Electrochemical production of hydroxylamine from nitrate on metal electrodes: A comparative study of selectivity and efficiency

Despite the great potential of electrochemical nitrate reduction as a hydroxylamine production method, this strategy has not been sufficiently examined, and the effects of electrode material type on the selectivity and efficiency of this reduction remain underexplored. To bridge this gap, the present study evaluated six metals (Ag, Cu, Ni, Sn, Ti, and Zn) as cathode materials for the electrochemical reduction of nitrate to hydroxylamine, showing that the selectivity of hydroxylamine production was maximal for Sn, while the corresponding faradaic and energy utilization efficiencies were maximal for Ti. Although all tested materials favored nitrate reduction over hydrogen evolution, the disparity in the onset potentials of these reactions did not adequately explain the variations in nitrate removal efficiency, which was found to be influenced by material resistance and charge-transfer properties. The rate constants of elementary nitrate reduction steps determined from the time-dependent concentrations of nitrate and its reduction products (nitrous acid, hydroxylamine, and ammonium) were used to calculate the selectivity and efficiency of hydroxylamine production for each electrode. In turn, these selectivities and efficiencies were correlated with the density functional theory-computed adsorption energies of a key hydroxylamine precursor on different electrodes to afford a volcano-type plot with Ti and Sn at its pinnacle. Thus, this study introduces valuable descriptors and methods for the further screening of electrocatalysts for hydroxylamine generation and the establishment of more environmentally friendly hydroxylamine production techniques utilizing sustainable electricity.

Highly selective electrochemical reduction of nitrate via CoO/Ir-nickel foam cathode to treat wastewater with a low C/N ratio

Thorough nitrate removal from reclaimed water by biological techniques without carbon sources is difficult. Flexible, controllable electrochemical nitrate reduction is widely researched. Herein, ultrathin CoO nanosheets were constructed through amino group induction and orientation. The interfacial electron transfer resistance of two-dimensional CoO was 43.4% lower than that of one-dimensional nanoparticles, resulting in higher current density and improved nitrate reduction efficiency. Nickel foam and IrO2-nickel foam electrodes have almost no effect on nitrate reduction. It is worth noting that iridium loading on CoO (nanosheet) regulated the electronic band structure and generated active atomic H* . The nitrate removal rate increased from 45.1% (CoO (nanoparticle)-nickle foam) and 63.8% (CoO (nanosheet)-nickle foam) to 94.64% (CoO/Ir10 wt%-nickle foam). The proton enhancement effect improved indirect nitrate reduction by atomic H* and increased the NO3--N removal rate to 99.8%. Active chlorine species generated by Cl- in the wastewater selectively converted more than 99% of nitrate to N2, exceeding previous Co-based cathode results. In situ DEMS indicated that electrochemical reduction of nitrate included deoxidation (NO3-→*NO2-→*NO→*N/*N2O→N2) and hydrogenation (*NH2→*NH3→NH4+). The NO3--N removal rate of CoO/Ir10 wt% exceeded 65% during treatment of wastewater treatment plant effluents, verifying the feasibility of electrochemical nitrate reduction with the CoO/Ir10 wt% cathode. A strategy for designing electrochemical nitrate reduction electrocatalysts with excellent potential for full-scale application to treat wastewater treatment plant effluent is provided.

Electrochemical nitrate reduction over bimetallic Pd-Sn nanocatalysts with tunable selectivity toward benign nitrogen

Nitrate is recognized as a highly impactful water contaminant among various pollutants in water. To address the ever-growing demand for water purification, this work investigates the bimetallic palladium (Pd) and tin (Sn) catalysts, which are electrochemically deposited on stainless steel mesh support (Pd-Sn/SS) for the selective conversion of harmful nitrate (NO3-) into benign nitrogen (N2) gas. Results indicate that the bimetallic composition in Pd-Sn/SS electrodes substantially influenced the reaction route for nitrate reduction as well as the performance of nitrate transformation and nitrogen selectivity. It is found that the electrode prepared from Pd:Sn = 1:1 (mole ratio) demonstrates an outstanding nitrate conversion of 95%, nitrogen selectivity of 88%, and nitrogen yield of 82%, which outperform many reported values in the literature. The electrochemically synthesized bimetallic electrode proposed herein enables a new insight for promoting the reactivity and selectivity of nitrate reduction in water.

