Interpretation of the difference of optimal Mo density in MoS<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and MoS<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> HDS catalysts

The first part of this paper deals with the morphology of the MoS₂ phase and its oxide precursor, the MoO₃ phase, mainly from a geometrical point of view. After giving a brief review of the literature describing the structure of these compounds, Mo densities in both phases were calculated along various crystallographic planes. Further, using structural models recently proposed by others, Mo densities in MoS₂ were also calculated in the case of an epitactic growth on γ-Al₂O₃ and TiO₂ model surfaces. Then, the calculated Mo densities were compared with experimental results (Mo density when HDS activity is maximal) previously obtained for catalysts constituted of MoS₂ supported on a low SSA TiO₂, a high SSA TiO₂ and a conventional γ-alumina. It was suggested that either on alumina or titania the MoS₂ phase is growing as (100) MoS₂ planes. However, while on the alumina the optimal MoS₂ phase might be constituted of dispersed MoS₂ slabs covering only a part of the alumina surface (2.9–3.9 Mo atoms/nm²), on titania the optimal MoS₂ phase might be constituted of a uniform MoS₂ monolayer (5.2 atoms/nm² for the high SSA titania, which is equal to the Mo density of a perfect MoS₂ (100) plane). This difference may originate in the creation of a 'TiMoS' phase enhancing the S atoms mobility over Mo/TiO₂-sulfided catalysts. Indeed, while in the case of a γ-alumina carrier the active sites (labile S atoms) are located on the edge of MoS₂ slabs making the ratio Mo_edge/Mo_total a crucial parameter for the catalytic performances, in the case of a titania carrier the labile sulfur atoms might be statistically distributed all over the TiMoS active phase. Further, the higher Mo density observed over the high SSA titania (5.2 atoms/nm²) when compared to that over the low SSA titania (4.2 atoms/nm²) was supposedly due to the pH-swing method advantageously used to prepare the former carrier. Indeed, this method allows giving a solid with enhanced mechanical properties providing a good stability to the derived catalysts under experimental conditions. In addition, this TiO₂ carrier exhibits a great homogeneity, with a surface structure substantially uniform, which might be adequate for a long-range growth of (100) MoS₂ slabs.     

Electrochromic properties of WO<Subscript>3</Subscript> and WO<Subscript>3</Subscript>:P thin films

WO₃ and WO₃:P (5 mol% H₃PO₄) thin films were prepared using the sol-gel route and the electrochromic properties of the films were investigated using in situ spectroelectrochemical methods. The measurements were performed in propylene carbonate solution with 0.1 M LiClO₄ as electrolyte. During the cathodic polarization at –0.8 V a blue coloration is observed with a reversible variation between 14% and 84% of the transmittance at λ=633 nm. The kinetics for the bleaching process is faster for the WO₃:P film than for the undoped WO₃ film. Electronic Publication     

Efficient solar photocatalyst based on Ag<Subscript>3</Subscript>PO<Subscript>4</Subscript>/graphene nanosheets composite for photocatalytic decolorization of dye pollutants

Silver orthophosphate-graphene nanosheets composite (Ag₃PO₄-GNs) has been fabricated using a facile hydrothermal method. The Ag₃PO₄-GNs were characterized using XRD, UV–vis DRS and SEM. The photocatalytic activity of Ag₃PO₄-GNs was evaluated by photocatalytic decolorization of dye aqueous solutions under simulated solar light irradiation. It was observed that Ag₃PO₄ nanoparticles in Ag₃PO₄-GNs were attached on the surface of graphene nanosheets.The introduction of graphene nanosheets enhanced remarkably the visible light absorption region of Ag₃PO₄-GNs compared with bare Ag₃PO₄. The photocatalytic activity of Ag₃PO₄-GNs is nearly twice as high as that of the pure Ag₃PO₄. The removal efficiency can reach more than 90 % by Ag₃PO₄-GNs under simulated solar light irradiation within 25 min, which might mainly be attributed to high adsorption capacity, extended light absorption range and efficient charge separation. After irradiation for 60 min, 84.70 % TOC mineralization was achieved by Ag₃PO₄-GNs. Based on the results of detection of active species, the direct oxidization of dye pollutants in aqueous solution by holes takes a major role in the whole decolorization process by Ag₃PO₄-GNs. As a result, Ag₃PO₄-GNs with the high photocatalytic activity are proven to be an excellent light photocatalyst for potentially scalable removal of dyes in aqueous solutions and other environmental remediation under simulated solar light irradiation.     

