The key factor determining the anodic deposition of vanadium oxides

This work demonstrates that anodic deposition of vanadium oxide (denoted as VO_x·nH₂O) can be considered as the chemical co-precipitation of V⁵⁺ and V⁴⁺ oxy-/hydroxyl species and the accumulation of V⁵⁺ species at the vicinity of electrode surface is the key factor for the successful anodic deposition of VO_x·nH₂O at a potential much more negative than the equilibrium potential of the oxygen evolution reaction (OER). The results of in situ UV-vis spectra show that the V⁴⁺/V⁵⁺ ratio near the electrode surface can be controlled by varying the applied potential, leading to different, three-dimensional (3D) nanostructures of VO_x·nH₂O. The accumulation of V⁵⁺ species due to V⁴⁺ oxidation at potentials ≥0.4 V (vs. Ag/AgCl) has been found to be very similar to the phenomenon by adding H₂O₂ in the deposition solution. The X-ray photoelectron spectroscopic (XPS) results show that all VO_x·nH₂O deposits can be considered as aggregates consisting of mixed V⁵⁺ and V⁴⁺ oxy-/hydroxyl species with the mean oxidation state significantly increasing with the applied electrode potential.     

Molten V2O5/Cs0.9K0.9Na0.2S2O7 and V2O5/K2S2O7 catalysts as electrolytesin an electrocatalytic membrane separation device for SO2 removal

Bench scale fuel cell tests have been carried out on the SO₂ oxidation catalyst systems V₂O₅/M₂S₂O₇ (M = alkali) used as electrolytes in a standard molten carbonate fuel cell (MCFC) fuel cell setup for removal of SO₂ from power plant flue gases. Porous Li _x Ni_(1–x)O electrodes were used both as anode and cathode. The cleaning cell removes SO₂ when a potential is applied across the membrane, potentially providing cheap and ecological viable means for regeneration of SO₂ from off-gases into high quality H₂SO₄. Results show that successful removal of up to 80% SO₂ at 450 °C can be achieved at approximately 5 mAcm^–2. However, the data obtained during the experiments explain the current limitations of the process, especially in terms of electrolyte wetting capability and acid/base chemistry of the electrolyte.     

First In Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts

In situ Raman spectroscopy at temperatures up to 500°C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO₂ oxidation. Vanadia/silica catalysts impregnated with Cs₂SO₄ were exposed to various SO₂/O₂/SO₃ atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported model V₂O₅–Cs₂SO₄ and V₂O₅–Cs₂S₂O₇ molten salts. The data indicate that (1) the V^V complex V^VO₂(SO₄)₂ ^3– (with characteristic bands at 1034 cm^–1 due to (V=O) and 940 cm^–1 due to sulfate) and Cs₂SO₄ dominate the catalyst surface after calcination; (2) upon admission of SO₃/O₂ the excess sulfate is converted to pyrosulfate and the V^V dimer (V^VO)₂O(SO₄)₄ ^4– (with characteristic bands at 1046 cm^–1 due to (V=O), 830 cm^–1 due to bridging S–O along S–O–V and 770 cm^–1 due to V–O–V) is formed and (3) admission of SO₂ causes reduction of V^V to V^IV (with the (V=O) shifting to 1024 cm^–1) and to V^IV precipitation below 420°C.     

Indirect anodic oxidation of anthracene to anthraquinone in a slurry electrolyte system in the presence of both surfactant and redox mediator

The anodic oxidation of anthracene has been studied in a slurry electrolyte containing both the redox mediator, Mn³⁺/Mn²⁺, and surfactant, dodecyl-benzene-sulphonate (DBS). The reaction mechanism, kinetics and the factors which affect the current efficiency of anthraquinone production were explored. Both the experimental results and the theoretical analysis show that the reaction is second order, i.e. first order with respect to Mn³⁺ and DBS, when the concentration of DBS is less than 1.64×10^–4M and is first order with respect to Mn³⁺ only when the DBS concentration is larger than 1.64×10^–4M. No anthraquinone was found in the absence of DBS. The main factors affecting the current efficiency of the anodic oxidation of anthracene to anthraquinone in the presence of both Mn³⁺/Mn²⁺ and DBS are H₂SO₄ concentration, current density, anthracene particle size, concentration of DBS and redox mediator as well as temperature. The current efficiency is slightly affected by the concentration of DBS and agitation rate when the concentration of DBS and agitation rate are higher than 1.64×10^–4M and 700 r.p.m., respectively.     

