| 1. |
- Nilsson, Jerker, et al.
(författare)
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Formation of active phases in the Sb-V-, Al-Sb-V-, and Al-Sb-V-W-oxide systems for propane ammoxidation
- 1997
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 110, s. 413-422
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Tidskriftsartikel (refereegranskat)abstract
- The Sb-V-O, Al-Sb-V-O and Al-Sb-V-W-O systems were investigated for the ammoxidation of propane to acrylonitrile. Compared with pure approximate to SbVO4, the approximate to SbVO4 that was obtained by co-calcination of pure approximate to SbVO4 with alpha-Sb2O4 at 800 degrees C and subsequent sieving was found to be much more selective to acrylonitrile. XRD data showed no variation of the rutile unit cell for synthesis ratios Sb:V greater than or equal to 1, indicating that the active phase is approximate to SbVO4 with a surface enriched with Sb. Characterisation of syntheses in the Al-Sb-V-O system allowed the identification of a trirutile-like phase with the composition Al1-xSbVxO4 (0 < x < 0.5). The synthesis of this phase, which is active and selective to acrylonitrile, requires excess of aluminium. Over a fresh preparation with Al, Sb, V and W the activity and the selectivity to acrylonitrile increased considerably with time-on-stream. This behaviour shows that the active rutile phase is formed in situ, and EDX analyses gave the average composition Al0.1Sb0.8V0.7W0.4O4. The highest yield to acrylonitrile that was observed for the three systems was 37 % and was obtained over the Al-Sb-V-W-oxide.
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| 2. |
- Westerberg, B, et al.
(författare)
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Kinetic study of the selective catalytic reduction of nitric oxides with hydrocarbon in diesel exhausts
- 1998
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 116, s. 317-326
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Tidskriftsartikel (refereegranskat)abstract
- The kinetics of the selective catalytic reduction of nitric oxides (NOx) on a proprietary high temperature catalyst with diesel as the reductant have been studied. The objective was to derive a kinetic model that can be used for real time simulation of the catalyst. In the extension, the real time simulation will be used when controlling the injection of reductant. This is a requirement for achieving a high efficiency and a low fuel penalty. The response time and the NOx conversion level upon transient diesel injection was found to be dependent on the temperature. At temperatures below 570 K very low or no NOx conversion was observed. Above 570 K a small conversion was observed. No direct response upon diesel injection could be distinguished and the NOx conversion was independent on the hydrocarbon concentration. As the temperature was increased the response became apparent and then faster and the conversion level gradually became more dependent on the hydrocarbon concentration. Above 700 K the response was immediate (response time less than 15 s) and the conversion level was directly dependent on the hydrocarbon concentration. It was concluded that the NOx reduction proceeds via the formation of a hydrocarbon intermediate and the successive reaction between the hydrocarbon intermediate and NOx. When this reaction mechanism was modeled many features of the catalyst behaviour were reproduced.
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| 3. |
- Li, Qinghua, et al.
(författare)
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The synthesis of small colloidal crystals of TPA-silicalite-1 with short synthesis times and high yields
- 1999
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 125, s. 133-140
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Tidskriftsartikel (refereegranskat)abstract
- A two-stage varying-temperature crystallization procedure can be used to synthesize particularly small colloidal crystals of TPA-silicalite-1 with reduced synthesis times and high yields. Using this procedure involves starting a synthesis at a reduced temperature which favours the nucleation of a large crystal population. The reduced temperature is maintained throughout the nucleation stage, after which an elevated temperature is used to accelerate the crystal growth and reach higher equilibrium yields favoured by higher temperatures
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| 4. |
- Tosheva, Lubomira, et al.
(författare)
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A method for the preparation of silicalite-1 microspheres
- 1999
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 125, s. 21-28
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Tidskriftsartikel (refereegranskat)abstract
- Silicalite-1 containing microspheres were prepared by a new method based on the use of anion exchange resins as shape directing templates. The resins were hydrothermally treated in a synthesis solution yielding silicalite-1 at 100°C and finally removed by calcination. The resultant material consists of hard, solid and white spherical particles with a shape and size identical to the ion exchange resin used. Characterization of samples taken out after different times of treatment showed that an amorphous silica is formed at the beginning of the treatment. This amorphous silica shows structural changes with time parallel to which silicalite-1 starts to form. The crystallization reaction discontinues after a treatment time of about 48 h. A calcined sample obtained after 48 h contains a large amount of amorphous matter and the additional hydrothermal treatment does not substantially improve the crystallinity of the product. Silicalite-1 microspheres of a very high crystallinity were obtained by a two-step procedure using an initial treatment at 100°C followed by a secondary treatment at a higher temperature.
