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.     

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 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: 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.     

Corrosion and electrochemical behavior of metal-oxide anodes during electrolysis with an ion-exchange membrane

The reasons for a specific behavior of anodes in chlorine electrolysis with an ion-exchange membrane are considered. The corrosion rate in modeling conditions and the predicted lifetime of anodes of different compositions are studied. The anodes’ coatings contain mixed oxides of Ir, Ru, Sn, or Ti. The effect of the anode’s position relative to the membrane and a relative stability of the anode coatings are examined in conditions simulating a heavy alkalization of the anolyte caused by a membrane rupture. A considerable advantage of using the anode with a coating containing 15, 15, and 70 mol % of RuO₂, IrO₂, and TiO₂ is demonstrated     

Formation,dissociation and expansion behavior of platinum group metal oxides (PdO,RuO2, IrO2)

The oxidation behavior of palladium, ruthenium and iridium powders of different grain sizes was investigated by TG, DTA and X-ray methods. The solid oxides formed during heating up (PdO, RuO₂, IrO₂) show different stability and decomposition temperatures depending on the oxygen pressure. The kinetics of the reaction MeO_x → Me+x/2 O₂ is discussed. High temperature X-ray studies confirmed the strong anisotropy of thermal expansion in the case of RuO₂ and IrO₂. The thermal expansion behavior of these oxides is compared to that of other rutile-type oxides.     

Energy resolved XPS depth profile of (IrO2, RuO2, Sb2O5, SnO2) electrocatalyst powder to reveal core‐shell nanoparticle structure

Synchrotron‐based energy resolved XPS was used to characterize the structure of IrO₂? RuO₂‐coated Sb₂O₅? SnO₂ nanoparticles. Samples were heat treated at 300, 350, 400, 450 and 500 °C after chloride Ir and Ru precursors were added to Sb₂O₅? SnO₂. Photoelectron kinetic energies of 100, 350 and 1400 eV were employed to obtain an indication of the depth of elemental distributions and chemical shifts. It was shown that the electrocatalyst consists of a core of Sb₂O₅? SnO₂ enriched with Sb₂O₅ towards the surface, with a shell of IrO₂? RuO₂ deposited on this core, and an outer layer of Sb₂O₅? SnO₂ over this shell. No significant chemical interaction occurs between IrO₂? RuO₂ and Sb₂O₅? SnO₂. The energy resolved XPS depth profile technique is effective for studying core‐shell materials. Copyright © 2010 John Wiley & Sons, Ltd.     

Investigation of RuO2-IrO2-SnO2 thin film evolution

The thermal evolution process of RuO₂–IrO₂–SnO₂ mixed oxide thin films of varying noble metal contents has been investigated under in situ conditions by thermogravimetry-mass spectrometry (TG-MS), infrared emission spectroscopy (IR) and cyclic voltammetry (CV). The gel-like films prepared from aqueous solutions of the precursor compounds RuOHCl₃, H₂IrCl₆ and Sn(OH)₂(CH₃COO)_2–xCl_x on titanium metal support were heated in an atmosphere containing 20% O₂ and 80% Ar up to 600°C. Chlorine evolution takes place in a single step between 320 and 500°C accompanied with the decomposition of the acetate ligand. The decomposition of surface species formed like carbonyls, carboxylates and carbonates occurs in two stages between 200 and 500°C. The temperature of chlorine evolution and that of the final film formation increases with the increase of the iridium content in the films. The anodic peak charge shows a maximum value at 18% iridium content.     

Low-temperature deposition and crystallization of RuO2/TiO2 on cotton fabric for efficient solar photocatalytic degradation of o-toluidine

