Vanadium Oxide Catalysts Research Articles

ChemInform Abstract: Vanadium Oxide Catalysts Supported on Laser-Synthesized Titania Powders: Characterization and Catalytic Activity in the Selective Reduction of Nitric Oxide.

Abstract Laser-activated pyrolysis was applied to the synthesis of ultrafine TiO2, producing a non porous monocrystalline material in anatase phase with high specific surface area and uniform particle size. This powder was used as support for the preparation of vanadia catalysts with different compositions by wet impregnation. The pure TiO2 and the vanadia catalysts were characterized by X-ray diffraction, transmission electron microscopy, Fourier transform IR, X-ray photoelectron spectroscopy, ammonia temperature-programmed desorption and were tested in the reaction of nitric oxide reduction by ammonia. It has been proposed that two different VOx species formed in the submonolayer region: a VIV containing species which prevails up to 6 wt.-% V2O5 and a VV containing species which develops from 6 wt.-% V2O5 up to monolayer completion. The structure of these species has been hypothesized. A reaction scheme has been proposed to interpret the catalytic activity for the selective catalytic reduction (SCR) of NOx. The selectivity of catalysts for nitrogen is related to the prevalence of the VIV containing species.

Ethylbenzene Dehydrogenation in the Presence of CO2 over MCF-Supported Vanadium Oxide Catalysts

A series of vanadia catalysts supported on mesocellular silica foam (MCF) with a V content ranging from 2% to 10% were studied with respect to their performance in the dehydrogenation of ethylbenzene (EB) to styrene in the presence of CO 2 . The structural and textural characterization of these catalysts was done using N 2 adsorption, X-ray diffraction, and temperature-programmed reduction. These catalysts were found to be effective for the dehydrogenation reaction and the 6%V/MCF catalyst showed the highest activity. The MCF-supported vanadia catalysts exhibited far higher activity than their conventional V/MCM-41 counterparts, which can be attributed to the higher reducibility and better diffusion of reactants and products in the former catalysts. A higher conversion was obtained during EB dehydrogenation with CO 2 than with N 2 . This is due to the oxidative dehydrogenation of EB with the help of oxygen that originates from CO 2 as well as the coupling of EB simple dehydrogenation with the reverse water-gas shift reaction.

Characterization and reactivity of sol–gel synthesized TiO 2–Al 2O 3 supported vanadium oxide catalysts

A 90 wt% TiO 2 in a titania–alumina mixed oxide support (90Ti–Al) was synthesized by the sol–gel method and characterized. Several 90Ti–Al supported vanadium oxide (vanadia) catalysts, xV90Ti–Al, were also synthesized, characterized and probed by the propane oxidative dehydrogenation (ODH) reaction. For the 90Ti–Al support and low vanadia loadings, no rutile peak was observed even after calcination at 1073 K. For vanadia loading greater than 7.5 wt%, rutile peaks were observed at 923 K and higher calcination temperatures. Characterization and reactivity studies suggest that molecularly dispersed, reducible surface vanadia species are present and active for the propane ODH reaction. However, the presence of alumina on the support surface decreased the catalytic activity relative to only titania. The activity variation with calcination temperature had a pronounced effect on the activity of some catalysts and is related to the presence/absence of the surface vanadia species and the surface Al/Ti ratio, both of which change with calcination temperature.

New silver- and vanadium-containing multimetal oxides for oxidation of aromatic hydrocarbons

Carboxylic acids and carboxylic anhydrides such as benzoic acid, maleic anhydride, phthalic anhydride or pyromellitic anhydride are produced industrially by catalytic gas-phase oxidation of aromatic hydrocarbons such as benzene, o-xylene, naphthalene, toluene or durene in fixed-bed multitube reactors. Although the processes for the oxidation of o-xylene and/or naphthalene to phthalic anhydride have been studied very intensively for decades, there still is a need for improved catalysts with a selectivity to phthalic anhydride which is higher than 82 mol%. We have found a novel multimetal oxide catalyst of the formula Ag a− b M b V 2O x which allow significantly higher phthalic anhydride yield, since the new silver vanadium oxide catalysts can oxidize o-xylene significantly more selectively to form phthalic anhydride or intermediates than by using catalysts of the vanadium oxide/anatase type. By combining the new silver vanadates with conventional V 2O 5/TiO 2 catalysts, a PA yield of more than 84 mol% could be reached.

