Higher Vanadia Loadings Research Articles

Effects of MoOx on dispersion of vanadia and low-temperature NH3-SCR activity of titania supported catalysts: Liquid acidity and steric hindrance

Monometallic (V/TiO2 and Mo/TiO2) and bimetallic (V-Mo/TiO2) catalysts were prepared for selective catalytic reduction (SCR) of NOx with NH3 at low temperatures (100–300 °C). The effect of MoOx on the dispersion of vanadia with different V2O5 loadings was investigated on the basis of X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometry (ICP-OES), N2 adsorption–desorption, Raman spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). As the molybdenum precursor, NH4Mo7O24 increased the solution acidity and promoted polymerization of VOx in liquid phase. At a low vanadia loading, polymerization of VOx, the adsorption rate of NH3 and transformation of active intermediate species NH2 were enhanced by adding MoOx, while at a high vanadia loading, the steric hindrance became dominant leading to the formation of crystalline V2O5 and MoO3 and caused decline in SCR activity.

Evidences for Different Reaction Sites for Dehydrogenation and Dehydration of Ethanol over Vanadia Supported on Titania

Vanadia supported on titania were prepared with increasing vanadia loadings up to 20 wt % to study the effect of the vanadia loading on catalytic reactivity. A competitive decomposition of ethanol into ethylene (dehydration) and acetaldehyde (dehydrogenation) over the catalyst occurs over the vanadia supported on TiO2. Dehydrogenation is generally favored over dehydration over a wide range of the vanadia loadings up to 20 wt % due to a lower energy barrier for the dehydrogenation channel. Both dehydration and dehydrogenation rates are enhanced in proportion to the vanadia loadings up to about 2 wt %. At higher vanadia loadings (>2 wt %), however, the dehydration rate decreases while the dehydrogenation rate saturates. X‐ray photoelectron spectroscopic analysis reveals that reduced V and Ti species are formed at the low vanadia contents and act as strong dissociative adsorption sites for H2O, while extended VOx clusters as well as three‐dimensional V2O5 islands formed at high vanadia loadings prevent the formation of such sites. Thus, the observed difference between the two reaction channels can be explained from the structural transition of the vanadia overlayers from highly dispersed small VOx clusters into larger polyvanadates or islands. At low vanadia loadings (<2 wt %), both the edge sites and the surface sites of the small VOx clusters grow in proportion to the loadings as the number of the clusters increases. Thus, both reaction channels are enhanced as well. At higher vanadia loadings (>2 wt %), however, the transformation into larger islands reduces the edge sites or the boundaries between the clusters and TiO, not the surface area. This implies that the active sites for the dehydrogenation are on the surfaces of the vanadia overlayers, while those for the dehydration are on the boundaries between the VOx and TiO2. Our results provide an additional insight into the active sites for dehydration and dehydrogenation reactions over the titania‐supported vanadia catalysts.

The promotion effect of ceria on high vanadia loading NH3-SCR catalysts

A high vanadia loading (5 wt%) catalyst was modified by ceria to improve the NH3-SCR performance. CeVO4 was generated after the addition of ceria to 5 wt% V2O5/TiO2, result not seen in the case of low-loading vanadia samples. CeVO4 presented stable Brønsted acid sites for NH3 adsorption and moderate redox sites for NH3 activation rather than polymeric VOx for the V-based catalyst. These properties effectively suppressed formation of N2O and NO (side reactions). Therefore, ceria preserved the low-temperature activity of V2O5/TiO2 and improved its high-temperature N2 selectivity. Furthermore, ceria promoted the resistance of V2O5/TiO2 to SO2 and H2O poisoning.

Surface chemistry and catalytic properties of VOX/Ti-MCM-41 catalysts for dibenzothiophene oxidation in a biphasic system