Mutual spontaneous and electrosurface processes at heterophase interfaces WO<Subscript>3</Subscript>|Me<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> (Me = In,Eu, Sc)

For the first time ever it is demonstrated in this work that, in spontaneous conditions and following the imposition of an electric field, mutual penetration of components of WO₃ and Me₂(WO₄)₃ occurs at heterophase interfaces WO₃|Me₂(WO₄)₃ where Me = In, Eu, or Sc. Tungstic oxide WO₃ is pulled onto the inner surface of ceramic Me₂(WO₄)₃ and, in turn, components of Me₂(WO₄)₃ penetrate onto the surface of grains of ceramic WO₃, which is confirmed by the method of x-ray—fluorescence analysis. Data concerning the conductivity and transport numbers of Eu₂(WO₄)₃ and a composite on its basis, which was manufactured as a result the electrosurface transport of WO₃, are obtained for the first time ever. With allowance made for the data on the O_2? character of the ionic conduction in MeWO₄ and Eu₂(WO₄)₃ it is concluded that the type of ionic carriers in tungstates (Me ⁿ⁺)_2/n [WO₄] makes no impact on the mechanism of spontaneous and field-induced processes that are developing at the (Me ⁿ⁺)_2/n [WO₄]|WO₃ interfaces.     

NaBO<Subscript>2</Subscript>-Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Na<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-Na<Subscript>2</Subscript>WO<Subscript>4</Subscript> quaternary system

This is the first study of the NaBO₂-Na₂CO₃-Na₂MoO₄-Na₂WO₄ quaternary system by differential thermal analysis. Na₂[MoO_4(x)WO_{4(1 − x)}] solid solutions in the quaternary system are found to not decompose.     

Four-component system LiF-K<Subscript>2</Subscript>WO<Subscript>4</Subscript>-CaF<Subscript>2</Subscript>-BaWO<Subscript>4</Subscript>

The four-component system LiF-K₂WO₄-CaF₂-BaWO₄ has been studied for the first time using physicochemical methods. The a priori prediction of the phase complex revealed the phase tree and crystallization path of the system. The prediction was verified experimentally, by construction of a topologic model of the phase diagram, and the solution of the equations of the general law of liquidus-surface formation. The density has been measured, and the heat-storage properties of eutectic mixtures have been estimated.     

Preparation and characterization of the Ti/IrO<Subscript>2</Subscript>/WO<Subscript>3</Subscript> as supercapacitor electrode materials

WO₃ films have been prepared onto IrO₂-coated Ti substrate by electro-deposition, and as-deposited and annealed films have been characterized by using Raman spectroscopy. It was found that the asdeposited film consists of orthorhombic WO₃ · H₂O phase, which transforms to amorphous WO₃ by annealing at 250°C and to monoclinic phase by annealing at and above 350°C. All electrochemical experiments were carried on Ti/IrO₂/WO₃ annealed at 450°C. The open-circuit potential could change significantly due to the hydration of the coating film. However this process is fairly slow. Reproducible voltammograms could be obtained quickly, further revealing high electrochemical stability of the Ti/IrO₂/WO₃ electrode. And the shapes of CV show the approximate rectangular mirror image, showing the typical characteristic of capacitive behavior. The specific capacitance obtained at a scan rate of 50 mV s⁻¹ is 46 F g⁻¹.     

