Efficient room temperature oxidation of cyclohexane over highly active hetero-mixed WO3/V2O5 oxide catalyst

An efficient room temperature catalyzed oxidation of cyclohexane to cyclohexanone (K) and cyclohexanol (A) was achieved over hetero-mixed tungsten–vanadia (WO₃/V₂O₅) using H₂O₂ oxidant. WO₃/V₂O₅ exhibited high catalytic activity to initiate the free-radical oxyfunctionalization of cyclohexane to afford up to 90% conversions within 6 h. The KA selectivity was found to depend on reaction time and the amount of catalyst. The WO₃/V₂O₅ catalyst was highly recyclable with consistent catalytic activity.     

Infrared and raman characterization of V<Subscript>2</Subscript>O<Subscript>5</Subscript> on zirconia modified with WO<Subscript>3</Subscript> and activity for acid catalysis

Vanadium oxide supported on zirconia modified with WO₃ was prepared by adding Zr(OH)₄ powder into a mixed aqueous solution of ammonium metavanadate and ammonium metatungstate followed by drying and calcining at high temperatures. The characterization of prepared catalysts was performed by using FTIR, Raman, and XRD. In the case of calcination temperature at 773 K, for samples containing low loading V₂O₅ below 18 wt%, vanadium oxide was in a highly dispersed state, while for samples containing high loading V₂O₅ equal to or above 18 wt%, vanadium oxide was well crystallized due to the high V₂O₅ loading on the surface of ZrO₂. The ZrV₂O₇ compound was formed through the reaction of V₂O₅ and ZrO₂ at 873 K, and the compound decomposed into V₂O₅ and ZrO₂ at 1,073 K, these results were confirmed by FTIR and XRD. Catalytic tests for 2-propanol dehydration and cumene dealkylation have shown that the addition of WO₃ to V₂O₅/ZrO₂ enhanced both catalytic activity and acidity of V₂O₅-WO₃/ZrO₂ catalysts. The variations in catalytic activities for both reactions are roughly correlated with the changes of acidity.     

Nonlinear Optical Properties of TeO2-Based Glasses: MOx-TeO2 (M = Sc, Ti, V, Nb, Mo, Ta, and W) Binary Glasses

The third-order nonlinear optical susceptibilities, X⁽³⁾, of TeO₂-based glasses containing transition metal oxides (M = Sc₂O₃, TiO₂, V₂O₅, Nb₂O₅, MoO₃, Ta₂O₅, and WO₃) glasses have been measured by the third harmonic generation (THG) method in order to investigate the effect of the empty d-orbital contributions to the third-order nonlinear optical susceptibilities. It is found that the addition of TiO₂, Nb₂O₅, and WO₃ to TeO₂ glass increases the X⁽³⁾ value as well as the refractive index, while others decrease both of them. The positive effect of the TiO₂, Nb₂O₅, or WO₃, on the X⁽³⁾ of TeO₂ glass was interpreted in terms of the cationic empty d-orbital contribution. There is an almost linear relation between the X⁽³⁾ and the term (n²_ω+ 2)³.(n²_ω -1).E_d/E²₀ containing three measurable parameters only, irrespective of the kinds of MO, which was derived based on the bond orbital theory developed by M. E. Lines. The largest X⁽³⁾ value obtained is 1.69 × 10⁻¹² esu for 30NbO_2.5.70TeO₂ glass, about 60 times larger than that of pure fused silica glass.     

Selective catalytic reduction (SCR) of NO by NH3 over TiO2-supported V2O5–WO3 and V2O5–MoO3 catalysts

A comparison between commercial and model WO₃–V₂O₅/TiO₂ and MoO₃–V₂O₅/TiO₂ SCR catalysts is considered in this study. The data indicate that WO₃ and MoO₃ behave as “structural” and “chemical” promoters for the catalysts. MoO₃-based catalysts are more active but less selective than WO₃–V₂O₅/TiO₂ catalysts in the SCR reaction, although in the presence of water the catalytic performances of the investigated samples are comparable. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterisation by EPR spectroscopy