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| 5. |
- Zsigmond, A, et al.
(författare)
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Aerobic oxidations on metal macrocycles encapsulated in zeolites
- 1995
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 94, s. 728-735
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Tidskriftsartikel (refereegranskat)abstract
- Zeolite-encapsulated Fe-phthalocyanine and Co-salophen catalysts were used in the palladium-catalyzed aerobic oxidation of hydroquinone to benzoquinone, in the oxidation of l-octene to 2-octanone and in the allylic oxidation of cyclohexene to 3-acetoxycyclohexene. These catalysts proved to be active in the above reactions and they were stable towards self-oxidation and it was possible to reuse them in subsequent runs. The specific activity of the encapsulated Fe-phthalocyanine catalyst was about four times higher than those of the free complex.
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| 6. |
- Zsigmond, A, et al.
(författare)
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Selective oxidation of benzyl alcohol on a zeolite ship-in-a-bottle complex
- 1997
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 108, s. 453-459
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Tidskriftsartikel (refereegranskat)abstract
- A Co(salophen)/zeolite catalyst was prepared by the template synthesis method. This catalyst proved to be active in the ruthenium catalyzed oxidation of benzyl alcohol. The heterogenized Co(salophen), having the same amount of complex produced a higher rate in the oxidation reactions than the free complex. It can be explained by the sites isolation theory. In the case of the heterogenized catalyst it was not necessary to use an extra axial ligand such as triphenylphosphine. It was also found that in the case of Co(salophen)/zeolite catalyst the choice of the solvent was not so critical, as in the case of the free complex.
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| 7. |
- Acke, Filip, 1968, et al.
(författare)
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Influence of the platinum-support interaction on the direct reduction of NOx under lean conditions
- 1998
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 116, s. 285-294
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Tidskriftsartikel (refereegranskat)abstract
- Catalysts containing Pt supported on SiC, Al2O3 and ZSM-5 were prepared and studied for NOx reduction by C3H6 in Oz excess under transient (temperature ramps) and steady-state conditions. The maximum NOx reduction activity in the heating ramp experiments was similar for Pt/SiC and Pt/ZSM-5, while Pt/Al2O3 showed higher maximum activity. Both N-2 and N2O formation was observed for all catalysts, although the respective amounts varied with the investigated system. Highest Nz selectivity was observed for Pt/Al2O3. When the NOx reduction activity was studied under steady-state conditions the activity of Pt/Al2O3 decreased substantially (mainly due to a loss in N-2 production). Pt/ZSM-5 became somewhat more selective towards Na production whereas the activity and selectivity of Pt/SiC remained at about the same values as far the heating ramp experiments. Adsorbed species on the surface of the different catalysts were investigated using in-situ FTIR in order to obtain information about the reaction mechanisms. The adsorption of species on Pt/SiC was negligible, while a number of absorption bands were observed for Pt/Al2O3 (N and C containing species, and -NCO) and Pt/ZSM-5 (HC).
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| 8. |
- Amberntsson, Annika, 1974, et al.
(författare)
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Sulphur dioxide deactivation of NOx storage catalysts
- 1999
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 126, s. 317-324
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Tidskriftsartikel (refereegranskat)abstract
- The influence of sulphur dioxide on the NOx storage performance of a Pt-Rh/BaO/Al2O3 model catalyst has been investigated. Addition of 2.5-25 vol.-ppm SO2 to a synthetic lean-burn exhaust, containing NO, C3H6, O2 and Ar, caused deactivation of the NOx storage function of the catalyst. The rate of deactivation was found to be proportional to the exposure of SO2. This effect is seen for temperatures between 250 and 450 degrees C. Together with in situ FTIR measurements, this leads to the conclusion that sulphur accumulates in the NOx storage component of the catalyst during the exposure. Furthermore, SO2 also inhibits the oxidation capacity of the catalyst during lean periods as is observed by a decreased NO oxidation activity. The presence of SO2 also reduces the reduction capacity of the catalyst under rich periods, which leads to an increased N2O formation and a decreased NO conversion.
