Electrochemical reduction of nitrate to ammonia at modified gold electrodes

Abstract

The electrocatalytic activity of Au electrodes modified with thiourea (TU), urea (U), iodide ions (I −) and sulfide ions (S 2−) towards reduction of NO 3 − to NH 3 have been tested. Cyclic voltammetry (CV) of the TU/Au electrode in 0.5 M NaNO 3 (N 2-saturated, pH 12.5) revealed an increase of the reduction peak of NO 3 − at ca. +0.75 V versus reversible hydrogen electrode (RHE) more than at the bare (i.e., unmodified) Au electrode. On the other hand, the intensity of this peak is more depressed at I −/Au, S 2−/Au and U/Au electrodes. The Tafel slope is almost the same for the two reactions, i.e., the hydrogen evolution reaction (HER) and the nitrate reduction reaction suggesting a common rate-determining step (r.d.s.) for both reactions. Galvanostatic electrolysis of the basic nitrate solution resulted in three major reduction products i.e., NH 3, NO 2 − and H 2. The TU/Au electrode showed the highest current efficiency, Q, of the NH 3 production (about 85%), which is about 6.5 times higher than that of the NO 2 − production. While the I −/Au and S 2−/Au showed a sharp increase in the current efficiency for the hydrogen evolution reaction to about 43 and 47%, respectively, with a concurrent depression of the total current efficiency of the nitrate reduction.