Photoelectrochemical characteristics of ferric tungstate

Active ferric tungstate was prepared by fusing an equimolar mixture of tungsten oxide and ferric oxide at 1100 °C and annealing at 800 °C for 20 h. Analysis of the electrode material by X-ray diffraction showed that its composition was Fe₂WO₆. When this material was illuminated by visible light in 0.1 M NaOH solution, an anodic photocurrent at a positive potential of 0.5 V (SCE) was obtained. Therefore, this material is considered as an n-type semiconductor. The d.c. conductivity of this material at 25 °C was 4 × 10⁻⁶ Ω⁻¹ cm⁻¹. In the dark, unexpectedly high anodic currents were observed at positive potentials of 0.8 V (SCE) in 0.1 M NaOH. These currents are attributed to the existence of a high density of electron-hole recombination centers within the band-gap of ferric tungstate. When dimethyl viologen (DMV) was used as an electroactive compound in the electrolyte, the anodic photocurrents increased significantly. The oxidation of DMV is thus expected to compete with the electron-hole recombination process. Furthermore, the process of electron-hole recombination was also predicted from the shape of the photocurrent transients under interrupted illumination. These transients exhibited first-order relaxation effects in the region of the onset time of the photocurrents. The band-gap energy of Fe₂WO₆ was found to be about 1.5 eV and its flat-band potential in 0.1 M NaOH was about −0.3 V (SCE). The photoelectrochemical properties of ferric tungstate are explained according to the formalism of the band model of the semiconductor/electrolyte interface. Received: 16 July 1997 / Accepted: 26 September 1997     

Methanol oxidative fuel cell: electrochemical synthesis and characterization of low-priced WO<Subscript>3</Subscript>-Pt anode material

Electroplating of WO₃-Pt has been carried out using a suitable plating bath solution and optimum working conditions. X-ray and SEM studies of the deposit reveal a smooth and uniform distribution of micro-particles on the surface. New phases of the plated material appear on heat treatment. The electrocatalytic activity of the WO₃-Pt co-deposit is considerably higher than for platinum alone. WO₃ in the WO₃-Pt co-deposit reduces the retardation effect of methanol oxidation by the reaction intermediate observed in the case of Pt alone. WO₃-Pt co-deposits exhibit low overpotential for methanol oxidation in both acidic and alkaline media at low temperature and have good corrosion resistance in electrolytic media. The electrochemical parameters for methanol oxidation of these co-deposits depend on trace amounts of the platinum, heat treatment and the microstructure of the coating.     

Practical and versatile oxidation of alcohol using novel Na2WO4–H2O2 system under neutral conditions

This paper presents a novel Na₂WO₄–H₂O₂ oxidation system. The oxidation of alcohol to ketone or aldehyde was carried out by using N,N-dimethylacetamide, hydrogen peroxide, and a catalytic amount of disodium tungstate dihydrate under neutral conditions. This method is very simple, practical for large-scale manufacturing, and applicable to a variety of substrates including an acid-sensitive substrate. Disodium tetraperoxotungstate dihydrate (Na₂[W(O₂)₄]·2H₂O) was isolated from a mixture of N,N-dimethylacetamide, hydrogen peroxide, and disodium tungstate dihydrate, and a proposal reaction mechanism is discussed in this paper.     

Luminescence in trilanthanumtrichlorotungstate (La3WO6Cl3)

The luminescence properties of La₃WO₆Cl₃ are reported and discussed. The tungstate group occurs as a trigonal prismatic WO^6?₆ complex. The blue luminescence is, for the greater part, quenched at room temperature. No energy migration occurs in this lattice. The decay times are discussed in terms of a simple molecular-orbital (MO) scheme. The luminescence of the following activating ions was studied: Mo⁶⁺, Bi³⁺, Eu³⁺, Sm³⁺, Ce³⁺, and Tb³⁺. The molybdate group produces a red emission with low efficiency. The Bi³⁺ ion induces a narrow band emission with small Stokes shift. This is interpreted using a Bi³⁺O^2?W⁶⁺ charge-transfer state. Except for Ce³⁺, the rare earth activators show luminescence, but the total transfer efficiency from tungstate to the rare-earth ions is low. This is not due to the one-step tungstate-rare-earth transfer (which is efficient), but to the localized nature of the tungstate excitation. The Eu³⁺ charge-transfer band is at very low energies.     