The fresh and used EUROCAT II V₂O₅–WO₃/TiO₂ catalysts have been studied by electron paramagnetic resonance. For both samples, V⁴⁺ in clusterlike arrangement and two sites corresponding to VO²⁺ ions in octahedral symmetry axially distorted were found. It has been shown that the TiO₂ phase transformation (anatase → rutile) occurs at lower temperature (1073 K) for the EUROCAT II catalysts than the WO₃/TiO₂ support. Impurities of Fe³⁺ ions were detected both on the catalysts and on the WO₃/TiO₂ support. Q-band measurements have allowed a difference in the environment of the Fe³⁺ ions caused by the SCR process to be observed. Finally, traces of Ti³⁺ ions in the anatase phase have been detected.     

WO3 and MoO3 addition effect on V2O5/TiO2 as promoters for removal of NOx and SOx from stationary sources

As an attempt to improve the catalytic activity at higher reaction temperatures between 300-450°C, various mole ratios of WO₃ were added to V₂O₅/TiO₂ catalytic systems. And also, in order to suggest a new mixed oxide catalyst system for simultaneous removal of NO_x and SO_x, from stationary sources, MoO₃-V₂O₅/TiO₂catalysts were prepared by a conventional impregnation method together with a newly introduced method of surface fixation (non-aqueous solution method). In case of WO₃ addition, at higher reaction temperature range (300–450°C), WO₃ and WO₃-V₂O₅/TiO₂ catalysts showed significant high conversion in NO reduction with NH₃ while V₂O₅/TiO₂ catalyst showed a significant change in selectivity mainly due to the excess side reaction of NH₃ oxidation. This difference in selectivity due to NH₃ oxidation at high temperature is supposed to be associated with the difference in values of surface excess oxygen between WO₃ and V₂O₅ on titania. The surface acidities of tested catalysts were relatively well correlated with the % conversion of NO at 400°C. In case of MoO₃ addition, the catalytic activity for the simultaneous removal of NO_x and SO_x were quite enhanced by the addition of MoO₃ into V₂O₅/TiO₂ catalysts. The enhanced activities were responsible for the formation of Mo=O bond on the intermediate species produced by solid solutions on MoO₃-V₂O₅/TiO₂ (aqueous). However, in the case of MoO₃-V₂O₅/TiO₂ (non-aqueous), the exact source of active site was not able to detect in IR spectra in spite of more enhanced activity was obtained in this study. After SO₂ contact, VOSO₄ is newly formed on the surface of catalyst, which supposed to be associated with the activity enhancement.     

Alkali resistivity of Cu based selective catalytic reduction catalysts: Potassium chloride aerosol exposure and activity measurements

The deactivation of V₂O₅–WO₃–TiO₂, Cu–HZSM5 and Cu–HMOR plate type monolithic catalysts was investigated when exposed to KCl aerosols in a bench-scale reactor. Fresh and exposed catalysts were characterized by selective catalytic reduction (SCR) activity measurements, scanning electron microscope–energy dispersive X-ray spectroscopy (SEM–EDX) and NH₃-temperature programmed desorption (NH₃-TPD). 95% deactivation was observed for the V₂O₅–WO₃–TiO₂ catalyst, while the Cu–HZSM5 and Cu–HMOR catalysts deactivated only 58% and 48%, respectively, after 1200 h KCl exposure. SEM analysis of the KCl aerosol exposed catalysts revealed that the potassium salt not only deposited on the catalyst surface, but also penetrated into the catalyst wall. Thus, the K/M ratio (M = V or Cu) was high on V₂O₅–WO₃–TiO₂ catalyst and comparatively less on Cu–HZSM5 and Cu–HMOR catalysts. NH₃-TPD revealed that the KCl exposed Cu–HZSM5 and Cu–HMOR catalysts only experienced a slight loss of acidity while the V₂O₅–WO₃–TiO₂ catalyst lost most of the acidity. High alkali resistivity seems to be characteristic of the zeolite supported SCR catalysts which thus could be attractive for flue gas cleaning in biomass plants.     

