Selective Nitrate-to-Ammonia Transformation on Surface Defects of Titanium Dioxide Photocatalysts

Nanoarchitectonics of S-Scheme Heterojunction Photocatalysts: A Nanohouse Design Improves Photocatalytic Nitrate Reduction to Ammonia Performance

Photocatalytic nitrate reduction reaction (NitRR) is a promising route for environment remediation and sustainable ammonia synthesis. To design efficient photocatalysts, the recently emerged nanoarchitectonics approach holds great promise. Here, we report a nanohouse-like S-scheme heterjunction photocatalyst with high photocatalytic NitRR performance. The nano-house has a floor of plate-like metal organic framework-based photocatalyst (NH2-MIL-125), on which another photocatalyst Co(OH)2 nanosheet is grown while ZIF-8 hollow cages are also constructed as the surrounding wall/roof. Experimental and simulation results indicate that the positively charged, highly porous and hydrophobic ZIF-8 wall can modulate the environment in the nanohouse by (i) NO3 - enrichment/NH4 + discharge and (ii) suppression of the competitive hydrogen evolution reaction. In combination with the enhanced electron-hole separation and strong redox capability in the NH2-MIL-125@Co(OH)2 S-scheme heterjunction confined in the nano-house, the designed photocatalyst delivers an ammonia yield of 2454.9 μmol g-1 h-1 and an apparent quantum yield of 8.02 % at 400 nm in pure water. Our work provides new insights into the design principles of advanced photocatalytic NitRR photocatalyst.

A hydrogen radical pathway for efficacious electrochemical nitrate reduction to ammonia over an Fe-polyoxometalate/Cu electrocatalyst

Electrochemical nitrate (NO3-) reduction to ammonia (NH3), which is a high value-added chemical or high-energy density carrier in many applications, could become a key process overcoming the disadvantages of the Haber-Bosch process; however, current electrocatalysts have severe drawbacks in terms of activity, selectivity, and stability. Here, we report the hydrogen radical (H*) pathway as a solution to overcome this challenge, as demonstrated by efficacious electrochemical NO3- reduction to NH3 over the Fe-polyoxometalate (Fe-POM)/Cu hybrid electrocatalyst. Fe-POM, composed of Preyssler anions ([NaP5W30O110]14-) and Fe cations, facilitates efficient H* generation via H2O + e- → H* + OH-, and H* transfer to the Cu sites of the Fe-POM/Cu catalyst enables selective NO3- reduction to NH3. Operando spectroelectrochemical spectra substantiate the occurrence of the H* pathway through direct observation of Fe redox related to H* generation and Cu redox related to NO3- binding. With the H* pathway, the Fe-POM/Cu electrodes exhibit high activity for NO3- reduction to NH3 with 1.44 mg cm-2 h-1 in a 500 ppm NO3-/1 M KOH solution at -0.2 V vs. RHE, which is about 36-fold higher than that of the pristine Cu electrocatalyst. Additionally, it attains high selectivity with a faradaic efficiency of up to 97.09% at -0.2 V vs. RHE while exhibiting high catalytic stability over cycles.

High Ammonia Yield Rate from Dilute Nitrate Solutions Using a Cu(100)-Rich Foil: A Step Closer to Large-Scale Production

The electrochemical reduction of nitrate (NO3-) ions to ammonia (NH3) provides an alternative method to eliminate harmful NO3- pollutants in water as well as to produce highly valuable NH3 chemicals. The NH3 yield rate however is still limited to the μmol h-1 cm-2 range when dealing with dilute NO3- concentrations found in waste streams. Copper (Cu) has attracted much attention because of its unique ability to effectively convert NO3- to NH3. We have reported a simple and scalable electrochemical method to produce a Cu foil having its surface covered with a porous Cu nanostructure enriched with (100) facets, which efficiently catalyzes NO3- to NH3. The Cu(100)-rich electrocatalyst showed a very high NH3 production rate of 1.1 mmol h-1 cm-2 in NO3- concentration as low as 14 mM NO3-, which is 4-5 times higher than the best-reported values. Increasing the NO3- concentration (140 mM) resulted in an NH3 production yield rate of 3.34 mmol h-1 cm-2. The durability test conducted for this catalyst foil in a flow cell system showed greater than 100 h stability with a Faradaic efficiency greater than 98%, demonstrating its potential to be used on an industrially relevant scale. Further, density functional theory (DFT) calculations have been performed to understand the better catalytic activity of Cu(100) compared to Cu(111) facets toward NO3-RR.

Photocatalytic NO x removal and recovery: progress, challenges and future perspectives

The excessive production of nitrogen oxides (NO x ) from energy production, agricultural activities, transportation, and other human activities remains a pressing issue in atmospheric environment management. NO x serves both as a significant pollutant and a potential feedstock for energy carriers. Photocatalytic technology for NO x removal and recovery has received widespread attention and has experienced rapid development in recent years owing to its environmental friendliness, mild reaction conditions, and high efficiency. This review systematically summarizes the recent advances in photocatalytic removal, encompassing NO x oxidation removal (including single and synergistic removal and NO3 - decomposition), NO x reduction to N2, and the emergent NO x upcycling into green ammonia. Special focus is given to the molecular understanding of the interfacial nitrogen-associated reaction mechanisms and their regulation pathways. Finally, the status and the challenges of photocatalytic NO x removal and recovery are critically discussed and future outlooks are proposed for their potential practical application.

