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Chemical and Physical Characterization of Electrocatalysts based on Iridium Oxide and prepared by Sol-Gel and Physical Vapor Deposition
Full text linkElectrokinetic remediation of soils polluted by heavy metals (mercury in particular)
No full textThe electrokinetic approach, for the extraction of contaminants from a soil matrix, requires the application of electric fields of suitable intensity, through saturated portions of the soil. When aimed at the removal of species present in metallic form (like mercury) or as poorly conductive minerals (like some
sulfides), pollutants require to be preliminary dissolved, an action that can be facilitated by adding appropriate chemicals. In this paper, we show that the presence of an electric field is decisive, possibly because the pollutant-containing particles were able to act as polarizable species (and thus as improper electrodes). At first, tests were performed on small amounts of soil (about 200 g, in plastic bench cells); then, the process was scaled up, testing approximately 400 kg of soil. A 60% of total mercury was removed, in less than 3 months, by adopting specific expedients, in terms of hydraulic control as well as of plant design
Preparation by physical vapor deposition and characterization of films of iridium-tin oxides
No full textThe request for electrodes exhibiting satisfactory catalytic activity and high stability, under particular conditions of potential and current density, has stimulated the synthesis of new materials with improved performances. Among them, devices based on RuO2 and IrO2 found wide application in the field of DSAs (dimensionally stable anodes) for their ability to associate a good electrocatalytic activity to a high resistance to corrosion. Often, a greater stability leads to a decrease of catalytic activity, but it is possible to find a compromise by using mixtures of oxides. Besides the conductive and electroactive oxide (typically noble metal oxides, such as IrO2, RuO2, PtOx and RhOx), other oxides are added to the mixture (these are oxides of so-called "valve" metals, like TiO2, SnO2, Ta2O5, Nb2O5 and ZrO2), which have the dual purpose of increasing the corrosion resistance of the electrode, and to dilute the primary oxide as to minimize the device production costs.
For the preparation of film electrodes, a physical vapor deposition (PVD) technique has been used: in particular, the attention was focused on the reactive sputtering. This method ensures a better homogeneity of the film, in terms of thickness and roughness, compared to conventional pyrolytic techniques used for electrodes preparation.
The synthesis of thin-film-electrodes was carried out at room temperature, and their characterization was attained through electrochemical as well as surface tests; then, further information was obtained studying the catalytic properties of films towards the chlorine evolution reaction
The PVD approach for the preparation of Ir-Sn dioxide thin films
No full textThe request for electrodes exhibiting satisfactory catalytic activity and high stability, under particular conditions of potential and current density, has stimulated the synthesis of new materials with improved performances. Among them, devices based on RuO2 and IrO2 found wide application in the field of DSAs (dimensionally stable anodes) for their ability to associate a good electrocatalytic activity to a high resistance to corrosion. Often, a greater stability leads to a decrease of catalytic activity, but it is possible to find a compromise by using mixtures of oxides. Besides the conductive and electroactive oxide (typically noble metal oxides, such as IrO2, RuO2, PtOx and RhOx), other oxides are added to the mixture (these are oxides of so-called "valve" metals, like TiO2, SnO2, Ta2O5, Nb2O5 and ZrO2), which have the dual purpose of increasing the corrosion resistance of the electrode, and to dilute the primary oxide as to minimize the device production costs.
For the preparation of film electrodes, a physical vapor deposition (PVD) technique has been used: in particular, the attention was focused on the reactive sputtering. This method ensures a better homogeneity of the film, in terms of thickness and roughness, compared to conventional pyrolytic techniques used for electrodes preparation.
