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: A Sublayer between the Active Coating and Titanium

An optimum composition and a technique for applying a protecting sublayer on titanium are substantiated experimentally. The sublayer prevents the oxidation of titanium during the production and application of highly porous metal oxide anodes. The formation of such a sublayer involves several stages: (a) coating chemically polished and etched in 5-% hydrofluoric acid titanium with hexachloroiridic acid, (b) drying hexachloroiridic acid, (c) a two-step treatment of anodes in argon with a low concentration of oxygen at 350°C, and (d) a brief annealing of the anodes in air at 400°C. The application of such a sublayer makes sense especially in the case of an anode with a thin highly porous active coating. The remarkable protecting properties of the sublayer are due to the formation of a dense coating on titanium. The coating consists of metallic iridium, titanium, and an amount of oxides of these metals. The titanium substrate itself undergoes minimum oxidation.     

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.     

Tantalum oxide effect on the surface structure and morphology of the IrO<Subscript>2</Subscript> and IrO<Subscript>2</Subscript> + RuO<Subscript>2</Subscript> + TiO<Subscript>2</Subscript> coatings and on the corrosion and electrochemical properties of anodes prepared from these

Alterations in the phase composition, porosity, and surface morphology of coatings are examined following the insertion of a quantity of Ta₂O₅ into active coatings prepared from IrO₂ or IrO₂ + RuO₂ + TiO₂ (OIRTA). It is shown that even an insignificant concentration of Ta₂O₅ in a coating renders it substantially amorphous and leads to the appearance of a large number of wide protracted cracks in the coating. The latter extends the surface of anodes and boosts their apparent catalytic activity in the chlorine evolution reaction. In addition, this accelerates the diffusion of chloride ions toward the front surface of anodes, which noticeably reduces the overvoltage of the chlorine evolution reaction when manufacturing sodium chlorate. The coatings’ amorphization and the development of their surface substantially reduce the corrosion resistance of these anodes as compared with OIRTA.     

Studies on the Surface Oxygen Species and the Catalytic Activity for CO Oxidation of La1-xSrxCo1-xMnxO3, a Perovskite-Type Oxides Catalyst

The surface oxygen distribution the active oxygen species for CO on the perovskite-type catalyst La_1-xSr_xCo_1-xMn_xO₃ and its catalytic oxidation activity with CO as probe were investigated by means of XRD, TPD and XPS in a continuous flow microreactor. Results showed that different adsorbed oxygen species and lattice oxygen were distributed on the catalyst surface. Meanwhile, the surface lattice oxygen of the oxides was reacting in the course of CO oxidation. This leads to the conclusion that, when x=0.6, the catalyst shows the best oxidative activity and lower starting temperature.     

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.