Physical and chemical characterization of surface vanadium oxide supported on titania: influence of the titania phase (anatase, rutile, brookite and B) (original) (raw)
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Surface reactivity and morphology of vanadia-titania catalysts
Surface Science Letters, 1991
Vanadia/ titania samples have been prepared from two different titania supports (P-25 from Degussa and anatase from Tioxide) by impregnation with aqueous solutions of NH,VO, and calcination at 723 K. A decrease in the specific surface area is observed on increasing the amount of vanadia; titania from Degussa being non-porous, only development of wide pores is observed upon supporting vanadia. However, while the unloaded Tioxide support shows pores with average diameters of 8 and 5 nm. these latter disappear upon incorporation of vanadia. The reactivity of the V,Os/TiO, sample prepared using the P-25 support in olefin oxidation has been studied by FTIR spectroscopy in the temperature range 150473 K. Ethylene and propene are adsorbed as such at low temperatures, become hydrated to alkoxy species at medium temperatures and later undergo oxidation to carbonyl compounds (acetaldehyde and acetone). Allylic oxidation of propene and oxidative breaking of the C=C bonds is also observed.
Thermochimica Acta, 2004
Titania supported vanadia catalysts (2.5, 5, and 11 wt.% V 2 O 5 ) were prepared by a wet impregnation technique and their thermal behavior, morphology as well as redox properties were examined by thermal analysis methods thermogravimetry (TGA), differential scanning calorimetry (DSC), temperature programmed-evolved gas analysis with mass spectroscopy, (EGA-MS), scanning electron microscopy (SEM), and temperature programmed reduction (TPR). The two Eurocat samples EL10V1 and EL10V8 containing 1 and 8 wt.% V 2 O 5 were also characterized using the same techniques. Thermal decomposition of vanadium oxide precursors (ammonium vanadyl oxalate) supported on TiO 2 as evidenced by thermal analysis, occurs in three successive steps, which are influenced by the surrounding atmosphere (oxidative, reductive, and inert). The presence of tower-like vanadia crystals in the sample with the highest vanadia loading (11 wt.% V 2 O 5 ) was identified by SEM. The H 2 -TPR experiments revealed that the reduction temperature is a factor of the vanadia loading and the type of support. Vanadia species supported on Norton titania are more reducible that those supported on Eurocat titania.
Advanced Synthesis and Characterization of Vanadia/Titania Catalysts through a Molecular Approach
Catalysts
Vanadia/titania catalysts were synthesized by the equilibrium deposition filtration (EDF) method, which is a synthesis route that follows a molecular-level approach. The type of interfacial deposition as well as the interfacial speciation of the deposited oxo-V(V) species were determined by means of a model that takes into account experimental “proton-ion” curves and “adsorption edges”. It is shown that at pH ≥ 9.5, the deposition proceeds exclusively through the formation of mono-substituted inner sphere monomeric species in an “umbrella”-like Ti–OV(OH)2O configuration, whilst with lowering of the pH, a second species, namely the disubstituted inner sphere quadrameric species in a (Ti-O)2V4O10 configuration possessing two mono-oxo V=O and two di-oxo V(=O)2 terminations gradually prevails, which is in co-existence with the monomeric species. Raman spectroscopy is used for verifying the solution speciation, which is different compared to the interfacial speciation of the deposited ox...
Applied Catalysis B: Environmental, 1995
Sub-monolayer V,Os/TiO, catalysts were prepared by the equilibrium adsorption method using a high surface area TiO, anatase powder. They were characterized by the following techniques: X-ray diffraction, BET surface area measurements, electrical conductivity, electron paramagnetic resonance, UV-VIS, laser-Raman spectroscopy and "V solid state nuclear magnetic resonance (NMR). The physico-chemical characterization shows that vanadium is present in the 5 + oxidation state and that the vanadium oxide phase is well dispersed on the surface of the support. The conditions of preparation markedly influence the nature of the surface species: alkaline pH promotes the formation of isolated tetrahedral species, whereas with acidic pH more condensed vanadium oxide species, containing octahedrally coordinated vanadium, are formed. The calcination gives rise to a further aggregation of the surface vanadium oxide phase. The activity of the catalysts in the selective reduction of NO by NH, in the presence of O2 was determined. The analysis of kinetic data combined with the results of physico-chemical characterization leads to a good relationship between the reaction rate and the amount of octahedrally coordinated vanadium, as evaluated by NMR spectra under ambient conditions.
Journal of Solid State Chemistry, 1996
High surface area titania-supported materials prepared from V(IV) precursors and calcined at high temperatures have been characterized by Vis–UV diffuse reflectance, FT Raman, electron spin resonance, and X-ray photoelectron spectroscopies and tested in the partial oxidation of methane. Vanadium oxide loading and calcination temperature determine the structure of V2O5/TiO2materials. Below theoretical surface monolayer coverage, V(IV) species closely interacting with the support are observed. Vanadiam oxide species anchor by reaction with titanium oxide surface hydroxyl groups. The V(IV) species are stabilized by interaction with titania support and further stabilization occurs at high calcination temperatures by their location in titania (rutile) lattice. Larger loadings of vanadium decrease the temperatures required for conversion of titania (anatase) to titania (rutile). At higher vanadium loading segregation into bulk V2O5oxide takes place, thus decreasing interaction with titania support. This enables a larger population of V(V) species than samples with surface dispersed vanadium oxide species. Although partial oxidation of methane is nonselective on titania (anatase), partial oxidation products are observed on titania (rutile)-supported vanadium oxide catalysts. The higher selectivity to partial oxidation product formaldehyde appears to be related to the high stability of V(IV) cations located on rutile lattice and the absence of V(V) sites.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 1995
Samples of anatase, prepared using different methodologies, and an industrial sample were characterized using X-ray photoelectron spectroscopy, X-ray diffraction analysis, electron microscopy, diffuse reflectance spectroscopy, temperature-programmed desorption of ammonia, nitrogen adsorption, microelectrophoresis and potentiometric titrations. It was found that anatase prepared by hydrolysis of the titanium isopropoxide exhibited the largest specific surface area, the highest Lewis acidity and the highest concentration of the protonated plus neutral surface hydroxyl groups. These hydroxyl groups are considered to be the deposition sites for the VO3 ions in the preparation of the VzOs/anatase catalysts by equilibrium deposition-filtration. The above findings as well as the absence of foreign ions on the surface of the anatase prepared by the hydrolysis of titanium isopropoxide renders this material the most suitable for preparing the above catalysts used in NO reduction.
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
Vanadia/titania catalysts for gas phase partial toluene oxidation
Catalysis Today, 2000
Formation of vanadia species during the calcination of ball milled mixture of V 2 O 5 with TiO 2 was studied by Raman spectroscopy in situ and at ambient conditions. It is found that calcination in air leads to fast (1-3 h) spreading of vanadia over TiO 2 followed by a slower process leading to the formation of a monolayer vanadia. The calcinated catalyst showed higher activity during toluene oxidation than the uncalcinated one, but the selectivity towards C 7-oxygenated products (benzaldehyde and benzoic acid) remains unchanged. The activity of the catalysts is ascribed to the formation of vanadia species in the monolayer. The details of the parallel-consecutive reaction scheme of toluene oxidation are presented from steady-state and transient kinetics studies. Different oxygen species seem to participate in the deep and partial oxidation of toluene. Coke formation was observed during the reaction presenting an average composition C 2n H 1.1n. The amount of coke on the catalyst was not dependent on the calcination step and the vanadium content in the catalyst. Coke formation was seen to be responsible for the deactivation of the catalyst.