Tungsten Loading Research Articles

Tungsten based catalysts for oxidative desulfurization: surface species and partially reduced systems as key features to improve the activity

Tungsten oxide based catalysts for the oxidative desulfurization (ODS) process of dibenzothiophene compounds using H2O2 as oxidizing agent were studied. Catalysts were characterized by TPR-H2, SEM-EDS, XRD, N2-physisorption and Raman spectroscopy. Different surface tungsten oxide species were detected and related to the ODS activity, furthermore, the predominance of the surface tungsten oxide species was associated with the tungsten loading. Moreover, in some catalysts, WO3 crystals were detected and their role in the performance of the catalysts was studied too. Finally, the catalysts were partially reduced and tested in the ODS reaction, the reduced catalysts exhibited high activity and an improvement of H2O2 to sulfones yield.

Induced effect of tungsten incorporation on the catalytic properties of CeVO4 systems for the selective reduction of NOx by ammonia

The SCR performances of aged W-doped CeVO4 catalysts were investigated in fast-, NO2- and standard conditions. Bulk CeV1-xWxO4 catalysts were prepared in soft conditions, according to a hydrothermal synthesis route at 180 °C leading to the ad hoc tetragonal structure. The aging procedure consisted in exposing the samples to 10 vol.% H2O diluted in air at 600 °C. The impact of this aging process was investigated as a function of tungsten loading, with x varying in the range 0.02–0.15. A clear beneficial effect is distinguishable at high temperature, i.e., above 300 °C, associated to a sharp selectivity enhancement irrespective of the operating conditions and tungsten loading. On the contrary, a slight detrimental effect on the reaction rate is clearly discernible in standard-conditions below 300 °C. It was found that the thermal aging induces more extensive diffusion of V5+ species assisted by the presence of substituted W6+. It was also demonstrated that tungsten diffuses more readily than V5+ leading ultimately to a significant surface enrichment and the formation of Ce4W9O33 mixed oxide evidenced from XRD and Raman spectroscopy. Hence, tungsten would rather act as a stabilizer than promoter avoiding the sintering of monomeric VOx species to more active polyvanadate species but also less selective. The gain observed in selectivity at high temperature can be equally explained by a greater stabilization of Brønsted acid sites and the preservation of the dispersion of monomeric vanadate species preventing the occurrence of ammonia oxidation.

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.

Novel tungsten-incorporated mesoporous silicates synthesized via evaporation-induced self-assembly: Enhanced metathesis performance

Tungsten-incorporated mesostructured silicates were successfully synthesized by evaporation-induced self-assembly (EISA) method. Tungsten loadings from 2.9 to 22.2wt% were achievable, with the corresponding BET surface areas decreasing from 520 to 284m2/g at increased W loading. The W-EISA samples displayed amorphous ordered pore structure with an average pore size distribution of 2.3–3.3nm. Hydrated isolated tetrahedral [WO4]2− species and hydrated octahedral polytungstate species were identified from diffuse reflectance UV/Vis spectra under ambient conditions. Uniform distribution of W species, mostly located near the surface, was evident from XPS W 4f photoelectron image. At 450°C and 1atm, for the reaction of ethylene+2-butene metathesis to form propene, the W-EISA catalysts outperform either impregnated catalysts (WO3/SiO2 and WO3/KIT-6) or one-pot synthesized W-KIT-6 catalyst, implying better W utilization in the W-EISA catalyst.

Effect of Tungsten Addition on Nickel-Based Catalyst for Combined Steam and CO2 Reforming of Methane.

In this study, we investigated the effect of tungsten addition on Ni-based catalyst in methane steam-CO₂ reforming. All the catalysts were prepared by the co-impregnation method with varied tungsten loading. The as-prepared catalysts were characterized by inductively coupled plasma atomic emission spectrometer, X-ray diffraction, temperature-programmed reduction and thermogravimetric analysis. The tungsten-loaded catalyst showed improved coke resistance and CO₂ reactivity by the conducting activity test under fixed conditions.

