| 1. |
- Abdulhamid, Hussam, 1974, et al.
(author)
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Influence of the type of reducing agent (H2, CO, C3H6 and C3H8) on the reduction of stored NOX in a Pt/BaO/Al2O3 model catalyst
- 2004
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In: Topics in Catalysis. - 1572-9028 .- 1022-5528. ; 30-31:1-4, s. 161-168
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Journal article (peer-reviewed)abstract
- In this investigation, a comparative study for a NOx storage catalytic system was performed focusing on the parameters that affect the reduction by using different reductants (H2, CO, C3H6 and C3H8) and different temperatures (350, 250 and 150 degrees C), for a Pt/BaO/Al2O3 catalyst. Transient experiments show that H2 and CO are highly efficient reductants compared to C3H6 which is somewhat less efficient. H2 shows a significant reduction effect at relatively low temperature (150 degrees C) but with a low storage capacity. We find that C3H8 does not show any NOx reduction ability for NOx stored in Pt/BaO/Al2O3 at any of the temperatures. The formation of ammonia and nitrous oxide is also discussed.
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| 2. |
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| 3. |
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| 4. |
- Amberntsson, Annika, 1974, et al.
(author)
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Influence of platinum and rhodium composition on the NOx storage and sulphur tolerance of a barium based NOx storage catalyst
- 2003
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In: Applied Catalysis B: Environmental. - 0926-3373 .- 1873-3883. ; 46:3, s. 429-439
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Journal article (peer-reviewed)abstract
- In the present work the influence of the type of noble metals present in barium oxide based NOx storage catalysts was investigated, regarding the NOx storage performance, NO oxidation, NO reduction, sulphur deactivation and sulphur regenerability. Monolith samples with combinations of platinum and rhodium, were prepared, tested in a flow-reactor, and characterised by XPS measurements. The flow-reactor experiments simulated NOx storage and reduction cycles at 400 degrees C in synthetic gas mixtures with oxygen, propene and nitric oxide. For the sulphur deactivation and regenerability investigations 25 ppm (v/v) SO2 was added to the feed gas stream. From the experiments, it was concluded that a combination of platinum and rhodium is required to achieve good NOx storage and reduction performance. The NOx storage capacity was, however, found higher for catalysts containing only platinum compared to catalysts including rhodium. When exposed to SO2 the NOx storage capacity also seemed to deactivate faster for the samples containing rhodium than for samples with platinum as the sole noble metal. Additionally, it was observed that platinum gives high NO oxidation activity during the lean periods both with and without SO2 present in the gas feed. During the rich periods, rhodium showed high activity for NO reduction in sulphur free gas feed as well as in the presence of SO2. Finally, the results implied that to provide good sulphur regeneration ability of the NOx storage catalyst, a combination of platinum and rhodium is necessary.
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| 5. |
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| 6. |
- Amberntsson, Annika, 1974, et al.
(author)
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Investigations of sulphur deactivation of NOx storage catalysts: influence of sulphur carrier and exposure conditions
- 2002
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In: Catalysis Today. - 0920-5861. ; 73:3-4, s. 279-286
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Journal article (peer-reviewed)abstract
- The influence of SO2, H2S and COS in low concentrations on the deactivation of PURh/BaO/Al2O3 NOx storage catalysts was investigated. Different samples of the catalyst were exposed to synthetic gas mixtures mimicking lean/rich engine cycling in a mixed lean application at 400degreesC. The lean gas mixture contained 8 vol.% O2, 500 vol-ppm C3H6 and 400 vol-ppm NO balanced to 100 vol.% with Ar. The rich excursions were performed by switching off the oxygen supply. Sulphur, 25 vol-ppm of either SO2, H2S or COS, was added to the gas flow either during the lean, the rich or both periods. This procedure aimed at investigating the influence of the exposure conditions and therefore the lean and rich periods were kept equally long (5 min). In addition, thermodynamical calculations for the prevailing conditions were performed.
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| 7. |
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| 8. |
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| 9. |
- Amberntsson, Annika, 1974, et al.
