Impregnating ionic Pt species on vanadium oxide nanotubes

Vanadium oxide nanotubes (VNT) have drawn increasing attention as vanadium compounds may be used in a wide range of applications. In this contribution, VNT was synthesized by a hydrothermal treatment from V₂O₅ using hexadecylamine as template. XRD, FTIR, SEM, TEM/HRTEM and TG analyses were used to investigate their chemical stability and suitability to adsorb platinum ionic species without damaging the nanostructure. It was shown that the nanostructures are open-ended multiwalled nanotubes, formed by 13–16 layered walls, with inner diameter of 30–50 nm and outer diameter of 150–200 nm. The impregnation of Pt was found to be feasible but their structure and morphology are sensitive to the pH of the impregnation media.     

Acidic properties of vanadium oxide on magnesium fluoride

Vanadium oxides supported on magnesium fluoride obtained by the impregnation and the precipitation-deposition method have been studied by IR spectroscopy. The adsorption of pyridine indicated the presence of Lewis acid centers on the impregnated samples, whereas on the samples prepared by precipitation both Lewis and Brønsted centers have been observed. From the adsorption coefficients of the 1540 cm^–1 (Brønsted site) and 1445 cm^–1 (Lewis site) bands the numbers of both sites have been estimated. The hydroxyl groups bound in the chains of the (VO₃)_n tetrahedra or those in the trigonal pyramids VO₅ are believed to be responsible for the protonic centers, whereas the aprotonic centers occur at the ionic vanadium with coordinatively unsaturated spheres.     

Characteristics of vanadium-substituted mesoporous silica with wormhole framework structure: effects of vanadium content

Vanadium-substituted wormhole framework structure (V-WMS) mesoporous silicas (V-WMS) with various Si/V ratios in the range of 15 and 200 were prepared at ambient temperature by neutral surfactant templating pathway. The materials were synthesized by using dodecylamine as a template and tetraethylorthosilicate as a silicon source. They were characterized by energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD), N₂ adsorption–desorption, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared absorption spectroscopy (FT-IR) and ultraviolet-visible absorption spectroscopy. V-WMS samples shown characters of larger framework wall thickness, small crystallite domain sizes, and complementary textural mesoporosities in comparison with M41S materials. These mesoporous V-WMS samples exhibited irregularly shaped mesoscale fundamental particles which aggregated into larger particles. They also demonstrated better thermal stability than MCM-41. An absorption band of FT-IR at ca. 960 cm⁻¹ was assigned to the vibration of Si–O–V linkages. These samples also showed one strong UV–visible absorbance with overlapping maxima at about 255 nm. The results show that vanadium was incorporated into the structure of wormhole mesoporous silica (WMS). However, for V-WMS with high vanadium content (Si/V < 25), a broad shoulder in XRD pattern was observed at about 3–4°, suggesting the presence of impurity phase of vanadium species in the sample. The efforts in preparing V-WMS specimens by neutral-template synthesis route had led to new mesoporous silica molecular sieves with catalytically active vanadium centers.     

Long-term electrolytic hydrogen permeation in nickel and the effect of vanadium species addition

Nickel cathodes have been found to become deactivated under long-term polarization in the H₂ evolution region during alkaline water electrolysis. The cause of deactivation was examined using steady state polarization and measurement of hydrogen permeation through nickel foil in 8 mol/l KOH at 70 °C and 100 mA/cm². The long-term (over 50 h) permeation behaviour was explained by formation and growth of a nickel hydride phase. The rise in hydrogen overpotential was ascribed to an increase of the hydrogen surface coverage on the newly formed hydride. The effect of an electrolyte additive (a vanadium salt) on the hydrogen overpotential and permeation rate was also investigated. Upon addition of dissolved V₂O₅, the permeation rate was found to increase quickly and then slowly decrease to a steady value close to that measured for hydride-free nickel. Meanwhile, the hydrogen overpotential was observed to recover back to nearly its initial value for fresh nickel. The exhibited behaviour was attributed to decomposition of the hydride phase, after deposition of a vanadium-bearing compound. The prolonged contact between Ni and V was proposed as the main reason for hydride decomposition. The addition of more vanadium had no further result on the hydrogen overpotential.     