V Loading Effect on V2O5/ZrO2 Catalysts for Oxidative Desulfurization

Vanadium oxide catalysts were prepared by incipient wetness impregnation of ammonium metavanadate (AM) on an amorphous zirconia with different vanadium loading, in order to increase the performance in oxidative desulfurization (ODS) of dibenzothiophene compounds present in diesel fuel. In addition, to evaluate the preparation method effect, V2O5/ZrO2 catalysts were prepared by coprecipitation of alkoxide precursors of V and Zr, and by impregnation of AM on a support prepared by the sol−gel method (zirconia or zirconia−alumina mixed oxide). Catalysts were characterized by several techniques: X-ray diffraction, Raman spectroscopy, temperature-programmed reduction, scanning electron microscopy−energy dispersive X-ray, and N2 adsorption−desorption. Results showed ODS performance depended upon the superficial V species, vanadium loading, and preparation method of the catalyst. The highest ODS activity was obtained with the catalyst prepared by impregnation on an amorphous zirconia and V loading close to a monolayer coverage.

Effect of Sulfates and Reduced-Vanadium Species on Oxidative Desulfurization (ODS) with V2O5/TiO2 Catalysts

Titania-supported vanadium oxide catalysts were prepared by thermal spreading and wet impregnation at different preparation conditions, to increase the oxidative desulfurization (ODS) activity of the sulfur compounds. The catalysts were modified introducing sulfates species and creating vanadium oxides partially reduced on titania. The oxidation states and reducibility of vanadium catalysts were studied by means of temperature-programmed reduction (TPR) and potentiometric titration. Catalysts were characterized by scanning electron microscopy (SEM-EDX), N2 adsorption−desorption technique, X-ray diffraction (XRD), and Raman spectroscopy. Catalytic performance was discussed in terms of vanadium species prevailing on the catalysts, which increase the reactivity of refractory hydrodesulfurization (HDS) compounds (DBT, 4-MDBT, and 4,6-DMDBT).

Operando XAS and Raman study on the structure of a supported vanadium oxide catalyst during the oxidation of H 2S to sulphur

The modification of crystalline phases of a vanadium oxide supported on mesoporous zirconium phosphate during the partial oxidation of H 2S to sulphur has been studied by using an operando Raman-GC approach and XAS in reaction conditions. The catalyst, mainly presenting crystalline V 2O 5, is transformed during the oxidation of H 2S at 200 °C, presenting crystals of V 4O 9, which is identified by the presence of a band at ca. 900 cm −1 in the Raman spectra (using a 785 nm line of an Argon ion laser) and by the presence of a pre-edge at 5469.8 eV (and a main-edge at 5482.2 eV) in XANES spectra. At the same time, it is observed a high conversion of H 2S to sulphur (the main reaction product) and SO 2 (as minority). Both activity and selectivity depend on the time on stream. In this way, the selectivity to SO 2 decreases from ca. 5 to 1% with the time on stream. This change could be explained on the basis of the nature of V-species: the initial presence of V 5+–O–V 5+ pairs and the appearance of V 5+–O–V 4+ pairs at high time on stream.

