Titanium Anodes with Active Coatings Based on Iridium Oxides: The Chemical Composition of the Coatings and the Distribution of Their Components over Depth on Anodes Made of IrO2, IrO2 + TiO2, IrO2 + RuO2 + TiO2, and IrO2 + RuO2 + TiO2 + Ta2O5

The distribution of components of active coatings over depth and the valence of metals that constitute the coatings on the IrO₂, IrO₂ + TiO₂, IrO₂ + RuO₂ + TiO₂, and IrO₂ + RuO₂ + TiO₂ + Ta₂O₅ anodes are established using Auger electron spectroscopy and x-ray photoelectron spectroscopy. It is shown that all metals, with the exception of tantalum, exist in a coating in a tetravalent state, in the form of relevant dioxides. Tantalum is present in the coatings in the form of Ta₂O₅. Etching the coatings with the argon and neon ions leads to the reduction of iridium and ruthenium dioxides to relevant metals and a partial reduction of TiO₂ to TiO. It follows that the x-ray photoelectron spectroscopy method allows one to determine the valence of metals that make up a coating only in the surface layers of the coatings. It is shown that for all the anodes, with the exception of anodes containing Ta₂O₅, the composition of a coating barely alters with depth and satisfactorily conforms to the composition specified by the coating formula. For the anodes whose coating is containing Ta₂O₅ there is observed high enrichment of surface layers of the coating by iridium and tantalum. This is probably explained by the system's multiphaseness and by a substantial difference in the temperatures at which the formation of relevant phases occurs in the course of pyrolysis.     

Titanium Anodes with an Active Coating Based on Iridium Oxides: The Effect of the Coating's Thickness,Porosity, and Morphology on Its Stability,Selectivity, and Catalytic Activity

The corrosion resistance and the electrochemical behavior of oxide iridium-ruthenium titanium anode (OIRTA) containing 4 mol % RuO₂ + 26 mol % IrO₂ + 70 mol % TiO₂, with a relatively thick active coating (iridium load 15 g m^?2) are studied in conditions of chlorine electrolysis. It is established that polarization curves for the chlorine evolution at low currents exhibit an extended “Tafel” segment with a “Nernstian” slope equal to 0.036 V and that the process rate is limited by the chlorine diffusion away from the electrode surface. In the region of high currents, which is precisely where the chlorine evolution reaction is realized in the industry, polarization curves start displaying an extended, practically horizontal, segment of low polarizability (SOLP) and the chlorine evolution occurs out of the entire depth of the coating. It is shown that the rate of iridium dissolution out of these anodes is in excess of the rate of its dissolution out of OIRTA with a thin coating and the larger the iridium load in the coating, the larger the excess. This phenomenon is attributed to a higher porosity of OIRTA with a thick coating and to the occurrence of the process of iridium dissolution out of the coating throughout the entire depth of the coating. As a result, such an increase in the coating's thickness is likely to lead to a decrease in the lifetime of the anodes. It is discovered that a prolonged polarization of OIRTA in the region of the SOLP leads to an increase in the overvoltage and to a practically complete disappearance of the SOLP from the polarization curves. All this served as the grounds for our drawing the conclusion that it would make no sense to enlarge the thickness and increase the porosity of the active coating of the OIRTA anodes in order to enhance their catalytic activity. It proved manageable to produce substantially more efficient anodes by depositing a thin active coating onto rough titanium out of a diluted covering solution. In so doing, the OIRTA anodes possessed a higher corrosion resistance and a better selectivity at a small iridium load in the coating.     

Titanium Anodes with Active Coatings Based on Iridium Oxides: The Corrosion Resistance and Electrochemical Behavior of Anodes Coated by Mixed Iridium,Ruthenium, and Titanium Oxides

A study of the corrosion resistance and electrochemical behavior of titanium anodes with active coatings prepared from mixed oxides iridium, ruthenium, and titanium (OIRTA) is continued. The dependence of the catalytic activity, selectivity, and corrosion resistance of these anodes with x mol % RuO₂ + (30 ? x ) mol % IrO₂ + 70 mol % TiO₂ is studied in conditions of chlorine electrolysis on the ratio of concentrations of IrO₂ and RuO₂ in them at a constant loading of iridium in the coatings. It is established that the maximum corrosion resistance and selectivity is inherent in OIRTA with the RuO₂ concentration close to 4 mol %. Partial curves, which describe the dependence of the rates of dissolution of iridium out of OIRTA and the evolution of chlorine and oxygen in them on the electrode potential, are obtained. The dependence of the rates of these processes on the solution pH, the concentration of NaCl in it, and the thickness of the active layer is studied. It is shown that the rate of dissolution of iridium out of OIRTA and the concentration of oxygen in chlorine at a constant potential increase approximately proportionally to the coating thickness, from whence it follows that the said processes proceed over the entire depth of the coating. An assumption is put forth that the chlorine evolution on OIRTA of the optimum composition, with a loading of iridium equal to 2.5 g m^?2, at high anodic currents occurs in an outer-kinetics regime in the presence of diffusion limitations on the removal of chlorine out of the coating's depth.     

