Ammonia (NH3) is a crucial chemical feedstock used in fertilizers, pharmaceuticals, and dyes, and also serves as an energy storage medium and carbon-free energy carrier. The current industrial-scale ammonia synthesis, the Haber-Bosch process, is energy-intensive, operating under harsh conditions (high temperature and pressure) and contributing significantly to global CO2 emissions. Electrochemical ammonia synthesis using renewable electricity offers an attractive alternative. While electrochemical nitrogen reduction reaction (NRR) using atmospheric nitrogen (N2) has been explored, it suffers from low selectivity and activity, often producing ammonia at rates too low to be reliably distinguished from contamination. Nitrate (NO3-), a prevalent water pollutant, presents an alternative nitrogen source for electrochemical ammonia synthesis. While various metal catalysts have been used to selectively convert NO3- to N2, with NH3 as a byproduct, the development of efficient catalysts for selective nitrate reduction to ammonia is needed to create a viable and environmentally beneficial process. The reduction of NO3- to NH3 involves eight electron transfers and multiple possible pathways, underscoring the need for a deep understanding of the elementary steps to enable the rational design of selective catalysts. Single-atom catalysts (SACs) have shown promise in catalysis, offering unique activity and selectivity due to their atomic structure and electronic properties. However, to the best of the authors' knowledge, transition metal SACs for electrocatalytic NO3--to-NH3 conversion have not been reported. This study investigates the potential of Fe single-atom sites, inspired by the active sites in Haber-Bosch catalysts and nitrogenase enzymes, for this transformation.

The literature extensively covers the Haber-Bosch process's limitations and the search for alternative ammonia synthesis methods. Several studies have focused on electrochemical NRR using N2 as a nitrogen source, but challenges persist in achieving high selectivity and activity due to the strong N≡N triple bond. The use of nitrate as an alternative nitrogen source has also received attention, with studies reporting the use of various metal catalysts for nitrate reduction, primarily focusing on the production of N2. However, research on selective electrocatalytic nitrate reduction to ammonia remains limited. The use of single-atom catalysts has emerged as a promising area in catalysis, with studies demonstrating their unique advantages in several reactions; but their application in nitrate reduction to ammonia was yet to be explored in this study.

The Fe SAC was synthesized using a transition metal-assisted carbonization method with SiO2 as a hard template. The process involved mixing FeCl3, o-phenylenediamine, and SiO2 powder in isopropyl alcohol, followed by pyrolysis, alkaline and acidic leaching to remove the SiO2 template, and a second pyrolysis. The synthesized Fe SAC was characterized using various techniques, including transmission electron microscopy (TEM), aberration-corrected medium-angle annular dark-field scanning transmission electron microscopy (AC MAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), N2 sorption, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). Electrocatalytic nitrate reduction was performed in a customized H-cell under ambient conditions using a three-electrode system. The Fe SAC was deposited onto a glassy carbon electrode, and linear sweep voltammetry (LSV) was conducted in K2SO4 electrolyte with and without KNO3. Product selectivity was determined by holding a constant potential for 0.5 h and quantifying the produced NH3 using UV-Vis spectrophotometry and NMR. The main byproduct, NO2, was also quantified using UV-Vis. Control experiments were conducted using NC and Fe nanoparticles supported on NC (FeNP/NC) to assess the role of the single-atom Fe sites. Density functional theory (DFT) calculations were performed to investigate the reaction mechanism and identify potential limiting steps. The DFT calculations used the Fe-N4 motif as the active site model and explored different reaction pathways for the formation of NH3, NO, N2O, and N2.

The Fe single-atom catalyst (Fe SAC) exhibited high selectivity and activity for electrocatalytic nitrate reduction to ammonia. A maximal NH3 Faradaic efficiency of ~75% was achieved at -0.66 V vs. RHE, with an NH3 yield rate of ~20,000 µg h⁻¹ mgcat⁻¹. The NH3 selectivity increased with increasing negative potential. NMR results independently confirmed the UV-Vis spectrophotometry data, and isotope labeling experiments confirmed that the produced NH3 originated from the NO3- ions. The main byproduct was NO2, which showed a FE decreasing with increasing negative potential. The Fe SAC showed stable performance over 20 consecutive cycles and 35 h continuous electrolysis, demonstrating its durability. Control experiments with NC and FeNP/NC showed that Fe single-atom sites were significantly more active than nanoparticles, yielding ~20 times higher NH3 yield rate per molar Fe, and exhibiting enhanced stability. DFT calculations revealed the minimum energy pathway for nitrate reduction to NH3 on the Fe single-atom site. The potential limiting steps were identified as the reduction of NO to HNO and HNO to N. The DFT calculations explained the higher activity of Fe SAC compared to Co and Ni SACs. The calculations also revealed that N-N coupling pathways were energetically unfavorable due to the lack of neighboring active sites in the single-atom catalyst. Additional experiments varied nitrate concentration and pH, showing that the Fe SAC maintained high selectivity over a range of nitrate concentrations, but higher nitrate concentration improved yield rates.

The high activity and selectivity of the Fe SAC for nitrate reduction to ammonia demonstrate the potential of single-atom catalysts for this environmentally and economically significant reaction. The findings address the need for efficient catalysts for converting a common water pollutant into a valuable chemical. The prevention of N-N coupling by the isolated Fe atoms is a key factor in the high NH3 selectivity. The DFT calculations provide mechanistic insight, identifying the potential limiting steps and validating the experimental results. The combination of experimental data and theoretical modeling establishes a strong foundation for further design and optimization of catalysts for nitrate-to-ammonia conversion. This electrochemical approach offers a green and sustainable alternative to traditional ammonia synthesis methods.

This study successfully demonstrated an Fe single-atom catalyst for the highly selective and efficient electrocatalytic reduction of nitrate to ammonia. The high ammonia yield rate and Faradaic efficiency, coupled with the catalyst's stability, underscore the potential of this approach for sustainable ammonia production. Future research should focus on further enhancing catalytic activity and selectivity, adapting the system for real wastewater treatment, and optimizing reactor designs for low-concentration nitrate sources.

The study primarily focused on laboratory-scale experiments. Further research is needed to assess the scalability and economic viability of this process for industrial applications. The effect of potential contaminants in real wastewater on the catalyst's performance requires investigation. More detailed mechanistic studies using advanced in situ and operando characterization techniques are needed to further refine the understanding of the reaction pathway.