Supported Vanadium Oxide Research Articles

Isolated Vanadium Surface Complexes on Aluminum Fluoride ‐ A Model for the Relevance of Oxygen Atoms of Aluminum Oxide Supports in Catalytic Oxidation Reactions

AbstractThe redox properties and reactivity of oxidation catalysts of isolated mononuclear oxovanadium complexes supported on aluminum fluoride, AlF3, and on aluminum oxide, Al2O3, were compared. AlF3 is an interesting model for a non‐oxide support to study the role of oxygen atoms of nonreducible oxide supports during catalytic oxidation with supported vanadium oxide catalysts. Solid‐state 1H NMR indicate the presence of reactive FH and OH groups on the surface of AlF3 and the presence of reactive OH groups on the surface of Al2O3. Oxovanadium(V) triisopropoxide, VO(OiPr)3, was grafted onto the surfaces of both supports. Immobilization led to mononuclear vanadium complexes with oxidation states of +V (major) and +IV (minor). In contrast to alumina, the vanadium surface species were neither reducible nor oxidizable on AlF3. Redox cycles were studied by electron paramagnetic resonance of vanadium(IV). In situ IR spectroscopic investigations showed high and comparable initial stoichiometric reactivity to propane for both catalysts. Reactions on AlF3 stopped after a short initial period as a result of the absence of surface oxygen of the support. The results indicate the relevance of surface oxygen of the alumina support in oxidation reactions. In contrast to vanadium on alumina, VOx/AlF3 is catalytically inactive in the oxidative dehydrogenation of propane. Only after partial oxidation of AlF3 to Al2O3 above 500 °C was catalytic activity observed. The investigations support a recent report contradicting common oxygen‐exchange models, which assume the involvement of only surface vanadium oxide layers, and thus it appears as though a more complex process is in operation.

Electronic Structure of Oxygen Radicals on the Surface of VOx/TiO2Catalysts and Their Role in Oxygen Isotopic Exchange

The electronic structure of oxygen radicals formed by adsorption of gas-phase oxygen on partially reduced sites of supported vanadium oxide catalyst V4+Ox/TiO2 has been studied by periodic DFT. The unpaired electron density in the radicals is transferred from the paramagnetic V4+(3d1) ion to the adsorbed oxygen atoms resulting in the formation of surface oxygen radicals: atomic O–, superoxide O2–, and ozonide O3–. These radical species exhibit higher reactivity compared to the surface oxygen species stabilized on fully oxidized diamagnetic V5+(3d0) ions. Oxygen isotopic exchange over O– radicals has been investigated by the climbing image nudged elastic band (CI-NEB) method. We show that molecular oxygen can exchange with the lattice oxygen of the surface paramagnetic radicals V5+O– with low activation energy of about 14 kcal/mol, close to the value experimentally observed for some heterolytic R1 oxygen exchange reactions on vanadia catalysts. The obtained data suggest that O– radicals formed as short-liv...

Catalysis science of supported vanadium oxide catalysts

Supported vanadium oxide catalysts contain a vanadium oxide phase deposited on a high surface area oxide support (e.g., Al2O3, SiO2, TiO2, etc.) and have found extensive applications as oxidation catalysts in the chemical, petroleum and environmental industries. This review of supported vanadium oxide catalysts focuses on the fundamental aspects of this novel class of catalytic materials (molecular structures, electronic structures, surface chemistry and structure-reactivity relationships). The molecular and electronic structures of the supported vanadium oxide phases were determined by the application of modern in situ characterization techniques (Raman, IR, UV-vis, XANES, EXAFS, solid state (51)V NMR and isotopic oxygen exchange). The characterization studies revealed that the supported vanadium oxide phase consists of two-dimensional surface vanadia sites dispersed on the oxide supports. Corresponding surface chemistry and reactivity studies demonstrated that the surface vanadia sites are the catalytic active sites for oxidation reactions by supported vanadia catalysts. Combination of characterization and reactivity studies demonstrate that the oxide support controls the redox properties of the surface vanadia sites that can be varied by as much as a factor of ~10(3).

