Sub-nanocatalysis for Efficient Aqueous Nitrate Reduction: Effect of Strong Metal-Support Interaction

Highly active electroreduction of nitrates to ammonia over a zeolitic imidazolium framework-derived Fe single-atom catalyst with sulfur-modified asymmetric active centers

The electroreduction of aqueous nitrate (NO3-) to ammonium is an energy-efficient process that helps protect the environment and facilitates ammonia production. However, a fine optimization of the catalyst structure containing active centers should be performed to improve the efficiencies of NO3- reduction and NH4+ production. Herein, a zeolitic imidazolate framework (ZIF)-derived sulfur-modified Fe single-atom catalyst is developed as an efficient and durable cathode material. Experimental and theoretical studies confirm the role of S-doping in modifying the electron density distribution of Fe centers, promoting the interaction between the Fe 3d orbital and O 2p orbital of NO3- and thereby enhancing its catalytic performance. A Faradaic efficiency of 93.9% for NH4+ production at - 0.47 V vs. the reversible hydrogen electrode is achieved, which remains at 91.0% even after six cycles. A synergistic effect between a defect-rich support and metal atom centers can be utilized to develop a new strategy for the facile design and implementation of high-performance electrocatalysts.

Electronic Metal-Support Interaction Triggering Interfacial Charge Polarization over CuPd/N-Doped-C Nanohybrids Drives Selectively Electrocatalytic Conversion of Nitrate to Ammonia

Selective electrocatalytic nitrate-to-ammonia conversion holds significant potential in treatment of nitrate wastewater and simultaneously produces high-value-added ammonia. However, today's development of nitrate-to-ammonia technology remains hindered by the lack of electrocatalysts with high activity and selectivity. In this work, metal-organic framework-derived CuPd bimetallic nanoparticles/nitrogen-doped carbon (CuPd/CN) hybrid nanoarrays for efficient ammonia electrosynthesis from nitrate are designed and synthesized. Systematic characterization reveals that the electronic metal-support interaction between the CuPd nanoparticles and N-doped nanocarbon matrix could trigger interfacial charge polarization over the CuPd/CN composite and make Cu sites electron deficient, which is conducive to the adsorption of nitrate ions. Moreover, the Pd atom sites separate by Cu atoms and could catalyze the dissociation of H₂ O molecules to form adsorbed H species, which evolves into hydrogen radicals and behaves as the dominant reactive species in accelerating nitrate-to-ammonia electrocatalysis. These advantages endow the CuPd/CN nanoarrays with high faradaic efficiency (96.16%), selectivity (92.08%) as well as excellent catalytic stability for electroreduction of nitrate to ammonia.

Facile Synthesis of Carbon Nanobelts Decorated with Cu and Pd for Nitrate Electroreduction to Ammonia

The electrocatalytic nitrate conversion of ammonia at ambient conditions provides not only a solution for restoring the imbalance in the global nitrogen cycle but also a sustainable alternative for the Haber-Bosch process. However, large-scale and efficient application of electrocatalytic denitrification has been limited by the lack of active catalysts with a high selectivity of nitrate reduction to N₂. In this work, we present a one-step solution processed synthetic strategy at low temperature to prepare carbon-nanobelts-supported uniform Cu and Pd nanoclusters. It is found that Cu catalyzed the formation of carbon nanobelts. The prepared samples were used for the green synthesis of ammonia from nitrate by electrocatalysis. For the nitrate reduction reaction (NO₃RR), Cu-Pd/C nanobelts show higher activity than Cu/C nanobelts, achieving a high yield of ammonia of 220.8 μg mg_cat^-1 h^-1 with a Faradaic efficiency (FE) of 62.3% at -0.4 V vs RHE (reversible hydrogen electrode), while for the nitrite reduction reaction (NO₂RR), a high FE of 95% at -0.2 V vs RHE can be obtained for Cu/C nanobelts with the yield of ammonia increased with the negative shift of the applied potentials. Theoretical calculations demonstrated that Pd and Cu are responsible for hydrogen evolution reaction (HER) and NO₃RR, respectively.

Engineering Nitrogen Vacancy in Polymeric Carbon Nitride for Nitrate Electroreduction to Ammonia

Electrocatalytic nitrate reduction to ammonia is of great interest in terms of energy conservation and environmental protection. However, the development of abundant metal-free electrocatalysts with high activity, selectivity, and stability is still a big challenge. Herein, polymeric graphitic carbon nitride (g-C₃N₄) with controllable numbers of nitrogen vacancies is reported to exhibit high Faradaic efficiency (89.96%), selectivity (69.78%), and stability toward nitrate-to-ammonia conversion. ¹⁵N isotope labeling experiments prove the produced ammonia originating from nitrate reduction. The combined results of ex situ and in situ characterizations unveil the reaction pathway based on the captured critical intermediates. Density functional theory calculations reveal that nitrogen vacancies could introduce a new electron state at the Fermi level and promote the adsorption, activation, and dissociation of nitrate. An appropriate content of nitrogen vacancies is beneficial for modulating the adsorption energies of reaction intermediates (*NO, *NOH, *NH₂, etc.), facilitating the enhancement in ammonia selectivity and Faradaic efficiency.

