Thermal behaviour of high surface area V2O5/TiO2 catalysts (original) (raw)
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Surface Characterisation of V2O5/TiO2 Catalytic System
physica status solidi (a), 2001
Samples of the V 2 O 5 /TiO 2 system were prepared by the sol-gel method and calcined at different temperatures. Surface species of vanadium, their dispersion, as well as the structural evolution of the system were analysed by XRD, Raman, EPR, and XPS techniques. The results of XRD showed the evolution of TiO 2 from anatase phase to rutile phase. The Raman spectra for calcination temperatures up to 500 C showed a good dispersion of vanadium over titania in the form of monomeric vanadyl groups (V 4þ ) and polymeric vanadates (V 5þ ). At least three families of V 4þ ions were identified by EPR investigations. Two kinds of isolated V 4þ species are placed in sites of octahedral symmetry, substituting Ti 4þ in the rutile phase. The third is formed by pairs of V 4þ species on the surface of titania. Above 500 C part of superficial V 4þ is inserted into the matrix of titania and part is oxidized to V 5þ . The XPS results showed that the V/Ti ratio rises with increasing calcination temperature, indicating a smaller dispersion of vanadium.
Chemical and structural characterization of V2O5/TiO2 catalysts
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2001
A series of V 2 O 5 /TiO 2 samples was synthesized by sol-gel and impregnation methods with different contents of vanadia. These samples were characterized by x-ray diffraction ͑XRD͒, Raman spectroscopy, x-ray photoelectron spectroscopy ͑XPS͒, and electronic paramagnetic resonance ͑EPR͒. XRD detected rutile as the predominant phase for pure TiO 2 prepared by the sol-gel method. The structure changed to anatase when the vanadia loading was increased. Also, anatase was the predominant phase for samples obtained by the impregnation method. Raman measurements identified two species of surface vanadium: monomeric vanadyl (V 4ϩ) and polymeric vanadates (V 5ϩ). XPS results indicated that Ti ions were in octahedral position surrounded by oxygen ions. The V/Ti atomic ratios showed that V ions were highly dispersed on the vanadia/titania surface obtained by the sol-gel method. EPR analysis detected three V 4ϩ ion types: two of them were located in axially symmetric sites substituting for Ti 4ϩ ions in the rutile structure, and the third one was characterized by magnetically interacting V 4ϩ ions in the form of pairs or clusters. A partial oxidation of V 4ϩ to V 5ϩ was evident from EPR analysis for materials with higher concentrations of vanadium.
UPS study of the thermal reduction of fully oxidized V 2 O 5/TiO 2 (001)-anatase model catalysts
The thermal reduction of thin vanadium oxide layers (8 and 16Å) deposited with magnetron sputtering on mineral anatase TiO 2 (0 0 1) was examined with ultraviolet photoelectron spectroscopy (UPS). Completely oxidized vanadium oxide layers were deposited. During heating, the vanadium oxide layers reduced and evaporated, the thinnest vanadium oxide layer (8Å) even vanished. A re-oxidation of the heated 16Å V 2 O 5 /TiO 2 could not restore the reduced vanadium oxide completely to V 5+ .
The Journal of Physical Chemistry B, 1997
A high surface area titania-silica binary oxide support was prepared according to the homogeneous precipitation method and was coated with a monolayer of vanadium oxide. The TiO 2 -SiO 2 support and the V 2 O 5 /TiO 2 -SiO 2 catalyst were then subjected to thermal treatments from 773 to 1073 K. The influence of heat treatments on the dispersion and thermal stability was investigated by X-ray diffraction, FT infrared, X-ray photoelectron spectroscopy, oxygen uptake, and BET surface area methods. The results suggest that TiO 2 -SiO 2 is quite thermally stable even up to 1073 K calcination temperature. However, the V 2 O 5 /TiO 2 -SiO 2 catalyst is stable, in terms of dispersion and surface area, only up to a calcination temperature of 873 K. Thermal treatments beyond this temperature transformed vanadia and titania into crystalline phases and then titania anatase into rutile phase.
Colloids and Surfaces, 1990
Solid state 51V wideline NMR studies show that under ambient conditions the vanadium (V) oxide surface phases on TiO,(anatase) and Ti02(rutile) supports predominantly possess distorted-octahedral coordination. However, the coordination environment of vanadia is markedly influenced by the presence of impurities in the support materials. Surface contaminants promote the formation of tetrahedral surface vanadia species, which preferentially form at low surface coverages. The presence of these surface impurities depends on the titania preparation method and overshadows the influence, if any, of the bulk Ti02 lattice structure (anatase versus rutile). Thus, the strong influence of surface impurities on the V205/Ti02 system is most likely responsible for the widely varying claims about differences in the catalytic properties of V,05/ Ti02 (anatase) versus V20S/Ti02 (rutile) samples. INTRODUCTION V,O, supported on TiOz is known to be an important oxidation catalyst [l-111, specifically for the partial oxidation of o-xylene to phthalic anhydride. Catalytic studies have suggested that V205/Ti02 (anatase) is a superior catalyst than V205/Ti02 (rutile) for this oxidation [ 121. Early studies attributed the higher activity of the V205/Ti02 (anatase) to the ease of oxygen evolution under inert environments [ 2,131. Vejux and Courtine [ 131 ascribe the higher activity of the Vz05/Ti02 (anatase) catalyst to the crystallographic fit between pure V205 (010 plane) and pure TiO, (anatase) (010 or 001 plane). Likewise, the lower activity of V205/Ti02 (rutile) was attributed to the misfit of the lattice parameters of the two corresponding bulk phases. Since then, these con
Chemical and structural characterization of V[sub 2]O[sub 5]/TiO[sub 2] catalysts
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2001
A series of V 2 O 5 /TiO 2 samples was synthesized by sol-gel and impregnation methods with different contents of vanadia. These samples were characterized by x-ray diffraction ͑XRD͒, Raman spectroscopy, x-ray photoelectron spectroscopy ͑XPS͒, and electronic paramagnetic resonance ͑EPR͒. XRD detected rutile as the predominant phase for pure TiO 2 prepared by the sol-gel method. The structure changed to anatase when the vanadia loading was increased. Also, anatase was the predominant phase for samples obtained by the impregnation method. Raman measurements identified two species of surface vanadium: monomeric vanadyl (V 4ϩ ) and polymeric vanadates (V 5ϩ ). XPS results indicated that Ti ions were in octahedral position surrounded by oxygen ions. The V/Ti atomic ratios showed that V ions were highly dispersed on the vanadia/titania surface obtained by the sol-gel method. EPR analysis detected three V 4ϩ ion types: two of them were located in axially symmetric sites substituting for Ti 4ϩ ions in the rutile structure, and the third one was characterized by magnetically interacting V 4ϩ ions in the form of pairs or clusters. A partial oxidation of V 4ϩ to V 5ϩ was evident from EPR analysis for materials with higher concentrations of vanadium.
