Electrocatalytic oxidation of coal on Ti-supported metal oxides coupled with liquid catalysts for H2 production

Electrocatalytic oxidation of coal on Ti-supported metal/metal oxides coupled with liquid catalysts is systematically investigated as a method of producing hydrogen at the cathode. The composition of the liquid catalyst was varied to determine its effect on the coal electrolysis. A spectrum of byproducts from the coal oxidation at the anode was analyzed. The Ti-supported metal oxide electrodes were prepared by thermal decomposition and characterized by scanning electron microscopy (SEM). X-ray diffraction results show that the composition of the electrodes was Ti/Pt, Ti/RuO₂, Ti/IrO₂ and Ti/IrO₂–RuO₂. Coal oxidation tests on these electrodes indicate that Ti/IrO₂ has the best electrocatalytic activity. Polarization curves reveal that redox catalysts, such as Fe³⁺, K₃Fe(CN)₆, KBr and V₂O₅, bridge the coal particles and the solid electrode surface, thus increasing the rates of coal oxidation. The dynamic transition of Fe³⁺/Fe²⁺ is proven by a KMnO₄ titration experiment, and the possible catalytic mechanism is discussed. Product analysis shows that pure H₂ is generated at the cathode and that CO₂ is the main product at the anode.     

Characterization of DSA®-type oxygen evolving electrodes: Choice of a coating

In the search for a DSA^®-type electrode for oxygen evolution in acidic solutions, nine binary coatings with IrO₂, RuO₂, Pt as conducting component, and TiO₂, ZrO₂, Ta₂O₅ as inert oxides, have been deposited on titanium, examined for their microstructural properties and tested for their electrocatalytic activity and anodic stability. Electrochemical true surfaces of the coatings were found to be dependent on structure and morphology: the mixtures that form a solid solution (RuO₂–TiO₂), or allow limited miscibility (IrO₂–TiO₂), show the lowest dispersion of active material. Differences in service lives, were attributed to differences in wear mechanism of the electrodes. It was found that Ti/IrO₂ (70 mol%)-Ta₂O₅ (30 mol%) is by far the best electrode.     

Comparison of Ti/BDD and Ti/SnO<Subscript>2</Subscript>–Sb<Subscript>2</Subscript>O<Subscript>5</Subscript> electrodes for pollutant oxidation

Anodic oxidation is a promising process for degrading toxic and biologically refractory organic pollutants present in wastewater treatment. Proper selection of electrodes is the key to reach effective and economic operation. In this study, two types of electrodes, i.e. the recently developed Ti/BDD and Ti/SnO₂–Sb₂O₅, which is generally believed to be superior to the conventional electrodes, were compared under the same conditions. It was found that the Ti/BDD electrode could mineralize both phenol and reactive dyes effectively. But the Ti/SnO₂–Sb₂O₅ electrode could only mineralize phenol. When oxidizing more refractory reactive dyes, it demonstrated very poor activity. In addition, the Ti/BDD electrode had a service life of 264 h in an accelerated life test, but the Ti/SnO₂–Sb₂O₅ was irreversibly damaged within several seconds. The direct experimental comparison in the present study indicates that the Ti/BDD electrode is much better than the Ti/SnO₂–Sb₂O₅ electrode for pollutant oxidation.     

The efficiency of chlorine evolution in dilute brines on ruthenium dioxide electrodes

The efficiency of chlorine evolution from dilute brines (0.5–0.1m) was studied on RuO₂ and IrO₂ coated Ti anodes at 20°C and 70°C. Efficiencies are generally much higher than on graphite electrodes. However at low current densities at 70°C the efficiency on RuO₂ is considerably lower than on IrO₂.     

Anodic oxidation of phenol in the presence of NaCl for wastewater treatment

The electrochemical oxidation of phenol in the presence of NaCl for wastewater treatment was studied at Ti/SnO₂ and Ti/IrO₂ anodes. The experimental results have shown that the presence of NaCl catalyses the anodic oxidation of phenol only at Ti/IrO₂ anodes due to the participation of electro-generated ClO^– in the oxidation. Analysis of the oxidation products has shown that initially organo-chlorinated compounds are formed in the electrolyte which are further oxidized to volatile organics (CHCl₃).     