(author)
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Sulphur deactivation of NOx storage catalysts; Influence exposure conditions and noble metal
- 2003
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In: Journal of Catalysis. - 0021-9517 .- 1090-2694. ; 217:2, s. 253-263
-
Journal article (peer-reviewed)abstract
- In the present study, barium-based NOx, storage catalysts containing platinum, rhodium, or both noble metals were investigated. The influence of SO2 exposure conditions on the performance of NOx storage catalysts was studied using flow reactor measurements, FTIR, and XPS where the samples were exposed to lean and/or rich SO2-containing gas mixtures, simulating the conditions in a mixed lean application. The main results show that all samples are sensitive to sulfur and that deactivation is faster when SO2 is present in the feed under rich conditions than under lean or continuous SO2 exposure. It was also found that SO2 affects the performance of noble metals strongly and that noble metal deactivation most likely occurs during the rich period of a NOx storage cycle. Additionally, the influence of the noble metals present in the catalysts was investigated with respect to sulfur sensitivity and it was found that a combination of platinum and rhodium seems to be preferable for retaining high performance (high NO oxidation and reduction activity) of the catalyst under SO2 exposure and subsequent regeneration.
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| 10. |
- Broqvist, Peter, 1975, et al.
(author)
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Characterization of NOx species adsorbed on BaO: Experiment and theory
- 2004
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In: Journal of Physical Chemistry B. - 1520-5207 .- 1520-6106. ; 108:11, s. 3523-3530
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Journal article (peer-reviewed)abstract
- Charged NO, species (x = 1,2,3) formed upon adsorption of NO2 on BaO are characterized by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and theoretical vibrational analysis using the density functional theory (DFT). Experiments at 30degreesC reveal nitrite formation. At T > 250degreesC, shifts associated with formation of nitrates are observed, indicating an interconversion of oxygen atoms among adsorbates. The theoretical study includes single and pairwise NO2 adsorption on (BaO)(9) clusters. As has been reported previously [Broqvist, P.; Panas, I.; Fridell, E.; Persson, H. J. Phys. Chem. B 2002, 106, 137], an additional energy gain is calculated for the second adsorbed NO2. A vibrational analysis of the investigated adsorption configurations supports the interpretation of nitrite to nitrate interconversion on the BaO surface. Moreover, the calculations demonstrate the sensitivity in the NO2 vibrational band splitting with respect to adsorption configuration.
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| 11. |
- Broqvist, Peter, 1975, et al.
(author)
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NOx storage on BaO(100) surface from first principles: a two channel scenario
- 2002
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:1, s. 137-145
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Journal article (peer-reviewed)abstract
- NO2 adsorption at a BaO(100) surface is investigated by means of spin polarized GGA density functional theory. A periodic supercell procedure is employed, and two redox reaction channels are mapped out, involving two chemisorbed NO2 molecules per supercell. The chemisorption is studied in two subsequent steps. The reaction paths are initiated by NO2 adsorption in the form of a nitrite over a Ba2+ site. This generates an electron hole among the surrounding surface oxygen atoms. A reaction path branching occurs as the second NO2 either (a) acts as surface oxidant, forming a surface nitrite−peroxide pair by releasing NO(g), or (b) binds to an O-surf site to form a formal surface nitrate. A redox reaction involving surface nitrite−nitrate interconversion is also addressed. The computed results are employed to interpret experimental observations of surface nitrites, peroxides, NO(g) desorption, and surface Ba(NO3)2 formation. The understandings are discussed in the context of the NOx storage concept of lean-burn catalysis.
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| 12. |
- Broqvist, Peter, 1975, et al.
(author)
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NOx storage on BaO: theory and experiment
- 2004
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In: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 96:1-2, s. 71-78
-
Journal article (peer-reviewed)abstract
- We review our understanding of the NO2 interaction with BaO. The presented picture has evolved from calculations using the density functional theory, reactor measurements and vibrational spectroscopy of surface species during NO2 storage on a BaO/Al2O3 catalyst and BaO powder. The DFT calculations predict nitrite formation on the BaO(1 0 0) surface upon NO2 adsorption. A particular stable adsorption configuration is a nitrate-nitrite pair geometry, with the nitrate bonded to a barium cation and the nitrite involving a surface oxygen anion. This configuration is supported by vibrational spectroscopy. Only nitrite formation is observed for NO exposure to BaO powder at low temperatures, whereas signatures at similar to250degreesC reveal the formation of surface nitrates. Continuous NO2 adsorption at this temperature results in surface Ba(NO3)(2), as well as bulk Ba(NO3)(2), formation.