A series of vanadium oxide supported on Ti-MCM-41 catalysts was synthesized via the incipient impregnation method by varying the vanadia loading from 5wt% to 10, 15, 20 and 25wt%. These catalysts were characterized by a variety of advanced techniques for investigating their crystalline structure, textural properties, and surface chemistry information including surface acidity, reducibility, vanadium oxidation states, and morphological features. The catalytic activities of the catalysts were evaluated in a biphasic reaction system for oxidative desulfurization (ODS) of a model diesel containing 300ppm of dibenzothiophene (DBT) where acetonitrile was used as extraction solvent and H2O2 as oxidant. ODS activity was found to be proportional to the V5+/(V4++V5+) values of the catalysts, indicating that the surface vanadium pentoxide (V2O5) was the active phase. Reaction temperature would influence significantly the ODS efficiency; high temperature, i.e., 80°C, would lead to low ODS reaction due to the partial decomposition of oxidant. All the catalysts contained both Lewis and Brønsted acid sites but the former was predominant. The catalysts with low vanadia loading (5 or 10 wt%V2O5) had many Lewis acid sites and could strongly adsorb DBT molecule via the electron donation/acceptance action which resulted in an inhibition for the reaction of DBT with the surface peroxometallic species. The catalyst with high vanadia loading (25wt%V2O5/Ti-MCM-41) showed the highest catalytic activity and could remove 99.9% of DBT at 60°C within 60min.

Potassium poisoning of titania supported deNOx catalysts: Preservation of vanadia and sacrifice of tungsten oxide

V2O5-WO3/TiO2 catalysts were prepared by a wet impregnation method, and the deactivation by KCl of their catalytic activity for selective catalytic reduction of NOx by NH3 (NH3-SCR) was investigated. The fresh and poisoned catalysts were characterized by inductively coupled plasma (ICP), N2 adsorption, Raman spectroscopy, H2 temperature-programmed reduction, IR spectroscopy of adsorbed NH3, and NH3 oxidation. Vanadia species, which are the active sites for the SCR reaction, were turned into inert potassium vanadate, but they were partially maintained on the catalyst at a high vanadia loading. Tungsten oxide acts as a sacrificial agent that reacts with potassium to form potassium tungstate in addition to its roles in increasing the surface acidity of the catalyst and facilitating the dispersion of vanadia. The V2O5-WO3/TiO2 catalyst at a high vanadia loading exhibited the best resistance to alkali poisoning.

Vanadia–titania thin films for photocatalytic degradation of formaldehyde in sunlight

Thin films of vanadia–titania with good adhesion to the substrates have been deposited on various substrates such as glass slides, glass helix and silica raschig rings by simple sol–gel dip coating process using vanadium and titanium peroxide gel. The optimum concentration of vanadia in titania for obtaining good uniform viscous gel was found to be 0.5–4 wt% beyond which the vanadia particles disturb the gel network, resulting in the formation of a gelatinous precipitate. The films of vanadia–titania as well as the dried powder of the bulk gel were characterized by different characterization techniques. Optical characterization by UV–vis spectrophotometer showed a shift in optical absorption wavelength to the visible region that may be due to the incorporation of vanadia into titania structure. The XRD revealed the formation of anatase phase in pure titania as well as titania with up to 2% vanadia loading, whereas formation of rutile as minor phase along with anatase as major phase was observed at higher vanadia loading. The XRD did not show any peaks of vanadia phase up to 5% vanadia loading indicating either incorporation of vanadia into titania structure or high dispersion of amorphous vanadia on titania support. The pure and vanadia doped TiO 2 thin films were evaluated for their photocatalytic activity for degradation of methylene blue as a model pollutant under sunlight. Doping of V 2O 5 in TiO 2 showed an increase in the photo-degradation rate of methylene blue by a factor of 3–6.6 times compared to pure TiO 2. The highest rate has been obtained for 4% V 2O 5-doped TiO 2 films. Vanadia doped TiO 2 thin films were also found to be very active for photocatalytic degradation of formaldehyde from aqueous solution in sunlight.

Vanadium loaded carbon-based monoliths for the on-board No reduction: Influence of vanadia and tungsten loadings

Carbon-coated catalysts doped with tungsten and vanadia oxides with different V and W loadings have been prepared by the ionic exchange method and characterized. The surface, structure and composition have been investigated by XPS, Raman, N 2 sorption at 77 K, TPD-NH 3 and reactivity tests for the SCR of NO with NH 3 at low temperatures. Under reaction conditions, NO conversions were found to go through a maximum with vanadia surface coverage at approximately half a monolayer. The observed decrease in the SCR activity at higher vanadia loadings can be attributed to either a loss of dispersion or loss of textural properties. Maximum NO conversion is ascribed to the higher Brönsted proton acidity (V 4+) of the centres that decreases with increasing vanadia loadings up to 3 wt% loading due to the increase of V 4+/V 5+ ratio. Large amounts of tungsten (5%, w/w) upon or before addition of vanadia do not provide an enhancement of activity. The results indicate that W addition increases surface acidity leading to stronger Brönsted or even Lewis acid centre creation.

