| 1. |
- Amberntsson, Annika, 1974, et al.
(författare)
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Sulphur deactivation of NOx storage catalysts; Influence exposure conditions and noble metal
- 2003
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Ingår i: Journal of Catalysis. - 0021-9517 .- 1090-2694. ; 217:2, s. 253-263
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Tidskriftsartikel (refereegranskat)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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| 2. |
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| 3. |
- Andersson, Arne, et al.
(författare)
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Activities of V-Ti-O Catalysts in the Ammoxidation of 3-Picoline
- 1980
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 65:1, s. 9-15
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Tidskriftsartikel (refereegranskat)abstract
- Ammoxidation of 3-picoline was studied on reduced V-Ti-O catalysts with V6O13 as the major vanadium oxide. The results showed that the initial activity as a function of the TiO2 content reached a maximum at 50–60 mole% TiO2. It is proposed that there is maximum contact between the vanadium and titanium phases at this composition, which results in a weakening of the (VO)3+ surface bond. The selectivity of formation of nicotinonitrile exhibited a maximum of 83% at 10 mole% TiO2 and minima of 73 and 75% at 0 and 30 mole% TiO2, respectively. At higher TiO2 concentrations the selectivity increased continuously to 83% at 90 mole% TiO2. The variation of the selectivity of formation of nicotinonitrile depends on the View the MathML source ratio in the TiO2 phase. It was also found that the conversion and yields varied with the reaction time, which could be explained by the fact that reduced vanadium oxides were oxidized to V2O5 during the ammoxidation process. This oxidation leads to the formation of active and highly selective boundary surfaces between the TiO2-promoted vanadium oxides V6O13 and V2O5.
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| 4. |
- Andersson, Arne
(författare)
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Activities of V-Ti-O Catalysts in the Ammoxidation of 3-Picoline. II. Acid-Base Properties and Infrared Spectra
- 1982
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 76:1, s. 144-156
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Tidskriftsartikel (refereegranskat)abstract
- The acidity and basicity of prereduced V-Ti-O catalysts were measured by adsorption of NH3 and CO2. It was found that a high activity in the ammoxidation of 3-picoline corresponds to a relatively small amount of acidic sites. A catalyst selective in the formation of nicotinonitrile requires high concentrations of both acidic and basic sites. These results are discussed, and can be correlated if the acidity is considered to be a measure of the hydroxyl group concentration, and the basicity a measure of oxygen vacancies in the vicinity of VO groups. Also, the infrared spectra of the prereduced catalysts were recorded. A new band was found at 995 cm−1. The same band appeared in a homogeneous mixture of V2O5 and V6O13. It is proposed that this band can be assigned to VO bonds in a reduced V2O5phase with a number of disordered vacancies in the lattice, or a nonstoichiometric V6O13 phase. The shift in frequency relative to that of (VO)3+ shows that these bonds are weaker, and probably also more active. The effect of TiO2 is to increase the amount of V4+ in the vanadium oxide lattice. This is caused by dissolution of Ti4+ in the V2O5 melt during the catalyst preparation.
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| 5. |
- Andersson, Arne, et al.
(författare)
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Catalytic Anisotropy of MoO3 in the Oxidative Ammonolysis of Toluene
- 1988
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 114:2, s. 332-346
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Tidskriftsartikel (refereegranskat)abstract
- The oxidative ammonolysis of toluene, i.e., ammoxidation without the presence of molecular oxygen, was studied over a series of samples of MoO3 crystals. The specific surface areas of the various faces were determined from SEM micrographs. Correlations between activities and surface planes were found. For the formation of nitrile the specific activity decreased in the order {001} and {h01} > {100} > {010}. Also, for the formation of carbon oxides the terminations in the [001] direction were found to be especially active. These results are discussed in relation to surface structures and bond strength values of various oxygen species. It is concluded that the presence of both oxygen vacancies and nucleophilic oxygen species is a prerequisite for selective reaction to occur and that electrophilic oxygen species are the source for formation of carbon oxides. The characteristics of the various faces, as they emerge from the results on oxidative ammonolysis of toluene, seem to be of general significance for reactions occurring at the same types of active sites. They are shown to be applicable to results published in the literature on the oxidation of both propene and isobutene.