Anomalous reactivity of supported V2O5 nanoparticles for propane oxidative dehydrogenation: influence of the vanadium oxide precursor

The oxidative dehydrogenation (ODH) of propane to propylene by supported vanadia catalysts has received much attention in recent years, but different reactivity trends have been reported for this catalytic reaction system. In the present investigation, the origin of these differing trends are investigated with synthesis of supported V/SiO2, V/TiO2, and V/Al2O3 catalysts prepared with three different vanadium oxide precursors (2-propanol/vanadyl triisopropoxide [VO(O-Pri)3] (VTI), oxalic acid/ammonium metavanadate [NH4VO3] (AMV), and toluene/vanadyl acetylacetonate [VO(C5H7O2)2] (VAA)) in order to elucidate the influence of the precursor on supported vanadia phase and propane ODH activity. In situ Raman spectroscopy revealed that the choice of vanadium precursor does not affect the dispersion of the supported vanadium oxide phase below 4 V nm(-2) (0.5 monolayer coverage), where only isolated and oligomeric surface VO4 species are present, and only the AMV precursor favors crystalline V2O5 nanoparticle (NP) formation below monolayer coverage (8 V nm(-2)). The propane ODH specific reactivity trend demonstrated that there is no significant difference in TOF for the isolated and oligomeric surface VO4 sites. Surprisingly, V2O5 NPs in the ∼1-2 nm range exhibit anomalously high propane ODH TOF values for the supported vanadia catalysts. This was found for all supported vanadium oxide catalysts examined. This comparative study with different V-precursors and synthesis methods and oxide supports finally resolves the debate in the catalysis literature about the dependence of TOF on the surface vanadium density that is related to the unusually high reactivity of small V2O5 NPs.

CVD Derived Vanadium Oxide Nano-Sphere-Carbon Nanotube (CNT) Nano-Composite Hetero-Structures: High Energy Supercapacitors

We present herein a facile two-step chemical vapor deposition (CVD) approach to synthesize carbon nanotube (CNT) supported vanadium oxide nanosphere nano-composite hetero-structures as electrochemical supercapacitors. The high electronic conductivity of CNTs has been exploited to provide a 3-D electron transport medium on which vanadium oxide nano-droplets interact with electrolyte to exhibit excellent supercapacitor behavior in aqueous media. Thin nano-scale droplets of vanadium oxide deposited on the CNTs exhibit excellent capacitance values as high as 1400 Fg−1 with good rate capability and excellent charge retention up to 200 cycles. The electrochemical mechanisms and materials properties responsible for the supercapacitor properties of CNT supported vanadium oxide have been discussed at length.

On the Nature of Surface Vanadium Oxide Species on Carbons

Vanadium is a key component for selective and environmental catalysts. Supported vanadium oxide catalysts find many industrial applications for selective oxidation, ammoxidation, and reduction reactions for bulk and environmental processes. The properties of molecularly dispersed vanadium oxide species depend strongly on the nature of the support. Carbon materials are a particularly valuable kind of catalytic support (Rodriguez-Reinoso, F. Carbon1998, 36, 159), especially activated carbons prepared from biomass waste valorization, which are a very useful and economical material. In addition, if the carbon material presents phosphorus groups at its surface, as have been reported previously in the literature, then this material would be made resistant under oxidizing environments. Thus, this activated carbon is a very useful material for catalytic applications. This work provides molecular information about the nature of vanadium oxide species dispersed on activated carbon supports. As far as we know, ther...

Selective Conversion of Furfural to Maleic Anhydride and Furan with VOx/Al2O3 Catalysts

Furfural can be converted into maleic anhydride (73 % yield) through selective gas phase oxidation at 593 K with O(2) by using VO(x)/Al(2)O(3) (10 at(V) nm(-2)) as solid catalysts. The use of lower temperatures and/or O(2) pressures result in the additional formation of furan (maximum 9 % yield). Mechanistically, furfural (C(5)H(4)O(2)) is oxidized stepwise to furan (C(4)H(4)O), 2-furanone (C(4)H(4)O(2)), and finally, maleic anhydride (C(4)H(2)O(3)). The specific structure of the supported vanadium oxides and reaction conditions (temperature and reactants pressures) all influence furfural oxidation catalysis. We have found that Al(2)O(3)-supported polyvanadates are intrinsically more active (2.70 mmol h(-1) g-at V(-1)) than monovanadates (VO(4)) and V(2)O(5) crystals (0.89 and 0.70 mmol h(-1) g-at V(-1), respectively) in maleic anhydride and furan formation rates (553 K, 1.6 kPa furfural, 2.5 kPa O(2)). Our alternative approach enables the use of biomass instead of petroleum to synthesize maleic anhydride and furan from furfural. The potential variety of industrial applications is of enormous interest for the development of future biorefineries.