Vanadium Oxides Supported on a Thin Silica Film Grown on Mo(112): Insights from Density Functional Theory

Low-coverage vanadium oxide species (monomers and dimers) as well as larger VmOn clusters (m = 4 and 6) supported on an ultrathin SiO2/Mo(112) film are investigated by density functional theory in combination with statistical thermodynamics. At low vanadium chemical potentials, monomeric species are stable, and with increasing vanadium or oxygen chemical potentials, dimeric VOx species become stable. These monomeric and dimeric species are created by replacing Si atoms in the two-dimensional SiO2 film by vanadyl (V═O) groups. At high vanadium chemical potentials (high vanadia loading), the largest vanadia clusters considered (V6O15) are stable. The frequencies calculated for the V4O10 cluster anchored to the silica surface by two V−O(2)−Si interface bonds account for all vibrational frequencies observed for the vanadia/SiO2/Mo(112) model system. Its calculated infrared spectrum shows an intense band at 1048 cm−1 corresponding to a stretching of the V═O groups and a broad band in the 630−730 cm−1 range attributed to V−O−V vibrations, well-known for the V2O3 bulk. The characteristic vibrations of the ultrathin SiO2/Mo(112) support are infrared-inactive, because of the metal surface selection rule, which is in full agreement with the experimental observations.

Adsorption and ODH reaction of alkane on sol–gel synthesized TiO 2–WO 3 supported vanadium oxide catalysts: In situ DRIFT and structure–reactivity study

A titania-based mixed-oxide support containing 90 wt% TiO 2, 90Ti–W, and several supported vanadium oxide (vanadia) catalysts, xV90Ti–W ( x = 1–12.5 wt% vanadia), were synthesized and characterized. The anatase phase in the 90Ti–W support remained unaffected up to 1073 K. The presence of vanadia, however, leads to an appearance of the rutile phase and a decrease in the catalyst surface area. The xV90Ti–W catalysts possess surface vanadium and tungsten oxide dispersed species at moderate calcination temperatures. At high vanadia loadings, some interaction between the surface vanadium and tungsten oxide species may exist though bulk supported oxides were not detected. During ethane and propane adsorption, at relatively low adsorption temperatures, acetaldehyde and acetone species were observed, which then oxidized to other adsorbed oxygenated species at higher temperatures. Propane oxidative dehydrogenation (ODH) reaction studies revealed that the propene yield measured at the same conversion increased with increasing loading up to 10 wt% vanadia and then decreased. Analysis of the kinetic parameters showed that the rate constant ratio of propene formation to propene combustion, k 1/ k 2, which represents the propene yield at the same conversion, also increased with vanadia loading. The 10V90Ti–W catalyst was the best amongst this series of catalysts for the propane ODH reaction in terms of activity and propene yield.

Selective catalytic oxidation of H2S using nonhydrolytic vanadia-titania xerogels

A series of vanadia-titania (V-Ti) xerogel catalysts were prepared by nonhydrolytic sol-gel method. These catalysts showed much higher surface area and total pore volumes than the conventional V2O5-TiO2 xerogel. Two species of surface vanadium in the xerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Xerogel catalysts from the nonhydrolytic method showed very high conversion of H2S without harmful emission of SO2. The conversion of H2S increased with increasing vanadia loading up to 10V-Ti; however, it decreased at higher vanadia loading (12V-Ti and 18V-Ti) probably due to the formation of crystalline V2O5.

V2O5-WO3/TiO2-SiO2-SO42− catalysts: Influence of active components and supports on activities in the selective catalytic reduction of NO by NH3 and in the oxidation of SO2

The V2O5-WO3 catalysts loaded on the Ti-rich TiO2-SiO2-SO42− prepared by the coprecipitation method were investigated for the influences of the active components and the supports on the activities in SCR of NO by NH3 and in the oxidation of SO2 in comparison with a commercial V2O5-WO3/TiO2-SO42− catalyst. The physico-chemical properties of the catalysts were characterized by BET, XRD, IR, Raman, NH3-TPD, XPS and acidity measurements. The incorporation of sulfate to TiO2-SiO2 considerably increases the acidity of TiO2-SiO2, especially Brønsted acidity of the corresponding catalyst, resulting in the remarkable increase of the SCR activity and simultaneously, the addition of WO3 significantly enhances the SCR activity. The SCR activity changes depending on the calcination temperature of TiO2-SiO2-SO42−, which influences its acidity, surface area and titania crystallinity. The higher SCR activity was observed at high vanadia loadings above 2wt.% as compared with the V2O5-WO3/TiO2-SO42− catalyst. This seems to be due to the fact that polymeric vanadates on the catalyst, which cause the oxidation of NH3 to N2O, are almost absent as evidenced by Raman analysis. The SO2 oxidation activity is increased by elevating the calcination temperature of TiO2-SiO2-SO42−, which shifts the oxidation state of vanadium species to the higher state, and the activity is slightly enhanced by WO3 but not changed by sulfate. The tendency of the increase in the SO2 oxidation activity with increasing V2O5 loadings is significantly smaller than the V2O5-WO3/TiO2-SO42− catalyst. This was attributed to the lower oxidation state of vanadium species, almost the absence of polymeric vanadates responsible for the SO2 oxidation and also a more strongly inhibiting effect of ammonia on the oxidation of SO2.