Polyoxometalate based mesoporous acidic catalyst from biogenic silica for vegetable oil methanolysis: Structure activity relationship study

A superacidic mesoporous catalyst WOX/RHS-MCM-41 was synthesized using high purity biogenic silica extracted from an agro-waste, rice husk ash. Catalyst acidity was enhanced by dispersion of surface WOX groups on mesoporous support and was used for methanolysis of soybean oil and free fatty acid. Catalyst activity was found to depend upon the structure of WOX species formed over catalyst surface. Characterization with XRD, SEM, TEM, UV–vis DRS, Raman spectroscopy and acidity determination revealed that formation of highly dispersed amorphous polyoxotungstate phase on the catalyst surface developed with a specific tungsten loading and originated strong acidic sites showing high activity. Kinetic analysis revealed that pseudo-first order kinetics was followed up to the optimum reaction time with an activation energy 42.7kJmol−1. Catalyst showed high activity giving ∼90% FAME yield and reusability for multiple cycles without regeneration. The high catalytic activity was retained even in presence of free fatty acid also.

The Promoting Role of Tungsten Oxides in the Anodic Oxidation of Methanol on Platinum-Based Catalysts

Carbon-supported Pt-WO3 catalysts formulated with different proportions of platinum and tungsten were synthesized by thermal decomposition of metal carbonyl precursors. The activity of the PtWO3/C catalysts towards the methanol electrooxidation reaction (MOR) was evaluated in half-cell configuration and as anode catalyst in a direct methanol fuel cell device (DMFC) with a homemade membrane electrode assembly (MEA). The presence of WO3 increases significantly the catalyst activity, expressed by higher oxidation currents at lower potential values than those obtained with Pt/C. DMFC power output is comparable to that obtained by using a commercial MEA containing twice of the Pt loading at the anode. Electron microscopy and X-ray diffraction analysis (XRD) revealed that monoclinic WO2.92 and hexagonal WO3 phases coexist in the PtWO3/C catalysts. In samples with equal Pt loadings, the Pt particle size increases and its active area decreased as tungsten is added to the catalyst formulation. As the tungsten loading is increased and carbon content is diminished, hexagonal WO3 appears as the predominant crystalline phase.

Effect of tungsten loading on the physiochemical properties of nanocatalysts of Ni–Mo–W/carbon nanotubes for the hydrodesulfurization of thiophene

In this paper, trimetallic nanocatalysts of nickel–molybdenum–tungsten have been precipitated on multi walled carbon nanotubes and the effect of tungsten loading on structure and catalytic performance was studied. The prepared catalysts with different amounts of tungsten (0–3–6–9 wt.%) were tested as hydrodesulfurization catalyst in a stirred slurry-tank reactor at atmospheric pressure and 160 °C to eliminate thiophene. The catalyst with 6 wt.% had the highest thiophene removal and butane/butene ratio. Lower or higher loadings led to lower hydrodesulfurization activity and selectivity. To find the explanation, the prepared catalysts were examined by X-ray diffraction, field emission scanning electron microscopy, nitrogen adsorption Brunauer-Emmett-Telleranalysis, Fourier-transform infrared spectroscopy and thermogravimetric analysis. The characterization results indicated that in the catalyst with 6 wt.% tungsten, the smallest particles with average particle size of 31.9 nm and uniform distribution were observed. This catalyst showed the sharpest O–H peak in the Fourier-transform infrared analysis as well. The effect of thiophene concentration on the activity and selectivity of this catalyst was examined and the results showed less activity and lower butane/butene ratio in high thiophene concentrations. The examination of this catalyst after hydrodesulfurization also indicated no sintering and coke deposition.