Photoconductivities in monocrystalline layered V2O5 nanowires grown by physical vapor deposition

Photoconductivities of monocrystalline vanadium pentoxide (V₂O₅) nanowires (NWs) with layered orthorhombic structure grown by physical vapor deposition (PVD) have been investigated from the points of view of device and material. Optimal responsivity and gain for single-NW photodetector are at 7,900 A W^-1 and 30,000, respectively. Intrinsic photoconduction (PC) efficiency (i.e., normalized gain) of the PVD-grown V₂O₅ NWs is two orders of magnitude higher than that of the V₂O₅ counterpart prepared by hydrothermal approach. In addition, bulk and surface-controlled PC mechanisms have been observed respectively by above- and below-bandgap excitations. The coexistence of hole trapping and oxygen sensitization effects in this layered V₂O₅ nanostructure is proposed, which is different from conventional metal oxide systems, such as ZnO, SnO₂, TiO₂, and WO₃.     

Self Assembly and Properties of C:WO3 Nano-Platelets and C:VO2/V2O5 Triangular Capsules Produced by Laser Solution Photolysis

Laser photolysis of WCl₆ in ethanol and a specific mixture of V₂O₅ and VCl₃ in ethanol lead to carbon modified vanadium and tungsten oxides with interesting properties. The presence of graphene’s aromatic rings (from the vibrational frequency of 1,600 cm⁻¹) together with C–C bonding of carbon (from the Raman shift of 1,124 cm⁻¹) present unique optical, vibrational, electronic and structural properties of the intended tungsten trioxide and vanadium dioxide materials. The morphology of these samples shows nano-platelets in WO _x samples and, in VO _x samples, encapsulated spherical quantum dots in conjunction with fullerenes of VO _x . Conductivity studies revealed that the VO₂/V₂O₅ nanostructures are more sensitive to Cl than to the presence of ethanol, whereas the C:WO₃ nano-platelets are more sensitive to ethanol than atomic C.     

Selective catalytic reduction of NO with ammonia over V2O5 doped TiO2 pillared clay catalysts

A series of vanadia doped TiO₂-pillared clay (TiO₂-PILC) catalysts with various amount of vanadia were studied for selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen. It was found that the V₂O₅/TiO₂-PILC catalysts were highly active for the SCR reaction. The catalysts showed a broad temperature window, and the maximum NO conversion was higher than that on V₂O₅/TiO₂ catalyst and was the same as the commercial V₂O₅ + WO₃/TiO₂ catalyst. The V₂O₅/TiO₂-PILC catalysts also had higher N₂/N₂O product selectivities as compared to V₂O₅ doped TiO₂ catalysts. In addition, H₂O + SO₂ slightly increased the activities at high temperatures (>350°C) for the V₂O₅/TiO₂-PILC catalysts. Addition of WO₃ to V₂O₅ further increased the activities of the PILC catalysts. These results indicate that TiO₂-PILC is a good support for vanadia catalysts for the SCR reaction. In situ FT–IR experiment indicated that both Brønsted acid sites and Lewis acid sites exist on the catalyst surface, but with a large proportion being Brønsted acid sites at low temperatures (e.g., 100°C). The reaction path for NO reduction by NH₃ on the V₂O₅/TiO₂-PILC is similar to that on V₂O₅/TiO₂ catalyst, i.e., N₂ originates from the reaction between gaseous NO and NH₃ adspecies.     

The promotional effect of MoO3 doped V2O5/TiO2 for chlorobenzene oxidation

A series of V₂O₅–MoO₃/TiO₂ catalysts prepared by an impregnation method was investigated on their catalytic performance of chlorobenzene oxidation. This study demonstrated that MoO₃ is a dopant, which can improve catalyst redox properties and enhance chlorobenzene oxidation catalytic activity at low temperature. Compared to V₂O₅–TiO₂ and V₂O₅–WO₃/TiO₂ catalysts, V₂O₅–MoO₃/TiO₂ catalysts showed outstanding activity on chlorobenzene oxidation, which is due to the promoting influence of MoO₃. For excellent catalytic performance and good N₂ selectivity with water, the V₂O₅–MoO₃/TiO₂ catalysts may have promising commercial potential for the synergetic control of dioxins and NO_x emitted from stationary sources.     