Oxydehydrogenation of Propane over Vanadium Oxide Supported on Kieselguhr or MCM-41

Supported vanadium oxide (5 wt%) on either Kieselguhr or mesoporous MCM-41 was prepared using impregnation method and tested as a catalyst in propane oxidative dehydrogenation (POD). The catalyst samples were characterized using X-ray elemental analysis, Brunauer-Emmett-Teller (BET) physisorption, and Z-ray Photoelectron Spectroscopy (XPS). After impregnation, the catalyst surface area decreased compared with that of the support. More drastic decrease was observed in the case of MCM-41 (77%) than the Kieselguhr supported sample (48%). There are also different degrees of vanadium oxide-support interaction as reflected by the XPS result. Si-O binding energy of 531.5 eV was observed on MCM-41-supported sample compared with 529.5 eV for the Kieselguhr-supported sample. The catalyst tests were conducted at atmospheric pressure, with a propane to oxygen ratio of 0.7 - 3.6 and a reaction temperature of 400 - 700 °C. Oxidative dehydrogenation and combustion products were observed. Minor cracking reaction products (methane, ethane, and ethene) were also produced above 550 °C. The highest propene yield of 14% was obtained from the Kieselguhr-supported sample at 700 °C and with a C3 H8 /O2 ratio of 1.5.

Lignocellulosic-derived mesoporous materials: An answer to manufacturing non-expensive catalysts useful for the biorefinery processes

The synthesis procedure and characterization of a mesoporous carbon obtained from biomass waste by a simple method, generating a low-cost material, with the corresponding waste revaluation are described. The use of phosphoric acid during the activation of the waste material induces the presence of stable phosphate groups, with acid characteristics, on the surface of the mesoporous carbon, which could find many applications as an acid solid catalyst. In addition, such phosphate groups increase the stability of the carbon material under oxidizing conditions at relatively high temperatures, increasing the range of catalytic applications in which this material could be used. This paper also shows how the redox functionality can be incorporated on the surface of this carbon material by impregnation with vanadium oxide species. The corresponding carbon supported vanadium oxide catalyst is active and relatively selective in the propylene partial oxidation to oxygenates. It is also discussed how the catalytic behavior of such vanadium-containing catalysts can be improved by the use of Zr as dopant.

Characterization and Quantification of Reduced Sites on Supported Vanadium Oxide Catalysts by Using High‐Frequency Electron Paramagnetic Resonance

AbstractVOx (1.4–1.7 V nm−2) supported on SBA‐15, Al2O3, or TiO2 was studied before and after exposure to oxidative dehydrogenation of propane (ODP), and pure hydrogen or propane. After treatment, samples were quenched and frozen in quartz vials and characterized by using high‐frequency electron paramagnetic resonance (HF‐EPR). For SBA‐15‐ and Al2O3‐supported vanadia, V4+ sites were the most abundant paramagnetic species, whereas Ti3+ was dominant in TiO2‐supported V2O5. For the quantification of paramagnetic reduced sites, Mn2+ was used as reference. The maximum relative numbers of reduced V4+ or Ti3+ sites were found to increase in the sequence SBA‐15 (11 % V4+/V)<Al2O3 (16 % V4+/V)<TiO2 (21 % Ti3+/V) if hydrogen was used as a reducing agent. Using propane as a reductant, the percentage ratios were found to be SBA‐15 (8 % V4+/V), Al2O3 (18 % V4+/V), and TiO2 (27 % Ti3+/V). The corresponding V4+/V or Ti3+/V percentage ratios for samples exposed to ODP were lower, at 0.08, 9, and 17 %, respectively. The number of paramagnetic non‐V4+ or non‐Ti3+ sites was negligible for Al2O3 and SBA‐15 and in the range of 12–37 % for TiO2‐supported vanadia. The number of reduced sites after oxidation in air and storage in sealed vials is negligible, which indicates that all reduced sites can generally be reoxidized. After reaction in an oxygen‐lean ODP atmosphere, a structureless HF‐EPR pattern was observed for SBA‐15‐supported vanadia, which is tentatively attributed to the formation of two‐dimensional polymeric structures of vanadia. For Al2O3‐supported vanadia, this loss of spectral resolution was less pronounced, which indicates a more defined environment of V4+ sites. The results are discussed in terms of a two‐parameter model, rationalizing the dual influences of reducibility and local structure of redox‐active sites on the ODP reaction.