Ensemble Effects on Methanol Oxidation to Formaldehyde on Ferric Molybdate Catalysts

The properties of Mo-doped iron oxide are compared with those of the single oxides of Fe and Mo, and with stoichiometric ferric molybdate for the selective oxidation of methanol. It is found that Mo oxide segregates to the surface of the iron oxide at low loadings, while at higher loadings, but below the stoichiometric ratio, presents layers of ferric molybdate at the surface. The relationship between bulk loading and surface Mo is explored, and it is concluded that the reactivity is dominated by ensemble effects. Simple modelling indicates that four or more Fe cation ensembles are required to combust methanol to CO₂, ensembles of two Mo cations are required for selective oxidation to formaldehyde, whereas it seems that isolated single sites of either Fe or Mo produce CO.     

Mercurous nitrate as a reductimetric reagent: The potentiometric titration of ferric iron in the presence of ions forming highly coloured thiocyanates

The reduction of ferric iron by means of mercurous salts in the presence of an excebs of ammonium, thiocyanate can be predicted on theoretical grounds. By consideration of the oxidation potentials involved it has been shown by calculation that quantitative reduction of the ferric iron is to be expected.—The titration may be effected potentiometrically using a bright platinum indicator electrode in conjunction with a silver/silver chloride electrode or a saturated calomel reference electrode. Ferric iron can be titrated in the presence of ions such as cobalt which form intensely coloured thiocyanate complexes.A possible method for the titration of molybdate by means of mercurous nitrate has been examined. The sensitivity of the method suffers from the instability of potential of the system under titration, but particularly since tungstate does not interfere, the method is worthy of further study.     

Microprobe study of enhanced Raman scattering effect on WO3/Ag thin films

Enhanced Raman scattering effect on WO₃ thin films, sputtered on a silicon substrate coated-silver thin film, has been evidenced using the MOLE microprobe. The silver surface is very rough and the observed ERS effect seems to be due to the formation of tungsten bronze at the WO₃/Ag interface. Thermal treatment favours silver diffusion in the WO₃ film and the formation of silver tungstate.     

Preparation and photoelectrocatalytic activity of a nano-structured WO3 platelet film

A tungsten trioxide (WO₃) film was prepared by calcination from a precursor paste including suspended ammonium tungstate and polyethylene glycol (PEG). The ammonium tungstate suspension was yielded by an acid-base reaction of tungstic acid and an ammonium solution followed by deposition with ethanol addition. Thermogravimetric (TG) analysis showed that the TG profile of PEG is significantly influenced by deposited ammonium tungstate, suggesting that PEG is interacting strongly with deposited ammonium tungstate in the suspension paste. X-ray diffraction (XRD) data indicated that the WO₃ film is crystallized by sintering over 400 °C. The scanning electron microscopic (SEM) measurement showed that the film is composed of the nano-structured WO₃ platelets. The semiconductor properties of the film were examined by Mott-Schottky analysis to give flat band potential E_FB=0.30 V vs. saturated calomel reference electrode (SCE) and donor carrier density N_D=2.5×10²² cm⁻³, latter of which is higher than previous WO₃ films by two orders of magnitude. The higher N_D was explained by the large interfacial heterojunction area caused by the nano-platelet structure, which apparently increases capacitance per a unit electrode area. The WO₃ film sintered at 550 °C produced 3.7 mA cm⁻² of a photoanodic current at 1.2 V vs. SCE under illumination with a 500 W xenon lamp due to catalytic water oxidation. This photocurrent was 4.5-12.8 times higher than those for the other control WO₃ films prepared by similar but different procedures. The high catalytic activity could be explained by the nano-platelet structure. The photocurrent was generated on illumination of UV and visible light below 470 nm, and the maximum incident photon-to-current conversion efficiency (IPCE) was 47% at 320 nm at 1.2 V. Technically important procedures for preparation of nano-structured platelets were discussed.     