Role of vanadium oxide on the lithium silicate glass structure and properties

The structural role of V in 28Li₂O–72SiO₂ (in mol%) lithium silicate glass doped with 0.5 mol% V₂O₅ was assessed using ²⁹Si and ⁵¹V Nuclear Magnetic Resonance (NMR), Fourier-transform infrared (FTIR), and X-ray photoelectron (XPS) spectroscopy techniques. Despite the low amount of V₂O₅ used, the structural information obtained or deduced from the statistical analysis of the NMR data could explain the evolution of glass properties after V₂O₅ addition. The XPS results indicated that all vanadium exists in 5+ oxidation state. Both the ²⁹Si NMR and FTIR data point toward an increase in the polymerization of the silicate network, caused by the V₂O₅ acting as network former, capable to form various tetrahedral units (for n = 0, 1, and 2) in the glasses. These units, which are similar to phosphate units, scavenge the Li⁺ ions and cause the silicate network to polymerize. However, in an overall balance, the entire glass network is depolymerized due to the additional nonbridging oxygens contributed by the vanadium polyhedra. The addition of vanadium causes the network to expand and increases the ionic conductivity.     

Direct synthesis of vanadium incorporated three-dimensional KIT-6: A systematic study in the oxidation of cyclohexane

The direct synthesis of three-dimensional (3-D) cubic V-KIT-6 was prepared using a Pluronic P123 triblock copolymer as the structure directing agent and n-butanol as the co-surfactant. The material obtained therein showed a very high specific surface area 1000 m²/g with pore diameters that could be tuned within a narrow size distribution of 5.7–6.0 nm. After calcination, Raman and UV–vis analysis revealed the presence of V⁵⁺ species in a highly dispersed state with much less crystalline V₂O₅ formation. ⁵¹V-NMR analysis showed that the vanadium species interacted directly with the silica framework in an almost symmetrical tetrahedral environment. NH₃-TPD analysis for V-KIT-6 showed a wide distribution of acid sites at temperatures ranging from 200 to 800 °C. The calcined V-KIT-6 materials showed excellent catalytic activity in the direct oxidation of cyclohexane using dilute aqueous H₂O₂ as the oxidant.     

Photocatalytic oxidation of alkane at a steady rate over alkali-ion-modified vanadium oxide supported on silica

Photocatalysts were applied to photocatalytic oxidation of propane in a fixed bed flow reactor. Titanium dioxide exhibited fairly high activity but the deep oxidation was predominant. Silica-supported vanadium oxide (VS) favors the partial oxidation to form propanone and variety of aldehydes and the fast deactivation took place accompanied by reduction of vanadium ions. On the other hand, alkali-ion-modified VS exhibited the highest activity and high selectivity to propanone. The activity was stable because vanadates in alkali-ion-modified VS do not change their structure or are not poisoned by water molecules throughout the reaction. The cleavage of C=C double bond to form aldehydes predominantly proceeded in the case of photo-oxidation of 1-butene over VS while in the case of photo-oxidation of Rb-modified VS, the oxidation of secondary carbon atom to form methyl vinyl ketone is favored. Methyl ethyl ketone was produced at a steady rate over Rb-VS in the photo-oxidation of n-butane.     

The states of vanadium species in V-SBA-15 synthesized under different pH values

V-SBA-15 (n_Si/V ≈ 10) mesoporous molecular sieves have been synthesized in different pH-value medias and characterized by XRD, N₂ adsorption, TEM, UV–vis, Raman, FT-IR, ESR, and NH₃-absorbed in situ FT-IR techniques. Results indicated that the pH value played an important role on the states of vanadium species in the prepared materials. As the pH value increased, the amount of the crystalline vanadium oxides decreased, the incorporated vanadium species with isolated tetrahedral (V⁵⁺) and square pyramidal coordination (VO²⁺) increased accordingly. At pH = 3.0, vanadium species were mainly in the silica framework and formed stronger acid sites, it seems to suggest that the linear configuration V⁵⁺–ONH₃H⁺–O–V⁴⁺ groups have been formed after ammonia was adsorbed. However, for the samples synthesized at the low pH values, the crystalline vanadium oxides existed and the amount of the dispersed tetrahedral vanadium species decreased, which caused the weaker acid sites.     

The effect of embedded vanadium catalyst on activated electrospun CFs for hydrogen storage

In this paper, the effect of embedding vanadium pentoxide in electrospun carbon fiber was investigated in relation to textural properties and hydrogen storage behavior. Electrospun carbon fibers involving vanadium were prepared from polyacrylonitrile/N,N-dimethyl formamide/vanadium pentoxide through electrospinning method and heat treatment. Chemical activation of electrospun carbon fibers was carried out in order to generate the pore by using potassium hydroxide as a chemical agent. Eventually, vanadium embedded activated electrospun carbon fibers (AECFs) with high specific surface area 2780 m²/g were prepared as a hydrogen storage medium. As the effects of vanadium pentoxide, it was found that dissociated oxygen from vanadium pentoxide in electrospun fibers would generate the ultra-micropore (0.6 nm) by forming carbon monoxide and carbon dioxide during the carbonization. Also, vanadium in electrospun carbon fibers is considered to act as a catalyst for the improved capacity of hydrogen adsorption. Vanadium embedded AECFs have the high capacity of hydrogen storage, about 2.5 wt% at room temperature and 100 bar.     