Concurrent Ammonia Synthesis and Alcohol Oxidation Boosted by Glutathione-Capped Quantum Dots under Visible Light

Mother nature accomplishes efficient ammonia synthesis via cascade N2 oxidation by lightning strikes followed with enzyme-catalyzed nitrogen oxyanion (NOx -, x = 2,3) reduction. The protein environment of enzymatic centers for NOx --to-NH4 + process greatly inspires the design of glutathione-capped (GSH) quantum dots (QDs) for ammonia synthesis under visible light (440 nm) in tandem with plasma-enabled N2 oxidation. Mechanistic studies reveal that GSH induces positive shift of surface charge to strengthen the interaction between NOx - and QDs. Upon visible light irradiation of QDs, the balanced and rapid hole and electron transfer furnish GS·radicals for 2e-/2H+ alcohol oxidation and H·for 8e-/10H+ NO3 --to-NH4 + reduction simultaneously. For the first time, mmol-scale ammonia synthesis is realized with apparent quantum yields of 5.45% ± 0.64%, and gram-scale synthesis of value-added acetophenone and NH4Cl proceeds with 1:4 stoichiometry and stability, demonstrating promising multielectron and multiproton ammonia synthesis efficiency and sustainability with nature-inspired artificial photocatalysts.

Sunlight-Driven Nitrate-to-Ammonia Reduction with Water by Iron Oxyhydroxide Photocatalysts

The photocatalytic reduction of harmful nitrates (NO3-) in strongly acidic wastewater to ammonia (NH3) under sunlight is crucial for the recycling of limited nitrogen resources. This study reports that a naturally occurring Cl--containing iron oxyhydroxide (akaganeite) powder with surface oxygen vacancies (β-FeOOH(Cl)-OVs) facilitates this transformation. Ultraviolet light irradiation of the catalyst suspended in a Cl--containing solution promoted quantitative NO3--to-NH3 reduction with water under ambient conditions. The photogenerated conduction band electrons promoted the reduction of NO3--to-NH3 over the OVs. The valence band holes promoted self-oxidation of Cl- as the direct electron donor and eliminated Cl- was compensated from the solution. Photodecomposition of the generated hypochlorous acid (HClO) produced O2, facilitating catalytic reduction of NO3--to-NH3 with water as the electron donor in the entire system. Simulated sunlight irradiation of the catalyst in a strongly acidic nitric acid (HNO3) solution (pH ∼ 1) containing Cl- stably generated NH3 with a solar-to-chemical conversion efficiency of ∼0.025%. This strategy paves the way for sustainable NH3 production from wastewater.

Highly efficient nitrogen fixation over S-scheme heterojunction photocatalysts with enhanced active hydrogen supply

Photocatalytic N2 fixation is a promising strategy for ammonia (NH3) synthesis; however, it suffers from relatively low ammonia yield due to the difficulty in the design of photocatalysts with both high charge transfer efficiency and desirable N2 adsorption/activation capability. Herein, an S-scheme CoSx/ZnS heterojunction with dual active sites is designed as an efficient N2 fixation photocatalyst. The CoSx/ZnS heterojunction exhibits a unique pocket-like nanostructure with small ZnS nanocrystals adhered on a single-hole CoSx hollow dodecahedron. Within the heterojunction, the electronic interaction between ZnS and CoSx creates electron-deficient Zn sites with enhanced N2 chemisorption and electron-sufficient Co sites with active hydrogen supply for N2 hydrogenation, cooperatively reducing the energy barrier for N2 activation. In combination with the promoted photogenerated electron-hole separation of the S-scheme heterojunction and facilitated mass transfer by the pocket-like nanostructure, an excellent N2 fixation performance with a high NH3 yield of 1175.37 μmol g-1 h-1 is achieved. This study provides new insights into the design of heterojunction photocatalysts for N2 fixation.

Enhanced interfacial effect via acid theory with boosted photocatalytic performance of nitenpyram removal

Surface reaction is a prominent aspect that affects the efficiency of photocatalysis. In this work, acid theory was employed to facilitate the reaction dynamics and enhance the interfacial effect between photocatalysts and target molecules. The photocatalytic removal efficiency of NTP was 66 % for bare CdS in 50 min with apparent rate constants of 0.023 compare to 96 % with apparent rate constants of 0.065 for 5% Ce-CdS. The introduced Ce atom as bifunctional active site reduces the energy barrier of O2 adsorption, strengthens the interfacial effect and accelerates the electrons transfer, which could facilitate surface reaction process and boost the photocatalytic performance.

