Selective Nitrate Reduction to Ammonia at Environmentally Relevant Concentrations with an Iron-Phthalocyanine Polymer

Electrochemical reduction of nitrate to ammonia is a promising strategy to simultaneously produce a valuable feedstock chemical while removing a widespread water pollutant. We report on an ammonia-selective iron phthalocyanine-based polymer synthesized through a Cu(I)-catalyzed 1,3-dipolar cycloaddition between 1,4-diethynylbenzene and tetraazidophthalocyanine Fe(II). The electrochemical performance was benchmarked against that of molecular iron phthalocyanine. The polymer achieves up to 95% ammonia selectivity following the tethering of phthalocyanine subunits and yields 1252 mg NH3-N L–1 gcat–1 from initial concentrations typically found in contaminated groundwater (30 mg NO3–-N L–1). XAS operando studies reveal a decrease in the oxidation state of the iron polymer during electrolysis followed by a prompt return to its original state after the current is removed, suggesting that Fe(II) is the active center. These findings demonstrate a strategy for limiting N–N coupling by increasing the intermolecular distances of molecular units through polymerization. This strategy enhances the stability and increases product selectivity toward ammonia by 20% for the polymer compared to the molecular unit.