| 1. |
- Hansen, Staffan, et al.
(författare)
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Ammoxidation of Toluene over Y-Ba-Cu-Co-O Perovskites
- 1990
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Ingår i: Catalysis Letters. - 1011-372X. ; 6:1, s. 33-42
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Tidskriftsartikel (refereegranskat)abstract
- mmoxidation of toluene over the perovskites YBa2Cu3O6.1, YBa2Cu2CoO6.7 and YBaCuCoO4.9 was investigated at 400 °C. At low partial pressures of O2 benzonitrile was selectively formed, while CO2 was the main product at high pressures of O2. Systematic differences in activity were observed for the three phases and are related to the crystal contents of Cu and Co. At low O2 pressures, Cu-sites are active for nitrile formation, while Co-sites give CO2. At high O2 pressures, the activity for CO2 of Cu-sites increases more than that of Co-sites due to filling of near-surface oxygen vacancies.
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| 2. |
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| 3. |
- Otamiri, JC, et al.
(författare)
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Ammoxidation of Toluene by YBa2Cu3O6 + x and Copper Oxides. Activity and XPS Studies
- 1990
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Ingår i: Applied Catalysis. - 0166-9834. ; 65:1, s. 159-174
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Tidskriftsartikel (refereegranskat)abstract
- The catalytic activities of YBa2Cu3O6+x and copper oxides have been studied in the ammoxidation of toluene. At low oxygen pressures, over YBa2Cu3O6+x selective ammoxidation to benzonitrile occurs, whereas at high pressures, only total combustion of toluene to carbon oxides is observed. X-ray photoelectron spectroscopy (XPS) analysis of YBa2Cu3O6+x catalysts indicates predominance of Cu(I) states under partial ammoxidation conditions, while under total oxidation conditions, Cu(II) states are predominant. Comparison with the catalytic performance of copper oxide catalysts shows that differences do exist. The catalysis by YBa2Cu3O6+x is greatly influenced by the oxygen content of the bulk material despite its surface composition.
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| 4. |
- Otamiri, JC, et al.
(författare)
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Preparation of YBa2Cu3O6 + x by a Formic Acid Method
- 1990
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Ingår i: Journal of Materials Research. - 0884-2914. ; 5:7, s. 1388-1391
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Tidskriftsartikel (refereegranskat)abstract
- Synthesis of YBa2Cu3O6+x from formic acid solutions of stoichiometric amounts of the initial materials has been investigated in the temperature range 750–950°C. The reaction is strongly influenced by the concentration of formic acid. Dilute solutions favor synthesis of purer samples under moderate conditions of temperature and sintering time. At lower temperature the same effect can be achieved if sintering time is longer, while at higher temperature, multiple grinding and heating at very short intervals are necessary to avoid formation of Y2BaCuO5. The observed effect of HCOOH concentration is explained by the different mechanisms involved in the decomposition of the formates obtained.
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| 5. |
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| 6. |
- Sanati, Mehri, et al.
(författare)
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Ammoxidation of Toluene over TiO2(B)-Supported Vanadium Oxide Catalysts
- 1990
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Ingår i: Journal of Molecular Catalysis. - 0304-5102. ; 59:2, s. 233-255
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Tidskriftsartikel (refereegranskat)abstract
- TiO2(B) was used as a support for vanadium oxide with loadings in the range 1/4 to 10 theoretical layers. The vanadium could be separated into two parts, which were soluble and insoluble in NH3(aq), respectively. Insoluble vanadium, in an amount corresponding to a complete monolayer at high loadings, was present as V4+ in freshly prepared catalysts. Diffuse reflectance FTIR spectroscopy was used to characterize the catalysts. The bands assigned to stretching vibrations of hydroxyl groups on TiO2(B) were found to decrease in intensity with increasing vanadium loading. Spectra in the V=O and V-O-V vibration region revealed a rather complex growth pattern. For a catalyst covered with a complete monolayer, vanadium strongly interacting with the support is present as interconnected tetrahedral V4+ units. When the loading is increased, several additional layers of V5+ in tetrahedral coordination can exist on top of the monolayer. At higher loadings, there is an abrupt change in structure, resulting in the formation of an amorphous phase built up from distorted VO6 octahedra. Also, crystalline V2O5 is formed. The catalysts were used in the ammoxidation of toluene to benzonitrile. Multilayer catalysts showed better catalytic performance in comparison with crystalline V2O5 and catalysts exposing monolayer species.