Identifying the active sites and intermediates on copper surfaces for electrochemical nitrate reduction to ammonia

Copper (Cu) is a widely used catalyst for the nitrate reduction reaction (NO3RR), but its susceptibility to surface oxidation and complex electrochemical conditions hinders the identification of active sites. Here, we employed electropolished metallic Cu with a predominant (100) surface and compared it to native oxide-covered Cu. The electropolished Cu surface rapidly oxidized after exposure to either air or electrolyte solutions. However, this oxide was reduced below 0.1 V vs. RHE, thus returning to the metallic Cu before NO3RR. It was distinguished from the native oxide on Cu, which remained during NO3RR. Fast NO3- and NO reduction on the metallic Cu delivered 91.5 ± 3.7% faradaic efficiency for NH3 at -0.4 V vs. RHE. In contrast, the native oxide on Cu formed undesired products and low NH3 yield. Operando shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) analysis revealed the adsorbed NO3-, NO2, and NO species on the electropolished Cu as the intermediates of NH3. Low overpotential NO3- and NO adsorptions and favorable NO reduction are key to increased NH3 productivity over Cu samples, which was consistent with the DFT calculation on Cu(100).

Photocatalytic boron nitride-based nanomaterials for the removal of selected organic and inorganic contaminants in aqueous solution: A review

Boron nitride (BN) coupled with various conventional and advanced photocatalysts has been demonstrated to exhibit extraordinary activity for photocatalytic degradation because of its unique properties, including a high surface area, constant wide-bandgap semiconducting property, high thermal-oxidation resistance, good hydrogen-adsorption performance, and high chemical/mechanical stability. However, only limited reviews have discussed the application of BN or BN-based nanomaterials as innovative photocatalysts, and it does not cover the recent results and the developments on the application of BN-based nanomaterials for water purification. Herein, we present a complete review of the present findings on the photocatalytic degradation of different contaminants by various BN-based nanomaterials. This review includes the following: (i) the degradation behavior of different BN-based photocatalysts for various contaminants, such as selected dye compounds, pharmaceuticals, personal care products, pesticides, and inorganics; (ii) the stability/reusability of BN-based photocatalysts; and (iii) brief discussion for research areas/future studies on BN-based photocatalysts.

Carbon doped hexagonal boron nitride as an efficient metal-free catalyst for NO capture and reduction

The electrochemical NO reduction reaction (NORR) towards NH3 is considered a promising strategy to cope with both NO removal and NH3 production. Currently, the research on NORR electrocatalysts mainly focuses on metal-based catalysts, while metal-free catalysts are quite scarce. In this work, we have systematically investigated the properties of pristine and C/O doped h-BN for efficient NO capture and reduction. Our results reveal that the basal plane of pristine h-BN is inert to the adsorption of NO, while doping C or O can significantly enhance the NO capture abilities of h-BN. Then, we highlight that C-doped h-BN exhibits excellent NORR catalytic performance with a relatively low limiting potential of -0.28 V. Further analysis shows that the suitable adsorption strength of NO on the C-doped h-BN surface is the prime reason for its excellent NO reduction activity, which is shown to be due to appropriate electronic interactions between the active site and NO. Last but not least, the catalytic selectivity of h-BN towards the NORR is confirmed by inhibiting the competing hydrogen evolution reaction. Our findings not only provide deeper insight into the essential effect of element doping on the catalytic activities of h-BN, but also propose general design principles for high-performance metal-free NORR electrocatalysts.

Unraveling the Activity Trends and Design Principles of Single-Atom Catalysts for Nitrate Electrocatalytic Reduction

Electrocatalytic nitrate (NO3-) reduction represents one of the most promising approaches to mitigate NO3- pollution and yield NH3, but it is still challenged by the atomic economy and selectivity issues of substantial active sites. Here, we describe a comprehensive investigation on a series of single-atom catalysts (SACs) using nitrogen-doped carbon as substrate (metal/NC). The essence of activity is related to the extent of the electron transfer capacity (SAs → NO3-). Among these examined SACs, the Cu/NC presents good performance toward NH3 synthesis, i.e., a maximum NH3 Faradaic efficiency of 100% with a high NH3 yield rate of up to 32,300 μg h-1 mgcat.-1. X-ray absorption fine structure spectra and density functional theory calculations provide evidence that the electronic structure of Cu-N4 coordination prohibits the formation of N2, N2O, and H2 and facilitates the orbital hybridization between the 2p orbitals of NO3- and 3d orbitals of Cu single-atom sites. Our study is believed to provide fundamental guidance for the future design of highly efficient electrocatalysts in NO3- reduction to NH3.

