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

Excessive use of anthropogenic nitrogen compounds has significantly disturbed the natural nitrogen cycle and detrimentally impacted local and global ecosystems.1-3 In particular, nitrate (NO3−) is a common nitrogen-containing contaminant in agricultural,4 industrial,5 and low-level nuclear wastewater6 that causes significant environmental damage including eutrophication when introduced into groundwater.7 Standard nitrate remediation techniques often involve separations that lead to highly concentrated nitrate brines that require further remediation. An alternate strategy is direct electrochemical nitrate remediation. In this work, we report a bio-inspired Cr-based molecular catalyst incorporated into a redox polymer that selectively and efficiently reduces aqueous NO3− to NH3, a desirable value-added fertilizer component and industrial precursor, at rates of ~0.29 mmol NH3 gcat-1 h-1 with >90% Faradaic efficiency for NH3. The NO3− reduction occurs via a cascade catalysis mechanism involving the stepwise reduction of NO3− to NH3 via observed NO2− and NH2OH intermediates. To our knowledge, this one of the first examples of a molecular catalyst, homogeneous or heterogenized, that reduces NO3− to NH3 at rates and 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 early transition-metal complexes for electrochemical nitrate remediation with nutrient recovery.