Generation of Strong Acid Sites on Yttrium-Doped Tetragonal ZrO2-Supported Tungsten Oxides: Effects of Dopant Amounts on Acidity, Crystalline Phase, Kinds of Tungsten Species, and Their Dispersion

A tungsten oxide catalysts supported on yttrium- or ytterbium-doped crystalline zirconium oxide (WOx/Y(Yb)-ZrO2) with different dopant concentrations but the same tungsten loadings and calcination temperatures were prepared to investigate the mechanism responsible for the strong solid acidity of so-called tungstated zirconia. The surface acid–base properties of WOx/Y(Yb)-ZrO2 and the support were characterized by catalyses of n-butane skeletal isomerization, alkylation of anisole with benzyl alcohol, and 2-butanol decomposition. The structural characterization and quantitative analysis were conducted by XRD, XAFS, and UV–Vis techniques. Crystalline zirconium oxides were effective as the starting material for tungsten–zirconium-based strong solid acid catalysts instead of amorphous Zr(OH)x if the crystalline form was tetragonal. Tetragonal-phase Y(Yb)-ZrO2 substitutional solid solutions with dopant concentrations of 3–5 mol % was a suitable support for stabilization of active small WO3 clusters. Tungsten species aggregated to form inactive large WO3 particles on the monoclinic ZrO2 polymorph with Y-dopant amount less than 2 mol % even though the surface density of tungsten species and calcination temperature were the same as those for the active catalysts. Inactive yttrium–tungsten binary oxides formed when the yttrium doping level exceeded 10 mol %. Intrinsic role of crystalline phase of tetragonal zirconia on generating the strong solid acidity is confirmed to stabilize WO3 small clusters without aggregation.

Esterification of cetyl alcohol with palmitic acid over WO3/Zr-SBA-15 and Zr-SBA-15 catalysts

Tungsten loaded and Zr incorporated SBA-15 catalysts (WO3/Zr-SBA-15 and Zr-SBA-15) were developed for esterification of cetyl alcohol and palmitic acid. The influence of the Zr content, tungsten loading amount, calcination temperature, feed composition and catalyst amount has been studied. Higher tungsten loading decreased the acidity due to formation of WO3 crystals whereas calcination temperature enhanced the acidity by favoring the dispersion of WOx species. Activities of the catalyst changed depending on their amount of Brønsted sites and total number of acid sites. Zr-SBA-15 catalyst which had the highest amount of Brønsted acid sites gave maximum cetyl palmitate yield (63.1%). This catalyst retained its activity up to 3 reuse cycles without significant loss of activity.

Methane dehydroaromatization over modified Mn/H-ZSM-5 zeolite catalysts: Effect of tungsten as a secondary metal

The influence of tungsten (0.5–1.5wt.%) on the MDA reaction was investigated at 800°C. Addition of W to a 2Mn/H-ZSM-5 catalyst led to a decrease in methane conversion from 6 to 4%. An aromatic selectivity of 74% was obtained with the unpromoted catalyst but this decreased (i.e. from 41 to 12%) with increase in tungsten loading. Addition of Pt or Ru to a 1.5W/2Mn/H-ZSM-5 catalyst led to an increase in aromatic selectivity from 12 to 55% (Pt) or 46% (Ru) while the coke selectivity decreased from 78 to 37% (Pt) or 42% (Ru) due to the hydrogenating activity of Pt and Ru.

Effect of Dispersion of the Active Phase on the Activity and Coke Formation over WO3/SiO2 Catalysts in the Metathesis of Ethylene and 2-Butene

WO3/SiO2 catalysts with various tungsten loadings were prepared by the incipient wetness impregnation method and studied in the metathesis of ethylene and trans-2-butene as feed to produce propylene. Two different calcination methods were used to modify the dispersion of the tungsten active phase. The catalysts were characterized by X-ray diffraction, transmission electron microscopy, nitrogen physisorption, ion-exchange titration, FT-Raman, ammonia temperature programmed desorption and temperature programmed oxidation. Better dispersion is obtained from low heating rate calcination. The dispersion of tungsten showed a direct effect on coke formation and was the key factor to improve the stability of the catalysts.