Effect of oxygen activity in the gaseous phase on the mechanism of reactions in the solid-state synthesis of In<Subscript>2</Subscript>(WO<Subscript>4</Subscript>)<Subscript>3</Subscript> and In<Subscript>6</Subscript>WO<Subscript>12</Subscript>

The effect of oxygen’s activity on the rate of In₂(WO₄)₃ and In₆WO₁₂ formation reactions was studied to determine the reaction mass transfer mechanism. It was established that the formation of In₂(WO₄)₃ in a model reaction cell is due to the transfer of WO ₄ ^2? components and electrons moving in opposite directions through the reaction product. The relation between the diffusion coefficients of the carriers was found. The rate of electron diffusion and the reaction rate were shown to vary according to the law \(K_p \approx D_{\lim } = D_e \sim a_{O_2 }^{ - 1/4} \). We conclude that the formation of electronic conductor In₆WO₁₂ is a two-region process: at the In₂(WO₄)₃ | In₆WO₁₂ interface, the product is formed on the In₂(WO₄)₃ surface due to {WO₃} escaping toward In₂O₃, and at the In₆WO₁₂ | In₂O₃ interface, the product is formed on the In₂O₃ surface via the reaction of diffuse {WO₃} with In₂O₃. The probable relationship between the diffusion coefficients of the In₆WO₁₂ components was obtained. A relation was developed for the process rate. The diffusion coefficients for the limiting component were calculated using the data on the estimated thickness of the product layers.     

Preparation of novel visible-light-driven In<Subscript>2</Subscript>O<Subscript>3</Subscript>–CaIn<Subscript>2</Subscript>O<Subscript>4</Subscript> composite photocatalyst by sol–gel method

Novel visible-light-activated In₂O₃–CaIn₂O₄ photocatalysts were developed in this paper through a sol–gel method. The photocatalytic activities of In₂O₃–CaIn₂O₄ composite photocatalysts were investigated based on the decomposition of methyl orange under visible light irradiation (λ > 400 nm). The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrum (EDS), X-ray photoelectron spectroscopy (XPS) and UV–vis diffused reflectance spectroscopy (DRS). The results revealed that the In₂O₃–CaIn₂O₄ composite samples with different In₂O₃ and CaIn₂O₄ content can be obtained by controlling the synthesis temperature, and the composite photocatalysts extended the light absorption spectrum toward the visible region. The photocatalytic tests indicated that the composite samples demonstrated high visible-light activity for decomposition of methyl orange. The significant enhancement in the In₂O₃–CaIn₂O₄ photo-activity under visible light irradiation can be ascribed to the efficient separation of photo-generated carriers in the In₂O₃ and CaIn₂O₄ coupling semiconductors.     

Preparation and Characterisation of Nano-Structured WO<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> Layers for Photoelectrochromic Devices

Nano-structured WO₃-TiO₂ layers were prepared by the sol-gel route. To obtain transparent, porous and crack free layers up to 0.8 μ m with a single dipping cycle a templating strategy was used. As a template three-dimensionally network based on organically modified silane was introduced to the WO₃ and TiO₂ sols. The WO₃ layers were dip-coated onto the conductive glass substrate (TCO) and the TiO₂ layers on the top of the WO₃ layer. The morphology and the structure of the layers were determined by Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), Energy Dispersive X-Ray Spectroscopy (EDXS), Auger and Infrared spectroscopy. SEM image of the WO₃-TiO₂ layer confirmed the nano-porosity of the layers and give the size of the particles of about 10 nm for TiO₂ and 30 nm for WO₃ layer. Further analysis indicated that the titanium sol penetrates the WO₃ layer. Particles in the WO₃ layer consist of a crystalline monoclinic WO₃ core surrounded by a 5–10 nm amorphous phase consisting of WO₃, TiO₂ and SiO₂. The WO₃-TiO₂ layers were used to assemble all solid state photoelectrochromic (PE) devices. Under 1 sun irradiation (1000 W/m²) the visible transmittance of the PE device changes from 62% to 1.6%. The colouring and bleaching processes last about 10 minutes.     

Solid-phase synthesis of sodium-bismuth tungstate NaBi(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>

Results of a solid-phase synthesis of polycrystalline sodium-bismuth tungstate NaBi(WO₄)₂ from NaNO₃, Na₂WO₄, Bi₂O₃, and WO₃ are presented. The optimal synthesis conditions are determined and a technological flow chart for synthesis of a single-phase product is suggested.     

