| 1. |
|
|
| 2. |
- Jansen, Susan, et al.
(författare)
-
EPR Studies of the Coverage Effects in TiO2(B)-Supported Vanadia Catalysts for Toluene Ammoxidation
- 1992
-
Ingår i: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 138:1, s. 79-89
-
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.
|
|
| 3. |
- Nilsson, Roland, et al.
(författare)
-
Ammoxidation of Propane over Antimony-Vanadium-Oxide Catalysts
- 1994
-
Ingår i: Journal of Catalysis. - : Elsevier BV. - 1090-2694 .- 0021-9517. ; 148:2, s. 501-513
-
Tidskriftsartikel (refereegranskat)abstract
- Catalysts belonging to the Sb-V-O system were prepared with various Sb/V ratios and were used for propane ammoxidation to acrylonitrile. XRD patterns of freshly prepared samples show those with excess vanadia to consist of V2O5 and SbVO4, while SbVO4 and α-Sb2O4 are constituents in the samples with a Sb/V ratio above unity. High rate and selectivity for propylene formation at low conversion are characteristic for samples with excess vanadia and considering XRD, Raman, infrared, and XPS results, this is explained by formation of amorphous vanadia spread over the surface of SbVO4. Catalysts with both α-Sb2O4 and SbVO4 phases are the most selective for acrylonitrile formation, a function that is linked to their ability to selectively transform intermediate propylene. XPS data suggest this function to be associated with the formation of suprasurface antimony sites on SbVO4 as a result of migration of antimony from α-Sb2O4 during the catalytic process. Raman and infrared spectral features revealed that compared with SbVO4, the samples with both α-Sb2O4 and SbVO4 are more efficiently reoxidised during propane ammoxidation. Rate dependences on the partial pressures of reactants over a sample with excess α-Sb2O4 show that the adsorption of propane is the rate limiting step for propylene formation, and that acrylonitrile and carbon oxides are predominantly formed from the intermediate propylene in routes comprising nonequilibrated steps. Addition of water vapour results in an increase of rate and selectivity for acrylonitrile formation. The kinetic dependences indicate that for acrylonitrile formation it is advantageous to have a feed rich in propane and to use recirculation for obtaining high productivity.
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
- Sanati, Mehri, et al.
(författare)
-
Vanadia Catalysts on Anatase, Rutile, and TiO2(B) for the Ammoxidation of Toluene: An ESR and High-Resolution Electron Microscopy Characterization
- 1991
-
Ingår i: Journal of Catalysis. - 1090-2694. ; 132:1, s. 128-144
-
Tidskriftsartikel (refereegranskat)abstract
- Three titania polymorphs, anatase, rutile, and TiO2(B), were used as supports for vanadia. Catalysts with nominal loadings corresponding to 1.5 and 5 theoretical layers were prepared. True monolayer samples were obtained by NH3(aq)-treatment of the samples with 5 nominal vanadia layers. Prepared catalysts were characterized by chemical analysis, ESR, and high-resolution electron microscopy. It was observed that an almost complete monolayer of vanadia can be formed on anatase and TiO2(B). The monolayer on TiO2(B) consists of V4+ species. A magnetic species attributable to tetrahedral states of vanadium was found to dominate the ESR resonance. Chemical analysis of the monolayer on anatase showed the presence of both V4+ and V4+ species. The ESR spectrum indicated a high degree of V4+-V4+ interaction. From the characterizations of rutile-supported samples it seems that the support interface is a solid solution of V4+. On all supports, vanadium on top of the monolayer is present as V5+. High-resolution micrographs of the samples with the highest loading revealed the formation of amorphous vanadia on both anatase and rutile. On TiO2(B), vanadia was found to grow coherently at the titania interface. The catalysts were used in the oxidation and ammoxidation of toluene to benzaldehyde and benzonitrile, respectively. Considering both activity and selectivity, multilayers of vanadia supported on TiO2(B) show a good performance for both reactions.
|
|
| 7. |
- Wallenberg, Reine, et al.
(författare)
-
A High-Resolution Electron Microscopy Investigation of TiO2(B)-Supported Vanadium Oxide Catalysts
- 1990
-
Ingår i: Journal of Catalysis. - 1090-2694. ; 126:1, s. 246-260
-
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.
|
|
