Electrochimica Acta 56 (2011) 1361–1365 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Electrochemical oxidation of ammonia (NH 4 + /NH 3 ) on thermally and electrochemically prepared IrO 2 electrodes Agnieszka Kapalka a,∗ , Stéphane Fierro a , Zacharias Frontistis b , Alexandros Katsaounis b,∗ , Stefano Neodo a , Olivier Frey c , Nico de Rooij c , Kai M. Udert d , Christos Comninellis a a Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland b Department of Environmental Engineering, Technical University of Crete, GR-73100 Chania, Greece c Institute of Microengineering (IMT), Ecole Polytechnique Fédérale de Lausanne (EPFL), 2000 Neuchâtel, Switzerland d Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland article info Article history: Received 21 July 2010 Received in revised form 15 October 2010 Accepted 23 October 2010 Available online 4 November 2010 Keywords: Oxidation of ammonia IrO2 electrode pH 9 Deactivation Surface redox couple abstract The electrochemical oxidation of ammonia (NH 4 + /NH 3 ) in sodium perchlorate was investigated on IrO 2 electrodes prepared by two techniques: the thermal decomposition of H 2 IrCl 6 precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(III) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes. On both electrodes, ammonia is oxidized in the potential region of Ir(V)/Ir(IV) surface redox couple activity, thus, may involve Ir(V). During ammonia oxidation, TDIROF is deactivated, probably by adsorbed products of ammonia oxidation. To regenerate TDIROF, it is necessary to polarize the electrode in the hydrogen evolution region. On the contrary, AIROF seems not to be blocked during ammonia oxidation indicating its fast regeneration during the potential scan. The difference between both electrodes results from the difference in the activity of the iridium oxide surface redox couples. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Discharges of ammonia to the aquatic environment cause eutrophication and fish toxicity. Today, large treatment plants with biological nitrification and denitrification are used to remove ammonia from municipal wastewater. Recently, the concept of urine separation has been proposed as a new technology to improve and simplify nitrogen removal from wastewater [1]. Several pro- cesses have been proposed and tested for nitrogen removal from urine, but none of them has been implemented on a large scale so far [2]. Electrochemical removal of ammonia has been shown to be a promising method for degradation of ammonia along with organic compounds present in wastewater [3]. The process might also be well suited to remove ammonia and organic substances from urine. The main advantage of this technique is that no chemicals or bac- teria are required. In fact, only electrical energy is consumed for elimination of pollutants. The mechanism of ammonia electro-oxidation has been stud- ied mainly on Pt electrodes and Pt–Me (Me = Ni, Ir, Ru, Cu) binary ∗ Corresponding authors. E-mail addresses: agnieszka.cieciwa@epfl.ch (A. Kapalka), alex.katsaounis@enveng.tuc.gr (A. Katsaounis). alloys [4]. The mechanism of ammonia electro-oxidation on Pt- based electrodes involves dehydrogenation of adsorbed ammonia and formation of N 2 as a final product (Eqs. (1)–(3)) [4–6]. NH 3,ads → NH 2,ads + H + + e - (1) NH 2,ads → NH ads + H + + e - (2) NH x,ads + NH y,ads → N 2 + (x + y)H + + (x + y)e - (3) NH ads → N ads + H + + e - (4) Besides N 2 , the oxygenated nitrogen species (such as NO and N 2 O) might also be formed when the electrode surface becomes oxidized [4]. The mechanism of electro-oxidation of ammonia on metal oxide type electrodes, such as RuO 2 or IrO 2 is much less understood. It is known, however, that on these anodes ammonia is oxidized through several steps to various nitrogen compounds (Eq. (5)) [7,8]. (5) With this study, we aimed to gain a better understanding of the electrochemical oxidation of ammonia on IrO 2 electrodes. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.10.071