pH-Dependent Electrocatalytic Aqueous Ammonia Oxidation to Nitrite and Nitrate by a Copper(II) Complex with an Oxidation-Resistant Ligand

Abstract

The electrocatalytic aqueous ammonia oxidation (AO) represents a more sustainable alternative to accessing nitrite (NO2-) and nitrate (NO3-). We now report that Cu(pyalk)2 {pyalk = 2-(pyridin-2-yl)propan-2-oate}, previously employed as a homogeneous water oxidation (WO) catalyst, is also active for selective AO in aqueous environments. The traditional Griess analytical test for NO2-/NO3- was modified to permit the operation in the presence of the otherwise interfering Cu2+ ion. Choosing the right pH is crucial for achieving high AO selectivity, with optimal formation of NO2- occurring at pH 9 (faradaic efficiency 62%). Electrochemical analysis reveals a monometallic reaction pathway and offers a plausible explanation for the chemoselectivity: at pH 9, AO is dominant, while at elevated pH 13, WO dominates.