Charge separation by tetrahexahedron-SrTiO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> heterojunction as an efficient photocatalyst

Charge separation plays a key role in the conversion of solar energy into chemical energy for use in the redox reaction and as well as in the photocatalytic activity. In this study, SrTiO₃ particles with different morphologies including irregular, tetrahexahedron, and cube were synthesized by an in situ solvothermal method. The photocatalytic activity of the synthesized nanoparticles was investigated in the photocatalytic decomposition of methylene blue under UV light irradiation. Tetrahexahedron SrTiO₃ particles exhibited high decomposition activity (70 %), which is about two times higher than those of the irregular and cubic SrTiO₃ particles. The high decomposition activity of tetrahexahedron SrTiO₃ particles could be attributed to the improvement of charge separation achieved on different crystal facets. To reach a good charge separation, tetrahexahedron SrTiO₃/TiO₂ coupled nanoparticles were fabricated by impregnation method. Results showed that coupling tetrahexahedron SrTiO₃ with TiO₂ could produce efficient charge separation between tetrahexahedron SrTiO₃ and TiO₂ due to their matched band edges. In order to achieve better charge separation, the tetrahexahedron SrTiO₃/90 %TiO₂ sample was calcined at different temperatures in the 450–750 °C range. Tetrahexahedron SrTiO₃/90 %TiO₂ coupled nanoparticles calcined at 650 °C show high photocatalytic activity compared with other samples. The prepared samples were characterized by using various techniques such as X-ray diffraction, scanning electron microscopy, photoluminescence emission spectra, and UV–Vis diffuse reflectance spectroscopy.     

Assessment of operating parameters for photocatalytic degradation of a textile dye by Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript>/clinoptilolite nanocatalyst using Taguchi experimental design

In the current study, a nanophotocatalyst doped with of TiO₂ and Fe₂O₃ nanoparticles supported on Iranian clinoptilolite was synthesized and characterized by XRD, XRF, SEM, and EDX analyses. The results suggested the successful loading of TiO₂ and Fe₂O₃ nanoparticles onto the surface of clinoptilolite. The SEM images confirmed the average size of nanoparticles deposited on zeolite, which was about 20–40 nm. Furthermore, application of the synthesized photocatalyst in photocatalytic degradation of Acid Black 172 dye was studied using the Taguchi method and the chosen parameters were as follows: pH (2–7), dye concentration (50–200 mg/l), irradiation time (30–120 min), and catalyst dosage (0.5–1.5 g/l). The results indicate that dye concentration, pH, and irradiation time are respectively the most effective factors in these experiments while with the minimum dosage of the catalyst (0.5 g/l), up to 90 % removal efficiency could be achieved. The optimum value for each parameter was pH = 2, dye concentration = 50 mg/l, catalyst dosage = 1 g/l and irradiation time = 60 min, and the dye removal efficiency reached up to 100 % at these optimal conditions. Furthermore, after five-times recycling and reusing the catalyst, the efficiency of the photocatalytic degradation was reduced from 91.5 to 65.9 %, which is still an acceptable value.     

Low-temperature catalytic preparation of multi-wall MoS<Subscript>2</Subscript> nanotubes

In the catalytic reduction atmosphere of H₂+CH₄+C₄H₄S, the ball-milled precursor (NH₄)₂MoS₄ is heated to 300°C for decomposition. The as-synthesized product is characterized by XRD, SEM, HRTEM, EDX, and BET. The results show that multi-wall MoS₂ nanotubes are obtained. The length of the nanotubes is around 3–5 μm. The diameters of the nanotubes are homogeneous, with an inner diameter of ∼15 nm, an outer diameter of ∼30 nm, and an interlayer (002) d-spacing of 0.63 nm. This catalytic thermal reaction occurring at low temperatures is important for the large-scale preparation of similar transition-metal disulfide nanotubes.     