The synthesis of thin-film-electrodes was carried out at room temperature, and their characterization was attained through electrochemical as well as surface tests; then, further information was obtained studying the catalytic properties of films towards the chlorine evolution reaction
Influence of the nature of the electrode material and process variables on the kinetics of the chlorine evolution reaction. (I) The case of IrO2-based electrocatalysts
No full textKinetic studies on the chlorine evolution reaction (ChlER) on oxide-based materials have been the subject of a number of papers since the seventies, following the introduction of DSAs (Dimensionally Stable Anodes) in chlor-alkali plants. On the basis of experimental data, different pathways have been proposed for the reaction over the years. Actually, specific experimental conditions and different approaches in sample preparation may lead to conflicting explanations. In the present paper, the ChlER kinetics has been studied at four electrode materials based on iridium and titanium oxides (with a 1:2 molar ratio). Electrodes were synthetized at two temperatures (350 and 450 °C) and by two different preparation methods: physical vapor deposition (rf-magnetron sputtering) and a conventional sol-gel technique, using special precursors developed in our laboratory. Both methodologies guarantee a high level of reproducibility. As also observed by other authors, experimental data have shown a lack of linearity in Tafel plots, high b slopes and reaction orders with respect to chloride ≤ 1, which have been justified on the basis of a Volmer-Heyrovsky pathway, by considering a model proposed by Tilak and Conway in 1992. This approach highlighted the role of the adsorbed intermediates, also at low overpotentials, for all electrode materials. To analyze further the kinetics, Langmuir and Frumkin models for intermediates adsorption were considered. Values for the lateral interaction parameter g were estimated, which ranged between 1 and 10, in all cases. Concerning the effect of pH, its influence on the ChlER rate seems to be related only with electrode surface modifications, without any involvement of protons in the rate determining step of the process. A slight inhibiting effect was assessed, by increasing the protons concentration. Eventually, impedance spectroscopy analysis did not appear sensitive to intermediate adsorption, plausibly because of the low variation of the coverage within the Tafel region; a poorly resolved contribution related to porosity was found in the case of samples prepared at 350 °C.Kinetic studies on the chlorine evolution reaction (ChlER) on oxide-based materials have been the subject of a number of papers since the seventies, following the introduction of DSAs (Dimensionally Stable Anodes) in chlor-alkali plants. On the basis of experimental data, different pathways have been proposed for the reaction over the years. Actually, specific experimental conditions and different approaches in sample preparation may lead to conflicting explanations. In the present paper, the ChlER kinetics has been studied at four electrode materials based on iridium and titanium oxides (with a 1:2 molar ratio). Electrodes were synthetized at two temperatures (350 and 450 °C) and by two different preparation methods: physical vapor deposition (rf-magnetron sputtering) and a conventional sol-gel technique, using special precursors developed in our laboratory. Both methodologies guarantee a high level of reproducibility. As also observed by other authors, experimental data have shown a ..
Procedimento per depurare una matrice terrosa contaminata da metalli pesanti
No full textLa presente invenzione concerne un procedimento per depurare una matrice terrosa contaminata da metalli pesanti comprendente almeno: (a) predisporre almeno un pozzetto catodico all’interno di detta matrice terrosa, detto pozzetto comprendendo almeno un catodo immerso in una soluzione elettrolitica (soluzione catodica) inclusa all’interno di un contenitore catodico permeabile ai liquidi e alle specie ioniche; (b) predisporre almeno un pozzetto anodico all’interno di detta matrice terrosa, detto pozzetto comprendendo almeno un anodo immerso in una soluzione elettrolitica (soluzione anodica) inclusa all’interno di un contenitore anodico permeabile ai liquidi e alle specie ioniche; (c) immettere acqua, preferibilmente acqua comprendente un agente complessante (soluzione complessante), in detta matrice terrosa così da impregnare detta matrice terrosa sino a raggiungere un grado di saturazione pari ad almeno il 70% e formare complessi ionici metallici con detti metalli contaminanti; (d) applicare una differenza di potenziale a detti anodo e catodo così da far migrare detti complessi ionici metallici; (e) recuperare detti metalli contaminanti da una o più fra dette soluzione anodica, soluzione catodica e acqua di impregnazione di detta matrice terrosa