Photoelectrochemical properties of tungsten trioxide thin film electrodes prepared from facet-controlled rectangular platelets

The control of anisotropic crystal growth is critical for directing the orientation of crystal lattice planes, and it plays a key role towards understanding the effects of different planes on chemical reactions. Here, we report on the photoelectrochemical properties of plate-structured tungsten trioxide (WO₃) thin films prepared from facet-controlled rectangular platelets of hydrotungstite (WO₃·2H₂O) and tungstite (WO₃·H₂O), which are directly grown on tungsten substrates. The WO₃ thin films, prepared via WO₃·2H₂O platelets, show relatively stable current for photoelectrochemical water splitting and methanol oxidation. On the other hand, the photocurrent of the WO₃ thin films prepared via WO₃·H₂O platelets was significantly decreased during the photoelectrochemical oxidation of water, which is likely due to the accumulation of partially oxidized intermediates such as peroxo species on the surface. These results indicate that the surface nanostructures of WO₃ may have a significant influence on photoelectrode efficiency and selectivity for the catalytic oxygen evolution reaction.     

Darstellung von Kupfer-, Mangan-, Cadmium- und Bleiwolframat durch tribochemische Reaktion

Preparation of copper, manganese, cadmium and lead tungstate by tribochemical reaction Stoichiometric mixtures of powders CuO/WO₃, MnO/WO₃, CdO/WO₃, PbO/WO₃ are grinded in a vibrating mill. Infrared spectra, X-ray diffraction, DTA and investigations of solubility show that on grinding in vibrating mill the tungstates M^IIWO₄ are formed from the oxide mixtures by tribochemical reaction.     

Vibrational spectra of compounds Ln2MoO6 and Ln2WO6

The vibrational spectra of compounds Ln₂WO₆ and Ln₂MoO₆ (Ln = lanthanide, including Y and Bi) are reported. Neglecting details it is possible to assign the internal vibrations of the molybdate and tungstate group. The results are in agreement with the structural data known up till now and suggest further that tungsten in the unknown Y₂WO₆ structure is in six-coordination, that Bi₂MoO₆ shows still another modification than koechlinite and that vibrational spectroscopy may yield additional structural information for compounds like those under consideration.     

Ein neuer Strukturtyp am Lanthanoid-Oxowolframat FeCe(WO4)W2O8 = FeCe(WO4)3

A New Structure Type for the Rare Earth Oxotungstate FeCe(WO₄)W₂O₈ = FeCe(WO₄)₃ Single crystals of the hitherto unknown compound FeCe(WO₄)₃ have been prepared by crystallization from melts of Fe₂O₃, CeO₂ and WO₃. It crystallizes with triclinic symmetry, space group P1 , a = 7,486(3); b = 7,528(1); c = 16,502(4) Å, α = 101,00(2); β = 96,62(3); γ = 98,62°; Z = 2. Tungsten shows octahedral and tetrahedral coordination by oxygen. The crystal structure is characterized by layers related to the Scheelite and Wolframite type. Thermogravimetric measurements led to a lost of oxygen during reaction. It results in a decrease of the oxidation states of Fe³⁺ and Ce⁴⁺ respectively, as will be discussed using magnetic measurements and calculations of the Coulomb terms of lattice energy. The structure contains a one-fold coordinated oxygen.     

2D Bi2WO6/MoS2 as a new photo-activated carrier for boosting electrocatalytic methanol oxidation with visible light illumination

In this paper, a new two-dimensional (2D)/2D composite of Bi₂WO₆/MoS₂ was facile synthesized, and then was used as supporting material for depositing Pt nanoparticles. The as-synthesized Pt-Bi₂WO₆/MoS₂ was extended into photo-assisted electrocatalytic oxidation of methanol, which is a model anode reaction for direct methanol fuel cell. Compare with traditional electrocatalytic process, Pt-Bi₂WO₆/MoS₂ displays 1.5 times enhanced electrocatalytic performance on methanol oxidation with assistance of visible light irradiation and 2.2 times for commercial Pt/C. Besides, from the results of chronoamperometric and chronopotentiometry experiments, the stability of Pt-Bi₂WO₆/MoS₂ electrode is clearly improved under visible light irradiation. The synergistic effects of photo- and electro-catalytic in the heterojunction of Pt-Bi₂WO₆/MoS₂ in favor of the above enhancement. This research gives more insights in the fields of photo-assisted traditional electrocatalytic application by constructing of semiconductor heterojunction carrier.     