Oxygen vacancy enhanced room temperature ferromagnetism in Ar+ ion irradiated WO3 films

Room-temperature ferromagnetism in WO₃ films was enhanced by 130 keV Ar⁺ ion irradiation. The X-ray diffraction (XRD) and Raman measurements not only confirmed the monoclinic phase of the irradiated WO₃ films, but also showed that oxygen vacancy (V_O) defects were formed. The analysis of photoluminescence spectra strongly reconfirmed the presence of oxygen vacancy. X-ray photoelectron spectroscopy (XPS) measurements revealed that the contents of V_O and induced W⁵⁺ ions increase with increasing irradiation fluence and rich W⁵⁺-V_O defect complexes in the irradiated WO₃ films were formed. Further, the magnetic measurements exhibited a 2-fold enhancement in the saturation magnetization at the largest fluence of 3 × 10¹⁶ ions/cm². At lower irradiation fluence, a bound magnetic polaron model was proposed to reveal the ferromagnetic exchange coupling resulting from overlapping of V_O⁺ and V_O⁺⁺ defect states, and 5d¹ states of W⁵⁺. At high irradiation fluence, the carrier concentration reaches 1.02 × 10²⁰/cm³ and carrier-mediated exchange interactions result in the film's ferromagnetism.     

Electrical properties of V2O5–WO3/TiO2 EUROCAT catalysts evidence for redox process in selective catalytic reduction (SCR) deNOx reaction

Electrical conductivity measurements on EUROCAT V₂O₅–WO₃/TiO₂ catalyst and on its precursor without vanadia were performed at 300°C under pure oxygen to characterize the samples, under NO and under NH₃ to determine the mode of reactivity of these reactants and under two reaction mixtures ((i) 2000 ppm NO + 2000 ppm NH₃ without O₂, and (ii) 2000 ppm NO + 2000 ppm NH₃ + 500 ppm O₂) to put in evidence redox processes in SCR deNO_x reaction.It was first demonstrated that titania support contains certain amounts of dissolved W⁶⁺ and V⁵⁺ ions, whose dissolution in the lattice of titania creates an n-type doping effect. Electrical conductivity revealed that the so-called reference pure titania monolith was highly doped by heterovalent cations whose valency was higher than +4. Subsequent chemical analyses revealed that so-called pure titania reference catalyst was actually the WO₃/TiO₂ precursor of V₂O₅–WO₃/TiO₂ EUROCAT catalyst. It contained an average amount of 0.37 at.% W⁶⁺dissolved in titania, i.e. 1.07 × 10²⁰ W⁶⁺ cations dissolved/cm³ of titania. For the fresh catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved in titania were found to be equal to 1.07 × 10²⁰ and 4.47 × 10²⁰ cm⁻³, respectively. For the used catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved were found to be equal to 1.07 × 10²⁰ and 7.42 × 10²⁰ cm⁻³, respectively. Since fresh and used catalysts have similar compositions and similar catalytic behaviours, the only manifestation of ageing was a supplementary progressive dissolution of 2.9 × 10²⁰ additional V⁵⁺ cations in titania.After a prompt removal of oxygen, it appeared that NO alone has an electron acceptor character, linked to its possible ionosorption as NO⁻ and to the filling of anionic vacancies, mostly present on vanadia. Ammonia had a strong reducing behaviour with the formation of singly ionized vacancies. A subsequent introduction of NO indicated a donor character of this molecule, in opposition to its first adsorption. This was ascribed to its reaction with previously adsorbed ammonia strongly bound to acidic sites. Under NO + NH₃ reaction mixture in the absence of oxygen, the increase of electrical conductivity was ascribed to the formation of anionic vacancies, mainly on vanadia, created by dehydroxylation and dehydration of the surface. These anionic vacancies were initially subsequently filled by the oxygen atom of NO. N^o atoms, resulting from the dissociation of NO and from ammonia dehydrogenation, recombined into dinitrogen molecules. The reaction corresponded to

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. In the presence of oxygen, NO did not exhibit anymore its electron acceptor character, since the filling of anionic vacancies was performed by oxygen from the gas phase. NO reacted directly with ammonia strongly bound on acidic sites. A tentative redox mechanism was proposed for both cases.     