CeO 2-supported vanadium oxide catalysts for soot oxidation: the roles of molecular structure and nanometer effect

The nanometer CeO 2 powder was prepared by the method of microwave-assisted heating hydrolysis, and the nanometer CeO 2- supported or ordinary CeO 2-supported vanadia catalysts with different vanadium loadings (atomic ratios: 100V/Ce=0.1, 1, 4, 10, and 20) were prepared by an incipient-wetness impregnation method. Spectroscopic techniques (XRD, FT-IR, Raman and UV-Vis DRS) were utilized to characterize the structures of VO x /CeO 2 catalysts. The results showed that the structures of CeO 2-supported vanadium oxide catalysts depended on the vanadium loading. Isolated mono-vanadate species were present on the surface of catalysts at low V loading, and poly-vanadate species existed at medium V loading, but CeVO 4 crystallites were formed at high V loading (V 10 and V 20). The catalytic performances of these catalysts for diesel soot oxidation were investigated with temperature-programmed oxidation reaction (TPO). For soot oxidation, the poly-vanadate species were mainly active sites in ordinary CeO 2-supported vanadyl catalysts, but nanometer CeO 2 was mainly active components in nanometer CeO 2-supported vanadium oxide catalysts. Due to the nanometer effect and the good contact between the catalyst and the soot, nanometer CeO 2 exhibited a very high catalytic activity for soot combustion.

Redistribution of Supported Vanadium Oxide Catalysts by Pattern Formation

Abstract The behavior of ultra-thin (<1 ML) films of vanadium oxide on Rh(111) during the H 2 + O 2 reaction was studied in the 10 −6 -10 −4 mbar range with photoemission electron microscopy (PEEM). One observes that under reaction conditions the V-oxide condenses forming a stripe pattern of macroscopic dimensions on the surface. The pattern is characterized by an intrinsic wave length which obeys a power law dependence on the reactants pressure. The origin of the pattern is discussed within the concept of reactive phase separation, i.e. the strongly attractive interactions between V and O and/or changes in the interfacial energies caused by reduction are presumably the main driving forces for condensation.Graphical Abstract[IMAGE]

Structural characterization of vanadium oxide catalysts supported on nanostructured silica SBA-15 using X-ray absorption spectroscopy

The local structure of vanadium oxide supported on nanostructured SiO2 (VxOy/SBA-15) was investigated by in situ X-ray absorption spectroscopy (XAS). Because the number of potential parameters in XAS data analysis often exceeds the number of "independent" parameters, evaluating the reliability and significance of a particular fitting procedure is mandatory. The number of independent parameters (Nyquist) may not be sufficient. Hence, in addition to the number of independent parameters, a novel approach to evaluate the significance of structural fitting parameters in XAS data analysis is introduced. Three samples with different V loadings (i.e. 2.7 wt %, 5.4 wt %, and 10.8 wt %) were employed. Thermal treatment in air at 623 K resulted in characteristic structural changes of the V oxide species. Independent of the V loading, the local structure around V centers in dehydrated VxOy/SBA-15 corresponded to an ordered arrangement of adjacent V2O7 units. Moreover, the V2O7 units were found to persist under selective oxidation reaction conditions.

Water Effect on the Electronic Structure of Active Sites of Supported Vanadium Oxide Catalyst VOx/TiO2(001)

The interaction of water molecules with the reduced and fully oxidized surface sites of the supported vanadium oxide catalyst VOx/TiO2 has been investigated by the Periodic DFT method. It has been found that the molecular structures of the surface VOx species are radically altered when adsorbed water is involved in the redox cycle. Water dissociates spontaneously on the reduced vanadium sites forming the surface hydroxyl groups OH. The following reoxidation by gas-phase oxygen results in the formation of active sites O═VO2(OH) including both the Bronsted acid sites OH and the vanadyl oxygen V═O more reactive than on the dehydrated surface. Gas-phase oxygen, embedded on the surface under oxidation, does not take part in the formation of surface hydroxyl groups. The hydroxylation−hydration reaction path on the fully oxidized VOx/TiO2 surface has been calculated. It has been found that the recombination reaction of the two surface hydroxyl groups V—OH to form water with the following water desorption prevail...