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| 13. |
- Carlsson, Per-Anders, 1972, et al.
(author)
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A transient in situ FTIR and XANES study of CO oxidation over Pt/Al2O3 catalysts
- 2004
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 226:2, s. 422-434
-
Journal article (peer-reviewed)abstract
- We report experimental results for the oxidation of CO over supported Pt/Al2O3 catalysts operating in oxygen excess at atmospheric pressure. To study the reaction kinetics under transient conditions we have employed step changes of the O2 concentration by intermittently switching off the O2 supply at various temperatures ranging from 523 to 623 K. Detailed in situ FTIR and XANES data for CO coverage and the chemical state of Pt, respectively, are presented together with the CO conversion, which in both cases was monitored by mass spectrometry. A red-shift of the vibrational frequency of linearly bonded CO which correlates with a blue-shift of the Pt LIII binding energy indicates that the Pt catalyst initially is partially oxidised and gradually reduced when the O2 supply is switched off. Control experiments with a NO2 oxidised Pt/Al2O3 catalyst support these findings. A hysteresis in the catalytic activity due to the different rates whereby Pt is oxidised and reduced as a function of gas-phase composition is observed. The activation energy for the Pt oxide reduction (decomposition) process is estimated to be about 50 kJ/mol. The results further emphasise that the conventional three-step Langmuir-Hinshelwood (LH) scheme used to interpret CO oxidation on Pt surfaces must be complemented by a Pt oxidation and reduction mechanism during transient conditions. Moreover, FTIR data suggest that during the extinction, the partially oxidised platinum surface is reduced by chemisorbed CO which should be explicitly accounted for in the modeling of the reaction mechanism.
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| 14. |
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| 15. |
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| 16. |
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| 17. |
- Carlsson, Per-Anders, 1972, et al.
(author)
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Induced low temperature catalytic ignition by transient changes in the gas composition
- 2002
-
In: Catalysis Today. - 0920-5861. ; 73:3-4, s. 307-313
-
Journal article (peer-reviewed)abstract
- The effect of gas composition changes on the low temperature activity for supported platinum model catalysts has been studied. By introducing well-controlled periodic O2 pulses to a simple diluted gas mixture of CO and O2, a substantial improvement of the low temperature oxidation activity was observed. The reason for low activity on noble metals at low temperatures is often attributed to self-poisoning by CO. The improved catalytic performance observed is proposed to origin from the transients causing a surface reactant composition that is favourable for the reaction rate, i.e. lower degree of self-poisoning. This was also confirmed by in situ Fourier transform infrared spectroscopy in combination with mass spectrometry measurements, which gave evidence for the existence of a strong interplay between the gas phase concentration and the adsorbate composition for these catalysts.
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| 18. |
- Carlsson, Per-Anders, 1972, et al.
(author)
-
Periodic control for improved low-temperature catalytic activity
- 2001
-
In: Topics in Catalysis. - 1572-9028 .- 1022-5528. ; 16-17:1-4, s. 343-347
-
Journal article (peer-reviewed)abstract
- The influence of transient changes in the gas composition on the low-temperature activity of a commercial three-way catalyst and a Pt/Al2O3 model catalyst has been studied. By introducing well-controlled periodic O2 pulses to simple gas mixtures of CO or C3H6 (in N2), a substantial improvement of the low temperature oxidation activity was observed for both catalysts. The reason for low activity at low temperatures is normally attributed to self-poisoning by CO or hydrocarbons. The improved catalytic performance observed here is suggested to origin from the transients causing a surface reactant composition that is favourable for the reaction rate.
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| 19. |
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| 20. |
- Carlsson, Per-Anders, 1972, et al.