Synthesis of C4 olefins from n-butane over a novel VOx/SnO2–ZrO2 catalyst using CO2 as soft oxidant

Oxidative dehydrogenation (ODH) of n-butane was investigated over ZrO2, SnO2 and SnO2–ZrO2 mixed oxide supported vanadium oxide catalysts with the aim of utilizing CO2 as the soft oxidant, and to study the effect of mixed oxide support on the ODH activity and C4 olefins selectivity. The acid–base properties, reducibility, surface area and morphology of the SnO2–ZrO2 mixed oxide and its individual component oxide supported vanadium oxide samples were thoroughly studied. The mixed oxide supported vanadia sample exhibited superior acid–base bifunctionality; in particular, more numbers of medium strength acid–base sites were observed. The reduction temperature of the vanadia was observed to decrease in the case of mixed oxide supported vanadia catalysts. The mixed oxide supported vanadia sample exhibited a high conversion and product selectivity than its individual component oxide supported vanadia samples. A significant difference in the catalyst activity was noted in the presence and absence (He) of CO2 feed gas. Among various catalysts evaluated, the VOx/SnO2–ZrO2 catalyst exhibited excellent performance (22.34 and 36.63% conversion of n-butane and selectivity of C4 olefins, respectively), which is attributed to a high specific surface area, superior acid–base bifunctionality and easily reducibility of the dispersed vanadium oxide.

Aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran on supported vanadium oxide catalysts: Structural effect and reaction mechanism

The structure–activity relationship and reaction mechanism for selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) in toluene were studied on vanadium oxide domains on TiO2, Al2O3, Nb2O5, ZrO2, and MgO and with a wide range of VOx surface densities. The structures of these catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance UV–vis spectroscopy (UV–vis DRS), and Raman spectroscopy, and their reducibility was probed by H2-temperature programmed reduction. The structures of the VOx domains evolved from monovanadate to polyvanadate structures with increasing the VOx surface densities, and finally to crystalline V2O5 clusters at surface densities above one-monolayer capacity. Within one-monolayer capacity, higher VOx surface densities and more reducible supports led to higher reducibility and reactivity of the VOx domains. The support surfaces covered with polyvanadates and V2O5 clusters and the supports with acidity favored the formation of DFF. The correlation between the reducibility and reactivity, together with the kinetic studies, suggests that the HMF oxidation to DFF proceeds via the redox mechanism involving the V5+/V4+ redox cycles and the reoxidation of V4+ to V5+ by O2 as the rate-determining step. These results may provide guidance for the design of more efficient catalysts for the HMF oxidation to synthesize DFF.

Design Strategies for the Molecular Level Synthesis of Supported Catalysts

Supported catalysts, metal or oxide catalytic centers constructed on an underlying solid phase, are making an increasingly important contribution to heterogeneous catalysis. For example, in industry, supported catalysts are employed in selective oxidation, selective reduction, and polymerization reactions. Supported structures increase the thermal stability, dispersion, and surface area of the catalyst relative to the neat catalytic material. However, structural and mechanistic characterization of these catalysts presents a formidable challenge because traditional preparations typically afford complex mixtures of structures whose individual components cannot be isolated. As a result, the characterization of supported catalysts requires a combination of advanced spectroscopies for their characterization, unlike homogeneous catalysts, which have relatively uniform structures and can often be characterized using standard methods. Moreover, these advanced spectroscopic techniques only provide ensemble averages and therefore do not isolate the catalytic function of individual components within the mixture. New synthetic approaches are required to more controllably tailor supported catalyst structures. In this Account, we review advances in supported catalyst synthesis and characterization developed in our laboratories at Northwestern University. We first present an overview of traditional synthetic methods with a focus on supported vanadium oxide catalysts. We next describe approaches for the design and synthesis of supported polymerization and hydrogenation catalysts, using anchoring techniques which provide molecular catalyst structures with exceptional activity and high percentages of catalytically significant sites. We then highlight similar approaches for preparing supported metal oxide catalysts using atomic layer deposition and organometallic grafting. Throughout this Account, we describe the use of incisive spectroscopic techniques, including high-resolution solid state NMR, UV-visible diffuse reflectance (DRS), UV-Raman, and X-ray absorption spectroscopies to characterize supported catalysts. We demonstrate that it is possible to tailor and isolate defined surface species using a molecularly oriented approach. We anticipate that advances in catalyst design and synthesis will lead to a better understanding of catalyst structure and function and, thus, to advances in existing catalytic processes and the development of new technologies.