An HREM study of the WO3/TiO2 monolayer catalyst system. Proposals for the overlayer structure

High resolution electron microscopy (HREM) has been used to characterise the WO₃/TiO₂ (anatase) catalyst system. Comparisons between pure samples of TiO₂ in the uncalcined and calcined states with titania in the catalyst (loaded with 10 wt% WO₃) indicate that the tungsten oxide overlayer preserves the surface roughness of the support (as observed in the pure uncalcined material). The calcination of pure anatase results in significant grain growth and surface smoothing. In the electron microscope, the tungsten oxide overlayer is revealed as 2D pseudo-hexagonal shaped clusters which appear to be epitaxially related to the support. After noting that the anatase support predominantly exposes {112}, {011}, {110} and (001) type facets we have combined this information with structural data on the overlayer derived from a previous EXAFS and XANES study by Hilbrig et al. that reported the presence of WO₄ species and six-coordinate WO₅ groups. By considering the arrangement of terminating oxygen atoms on each of the aforementioned anatase surfaces, we suggest ways in which the WO _x species may be linked together to form the tungsten oxide overlayer. This approach has led us to conclude that, with the exception of the (001) surface, the overlayer may consist of WO₄ dimers rather than chains of linked WO₅ groups terminated by WO₄ species.     

Sodium doped-V2O5 nanorods for visible light irradiated photocatalytic performance for the degradation of Rh-dye

The sodium doped vanadium pentaoxide (Na.V₂O₅) nanorods were synthesized through hydrothermal method. The sodium (Na) enriched V₂O₅ nanorods with different concentration of Na varied from 1 wt%, 3 wt% and 5 wt%. X-ray diffraction, field emission scanning electron microscopy, UV–Visible spectroscopy and fourier transform infrared spectroscopy were used to investigate the structural, morphological and optical properties of the synthesized pure V₂O₅ and Na-doped V₂O₅ (1, 3 and 5 wt%). The microstructural studies confirmed the formation of nanorods structure for Na doped V₂O₅. The photocatalytic performance of these synthesized photocatalysts were studied to degrade Rhodamine B dye in visible light region. Their efficiency to perform the photodegradation was improved in all Na doped V₂O₅ samples (1, 3 and 5 wt%) than that of pure V₂O₅ nanorods. Among all concentration of Na in V₂O₅ nanorods, the Na.V₂O₅ (5 wt%) exhibited the maximum degradation of the rhodamine dye having 88.9% degradation with rate constant 0.0619 min^?1 of a reaction.     

The reactivity of V2O5-WO3-TiO2 catalyst supported on a ceramic filter candle for selective reduction of NO

For realizing the environmental issues and constituting an economical treatment system, a catalytic filter based on V₂O₅/TiO₂ supported on tubular filter elements has many advantages by removing NO_x and particulate simultaneously from flue gas. In order to improve the activity of a catalytic filter based on V₂O₅/TiO₂ supported on a commercial high temperature filter element (PRD-66), the promoting effects of WO₃ were investigated in an experimental unit. PRD-66 presented very good properties for SCR catalyst carrier since it contains much active material such as A1₂O₃ SiO{om2}, and MgO whose contributions were remarkable. For additional catalyst carrier, TiO₂ particles were coated in the pores of PRD-66 with relatively good distribution of the particle size less than 1 μm, by a coating process applying centrifugal force. WO₃, in the V₂O₅-WO₃-TiO₂/PRD-66 catalytic filter system, increased the SCR activity significantly and broadened the optimum temperature window. The catalytic filter shows the maximum NO conversion of more than 95% for NO concentration of 700 ppmv at face velocity of 0.02 m/sec, which is comparable to the current commercial catalytic filters of plate form.     