On the electrolysis of dilute chloride solutions: Influence of the electrode material on Faradaic efficiency for active chlorine, chlorate and perchlorate
No full textIn the present work, the electrolytic process of diluted aqueous chloride solutions was investigated at Ti/RuO2.2SnO2 and Ti/Pt electrodes, at different values of current density, temperature and electrolysis time. The time evolution of chlorine-related species (i.e. active chlorine (dissolved Cl2, HClO, OCl-), chlorite, chlorine dioxide, chlorate and perchlorate) was investigated in
order to establish whether their formation and consumption was related to either chemical or electrochemical path of reactions. The estimated faradic efficiencies demonstrated the better catalytic activity of the Ti/RuO2.2SnO2 electrode towards the chlorine evolution reaction with respect to the Ti/Pt anode, and the key-role played by the temperature, which reflects the different activation energies of the two competing electrochemical reactions, i.e. chloride and water oxidations. The concentration trends of chlorate and perchlorate indicated that the electrochemical route was responsible for their presence in the bulk solution, instead of a chemical path. The low concentration
levels assessed for chlorites throughout the tests did not suggest the preponderance of the chemical over the electrochemical depletion process; however, further potentiodynamic tests suggested their high reactivity towards both anodic and cathodic surfaces, thus suggesting that the electrochemical path of depletion could prevail over the chemical. Conversely, due to a solution pH unfavourable to the stability of chlorine dioxide, its low concentration level was associated to a chemical depletion route
Electrochemical properties of stoichiometric RuN film prepared by rf-magnetron sputtering: A preliminary study
No full textThe electrochemical properties and stability of ruthenium mononitride (RuN) thin films were studied. Coatings of RuN were synthesized on electropolished titanium supports by rf-magnetron sputtering. RuN electrodes appear rather stable against dissolution, independently of pH, but show to possess the greatest stability only in alkaline environment. Under hydrogen evolution conditions the films show relevant catalytic properties, comparable with Pt, Pd and Ru/Ir derivatives; a significant coverage by adsorbed reaction intermediates is involved. These electrodes are of potential application in energetics and sensoring
Investigation on the active chlorine production from diluted chloride solutions and electrochemical reactivity of their side-products
No full textThe electrolytic process of diluted aqueous chloride solutions (0.07 M) was investigated at Ti/RuO2.2SnO2 and Ti/Pt electrodes, at different values of current density (250, 750 and 1500 A m-2), temperature (10, 25 and 65 °C) and electrolysis time (0 – 180 minutes).
The time evolution of chlorine species (i.e. active chlorine, chlorite, chlorine dioxide, chlorate and perchlorate), as well as their faradic efficiency, were investigated to understand whether their formation and consumption was either chemical or electrochemical.
Different current efficiency profiles were generally observed for the two explored electrode materials, when the applied current density and the temperature were varied in the above reported experimental intervals. At the Ti/RuO2.2SnO2 (especially at the lowest current density), similar current efficiencies for the synthesis of active chlorine and chlorate were estimated. On the contrary, rather low current efficiencies for active chlorine and high current efficiencies for chlorate and perchlorate were measured at the Ti/Pt electrode.
The concentration trends of chlorate and perchlorate indicated that the electrochemical route was responsible for their presence in the bulk solution, instead of a chemical path. This insight was corroborate by the measured anodic potentials: high values for the anode potential were indeed related to high current efficiencies for chlorate and perchlorate.
The low concentration levels (and thus low current efficiencies) assessed for chlorite and chlorine dioxide throughout the tests were not useful for discriminating between a chemical and an electrochemical depletion processes; however, further potentiodynamic tests suggested that chlorites are highly reactive towards both anodic and cathodic surfaces, thus suggesting that the electrochemical path of depletion could prevail over the chemical one. Conversely, due to a solution pH quite unfavourable to the stability of chlorine dioxide, its low concentration level was justified by a chemical depletion route
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