Studies on promising cell performance with H2SO4 as the catholyte for electrogeneration of Ag2+ from Ag+ in HNO3 anolyte in mediated electrochemical oxidation process

Electrochemical performance of a divided cell with electrogeneration of Ag²⁺ from Ag⁺ in 6 M HNO₃ anolyte has been studied with 6 M HNO₃ or 3 M H₂SO₄ as the catholyte. This work arose because in mediated electrochemical oxidation (MEO) processes with Ag(II)/Ag(I) redox mediator, HNO₃ is generally used as catholyte, which, however, produces NO _x gases in the cathode compartment. The performance of the cell with 6 M HNO₃ or 3 M H₂SO₄ as the catholyte has been compared in terms of (i) the acid concentration in the cathode compartment, (ii) the Ag⁺ to Ag²⁺ conversion efficiency in the anolyte, (iii) the migration of Ag⁺ from anolyte to catholyte across the membrane separator, and (iv) the cell voltage. Studies with various concentrations of H₂SO₄ catholyte have been carried-out, and the cathode surfaces have been analyzed by SEM and EDXA; similarly, the precipitated material collected in the cathode compartment at higher H₂SO₄ concentrations has been analyzed by XRD to understand the underlying processes. The various beneficial effects in using H₂SO₄ as catholyte have been presented. A simple cathode surface renewal method relatively free from Ag deposit has been suggested.     

Application of Combinatorial Methodologies to the Synthesis and Characterization of Electrolytic Manganese Dioxide

A combinatorial method has been used to investigate the effects of anodic current density, and Mn(II) and H₂SO₄ concentrations on the electrochemical synthesis and characterization of electrolytic manganese dioxide (EMD). The combinatorial method involved rapid parallel and series electrochemical deposition of EMD from electrolytes with various Mn(II)(0.15–1.82 M) and H₂SO₄(0.05–0.51 M) concentrations, at various anodic current densities (25–100 A m^–2), onto individual 1 mm² titanium electrodes, in an overall array consisting of 64 electrodes. Electrode characterization was then by average plating voltage (recorded during deposition), and open circuit voltage and chronoamperometric discharge in 9 M KOH. The applicability and benefit of the method was demonstrated by identifying the conditions of 0.59 M Mn(II), 0.17 M H₂SO₄ and 62.5 A m^–2 anodic current density as leading to the best performing EMD. These are comparable with existing knowledge regarding the synthesis and electrochemical performance of EMD, demonstrating clearly the capabilities of the combinatorial method, and providing a starting point for future experimentation. An added benefit of the method in this work was the considerable time saved during experimentation.     

Selective oxidation of H2S to sulfur over vanadia supported on mesoporous zirconium phosphate heterostructure

Vanadium oxide supported on mesoporous zirconium phosphate catalysts has been synthesized, characterized and tested in the selective oxidation of H₂S to sulfur. The nature of the vanadium species depends on the V-loading of catalyst. Catalysts with a V-content lower than 4wt% present both isolated vanadium species and V₂O₅ crystallites. However, V₂O₅ crystallites have been mainly observed in catalysts with higher V-content, although the presence of isolated V-species on the surface of the metal oxide support cannot be completely ruled out. The catalytic behaviour also depends on V-loading of catalysts. Thus, while the catalytic activity of catalysts can be related to the number of V-sites, the catalyst decay is clearly observed in samples with low V-loading. The characterization of catalysts after the catalytic tests indicates the presence of sulfur on the catalyst, which is favoured on catalysts with low V-loading. However, a clear transformation of V₂O₅ to V₄O₉ can be proposed according to XRD and Raman results of used catalysts with high V-loading. The importance of V⁵⁺–O–V⁴⁺ pairs in activity and selectivity is also discussed.     

SO2 oxidation over the V2O5/TiO2 SCR catalyst

The effects of V₂O₅ loading of the V₂O₅/TiO₂ SCR catalyst on SO₂ oxidation activity were examined by infrared spectroscopy (DRIFT) and SO₂ oxidation measurement. Vanadium oxide added to the catalyst was found to be well dispersed over the TiO₂ carrier until covered with monolayer V₂O₅. The rate of SO₂ oxidation increased almost linearly with V₂O₅ loading below the monolayer capacity and attained saturation with further increase. The hydroxyl groups bonded to vanadium atoms, V–OH, might be altered by SO₂ oxidation. Both V=O and V–OH groups are likely involved in the adsorption and desorption of SO₂ and SO₃.     