(author)
-
Periodic control for low-temperature ignition
- 2000
-
In: Proceedings of the 5th International Congress on Catalysis and Automotive Pollution Control (CAPoC5), Brussels, Belgium, April 12-14.
-
Conference paper (peer-reviewed)
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| 21. |
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| 22. |
- DAWODY, JAZAER, 1959, et al.
(author)
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The effect of metal oxide additives (WO3, MoO3, V2O5, Ga2O3) on the oxidation of NO and SO2 over Pt/Al2O3 and Pt/BaO/Al2O3 catalysts
- 2003
-
In: 6th European Congress on Catalysis, Innsbruck, Austria, August 31- September 4, 2003.
-
Conference paper (peer-reviewed)
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| 23. |
- DAWODY, JAZAER, 1959, et al.
(author)
-
The effect of metal oxide additives (WO3, MoO3, V2O5, Ga2O3 on the oxidation of NO and SO2 over Pt/BaO/Al2O3 catalysts
- 2004
-
In: Journal of Molecular Catalysis A: Chemical. - : Elsevier BV. - 1381-1169. ; 209:1-2, s. 215-225
-
Journal article (peer-reviewed)abstract
- For NOx storage, the oxidation of NO to NO2 is an important step in the storage mechanism. In the firstpart of this work, model WO3/Pt/Al2O3, MoO3/Pt/Al2O3, V2O5/Pt/Al2O3, Ga2O3/Pt/Al2O3 and Pt/Al2O3 catalysts were prepared and used in flow reactor experiments. Heating ramp oxidation and steady state oxidation experiments were performed in order to investigate the catalytic activity for NO oxidation both in the presence and absence of SO2 in the reaction gas mixture. In the absence of SO2, the WO3 and MoO3 containing catalysts showed a significantly higher NO oxidation activity than the other catalysts. When SO2 was included in the reaction gas mixture, the NO oxidation activity decreased for all catalysts, but the MoO3 containing catalyst was less affected by the presence of SO2 and also showed the lowest SO2 oxidation activity. In the second part of this work, model Pt/BaO/Al2O3 and Pt/BaO/MoO3/Al2O3 catalysts were prepared and used in heating ramp oxidation and transient flow reactor experiments in order to investigate the effects of NO oxidation activity enhancement and suppression of the activity for SO2 oxidation on NOx storage capacity. Both catalysts showed equal NOx storage capacity in the sulphur free cycles. When SO2 was included in the reaction gas mixture, the NOx storage capacity declined faster for the Pt/BaO/MoO3/Al2O3 catalyst than for the Pt/BaO/Al2O3 catalyst, due to sulphur poisoning which resulted in a faster decrease in the regeneration capacity during rich periods.
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| 24. |
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| 25. |
- Fridell, Erik, 1963, et al.
(author)
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Model studies of NOx storage
- 2001
-
In: ENVICAT2001, Amolfi, Italy, May 2-6, 2001.
-
Conference paper (peer-reviewed)
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| 26. |
- Fridell, Erik, 1963, et al.
(author)
-
Model studies of NOx storage and sulphur deactivation of NOx storage catalysts
- 2001
-
In: Topics in Catalysis. - 1572-9028 .- 1022-5528. ; 16-17:1-4, s. 133-137
-
Journal article (peer-reviewed)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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| 27. |
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| 28. |
- Fridell, Erik, 1963, et al.
(author)
-
Model studies of sulfur deactivation of NOx storage catalysts
- 2004
-
In: Society of Automotive Engineers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0099-5908. - 0768013194 ; SP-1860, s. 115-119
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Journal article (peer-reviewed)abstract
- Model NOx storage catalysts were used to study the sulfur deactivation of these systems using reactor studies, FTIR and XPS. It is found that the deactivation rate shows a linear dependence on the SO2 concentration and that bulk sulfates are easily formed. Experiments with SO2 present either during lean or during rich conditions show a more rapid deactivation in the latter case.
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| 29. |
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| 30. |
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| 31. |
- Fridell, Erik, 1963, et al.
