Tungsten Species Research Articles

Entrapped Single Tungstate Site in Zeolite for Cooperative Catalysis of Olefin Metathesis with Brønsted Acid Site

Industrial olefin metathesis catalysts generally suffer from low reaction rates and require harsh reaction conditions for moderate activities. This is due to their inability to prevent metathesis active sites (MASs) from aggregation and their intrinsic poor adsorption and activation of olefin molecules. Here, isolated tungstate species as single molecular MASs are immobilized inside zeolite pores by Brønsted acid sites (BASs) on the inner surface. It is demonstrated that unoccupied BASs in atomic proximity to MASs enhance olefin adsorption and facilitate the formation of metallocycle intermediates in a stereospecific manner. Thus, effective cooperative catalysis takes place over the BAS-MAS pair inside the zeolite cavity. In consequence, for the cross-metathesis of ethene and trans-2-butene to propene, under mild reaction conditions, the propene production rate over WO x/USY is ca. 7300 times that over the industrial WO3/SiO2-based catalyst. A propene yield up to 79% (80% selectivity) without observable deactivation was obtained over WO x/USY for a wide range of reaction conditions.

Preparation, characterization of highly active recyclable zirconium and tin tungstate catalysts and their application in Pechmann condensation reaction

In this contribution, a facile and efficient one-step method for the preparation of mesoporous zirconium and tin tungstate with different Zr or Sn:W molar ratios. The prepared catalysts were characterized by different textural and spectroscopic techniques such as XRD, Raman spectra, SEM, EDX, TEM and N2 adsorption–desorption at − 196 °C. The surface acidity was measured by nonaqueous potentiometric titration of n-butylamine and FTIR spectra of chemisorbed pyridine. XRD results showed that all the prepared samples calcined at 400 °C are amorphous, no evidence for formation of big aggregates of WOx species on the catalyst surfaces, but all WOx species are homogeneously dispersed as small particles on the surfaces. At higher ratio of W (ZrW3 and SnW3) the excess WOx species are distributed inside the pores or block the pores by covering the necks of the pores, which leads to decrease in the specific area as evident from N2 adsorption- desorption study. Raman spectra showed that the tungstate species has WO4 distorted tetrahedral structure. The surface acidity measurement showed that the catalysts contain mixture of Lewis and Bronsted acid sites with different ratios, the maximum B/L ratio reach maximum value at molar ratio 1:2 (ZrW2 and SnW2). The results showed that zirconium tungstate samples have higher surface acidities than tin tungstate samples. The prepared catalysts showed high catalytic activities for the synthesis of 7-hydroxy-4-methyl coumarin via the Pechmann condensation reaction of resorcinol with ethyl acetoacetate. High yield (~ 94%) with 100% selectivity of 7-hydroxy-4-methyl coumarin was obtained under solvent free conditions.

Cu‐WOx‐TiO2 Catalysts by Modified Evaporation‐Induced Self‐Assembly Method for Glycerol Hydrogenolysis to 1, 3‐Propanediol

AbstractA series of Cu‐WOx‐TiO2 catalysts for heterogeneous catalysis with different molar ratios of copper and tungsten species loadings were prepared by modified evaporation‐induced self‐assembly method. Their physical and chemical properties were characterized by X‐ray diffraction, liquid N2 sorption, Py‐IR, Raman, inductive coupled plasma, X‐ray photoelectron spectroscopy, scanning electron microscope‐energy dispersive X‐ray spectroscopy and transmission electron microscope techniques, and their catalytic performances were investigated in the hydrogenolysis of glycerol. The mesoporous framework for Cu‐WOx‐TiO2 materials can be kept well when the molar ratio of tungsten species is lower than 6%. Although the tungsten species loading is the same, the 2Cu‐6WOx‐ TiO2 sample has distinctly more Brønsted acid sites than the 2Cu‐6WOx/TiO2‐I sample prepared by impregnation method, which is owing to homogeneous tetrahedrally WOx species dispersion resulting in the formation of W5+ species. The 2Cu‐6WOx‐TiO2 sample has a higher catalytic performance than other samples, and the glycerol conversion of 12.7%, 1, 3‐Propanediol selectivity of 32.3% and the ratio of 1, 3‐Propanediol to 1, 2‐Propanediol of 1.17 can be obtained. The 2Cu‐6WOx‐TiO2 sample shows good catalytic reusability, whose glycerol conversion 12.1% and 1, 3‐PDO selectivity 30.8% can be reached even though after repeated reaction for 5 times.