Oxidative dehydrogenation of cyclohexane over alumina-supported vanadium oxide nanoliths

Featuring highly ordered one-dimensional nanopores, anodic aluminum oxide (AAO) makes an ideal substrate for fabrication of catalysts by atomic layer deposition (ALD). Vanadium oxide (VOx) catalysts supported on AAOs and prepared by ALD and incipient wetness impregnation are characterized by X-ray fluorescence and ultraviolet–visible (UV–Vis) spectroscopy. At low loadings (<4 V/nm 2) the supported VOx are mostly isolated monomers; polyvanadate domains are gradually formed as the surface vanadium content increases. The catalytic performance at a series of loadings (<3–32 V/nm 2), and hence different forms of VOx, for the oxidative dehydrogenation (ODH) of cyclohexane are investigated. Compared to the catalysts prepared by incipient wetness impregnation, the ALD VOx catalysts show specific activities that are between 2 and 7.5 times higher. This reflects a better dispersion of the catalytic species on the surface as synthesized by ALD. In the cyclohexane ODH reaction with the supported ALD VOx, the kinetic orders and activation energies are comparable to kinetics data reported previously for the supported VOx. The results indicate that the ALD technique can be applied as an alternative approach to synthesize the supported VOx catalysts and achieves very good dispersion even at loadings above one monolayer (8 V/nm 2). In the ODH reaction, polyvanadate sites are shown to be more active, overall, than monovanadate sites. However, numerical modeling of the reaction pathways indicates that the olefin formation rate is ∼3 times faster on monomeric VOx sites than on polymeric VOx. By comparing the ODH of cyclohexane and the oxidations of cyclohexene and benzene, we find that both the sequential path and the direct path (the direct conversion from cyclohexane to benzene) are important in the oxidation process of cyclohexane.

Structure and catalytic properties of MgO-supported vanadium oxide in the selective oxidation of cyclohexane

Abstract MgO-supported vanadium oxide catalysts with different vanadium loadings have been prepared by wet impregnation and grafting using ammonium metavanadate or VOCl 3 as precursor, respectively. The prepared catalysts have been characterized by several techniques such as XRD, FT-IR, DR-spectroscopy, BET, 51 V solid-state NMR, SEM and EDX. For comparison, three different phases of magnesium vanadate (meta-β-MgV 2 O 6 , pyro-α-Mg 2 V 2 O 7 , and ortho-Mg 3 V 2 O 8 ) have been synthesized as reference materials for the structural characterization. The catalytic activity of the prepared catalysts was evaluated for the liquid-phase oxidation of cyclohexane as a model reaction to obtain cyclohexylhydroperoxide, cyclohexanol and cyclohexanone with PCA as co-catalyst. The results show that VO x /MgO catalysts containing isolated VO 4 species either impregnated or grafted do not show considerable activity in the oxidation process. In contrary, catalysts with higher vanadium loading, which contain ortho-Mg 3 V 2 O 8 , pyro-Mg 2 V 2 O 7 and V 2 O 5 phases as surface species show conversions up to 70% and TONs up to 34,700. Compared to previously studied support materials for this catalytic system the leaching behavior is significantly improved.

Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

The molecular structure of silica-supported vanadium oxide (VOx) catalysts over wide range of surface VOx density (0.0002−8 V/nm2) has been investigated in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV−vis diffuse reflectance spectroscopy. Resonance Raman scattering is clearly observed using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the three visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VOx Raman bands (920, 1032, and 1060 cm−1) whose assignments have been controversial. The resonance Raman spectra of dehydrated VOx/SiO2 show a V═O band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of two different surface VOx species on dehydrated SiO2 even at submonolayer VOx loading. Quantitat...