Promoting the efficiency and selectivity of NO3--to-NH3 reduction on Cu-O-Ti active sites via preferential glycol oxidation with holes

The combined reductive and oxidative reaction is the essence of a solar-driven photoredox system. Unfortunately, most of these efforts focus on the specific half-reactions, and the key roles of complete photoredox reactions have been overlooked. Taking the nitrate reduction reaction (NO3-RR) as a typical multiple-electrons involved process, the selective reduction of the NO3- into ammonia (NH3) synthesis with high efficiency is still a grand challenge. Herein, a rational oxidative half-reaction is tailored to achieve the selective conversion of NO3- to NH3 on Cu-O-Ti active sites. Through the coupled NO3-RR with glycol oxidation reaction system, a superior NH3 photosynthesis rate of 16.04 ± 0.40 mmol gcat-1 h-1 with NO3- conversion ratio of 100% and almost 100% of NH3 selectivity is reached on Cu-O-Ti bimetallic oxide cluster-anchored TiO2 nanosheets (CuOx@TNS) catalyst. A combination of comprehensive in situ characterizations and theoretical calculations reveals the molecular mechanism of the synergistic interaction between NO3-RR and glycol oxidation pair on CuOx@TNS. The introduction of glycol accelerates the h+ consumption for the formation of alkoxy (•R) radicals to avoid the production of •OH radicals. The construction of Cu-O-Ti sites facilitates the preferential oxidation of glycol with h+ and enhances the production of e- to participate in NO3-RR. The efficiency and selectivity of NO3--to-NH3 synthesis are thus highly promoted on Cu-O-Ti active sites with the accelerated glycol oxidative half-reaction. This work upgrades the conventional half photocatalysis into a complete photoredox system, demonstrating the tremendous potential for the precise regulation of reaction pathway and product selectivity.

Synergy between Cu and Co in a Layered Double Hydroxide Enables Close to 100% Nitrate-to-Ammonia Selectivity

Nitrate electroreduction (NO3RR) holds promise as an energy-efficient strategy for the removal of toxic nitrate to restore the natural nitrogen cycle and mitigate the adverse impacts caused by overfertilization from suboptimal agricultural practices. However, existing catalysts suffer from limited electrocatalytic activity, poor selectivity, inadequate durability, and low scalability. To address this quadrilemma, in this study, we developed a cost-effective layered double hydroxide (LDH) electrocatalyst with a lamellar structure that presents trimetallic CuCoAl active sites on the nanomaterial surface. This codoping design enabled electrochemical upcycling of nitrate into ammonia exclusively and efficiently with an onset potential at 0 V vs RHE, where the electrocatalytic process is less energy intensive and has a lower carbon footprint than conventional practices. The synergistic interaction among Cu, Co, and Al further afforded a 99.5% Faradic efficiency (FE) and a yield rate of 0.22 mol h-1 g-1 for nitrate-to-ammonia electroreduction, surpassing the performance of state-of-the-art nonprecious metal NO3RR electrocatalysts over an extended operation period. To gain insights into the origin of the catalytic performance observed on LDH, control materials were employed to elucidate the roles of Cu and Co. Cu was found to improve the NO3RR onset potential despite displaying limited FE for ammonia synthesis, while Co was discovered to suppress the formation of nitrite byproduct though requiring large overpotential. Simulated wastewater containing phosphate and sulfate, which are typically present in industrial effluents, was used to further investigate the effect of electrolytes on NO3RR. Intriguingly, the use of phosphate buffer resulted in a superior yield rate and FE for ammonia production while simultaneously inhibiting nitrite byproduct formation compared with the sulfate case. These experimental findings were supported by density functional theory (DFT) calculations, which explored the adsorption strength of nitrate adducts adjacent to coadsorbed electrolytes on the LDH surface. Additionally, the relative free energies of NO3RR species were also computed to examine the proton-coupled electron transfer (PCET) mechanism on CuCoAl LDH, shedding light on the potential-dependent step (PDS) and the exclusive selectivity for nitrate-to-ammonia conversion. The CuCoAl LDH developed here offers scalability by eliminating the need for precious metals, rendering this earth-abundant catalyst particularly appealing for sustainable nitrate electrovalorization technology.

Advancing nitrate reduction to ammonia: insights into mechanism, activity control, and catalyst design over Pt nanoparticle-based ZrO2

The reduction of nitrogen oxides (NOx) to NH3, or N2 represents a crucial step in mitigating atmospheric NO3 and NO2 emissions, a significant contributor to air pollution. Among these reduction products, ammonia (NH3) holds particular significance due to its utility in nitrogen-based fertilizers and its versatile applications in various industrial processes. Platinum-based catalysts have exhibited promise in enhancing the rate and selectivity of these reduction reactions. In this study, we employ density functional theory (DFT) calculations to explore the catalytic potential of Pt nanoparticle (PtNP)-supported ZrO2 for the conversion of NO3 to NH3. The most favorable pathway for the NO3 reduction to NH3 follows a sequence, that is, NO3 → NO2 → NO → ONH → ONH2/HNOH → NH2/NH → NH2 → NH3, culminating in the production of valuable ammonia. The introduction of low-state Fe and Co dopants into the ZrO2 support reduces energy barriers for the most challenging rate-determining hydrogenation step in NOx reduction to NH3, demonstrating significant improvements in catalytic activity. The incorporation of dopants into the ZrO2 support results in a depletion of electron density within the Pt cocatalyst resulting in enhanced hydrogen transfer efficiency during the hydrogenation process. This study aims to provide insights into the catalytic activity of platinum-based ZrO2 catalysts and will help design new high-performance catalysts for the reduction of atmospheric pollutants and for energy applications.