Synthesis of Tungsten Oxide Nanorods by the Controlling Precipitation Reaction: Application for Hydrogen Evolution Reaction on a WO3 Nanorods/Carbon Nanotubes Composite Film Modified Electrode

Tungsten trioxide nanorods (WO₃) were prepared by controlling the precipitation reaction of sodium tungstate dehydrates solution and hydrothermal process. The synthesized tungsten trioxide nanorods have been characterized by X‐ray diffraction (XRD), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), cyclic voltammetry (CV) and linear sweep voltammetry (LSV). Electrochemical activity for hydrogen evolution reaction (HER) on WO₃ nanorods / carbon nanotube (WO₃/CNT) composite electrocatalyst was first studied in acid solution (0.1 M H₂SO₄) at room temperature. The overall experimental results revealed that the electrocatalytic activity for HER on WO₃/CNT is two order magnitude higher than those obtained with WO₃ nanorods and is four times than in the case of commercial WO₃ in 0.1 M H₂SO₄. On the other hand, the kinetic reaction mechanisms were discussed on WO₃/CNT electrocatalysts in acid solution for HER. However, the rate‐determining step carries through Tafel reaction on commercial WO₃, CNT, WO₃ nanorods and WO₃/CNT electrocatalysts in acidic solution was introduced.     

Mechanism of soft solution processing formation of alkaline earth metal tungstates: an electrochemical and in situ AFM study

The mechanism of alkaline earth metal tungstate formation during soft solution processing was studied by cyclic voltammetry, electrochemical impedance spectroscopy and by direct in situ observation of the surface changes using atomic force microscopy. The electrochemical oxidation of W to WO₃ was followed by dissolution of WO₃ and, with some delay, by precipitation of tungstates at the metal surface. The same Tafel slopes observed in Li⁺, Ba²⁺, Sr²⁺ and Ca²⁺ containing solutions indicate that the course of the oxidation process is independent of the cation present in solution. The observed differences in the current-voltage curves outside the Tafel region are accounted for by the different film-forming tendencies of the various alkaline earth metal cations. The growth of tungstate layers at the W substrate decreases the electrochemically active area and limits the production of WO₄ ^2– at later stages of deposition. At low potentials (E<0.2 V) the oxidation of W is the rate-controlling step. At higher potentials, however, the dissolution process slows down due to a relative decrease of the pH in the electrode vicinity and dissolution becomes the rate-limiting step. Electronic Publication     

Kinetics of FeSe2 oxidation by ferric iron and its reactivity compared with FeS2

The mobility and bioavailability of selenium is a major health and environmental issue and a main concern for geological disposal of high-level radioactive waste. Chemically and/or microbially mediated oxidation of insoluble Se-bearing particulate, such as iron selenides, to dissolved and mobile phases controls the transport and distribution of Se in the environment. The oxidation of ferroselite(FeSe2) by ferric iron was investigated in anoxic conditions. The redox reaction can be represented by: FeSe2 + 2Fe3+ = 2Se0 + 3Fe2+. Kinetic studies indicated that the reaction can be described by second-order rate law, with rate constants of 0.49±0.01, 0.85±0.02, 1.84±0.04, and 3.29±0.13 L mol-1 s-1 at pH 1.62, 1.87, 2.23, and 2.49, respectively. The positive correlation between reaction rate and pH implies that diffusion of Fe3+ oxidant to the mineral surface is the rate-determining step. The strong reactivity of FeSe2 towards Fe3+ suggests that ferric iron may play a significant role in FeSe2 oxidation process(e.g., by Se4+, O2, etc.) and Se0 should be the first reaction product. Also, it was shown that the reduction rate of Fe3+ or Se4+ by pyrite(FeS2) can be significantly increased in the presence of FeSe2, suggesting a stronger reactivity of FeSe2 compared with pyrite. The results obtained extend our knowledge about the subtle interaction between Se, pyrite and iron selenides in the environment, and give insight into the transfer of selenium from iron selenides to bio-available selenium(i.e., selenite and selenate) in the Se-rich environment.     