Effects of experimental parameters on the physical properties of non-stoichiometric sputtered vanadium nitrides films

Polycrystalline vanadium nitrides thin films were deposited onto (1 0 0)-oriented silicon wafers by reactive dc planar magnetron sputtering. The influence of the nitrogen gas flow (from 0 to 15 sccm) was studied. Several substrate temperatures were investigated: 150, 400 and 650 °C. Analytical techniques including X-ray diffraction and reflectivity, atomic force microscopy and optical photospectrometry were used to characterize the structure, the morphology and the optical properties of the films. The measured thickness indicates that the deposition rate is decreased (from 3.5 Å for 0 sccm to 1.5 Å for 15 sccm) with increasing nitrogen gas flow. Obtained structures depend on the substrate temperature. The structure of pure vanadium (0 sccm) varies from amorphous phase at 150 and 400 °C to -V phase at 650 °C. The films crystallize dominantly in β-V₂N_1−x phase at low nitrogen gas flows and in δ-VN_1−x phase at high nitrogen gas flows. The as-deposited VN films were highly textured. The texture seems to depend on the nitrogen gas flow. The root mean square (rms) derived from atomic force microscopy (AFM) varies with the nitrogen gas flow. The optical reflectivity of VN films shows high values in the infrared region.     

Remarkable effect of the preparation method on the state of vanadium in BEA zeolite: Lattice and extra-lattice V species

The state of vanadium in two BEA zeolites is investigated by XRD, FTIR, DR UV–vis and EPR. One of the samples, VAlBEA (1.3 wt.% of V), is prepared by conventional ion exchange and the other, VSiBEA (2.0 wt.% of V), by a two-step postsynthesis method involving dealuminated BEA zeolite. No structural changes are observed after incorporation of vanadium into AlBEA zeolite by ion-exchange method. In contrast, the impregnation of SiBEA with V(IV) (VOSO₄) precursor leads to an increase of unit cell parameters of the BEA, to the consumption of silanol groups in vacant T-sites and incorporation of V in the framework of BEA zeolite as well dispersed tetrahedral V(V) species. NO and CO used as IR probe molecules, DR UV–vis and EPR allow to establish the oxidation state of vanadium in as prepared, oxidized, activated and reduced VAlBEA and VSiBEA zeolites. The IR spectra of oxidized, activated and reduced VAlBEA samples are very similar. It suggests that V introduced by ion exchange in extra-lattice position is stabilized on all samples in similar oxidation state. CO adsorption evidence the presence of vanadium in IV oxidation state via IR bands at about 2200 and 2180 cm⁻¹ assigned to V(IV)–CO monocarbonyl and V(IV)–(CO)₂ dicarbonyl species. In contrast, the oxidation state of V in VSiBEA changes strongly in function of calcinations in oxygen, outgassing at high temperature (773 K) and reducing with hydrogen at high temperature (873 K). This shows that lattice tetrahedral V species change easily oxidation state and this property allows them to be good candidate as active site of selective redox reactions.     

The effect of the nature of vanadium species on benzene total oxidation

The nature of the vanadium species present on V₂O₅/Al₂O₃ catalysts was investigated by using solid state ⁵¹V NMR, diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD) and temperature programmed reduction (TPR). ⁵¹V NMR and DRS analyses indicated the presence of V⁵⁺ in tetrahedral symmetry at low vanadium loading. A surface polymeric vanadium species and/or the bulk crystalline V₂O₅ were mainly observed at high vanadium loading as also detected by XRD. The positions of the absorption edges determined through the UV–VIS spectra allowed distinguishing between various tetrahedral symmetries. After TPR, the average oxidation state of vanadium depended on the vanadium content. The nature of vanadium species was related to the catalyst behavior on the benzene oxidation reaction. The catalysts containing high vanadium content were more active suggesting that a high amount of V⁴⁺ is responsible for the higher activity.     