Development of a novel redox flow battery for electricity storage system

A novel cylindrical battery which uses carbon fibres with high specific surface area as electrodes and a porous silica glass with high chemical stability as membrane has been fabricated. The results obtained from electrolysis of 0.5 M K₃Fe(CN)₆–0.5 M KCl and of 85 mM V(IV)–1 M H₂SO₄ indicate that the cell possesses excellent electrolytic efficiency. As a redox flow battery (RFB) its performance was investigated by employing all-vanadium sulfate electrolytes. The results of the cyclic voltammetry measurements indicate that at a glassy carbon electrode the electrochemical window for 2 M H₂SO₄ solution could reach 2.0 2.4 V. Constant current charging–discharging tests indicate that the batteries could deliver a specific energy of 24 Wh L^–1 at a current density of 55 mA cm^–2. The open-circuit cell voltage, after full charging, remained constant at about 1.51 V for over 72 h, while the coulombic efficiency was over 91%, showing that there was negligible self-discharge due to active ions diffusion through the membrane during this period.     

Cathodic reduction of 4-nitroso-N,N-dimethylaniline at various electrode materials

This study is concerned with the cathodic reduction of 4-nitroso-N,N-dimethylaniline to 4-amino-N,N-dimethylaniline in 1.0 M HCl and 1.0 M H₂SO₄ containing 10% MeOH. The effect of cathode material, supporting electrolyte, electroactive compound concentration, temperature and current density in the electrochemical synthesis of 4-amino-N,N-dimethylaniline were investigated. The best cathode material was tin, which gave a selectivity of 92.7% with a current yield of 90.3% at a current density of 6 mA cm^–2. The final electrolysis product was obtained as its respective HCl and H₂SO₄ salts and their structures were elucidated using spectrochemical techniques.     

Investigation on V(IV)/V(V) species in a vanadium redox flow battery

Stokes radii of V(IV) and V(V) species in concentrated sulfuric acid solutions were determined from their diffusion limited current densities on a rotating platinum disk electrode and the solution viscosity. In addition, V(IV) and V(V) species were estimated based on their solubility, UV-Vis spectra, and cyclic voltammetric data. The possible ion-pair formation of V(IV) cation with SO₄²⁻ and/or HSO₄⁻ and the spontaneous polymerization of V(V) at a low H₂SO₄ concentration were discussed.     

Electrodeposition of Co–Cu alloy coatings from glycinate baths

The cathodic polarization, cathodic current efficiency of codeposition, composition and structure of Co–Cu alloy as a function of bath composition, current density and temperature were studied. Electrodeposition was carried out from solutions containing CuSO₄ · 5H₂O, CoSO₄ · 7H₂O, Na₂SO₄ and NH₂CH₂COOH. The cathodic current efficiency of codeposition (CCE) was high and it increased with increasing temperature and Cu²⁺ content in the bath, but it decreased with current density. The codeposition of Co–Cu alloys from these baths can be classified as regular. The Co content of the deposit increased with Co²⁺ content and current density and decreased with glycine concentration and temperature. The structure of the deposited alloys was characterized by anodic stripping and X-ray diffraction techniques. The data showed that the deposited alloys consisted of a single solid solution phase with a face-centred cubic (f.c.c.) structure.     

Influence of surfactants on the electroreduction of oxygen to hydrogen peroxide in acid and alkaline electrolytes

The effect of surfactants on the electroreduction of O₂ to H₂O₂ was investigated by cyclic voltammetry and batch electrolysis on vitreous carbon electrodes. The electrolytes were either 0.1 M Na₂CO₃ or 0.1 M H₂SO₄ at 295 K, under 0.1 MPa O₂. Electrode kinetics and mass transport parameters showed the influence of surfactants on the O₂ electroreduction mechanism. The cationic surfactant (Aliquat 336^®, tricaprylmethylammonium chloride), at mM levels, increased the standard rate constant of O₂ electroreduction to H₂O₂ 15 times in Na₂CO₃ and 1900 times in H₂SO₄, to 1.8 × 10^–6 m s^–1 and 9.9 × 10^–10 m s^–1, respectively. This effect on the reaction rate might be due to an increase of the surface pH, induced by the Aliquat 336^® surface film. The nonionic (Triton X-100) and anionic (sodium dodecyl sulfate) surfactants retarded the O₂ electroreduction, presumably by forming surface structures, which blocked the access of O₂ to the electrode. Ten hour batch electrosynthesis experiments performed at 300 A m^–2 superficial current density, 0.1 MPa O₂, 300 K, on reticulated vitreous carbon (30 ppi), showed that compared to the values obtained in the absence of surfactant, mM concentrations of Aliquat 336^® increased the current efficiency for peroxide from 12% to 61% (0.31 M H₂O₂) in 0.1 M Na₂CO₃ and from 14% to 55% (0.26 M H₂O₂) in 0.1 M H₂SO₄, respectively.     