Electrochemical reaction of lithium with orthorhombic bismuth tungstate thin films fabricated by radio-frequency sputtering

Bi₂WO₆ thin films with fast deposition rate have been fabricated by radio-frequency (R.F.) sputtering deposition, and are used as positive electrodes in rechargeable thin film lithium batteries. An initial discharge capacity of 113 μAh/cm²-μm is obtainable for Bi₂WO₆ film electrode with good capacity reversibility. A multiple-center reactive mechanism associated with both Bi³⁺/Bi⁰ and W⁶⁺/W^x+(x < 6) is investigated by ex situ X-ray diffraction patterns, transmission electron microscopy and selected area electron diffraction techniques, apart from the direct comparison of Bi₂WO₆ electrochemical performance with those of Bi₂O₃ and WO₃ thin films. A possible explanation about smooth capacity loss of Bi₂WO₆ after long-term cycling is suggested from the incomplete reaction of Bi component. The advantages of Bi₂WO₆ thin films over the singer-center Bi₂O₃ or WO₃ thin films are shown in both the aspects of volumetric capacity and cycling life.     

Influence of the V2O5 Loading on the Structure and Activity of V2O5/TiO2 SCR Catalysts for Vehicle Application

In this paper the effect of the vanadium oxide loading on the surface vanadia structure and the activity as well as selectivity in the catalytic reduction of NO with NH₃ was studied for a V₂O₅/TiO₂ model system. A series of TiO₂ (WO _x stabilized anatase) supported vanadia catalysts with varying loadings were characterized by laser Raman spectroscopy, ⁵¹V MAS-NMR, V K XANES. To determine the acidic properties, DRIFTS measurements were done with pyridine adsorbed on the samples. The measurements indicate that with increasing active phase loading square pyramidal coordinated surface vanadia species are replaced by an amorphous highly dispersed vanadium oxide phase with a coordination like V₂O₅. In addition, the ratio of Brønsted to Lewis acid sites is shifted from a comparatively low to an equal level at high loadings. This structural change is accompanied by a clearly improved catalytic activity and selectivity.     

Novel insights into electrical transport mechanism in ionic‐polaronic glasses

Transformation of electrical transport from ionic to polaronic in glasses, which are a potential class of new cathode materials, has been investigated in four series containing WO₃/MoO₃ and Li⁺/Na⁺ ions, namely: xWO₃–(30?0.5x)Li₂O–(30?0.5x)ZnO–40P₂O₅, xWO₃–(30?0.5x)Na₂O–(30.5x)ZnO–40P₂O₅, xMoO₃–(30?0.5x)Li₂O–(30?0.5x)ZnO–40P₂O₅, and xMoO₃–(30?0.5x)Na₂O–(30?0.5x)ZnO–40P₂O₅, 0 ≤ x ≤ 60, (mol%). This study reports a detailed analysis of the role of structural modifications and its implications on the origin of electrical transport in these mixed ionic‐polaron glasses. Raman spectra show the clustering of WO₆ units by the formation of W–O–W bonds in glasses with high WO₃ content while the coexistence of MoO₄ and MoO₆ units is evidenced in glasses containing MoO₃ with no clustering of MoO₆ octahedra. Consequently, DC conductivity of tungstate glasses with either Li⁺ or Na⁺ exhibits a transition from ionic to polaronic showing a minimum at about 20‐30 mol% of WO₃ as a result of ion‐polaron interactions followed by a sharp increase for six orders of magnitude as WO₃ content increases. The formation of WO₆ clusters involved in W‐O‐W linkages for tungsten glasses plays a key role in significant increase in DC conductivity. On the other hand, DC conductivity is almost constant for glasses containing MoO₃ suggesting an independent ionic and polaronic transport pathways for glasses containing 10‐50 mol% of MoO₃.     

Performance impact and poisoning mechanism of arsenic over commercial V2O5–WO3/TiO2 SCR catalyst

The performance impact and poisoning mechanism of arsenic on commercial V₂O₅–WO₃/TiO₂ SCR catalysts were studied. XRD, BET, FT-IR and XPS were used to characterize its physical–chemical properties. The results showed that As₂O₃ would be adsorbed on the catalyst surface and most oxidized by surface chemisorption oxygen to ultimately form As₂O₅. Lattice oxygen of V₂O₅ may also be transformed into surface sorption oxygen to oxidize As₂O₃, but tungsten and titanium species showed no changes in this progress. The creating As₂O₅ formed dense layer on catalyst surface, preventing NH₃ adsorption and active sites recovery, which finally resulted in catalytic deactivation.