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| 2. |
- Auvray, Xavier, 1986, et al.
(författare)
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Kinetic modeling of NH3-SCR over a supported Cu zeolite catalyst using axial species distribution measurements
- 2015
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 163, s. 393-403
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Tidskriftsartikel (refereegranskat)abstract
- In this study, a kinetic model is developed for NH3-SCR over a honeycomb-monolith-supported Cu-zeolites using intra-catalyst axial species distribution measurements. An ammonia TPD experiment, together with micro calorimetry data were used for tuning the ammonia adsorption and desorption properties. The spatial distribution for NO oxidation, NH3 oxidation and NH3 "Standard" SCR were modeled between 200 and 400 degrees C. Four-step protocol measurements were employed in order to validate the transient functions of the model. The resulting kinetic model provides good spatiotemporal simulation of the SCR reaction and component reactions throughout the monolith catalyst system.
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| 3. |
- Auvray, Xavier, 1986, et al.
(författare)
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Local ammonia storage and ammonia inhibition in a monolithic copper-beta zeolite SCR catalyst
- 2012
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Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 126, s. 144-152
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Tidskriftsartikel (refereegranskat)abstract
- Selective catalytic reduction of NO with NH3 was studied on a Cu-beta zeolite catalyst, with specific focus on the distributed NH3 capacity utilization and inhibition. In addition, several other relevant catalyst parameter distributions were quantified including the SCR zone, or catalyst region where SCR occurs, and NO and NH3 oxidation. We show that the full NH3 capacity (100% coverage) is used within the SCR zone for a range of temperatures. By corollary, unused NH3 capacity exists downstream of the SCR zone. Consequently, the unused capacity relative to the total capacity is indicative of the portion of the catalyst unused for SCR. Dynamic NH3 inhibition distributions, which create local transient conversion inflections, are measured. Dynamic inhibition is observed where the gas phase NH3 and NO concentrations are high, driving rapid NH3 coverage buildup and SCR. Accordingly, we observe dynamic inhibition at low temperatures and in hydrothermally aged states, but predict its existence very near the catalyst front in higher conversion conditions where we did not specifically monitor its impact. While this paper addresses some general distributed SCR performance parameters including Oxidation and SCR zone, our major new contributions are associated with the NH3 capacity saturation within the SCR zone and dynamic inhibition distributions and the associated observations. These new insights are relevant to developing accurate models, designs and control strategies for automotive SCR catalyst applications.
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| 4. |
- Currier, Russell W, et al.
(författare)
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The evolution of infectious agents in relation to sex in animals and humans: brief discussions of some individual organisms.
- 2011
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Ingår i: Annals of the New York Academy of Sciences. - : Wiley. - 1749-6632 .- 0077-8923. ; 1230, s. 74-107, s. 74-107
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Forskningsöversikt (refereegranskat)abstract
- The following series of concise summaries addresses the evolution of infectious agents in relation to sex in animals and humans from the perspective of three specific questions: (1) what have we learned about the likely origin and phylogeny, up to the establishment of the infectious agent in the genital econiche, including the relative frequency of its sexual transmission; (2) what further research is needed to provide additional knowledge on some of these evolutionary aspects; and (3) what evolutionary considerations might aid in providing novel approaches to the more practical clinical and public health issues facing us currently and in the future?
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| 5. |
- Leistner, Kirsten, 1984, et al.
(författare)
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Comparison of Cu/BEA, Cu/SSZ-13 and Cu/SAPO-34 for ammonia-SCR reactions
- 2015
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 258, s. 49-55
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the ammonia-SCR process was investigated using 2.5 wt.% Cu/BEA, 2.6 wt.% Cu/SAPO-34 and 3.1 wt.% Cu/SSZ-13. Several reactions such as NO oxidation, ammonia oxidation, standard SCR, fast SCR and NO2 SCR were studied to understand the effect of zeolite type. It was found that the small-pore zeolites/silicoaluminophosphates with CHA structure (Cu/SAPO-34 and Cu/SSZ-13) exhibited higher SCR activity at 150°C and lower selectivity toward N2O formation during standard SCR conditions than Cu/BEA. However, formation of ammonium nitrate species during fast SCR conditions at 150°C occurred over Cu/CHA catalysts, which resulted in a gradual decrease of the NOx conversion. Such blocking was also observed over Cu/BEA, albeit to a minor extent. The ammonium nitrate formation and its decomposition temperature regimes resulted in that the Cu/BEA was catalytically more active at lower temperature than either Cu/SAPO-34 or Cu/SSZ-13 during fast SCR conditions. Additionally, our results show that the ammonium nitrate species were more stable on the small-pore zeolites than on Cu/BEA. Comparing the two Cu/CHA catalysts, Cu/SAPO-34 and Cu/SSZ-13, it was found that ammonia oxidation at high temperatures and ammonia SCR at 150°C was higher on Cu/SAPO-34. Further, TPR experiments showed that Cu in Cu/SAPO-34 is more easily reduced compared to Cu/SSZ-13. This can facilitate the redox processes and can thereby be a reason for the higher activity at 150°C for Cu/SAPO-34. In addition, Cu/SAPO-34 forms less N2O and this might be a result of the formation of more stable ammonium nitrates. To conclude, the choice of the type of zeolite/silicoaluminophosphates affects the activity and selectivity of the different steps in the SCR process.