Gas-phase elemental mercury capture by a V2O5/AC catalyst

Gas-phase elemental mercury (Hg⁰) capture by an activated coke (AC) supported V₂O₅ (V₂O₅/AC) catalyst was studied in simulated flue gas and compared with that by the AC. The study on the influences of V₂O₅ loading, temperature, capture time and flue gas components (O₂, SO₂, H₂O and N₂) shows that the Hg⁰ capture capability of V₂O₅/AC is much higher than that of AC. It increases with an increase in V₂O₅ loading and is promoted by O₂, which indicates the important role of V₂O₅ in Hg⁰ oxidation and capture; it is promoted slightly by SO₂ but inhibited by H₂O; it increases with an increase in temperature up to 150 °C when Hg desorption starts. X-ray photoelectron spectroscopy analysis and sequential chemical extraction experiments indicate that the main states of Hg captured on V₂O₅/AC are HgO and HgSO₄. Temperature programmed desorption experiments were also made to understand the stability of the Hg captured.     

Sorption of ethene and ethane on the V2O5(001)/TiO2(001) anatase interface

Simulation techniques have been employed to investigate the differences in the low energy adsorption configurations of ethene and ethane on the TiO₂ supported and unsupported V₂O₅(001) surface. We find that the ethene molecule approaches much closer to thesupported V₂O₅(001) surface which is reflected in the 40 kj mol^–1 higher adsorption energy. The low energy adsorption configuration located for ethane on the supported V₂O₅ shows that the molecule does not approach as close to the supported V₂O₅ surface as does ethene, resulting in the adsorption energy of ethane being 52 kJ mol^–1 lower than that of ethene on the supported V₂O₅ surface.     

Effect of ceria on the structure and catalytic activity of V2O5/TiO2–ZrO2 for oxidehydrogenation of ethylbenzene to styrene utilizing CO2 as soft oxidant

The present study was undertaken to investigate the influence of ceria on the physicochemical and catalytic properties of V₂O₅/TiO₂–ZrO₂ for oxidative dehydrogenation of ethylbenzene to styrene utilizing CO₂ as a soft oxidant. Monolayer equivalents of ceria, vanadia and ceria–vanadia combination over TiO₂–ZrO₂ (TZ) support were impregnated by a coprecipitation and wet impregnation methods. Synthesized catalysts were characterized by using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, temperature programmed reduction, transmission electron microscopy and BET surface area methods. The XRD profiles of 550 °C calcined samples revealed amorphous nature of the materials. Upon increasing calcination temperature to 750 °C, in addition to ZrTiO₄ peaks, few other lines due to ZrV₂O₇ and CeVO₄ were observed. The XPS V 2p results revealed the existence of V⁴⁺ and V⁵⁺ species at 550 and 750 °C calcinations temperatures, respectively. TEM analysis suggested the presence of nanosized (<7 nm) particles with narrow range distribution. Raman measurements confirmed the formation ZrTiO₄ under high temperature treatments. TPR measurements suggested a facile reduction of CeO₂–V₂O₅/TZ sample. Among various samples evaluated, the CeO₂–V₂O₅/TZ sample exhibited highest conversion and nearly 100% product selectivity. In particular, the addition of ceria to V₂O₅/TZ suppressed the coke deposition and allowed a stable and high catalytic activity.     

Characterization of metal (Ba, Al, Si, V, and W)-incorporated TiO2 and toluene photodecomposition in the presence of H2O