(author)
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Platinum oxidation and sulphur deactivation in NOx storage catalysts
- 2004
-
In: Topics in Catalysis. - 1572-9028 .- 1022-5528. ; 30-31:1-4, s. 143-146
-
Journal article (peer-reviewed)abstract
- Flow reactor experiments and X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the importance of platinum oxide formation on Pt/BaO/Al2O3 NOx storage catalysts during reactions conditions. The reaction studied was NO(g) + 1/2 O-2(g) NO2 (g). During NO2 exposure of the catalyst the NO2 dissociation rate decreased during the reaction. This activity decrease with time was also studied with XPS and it was found to be due to platinum oxide formation. The influence of sulphur exposure conditions on the performance of the NOx storage catalysts was studied by exposing the samples to lean and/or rich gas mixtures, simulating the conditions in a mixed lean application, containing SO2 The main results show that all samples are sensitive to sulphur and that the deactivation proceeds faster when SO2 is present in the feed under rich conditions than under lean or continuous SO2 exposure. Additionally, the influence of the noble metals present in the catalysts was investigated regarding sulphur sensitivity and it was found that a combination of platinum and rhodium seems to be preferable to retain high performance of the catalyst under SO2 exposure and subsequent regeneration. Finally, the behaviour of micro-fabricated model NOx storage catalysts was studied as a function of temperature and gas composition with area-resolved XPS. These model catalysts consisted of a thin film of Pt deposited on one-half of a BaCO3 pellet. It was found that the combination of SO2 and O-2 resulted in migration of Pt on the BaCO3 support up to one mm away from the Pt/BaCO3 interface.
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| 32. |
- Fridell, Erik, 1963, et al.
(author)
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Regeneration of NOx storage catalysts
- 2004
-
In: 13th International Congress on Catalysis, Paris, France, July 11-16, 2004.
-
Conference paper (peer-reviewed)
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| 33. |
- Fridell, Erik, 1963, et al.
(author)
-
The Mechanism for NOx Storage
- 2000
-
In: Catalysis Letters. - 1572-879X .- 1011-372X. ; 66:1-2, s. 71-74
-
Journal article (peer-reviewed)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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| 34. |
- Hinz, Andreas, et al.
(author)
-
An investigation of the reaction mechanism for the promotion of propane oxidation over Pt/Al2O3 by SO2
- 2001
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 201:2, s. 247-257
-
Journal article (peer-reviewed)abstract
- Total oxidation of propane with oxygen in the absence and the respective presence of SO2 in the feed gas was studied over 1 wt% Pt on gamma-Al2O3 using transient experiments with TAP (temporal analysis of products) and in situ DRIFT spectroscopy. The TAP experiments confirm the promoting role of SO2 on the conversion of propane and, moreover, show an inhibiting effect from SO2 on the conversion of the intermediate products propene and CO. In situ DRIFT spectroscopy reveals the formation of sulphate species on the catalyst surface and indicates the formation of an allylic intermediate in the absence of SO2 only. Interaction between hydrocarbon intermediates and the sulphate species is confirmed by the appearance of thiol fragments in the mass spectra. The transient experiments show that propane in the first step is irreversibly adsorbed at the surface. Analysis of the surface residence times of the products formed at the propane pulse in pump-probe experiments provides information about the reaction pathways. Without S02 in the feed gas, propane reacts consecutively in the sequence propane --> propene -->. ethane --> CO --> CO2. In the presence of SO2 the first step of the main route is the breaking of a C-C bond in propane producing ethane and a C-1 fragment, which then form CO and eventually CO2. It is suggested that the promoting effect by SO2 on the conversion of propane is due to the formation of an acidic site at the Pt/Al2O3/SO42-interface.
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| 35. |
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| 36. |
- Härelind Ingelsten, Hanna, 1973, et al.
