Cyclic voltammetric studies of nitrate and nitrite ion reduction at the surface of iridium-modified carbon fiber electrode

The electroreduction of nitrate and nitrite on iridium-deposited carbon fiber electrode in aqueous sodium perchlorate solutions of pH 7 was studied using cyclic voltammetric techniques. Results obtained on the modified electrode support a two-step nitrate reduction mechanism in which nitrate is first reduced to nitrite at E_peak = -800 mV (Ag/AgCl) during the negative scan and further reduced to ammonia at E_peak = -775 mV (Ag/AgCl) during the anodic scan. An oxidation peak for the unreduced nitrite was observed at -300 mV (Ag/AgCl). A Tafel slope of approximately 178 mV/dec and a pseudo-first-order rate constant of k° = 4.18×10^-7 cm s^-1 were determined for the cathodic reduction peak while Tafel slope of 18.9 mV/dec and k° = 2.62×10^-23 cm s^-1 were obtained for the anodic reduction peak. The Tafel slopes obtained also support the two-step mechanism. The results obtained strongly suggest a minimal effect of pH on nitrate reduction in the pH range of 2.0-13.0 in NaClO₄ solution at the surface of iridium-coated carbon fiber electrode.