Tungsten doped mesoporous SBA‐16 as novel heterogeneous catalysts for oxidation of cyclopentene to glutaric acid

Novel heterogeneous tungsten species in mesoporous silica SBA‐16 catalysts based on ship‐in‐a‐bottle methodology are originally reported for oxidizing cyclopentene (CPE) to glutaric acid (GAC) using hydrogen peroxide (H2O2). For all W‐SBA‐16 catalysts, isolated tungsten species and octahedrally coordinated tungsten oxide species are observed while WO3 crystallites are detected for the W‐SBA‐16 catalysts with Si/ W = 5, 10, and 20. The specific surface areas and the corresponding total pore volumes decrease significantly as increasing amounts of tungsten incorporated into the pores of SBA‐16. Using tungsten‐substituted mesoporous SBA‐16 heterogeneous catalysts, high yield of GAC (55%) is achieved with low tungsten loading (for Si/W = 30, ~13 wt%) for oxidation of CPE. The W‐SBA‐16 catalysts with Si/W = 30 can be reused five times without dramatic deactivation. In fact, low catalytic activity provided by bulk WO3 implies that the highly distributed tungsten species in SBA‐16 and the steric confinement effect of SBA‐16 are key elements for the outstanding catalytic performance.

Highly increased breakdown potential of anodic films on aluminum using a sealed porous layer

The growth of a uniform barrier-type anodic film on aluminum is usually terminated by electric breakdown, which is controlled by the resistance of electrolyte or anion concentration. In this study, highly resistive porous layers have been introduced by anodizing aluminum in sulfuric acid electrolyte followed by boiling water treatment to examine their influence on the electric breakdown potential. The pores of the porous alumina film are sealed by forming hydrated alumina (pseudo-boehmite) after the boiling water treatment. The breakdown potential increases to over 1500 V for the pore-sealed aluminum specimens on anodizing in sodium tungstate electrolyte. The electrochemical impedance spectroscopy measurements revealed an increased resistance of the porous layer after the pore-sealing treatment. GDOES depth profile analysis disclosed that the sealed porous layer impedes the incorporation of tungsten species into the barrier layer. The introduction of a highly resistive layer that also suppresses the anion incorporation on aluminum is effective in increasing the breakdown potential of anodic films.

Structure evolution of mononuclear tungsten and molybdenum species in the protonation process: Insight from FPMD and DFT calculations

In this work, we apply static density functional theory (DFT) calculations, as well as classical and first-principles molecular dynamics (FPMD) simulations, using the free-energy perturbation method to study the protonation ability, active site and structures of W(VI) and Mo(VI) in acidic aqueous solution. Using FPMD simulations, utilizing the pKa's calculation technique, we concluded that the octahedral WO2(OH)2(H2O)2 is the true formula for tungstic acid (H2WO4), and the hydroxyl ligands are the acidic site. This aqueous structure of H2WO4 is analogous to the previously reported structure of molybdic acid (H2MoO4). The FPMD trajectories of the tungstic acid deprotonation show that the mono-protonated monotungstate ion (HWO4−) may partially exist as a five-coordinated WO3(OH)(H2O)− species except for the four-coordinated WO3(OH)− species. This result is supported by DFT calculations, with an isoenergetic point (ΔE = 1.9 kcal·mol−1) for the WO3(OH)(H2O)− and WO3(OH)− species, when explicit solvent molecules are taken into account. In contrast, for the H2MoO4 acid, FPMD trajectories during the deprotonation process show that two H2O ligands immediately escape from the first coordinated sphere of Mo(VI) to form the four-coordinated MoO3(OH)− species. This difference indicates that structural expansion of W(VI) began in the first protonated step, while that of Mo(VI) only occurs in the second step. In addition, our calculated first and second acid constants for tungstic acid are higher than previously reported values for molybdic acid. This result suggests that WO42− is more easily protonated than the MoO42− anion in the same acidic solution, which is further confirmed by DFT calculations of hydrated oxoanions and its protonated species, based upon the hydration energy.