(author)
-
Influence of the Support Acidity of Pt/Aluminum-Silicate Catalysts on the Continuous Reduction of NO under Lean Conditions
- 2003
-
In: Applied Catalysis B: Environmental. - 0926-3373 .- 1873-3883. ; 41:3, s. 287-300
-
Journal article (peer-reviewed)abstract
- This study focuses on the influence of the support acidity on the continuous reduction of nitric oxide by hydrocarbons over supported Pt catalysts under lean conditions. The support materials used were alumina, silica and co-precipitated aluminum-silicates. Activity studies of NO reduction with either propene or propane as the reducing agent, CO chemisorption, CO oxidation, isopropylamine temperature programmed desorption (TPD) and ammonia TPD experiments have been performed. The isopropylamine TPD experiments indicated the presence of Bronsted acid sites in the samples that contained alumina. The activity for NO reduction and the selectivity for N-2 formation with propene as the reducing agent showed no pronounced differences between the samples studied. For propane, on the other hand, expressed differences in NO reduction activity between the samples were observed. The activation of propane and consequently the reduction of NO, seemed to be promoted by the presence of acidic sites on the surface of the support. The selectivity towards N2 formation seemed to be strongly connected to the amount of Bronsted acid sites present in the samples.
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| 37. |
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| 38. |
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| 39. |
- Härelind Ingelsten, Hanna, 1973, et al.
(author)
-
The influence of surface acidity on NO2 reduction by propane under lean conditions
- 2004
-
In: Journal of Molecular Catalysis A: Chemical. - : Elsevier BV. - 1381-1169. ; 209:1-2, s. 199-207
-
Journal article (peer-reviewed)abstract
- This investigation focuses on the influence of surface acidity on the continuous catalytic reduction of NOx by propane under lean conditions. The acidity was varied by studying noble metal free alumina, silica and co-precipitated aluminium-silicates with varying alumina-silica ratio. The catalysts were characterised by isopropylamine temperature programmed desorption (TPD), with respect to NOx reduction (NO2 and propane) and by Fourier Transform Infrared (FTIR) spectroscopy. The isopropylamine TPD was used to determine the sample acidity. The amount of Bronsted acid sites increased with increasing amount of alumina. The sample activity for NOx reduction with propane was found to correlate with the Bronsted-site density. Furthermore, no N2O was formed during the activity studies. The FTIR measurements implied formation of isocyanate species on the surface of the catalyst sample when NO/NO2, propane and oxygen were present in the gas phase. FTIR data also reveal an increasing amount of adsorbates on the surface with increasing amount of alumina in the sample. Moreover, with propane present, NO-species were consumed to a higher extent over samples containing a larger amount of alumina. Our results suggest a connection between the Bronsted-site density and the NOx reduction with propane under lean conditions.
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| 40. |
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| 41. |
- Jansson, Jonas, 1973, et al.
(author)
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A mechanistic study of low temperature CO oxidation over cobalt oxide
- 2001
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In: Topics in Catalysis. - 1572-9028 .- 1022-5528. ; 16-17:1-4, s. 385-389
-
Journal article (peer-reviewed)abstract
- The CO oxidation over Co3O4 and Co3O4/Al2O3 has been investigated using flow reactor and in situ FTIR studies. Cobalt oxide shows very high activity even at room temperature. However, a gradual deactivation takes place during reaction. The deactivated catalyst shows the presence of two different carbonate species and one graphite-like species. A possible mechanism for the deactivation is discussed.
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| 42. |
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| 43. |
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| 44. |
- Jansson, Jonas, 1973, et al.
(author)
-
On the catalytic activity of Co3O4 in low-temperature CO oxidation
- 2002
-
In: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 211:2, s. 387-397
-
Journal article (peer-reviewed)abstract
- Oxidation of CO over Co3O4 at ambient temperature was studied with flow reactor experiments, and in-situ spectroscopic and structural methods. The catalyst deactivates during the reaction. The rate of deactivation increased with increasing CO or CO2 gas-phase concentration but decreased with increased 02 concentration or increased temperature. Regeneration of the catalyst in 10% O-2/Ar was more efficient than regeneration in Ar alone. The presence of carbonates and surface carbon on the deactivated catalyst was concluded from TPO experiments. None of these species could, however, be correlated with the deactivation of the catalyst. In-situ FTIR showed the presence of surface carbonates, carbonyl, and oxygen species. The change in structure and oxidation state of the catalyst was studied by in-situ XRD, in-situ XANES, XPS, and flow reactor experiments. One possible explanation for the deactivation of the catalyst is a surface reconstruction hindering the redox cycle of the reaction. (C) 2002 Elsevier Science (USA).