Design and electrochemical study of SnO2-based mixed oxide electrodes

For the electrochemical treatment of wastewater, it is critical to develop electrodes with a high activity for the oxidation of pollutants, long lifetimes, and low cost. In the present study, we have fabricated four different SnO₂-based electrodes (Ti/SnO₂-Sb₂O₅, Ti/SnO₂-Sb₂O₅-PtO_x, Ti/SnO₂-Sb₂O₅-RuO₂ and Ti/SnO₂-Sb₂O₅-IrO₂) using the thermal decomposition method and, for the first time, systemically studied their stability and electrocatalytic activity towards the degradation of 2-nitrophenol (2-NPh), 3-nitrophenol (3-NPh) and 4-nitrophenol (4-NPh). Scanning electron microscope (SEM) and X-ray energy dispersive spectrometry (EDS) were used to characterize the morphology and composition of the four different SnO₂-based electrocatalysts. Lifetime tests show that doping IrO₂ or RuO₂ greatly improves the stability of the SnO₂-based electrodes. The electrochemical activities of the prepared SnO₂-based electrodes were characterized using the degradation of 2-NPh, 3-NPh and 4-NPh. In situ UV/vis spectroscopy was used to monitor the concentration changes of the nitrophenols with time showing that the rate constants for the electrochemical oxidation of the nitrophenols decrease in the order of: 2-NPh > 4-NPh > 3-NPh. The effect of the applied current densities and initial concentrations of nitrophenols have also been investigated. Our study has shown that the fabricated Ti/SnO₂-Sb₂O₅-IrO₂ electrodes are very promising for the electrochemical treatment of wastewater.     

Electrochemical oxidation of p-chlorophenol on SnO2–Sb2O5 based anodes for wastewater treatment

The influence of an IrO₂ interlayer between the Ti substrate and the SnO₂–Sb₂O₅ coating on the electrode service life and on the efficiency of p-chlorophenol (p-CP) oxidation for wastewater treatment has been investigated. The results have shown that if the loading of the SnO₂–Sb₂O₅ coating relative to the IrO₂ interlayer loading ( ratio defined by Equation 1) is high ( = 20–30) the service life of the electrode can be increased without modification of the ability of this electrode to perform p-CP oxidation. This suggests that the oxidation of p-CP using a Ti/IrO₂/SnO₂–Sb₂O₅ electrode with high ratio ( > 20) occurs only through the SnO₂–Sb₂O₅ component of the electrode, with no interference of the IrO₂ interlayer. However, the electrode potential at a given current density is considerably lower in the case of the Ti/IrO₂/SnO₂–Sb₂O₅ electrode. In order to explain this decrease in electrode potential we speculate that water is firstly discharged on IrO₂, which is present in small amounts on the surface, forming hydroxyl radicals at a relatively low potential. These active hydroxyl radicals then migrate (spill over) towards the SnO₂–Sb₂O₅ coating, where they are physiosorbed and react with p-CP leading to complete combustion.     

Electrochemical degradation of Reactive Red 120 using DSA and BDD anodes

Electrochemical oxidation of an azo dye (Reactive Red 120) was studied in acidic media (1 M HClO₄) using DSA type (Ti/IrO₂–RuO₂) and boron doped diamond (BDD) anodes. Ti/IrO₂–RuO₂ exhibited low oxidation power with high selectivity to organic intermediates and low TOC removal (10% at 25 °C and 40% at 80 °C). On the other hand BDD was found to be suitable for total mineralization of the organic loading to CO₂. In both cases, the decoloration of the solution was almost 100% achieved very quickly with BDD (2 Ah L⁻¹) but only after long treatment with Ti/IrO₂–RuO₂ (25 Ah L⁻¹). The instantaneous current efficiency (ICE) was up to 0.13 in the case of Ti/IrO₂–RuO₂ and up to 0.45 in the case of BDD.     

Comparison of platinum with IrO2–Ta2O5 system for the stannous ion consumption in methane sulfonic acid baths with and without catechol

The stannous ion consumption in methane sulfonic acid (MSA) baths with and without catechol was quantitatively evaluated using platinum and IrO₂–Ta₂O₅/Ti anodes. The stannous ion is consumed at the same rate with both anodes, accompanied by the generation of tin sludge on the anode surfaces in the bath without catechol, while these undesirable phenomena are significantly suppressed in the bath containing catechol. Spectroscopic measurements indicate that stannous ion and catechol form a complex at the mole ratio of 1:2, thus preventing the oxidation of stannous ions. This complexation effect is independent of the anode material at low current density, but depends on the anode material at high current density. The stannous ion consumption at high current density is sufficiently inhibited only with the IrO₂–Ta₂O₅/Ti anode due to the low oxygen evolution potential. Voltammetric measurements also suggest that the continuous oxidation of stannous ion in the catechol-containing bath is simultaneous with oxygen evolution.     