This study focused on toluene photodecomposition in the presence of H₂O over metal (Ba, Al, Si, V, and W)-incorporated TiO₂. The nanometer-sized, metal-TiO₂ photocatalyst samples, including Ba²⁺, Al³⁺, Si⁴⁺, V⁵⁺, and W⁶⁺ ions, were prepared by using the solvothermal method. The X-ray photoelectron spectroscopy (XPS) results showed that the Ti-OH peak, which indicates hydrophilicity, increased with increasing Al and Si ion components but decreased with increasing Ba, V, and W ion components. The contact angles were distributed over the range of 0–10° on almost all films (200-nm thick) after irradiation for 2 h, and in particular approached 0° on the Al-TiO₂ and Si-TiO₂ nanometer-sized films after just 30 min. The toluene (100 ppm) photodecomposition in the continuous system increased in the order of Al-TiO₂>Si-TiO₂>pure TiO₂>W-TiO₂>Ba-TiO₂>V-TiO₂, and the maximum toluene conversion rate achieved was 45% over Al-TiO₂ film after 120 min. The toluene conversion remarkably increased; however, over all photocatalysts, with H₂O addition during the toluene photo-decomposed reaction, and in particular, the conversion reached up to 90% after 120 min over Al-TiO₂ and Si-TiO₂ with increased hydrophilicity. After photoreaction for 24 h, minimal carbon was deposited on the photocatalyst under both reaction conditions, with and without H₂O addition, although the deposited carbon amounts were smaller for the former. These results confirmed that the hydrophilicity of the photocatalyst had a greater effect on toluene decomposition, while the photocatalytic deactivation could be retarded by H₂O supplementation during toluene decomposition.     

High speed zinc electrowinning system using a hydrogen anode and a rotating aluminium disc cathode

The performance of a novel high speed zinc electrowinning system using a hydrogen anode and an aluminium rotating disc cathode (1 m diam.) was investigated under various experimental conditions. This new type of zinc electrowinning system was continuously operated at a current density of 70 A dm^–2, which is twelve times higher than that usually employed. Current efficiency is 90% at 50 A dm^–2 in an electrolyte containing 60 g dm^–3 Zn + 160 g dm^–3 H₂SO₄, the zinc purity being at least 99.999%. The energy usage of the system is 1650 kWh per tonne of zinc, 380 m³ of H₂ gas being required.     

Near-infrared quantum cutting in Ho3+, Yb3+-codoped BaGdF5 nanoparticles via first- and second-order energy transfers

Infrared quantum cutting involving Yb³⁺ 950–1,000 nm (² F_5/2 → ² F_7/2) and Ho³⁺ 1,007 nm (⁵S₂,⁵F₄ → ⁵I₆) as well as 1,180 nm (⁵I₆ → ⁵I₈) emissions is achieved in BaGdF₅: Ho³⁺, Yb³⁺ nanoparticles which are synthesized by a facile hydrothermal route. The mechanisms through first- and second-order energy transfers were analyzed by the dependence of Yb³⁺ doping concentration on the visible and infrared emissions, decay lifetime curves of the ⁵ F₅ → ⁵I₈, ⁵S₂/⁵F₄ → ⁵I₈, and ⁵ F₃ → ⁵I₈ of Ho³⁺, in which a back energy transfer from Yb³⁺ to Ho³⁺ is first proposed to interpret the spectral characteristics. A modified calculation equation for quantum efficiency of Yb³⁺-Ho³⁺ couple by exciting at 450 nm was presented according to the quantum cutting mechanism. Overall, the excellent luminescence properties of BaGdF₅: Ho³⁺, Yb³⁺ near-infrared quantum cutting nanoparticles could explore an interesting approach to maximize the performance of solar cells.     

Influence of Nafion film on oxygen reduction reaction and hydrogen peroxide formation on Pt electrode for proton exchange membrane fuel cell

The influence of Nafion^® film on ORR kinetics and H₂O₂ formation on a Pt electrode was investigated using RRDE in 0.1 M HClO₄. It was found that the Nafion^®-coated Pt system showed lower apparent ORR activity and more H₂O₂ production than the bare Pt electrode system. From the temperature sensitivity, it was revealed that the apparent activation energies of ORR in the Nafion^®-coated Pt system were lower than the bare Pt electrode system, and the H₂O₂ formation was suppressed with the increase of the temperature. In order to analyze the results furthermore, other systems (0.1/1.0 M, HClO₄/CF₃SO₃H) with the bare Pt electrodes were also examined as references. It was exhibited that the ORR kinetic current, the H₂O₂ formation, and the apparent activation energies of 1.0 M CF₃SO₃H system were close to those of the Nafion^®-coated Pt system. We concluded that the orientation of anion species of Nafion^® and CF₃SO₃H to the Pt surface via water molecules, as well as a fluorocarbon polymer network of Nafion^®, might block O₂ adsorption, resulting in the smaller effective surface area of the Pt electrode for ORR, the smaller ORR kinetic current, and the more H₂O₂ production.