Science China-chemistry, 2002
The dispersion state and catalytic properties of anatase-supported vanadia species are studied by means of X-ray diffraction (XRD), laser Raman spectroscopy (LRS), H2 temperature-programmed reduction (TPR) and the selective oxidation of o-xylene to phthalic anhydride. The almost identical values of the experimental dispersion capacity of V2O5 on anatase and the surface vacant sites available on the preferentially exposed (001) plane of anatase suggest that the highly dispersed vanadium cations are bonded to the vacant sites on the surface of anatase as derived by the incorporation model. When the loading amount of V2O5 is far below its dispersion capacity, the dispersed vanadia species might mainly consist of isolated VOx species bridging to the surface through V-O-Ti bonds. With the increase of V2O5 loading the isolated vanadia species interact with their nearest neighbors (either isolated or polymerized vanadia) through bridging V-O-V at the expenses of V-O-Ti bonds, resulting in the increase of the ratio of polymerized to isolated vanadia species and the decrease of the reactivity of the associated surface oxygen anions and, consequently, although the activity increases with loading to reach a maximum value, the turn over number (TON) of the V2O5/TiO2 catalyst decreases linearly. When the loading amount of V2O5 is higher than its dispersion capacity, the turn over number decreases more rapidly with the increase of V2O5 loading due to the formation of V2O5 crystallites in which the oxygen anions associated with V-O-V bonds are less reactive and only partially exposed on the surface.
Reaction Kinetics and Catalysis Letters, 2004
The properties of the catalysts for partial oxidation of o-xylene depend on the structure of the supported vanadium sites. The structure itself is strongly dependent on the calcination temperature of the catalyst at which thermal deposition of the metal oxide on the oxide support takes place. We have investigated the effect of calcination temperature on the activity and selectivity of industrial V2O5-TiO2 (anatase) supported catalysts designed for partial oxidation of o-xylene in their application to methanol oxidation.
Quantitative structural analysis of dispersed vanadia species in TiO2(anatase)-supported V2O5
Journal of Catalysis, 1992
VzOs-TiO2(anatase) catalysts have been studied under oxidizing and reducing conditions using in situ laser Raman spectroscopy (LRS) and temperature-programmed reduction (TPR) and oxidation (TPO). Quantitative Raman and TPO analysis of the oxidized samples show that these materials are comprised of a distribution of monomeric vanadyls, polymeric vanadates, and crystallites of V205. At low loadings, the predominant species are monomeric vanadyls, with the remaining vanadia being present in the form of polymeric vanadates. As the surface concentration of V205 increases, a maximum in the concentration of the polymeric vanadates is detected. Crystallites of V205 form at the expense of the polymeric vanadates as the loading is raised above the dispersive capacity of the TiO2(anatase) support. An equilibrium polymerization model is proposed to account for the observed concentration of vanadia species, which leads to an initial polymer size of-2 at low loadings, consistent with the formation of dimeric species. Raman and TPR/TPO studies of the reduction process indicate that the terminal V = O groups of the monomeric and polymeric vanadia species are removed preferentially to the bridging oxygen atoms of the polymeric species. The maximum loss of oxygen upon reduction is one oxygen atom per vanadium atom.
Influence of Potassium Doping on the Formation of Vanadia Species in V/Ti Oxide Catalysts
Langmuir, 2001
The influence of potassium on the formation of surface vanadia species on V/Ti oxide catalysts containing from 0.2 to 5 monolayers of vanadia (K/V atomic surface ratio e1) has been investigated by temperature programmed reduction in hydrogen and by FT-Raman spectroscopy under dehydrated conditions. In the pure catalysts, monomeric and polymeric (metavanadate-like) species, "amorphous" and bulk crystalline V2O5 were detected depending on the surface vanadia loading. In the K-doped catalysts, vanadia species formed on the surface depend also on the K/V atomic ratio. Even at small K/V ratios, K inhibits the formation of the polymeric species in favor of the "K-doped" and/or "K-perturbed" monomeric species. These species possess lengthened VdO bonds with respect to the monomeric species in the undoped V/Ti oxides. At K/V ) 1, the "K-doped" monomeric species and "amorphous" KVO3 are mainly present on the surface. Reduction of vanadia forms in the K-doped catalysts takes place at higher temperatures than in the catalysts where potassium was absent. The monomeric and polymeric species, which are the active sites in partial catalytic oxidation, have the lowest reduction temperature. Vanadia species formed on the commercial titania, containing K, were also elucidated. The catalysts were characterized via X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and Brunauer-Emmett-Teller surface area measurements.