The nitrate reduction process: A way for increasing interfacial pH

This paper is devoted to the electrochemical study of the nitrate reduction in aqueous media. The reduction process allows the production of hydroxyl ions at the electrode surface, increasing the local pH. This process provides a useful tool for the precipitation of metallic oxide/hvdroxide such as ZnO. When the interfacial pH was high enough, the chemical equilibrium was shifted towards the crystallization. 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 KNO3 instead of 10.4 in the presence of only oxygen. Thus, nitrate reduction 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. (c) 2006 Elsevier B.V. All rights reserved.