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| 6. |
- Li, J. H., et al.
(författare)
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Characterization of Active Species in Cu-Beta Zeolite by Temperature-Programmed Reduction Mass Spectrometry (TPR-MS)
- 2013
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Ingår i: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1572-9028 .- 1022-5528. ; 56:1-8, s. 201-204
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Tidskriftsartikel (refereegranskat)abstract
- The number and type of copper species present in an under-exchanged Cu-Beta zeolite catalyst were characterized using temperature-programmed reduction mass spectrometry (TPR-MS). Both H-2 consumption and H2O evolution were tracked, yielding additional insights into the nature of Cu species. Prior to the TPR-MS tests, catalyst samples were subjected to various high- temperature aging treatments in the presence of O-2 in He, in order to assist with interrogating and resolving different types of sites. Absence of reducible species in the precursor material was confirmed by subjecting the H-form of the same zeolite to the TPR. Upon aging at temperatures below 700 A degrees C, the TPR pattern showed several distinct peaks of different intensities. Aging at 800 A degrees C led to shifting and broadening of these H-2 consumption peaks, as well as to some reduction of the integral amount of the reducible sites. The 900 A degrees C exposure resulted in a drastic change of the TPR pattern, with several new well-resolved H-2 consumption peaks, indicative of drastic changes in the catalyst structure. The integral amount of H-2 consumed quantitatively matched the amount of Cu species present in the catalyst, assuming the stoichiometry. The combination of experimental findings in this work lends support to a hypothesis that different TPR peaks are indicative of different types of sites rather than of step-wise reduction of copper species.
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| 7. |
- Mihai, Oana, 1975, et al.
(författare)
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The effect of Cu-loading on different reactions involved in NH3-SCR over Cu-BEA catalysts
- 2014
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 311, s. 170-181
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we investigate the effect of varying copper loading in Cu-BEA on different reactions involved in the SCR mechanism. The catalysts were characterized by BET, ICP-AES, and UV-Vis. Enlarged ammonia storage was observed when increasing the copper loading mainly owing to loosely bonded NH3, but for over-exchanged samples, a new high-temperature shoulder was also observed. The N2O production significantly grew with increasing copper loading. In addition, the reaction rates of NO oxidation and NH3 oxidation per Cu site were higher on the high-loaded copper samples compared to those with lower loading. However, the opposite results were found for NH3 SCR, but differences were small for this reaction. Ammonium nitrate formation and decomposition were examined during a variety of temperature-programmed desorption (TPD) experiments, and three regions related to the presence of acid sites and low- versus high-loaded copper sites were observed. When dosing NO2 before NH3, ammonium nitrate species were formed but in lower amounts than if NH3 and NO2 were dosed simultaneously.
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| 8. |
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| 9. |
- Mihai, Oana, 1975, et al.
(författare)
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The Effect of NO2/NOx Feed Ratio on the NH3-SCR System Over Cu–Zeolites with Varying Copper Loading
- 2014
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Ingår i: Catalysis Letters. - : Springer Science and Business Media LLC. - 1572-879X .- 1011-372X. ; 144:1, s. 70-80
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Tidskriftsartikel (refereegranskat)abstract
- In this study we examine the NO/NO2–NH3-SCR system over Cu–BEA with varying Cu loading. Significantly higher selective catalytic reduction (SCR) activity is observed at low temperature on highly loaded copper samples, whereas the reverse trend is noticed at high temperature. The N2O formation is substantially increased over “over-exchanged” Cu sites, where Cu co-ordinate to one Al and charge-balanced with one OH-group. This is also the case for NO2 reaction with NH3 to produce NO. Using transient experiments the formation/decomposition of ammonium nitrate species are examined. The decomposition depends on the temperature, the sequence of the feed as well as the type of copper species present.
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| 10. |
- Nedyalkova, Radka, 1976, et al.
(författare)
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Experimental evidence of the mechanism behind NH3 overconsumption during SCR over Fe-zeolites
- 2013
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 299, s. 101-108
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Tidskriftsartikel (refereegranskat)abstract
- Isotope labeled (NO)-N-15 was used to investigate the mechanism of the unusual overconsumption of NH3 during standard SCR over Fe-zeolite, under conditions during which NH3 oxidation with O-2 alone (without NO) was unfavorable. When the Fe-BEA catalyst was exposed to the (NO)-N-15 + (NH3)-N-14 + O-2 gas mixture at 250 and 300 degrees C, the resulting products included (NO)-N-14, a product of (NH3)-N-14 oxidation under SCR conditions. However, (NO)-N-14 was not detected when Fe-BEA was exposed to (NH3)-N-14 + O-2. Since the only source of N-14 derives from (NH3)-N-14, with the labeled gas mixture used, the (NO)-N-14 during SCR must have originated from oxidation of (NH3)-N-14. Furthermore, twice as much (NN)-N-14-N-14 was observed at 300 degrees C under SCR conditions in comparison with NH3 oxidation using O-2 alone. Under SCR conditions, the (NO)-N-14 formed through the unusual oxidation route further reacted with (NH3)-N-14 to produce N-14(2). Thus, for the first time, we have experimental evidence for the unusual overconsumption of ammonia during SCR over Fe-zeolites.
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