Orientation of tungsten trioxide thin films fabricated by sol–gel method using aqueous sols of colloidal layered tungstates

Oriented monoclinic WO₃ thin films were fabricated by sol–gel method using aqueous sols of colloidal layered tungstates. The colloidal tungstate sols were prepared by reacting different alkylamines with layered tungstic acid H₂WO₄ in water. With decreasing the alkyl chain length of the alkylamines, the colloidal layered tungstate became smaller. Alkylamines with a short alkyl chain provided transparent aqueous sols. Furthermore, the WO₃ thin films fabricated from the obtained aqueous sols had a high (100)-orientation. However, upon annealing H₂WO₄ crystals applied on a glass substrate with the tungstate layers parallel to the substrate, highly (001)-oriented WO₃ layers were obtained. Since both of the A- and C-planes of WO₃ have a similar structure to the layers of H₂WO₄, the orientation of the WO₃ thin films and layers probably resulted from the topotactic structural conversion of the tungstates. Interestingly, the preferential orientation of the thin films was dependent on the presence or absence of interlayer alkylamines in the tungstates.     

Orthophosphoric acid solutions of sodium orthovanadate,sodium tungstate,and sodium molybdate as potential corrosion inhibitors of the Al2Cu intermetallic phase

Orthophosphoric acid solutions of sodium orthovanadate, sodium tungstate, and sodium molybdate are tested as potential corrosion inhibitors of the Al₂Cu intermetallic phase. Corrosion inhibition is observed for 0.2 M solutions of Na₃VO₄ and Na₂WO₄ by increasing the pH to > 2. When the pH is < 2, the aforementioned salts increase the corrosion rate of the intermetallic phase. A 0.2 M solution of Na₃VO₄ causes the precipitation of vanadium phosphate on the surface of the Al₂Cu phase at pH = 1.

     

纳米三氧化钨复合催化剂的制备及对甲醇电催化性能

采用浸渍沉淀法制备出WO₃-碳纳米管(WO₃-CNTs)纳米复合材料, 微波辅助乙二醇法在其表面负载活性成分Pt, 得到纳米Pt/WO₃-CNTs 催化剂. 采用X射线衍射(XRD), 透射电子显微镜(TEM)和X射线光电子能谱(XPS)等测试手段对催化剂的结构和形貌进行表征, 结果表明Pt 纳米粒子为面心立方晶体结构, 粒径大小在3-5 nm之间, 均匀地分布在WO₃-CNTs 纳米复合材料表面, 同时发现催化剂中的Pt 主要以金属态的形式存在. 采用循环伏安和计时电流法研究了在酸性溶液中Pt/WO₃-CNTs 催化剂对甲醇的电催化氧化性能, 结果表明Pt/WO₃-CNTs 催化剂比用硝酸处理的碳纳米管载铂催化剂(Pt/CNTs)对甲醇呈现出更高的电催化氧化活性和抗CO中毒性能.     

Hydrogenation of Cinnamaldehyde on Ir/γ-Al2O3 Catalysts. Influence of the Surface Acidity

The oxidation of methane has been studied in a flow system as a function of the chemical composition of zeolite catalyst using nitrous oxide as oxidant. It is concluded that methanol is a primary oxidation product which may undergo further oxidation to formaldehyde and to carbon oxides. However, it may also undergo conversion over the acidic catalyst to higher hydrocarbons. Reaction with nitrous oxide resulted in the production of carbon oxides, methanol, formaldehyde, C₂ - C₄ , C₅ - C₇ nonaromatics, and aromatics. The effect of Fe₂ O₃ and Al₂ O₃ , with or without, over HZSM5 on products was studied.