx)-trap (LNT) and selective catalytic reduction (SCR) are efficient systems for the abatement of NOx. The combination of LNT and SCR catalysts improves overall NOx removal, but there is a risk that the SCR catalyst will be exposed to high temperatures and rich exhaust during the LNTs sulfur regeneration. Therefore, the effect of exposure to various rich conditions and temperatures on the subsequent SCR activity of a Cu-exchanged chabazite catalyst was studied. CO, H2, C3H6, and the combination of CO + H2 were used to simulate rich conditions. Aging was performed at 800 °C, 700 °C, and, in the case of CO, 600 °C, in a plug-flow reactor. Investigation of the nature of Cu sites was performed with NH3-temperature-programed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) of probe molecules (NH3 and NO). The combination of CO and H2 was especially detrimental to SCR activity and to NH3 oxidation. Rich aging with low reductant concentrations resulted in a significantly larger deactivation compared to lean conditions. Aging in CO at 800 °C caused SCR deactivation but promoted high-temperature NH3 oxidation. Rich conditions greatly enhanced the loss of Brønsted and Lewis acid sites at 800 °C, indicating dealumination and Cu migration. However, at 700 °C, mainly Brønsted sites disappeared during aging. DRIFT spectroscopy analysis revealed that CO aging modified the Cu2+/CuOH+ ratio in favor of the monovalent CuOH+ species, as opposed to lean aging. To summarize, we propose that the reason for the increased deactivation observed for mild rich conditions is the transformation of the Cu species from Z2Cu to ZCuOH, possibly in combination with the formation of Cu clusters. © 2019, MDPI AG. All rights reserved.
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| 3. |
- Auvray, Xavier, 1986, et al.
(författare)
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Lean and rich aging of a Cu/SSZ-13 catalyst for combined lean NO x trap (LNT) and selective catalytic reduction (SCR) concept
- 2019
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Ingår i: Catalysis Science and Technology. - : Royal Society of Chemistry (RSC). - 2044-4753 .- 2044-4761. ; 9:9, s. 2152-2162
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Tidskriftsartikel (refereegranskat)abstract
- © 2019 The Royal Society of Chemistry. In the combined lean NO x trap (LNT) and selective catalytic reduction (SCR) concept, the SCR catalyst can be exposed to rich conditions during deSO x of the LNT. Aging of Cu/SSZ-13 SCR catalysts, deposited on a cordierite monolith, was therefore studied in rich, lean and cycling lean/rich operations at 800 °C (lean condition: 500 ppm NO, 8% O 2 , 10% H 2 O and 10% CO 2 ; rich condition: 500 ppm NO, 1% H 2 , 10% H 2 O and 10% CO 2 ). The structure of the catalyst was investigated by X-ray diffraction (XRD), surface area measurements and scanning transmission electron microscopy (STEM). In general, aging decreased the SCR activity and NH 3 oxidation. However, rich conditions showed a very rapid and intense deactivation, while lean aging led to only a small low-temperature activity decrease. The XRD results showed no sign of structure collapse, but the number of active sites, as titrated by NH 3 temperature-programed desorption (NH 3 -TPD) and in situ DRIFTS, revealed an important loss of acid sites. NH 3 storage was significantly more depleted after rich aging than after lean aging. The Lewis sites, corresponding to exchange Cu 2+ , were preserved to some extent in lean conditions. Lean aging also decreased the enthalpy of NH 3 adsorption from -158 kJ mol -1 to -136 kJ mol -1 . Moreover, a comparison of aging in lean-rich cycling conditions with aging only in rich conditions revealed that adding lean events did not hinder or reverse the deactivation, and it was mainly the time in rich conditions that determined the extent of the deactivation. The STEM images coupled with elemental analysis revealed the formation of large Cu particles during rich aging. Conversely, Cu remained well dispersed after lean aging. These results suggest that the copper migration and agglomeration in large extra-framework particles, accelerated by the action of hydrogen, caused the observed severe deactivation.
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| 4. |
- Fridell, Erik, 1963, et al.
(författare)
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Model studies of NOx storage
- 2001
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Ingår i: ENVICAT2001, Amolfi, Italy, May 2-6, 2001.
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Konferensbidrag (refereegranskat)
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| 5. |
- Fridell, Erik, 1963, et al.
(författare)
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Model studies of NOx storage and sulphur deactivation of NOx storage catalysts
- 2001
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Ingår i: Topics in Catalysis. - 1572-9028 .- 1022-5528. ; 16-17:1-4, s. 133-137
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Tidskriftsartikel (refereegranskat)abstract
- The storage of NOx under lean conditions in model NOx storage catalysts as well as the deactivation by sulphur have been studied. We find that NO2 plays an important role in the storage mechanism as an oxidising agent. Two different mechanisms for this are discussed: the formation of surface peroxides and the oxidation of nitrites to nitrates, FTIR studies show that NOx is stored as surface nitrates, The sulphur deactivation is found to be more severe when SO2 is added during the rich phase than when SO2 is added during the lean period. FTIR shows the formation of bulk sulphates both under lean and rich conditions.
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| 6. |
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| 7. |
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| 8. |
- Fridell, Erik, 1963, et al.
(författare)
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The Mechanism for NOx Storage
- 2000
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Ingår i: Catalysis Letters. - 1572-879X .- 1011-372X. ; 66:1-2, s. 71-74
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Tidskriftsartikel (refereegranskat)abstract
- The mechanisms for storing of NOx in platinum-barium-alumina catalysts during lean-rich transients are investigated. Oxidation of NO to NO2 is found to be an important step. NO2 is found to be important for oxidation of the catalyst or of nitrites to form nitrates. NOx is then stored in the form of surface nitrates. FTIR studies show no formation of bulk nitrates in these experiments.
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| 9. |
- Olsson, Louise, 1974, et al.
(författare)
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A kinetic study of oxygen adsorption/desorption and NO oxidation over Pt/Al2O3 catalysts
- 1999
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Ingår i: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 103:47, s. 10433-10439
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Tidskriftsartikel (refereegranskat)abstract
- Laboratory tests and kinetic modeling were carried out in order to provide kinetic input data to a systematic investigation of the mechanism of nitrogen oxides (NO,) storage in catalysts used for lean-burn engines. In particular, we present a kinetic model of the NO oxidation to NO2 over a Pt/Al2O3 model catalyst for the temperature range 250-450 degrees C. Since the oxygen behavior at atmospheric pressure is critical for such a model, we have also studied the adsorption/desorption of oxygen by temperature-programmed desorption (TPD) experiments. The experiments show that oxygen starts to desorb at about 300 degrees C. Furthermore, the NO oxidation was studied in a temperature ramp with NO and oxygen in the gas feed. The data from this experiment and the above-determined values for the oxygen adsorption/desorption were used to construct a kinetic model for the NO oxidation. Finally, the model was validated with some transient experiments with either NO or NO2 and different oxygen concentrations in the gas feed. We found a good agreement between these experiments and the model.
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| 10. |
- Skoglundh, Magnus, 1965, et al.
(författare)
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Model studies of NOx storage catalysts
- 2000
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Ingår i: 2000 International Chemical Congress of Pacific Basin Societies (PACIFICHEM’2000) in Honolulu, Hawaii, USA, December 14-19, 2000..
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Konferensbidrag (refereegranskat)
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