Characterization and reactivity of Al 2O 3–ZrO 2 supported vanadium oxide catalysts

Vanadium oxide catalysts with V 2O 5 loadings ranging from 2.5 to 20 wt.% supported on Al 2O 3–ZrO 2 (1:1 wt.%) mixed oxide have been prepared by wet impregnation method. The calcined samples were characterized by X-ray diffraction (XRD), BET surface area, pulse oxygen chemisorption, electron spin resonance (ESR), temperature-programmed reduction (TPR) of H 2, X-ray photoelectron spectroscopy (XPS), and UV–vis diffuse reflectance spectroscopy (UVDRS). The catalytic properties have been evaluated for vapor phase ammoxidation of toluene to benzonitrile. Dispersion of vanadia was determined by the pulse oxygen chemisorption method at 643 K. Vanadia is found to be present in a highly dispersed state at lower loadings and dispersion decreases steadily with increase of vanadia loading. The ESR spectra obtained under ambient conditions shows the presence of V 4+ in tetrahedral symmetry. The TPR results show a single reduction peak corresponding to V 5+→V 3+. XPS results reveal that vanadium is present in a fully oxidized state (+5) in all the samples. The intensity ratio V 2p 3/2/Al 2p 3/2 and V 2p 3/2/Zr 3d 5/2 is found to increase with increase in vanadia loading up to 12.5 wt.% and levels off at higher vanadia loadings. The UV–vis diffuse reflectance spectra indicate the existence of isolated and clusterized tetrahedral (Td) V 5+ species in all catalysts. Ammoxidation activity increases with vanadia loading up to 12.5 wt.%, which corresponds to monolayer coverage and remains constant at higher vanadia loadings. The catalytic activity in ammoxidation of toluene to benzonitrile has been correlated to the oxygen chemisorption sites.

Vapor phase oxidation of 4-fluorotoluene over vanadia–titania catalyst

The vapor phase oxidation of 4-fluorotoluene has been carried out over vanadia–titania catalysts with moderate conversion and selectivity for 4-fluorobenzaldehyde. Two series of V 2O 5/TiO 2 catalysts with 1–10 mol% vanadia were prepared by sol–gel technique using vanadium and titanium peroxide as vanadia and titania precursors respectively and by impregnation technique using vanadium peroxide on anatase titania support. The samples were characterized by X-ray diffraction, NH 3-TPD, FT-IR and BET surface area measurements. The XRD of the catalysts prepared by impregnation technique showed retention of the anatase titania whereas the catalysts prepared by sol–gel technique showed the formation of rutile titania with minor amount of anatase phase at lower vanadia content (1–3%), which completely transformed into anatase phase at higher vanadia loading. The samples prepared by sol–gel method showed higher acidity and surface area compared to the samples prepared by impregnation. Pyridine adsorption study by FT-IR revealed the presence of Lewis acidity at lower vanadia loading (1–3%) and presence of both Lewis as well as Bronsted acidity at higher vanadia loading. The catalytic activity for oxidation of 4-fluorotoluene increased with vanadia loading in the sol–gel catalysts. The catalysts prepared by impregnation technique were found to be less active. However the selectivity for 4-fluorobenzaldehyde decreased with increase in vanadia content. The influence of vanadia loading, reaction temperature and contact time on the catalytic activity for 4-fluorotoluene oxidation has been investigated. The structure of the catalyst and its catalytic activity has been correlated.