Precursor Effect on the Molecular Structure, Reactivity, and Stability of Alumina-Supported Vanadia

This work studies the influence of specific vanadium precursors on alumina-supported vanadium oxide catalysts. Vanadyl sulfate, ammonium metavanadate, and vanadyl acetylacetonate were used as precursors. All catalysts have the same surface vanadium coverage (4 V atoms/nm2). The structural features of surface vanadium oxide species were studied by 51V-NMR, UV−vis, and Raman spectroscopy. The vanadium precursor determines the strength of interaction between supported VOx species and alumina. The reducibility of vanadium ions was estimated by conventional TPR/TPO cycles. The structural and electronic changes during reduction/oxidation cycles were studied by in situ Raman and UV−vis spectroscopy (TPR/TPO-Raman, TPR/TPO-UV−vis). Surface sulfate species reduce concomitantly to surface vanadium oxide species.

Surface and Bulk Aspects of Mixed Oxide Catalytic Nanoparticles: Oxidation and Dehydration of CH3OH by Polyoxometallates

The molecular structures and surface chemistry of mixed metal oxide heteropolyoxo vanadium tungstate (H(3+x)PW(12-x)V(x)O(40) with x = 0, 1, 2, and 3) Keggin nanoparticles (NPs), where vanadium is incorporated into the primary Keggin structure, and supported VO(x) on tungstophosphoric acid (TPA, H(3)PW(12)O(40)), where vanadium is present on the surface of the Keggin unit, were investigated with solid-state magic angle spinning (51)V NMR, FT-IR, in situ Raman, in situ UV-vis, CH(3)OH temperature-programmed surface reaction (TPSR), and steady-state methanol oxidation. The incorporated VO(x) unit possesses one terminal V horizontal lineO bond, four bridging V-O-W/V bonds, and one long V-O-P bond in the primary Keggin structure, and the supported VO(x) unit possesses a similar coordination in the secondary structure under ambient conditions. The specific redox reaction rate for VO(x) in the Keggin primary structure is comparable to that of bulk V(2)O(5) and the more active supported vanadium oxide catalysts. The specific acidic reaction rate for the WO(x) in the TPA Keggin, however, is orders of magnitude greater than found for bulk WO(3), supported tungsten oxide catalysts, and even the highly acidic WO(3)-ZrO(2) catalyst synthesized by coprecipitation of ammonium metatungstate and ZrO(OH)(2). From CH(3)OH-TPSR and in situ Raman spectroscopy it was found that incorporation of vanadium oxide into the primary Keggin structure is also accompanied by the formation of surface VO(x) species at secondary sites on the Keggin outer surface. Both CH(3)OH-TPSR and steady-state methanol oxidation studies demonstrated that the surface VO(x) species on the Keggin outer surface are significantly less active than the VO(x) species incorporated into the primary Keggin structure. The presence of the less active surface VO(x) sites in the Keggins, thus, decreases the specific reaction rates for both methanol oxidation and methanol dehydration. During methanol oxidation/dehydration (O(2)/CH(3)OH = 2.17, T = 225 degrees C), in situ UV-vis diffuse reflectance spectroscopy revealed that vanadium oxide is primarily present as the V(+5) cation, which reflects the Mars-van Krevelen redox mechanism and rapid reoxidation by molecular O(2). The bulk TPA Keggin structure becomes more disordered and less thermally stable as the vanadium content increases. Although surface polyaromatic carbon forms during methanol oxidation on the Keggin surfaces, its influence on the reaction kinetics seems minimal as the carbon content diminishes as the vanadium oxide content increases and the reaction temperature is raised. No relationships were found between the electronic structure (UV-vis E(g) values) and TOF(redox) and TOF(acid) (TOF = turnover frequency) kinetics, which reflect the complexity of H(3+x)PW(12-x)V(x)O(40) Keggins. The overall catalytic performance of the H(3+x)PW(12-x)V(x)O(40) Keggin materials results from a complex interplay among the presence of redox vanadium (as secondary surface VO(x) species and substituted VO(x) sites in the primary Keggin NP structure), structural disorder of the Keggin NPs, exposed surface acid and redox sites, and coke deposition. These new insights reveal that the Keggin heteropolyoxometallates are much more complex than originally thought and that care must be taken in using Keggins as model mixed metal oxide NPs in catalytic kinetic and theoretical studies because their surface and bulk structures are dynamic under the reaction conditions.