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| 7. |
- Wallenberg, Reine, et al.
(författare)
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A High-Resolution Electron Microscopy Investigation of TiO2(B)-Supported Vanadium Oxide Catalysts
- 1990
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Ingår i: Journal of Catalysis. - 1090-2694. ; 126:1, s. 246-260
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Tidskriftsartikel (refereegranskat)abstract
- TiO2(B) crystals were found to be isomorphic with those of its precursor, K2Ti4O9. The former crystals had a large number of facetted voids, 3–30 nm, formed as a result of the removal of potassium and water in preceding hydrolysis and calcination steps, respectively. TiO2(B)-supported vanadium oxide catalysts with loadings in the range 0.25−10 theoretical layers were prepared by impregnation of the support with an oxalic acid solution of NH4VO3 followed by calcination in air. HREM micrographs of catalysts with a low vanadium loading, recorded using a low electron-dose imaging technique, showed that the surfaces, in the initial stage, were without any anomalous surface structure. This observation may be due to a similar structure of the support and the deposited vanadia phase. At high vanadium loadings, both amorphous and crystalline particles were seen, in agreement with the features revealed by the use of IR spectroscopy. For catalysts with low vanadium loadings, the IR difference spectra showed the presence of tetrahedrally coordinated V4+ and V5+ species. In a fully converged electron beam, reduction of the support and the vanadia phases occured, resulting in the formation of small crystallites. The catalysts were used for the oxidation of toluene to benzaldehyde. However, contrary to what has been observed for the ammoxidation producing benzonitrile, no enhanced catalytic properties, in comparison with those of crystalline V2O5, were obtained using TiO2(B) as support.
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| 8. |
- Wallenberg, Reine, et al.
(författare)
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On the Transformation Mechanism of K2Ti4O9 to TiO2(B) and Formation of Microvoids
- 1990
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Ingår i: Microscopy Microanalysis Microstructures. - : EDP Sciences. - 1154-2799. ; 1:5-6, s. 357-364
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Tidskriftsartikel (refereegranskat)abstract
- TiO2(B), a potential catalyst support, has been synthesized from a support precursor, K2Ti4O9. High resolution electron microscopy revealed that the 32% volume loss in this transformation is achieved by formation of facetted microvoids, around 10 nm in size, maintaining the morphology and size of the original crystals. A structural mechanism, including a hypothetical structure for a K2Ti 2O5 intermediate, is suggested.
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| 9. |
- Wallenberg, Reine, et al.
(författare)
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Vanadium Oxide on TiO2(B) - a HREM Study of Catalysis by Support Interaction
- 1990
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Ingår i: Ultramicroscopy. - 0304-3991. ; 34:1-2, s. 33-40
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Tidskriftsartikel (refereegranskat)abstract
- A model catalyst system with a loading range between 1/4 to 10 theoretical layers of vanadium oxide on TiO2(B ) support was tried for selective ammoxidation and oxidation of toluene. In the case of ammoxidation, a peak in conversion rate per unit surface area was found for a catalyst with a coverage of about 2 theoretical layers, indicating a strong interaction between the catalyst and the otherwise inactive support. The presence and amount of vanadium oxide on the support surface was verified by diffuse reflectance infrared spectroscopy, energy-dispersive elemental X-ray mapping and chemical analysis. By high resolution electron microscopy, the 32~ volume loss of the K2Ti409 support precursor on calcination was shown to be achieved by formation of microscopic, facetted voids ("negative crystals") in the bulk of the material. Initially, surfaces appeared dean and without any anomalous surface features. This can be explained by similar scattering power of V and Ti, and similar structure types. However, the active phase could be "developed" to visibility by prolonged exposure to the electron beam, which produced 1-2 am particles of reduced vanadium oxides on the surface.
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