Bioinspired Tandem Electrode for Selective Electrocatalytic Synthesis of Ammonia from Aqueous Nitrate

The electrocatalytic nitrate reduction reaction (NO3RR) has recently emerged as a promising technique for readily converting aqueous nitrate (NO3-) pollutants into valuable ammonia (NH3). It is vital to thoroughly understand the mechanism of the reaction to rationally design and construct advanced electrocatalytic systems that can effectively and selectively drive the NO3RR. There are several natural enzymes that incorporate molybdenum (Mo) and that can activate NO3-. Based on this, a cadmium (Cd) single-atom anchored Mo2TiC2Tx electrocatalyst (referred to as CdSA-Mo2TiC2Tx) through the NO3RR to generate NH3 was rationally designed and demonstrated. In an H-type electrolysis cell and at a current density of 42.5 mA cm-2, the electrocatalyst had a Faradaic efficiency of >95% and an impressive NH3 yield rate of 48.5 mg h-1 cm-2. Moreover, the conversion of NO3- to NH3 on the CdSA-Mo2TiC2Tx surface was further revealed by operando attenuated total reflection Fourier-transform infrared spectroscopy and an electrochemical differential mass spectrometer. The electrocatalyst significantly outperformed Mo2TiC2Tx as well as reported state-of-the-art catalysts. Density functional theory calculations revealed that CdSA-Mo2TiC2Tx decreased the ability of the d-p orbital to hybridize with NH3* intermediates, thereby decreasing the activation energy of the potential-determining step. This work not only highlights the application prospects of heavy metal single-atom catalysts in the NO3RR but also provides examples of bio-inspired electrocatalysts for the synthesis of NH3.

Electrochemical nitrate reduction to high-value ammonia on two-dimensional molybdenum carbide nanosheets for nitrate-containing wastewater upcycling

Electrochemical conversion of nitrate wastewater into high-value ammonia fertilizer has attracted extensive attention in wastewater treatment and resource recovery, but presents great challenges due to complicated reaction pathways and competing side reactions. Herein, we report a feasible method for the successful fabrication of Mo₂C nanosheets (Mo₂C NSs) as electrocatalyst for the electroreduction of nitrate to ammonia. Compared to Mo₂C nanoparticles, the Mo₂C NSs exhibited superior activity and selectivity in NH₃ electrosynthesis with an NH₃ yield rate of 25.2 mg·h^-1·mg^-1_cat. at -0.4 V and a Faradaic efficiency of 81.4 % at -0.3 V versus reversible hydrogen electrode. The X-ray diffraction and transmission electron microscopy characterization verifted the controllable conversion of 2D MoO₂ NSs into 2D Mo₂C NSs. In situ spectroscopic studies and on-line differential electrochemical mass spectrometry revealed the proposed reaction pathway of NO₃^- to NH₃ conversion, *NO₃^- → *NO₂^- → *NO→*NOH → *NH₂OH → *NH₃. Density functional theory calculations further verified the effective N-end NOH pathway with the conversion of *NH₂OH to *NH₂ as the rate-determining step requiring a low energy barrier of 0.58 eV. Importantly, the key hydrogenation of *NO to form *NOH species underwent a lower energy barrier of 0.39 eV compared with the formation of *ONH species (1.06 eV).

Revealing the activity origin of ultrathin nickel metal-organic framework nanosheet catalysts for selective electrochemical nitrate reduction to ammonia: Experimental and density functional theory investigations

The electrochemical nitrate reduction reaction (NitRR) affords a sustainable way for nitrate mitigation and ammonia synthesis, but there are still some problems such as poor nitrate conversion, low ammonia selectivity, and slow reaction kinetics. A clear structure-performance relationship is essential for designing efficient catalysts and understanding the reaction mechanisms. Herein, ultrathin nickel metal-organic framework (Ni-MOF) nanosheets supported on Ni foam featuring a well-defined stable structure, large electrochemically active surface area, and low electron transport resistance were prepared by a one-step solvothermal process. At -1.4 V, the nitrate reduction, rate constant, ammonia selectivity, and yield reached 96.4%, 0.448 h^-1, 80%, and 110.13 ug·h^-1·cm^-2, respectively. Experimental and theoretical studies demonstrated that the hydroxyl-ligated Ni atoms exhibited higher nitrate adsorption properties and lower activation energy towards NitRR compared to carboxylic acid-ligated Ni atoms. Mechanism investigations revealed a nitrate-to-ammonia reaction pathway involving multiple intermediate species on Ni-MOF nanosheet catalysts. This work offers a new avenue to construct highly efficient electrocatalysts for the selective transformation of nitrate to valuable ammonia.

Enhancing Electrochemical Nitrate Reduction to Ammonia over Cu Nanosheets via Facet Tandem Catalysis

Electrochemical conversion of nitrate (NO3-) into ammonia (NH3) represents a potential way for achieving carbon-free NH3 production while balancing the nitrogen cycle. Herein we report a high-performance Cu nanosheets catalyst which delivers a NH3 partial current density of 665 mA cm-2 and NH3 yield rate of 1.41 mmol h-1 cm-2 in a flow cell at -0.59 V vs. reversible hydrogen electrode. The catalyst showed a high stability for 700 h with NH3 Faradaic efficiency of ~88% at 365 mA cm-2. In situ spectroscopy results verify that Cu nanosheets are in situ derived from the as-prepared CuO nanosheets under electrochemical NO3- reduction reaction conditions. Electrochemical measurements and density functional theory calculations indicate that the high performance is attributed to the tandem interaction of Cu(100) and Cu(111) facets. The NO2- generated on the Cu(100) facets is subsequently hydrogenated on the Cu(111) facets, thus the tandem catalysis promotes the crucial hydrogenation of *NO to *NOH for NH3 production.