Kinetically matched C-N coupling toward efficient urea electrosynthesis enabled on copper single-atom alloy

Chemical C-N coupling from CO2 and NO3-, driven by renewable electricity, toward urea synthesis is an appealing alternative for Bosch-Meiser urea production. However, the unmatched kinetics in CO2 and NO3- reduction reactions and the complexity of C- and N-species involved in the co-reduction render the challenge of C-N coupling, leading to the low urea yield rate and Faradaic efficiency. Here, we report a single-atom copper-alloyed Pd catalyst (Pd4Cu1) that can achieve highly efficient C-N coupling toward urea electrosynthesis. The reduction kinetics of CO2 and NO3- is regulated and matched by steering Cu doping level and Pd4Cu1/FeNi(OH)2 interface. Charge-polarized Pdδ--Cuδ+ dual-sites stabilize the key *CO and *NH2 intermediates to promote C-N coupling. The synthesized Pd4Cu1-FeNi(OH)2 composite catalyst achieves a urea yield rate of 436.9 mmol gcat.-1 h-1 and Faradaic efficiency of 66.4%, as well as a long cycling stability of 1000 h. In-situ spectroscopic results and theoretical calculation reveal that atomically dispersed Cu in Pd lattice promotes the deep reduction of NO3- to *NH2, and the Pd-Cu dual-sites lower the energy barrier of the pivotal C-N coupling between *NH2 and *CO.

Breaking Local Charge Symmetry of Iron Single Atoms for Efficient Electrocatalytic Nitrate Reduction to Ammonia

The electrochemical conversion of nitrate pollutants into value-added ammonia is a feasible way to achieve artificial nitrogen cycle. However, the development of electrocatalytic nitrate-to-ammonia reduction reaction (NO3 - RR) has been hampered by high overpotential and low Faradaic efficiency. Here we develop an iron single-atom catalyst coordinated with nitrogen and phosphorus on hollow carbon polyhedron (denoted as Fe-N/P-C) as a NO3 - RR electrocatalyst. Owing to the tuning effect of phosphorus atoms on breaking local charge symmetry of the single-Fe-atom catalyst, it facilitates the adsorption of nitrate ions and enrichment of some key reaction intermediates during the NO3 - RR process. The Fe-N/P-C catalyst exhibits 90.3 % ammonia Faradaic efficiency with a yield rate of 17980 μg h-1  mgcat -1 , greatly outperforming the reported Fe-based catalysts. Furthermore, operando SR-FTIR spectroscopy measurements reveal the reaction pathway based on key intermediates observed under different applied potentials and reaction durations. Density functional theory calculations demonstrate that the optimized free energy of NO3 - RR intermediates is ascribed to the asymmetric atomic interface configuration, which achieves the optimal electron density distribution. This work demonstrates the critical role of atomic-level precision modulation by heteroatom doping for the NO3 - RR, providing an effective strategy for improving the catalytic performance of single atom catalysts in different electrochemical reactions.

Renewable energy driven electroreduction nitrate to ammonia and in-situ ammonia recovery via a flow-through coupled device

Green ammonia production from wastewater via electrochemical nitrate reduction contributes substantially to the realization of carbon neutrality. Nonetheless, the current electrochemical technology is largely limited by the lack of suitable device for efficient and continuous electroreduction nitrate into ammonia and in-situ ammonia recovery. Here, we report a flow-through coupled device composed of a compact electrocatalytic cell for efficient nitrate reduction and a unit to separate the produced ammonia without any pH adjustment and additional energy-input from the circulating nitrate-containing wastewater. Using an efficient and selective Cl-modified Cu foam electrode, nearly 100% NO3- electroreduction efficiency and over 82.5% NH3 Faradaic efficiency was realized for a wide range of nitrate-containing wastewater from 50 to 200 mg NO3--N L-1. Moreover, this flow-through coupled device can continuingly operate at a large current of 800 mA over 100 h with a sustained NH3 yield rate of 420 μg h-1 cm-2 for nitrate-containing wastewater treatment (50 mg NO3--N L-1). When driven by solar energy, the flow-through coupled device can also exhibit exceptional real wastewater treatment performance, delivering great potential for practical application. This work paves a new avenue for clean energy production and environmental sustainability as well as carbon neutrality.

Cr3C2 nanoparticles decorated carbon nanofibers for efficient nitrate reduction to ammonia at ambient conditions

Electrochemical nitrate (NO₃^-) reduction is a promising approach to relieve nitrate pollution and produce value-added ammonia (NH₃), but efficient and durable catalysts are required due to the large bond dissociation energy of nitrate and low selectivity. Herein, we propose chromium carbide (Cr₃C₂) nanoparticles loaded carbon nanofibers (Cr₃C₂@CNFs) as electrocatalysts to convert nitrate to ammonia. In phosphate buffer saline containing 0.1 mol L^-1 NaNO₃, such catalyst achieves a large NH₃ yield of 25.64 mg h^-1 mg^-1_cat. and a high faradaic efficiency of 90.08% at -1.1 V vs the reversible hydrogen electrode, which also shows excellent electrochemical durability and structural stability. Theoretical calculations reveal the adsorption energy for nitrate at Cr₃C₂ surfaces reaches -1.92 eV and the potential determining step (*NO→*N) for Cr₃C₂ hits a low energy increase of 0.38 eV.