Probing a Redox Behavior of TiO2/SBA-15 Supported VxOyCatalyst Using an Electron Beam in a 200 kV Transmission Electron Microscope

Supported vanadium oxides are becoming the model catalysts for numerous selective oxidation reactions. To exploit redox chemistry of well-known titania-supported vanadia is currently one of the most fascinating topics. Herein, by applying in situ electron energy-loss spectroscopy, we succeeded in stimulating a new redox reaction in a routine transmission electron microscope; the electron beam can boost further oxidation from an amorphous lower-oxidation-state vanadia to maximal V2O5 with an expense of oxygen from its support of TiO2 to be reduced. Our findings could be extended into a new strategy: through utilizing an electron beam, and probing the redox chemistry between catalysts and their support in a wide class of heterogeneous catalyst systems, such as various oxide/oxide, metal/oxide, and oxide/metal catalysts.

Conceptual chemistry approach towards the support effect in supported vanadium oxides: Valence bond calculations on the ionicity of vanadium catalysts

The concept of bond ionicity, obtained via a valence bond analysis, is invoked in the interpretation of the catalytic activity of supported vanadium oxides, in analogy with previous work conducted within the framework of conceptual DFT. For a set of model clusters representing the vanadium oxide supported on SiO 2, Al 2O 3, TiO 2, ZrO 2, the ionic character of the vanadium-oxygen bond, involved in the dissociative adsorption of methanol on the catalyst, was quantified. Detailed scrutiny shows that this ionicity increases from the Al through the Zr support, in agreement with the increasing catalytic activity through this series; the case of the Si supported oxide is found to be an exception however, giving rise to the most ionic V–O bond of the different compounds studied. This finding is confirmed by calculations on smaller clusters focusing on detail in the π back bonding.

Surface Optimization and Redox Behavior of Vanadium Oxides Supported on γ-Al2O3

This report describes optimization of the synthesis of γ-Al2O3 supported vanadium oxides using three different vanadium precursors: OV(OiPr)3, OV(OEt)3 and OV(OPr)3. It was observed that the actual loading of the metal oxide is controlled by a number of factors, including the kinetics and thermodynamics of the grafting process, the time allowed for the grafting reaction to proceed, the amount of available precursor, the concentration of the grafting solution, and temperature at which grafting is pursued. From TPR (temperature programmed reduction) and XPS (X-ray photoelectron spectroscopy) studies it was observed that V4+ and V5+ were the predominant oxidation states present under both normal and rigorous reducing conditions simulating those prevalent in the steam reforming process.

DR UV–vis Study of the Supported Vanadium Oxide Catalysts

A new methodology of the measurement and evaluation of DR UV–vis spectra of VOx species supported on hexagonal mesoporous silica was developed. The suggested methodology utilizes the benefit of the well resolved absorption bands in the spectra of highly diluted samples. Methodology was tested on two sets of V-materials prepared by the impregnation and direct synthesis method. It was observed that both types of materials consist of the same type of tetrahedrally coordinated monomeric and oligomeric species, but they differ in the population of these species. In particular, the octahedrally coordinated species and tetrahedral oligomeric units of higher polymerization degree were significantly more formed on the surface of V-materials prepared by impregnation method.

Electron Distribution in Partially Reduced Mixed Metal Oxide Systems: Infrared Spectroscopy of CemVnOo+ Gas-Phase Clusters

Vibrational predissociation spectra of rare-gas-tagged [(CeO(2))(VO(2))(1-2)](+) and [(Ce(2)O(3))(VO(2))](+) clusters are measured in the 400-1200 cm(-1) region. Density functional theory (DFT) is used to determine the geometric and electronic structure of low-energy isomers of the partially reduced clusters. Comparison of experimental and simulated spectra provides evidence for the larger stability of Ce(+3)/V(+5) compared to that of Ce(+4)/V(+4), which confirms that the exceptionally high reducibility of Ce(+4) accounts for the promoting role of ceria in supported vanadium oxide catalysts.

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Quantitative Chemical State Analysis of Supported Vanadium Oxide Catalysts by High Resolution Vanadium Kα Spectroscopy

Oxidation states of vanadium species on Al(2)O(3), SiO(2), and TiO(2) were quantitatively analyzed by least-squares fitting of V Kα spectra recorded with a two-crystal X-ray fluorescence spectrometer. Uncertainties of analytical results by the normalization procedure, and coefficient of validation and the reduction behavior of vanadium species by X-ray irradiation were discussed. The V(5+)/V(4+)/V(3+) ratios on Al(2)O(3), SiO(2), and TiO(2) calcined at 773 K in air were determined to be ca. 6/3/1, 3/6/1, and 5/4/1, respectively. The possible chemical states of vanadium species on supports were proposed.