The roles of phosphate and tungstate species in surface acidities of TiO2-ZrO2 binary oxides – A comparison study

Porous tungstated and phosphated TiO2-ZrO2 (TZ) binary oxides with high and strong acidity were successfully prepared by means of sol-gel or impregnation approaches. In addition, the influences of the two types of modifiers on the microstructures and acidity were systematically examined, compared, and clarified. The TZ oxide derived from a surfactant-templating method exhibited a high surface area of 195 m2/g with a pore size of 6.3 nm. Moreover, it had a high acidity of 859 µmol/g with a density of 4.4 µmol/nm2 because of defective surface. Phosphation significantly increased the acidity to 1547 µmol/g and showed the highest acid density of 6.7 µmol/nm2 at a surface P density of 22.7P/nm2. On the other hand, tungstated compounds just showed the highest acidity of 972 µmol/g and the highest acid density of 4.8 µmol/nm2 at 4.7 W/nm2. Compared to tungstate species, phosphate anions are more capable of promoting the acidity because they are able to distort the host network and inhibit elemental rearrangement. While Lewis acidity prevailed in the tungstated compounds, Brønsted acidity was dominant in the phosphated oxides. The WO and POH groups were responsible for strong acidity in the modified compounds. Phosphated compounds formed strong Brønsted acid sites on the POH groups with a particular strength, and tungstation produced Lewis acid sites with a continuous strength on the metal ions adjacent to the tungstate moieties. Cyclic NH3 adsorption-desorption processes revealed that the active sites for NH3 adsorption were stable in both the tungstate and phosphate modified compounds, revealing that these solid acids are promising as the adsorbents for removal of base gases.

Hydrogen abstraction from methane on cristobalite supported W and Mn oxo complexes: A DFT study

The oxidative coupling of methane using the MnNaW/SiO2 catalyst was considered for the W-Mn two-metal-site model on the (111) surface of α-cristobalite by DFT means in the cluster approximation. The crucial step of this process, namely, the hydrogen abstraction from methane is assumed to proceed on whether the terminal WO or the MnO group. The energy barrier for the methane dissociation on the tungsten species was predicted to be much higher than that for manganese. This allows one to suggest that the Mn species are responsible for hydrogen abstraction from methane. The W species are suggested to play a role in the ethane formation on the surface from tungsten bound methoxy group and govern selective oxidative dehydrogenation of ethane.

In-depth characterization of as-deposited and annealed Fe-W coatings electrodeposited from glycolate-citrate plating bath

Fe-W coatings with 4, 16 and 24 at.% of W were electrodeposited under galvanostatic conditions from a new environmental friendly Fe(III)-based glycolate-citrate bath. This work aims to find correlations between composition including the light elements, internal structure of the electrodeposited Fe-W alloys and functional properties of material. The obtained alloys were characterized by Glow Discharge Optical Emission Spectrometry (GD-OES), Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), and X-ray Diffraction (XRD). Compositional depth profiles of 10 μm thick coatings obtained by GD-OES show that the distribution of metals is uniform along the entire film thickness, while SEM imaging depicted the presence of cracks and O- and W-rich areas inside the Fe-W coating with 4 at.% W. In the samples with 16 and 24 at.% of W, oxygen and hydrogen are present mostly at the surface about 1 μm from the top while traces of carbon are distributed within the entire coatings. With increasing W content, the structure of the coatings changes from nanocrystalline to amorphous which was shown by XRD and TEM analysis. Also, the surface of coatings becomes smoother and brighter, that was explained based on the local adsorption of intermediates containing iron and tungsten species. Annealing experiments coupled with XRD analysis show that the thermal stability of Fe-W alloys increases when the W content increases, i.e. the coating with 24 at.% W retains the amorphous structure up to 600 °C, where a partially recrystallized structure was observed. Upon recrystallization of the amorphous samples the following crystalline phases are formed: α-Fe, Fe2W, Fe3W3C, Fe6W6C, and FeWO4. Hence, the Fe-W coatings with higher W content (>25 at.%) can be considered as suitable material for high temperature applications.