Efficient Electrocatalytic Nitrate Reduction to Ammonia Based on DNA-Templated Copper Nanoclusters

In alkaline solutions, the electrocatalytic conversion of nitrates to ammonia (NH₃) (NO₃RR) is hindered by the sluggish hydrogenation step due to the lack of protons on the electrode surface, making it a grand challenge to synthesize NH₃ at a high rate and selectivity. Herein, single-stranded deoxyribonucleic acid (ssDNA)-templated copper nanoclusters (CuNCs) were synthesized for the electrocatalytic production of NH₃. Because ssDNA was involved in the optimization of the interfacial water distribution and H-bond network connectivity, the water-electrolysis-induced proton generation was enhanced on the electrode surface, which facilitated the NO₃RR kinetics. The activation energy (E_a) and in situ spectroscopy studies adequately demonstrated that the NO₃RR was exothermic until NH₃ desorption, indicating that, in alkaline media, the NO₃RR catalyzed by ssDNA-templated CuNCs followed the same reaction path as the NO₃RR in acidic media. Electrocatalytic tests further verified the efficiency of ssDNA-templated CuNCs, which achieved a high NH₃ yield rate of 2.62 mg h^-1 cm^-2 and a Faraday efficiency of 96.8% at -0.6 V vs reversible hydrogen electrode. The results of this study lay the foundation for engineering catalyst surface ligands for the electrocatalytic NO₃RR.

Microenvironment Engineering to Promote Selective Ammonia Electrosynthesis from Nitrate over a PdCu Hollow Catalyst

The electrosynthesis of recyclable ammonia (NH₃ ) from nitrate under ambient conditions is of great importance but still full of challenges for practical application. Herein, an efficient catalyst design strategy is developed that can engineer the surface microenvironment of a PdCu hollow (PdCu-H) catalyst to confine the intermediates and thus promote selective NH₃ electrosynthesis from nitrate. The hollow nanoparticles are synthesized by in situ reduction and nucleation of PdCu nanocrystals along a self-assembled micelle of a well-designed surfactant. The PdCu-H catalyst shows a structure-dependent selectivity toward the NH₃ product during the nitrate reduction reaction (NO₃ ^- RR) electrocatalysis, enabling a high NH₃ Faradaic efficiency of 87.3% and a remarkable NH₃ yield rate of 0.551 mmol h^-1 mg^-1 at -0.30 V (vs reversible hydrogen electrode). Moreover, this PdCu-H catalyst delivers high electrochemical performance in the rechargeable zinc-NO₃ ^- battery. These results provide a promising design strategy to tune catalytic selectivity for efficient electrosynthesis of renewable NH₃ and feedstocks.

Theoretical insights into dissociative-associative mechanism for enhanced electrochemical nitrate reduction to ammonia

The development of electrochemical nitrate reduction reaction (NO₃RR) is a "two birds-one stone" method, which can not only remove NO₃^- pollutant, but also produce valuable ammonia (NH₃). However, a mechanistic understanding of the nitrate reduction process remains very limited. Herein, we highlighted a dissociative-associative mechanism for the NO₃RR, in which the N-O bond of nitrate is initially broken to form *O and *NO₂ intermediate adsorbed on two active sites (dissociation process) and then subsequently hydrogenated and reduced to ammonia (association process). By taking a series of diatomic site catalysts (CuTM/g-CN and CuTM/N₆C, TM= Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) as models, we systematically investigate the dissociative-associative mechanism for the NO₃RR and compared with the Cu-based single-atom catalysts which follows the traditional directly associative mechanism. Density functional theory (DFT) calculations show that dissociative-associative mechanism is energetically favorable on seven catalysts (CuTi/g-CN, CuV/g-CN, CuMn/g-CN, CuCo/g-CN, CuV/N₆C, CuCr/N₆C and CuFe/N₆C) with the significantly reduced limiting potential of - 0.14 V to - 0.47 V. Specifically, an efficiently screening strategy was proposed to determine the dissociative-associative or directly associative mechanism for NO₃RR. This work can provide useful guideline for the rational design and development of NO₃RR electrocatalysts.

Analysis of the Scale of Global Human Needs and Opportunities for Sustainable Catalytic Technologies

We analyzed the enormous scale of global human needs, their carbon footprint, and how they are connected to energy availability. We established that most challenges related to resource security and sustainability can be solved by providing distributed, affordable, and clean energy. Catalyzed chemical transformations powered by renewable electricity are emerging successor technologies that have the potential to replace fossil fuels without sacrificing the wellbeing of humans. We highlighted the technical, economic, and societal advantages and drawbacks of short- to medium-term decarbonization solutions to gauge their practicability, economic feasibility, and likelihood for widespread acceptance on a global scale. We detailed catalysis solutions that enhance sustainability, along with strategies for catalyst and process development, frontiers, challenges, and limitations, and emphasized the need for planetary stewardship. Electrocatalytic processes enable the production of solar fuels and commodity chemicals that address universal issues of the water, energy and food security nexus, clothing, the building sector, heating and cooling, transportation, information and communication technology, chemicals, consumer goods and services, and healthcare, toward providing global resource security and sustainability and enhancing environmental and social justice.