Long service life IrO2/Ta2O5 electrodes for electroflotation

Ti/IrO₂-Ta₂O₅ electrodes prepared by thermal decomposition of the respective chlorides were successfully employed as oxygen evolving electrodes for electroflotation of waste water contaminated with dispersed peptides and oils. Service lives and rates of dissolution of the Ti/IrO₂-Ta₂O₅ electrodes were measured by means of accelerated life tests, e.g. electrolysis in 0.5M H₂SO₄ at 25°C and j = 2 A cm^–2. The steady-state rate of dissolution of the IrO₂ active layer was reached after 600–700 h (0.095 g Ir h^–1 cm^–2) which is 200–300 times lower than the initial dissolution rate. The steady-state rate of dissolution of iridium was found to be proportional to the applied current density (in the range 0.5–3 A cm^–2 ). The oxygen overpotential increased slightly during electrolysis (59–82 mV for j = 0.1 A cm^–2 ) and the increase was higher for the lower content of iridium in an active surface layer. The service life of Ti/IrO₂ (65 mol%)-Ta₂O₅ (35 mol%) in industrial conditions of electrochemical devices was estimated to be greater than five years.List of symbols a constant in Tafel equation (mV) - b slope in Tafel equation (mV dec^–1) - E potential (V) - f mole fraction of iridium in the active layer - j current density (A cm^–2) - l number of layers - m _Ir content of iridium in the active layer (mg cm^–2) - r dissolution rate of the IrO₂ active layer (g Ir h^–1 cm^–2) - T _c calcination temperature (°C) - _{O 2} oxygen overpotential (mV) - _{O 2} difference in oxygen overpotential (mV) - _A service life in accelerated service life tests (h) - _S service life in accelerated service life tests related to 0.1 mg Ir cm^–2 (h) - _p polarization time in accelerated service life tests (h)     

Influence of rare earths doping on the structure and electro-catalytic performance of Ti/Sb-SnO2 electrodes

Rare earth Ce, Eu, Gd and Dy doped Ti/Sb-SnO₂ electrodes were prepared by thermal decomposition and the performance of electrodes for the electro-catalytic decomposition of a model pollutant (phenol) was investigated. Phenol degradation and TOC removal followed pseudo-first-order kinetics in the experimental range, with the maximum rate achieved using Gd-doped electrode and the minimum rate obtained by Ce-doped electrode. Electrodes were characterized by cyclic voltammetry, X-ray diffraction, electron dispersive spectrometry, and X-ray photoelectron spectroscopy. It was suggested that the enhanced performance of the Gd-doped Ti/Sb-SnO₂ electrode arose from the increased adsorption capacity of hydroxyl radicals on the electrode surface and the lower mobility of oxygen atoms in SnO₂ lattice. The redox couple of Ce⁴⁺/Ce³⁺ on Ce-doped Ti/Sb-SnO₂ electrode surface functioned as mediators in the electrochemical oxidation process, allowing oxygen transfer in the SnO₂ lattice, and lowered the electro-catalytic ability of the electrode on phenol mineralization.     

Effect of substitution of SnO2 for TiO2 on the surface and electrocatalytic properties of RuO2+TiO2 electrodes

The effect of substituting SnO₂ for TiO₂ in RuO₂+TiO₂ electrodes has been studied by varying the SnO₂ content systematically in a series of oxides of general composition 30 mol% RuO₂+x mol% SnO₂+(70-x) mol% TiO₂. The surface properties have been investigated by voltammetric curves, the electrocatalytic activity by using O₂ evolution from 1 mol dm^–3 HClO₄ solutions as a test reaction. It has been observed that only the surface area changes at intermediate compositions as a result of morphological modifications, while the electrocatalytic activity increases dramatically as the substitution of SnO₂ for TiO₂ becomes complete. Reasons for that are discussed. The present results do not support the claim that SnO₂ depresses the electrocatalytic activity of oxide electrodes for oxygen evolution.Dedicated to Professor Dr Fritz Beck on the occasion of his 60th birthday.     