Metal particles supported on SiO2-OH nanosphere: New insight into interactions with metals for cellulose conversion to ethylene glycol

The catalytic transformation of cellulose to polyols is one of the significant strategies to overcome the shortage of fossil fuels. Here we report a simple but effective preparation method for Ni-W/SiO2 catalysts by introduction of chelating agents (ammonium hydroxide and ethanediamine) coupled with organic reducing agent (ethylene glycol). The ethylene glycol (EG) yield from microcrystalline cellulose could reach up to 63.3% at 240 °C, 5.0 MPa H2 for 2.0 h over 15%Ni-20%W/SiO2-EEG catalyst. Moreover, some key parameters of process conditions including calcining temperature, reaction temperature and time, H2 pressure and the recycling use of catalysts were investigated. Both theoretical and experimental studies suggested that 15%Ni-20%W/SiO2-EEG catalyst showed the best activity and stability for cascading steps for EG formation at this reaction condition. By the detailed analysis of Ni-W/SiO2 catalysts after calcination via FT-IR, XRD, XPS, thermogravimetry (TG), TEM and element mapping, it could be concluded that on the surface of 15%Ni-20%W/SiO2-EEG catalyst, the particles showed less agglomeration and more homogeneous on the surface of SiO2. Some NixWy alloys (NiW, Ni2W4C) were also formed. Furthermore, it was found that the introduction of ethanediamine could promote the actual loading of nickel and tungsten species slightly according to the analysis of ICP-OES test.

Formation of N2O greenhouse gas during SCR of NO with NH3 by supported vanadium oxide catalysts

Selective catalytic reduction of NO by NH3 over supported vanadium oxide catalysts has been studied for decades, but the studies mostly concentrated on the dominant N2 product with much less attention paid to the formation of the undesired N2O product. In the present study, fundamental aspects of the N2O formation reaction were elucidated by a series of temperature-programmed surface reaction studies with isotopic labelled reactants. The surface vanadium oxide species on the TiO2 support are active sites for the N2O formation reaction, while tungsten species mainly function as promoters. Oxygen from NO, gaseous O2 and catalyst oxygen all function as oxygen sources for formation of N2O (∼50%, ∼30% and ∼20%, respectively). The rate-determining-step for N2O formation involves breaking of the ammonia NH bond. These new molecular level insights have the potential to guide the rational design of improved SCR catalysts for diesel engines with reduced N2O produced.

Silicotungstic acid immobilized on lamellar hexagonal boron nitride for oxidative desulfurization of fuel components

Heteropoly acids have attracted increasing attentions in oxidative desulfurization (ODS) field, but the solubility and low surface area hindered their applications. Moreover, bulk silicotungstic acid (HSiW) was less researched, owing to its relative lower activity. Herein, a layered-structure hexagonal boron nitride (h-BN) was employed for dispersion of HSiW by impregnation method. Interestingly, the immobilization process not only gave rise to the promising dispersion, but also produced valence alternation of tungsten species in HSiW. Detailed experiments found that both the factors led to a high catalytic activity in ODS, resulting in a 100% of sulfur removal and 9 times recycling performance. Additionally, the current designed ODS system showed excellent resistibility to interference substrates. Furthermore, the catalytic process and oxidizing product were further studied by gas chromatography–mass spectrometry (GC–MS).

Optimized low-temperature fabrication of WO3 films for electrochromic devices

We investigated the effect of process conditions on the electrochromic (EC) properties of tungsten trioxide (WO3) films. When WO3 films deposited using a sol-gel method were thermally treated in air at 150 °C, the majority of tungsten species in the films became W6+, which is important for the realization of an optically transparent bleached state. On the other hand, annealing in a vacuum required only 60 °C to induce a similar level of W6+ in the WO3 films. However, a cracked film morphology was observed at higher temperatures, regardless of whether the films were annealed in air or vacuum. Using the WO3 films prepared under various conditions, EC devices (ECDs) were fabricated to evaluate EC properties. We concluded that the optimal annealing conditions for WO3 films for ECDs are 60 °C in vacuum, at which the highest coloration efficiency, largest transmittance difference, and fastest bleaching/coloration dynamics were obtained. These mild fabrication conditions at a low temperature (60 °C) provide the opportunity to utilize flexible electrodes on plastic. Therefore, we successfully demonstrated a flexible WO3-based ECD.