The roles of redox and acid–base properties of silica-supported vanadia catalysts in the selective oxidation of ethane

The redox and acid–base characters of silica-supported vanadium or alkali-modified vanadium catalysts by varying vanadium loading were studied with the methods of H 2-TPR, NH 3-TPD and CO 2-TPD combining other structural characterization techniques of ESR, UV–vis spectroscopy. The effects of these properties on their catalytic performances for the ethane oxidation by oxygen were investigated. For both the unpromoted V/SiO 2 (V:Si = x:Si) and promoted Cs–V/SiO 2 (Cs:V:Si = 1: x:100) systems, the reduction extent and the reducibility increase with increasing vanadia loading and the presence of cesium enhances the reduction extent. The structures of the vanadyl species have large effect on the vanadia reducibility and the isolated surface vanadyl species are less reducible than the polymeric vanadyl species including microcrystalline vanadia. For the samples with high vanadia loading (≥2.0%), the reducibility is high and thus the redox characteristic of the catalyst is one of the most important factors that govern the catalytic reactivity. In the samples with low vanadia loading (<0.5%), the reducibility is low. Therefore, the basicity or acidity characteristics of the catalyst become the major factors that control the catalytic reactivity. The cesium promoted samples Cs–V/SiO 2 (2.0 and 10.0%) exhibit high reducibilities, which qualitatively suggests that their oxygen mobility is very high and may result in deep oxidation of ethane. In contrast, those samples with low reducibility are selective for the oxidation of ethane including ODH to ethylene and oxygenate formation.

VO x species on alumina at high vanadia loadings and calcination temperature and their role in the ODP reaction

Vanadium species formed on the alumina support after calcination between 773 and 973 K at vanadium loadings between 4.45 and 13.2 wt.% were characterized with different methods ( 51 V NMR, ESR, H 2-TPR and in situ UV–vis-DRS) and tested in the oxidative dehydrogenation of propane. By calcination at 873 K, beside dispersed isolated and polymerized V 5+ surface species mainly in tetrahedral coordination, a crystalline AlVO 4 phase was formed. The portion of vanadium present as AlVO 4 increases with increasing vanadium loadings (e.g. about 60 at 13.2 wt.%). The domain size of the polymerized V 5+ species is only weakly influenced by the vanadium loadings at calcination up to 873 K. It is suggested that both isolated and polymerized tetrahedrally coordinated V 5+ species are active in propane dehydrogenation. The AlVO 4 phase and bulk-like V 2O 5 have only a low catalytic activity. The highest propene selectivity was obtained for the sample with the highest dispersion of the isolated and polymerized tetrahedrally coordinated V 5+ species.

Dispersion and reactivity of V 2O 5 catalysts supported on Al 2O 3–ZrO 2

Vanadium oxide catalysts with V 2O 5 loadings ranging from 2.5 to 20 wt% supported on Al 2O 3–ZrO 2 (1:1 wt%) mixed oxide have been prepared by wet impregnation method. The calcined samples were characterized by X-ray diffraction, BET surface area, pulse oxygen chemisorption, and temperature programmed desorption (TPD) of NH 3. Dispersion of vanadia was determined by the pulse oxygen chemisorption method at 643 K. At lower loadings, vanadia is found to be present in a highly dispersed state and dispersion decreases steadily with increase in vanadia loadings. X-ray diffraction results also suggest that vanadium oxide exists in a highly dispersed amorphous state at lower loadings and in a crystalline V 2O 5 phase at higher vanadia loadings. The effect of vanadia loading has been studied on the ammoxidation of toluene to benzonitrile over V 2O 5/Al 2O 3–ZrO 2 catalysts. The catalytic activity for the ammoxidation of toluene increases with vanadia loading up to 12.5 wt%, which corresponds to monolayer coverage and levels off at higher vanadia loadings. The correlation of catalytic activity with oxygen chemisorption sites and ammonia TPD measurements was discussed.