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| 6. |
- Andersson, Arne
(författare)
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Structural Dynamics of a V2O5/SnO2 Catalyst in the Ammoxidation of 3-Picoline
- 1981
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 69:2, s. 465-474
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the reaction parameters on the conversion of 3-picoline and the yield of nicotinonitrile in the ammoxidation of 3-picoline on a View the MathML source catalyst was studied. The highest yield of nicotinonitrile was 82%. This was obtained at 400 °C, with a space velocity of 4000 h−1 and the following mole ratios: ammonia/3-picoline = 12, air/3-picoline = 210, and water vapour/3-picoline = 40. It was found that the composition of the surface of the catalyst depended strongly on the values of the reaction parameters. The dynamic structure of the catalyst surface was studied by determining the phases present by X-ray diffraction analysis and the average oxidation number of vanadium by titrimetric methods. These results were correlated, showing that catalysts with both V2O5 and V6O13 were especially active and selective. The composition of the SnO2 phase was analyzed and was found to be V0.03Sn0.97O2, and did not depend on the reaction parameters. Infrared investigations showed that the role of SnO2 was to weaken the short VO bonds. This can probably be seen as a consequence of incorporation of Sn4+ in the V2O5 lattice.
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| 7. |
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| 8. |
- Jansen, Susan, et al.
(författare)
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EPR Studies of the Coverage Effects in TiO2(B)-Supported Vanadia Catalysts for Toluene Ammoxidation
- 1992
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 138:1, s. 79-89
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Tidskriftsartikel (refereegranskat)abstract
- EPR spectroscopy has been employed to determine the nature of vanadium (+4) species in the TiO2(B)-supported vanadia catalysts. IR and EPR studies show that multiple vanadia species are observed as a function of loading. At coverages greater than 3.5 theoretical layers, the EPR signal is reminiscent of bulk V205. At lower coverages, a different magnetic species dominates the resonance and may be attributable to tetrahedral states of vanadium at the oxide interface. The spin concentration and (g) suggest two distinct vanadia phases as a function of coverage. In this study, we have also analyzed catalysts after use in ammoxidation of toluene. These samples show strikingly different features compared with the freshly prepared samples. Changes in both the g-anisotropy and hyperfine coupling were observed relative to the fresh catalysts. Though EPR is not surface sensitive, inference into the nature of the catalytic surface can be made by comparison of catalysts of different coverage and studies of monolayer samples. A study of the TiO2(B)-vanadia interface was made by comparing monolayer catalysts prepared from slightly different technique. Our measurements show strikingly different features in V4+ species as a function of preparation.
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| 9. |
- Wallin, Mikaela, 1975, et al.
(författare)
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Selective catalytic reduction of NOx with NH3 over H-ZSM-5: Influence of transient ammonia supply
- 2003
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Ingår i: Journal of Catalysis. - 0021-9517 .- 1090-2694. ; 218:2, s. 354-364
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Tidskriftsartikel (refereegranskat)abstract
- The effect of ammonia supply on the selective catalytic reduction of NOX over zeolite H-ZSM-5 was investigated using step response experiments between 200 and 500 degrees C. For inlet NO:NO2 ratios > 1, the activity for NOX reduction transiently increased when NH3 was removed from the feed. For NO:NO2 ratios less than or equal to 1, the NOX reduction however decreased. By pulsing NH3 to the feed, the activity for NO reduction was enhanced up to five times compared to continuous supply of ammonia. For NO:NO2 ratios exceeding one, also the selectivity towards N2O formation was lower with transient ammonia supply. Temperature programmed reaction experiments with preadsorbed NH3 showed highest initial NOX reduction activity when ammonia had been adsorbed at 300 or 250 degrees C compared to 200 degrees C. A minimum in NO reduction was observed at 130 degrees C independent of the ammonia adsorption temperature. For NO:NO2 ratios > 1, the results strongly indicate that NO oxidation is the rate determining step in the ammonia selective catalytic reduction (NH3-SCR) reaction over H-ZSM-5.
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| 10. |
- Agrell, J., et al.
(författare)
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Production of hydrogen from methanol over Cu/ZnO catalysts promoted by ZrO2 and Al2O3
- 2003
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 219:2, s. 389-403
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Tidskriftsartikel (refereegranskat)abstract
- Production of H-2 from methanol by steam reforming, partial oxidation, or a combination thereof was studied over Cu/ZnO-based catalysts. The catalysts were characterized by a variety of techniques, including N2O chemisorption, X-ray photoelectron spectroscopy, X-ray diffraction, and temperature-programmed reduction/oxidation. The influence of feed composition, reaction temperature, and catalyst formulation on H-2 production rate, product distribution, and catalyst lifetime was investigated. Distinct differences between the processes were observed with respect to catalyst behavior. ZrO2-containing catalysts, especially Cu/ZnO/ZrO2/Al2O3, exhibit the best performance in the steam reforming reaction. During partial oxidation, however, a binary Cu/ZnO catalyst exhibits the lowest light-off temperature and the lowest level of CO by-product. The redox properties of the catalyst appear to play a key role in determining the pathway for H-2 production. In particular. the extent of methanol and/or H-2 combustion at differential O-2 conversion is strongly dependent on the ease of copper oxidation in the catalyst.
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