Diallyl disulphide as natural organosulphur friction modifier via the in-situ tribo-chemical formation of tungsten disulphide

Abstract The present work shows a novel method for generating in-situ low friction tribofilms containing tungsten disulphide in lubricated contacts using diallyl disulphide as sulphur precursor. The approach relies on the tribo-chemical interaction between the diallyl disulphide and a surface containing embedded sub-micrometer tungsten carbide particles. The results show that upon sliding contact between diallyl disulphide and the tungsten-containing surface, the coefficient of friction drops to values below 0.05 after an induction period. The reason for the reduction in friction is due to tribo-chemical reactions that leads to the in-situ formation of a complex tribofilm that contains iron and tungsten components. X-ray photoelectron spectroscopy analyses indicate the presence of tungsten disulphide at the contact interface, thus justifying the low coefficient of friction achieved during the sliding experiments. It was proven that the low friction tribofilms can only be formed by the coexistence of tungsten and sulphur species, thus highlighting the synergy between diallyl disulphide and the tungsten-containing surface. The concept of functionalizing surfaces to react with specific additives opens up a wide range of possibilities, which allows tuning on-site surfaces to target additive interactions.

A nanocellulose template strategy for the controllable synthesis of tungsten-containing mesoporous silica for ultra-deep oxidative desulfurization

Nanocellulose is a sustainable and ecofriendly nanomaterial derived from renewable biomass. Herein, we developed a novel controllable method for the synthesis of tungsten-containing mesoporous silica catalysts by using nanocellulose templates. Disordered worm-like mesoporous catalysts with a high dispersion of tungsten species were obtained by this method. In particular, the as-prepared catalysts presented high specific surface areas (344–535m2/g), and their pore sizes could be efficiently tuned from 2nm to 10nm by adjusting the amount of bio-template in the synthesis system. The resultant samples were used asa catalyst for oxidative desulfurization (ODS) without any organic solvents as an extractant. The results showed that the catalysts with a large pore size (10nm) and an optimal Si/Wmol ratio (20) exhibited high catalytic activity, and dibenzothiophene could be completely removed at 60°C in 10min. The catalytic activity on different sulfur compounds increased in the order of benzothiophene<4,6-dimethyldibenzothiophene<dibenzothiophene. GC–MS analysis of the oxidation products revealed that the catalysts simultaneously acted as both a catalyst and an adsorbent. Moreover, after recycling for five times, the removal of the oxidation desulfurization system could still reach 94%. As this method is sustainable, facile and controllable, the as-prepared catalysts could have potential for practical applications.

Tungsten-doped TiO2/reduced Graphene Oxide nano-composite photocatalyst for degradation of phenol: A system to reduce surface and bulk electron-hole recombination

Recombination of photogenerated charges is the main factor affecting the photocatalytic activity of TiO2. Here, we report a combined strategy of suppressing both the bulk as well as the surface recombination processes by doping TiO2 with tungsten and forming a nanocomposite with reduced graphene oxide (rGO), respectively. Sol-gel method was used to dope and optimize the concentration of W in TiO2 powder. UV-Vis, XPS, PL and time resolved PL spectra along with DFT calculations indicate that W6+ in TiO2 lattice creates an impurity level just below the conduction band of TiO2 to act as a trapping site of electrons, which causes to improve the lifetime of the photo-generated charges. Maximum reduction in the PL intensity and the improvement in charge carrier lifetime was observed for TiO2 doped with 1 at.% W (1W-TiO2), which also displayed the highest photo-activity for the degradation of p-nitro phenol pollutant in water. Tuning of rGO/TiO2 ratio (weight) disclosed that the highest activity can be achieved with the composite formed by taking equal amounts of TiO2 and rGO (1:1), in which the strong interaction between TiO2 and rGO causes an effective charge transfer via bonds formed near the interface as indicated by XPS. Both these optimized concentrations were utilized to form the composite rGO/1W-TiO2, which showed the highest activity in photo-degradation of p-nitro phenol (87%) as compared to rGO/TiO2 (42%), 1W-TiO2 (62%) and pure TiO2 (29%) in 180 min. XPS and PL results revealed that in the present nanocomposite, tungsten species traps the excited electron to reduce the interband recombination in the bulk, while the interaction between TiO2 and rGO creates a channel for fast transfer of excited electrons towards the latter before being recombined on the surface defect sites.

