Selective Reduction of Aqueous Nitrate to Ammonium with an Electropolymerized Chromium Molecular Catalyst

Abstract

Nitrate (NO₃^-) is a common nitrogen-containing contaminant in agricultural, industrial, and low-level nuclear wastewater that causes significant environmental damage. In this work, we report a bioinspired Cr-based molecular catalyst incorporated into a redox polymer that selectively and efficiently reduces aqueous NO₃^- to ammonium (NH₄⁺), a desirable value-added fertilizer component and industrial precursor, at rates of ∼0.36 mmol NH₄⁺ mg_cat^-1 h^-1 with >90% Faradaic efficiency for NH₄⁺. The NO₃^- reduction reaction occurs through a cascade catalysis mechanism involving the stepwise reduction of NO₃^- to NH₄⁺ via observed NO₂^- and NH₂OH intermediates. To our knowledge, this is one of the first examples of a molecular catalyst, homogeneous or heterogenized, that is reported to reduce aqueous NO₃^- to NH₄⁺ with rates and Faradaic efficiencies comparable to those of state-of-the-art solid-state electrocatalysts. This work highlights a promising and previously unexplored area of electrocatalyst research using polymer-catalyst composites containing complexes with oxophilic transition metal active sites for electrochemical nitrate remediation with nutrient recovery.