Electrochemical reduction of nitrate and nitrite at metal and metallic oxide electrodes.

Nickel and copper electrodes have been investigated as active cathode material for the electrochemical reduction of nitrate and nitrite in weakly acidic aqueous solution. The current density at - 1.4 V for nitrite reduction is slightly larger than that of nitrate for a Ni electrode whereas the reverse behavior is observed for a copper electrode (eg, current for nitrate > current for nitrite). The electrochemical oxidation of the Ni electrode led to a decrease of the electrocatalytic activity upon continuous cycling. This decrease is related to the presence of a nickel hydroxide layer which was confirmed by X-ray photoelectron spectroscopy (XPS). Indeed, nickel 2p core level spectrum of a nickel electrode following cycling between 0.1 and - 1.4 V shows peaks associated to nickel hydroxide. On the other hand, the electrochemical oxidation of copper has no detrimental effect on the electrocatalytic activity of copper electrode. On the contrary, an increase of the current at -1.4 V is noticed. This can be explained by the reduction of the copper oxide(s) during the cathodic scan and a slight increase of the surface area of the copper electrode. XPS does not show any evidence for thick copper oxide(s) formation when the Cu electrode was cycled between 0.1 (or -1)and 1.4 V. The role of the oxide/hydroxide and the nature of the metal on the electrocatalytic activity for nitrate and nitrite reduction is discussed.