Titanium Anodes with Active Coatings Based on Iridium Oxides: An Attempt to Elucidate the Possibility of Enhancing Catalytic Activity and Corrosion Resistance of Anodes at the Expense of Variations in the Formation Regime of the Coatings

On the basis of the polarization, corrosion, and radiotracer measurements it is established that the optimum conditions for the deposition of active coatings consisting of IrO₂ and IrO₂ + TiO₂ onto titanium anodes are the performing of the pyrolysis in air at T = 350°C for 15 min with a final anneal in the same environment at T = 450°C for 1 h. Removing the final anneal or reducing its temperature enhances the catalytic activity of the anodes but at the same time reduce their corrosion resistance. Raising the anneal temperature above 450°C makes no sense, as the catalytic activity of the anodes toward the chlorine evolution reaction substantially diminishes and the titanium support undergoes oxidation starting with 500°C.     

Sol–gel processing of IrO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> mixed metal oxides based on a hexachloroiridate precursor

Mixed IrO₂–TiO₂ oxides were prepared by the sol–gel method upon acid-catalysed hydrolysis of an iridium solution in ethanol mixed with titanium tetraethoxide in ethanol. The iridium solution was obtained by reaction of the sodium hexachloroiridate(IV) precursor in the presence of sodium ethoxide in ethanol. Gels were formed in all but the high-Ir samples. Analysis of the dried gels showed minority-phase enrichment at the surface and the presence of Ir(III), while microscopy showed evidence for dispersed iridium-containing nanoparticles (1–20 nm in diameter). XRD powder patterns of the calcined material showed peaks due to a small amount of crystalline NaCl impurity which could be removed by washing. This left amorphous phases, except in the Ir:Ti 3:2 case, which showed evidence for the presence of separate crystalline oxide phases: anatase, IrO₂ and Ti _x Ir_1−x O₂.     

Development of nano IrO<Subscript>2</Subscript> composite-reinforced nickel–phosphorous electrodes for hydrogen evolution reaction

Electroless and electroplated nickel electrodes are extensively used for hydrogen evolution reaction (HER). In the present work, TiO₂-supported IrO₂ mixed oxide composite was prepared and used to reinforce Ni–P electroless plates to be used as catalytic electrodes for HER. The electrodes exhibited high electrocatalytic activity when the electrodes were used for HER. All the parameters including particle size of the catalyst, surface roughness, and surface active sites were studied. The particle size of the IrO₂ catalyst in the mixed oxide was found to have high influence on the catalytic activity of the electrodes. Low overpotential as low as 70 mV at a current density of 200 mA cm⁻² was achieved with the mixed oxide-reinforced Ni–P electrodes.     

Effect of SO<Subscript>2</Subscript> on chlorination of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> + Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> mixtures

The reaction of Bi₂O₃ + Fe₂O₃ mixtures with chlorine and SO₂ at 250–700°C is studied. At 300–500°C, the degree of bismuth chloride sublimation from the oxide mixture increases in the presence of SO₂. Chemical sublimation of FeCl₃ occurs after BiCl₃ is virtually completely recovered from the solid phase.     

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.     

Phase formation in the V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript> system

Solid-phase interactions in the V₂O₅-Ta₂O₅-MoO₃ system were studied. The formation of com- pounds TaVO₅ and VTa₉O₂₅ in the V₂O₅-Ta₂O₅ binary system was verified. Tetragonal VTa₉O₂₅-base solid solutions of the general formula Ta_{5 + 4x} V_{5 − 4x} O₂₅ (x = 0.25–1) and TaVO₅-base solid solutions of the general formula Ta _x Mo_{1 − x} V_{2 − x} O_{8 − 3x} (x = 0.625–1) were found to form. Subsolidus phase equilibria in the V₂O₅-Ta₂O₅-MoO₃ were determined.     