Effects of the geometry and operating temperature on the stability of Ti/IrO<Subscript>2</Subscript>–SnO<Subscript>2</Subscript>–Sb<Subscript>2</Subscript>O<Subscript>5</Subscript> electrodes for O<Subscript>2</Subscript> evolution

Ternary IrO₂–Sb₂O₅–SnO₂ anode has shown its superiorities over IrO₂ and many other electrocatalysts for O₂ evolution, in terms of electrochemical stability, activity and cost. The performance of IrO₂–Sb₂O₅–SnO₂ anodes is affected by its electrochemical properties and operating conditions. In this paper, the electrochemical stability and activity of the Ti/IrO₂–Sb₂O₅–SnO₂ anodes prepared with three different geometries were investigated under different operating conditions. It was found that anodes with large mean curvature have high electrochemical stability. Although increasing temperature results in a decrease in the stability of Ti/IrO₂–Sb₂O₅–SnO₂, the anode with a mean curvature of 200 m⁻¹ still shows acceptable service life even at 70 °C. This tolerance of high temperature was attributed to the thermal expansion difference between the substrate and the coating layer, the redox window for Ir(V)/Ir(IV) conversion, and the redox reversibility of Sb and Sn species in the coating layer.     

Preparation and evaluation of RuO2–IrO2, IrO2–Pt and IrO2–Ta2O5 catalysts for the oxygen evolution reaction in an SPE electrolyzer

IrO₂–RuO₂, IrO₂–Pt and IrO₂–Ta₂O₅ electrocatalysts were synthesized and characterized for the oxygen evolution in a Solid Polymer Electrolyte (SPE) electrolyzer. These mixtures were characterized by XRD and SEM. The anode catalyst powders were sprayed onto Nafion 117 membrane (catalyst coated membrane, CCM), using Pt catalyst at the cathode. The CCM procedure was extended to different in-house prepared catalyst formulations to evaluate if such a method could be applied to electrolyzers containing durable titanium backings. The catalyst loading at the anode was about 6 mg cm⁻², whereas 1 mg cm⁻² Pt was used at the cathode. The electrochemical activity for water electrolysis was investigated in a single cell SPE electrolyzer at 80 °C. It was found that the terminal voltage obtained with Ir–Ta oxide was slightly lower than that obtained with IrO₂–Pt and IrO₂–RuO₂ at low current density (lower than 0.15 A cm⁻²). At higher current density, the IrO₂–Pt and IrO₂–RuO₂ catalysts performed better than Ir–Ta oxide.     

Problems in DSA® coating deposition by thermal decomposition

Several DSA^® coatings were prepared by thermal decomposition of metallic chloride salts on titanium base. It was shown that the painting composition does not necessarily correspond to the final coating composition. Thermogravimetric and fluorescent X-ray measurements were used to identify and characterize the material losses. It was found that IrO₂, Ta₂O₅ and RuO₂ compounds can be deposited with almost 100% yield, while SnO₂ and Pt precursors give high material losses. The influence of parameters affecting the deposition yield are discussed.     

Synthesis and characterization of mesoporous Ta2O5–TiO2 photocatalysts for water splitting

Two series of Ta₂O₅–TiO₂ photocatalysts (Ta:Ti = 4:1, 1:1 and 1:4) were prepared by sol–gel technique applying triblock copolymer of Pluronic P123 and were tested in platinized form (0.3 wt.%) in photodecomposition of water under ultraviolet and visible light (λ > 300 nm). It was found the mesoporous character of tantalum containing catalysts with relatively high surface area (100–130 m² g⁻¹) of these samples. However, higher concentration of TiO₂ in mixed oxides leads to the destruction of mesoporous character of synthesized photocatalysts. All samples were characterized with thermogravimetry, XRD, N₂ physisorption, DR-UV–vis and FTIR spectroscopy. The mixed oxides of Ta₂O₅–TiO₂ system showed much lower band-gap than pure Ta₂O₅ and relatively high activity in platinized state in photocatalytic hydrogen generation under visible. Doping of pure oxides and mixed systems with sulfur resulted in lowering of the band-gap values below 3 eV and much better activity in H₂ evolution reaction. Non-platinized photocatalysts showed activity in liquid phase cyclohexene photooxidation at 305 K.     

