Dispersion state and catalytic properties of vanadia species on the surface of V 2 O 5 /TiO 2 catalysts (original) (raw)
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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.
J Catal, 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.
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
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.
Theoretical Study of the Effect of (001) TiO 2 Anatase Support on V 2 O 5
The Journal of Physical Chemistry C, 2010
The effect of (001) TiO 2 anatase support on the electronic and catalytic properties of a V 2 O 5 monolayer is analyzed using density functional theory (DFT). The catalyst is represented by both clusters and periodic slabs. Using two experimentally relevant models of monolayer V 2 O 5 /TiO 2 (anatase) catalyst, both weak and strong interactions between a V 2 O 5 monolayer and the TiO 2 support have been investigated. In the first model, where a crystallographic (001) V 2 O 5 layer is placed on top of the (001) TiO 2 support, the weak interaction between vanadia and titania does not result in a major reconstruction of the active phase. Nevertheless, the changes in the electronic properties of the system are evident. The deposition of the vanadia monolayer on the titania substrate results in charge redistribution, enhancing the Lewis acidity of vanadium and the chemical hardness above the vanadyl oxygen, and in a shift of the Fermi level to lower binding energies accompanied by a reduction in the band gap. In the second model, where the (001) titania anatase structure is extended with a VO 2 film terminated by half a monolayer of vanadyl oxygen, apart from a similar electronic effect, the strong interaction of the vanadia phase with the titania support resulting from a high order of epitaxy has an important effect on the structure of the active phase. Atomic hydrogen adsorption is most favorable on the vanadyl oxygen of all the investigated surfaces, while the adsorption energy on this site increases by ∼10 kJ/mol due to the weak interaction between vanadia and titania and is further increased by ∼50 kJ/mol as a stronger interaction between the two phases is achieved, all in agreement with the increase in the negative electrostatic potential above the vanadyl site. The observed trends in the reactivity of the oxygen sites in H adsorption for the different catalyst models are successfully explained in terms of a frontier orbital analysis.
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.
Preparation, characterization, and activity of Al 2 O 3-supported V 2 O 5 catalysts
A series of activated alumina-supported vanadium oxide catalysts with various V 2 O 5 loadings ranging from 5 to 25 wt% have been prepared by wet impregnation technique. A combination of various physicochemical techniques such as BET surface area, oxygen chemisorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), thermal gravimetric analysis (TGA), and Fourier transform infrared (FTIR) were used to characterize the chemical environment of vanadium on the alumina surface. Oxygen uptakes were measured at 370 • C with prereduction at the same temperature, which appears to yield better numerical values of dispersion and oxygen atom site densities. XRD and FTIR results suggest that vanadium oxide exists in a highly dispersed state below 15 wt% V 2 O 5 loading and in the microcrystalline phase above this loading level. TPR profiles of V 2 O 5 /Al 2 O 3 catalysts exhibit only a single peak at low temperature up to 15 wt% V 2 O 5 . It is suggested that the low-temperature reduction peak is due to the reduction of surface vanadia, which has been ascribed to the tetrahedral coordination geometry of the V ions. TPR of V 2 O 5 /Al 2 O 3 at higher vanadia loadings exhibited three peaks at reduction temperatures, indicating that bulk-like vanadia species are present for these catalysts only at higher vanadia loadings, with V ions in an octahedral coordination. The TPR profiles of V 2 O 5 /Al 2 O 3 catalysts indicate that at loadings lower than 15% vanadia forms isolated surface vanadia species, while two-dimensional structure and V 2 O 5 crystallites become prevalent in highly loaded (>15% V 2 O 5 ) systems. Liquid-phase oxidation of ethylbenzene to acetophenone has been employed as a chemical probe reaction to examine the catalytic activity. Ethylbenzene oxidation results reveal that 15%V 2 O 5 /Al 2 O 3 is more active than higher vanadia loading catalysts.
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.