Formation of Subsurface W5+ Species in Gasochromic Pt/WO3 Thin Films Exposed to Hydrogen

M/WO3 (M = Pt, Pd) systems formed by a porous WO3 thin film decorated by metal nanoparticles are known for their reversible coloring upon exposure to H2 at room temperature. In this work, this gasochromic behavior is investigated in situ by means of near-ambient photoemission (NAPP). Pt/WO3 systems formed by very small Pt nanoparticles (10 ± 1 nm average size) incorporated in the pores of nanocolumnar WO3 thin films prepared by magnetron sputtering at an oblique angle have been exposed to a small pressure of hydrogen at ambient temperature. The recorded UV–vis transmission spectra showed the reversible appearance of a very intense absorption band responsible for the blue coloration of these gasochromic films. In an equivalent experiment carried out in the NAPP spectrometer, W 4f, O 1s, Pt 4f, and valence band photoemission spectra have been recorded at various photon energies to follow the evolution of the reduced tungsten species and hydroxyl groups formed upon film exposure to hydrogen. The obtained res...

Structural Characterization of AgI–AgPO3–Ag2WO4 Superionic Conducting Glasses by Advanced Solid-State NMR Techniques

Glass samples of composition 40AgI–(60–x)AgPO3–xAg2WO4 (0 ≤ x ≤ 25 mol %) have been prepared by the conventional melt-quenching method. These glasses receive renewed interest due to their ionic conductivity and transparency in the visible range. Because the physical and optical properties of these glasses are highly dependent on composition in this system, a comprehensive structural study has been carried out using Raman spectroscopy and 1D and 2D NMR of the 31P and 109Ag nuclei. With increasing Ag2WO4 content, the network is modified from a 1D Q(2)-like chain structure to a topology in which Q(1) and Q(0) species linked to octahedrally coordinated tungsten species dominate. This structural transformation increases the glass rigidity and stability against hydrolysis reactions. The compositional evolution of the phosphate speciation (in terms of Q(n)mW units) is consistent with maximum tungstate dispersion in glasses with x ≤ 10, while for glasses with higher tungstate content the data are more consistent ...

Enhanced activity and stability of the monolithic Pt/SiO2–Al2O3 diesel oxidation catalyst promoted by suitable tungsten additive amount

Abstract A series of the modified Pt/SiO 2 –Al 2 O 3 diesel oxidation catalysts with different WO 3 loading were prepared through co-impregnation method. With an optimum additive loading of 2 wt% WO 3 , the catalyst significantly improved the performance and its stability. Compared to the free-tungsten sample, the catalyst increased the maximum conversion of NO by 5–10% and decreased the reaction temperature for 30% NO conversion by 7–28 °C. It was found that there exist the electron and structure interaction between platinum and tungsten species. The interactions can generate and maintain more Pt sites in metallic state for the catalytic oxidation with enhanced activity and stability.

Solvent Extraction of Tungsten(VI) from Moderate Hydrochloric Acid Solutions with LIX 63

The solvent extraction of tungsten(VI) from hydrochloric acid solutions using 5,8-diethyl- 7-hydroxydodecan-6-one oxime (LIX 63) was analyzed in solutions having an initial pH range from 2 to 5, by varying the concentration of metal and extractant. In our experimental range, the cationic exchange reaction as well as the solvation reaction occurred simultaneously. The cation exchange reaction was identified by applying a slope analysis method to the extraction data. The existence of cationic tungsten(VI) species was confirmed by ion exchange experiments with Diphonix resin at pH 3. Further study is needed to identify the nature of this tungsten cationic species. (Received November 23, 2016; Accepted January 10, 2017)