Reversible,repeatable and low phase transition behaviour of spin coated nanostructured vanadium oxide thin films with superior mechanical properties

Smooth, uniform and crystalline vanadium oxide thin films were deposited on quartz by spin coating technique with four different rpm i.e., 1000, 2000, 3000 and 4000 and subsequently post annealed at 350, 450 and 550?°C in vacuum. Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) techniques were utilized for microstructural characterizations and phase analysis, respectively, for vanadium oxide powder and deposited film. Nanorods were observed to be grown after vacuum annealing. X-ray photoelectron spectroscopy (XPS) technique was utilized to study the elemental oxidation state of deposited vanadium oxide films. Thermo-optical and electrical properties such as solar transmittance (τ_s), reflectance (ρ_s), absorptance (α_s), infrared (IR) emittance (ε_ir) and sheet resistance (R_s) of different thin films were evaluated. Based on the optical characteristics the optimized condition of the film processing was identified to be spin coated at 3000?rpm. Subsequently, the nanoindentation technique was utilized to measure hardness and Young's modulus of the optimized film. The measured nanomechanical properties were found to be superior to those reported for sputtered vanadium oxide films. Finally, temperature dependent phase transition characteristics of optimized vanadium oxide films were studied by differential scanning calorimetry (DSC) technique. Reversible and repeatable phase transition was found to occur in the range of 44–48?°C which was significantly lower than the phase transition temperature (i.e., 68?°C) of bulk VO₂.     

The selective oxidative dehydrogenation of propane on vanadium aluminophosphate catalysts

VAPO-5 and V/ ALPO-5 catalysts have been tested for the oxidative dehydrogenation of propane. Depending on the vanadium contents and the preparation procedure, different vanadium species and different catalytic behavior are observed. The catalyst containing V⁵⁺ species with a tetrahedral coordination presents the higher yield of propene in the oxidative dehydrogenation of propane. The same yields of CO₂ are observed on all vanadium aluminophosphate catalysts, while the higher the yield of propene the lower the yield of CO is.     

钒/二溴羟基苯基荧光酮/溴化十六烷基三甲基铵体系显色反应的研究

详细研究了显色剂二溴羟基苯基荧光酮(DBHPF)在表面活性剂溴化十六烷基三甲基铵(CTMAB)存在下与钒(V)形成配合物的条件。在pH=5.0的乙酸—乙酸钠缓冲液中,在CTMAB存在下,钒(V)与DBHPF生成紫红色络合物,其最大吸收峰在562nm波长处,摩尔吸光系数ε=5.46×104L·mol-1·cm-1。钒含量在0～10mg/25ml范围内符合比耳定律。如采用适当的分离手段,可用于测定某些复杂试样中的微量钒。     

酸处理对蒙脱石结构及理化性能的影响

膨润土原矿经不同浓度的硫酸处理后得到一系列成分、结构及物化性能有差异的样品,通过对各阶段反应产物的物相结构、化学成分、显微形貌等的分析研究,以及对硫酸处理前后样品的阳离子交换容量、吸蓝量、膨胀容等性能的测定发现:硫酸用量少时,不会破坏蒙脱石的铝氧八面体结构;但随着硫酸用量的增加,溶液中的H+会不断侵蚀并破坏蒙脱石的双八...     

Spark plasma sintering of WC-based cermets/titanium and vanadium added composites: A comparative study on the microstructure and mechanical properties

In an attempt to develop the composition and properties of W₂C-(W,Ti)C-TiC and WC-WC_1-x-VC-V super hardmetals, spark plasma sintering (SPS) method was implemented. WC powders were mixed separately with 10?wt% Ti and 10?wt% V in a high energy mixer mill and sintering processes were performed at temperatures of 2150 and 2000?°C, respectively. XRD investigations revealed the formations of TiC and (Ti,W)C as the reaction products in WC-10?wt% Ti composite. Moreover, the interfacial reaction between WC and V led to the formation of WC1-x and VC compounds. A higher bending strength (613?±?25?MPa) and fracture toughness (4.1?±?0.58?MPa?m^1/2) were obtained for WC-10?wt% V samples compared to WC-10?wt% Ti, While the WC-10?wt% Ti composite showed a higher value of hardness (3128?±?42 Vickers) in comparison to WC-10?wt% V (2632?±?39 Vickers), which can act as a super hard cermet.     

Study of the effect of isomorphic substitutions in the framework of zeolites with a Beta structure on their porosity and sorption characteristics

Zeolites of a Beta structure with different degrees of substitution of aluminum atoms by those of boron in the aluminum–silicon–oxygen framework have been investigated. The characteristics of the porosity-texture of the fabricated zeolites and the surface properties and sorption capacities of samples to a model drug substance (thiamine hydrochloride) have been examined. It has been demonstrated that isomorphic substitutions could affect the properties of the zeolite under study, whereas the largest effect is associated with the simultaneous presence of aluminum and boron atoms in the zeolite framework. Such samples are characterized with a larger specific area and pore diameters, a high concentration of Lewis acidic sites on the surface, and high adsorption capacity.