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| 9. |
- Erkfeldt, Sara U, 1974, et al.
(författare)
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Poisoning and regeneration of NOx adsorbing catalysts for automotive applications
- 1999
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 126, s. 211-218
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Tidskriftsartikel (refereegranskat)abstract
- The efficiency and product formation as a function of temperature, time and gas stoichiometry, in the regeneration of an SO2 poisoned NOx storage and reduction (NSR) catalyst has been studied. Using these results a simple model for sulphur poisoning and regeneration of this type of catalyst is proposed. SO2 poisons the catalyst, which results in a decrease of the NOx conversion with increasing SO2 exposure time. The NOx storage capacity can be regained after regeneration of the catalyst. The regeneration is most effective using long regeneration time, high temperature and low lambda value. A measure of the regeneration efficiency, activity based sulphur removal (ASR), is defined. ASR is used to model the regeneration efficiency with the independent variables; regeneration time, temperature and lambda value. The model describes the experimental results well. The amounts of released SO2, H2S and COS, during the regeneration differ depending on regeneration conditions. Low lambda values give high amounts of H2S and low amounts of SO2, whereas the reverse is true for high lambda values. The production of COS is low in all cases. The results can schematically be described with a model where sulphur can migrate between deactivating and non-deactivating positions on the catalyst surface.
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| 10. |
- Fridell, Erik, 1963, et al.
(författare)
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Investigations of NOx storage catalysts
- 1998
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 116, s. 537-547
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Tidskriftsartikel (refereegranskat)abstract
- NOx storage catalysts are used to reduce nitrogen oxides from lean-burn vehicles. The nitrogen oxides are stored in the catalyst during lean conditions and subsequently released and reduced during short periods of rich conditions. In the present study, we systematically investigate the sequence of elementary steps in the NOx reduction cycle, and the extent to which these steps influence the maximum NOx, reduction potential of the catalyst. As a model system, we use barium oxide as the NOx, storing compound in a Pt/Rh/Al2O3 system. Kinetics of NO oxidation, NO and NO2 adsorption, NO and NO2 release and reduction are studied under controlled conditions with systematic variations of temperature, gas composition, and storing/release times. The transient experiments comprise a storing phase using a lean NO/C3H6/O2/N2 gas mixture, and a regenerating phase where the O2 now is turned off. Experimentally, a significant amount of NOx is found to be stored in the Ba-containing material. A maximum in NOx storage is observed around 380 degrees C. For most of the experiments, there are clear NO and NO2 desorption peaks upon switching from the storing to the regeneration phase. TPD studies of NO and NO2 reveal a significant difference between prereduced and pre-oxidised samples where the former produce predominantly N2 and N2O at around 200 degrees C while NO and O2 desorb from the latter around 500 degrees C. In situ FTIR spectra show nitrate peaks in the region 1300-1400 cm(-1) when NOx is stored under lean conditions.
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| 11. |
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| 12. |
- Skoglundh, Magnus, 1965, et al.
(författare)
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Light-off performance over cobalt oxide- and ceria-promoted platinum and palladium catalysts
- 1998
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Ingår i: Studies in Surface Science and Catalysis. - 0167-2991. ; 116, s. 113-122
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Tidskriftsartikel (refereegranskat)abstract
- Monolith catalysts containing Co, Ce, Pt and Pd supported on alumina were prepared and tested with respect to low-temperature activity for oxidation of CO and propene. The catalysts were either pre-oxidised or pre-reduced prior to evaluation with respect to light-off performance, using net oxidising and net reducing CO/C3H6/O2/N2 gas mixtures. Promotion of Pt and Pd with cobalt or cerium oxide, favoured the low temperature activity significantly. Pre-reduction of Co- and Ce-promoted noble metals shifted the conversion starts of CO and propene toward lower temperatures compared with pre-oxidised samples. Pre-reduction of cobalt oxide, without Pt or Pd, yielded a dramatic improvement of the low-temperature catalytic performance compared with pre-oxidation of the said oxide. The catalysts were characterised by temperature programmed desorption of CO and specific surface area measurements. The high activity over the pre-reduced cobalt containing catalysts is suggested to be due to the presence of reduced cobalt oxide sites on those samples.
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