Stable Ti/RuO2-Sb2O5-SnO2 electrodes for O2 evolution

RuO₂-based electrodes are generally known to be unstable for O₂ evolution. In this paper, a stable type of RuO₂-based electrode, Ti/RuO₂-Sb₂O₅-SnO₂, is demonstrated for O₂ evolution. In the ternary oxide coating, RuO₂ serves as the catalyst, SnO₂ as the dispersing agent, and Sb₂O₅ as the dopant. The accelerated life test showed that the Ti/RuO₂-Sb₂O₅-SnO₂ electrode containing 12.2 molar percent of RuO₂ nominally in the coating had a service life of 307 h in 3 M H₂SO₄ solution under a current density of 0.5 A cm⁻² at 25 °C, which is more than 15 times longer than other types of RuO₂-based electrodes. Instrumental analysis indicated that RuO₂-Sb₂O₅-SnO₂ was a solid solution with a compact structure, which contributed to the stable nature of the electrode.     

Orthorhombic tantalum pentoxide nanorods for electrochemical applications

Water splitting is an important method for hydrogen production. Notably, tantalum oxide has the potential to employ extensive variety appliances because of its outstanding electrical and optical properties. Tantalum pentoxide (Ta₂O₅) nanopowders were produced using the sol–gel process. The effect of calcination time plays a main role on Ta₂O₅ crystal structure configuration. Transmission electron microscope images explored obtained Ta₂O₅ nanorod formation and investigated by electrochemical studies for its use in electrochemical water-splitting applications. The calculated specific capacitance values of Ta₂O₅ electrodes at different temperature conditions were 146.4, 191.7, and 221.7 F/g. Fabricated Ta₂O₅ electrodes showed overpotential of 304, 278, and 267 mV. Current densities of Ta₂O₅ electrodes at different calcinations times were 353, 419, and 461 mA/g. Ta₂O₅ powder calcined for 6 h revealed high specific capacitance and low overpotential, indicating better electrochemical reactivity suitable for water oxidation applications.     

Preparation of Ti/SnO2-Sb2O4 photoanode by electrodeposition and dip coating for PEC oxidations

Thin films of antimony-doped SnO₂ on titanium substrate with a doping range of 1.5-8 mol% were prepared by an electrodeposition and dip coating method. The prepared Ti/SnO₂-Sb₂O₄ thin films were tested as a photoanode in the photoelectrocatalytic(PEC) experiments to degrade phenol in aqueous solution in order to evaluate their PEC performance. The photocatalytic (PC), electrocatalytic (EC) and PEC activity of Ti/SnO₂-Sb₂O₄ thin films was compared in the degradation processes. And the effect of annealing temperature on their PEC activity was also investigated. The experimental results confirmed that the Sb-doped Ti/SnO₂ thin films enhanced the phenol degradation and the Ti/SnO₂-Sb₂O₄ film containing 6 mol% of Sb calcinated at 450 °C achieved the best performance for phenol degradation. The degradation experiments also demonstrated that the Ti/SnO₂-Sb₂O₄ film achieved faster degradation of phenol in the PEC process than in the PC and EC processes. Compared with Ti/TiO₂ and Ti/SnO₂ photoanodes, the Ti/SnO₂-Sb₂O₄ photoanode showed higher activity.     

Faradaic impedance investigation of the deactivation mechanism of Ir-based ceramic oxides containing TiO2 and SnO2

Ti-supported IrO₂/SnO₂/TiO₂ coatings were prepared by thermal decomposition of the chloride precursor mixtures (400 C). The effect of the replacement of TiO₂ by SnO₂ on the service life of the Ti/IrO₂ + TiO₂ coating was investigated under galvanostatic polarization at 0.8 A cm^–2 in 1.0 mol dm^–3 HClO₄. The deactivation mechanism of the electrodes, using OER as the reaction model, was investigated through the periodic recording of voltammetric curves (VC) and impedance spectra (EIS) as a function of the anodization time. Bulk composition and morphology of the electrode coatings were investigated by energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) before and after accelerated life tests. The service life showed a dependence on the electrode composition. Replacement of TiO₂ by SnO₂ in the Ir_0.3Ti_0.7O₂ electrode led to a decrease in the service life of these systems above 30% mol SnO₂ content. This behaviour is directly related to morphological factors and the absence of synergetic effects. The behaviour of the parameters supplied by the EIS, VC, SEM, and EDX analyses made it possible to describe the overall deactivation mechanism.