Preparation and characterization of tungsten/epoxy composites for γ-rays radiation shielding

Tungsten/epoxy composites were prepared by blending epoxy resin with different weight percent of tungsten powder. The effect of filler loading on shielding and mechanical properties of composites was investigated by using two different activities of Co-60 source. And radiation degradation mechanism was studied according to the changes of free radicals concentrations, mechanical properties and thermal properties of composites. The results show that with the increment of tungsten loading, shielding property of composites increases. However, with the increase of irradiation dose, thermal stability and mechanical properties of composites decrease firstly, then increase slightly, and decline sharply at the end due to competition reaction between chain scission and cross-linking originated from γ-rays radiation.

Influence of tungsten on the activity of a Mn/Ce/W/Ti catalyst for the selective catalytic reduction of NO with NH3 at low temperatures

The influence of tungsten in the Mn/Ce/W/Ti catalyst system on the activity and SO2 induced deactivation of the catalyst was evaluated. The properties of the catalyst were evaluated using physicochemical measurements, including Brunauer–Emmett–Teller (BET) surface area, X-ray photoelectron spectroscopy (XPS), NH3 temperature-programmed desorption (TPD), X-ray diffraction (XRD), and H2 temperature-programmed reduction (H2-TPR) analyses. The Mn4+ ratio increased with the addition of Ce and W to the Mn/Ti system. The Mn/Ce/W/Ti catalyst, prepared by controlling the Mn4+ ratio through the addition of Ce and W to Mn/Ti, exhibited excellent activity in the range of 120–200°C. The catalyst efficiency was positively correlated with NH3 adsorption. The catalytic activity and surface area of the Mn/Ce/5W/Ti catalyst (containing 5wt% tungsten) were optimal, but decreased at higher tungsten loadings. Increasing the Ce3+ ratio in Mn/Ce/W/Ti increased the selective catalytic reduction (SCR) activity. Mn/Ti and Mn/Ce/Ti were rapidly deactivated upon the injection of SO2 in the SCR reaction. However, the Mn/Ce/5W/Ti catalyst exhibited excellent resistance to SO2 induced deactivation. Thus, the addition of tungsten to Mn/Ce/Ti resulted in excellent NOx conversion and SO2 resistance.

Valorisation of Vietnamese Rice Straw Waste: Catalytic Aqueous Phase Reforming of Hydrolysate from Steam Explosion to Platform Chemicals

A family of tungstated zirconia solid acid catalysts were synthesised via wet impregnation and subsequent thermochemical processing for the transformation of glucose to 5-hydroxymethylfurfural (HMF). Acid strength increased with tungsten loading and calcination temperature, associated with stabilisation of tetragonal zirconia. High tungsten dispersions of between 2 and 7 W atoms·nm−2 were obtained in all cases, equating to sub-monolayer coverages. Glucose isomerisation and subsequent dehydration via fructose to HMF increased with W loading and calcination temperature up to 600 °C, indicating that glucose conversion to fructose was favoured over weak Lewis acid and/or base sites associated with the zirconia support, while fructose dehydration and HMF formation was favoured over Brönsted acidic WOx clusters. Aqueous phase reforming of steam exploded rice straw hydrolysate and condensate was explored heterogeneously for the first time over a 10 wt% WZ catalyst, resulting in excellent HMF yields as high as 15% under mild reaction conditions.

Anisotropic tensile properties of tungsten fiber reinforced Zr based metallic glass composites

Abstract The tensile properties and deformation behaviors of Zr based metallic glass composites containing different tungsten fiber orientations were investigated. The angles (θf) between tungsten fiber orientation and loading axial direction are 0°, 15°, 30°, 45°, 60°, 75° and 90°, respectively. The results show that the strength and the failure modes vary with θf. The tensile strength of the composite decreases as the θf increases. The tensile strength of the composite at θf=90° is only 253 MPa. The plasticity of all the composites do not be improved. The composites failed approximately 90° normal fault at θf=0°/15°, while the composites failed along the tungsten fiber when the θf is bigger than or equal to 30° because of the low axial grain boundary strength of the elongated W grains.

