The nitrate reduction process: A way for increasing interfacial pH Myriam Nobial a , Olivier Devos a , Oscar Rosa Mattos b , Bernard Tribollet a, * a UPR 15 du CNRS ‘‘Laboratoire Interfaces et Syste ` mes Electrochimiques’’, Universite ´ Pierre et Marie Curie, Case 133, 4 place Jussieu, 75252 Paris Cedex 05, France b Laboratorio de Corrosa ˜o Manuel de Castro, PEMM/COPPE Universidade Federal do Rio de Janeiro, Brazil Received 20 December 2005; received in revised form 28 February 2006; accepted 7 March 2006 Available online 24 April 2006 Abstract This paper is devoted to the electrochemical study of the nitrate reduction in aqueous media. The reduction process allows the pro- duction of hydroxyl ions at the electrode surface, increasing the local pH. This process provides a useful tool for the precipitation of metallic oxide/hydroxide such as ZnO. When the interfacial pH was high enough, the chemical equilibrium was shifted towards the crys- tallization. The nitrate reduction was carried out in the presence of dissolved oxygen. The investigated potential range corresponded to the mixed reduction of oxygen and nitrate where both reactions produced OH  and increased the interfacial pH. In this case, it was shown that the pH could reach a value higher than 12 in 1 M KNO 3 instead of 10.4 in the presence of only oxygen. Thus, nitrate reduc- tion was useful for the precipitation occurring at pH higher than 10.4. The electrochemical mechanism was studied by electrochemical impedance spectroscopy in a potential range corresponding to the first step of the oxygen reduction. The stationary current–voltage curve did not highlight any current due to the nitrate reduction. The impedance diagrams revealed two capacitive loops characterizing the reduction process of oxygen, namely the charge transfer process at high frequencies and the diffusion process at low frequencies according to the expectations. In the lowest frequency domain, an inductive loop was observed with an amplitude depending on the potential. It was shown that this response was due to the presence of adsorbed species which blocked a part of the active surface for the oxygen reduction. These adsorbed species came from nitrate reduction even if the stationary current of this reaction was negligible in the corresponding potential range. The decrease of the double layer capacity confirmed this approach. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Nitrate reduction; Adsorption; Impedance; Interfacial pH 1. Introduction Precipitation of different materials occurs when the pH is high enough. The shift to the basic pH can be obtained by imposing a cathodic reaction on the electrode. These materials include individual oxides (ZrO 2 , PbO, ZnO [1– 4]), scale deposits as CaCO 3 , [5,6] as well as compounds including BaTiO 3 [7], superconductors [8] and biomaterials [9,10]. The deposition is achieved from the metal ions by electro-generating hydroxyl ions to form metal oxide/ hydroxide films on a cathodic substrate. Different species for producing hydroxyl anions were proposed in the litera- ture like oxygen [11], water, hydrogen peroxide [12] or nitrate ions [13–16]. Reduction of these species allows a controlled pH value to be reached at the electrode/solution interface. As an example, the pH value corresponding to the precipitation of calcium carbonate occurs at a pH value close to 8 depending on the ionic strength of the carbonate- free water [17]. Thus, the reduction of the dissolved oxygen which produced OH  and a maximum pH of 10.4 was suf- ficient to form the scale deposition [18]. In the case of other metal oxide/hydroxide compounds, such as ZnO and ZrO 2 , the pH must be higher [1]. As a consequence, additional reduction processes producing hydroxyl ions are necessary, such as water or other soluble species reduction. Water reduction produces OH  and dihydrogen gas simulta- neously. However, the resulting H 2 bubbling on a substrate is not appropriate for precipitate formation. Another pos- sibility is the oxygen saturation of the solution which 0022-0728/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2006.03.003 * Corresponding author. Tel.: +33 1 44274170; fax: +33 1 44274074. E-mail address: bt@ccr.jussieu.fr (B. Tribollet). www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 600 (2007) 87–94 Journal of Electroanalytical Chemistry