Electrocatalytic reduction of nitrate ions in neutral medium at coinage metal-modified platinum electrodes

Nitrate is a water-soluble toxic pollutant that needs to be excluded from the environment. For this purpose, several electrochemical studies have been conducted but most of them focused on the nitrate reduction reaction (NRR) in alkaline and acidic media while insignificant research is available in neutral media with Pt electrode. In this work, we explored the effect of three coinage metals (Cu, Ag, and Au) on Pt electrode for the electrochemical reduction of nitrate in neutral solution. Among the three electrodes, Pt-Cu exhibited the best catalytic activity toward NRR, whereas Pt-Au electrode did not show any reactivity. An activity order of Pt-Cu > Pt–Ag > Pt-Au was observed pertaining to NRR. The Pt–Ag electrode produces nitrite ions by reducing nitrate ions (NO3-→NO2-). Meanwhile, at Pt-Cu electrode, nitrate reduction yields ammonia via both direct (NO3-→NH3) and indirect (NO3-→NO2-→NH3) reaction pathways depending on the potential. The cathodic transfer coefficients were estimated to be ca. 0.40 and ca. 0.52, while the standard rate constants for nitrate reduction were calculated as ca. 2.544 × 10^–2 cm.s⁻¹ and ca. 1.453 × 10^–2 cm.s⁻¹ for Pt-Cu and Pt–Ag electrodes, respectively. Importantly, Pt-Cu and Pt–Ag electrodes execute NRR in the neutral medium between their respective Hydrogen-Evolution Reaction (HER) and Open-Circuit Potential (OCP), implying that on these electrodes, HER and NRR do not compete and the latter is a corrosion-free process.