A green low-temperature deposition and crystallization method was developed to uniformly coat RuO₂/TiO₂ nanocomposite onto cotton fabrics for efficient solar photocatalysis. The sequential growth of anatase TiO₂ and rutile RuO₂ on the surface of the cotton was confirmed by XRD, Raman and XPS characterizations. After the deposition of RuO₂, the optical properties of RuO₂/TiO₂/Cotton revealed better visible light absorption and higher charge mobility, and XPS spectra showed that the peaks of Ti 2p_3/2 and O 1 s shifted towards the lower binding energies due to the interfacial charge transfer at the robust RuO₂/TiO₂ mediated with Ti–O–Ru bonding. The photocatalytic performances of the RuO₂/TiO₂/Cotton were evaluated towards the photodegradation of o-toluidine (o-TD), an aromatic amine widely used in the chemical industry. Compared with TiO₂/Cotton, RuO₂/TiO₂/Cotton exhibited a remarkable improvement in the photocatalytic activity. The presence of RuO₂ on the surface of TiO₂/Cotton narrowed the band gap and improved the absorption of visible light. Moreover, the successful formation of a robust heterogeneous interface between TiO₂ and RuO₂ suppressed the charge carrier (e^–/h⁺) recombination effectively. With the RuO₂/TiO₂ coating chemically bound to the cotton fibers, RuO₂/TiO₂/Cotton delivered long-term stability in photocatalytic activity and high mechanical durability even after 20 washing times. Our facile and scalable synthesis strategy paved a universal route to efficient immobilization of visible-light-responsible TiO₂-based photocatalysts on the low-heat-resistant substrates for various applications.

Graphical abstract

     

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.     

Thermolytic formation and microstructure of IrO2 + Ta2O5 mixed oxide anodes from chloride precursors

The thermolytic formation of IrO₂+Ta₂O₅ mixed oxides from chloride precursors is studied by thermogravimetry (TGA) and differential thermal analysis (DTA). The structure and morphologies of the corresponding oxide films coated on titanium bases are determined by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM), respectively. The experimental results showed that, as a result of the interaction between Ir and Ta components, especially, the formation of solid solution phases during the thermolysis processes, the oxidative dissociation of the H₂IrCl₆+TaCl₅ mixture is facilitated. The catalytic effect reached the maximum at a nominal IrO₂ content of 70 mol% in the expected product, i.e. IrO₂+Ta₂O₅ mixed oxides, accompanied by the highest solid solubility between the two oxides and the finest rutile-structured crystalline grains in the oxides. For the mixed precursors with a low iridium content (e.g. 10 mol% nominal IrO₂ in IrO₂+Ta₂O₅) or a low tantalum content (e.g. 80 mol% nominal IrO₂), however, the decomposition of the major component is inhibited by the minor one at high temperatures (610-800 °C). The results show that the solid solution at low Ir contents (<30 mol% IrO₂) is unstable since it decomposes at high temperatures (≥750 °C). Two or more IrO₂ based rutile-constructed solid solution phases are thermolytically formed from the mixed precursors with nominal IrO₂ contents ≥30 mol%. The rutile-structured phases stably exist only in the case of IrO₂ contents ≥60 mol%.     

Thin and Thick RuO2-TiO2 Coatings on Titanium Substrates by the Sol-Gel Process

Thin and thick 20 mol% RuO₂–80 mol% TiO₂ coatings have been obtained through a combination of different preparation parameters, including catalyst and oxide concentrations, using ruthenium trichloride trihydrate and titanium n-butoxide. Acid catalysed solutions produced uniform, crack-free coatings that were too thin for use on electrocatalysis electrodes. On the other hand, base catalysed solutions produced thicker, cracked coatings. Electrodes prepared with base catalysed solutions performed well in electrocatalysis lifetime testing for corrosion stability.     

RuO2-doping into high-aspect-ratio anodic TiO2 nanotubes by electrochemical potential shock for water oxidation

A novel method for homogenous incorporation of Ru (RuO₂, or RuO₃) into high aspect ratio anodic TiO₂ NTs was studied. TiO₂ NTs were prepared by anodization in HF based electrolyte, after which very short high applied potential, referred to as potential shock, was imposed on the TiO₂ NTs in KRuO₄ electrolyte. The high potential shock induced massive flow of RuO₄⁻ to positively-biased TiO2 NTs, resulting in the incorporation of Ru as a form of Ru, RuO₂, and RuO₃ in the TiO₂ NTs. Optimal potential shock, which allowed the most suitable amount and incorporation state of Ru catalysts in TiO₂ NTs, was determined by SEM, TEM, EDS, XPS, and LSV. It was demonstrated that electrochemical potential shock (simply imposed on the anodic TiO₂ for a few seconds in the electrolyte of KRuO₄) resulted in homogenous incorporation of Ru into the whole nanotubes without the need for any complicated steps or facilities.     