Photocatalytic and Antistain Properties of Ceramic Tiles Functionalized with Tungsten-doped Tio2

Tiles functionalized with W-doped TiO2 film has been prepared and tested in the discoloration of aqueous solution of methylene blue (MB) used as model pollutants, in presence of UV light. The influence of tungsten and its loading on tiles surface were evaluated. Photocatalytic tests, performed at controlled temperature and atmospheric pressure, have shown that the increase in the tungsten loading strongly influences the photocatalytic activity; higher content of tungsten induces an increase of the discolouration rate. Moreover, the addition of tungsten modifies the structure of titania determining a remarkably photocatalytic activity.

Isomerization of n-hexane on heteropolyacids supported on SBA-15. 1. Monofunctional impregnated catalysts

Monofunctional hybrid materials based on Keggin-type heteropolyacids supported on mesostructured silica were used in isomerization of n-hexane. The heteropolyacids, namely H3PW12O40 or H4SiW12O40, were immobilized onto SBA-15 type silica by incipient wetness impregnation. The resulting catalysts were thoroughly characterized by N2 adsorption–desorption isotherms, XRD, 31P NMR, TGA and FT-IR. These hybrid materials were active for the gas-phase isomerization of n-hexane. Various reaction parameters and catalyst features were investigated: tungsten loading, reaction temperature, contact time, pressure and nature of the polyanion. All results could be explained by a competition of the mono- and bimolecular mechanisms. Supported tungstosilicic acid was intrinsically more selective than phosphotungstic counterpart and should be preferred for applications in isomerization of alkanes. A deactivation of the catalysts was observed due to the poisoning of acid sites by coke formation.

Synthesis, Characterization, and Epoxidation Activity of Tungsten-Incorporated SBA-16 (W-SBA-16)

Tungsten, in varying amounts, was incorporated into a SBA-16 structure via a one-pot direct synthesis method under an acidic medium using Pluronic F127 triblock co-polymer as a template and n-butanol as a co-surfactant. Tetraethyl orthosilicate (TEOS) and sodium tungstate were used as the Si and W sources, respectively. The resulting materials (denoted as W-SBA-16) are characterized for structural ordering, textural properties, and types of tungsten incorporation by techniques such as small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), N2 sorption, high-resolution transmission electron microscopy (HR-TEM), diffuse-reflectance ultraviolet–visible light (DR-UV-vis) microscopy, temperature-programmed reduction in a hydrogen atmosphere (H2-TPR), and temperature-programmed desorption of ammonia (NH3-TPD). The surface area (823–354 m2/g) and pore volume (0.71–0.44 cm3/g) of the W-SBA-16 materials are found to decrease with an increase in tungsten loading (from 2.7 wt % to 30.4 wt % of the total sample...

Physicochemical properties of WOx/ZrO2 catalysts for palmitic acid esterification

A series of WOx/ZrO2 with various tungsten loadings was prepared via incipient-wetness impregnation of zirconium hydroxide. The resulting thermally processed materials were characterised by XRD, XPS, porosimetry, NH3-TPD and pyridine FTIR spectroscopy to elucidate their composition, morphology and acidity, and subsequently tested in the esterification of palmitic acid with methanol. Catalytic performance was strongly dependent upon calcination temperature and W surface density. Esterification activity increased with increasing surface W density, reaching a maximum at 8.9Wnm−2 corresponding to near monolayer coverage. Subsequent growth of crystalline WO3 lowered activity, consistent with a decrease in the density of active surface sites. Calcination temperatures as high as 800°C increased surface acidity and hence catalytic activity. The formation of polymeric tungstate species on zirconia is necessary to generate the Brönsted acid sites responsible for palmitic acid esterification under mild conditions.