Interfacial Reduction Nucleation of Noble Metal Nanodots on Redox-Active Metal-Organic Frameworks for High-Efficiency Electrocatalytic Conversion of Nitrate to Ammonia

Electrochemically converting nitrate to ammonia is a promising route to realize artificial nitrogen recycling. However, developing highly efficient electrocatalysts is an ongoing challenge. Herein, we report the construction of stable and redox-active zirconium metal-organic frameworks (Zr-MOFs) based on Zr₆ nanoclusters and redox-reversible tetrathiafulvalene (TTF) derivatives as inorganic nodes and organic linkers, respectively. The redox-active Zr-MOF can facilitate the in situ reduction of noble metal precursors free of external reductants and realize the uniform nucleation of noble metal nanodots (NDs) on Zr-MOF, achieving the preparation of M-NDs/Zr-MOF (M = Pd, Ag, or Au). The highly porous Zr-MOF with good conductivity can facilitate the mass transfer process. Among the M-NDs/Zr-MOF catalysts, Pd-NDs/Zr-MOF exhibits the highest electrocatalytic activity, delivering a NH₃ yield of 287.31 mmol·h^-1·g^-1_cat. and a Faradaic efficiency of 58.1%. The proposed interfacial reduction nucleation strategy for anchoring M NDs on Zr-MOFs can be applied to other challenging energy conversion reactions.

Combining electrochemical nitrate reduction and anammox for treatment of nitrate-rich wastewater: A short review

Treatment of nitrate-rich wastewater is important but challenging for the conventional biological denitrification process. Here, we propose combining the electrochemical reduction and anaerobic ammonium oxidation (anammox) processes together for treatment of nitrate-rich wastewater. This article reviews the mechanism and current research status of electrochemical reduction of nitrate to ammonium as well as the mechanism and applicability of the anammox process. This article discusses the principles, superiorities and challenges of this combined process. The feasibility of the combined process depends on the efficiency of electrochemical nitrate reduction to ammonium and the conditions in the anammox process to use the reduced ammonium as the substrate to achieve deep nitrogen removal. The article provides a feasible strategy for using the electrochemical reduction and anammox combined process to treat nitrate-rich wastewater.

Electrochemical nitrate removal with simultaneous magnesium recovery from a mimicked RO brine assisted by in situ chloride ions

Electrochemical reduction is effective to remove nitrate but byproducts such as ammonia and nitrite would need chloride addition for indirect oxidation to nitrogen gas. Herein, electrochemical nitrate reduction was investigated to remove nitrate from a mimicked reverse osmosis (RO) brine containing chloride that eliminates the need for external chloride addition. Both Cu/Zn and Ti nano cathodes exhibited the best performance of nitrate removal with >97 % removal in either Na₂SO₄ or NaCl electrolyte, although with different products. Complete nitrate reduction to nitrogen gas was realized in the RO brine whose complex composition decreased the electrode efficiency, for example from 71.4 ± 0.2%-49.4 ± 0.3 % with the Cu/Zn cathode after 5 cycles of operation. Magnesium was recovered at the same time of nitrate removal and the purity of Mg(II) could reach 96.8 ± 2.0 % after proper pH pre-treatment. In a preliminary adsorption study, a key byproduct - chlorate was reduced by 49.8 ± 2.7 % after 3-h adsorption by 100 g L^-1 activated carbon. These results have demonstrated the simultaneous electrochemical nitrate removal and resource recovery from a complex water like a RO brine and provided new information such as byproduct management and electrode deterioration.

Highly active and durable carbon electrocatalyst for nitrate reduction reaction

The development of a new class of carbon electrocatalysts for nitrate reduction reaction (NRR) that have high activity and durability is extremely important, as currently reported metallic electrocatalysts show a main drawback of low stability owing to leaching and oxidation. Herein, we demonstrate that a unique N-doped graphitic carbon-encapsulated iron nanoparticles can be utilized as a promising NRR electrocatalyst. The resulting Fe(20%)@N-C achieves a better nitrate removal proportion of 83.0% (attained in the first running cycle) compared to the efficiencies of other reference catalysts, including those with lower entrapped Fe content. The nitrogen selectivity is 25.0% in the absence of Cl^- and increases to 100% when supplemented with 1.0 g L^-1 NaCl. More importantly, there is no statistically significant difference (at a 95% confidence interval) regarding the removal percentage recorded over 20 cycles for the Fe(20%)@N-C cathode. We propose that the iron nanoparticles could attenuate the work function on the neighboring carbon atoms, which are the reactive sites for NRR, and that the graphitic shells hinder the access of the electrolyte, thus protecting the iron particles from dissolution and oxidation. Testing with the real industrial wastewater further demonstrates the superiority of Fe(20%)@N-C cathode towards NRR, as evidenced by efficient removal of nitrate available in the biological effluent from a local coking wastewater treatment plant.