Studies on catalytic functionality of V2O5/Nb2O5 catalysts

Abstract A series of V 2 O 5 /Nb 2 O 5 catalysts with V 2 O 5 loadings ranging from 2 to 12 wt.% of V 2 O 5 were prepared by wet impregnation method. These catalyst samples were characterized by X-ray diffraction (XRD), oxygen chemisorption, electron spin resonance (EPR), temperature-programmed desorption (TPD) of ammonia, and scanning electron microscopy (SEM). The catalytic properties of these catalysts were tested for vapor phase ammoxidation of toluene to benzonitrile and compared with more traditional supports, such as Al 2 O 3 , SiO 2 and TiO 2 . Dispersion of vanadia was determined by the static oxygen chemisorption method at 640 K on the samples prereduced at the same temperature. At low vanadia loadings ( 2 O 5 phase at higher loadings of V 2 O 5 . The EPR spectra obtained under ambient conditions for the catalysts show the presence of V 4+ ions in a distorted tetrahedral symmetry. SEM results suggest that at low vanadia loadings vanadia is well dispersed and at higher loadings dense clusters are formed. The results of temperature programmed desorption of ammonia indicate that the acidity increases with vanadia loading up to 6 wt.% V 2 O 5 and decreases at higher vanadia loadings. The ammoxidation activity also increases with vanadia loading up to 6 wt.%, which corresponds to monolayer coverage and remains constant at higher vanadia loadings. The catalytic properties during ammoxidation of toluene are correlated to the oxygen chemisorption sites and acidic sites.

Characterization and reactivity of vanadium oxide catalysts supported on niobia

A series of V 2O 5/Nb 2O 5 catalysts with vanadia loading varying from 2 to 12 wt.% were prepared and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), BET surface area and oxygen chemisorption at 640 K. The catalytic properties have been evaluated for vapor phase ammoxidation of 3-picoline to nicotinonitrile. XRD results suggest the formation of β-(Nb,V) 2O 5 phase at higher loadings. TPR profiles showed two peaks: the low temperature peak is due to reduction of surface vanadia species and the high temperature peak is due to reduction of Nb 2O 5. XPS results reveal that both vanadia and niobia are present in fully oxidized state (5+) in all the samples. The intensity ratio V 2p 3/2:Nb 3d 5/2 is found to increase with increase in vanadia loading up to 6 wt.% and to remain constant at higher vanadia loadings. The oxygen uptake increases with increase of vanadia loading on niobia, whereas the dispersion of vanadia decreases. The dispersion of vanadia measured by oxygen chemisorption method is in good agreement with the dispersion determined from XPS. The ammoxidation activity increases with vanadia loading up to 6 wt.%, which corresponds to monolayer coverage and remains constant at higher vanadia loadings. The catalytic properties during ammoxidation of 3-picoline are related to the oxygen chemisorption sites.

Vapour phase ammoxidation of toluene over vanadium oxide supported on Nb2O5–TiO2

Vanadium oxide catalysts with V2O5 loadings ranging 1 to 9% (w/w) supported on 1∶1 wt% Nb2O5–TiO2 mixed oxide have been prepared by the wet impregnation method. The calcined samples were characterized by X-ray diffraction (XRD), pulse oxygen chemisorption and temperature programmed desorption (TPD) of NH3. Dispersion of vanadia was determined by oxygen chemisorption at 643 K in a dynamic method. At low V2O5 loadings, vanadia is found to be present in a highly dispersed state. X-Ray diffraction results also suggest that vanadium oxide exists in a highly dispersed state at lower loadings and in a crystalline V2O5 phase at higher vanadia loadings (5 wt% and above). The dispersion of vanadia was found to decrease with V2O5 loading due to the formation of V2O5 crystallites. The catalytic properties were evaluated for the vapor phase ammoxidation of toluene to benzonitrile and correlated with characterization results.

Synthesis, characterization and ethanol partial oxidation studies of V 2O 5 catalysts supported on TiO 2–SiO 2 and TiO 2–ZrO 2 sol–gel mixed oxides

Sol–gel derived titania based mixed oxides TiO 2–SiO 2 and TiO 2–ZrO 2 were used as supports for preparing a series of catalysts with vanadia contents varying between 1–25 wt.%. The supports and the catalysts were characterized by employing 51 V and 1 H solid-state MAS NMR, diffuse reflectance FT-IR, and BET surface area measurements. The partial oxidation activities of the catalysts were tested using ethanol oxidation as the model reaction. 51 V solid-state NMR studies of the calcined catalysts showed the peaks corresponding to the presence of polymeric tetrahedral vanadia species at low vanadia loadings and 51 V chemical shifts corresponding to amorphous V 2O 5 like phases were observed in the catalysts with high V 2O 5 contents. After outgassing the catalysts, a broadening of the peaks was observed indicating a change in the coordination environment around vanadium. DRIFTS studies of the catalysts indicated the vibrations corresponding to VO bonds of V 2O 5 agglomerates, in the catalysts with high vanadia loadings. Acetaldehyde was obtained as the major product with traces of ethylene, ether, acetic acid, ethyl acetate, and CO x during the ethanol partial oxidation activity studies of mixed oxide supported vanadia catalysts.