A study on performance improvement of Ir oxide-coated titanium electrode for organic destruction
Introduction
The Dimensionally Stable Anode (DSA) of catalytic oxide electrode, which can generate an active hydroxyl radical and active chloride species to destroy refractory organic waste into carbon dioxide and water with a relatively low overpotential to oxygen and chlorine evolutions and with a long lifetime, has been widely used in the fields of oxygen, chlorine production, and waste water treatment in the past two decades [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. The most representative DSA are RuO2/Ti and IrO2/Ti of a rutile structure. It is known that catalytic activity of RuO2 is higher than that of IrO2, whereas the IrO2 electrode is more stable and has a much longer lifetime than the RuO2 electrode so that IrO2 electrode has attracted a greater deal attention than RuO2 electrode in view of the commercial application of DSA [10], [15], [16], [17], [18]. However, despite the long lifetime property of the IrO2 anode, cases of its application to organic waste destruction are very rare, compared with that of RuO2 anode. Also, organic destruction yield by the IrO2 electrode has been confirmed in our laboratory to be much lower than those by RuO2 and SnO2 electrodes. The DSA electrode is generally evaluated in terms of electrochemical properties such as voltammetric charge capacity representing active surface sites of the oxide electrode, Tafel slope and overpotential for oxygen or chlorine evolution, electrode surface impedance, and so on [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Even though a DSA has good electrochemical and material properties, the electrode fabricated under those conditions may have a bad result for organic waste destruction. For example, the sintering temperature range of 400–550 °C recommended in the literature [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [15], [16], [17], [18], [19], [20], [21] for the fabrication of the Ir oxide electrode has been found in our experiments not to be the best temperature for the oxidative destruction of 4-chlorophenol. Therefore, the best fabrication condition of a catalytic oxide electrode for organic waste destruction should be decided after simultaneous evaluations of the material, electrochemical, and organic waste destruction properties of the electrode.
In this work, an improvement of Ir oxide electrode performance for organic destruction while keeping a long lifetime of the electrode was investigated by evaluating the several electrode properties mentioned above.
Section snippets
Experimental
All reagents for the precursor solutions used in this work were chemical reagent grade and used as received. RuCl3 (Adlich), TiCl4 (Yakuri Chem.) and IrCl3 (Alfa Aesar) were dissolved in 1:1 v/v HCl to prepare the precursor stock solutions of 0.2 M. Demineralized water of 18.2 MΩ prepared by distilling twice and an ion-exchanger (Mill-Q plus) was used for the preparation of the precursor solution and washing the Ti substrate.
The Ti substrate of the oxide electrode for the measurements of
Results and discussion
The final sintering temperature for the fabrication of an Ir oxide electrode has been known to have to be between 400 and 550 °C, in other words, not to exceed 600 °C, in the most papers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [15], [16], [17], [18], [19], [20], [21]. It is considered that the temperature was decided as the temperature which gave the electrode a low surface resistance and an appropriate electrochemical activity. Fig. 1 shows cyclic voltammograms of Ir oxide electrodes
Conclusions
An Ir oxide electrode sintered at around 650°C rather than 400–550 °C suggested in the literature enhanced organic destruction because the electrode surface was sufficiently converted to IrO2 from the IrCl3 of the precursor solution. The additional TiO2-screening layer between the IrO2 layer and the Ti substrate, preventing the solid diffusion of TiO2 oxidized from the Ti substrate during high-temperature sintering, further enhanced the organic destruction. The 4CP destruction yield on such
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