Preparation and phase composition of Ta2O5:Zn alloys having low Zn2+ concentrations

Methods were developed for preparing Ta₂O₅:Zn alloys containing less than 3 wt % Zn²⁺ for the purpose of using them further in preparing lithium tantalate batches and growing from them single crystals having improved properties. A method where zinc is doped directly into a tantalum-containing back-extract followed by precipitation of tantalum and zinc hydroxides with ammonia is confined to a Zn²⁺ concentration of 1.7 wt % in Ta₂O₅; at higher concentrations, Zn²⁺ forms soluble ammine complexes. A method where Zn²⁺ is extracted by high-purity tantalum hydroxide is applicable within the range of Zn²⁺ concentrations studied. Optimal conditions were found for preparing Ta₂O₅:Zn²⁺ alloys of various compositions. X-ray powder diffraction and IR spectroscopy were used to study the phase composition of the alloys synthesized, and Zn²⁺ concentrations were determined at which a ZnTa₂O₆ phase was formed along with the major Ta₂O₅ phase.     

Activation of aromatic rings by early transition metal ions in the gas phase. Implications for the metal-catalyzed [2 + 2 + 2] cycloaddition of alkynes and nitriles

The reactions of a series of monocyclic and bicyclic arenes with early transition metal ions (Sc⁺, Y⁺, Nb⁺ and Ta⁺) and their oxides and dioxides were studied in a Fourier transform ion cyclotron resonance mass spectrometer. Ring cleavage of the nitrogen-containing heterocycles results in loss of HCN as the dominant pathway. Thermochemical considerations, secondary reactions and correlations with solution cyclotrimerization reactions indicate that the MC₄H₄⁺ product is a metallacyclopentadiene. Based on correspondence between the reactivities of a series of early metals with their valence electron counts, the reactivities of quinoline and isoquinoline and the decomposition behavior of the products, a metallacycloheptatriene intermediate is proposed for the heteroaromatic ring cleavage reaction. These results are compared to metal complexes in solution which catalyze the [2 + 2 + 2] cyclotrimerization of alkynes and nitriles.     

Study of the Efficiency of UV and Visible‐Light Photocatalytic Oxidation of Methanol on Mesoporous RuO2–TiO2 Nanocomposites

Mesoporous RuO₂–TiO₂ nanocomposites at different RuO₂ concentrations (0–10 wt %) are prepared through a simple one‐step sol–gel reaction of tetrabutyl orthotitanate with ruthenium(III) acetylacetonate in the presence of an F127 triblock copolymer as structure‐directing agent. The thus‐formed RuO₂–TiO₂ network gels are calcined at 450 °C for 4 h leading to mesoporous RuO₂–TiO₂ nanocomposites. The photocatalytic CH₃OH oxidation to HCHO is chosen as the test reaction to examine the photocatalytic activity of the mesoporous RuO₂–TiO₂ nanocomposites under UV and visible light. The photooxidation of CH₃OH is substantially affected by the loading amount and the degree of dispersion of RuO₂ particles onto the TiO₂, which indicates the exclusive effect of the RuO₂ nanoparticles on this photocatalytic reaction under visible light. The measured photonic efficiency ξ=0.53 % of 0.5 wt % RuO₂–TiO₂ nanocomposite for CH₃OH oxidation is maximal and the further increase of RuO₂ loading up to 10 wt % gradually decreases this value. The cause of the visible‐light photocatalytic behavior is the incorporation of small amounts of Ru⁴⁺ into the anatase lattice. On the other hand, under UV light, undoped TiO₂ shows a very good photonic efficiency, which is more than three times that for commercial photocatalyst, P‐25 (Evonik–Degussa); however, addition of RuO₂ suppresses the photonic efficiency of TiO₂. The proposed reaction mechanism based on the observed behavior of RuO₂–TiO₂ photocatalysts under UV and visible light is explored.     

Ca4IrO6, Ca3MgIrO6 and Ca3ZnIrO6

Single crystals of tetracalcium iridium hexaoxide, Ca₄IrO₆, tricalcium magnesium iridium hexaoxide, Ca₃MgIrO₆, and tricalcium zinc iridium hexaoxide, Ca₃ZnIrO₆, were prepared via high-temperature flux growth and structurally characterized by single-crystal X-ray diffraction. The three compounds are isostructural and adopt the K₄CdCl₆ structure type, comprised of chains of alternating face-shared [CaO₆], [MgO₆] or [ZnO₆] trigonal prisms and [IrO₆] octahedra, surrounded by columns of Ca²⁺ ions.     