Enhanced Photocatalytic Degradation of Methylene Blue over Hexagonal WO<Subscript>3</Subscript>/Graphene under Visible-Light Irradiation

A series of composites containing hexagonal tungsten trioxide (h-WO₃) and reduced graphene oxide (rGO) sheets are synthesized via a modified one-step hydrothermal route without assisted additive. The composites are characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and UV-vis absorption spectroscopy. The new procedure made it possible to increase the reduction degree of GO. Based on the evidence presented hexagonal WO₃ grows on the surface of graphene through chemical interactions with the surface. The visible-light photocatalytic degradation of methylene blue shows that the h-WO₃/rGO composites exhibit superior photocatalytic performance of 96% with a maximum degradation rate achieved under visible-light irradiation for 6 h. The speculations concerning the mechanism of photocatalytic reactions are discussed. The improved photocatalytic activity can be accounted for by the increased adsorption toward chemical species, the enhanced light absorption and an efficient separation of photogenerated electron-hole pairs and transfer of charge carriers.     

Five-component reciprocal systems Na,K∥Cl,CO<Subscript>3</Subscript>,MoO<Subscript>4</Subscript>,WO<Subscript>4</Subscript> and Na,K∥F,CO<Subscript>3</Subscript>,MoO<Subscript>4</Subscript>,WO<Subscript>4</Subscript>

Five-component reciprocal systems Na,K∥Cl,CO₃,MoO₄,WO₄ and Na,K∥F,CO₃,MO₄,WO₄ have been studied by differential thermal analysis (DTA) and X-ray powder diffraction (XPD). The systems have been triangulated to phase simplexes. The main reciprocal and complex-formation reactions have been revealed. The stability of [Na,K]₂CO₃, Na₂[Mo,W]O₄, and K₂[Mo,W]O₄ binary solid solutions and the nonexistence of quintuple invariant points in the title systems have been verified.     

Effect of tungsten oxide loading on metathesis activity of ethene and 2-butene over WO<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> catalysts

The metathesis of ethene and 2-butene to propene was studied over WO₃/SiO₂ catalysts with various WO₃ loadings (2, 4, 8, 12, 16, and 24 wt%). The 2-butene conversion and propene selectivity increased greatly with WO₃ loading increasing from 2 to 8 wt%, reached maximum at 8–12 wt% WO₃ loading, and then decreased when the WO₃ loading was higher than 12 wt%. From the above results and taking the economics into account, the optimal amount of WO₃ loading was ~8 wt%. The catalysts were characterized by physico-chemical and spectroscopic techniques to elucidate the effect of different tungsten oxide loadings on the metathesis reactivity of ethene and 2-butene. The characterization data indicated that three types of tungsten species (i.e., surface tetrahedral tungsten species, surface octahedral polytungstate species, and WO₃ crystallites) were present in the catalysts. It was found that WO₃ was not the active centers, and surface tetrahedral tungsten species might be more active than octahedral polytungstate species in metathesis reaction. The reduced form of tungsten species [W⁺⁴, W⁺⁵, and W^+(6−y) (0 < y < 1)] may be the suitable state of W species acting as metathesis active centers.     

Density functional theory (DFT) study of O<Subscript>3</Subscript> molecules adsorbed on nitrogen-doped TiO<Subscript>2</Subscript>/MoS<Subscript>2</Subscript> nanocomposites: applications to gas sensor devices

Density functional theory calculations were carried out to investigate the adsorption behaviors of O₃ molecules on the undoped and N-doped TiO₂/MoS₂ nanocomposites. With the inclusion of vdW interactions, which correctly account the long-range dispersion energy, the adsorption energies and final geometries of O₃ molecules on the nanocomposite surfaces were improved. For O₃ molecules on the considered nanocomposites, the binding sites were located on the fivefold coordinated titanium atoms of the TiO₂ anatase. The structural properties of the adsorption systems were examined in view of the bond lengths and bond angles. The variation of electronic structures was also discussed in view of the density of states, molecular orbitals and distribution of spin densities. The results suggest that the adsorption of the O₃ molecule on the N-doped TiO₂/MoS₂ nanocomposite is more favorable in energy than that on the pristine one, indicating that the N-doped nanocomposite has higher sensing capability than the pristine one. This implies that the N-doped TiO₂/MoS₂ nanocomposite would be an ideal O₃ gas sensor. However, our calculations thus provide a theoretical basis for the potential applications of TiO₂/MoS₂ nanocomposites as efficient O₃ sensors, leading to very interesting results in the context of air quality measurement.