Achievements, Challenges, and Perspectives on Nitrogen Electrochemistry for Carbon-Neutral Energy Technologies

Unrestrained anthropogenic activities have severely disrupted the global natural nitrogen cycle, causing numerous energy and environmental issues. Electrocatalytic nitrogen transformation is a feasible and promising strategy for achieving a sustainable nitrogen economy. Synergistically combining multiple nitrogen reactions can realize efficient renewable energy storage and conversion, restore the global nitrogen balance, and remediate environmental crises. Here, we provide a unique aspect to discuss the intriguing nitrogen electrochemistry by linking three essential nitrogen-containing compounds (i.e., N₂ , NH₃ , and NO₃ ^- ) and integrating four essential electrochemical reactions, i.e., the nitrogen reduction reaction (N₂ RR), nitrogen oxidation reaction (N₂ OR), nitrate reduction reaction (NO₃ RR), and ammonia oxidation reaction (NH₃ OR). This minireview also summarizes the acquired knowledge of rational catalyst design and underlying reaction mechanisms for these interlinked nitrogen reactions. We further underscore the associated clean energy technologies and a sustainable nitrogen-based economy.

Heterogenous Cu@ZrO2 nanofibers enable efficient electrocatalytic nitrate reduction to ammonia under ambient conditions

In this study, we construct a Cu@ZrO₂ heterogenous structure as a new catalyst that achieves a large NH₃ yield of 15.4 mg h^-1 mg^-1_cat. and a high faradaic efficiency of 67.6% at -0.7 V vs. RHE in 0.1 M PBS with 0.1 M NaNO₃, and it also shows excellent electrochemical durability and structural stability. Theoretical calculations reveal an extremely low adsorption energy of -1.54 eV at Cu surfaces and Cu can significantly reduce the applied overpotential and correspondingly promote the catalytic activity.

Catalytic transfer hydrogenation of N2 to NH3 via a photoredox catalysis strategy

Inspired by momentum in applications of reductive photoredox catalysis to organic synthesis, photodriven transfer hydrogenations toward deep (>2 e^-) reductions of small molecules are attractive compared to using harsh chemical reagents. Noteworthy in this context is the nitrogen reduction reaction (N₂RR), where a synthetic photocatalyst system had yet to be developed. Noting that a reduced Hantzsch ester (HEH₂) and related organic structures can behave as 2 e^-/2 H⁺ photoreductants, we show here that, when partnered with a suitable catalyst (Mo) under blue light irradiation, HEH₂ facilitates delivery of successive H₂ equivalents for the 6 e^-/6 H⁺ catalytic reduction of N₂ to NH₃; this catalysis is enhanced by addition of a photoredox catalyst (Ir). Reductions of additional substrates (nitrate and acetylene) are also described.

In situ and Operando Spectroscopies in Photocatalysis: Powerful Techniques for a Better Understanding of the Performance and the Reaction Mechanism

In photocatalysis, a set of elemental steps are involved together at different timescales to govern the overall efficiency of the process. These steps are divided as follow: (1) photon absorption and excitation (in femtoseconds), (2) charge separation (femto- to picoseconds), (3) charge carrier diffusion/transport (nano- to microseconds), and (4 and 5) reactant activation/conversion and mass transfer (micro- to milliseconds). The identification and quantification of these steps, using the appropriate tool/technique, can provide the guidelines to emphasize the most influential key parameter that improve the overall efficiency and to develop the "photocatalyst by design" concept. In this review, the identification/quantification of reactant activation/conversion and mass transfer (steps 4 and 5) is discussed in details using the in situ/operando techniques, especially the infrared (IR), Raman, and X-ray absorption spectroscopy (XAS). The use of these techniques in photocatalysis was highlighted by the most recent and conclusive case studies which allow a better characterization of the active site and reveal the reaction pathways in order to establish a structure-performance relationship. In each case study, the reaction conditions and the reactor design for photocatalysis (pressure, temperature, concentration, etc.) were thoroughly discussed. In the last part, some examples in the use of time-resolved techniques (time-resolved FTIR, photoluminescence, and transient absorption) are also presented as an author's guideline to study the elemental steps in photocatalysis at shorter timescale (ps, ns, and µs).