Light energy accumulation using Ti/RuO2 electrode as capacitor

In the present study, the possibility to use Ti/RuO₂ electrode as capacitor for storage of photoelectrons generated under UV irradiation in Ti/TiO₂ photoelectrode has been investigated. A light-sensitive TiO₂ layer has been formed by means of anodizing Ti electrode in the solution of 0.5 M H₂SO₄. A layer of RuO₂, exhibiting the properties of electrochemical capacitor, has been formed by means of thermal decomposition of RuOHCl₃ also on Ti substrate. The photocharging capability of RuO₂ has been studied by means of short-circuiting Ti/RuO₂ electrode with Ti/TiO₂ photoelectrode in deaerated solution of 0.1 M KOH. It has been shown that the intensity of photocurrent flowing from Ti/TiO₂ to Ti/RuO₂ electrode depends mainly on the potential of the latter. Maximum value of photocurrent density was ∼180 μA cm⁻², which corresponded to maximum value of photon-to-electron conversion efficiency (IPCE) of about 60%. The amount of photogenerated charge Q _ph, which can be stored, depends on the capacitance of RuO₂ coating. Under the conditions of the experiment, Q _ph ranged from ∼35 to ∼50 mC, which corresponded to a specific charge of RuO₂ coating ranging between ∼20 and ∼30 mAh g⁻¹.     

Reasons for the DSA Passivation during Chlorate Electrolysis and the Means for Extending the Anode Service Life

Based on experiments and analysis of literature data, it is shown that a poor utilization of the active mass of the coating on metal oxide anodes of the DSA type during the electrochemical sodium chlorate production is due to pseudopassivation (an increase in the anodic potential due to a predominant corrosion dissolution of RuO₂as compared with TiO₂in the outer working zone of the coating), which develops with time. The true irreversible anode passivation, resulting from the formation of a stable barrier layer of higher titanium oxides at the coating/substrate interface, occurs when the DSA active mass wears out to a residual ruthenium content below 1.4 g/m²in it. The coating service life can be extended with no increase in the potential and with a minimum active mass consumption by using, concurrently with RuO₂and TiO₂, catalytically active additives with higher corrosion resistance than that of RuO₂at pH 6 to 7. This prevents the anode pseudopassivation and, with an increased initial amount of the active mass, makes it possible to use the anodes until its residual content exceeds the limit below which the true passivation starts, and regenerate the coating by applying fresh mass on the residue of the existing mass.     

中空Ta2O5/TiO2复合光催化剂的可控氧化制备及性能

以钛粉、钽粉为原料,炭黑作为反应性模板,通过熔盐法在炭黑表面原位生长了TaTiC₂纳米碳化物涂层,并以所得TaTiC₂/C复合物为碳化物前驱体,再经可控氧化制备出中空Ta₂O₅/TiO₂复合光催化剂。采用X射线粉末衍射（XRD）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）、紫外-可见（UV-Vis）漫反射（DRS）及N₂物理吸附等手段对所制备的光催化剂进行形貌、显微结构及孔结构表征。以高压汞灯为紫外光源,以亚甲基蓝为目标降解物,通过光催化降解实验评价中空Ta₂O₅/TiO₂复合光催化剂的光催化活性。结果表明,熔盐法生长碳化物涂层厚度均匀（20~30 nm）,碳化物主要以TaTiC₂晶相存在且具有纳米级的颗粒尺寸。中空Ta₂O₅/TiO₂复合光催化剂同时具有200 nm左右的中空大孔结构及壳层10 nm左右的介孔结构。中空大孔和介孔的存在提高了所制备催化剂对亚甲基蓝的吸附能力。此外,TiO₂与Ta₂O₅通过电子能带结构的耦合,有效提高了光生电子和空穴的分离效率,从而显著提高了光催化活性。n_Ti∶n_Ta=2.5∶1.5时,相应的中空Ta₂O₅/TiO₂复合光催化剂表现出最佳的光催化活性,对亚甲基蓝的紫外光催化降解率高达97%。