| 171. |
- Andersson, Arne, et al.
(författare)
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Ammoxidation of 3-Picoline: An Activity and High-Resolution Electron Microscopic Investigation of Vanadium Oxide Catalysts
- 1986
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Ingår i: Journal of Catalysis. - 1090-2694. ; 98:1, s. 204-220
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Tidskriftsartikel (refereegranskat)abstract
- A V2O5 catalyst was used in the ammoxidation of 3-picoline to nicotinonitrile. It was observed that the selectivity for the formation of CO2 as a function of temperature passed through a minimum. This is explained to be due to the existence of weakly bonded electrophilic oxygen species at low temperatures, and an increasing degradation involving O2− at high temperatures. A comparison of two different V2O5 preparations shows the beneficial effect of the V2O5(010) plane on the formation of nicotinonitrile. The exposure of planes other than the (010) plane as the source of formation of CO2 is discussed by consideration of bond strength values. The activity, selectivity, and composition of the charged V2O5 catalyst were followed as a function of time-on-stream at various temperatures. It was found that the V2O5 phase was reduced in the course of the reaction. V4O9, VO2(B), VO2 (tetragonal), and even more reduced phases were formed depending upon the reaction temperature used. Of the pure oxides, V4O9 was found to be both less active and less selective than V2O5. VO2(B), however, is more active but less selective compared to V2O5. The phases formed were characterized by various methods including high-resolution transmission electron microscopy (HRTEM). This technique made it possible to image the View the MathML source phase boundary for the first time. The general direction of this boundary is parallel to the (301) plane of V2O5. Micrographs of VO2(B) show that the nature of defects formed depends on the reaction temperature. After use at 695 K two types of planar twin lamellae were formed. At a slightly higher temperature partly amorphous defects appeared. The influence on the catalytic reaction of the phase boundaries and defects formed is discussed.
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| 172. |
- Andersson, Arne, et al.
(författare)
-
Ammoxidation of toluene over molybdenum oxides
- 1988
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Ingår i: Catalysis Letters. - 1011-372X. ; 1:11, s. 377-383
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Tidskriftsartikel (refereegranskat)abstract
- The ammoxidation of toluene was studied both in presence and absence of gaseous oxygen over MoO3 and in situ prepared Mo4O11 (orth.) and MoO2. Irrespective of the partial pressure of oxygen, total reaction rates decrease in the following order: MoO3 > Mo4O11 > MoO2. The corresponding sequence for the selectivity towards formation of benzonitrile was found to depend on the partial pressure of oxygen. In two phase samples, no synergistic effects were observed for the formations of nitrile and carbon oxides. A comparison of rates for selective and nonselective products obtained in presence of gaseous oxygen with those obtained in its absence yield information as to whether the various products are formed at identical or different crystal faces. In the case of MoO3, nitrile and carbon oxides were found to be competitively formed at the same faces, while over MoO2 they are formed at different faces. Both options seem to prevail on
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| 173. |
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| 174. |
- Andersson, Arne
(författare)
-
An Oxidized Surface State Model of Vanadium Oxides and Its Application to Catalysis
- 1982
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596. ; 42:3, s. 263-275
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Tidskriftsartikel (refereegranskat)abstract
- The charge distribution around the VO bonds in V2O5, V6O13, and V2O4 was calculated by using an empirical formula. The same expression was also used to calculate the oxygen bond strengths on the surface. The surfaces of lower oxides were treated as though they were in an oxidized state which is believed to correspond to the conditions in oxidation and ammoxidation processes. The result is that O2−, in the form of VO surface groups, is responsible for the catalytic oxidation of hydrocarbons. O− is hindered by the formation of stable OH− groups. The positions of lower valent oxygens are considered to be vacant because of a slow reoxidation rate. In V2O5 the VO groups are located on the (010) plane, while in V6O13 they are mainly located on the (001) surface plane. But in this case the (100) and (010) surface planes also have some VO groups. The catalytic activity of the rutile form of V2O4 is limited by sterical factors, but the (110) surface plane has oxygens pointing perpendicular out of the surface.
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| 175. |
- 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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| 176. |
- 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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| 177. |
- Andersson, Anders, et al.
(författare)
-
The epidermal indeterminate cell- a special cell type?
- 1981
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Ingår i: Acta dermato-venereologica. Supplementum. - 0365-8341. ; 99, s. 41-48
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Tidskriftsartikel (refereegranskat)abstract
- Electron microscopy of normal human epidermiscomprising very large series of sections in combination withtilting of the sections has revealed that all "indeterminatecells" carry the specific markers of either melanocytes orLangerhans cells. The term "indeterminate cells" can thusbe abandoned. In addition, it is suggested that an epidermaldendritic cell lacking specific markers in the section underobservation can be characterized by the amount and distributionpattern of its microfilaments, an additional criterionbeing that the melanocytes carry a ciliunu
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| 178. |
- Andersson, Bo, et al.
(författare)
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A general model for jet fragmentation
- 1983
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Ingår i: European Physical Journal C. - 0170-9739. ; 20:4, s. 317-329
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Tidskriftsartikel (refereegranskat)abstract
- We present a general condition on quark fragmentation which gives a hadron distribution satisfying Lorentz invariance and causality. The hadronization can be described as an iterative cascade process, symmetric with respect to iteration from the quark and the antiquark ends. The possible particle distributions are strongly restricted, with few free parameters related to the total multiplicity and corelations in rapidity. These parameters can be given an appealing interpretation in terms of the expected area and perimeter dependence of Wilson loop integrals.
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| 179. |
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| 180. |
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