Atomically dispersed Fe atoms anchored on S and N-codoped carbon for efficient electrochemical denitrification

Nitrate, a widespread contaminant in natural water, is a threat to ecological safety and human health. Although direct nitrate removal by electrochemical methods is efficient, the development of low-cost electrocatalysts with high reactivity remains challenging. Herein, bifunctional single-atom catalysts (SACs) were prepared with Cu or Fe active centers on an N-doped or S, N-codoped carbon basal plane for N₂ or NH₄ ⁺ production. The maximum nitrate removal capacity was 7,822 mg N ⋅ g^-1 Fe, which was the highest among previous studies. A high ammonia Faradic efficiency (78.4%) was achieved at a low potential (-0.57 versus reversible hydrogen electrode), and the nitrogen selectivity was 100% on S-modified Fe SACs. Theoretical and experimental investigations of the S-doping charge-transfer effect revealed that strong metal-support interactions were beneficial for anchoring single atoms and enhancing cyclability. S-doping altered the coordination environment of single-atom centers and created numerous defects with higher conductivity, which played a key role in improving the catalyst activity. Moreover, interactions between defects and single-atom sites improved the catalytic performance. Thus, these findings offer an avenue for high active SAC design.

Selective Electrocatalytic Reduction of Nitrate to Ammonia with Nickel Phosphide

Liquid ammonia is considered a sustainable liquid fuel and an easily transportable carrier of hydrogen energy; however, its synthesis processes are energy-consuming, high cost, and low yield rate. Herein, we report the electrocatalytic reduction of nitrate (NO₃^-) (ERN) to ammonia (NH₃) with nickel phosphide (Ni₂P) used as a noble metal-free cathode. Ni₂P with (111) facet was grown in situ on nickel foam (NFP), which was regarded as a self-supporting cathode for ERN to synthesis NH₃ with high yield rate (0.056 mmol h^-1 mg^-1) and superior faradaic efficiency of 99.23%. The derived atomic H (*H), verified by a quenching experiment and an electron spin resonance (ESR) technique, effectively enhanced the high selectivity for NH₃ generation. DFT calculations indicated that *NO₃ was deoxygenated to *NO₂ and *NO, and *NO was subsequently hydrogenated with *H to generate NH₃ with an energy releasing process (ΔG < 0). OLEMS also proved that NO was the merely gas intermediate. NFP exhibited the unique superhydrophilic surface, metallic properties, low impedance, and abundant surface sites, favorable for adsorption of NO₃^-, generation of *H, and then hydrogenation of NO₃^-. Hence, NFP cathode showed high selectivity for NH₃ (89.1%) in ERN. NFP with long-term stability and low energy consumption provides a facile strategy for synthesis of NH₃ and elimination of NO₃^- contamination.

Synergistic Effect of Co(III) and Co(II) in a 3D Structured Co3O4/Carbon Felt Electrode for Enhanced Electrochemical Nitrate Reduction Reaction

As nitrate contamination causes serious environmental problems, it is necessary to develop stable and efficient electrocatalysts for efficient electrochemical nitrate reduction reaction (ENRR). Here, a nonprecious Co₃O₄/carbon felt (CF) electrode with a 3D structure was prepared by integrating electrodeposition with calcination methods. This 3D structured Co₃O₄/CF electrode exhibits a high-rate constant of 1.18 × 10^-4 s^-1 cm^-2 for the ENRR, surpassing other Co₃O₄ electrodes in previous literature. Moreover, it also has an excellent stability with a decrease of 6.4% after 10 cycles. Density functional theory calculations, electron spin resonance analysis, and cyclic voltammetry were performed to study the mechanism of the ENRR on the Co₃O₄/CF electrode, proving that atomic H* (indirect pathway) plays a prominent role in NO₃^- reduction and clarifying the synergistic effect of Co(III) and Co(II) in the Co(II)-Co(III)-Co(II) redox cycle for the ENRR: Co(III) prefers the adsorption of NO₃^- and Co(II) favors the production of H*. Based on this synergy, a relatively large amounts of Co(II) on the surface of the Co₃O₄/CF electrode (1.3 Co(II)/Co(III) ratio) was maintained by controlling the temperature of calcination to 200 °C with a lower energy barrier of H* formation of 0.46 eV than other ratios, which is beneficial for forming H* and enhancing the performance of the ENRR. Thus, this study suggests that building 3D structure and optimizing Co(II)/Co(III) ratio are important for designing efficient Co₃O₄ electrocatalyst for ENRR.

Enhancement in the rate of nitrate degradation on Au- and Ag-decorated TiO2 photocatalysts

The solar-driven reduction of nitrate to nitrogen has been studied in the presence of a formate hole scavenger, over a series of Au- and Ag-decorated TiO₂ catalysts.