Comparison of morphology, stability and electrocatalytic properties of Ru0.3Ti0.7O2 and Ru0.3Ti0.4Ir0.3O2 coated titanium anodes

This paper presents the comparative study of two dimensionally stable anodes (DSA?) of nominal composition Ti/Ru_0.3Ti_0.7O₂ and Ti/Ru_0.3Ti_0.4Ir_0.3O₂, prepared by thermal decomposition of the chloride precursor mixtures at 450°C. The materials were studied by scanning electron microscopy (SEM), cyclic voltammetry and Tafel measurements to obtain information about their surface and electrocatalytic properties towards Cl₂ evolution reaction. The stability of the prepared anodes was investigated under accelerated conditions. It has been observed that the coating surface with 30% mole IrO₂ possesses more compact structure with less cracks. Furthermore, it had excellent electrocatalytic activity for the chlorine evolution. Accelerated stability tests showed long lifetime for Ti/Ru_0.3Ti_0.4Ir_0.3O₂ electrode. On the other hand, besides the excellent improvement of catalytic activity, the stability of the ternary oxide electrode increases compared to the binary oxide one. The deactivation rate appeared to be a consequence of the different morphology and synergetic effects of the prepared coatings.     

RuO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> mixed oxide composite coating for improvement of Al-alloy sacrificial anodes

It has been recently proved that RuO₂ can act as an effective surface activator of aluminum alloy sacrificial anodes. TiO₂ has the property of stabilizing RuO₂ coating and resisting biofouling on metal surfaces. Hence, a mixed oxide catalytic coating of TiO₂ and RuO₂ can enhance the galvanic performance of aluminum alloy sacrificial anodes and resists biofouling on the anode surface. In the present work RuO₂–TiO₂ mixed oxide was coated on aluminum alloy sacrificial anodes. The large and uniform porous nature of the coating was found to facilitate efficient ion diffusion. The coating was found to persist on the anode even after 3 months of galvanic exposure. The anode having an optimum combination of the mixed oxide had 70% TiO₂ as the major component in the coating. The catalytic coating significantly improved the performance of the anodes to a large extent.     

Effects of the Chemical Modifier on the Thermal Evolution of SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> Precursor Powders

The effects of the chelating agent on the thermal evolution of SrBi₂Ta₂O₉ precursor powders were investigated. The precursor solutions were prepared from non-hydrolyzing precursors of bismuth and strontium and a tantalum alkoxide. The utilization of diethanolamine or triethanolamine as chelating agent was found to produce the segregation of metallic bismuth in the as-prepared powders, which led to the formation of a multiphase system. On the other hand, acetoin, one of the α-hydroxyketones, showed outstanding characteristics for the low-temperature synthesis of SrBi₂Ta₂O₉: elimination of residual organics at low temperature, an earlier onset of crystallization, and no segregation of secondary phases during the whole crystallization process.     

ESR Study of the Formation of O<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> Radical Anions on Oxidized CeO<Subscript>2</Subscript> and CeO<Subscript>2</Subscript>/ZrO<Subscript>2</Subscript> Adsorbing a CO + O<Subscript>2</Subscript> Mixture

It is demonstrated by ESR measurements that O ₂ ⁻ (CO + O₂) radical anions result from CO + O₂ adsorption on the oxidized surface of CeO₂. These radical anions are stabilized in the coordination sphere of Ce⁴⁺ cations located in isolated and associated anionic vacancies. This reaction shows an activation behavior determined by CO adsorption. The variation of O ₂ ⁻ (CO + O₂) concentration with CO adsorption temperature suggests that surface carbonates and carboxylates participate in this reaction. In the (0.5– 10.0)%CeO₂/ZrO₂ system, O ₂ ⁻ forms on supported CeO₂ and is stabilized on Ce⁴⁺ and Zr⁴⁺ cations. The stability of O ₂ ⁻ -Ce⁴⁺ complexes is lower on supported CeO₂ than on unsupported CeO₂, indicating a strong interaction between the cerium cations and the support.__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 423–429.Original Russian Text Copyright © 2005 by Il’ichev, Kuli-zade, Korchak.     

Yttria stabilized zirconia solid electrolyte surface modification with ZrO<Subscript>2</Subscript>, Y<Subscript>2</Subscript>O<Subscript>3</Subscript>, and ZrO<Subscript>2</Subscript> + 9 mol % Y<Subscript>2</Subscript>O<Subscript>3</Subscript> films

The surface of ceramic electrolyte ZrO₂ + 9 mol % Y₂O₃, hereinafter referred to as YSZ (abbreviated yttria stabilized zirconia), was modified with 0.1 to 0.2 μm oxide films of ZrO₂, Y₂O₃, and YSZ (same composition as substrate) by dip coating in alcohol solutions of the relevant salts and further annealing. The results of scanning electronic microscopy and X-ray diffraction evidence epitaxial film growth. By means of impedance spectroscopy at the temperatures of 500 to 600°C, the effect of YZS electrolyte surface modification with ZrO₂, Y₂O₃, and YSZ films to the polarization resistance of silver electrode was studied.     