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| 45. |
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| 46. |
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| 47. |
- Olsson, Louise, 1974, et al.
(author)
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A Kinetic Study of NO Oxidation and NOx Storage on Pt/Al2O3 and Pt/BaO/Al2O3
- 2001
-
In: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 105:29, s. 6895-6906
-
Journal article (peer-reviewed)abstract
- Modeling and flow reactor experiments were used to study the kinetics of NO, storage/release on a Pt/BaO/Al2O3 model catalyst. The mechanism for this concept can be divided into four steps: (i) NO to NO2 oxidation on Pt, (ii) NO2 storage on BaO, (iii) NO, release, and (iv) NO, reduction to N2. In this paper, we have focused on the first three steps. From the NO oxidation study on Pt/Al2O3 compared to Pt/BaO/Al2O3, we observed that the presence of BaO decreases the formation of NO2. To test the importance of this step for effective storage, experiments were performed with a Pt/Al2O3 catalyst placed before the Pt/BaO/Al2O3 catalyst. This resulted in increased NO, storage for the combined system compared to the Pt/BaO/Al2O3 case. To resolve the second and third steps, an experimental investigation of NOx storage/release on BaO/Al2O3 was performed using only NO2 and N2 in the gas feed. We propose a kinetic model, which first includes adsorption of NO2, which oxidizes the surface, followed by nitrate formation. Finally, NO3-BaO-NO2, i.e., Ba(NO3)2, is formed. By using the kinetic parameters from the NO oxidation on Pt/BaO/Al2O3 and the NO, storage on BaO/Al2O3, a kinetic model was constructed to describe NO, storage/release experiments on Pt/BaO/Al2O3. However, the rate for NO, release was increased when Pt was present, and the kinetic model could not accurately describe this phenomenon. Therefore, the mechanism was modified by including a reversible surface spillover step of NO2 between Pt sites and BaO sites. Further, experiments with NO2 exposure followed by a temperature ramp with NO/N2 showed that the desorption behaviors from the BaO/Al2O3 and Pt/BaO/Al2O3 were significantly different, which further supports the spillover mechanism. Finally, the models describing NO, storage on BaO/Al2O3 and on Pt/BaO/Al2O3 were successfully validated with independent experiments.
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| 48. |
- Olsson, Louise, 1974, et al.
(author)
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Mean Field Modelling of NOx Storage on Pt/BaO/Al2O3
- 2002
-
In: Catalysis Today. - 0920-5861. ; 73:3-4, s. 263-270
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Journal article (peer-reviewed)abstract
- A mean field model, for storage and desorption of NOx in a Pt/BaO/Al2O3 catalyst is developed using data from flow reactor experiments. This relatively complex system is divided into five smaller sub-systems and the model is divided into the following steps: (i) NO oxidation on Pt/Al2O3; (ii) NO oxidation on Pt/BaO/Al2O3; (iii) NOx storage on BaO/Al2O3; (iv) NOx storage on Pt/BaO/Al2O3 with thermal regeneration and (v) NOx storage on Pt/BaO/Al2O3 with regeneration using C3H6. In this paper, we focus on the last sub-system. The kinetic model for NO, storage on Pt/BaO/Al2O3 was constructed with kinetic parameters obtained from the NO oxidation model together with a NO, storage model on BaO/Al2O3. This model was not sufficient to describe the NO, storage experiments for the Pt/BaO/Al2O3, because the NO, desorption in TPD experiments was larger for Pt/BaO/Al2O3, compared to BaO/Al2O3. The model was therefore modified by adding a reversible spill-over step. Further, the model was validated with additional experiments, which showed that NO significantly promoted desorption of NOx from Pt/BaO/Al2O3. To this NOx storage model, additional steps were added to describe the reduction by hydrocarbon in experiments with NO2 and C3H6. The main reactions for continuous reduction of NOx occurs on Pt by reactions between hydrocarbon species and NO in the model. The model is also able to describe the reduction phase, the storage and NO breakthrough peaks, observed in experiments.
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