Understanding the nature of vanadium species supported on activated carbon and its catalytic properties in the aerobic oxidation of aromatic alcohols

Vanadium oxide catalysts supported on activated carbon (V/AC) with V loadings ranging from 1 to 20 wt.% were prepared by a wet-impregnation method. Various physicochemical characterization techniques, including nitrogen physisorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption (XANES and EXAFS), X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR), were employed to understand the nature of vanadium species on activated carbon. The results revealed that vanadium oxide mainly existed in a highly dispersed state for 10 wt.% or less vanadium loadings; a large amount of vanadium resulted in aggregated microcrystalline phase. Vanadium species on activated carbon surface showed a similar local coordination structure to that of NH 4VO 3 with a distorted tetrahedral symmetry at low vanadium loadings, whereas octahedral coordination was dominant at high vanadium loadings (>10 wt.%). All V/AC samples showed V 5+ as the major oxidation state. Nevertheless, V 4+ centered in a distorted tetrahedral symmetry could be detected at a vanadium loading greater than 4 wt.%. The catalytic activity for the benzyl alcohol oxidation largely depended on the dispersion, oxidation state, and local coordination of vanadium oxides on activated carbon. Highly dispersed vanadium (5+) species with a distorted tetrahedral coordination were postulated to account for the excellent catalytic performances of V/AC catalysts (TOF = 39.1 h −1).

Insights into Oxygen Exchange Between Gaseous O2and Supported Vanadium Oxide Catalysts via17O NMR

Vanadium oxide reference compounds, KVO3 and V2O5, and supported vanadium oxide catalysts (Al2O3, TiO2, and SiO2) were investigated using magic angle sample spinning 17O NMR. All samples were 17O-enriched using gas−solid exchange. Extraction of chemical shift and quadrupolar coupling information for the model compounds KVO3 and V2O5 was performed via the simulation of MAS spectra obtained in one-pulse experiments and the observations were consistent with their known bulk structures. For the supported vanadia catalysts, it was found that the oxygen exchange process is dominated by 17O signal from the catalyst oxide supports. Spectra obtained via rotor-synchronized spin echoes revealed additional wide lines for Al2O3 and TiO2 supported vanadia catalysts that arise from 17O in the surface vanadia species of the catalysts. Additional 17O−51V TRAPDOR (TRAnsfer of Populations in DOuble Resonance) experiments support this assignment. The wide lines suggest that the local environments of the 17O nuclei associated...

ChemInform Abstract: Fifty Years of My Romance with Vanadium Oxide Catalysts

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Bulk and surface structures of V 2O 5/ZrO 2 catalysts for n-butane oxidative dehydrogenation

The present investigation focuses on the structural properties and reactivity of zirconia-supported vanadium oxide catalysts, prepared by equilibrium adsorption in basic (pH 10) or in acid (pH 2.7) conditions with vanadium content up to 6 wt.% (pH 10) and up to 11.6 wt.% (pH 2.7). The samples, heated at 823 K for 5 h in air, were characterized by X-ray diffraction, Raman spectroscopy and TPR, both as prepared and after leaching with an ammonia solution to remove species not anchored to the zirconia surface. Some representative samples were also tested for the n-butane oxidative dehydrogenation (ODH) reaction. Depending on vanadium content, various vanadium species were identified by Raman spectroscopy that reacted differently on exposure to H 2. At similar loading, the fraction of vanadium in a dispersed state and thus interacting with the support was higher in samples prepared at pH 10 than in those at pH 2.7. Samples prepared at pH 2.7 contained a higher fraction of large polymeric structures in addition to ZrV 2O 7 and V 2O 5. In line with literature data for propane ODH on similar catalysts, our catalytic results suggested that the active sites for the ODH reaction are associated with the V–O–V bonds of the polymeric exposed structures, whereas the Zr–O–V sites favour alkane combustion.