Effect of SO<Subscript>2</Subscript> on chlorination of zinc ferrite ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> and a mixture of ZnO and Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

Results of thermodynamic calculations and kinetic studies of the reaction of zinc ferrite ZnFe₂O₄ and of a mixture of oxides, ZnO and Fe₂O₃, with chlorine and SO₂ are presented.     

Specific heat on Sr<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>7</Subscript> and Sr<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>7</Subscript>

Summary Specific heats on the single crystals of Sr₂Nb₂O₇, Sr₂Ta₂O₇ and (Sr_1-xBa_x)₂Nb₂O₇ were measured in a wide temperature range of 2-600 K. Heat anomalies of a λ-type were observed at the incommensurate phase transition of T_INC (=495 K) on Sr₂Nb₂O₇ and at the super-lattice phase transition of T_SL (=443 K) on Sr₂Ta₂O₇; the transition enthalpies and the transition entropies were estimated. Furthermore, a small heat anomaly was observed at the low temperature ferroelectric phase transition of T_LOW (=95 K) on Sr₂Nb₂O₇. The transition temperature T_LOW decreases with increasing Ba content x and it vanishes for samples of x>2%.     

The decomposition of gaseous products of the chemical transport reaction between H<Subscript>2</Subscript>O<Subscript>2</Subscript> and ZnO on the surface of silica gel

The decomposition of gaseous products of the chemical transport reaction that occurs in the interaction between H₂O₂ vapor and ZnO was studied on the surface of silica gel. At the initial stage of the decomposition of the intermediate complex formed in the chemical transport reaction between H₂O₂ and ZnO, the specific surface area of the sorbent increases noticeably. The pore size distribution maximum simultaneously shifts toward smaller values. The opposite effect is observed as the time of holding silica gel in a flow of gaseous products of the chemical transport reaction between H₂O₂ and ZnO increases. The treatment of the silica sorbent by the intermediate complex formed in the reaction between H₂O₂ and ZnO substantially influences the fractal dimension of its surface.     

Solid Solutions of the System V<Subscript>2</Subscript>O<Subscript>5</Subscript>-V<Subscript>2</Subscript>O<Subscript>4</Subscript>-TiO<Subscript>2</Subscript> in Air

Gravimetry in combination with X-ray phase analysis, X-ray crystallography, and X-ray densitometry were used to determine the contents of V⁵⁺, V⁴⁺, and Ti⁴⁺ ions and vacancies in solid solutions formed by the reaction of V₂O₅ with TiO₂ in air at atmospheric pressure.     

Electrochemical Characterisations of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–13%TiO<Subscript>2</Subscript> Coated by Atmospheric Plasma Spray

Atmospheric plasma sprayed alumina–titania (Al₂O₃–13%TiO₂), coated on stainless steel (XC18), were characterized. The coating structure and morphology were studied by scanning electron microscopy. Their presented micro cracks, laminar splats. The coatings were studied by X-ray diffraction. The main phase transformation is that of α-Al₂O₃ into metastable γ-Al₂O₃. The α-Al₂O₃ phase is due to the occurrence of partially melted particles Electrochemical behaviours of coatings were mainly investigated by potentiodynamic polarization and electrochemical impedance spectroscopy in 0.01 M [K₃Fe(CN)₆/K₄Fe(CN)₆] as a function of process parameters. Also, schematic equivalent circuit was proposed. The results were expected to facilitate the understanding and improvement of the coating behaviours.     

Properties of the surface layer and catalytic activity of Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with Pt or Pd additives in the oxidation of hydrogen

The catalytic activity in the oxidation of hydrogen (in the gaseous state in the presence of excess oxygen) has been studied for samples of Pt(Pd)/Ta₂O_5−x, formed by reduction with hydrogen. The samples obtained had greater activity than the traditional catalysts Pt(Pd)/Al₂O₃. According to X-ray diffraction analysis and electron microscopic studies, Ta₂O_5−x becomes amorphous with the formation of more reduced non-stoichiometric oxygen-deficient tantalum oxides with a surface layer of catalyst. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 3, pp. 180–185, May–June, 2008.