A theoretical descriptor for screening efficient NO reduction electrocatalysts from transition-metal atoms on N-doped BP monolayer

An electrochemical nitric oxide (NO) reduction reaction (NORR) is proposed as an attractive method for simultaneous realization of NO removal and ammonia (NH₃) synthesis. Here, the potentials of 29 transition-metal atoms anchored on the nitrogen-doped BP monolayer (MN₃/BP) as efficient NORR catalysts are systematically examined using first-principles calculations. Combining the adsorption Gibbs free energies of the N and OH species, a simple descriptor is constructed and a volcano plot of the NORR limiting potentials on the single atom catalysts (SACs) is established. Consequently, the MoN₃/BP and IrN₃/BP SACs are picked out as promising NORR electrocatalysts for NH₃ synthesis with the limiting potentials of -0.10 V and -0.06 V, respectively. Their corresponding rate constants are significantly larger than or close to that of the excellent Pt(111) surface. The electronic analysis shows that the Mo-4d or Ir-5d orbitals can be well hybridized with the NO-2p orbitals, sufficiently activating the adsorbed NO species. Particularly, the MoN₃/BP and IrN₃/BP SACs possess high thermal stabilities and can be easily synthesized by using MoCl₃ and IrCl₃ as precursors, respectively. This work not only offers a simple descriptor to efficiently design NORR electrocatalysts but also provides a comprehensive atomic understanding on the mechanism of NO-to-NH₃ conversion.

Mn-vacancy birnessite for photo-assisted elimination of formaldehyde at ambient condition

Visible light-assisted catalysis has recently attracted considerable attention because it is efficient, cost effective, and does not cause indoor air pollution. Several birnessite-type MnO₂ catalysts with different numbers of manganese vacancies (MVs) were synthesized in this study and used for photo-assisted catalytic oxidation of HCHO. Under visible light irradiation, MVs act as trapping centers to accelerate electrons transport and produce abundant reactive radicals to boost the activation of molecular oxygen, thereby improving the catalytic HCHO oxidation. The birnessite with the highest number of MVs exhibits remarkable oxidation activity with 80 ppm of HCHO (42% HCHO conversion was attained at ambient temperature) and a corresponding gas hourly space velocity (GHSV) of 60 L/(g·h) in a dynamic experiment. Moreover, it mineralizes 80 ppm of HCHO within 160 min in a static experiment, whereas it only takes 90 min under the same conditions with the visible light irradiation. The activity factor of birnessite with the highest MV content under visible light irradiation is 2.2 times that observed under dark conditions. Overall, this study elucidates the photothermal catalytic oxidation of HCHO, and concludes that the birnessite comprising MVs is a promising material for air purification applications.

Electrocatalytic reduction of nitrate - a step towards a sustainable nitrogen cycle

Nitrate enrichment, which is mainly caused by the over-utilization of fertilisers and industrial sewage discharge, is a major global engineering challenge because of its negative influence on the environment and human health. To solve this serious problem, many technologies, such as the activated sludge method, reverse osmosis, ion exchange, adsorption, and electrodialysis, have been developed to reduce the nitrate levels in water bodies. However, the applications of these traditional techniques are limited by several drawbacks, such as a long sludge retention time, slow kinetics, and undesirable by-products. From an environmental perspective, the most promising nitrate reduction technology is enabled to convert nitrate into benign N₂, and features low cost, high efficiency, and environmental friendliness. Recently, electrocatalytic nitrate reduction has been proven by satisfactory research achievements to be one of the most promising methods among these technologies. This review provides a comprehensive account of nitrate reduction using electrocatalysis methods. The fundamentals of electrocatalytic nitrate reduction, including the reaction mechanisms, reactor design principles, product detection methods, and performance evaluation methods, have been systematically summarised. A detailed introduction to electrocatalytic nitrate reduction on transition metals, especially noble metals and alloys, Cu-based electrocatalysts, and Fe-based electrocatalysts is provided, as they are essential for the accurate reporting of experimental results. The current challenges and potential opportunities in this field, including the innovation of material design systems, value-added product yields, and challenges for products beyond N₂ and large-scale sewage treatment, are highlighted.

Subnanometric alkaline-earth oxide clusters for sustainable nitrate to ammonia photosynthesis

The limitation of inert N2 molecules with their high dissociation energy has ignited research interests in probing other nitrogen-containing species for ammonia synthesis. Nitrate ions, as an alternative feedstock with high solubility and proton affinity, can be facilely dissociated for sustainable ammonia production. Here we report a nitrate to ammonia photosynthesis route (NO3-RR) catalyzed by subnanometric alkaline-earth oxide clusters. The catalyst exhibits a high ammonia photosynthesis rate of 11.97 mol gmetal-1 h-1 (89.79 mmol gcat-1 h-1) with nearly 100% selectivity. A total ammonia yield of 0.78 mmol within 72 h is achieved, which exhibits a significant advantage in the area of photocatalytic NO3-RR. The investigation of the molecular-level reaction mechanism reveals that the unique active interface between the subnanometric clusters and TiO2 substrate is beneficial for the nitrate activation and dissociation, contributing to efficient and selective nitrate reduction for ammonia production with low energy input. The practical application of NO3-RR route in simulated wastewater is developed, which demonstrates great potential for its industrial application. These findings are of general knowledge for the functional development of clusters-based catalysts and could open up a path in the exploitation of advanced ammonia synthesis routes with low energy consumption and carbon emission.

Enhanced photocatalytic benzyl alcohol oxidation over Bi4Ti3O12 ultrathin nanosheets

Ultrathin Bi₄Ti₃O₁₂ nanosheets (NS) with the thickness about 3.9 nm were successfully synthesized by a hydrothermal method and were used as a photocatalyst for the oxidation of benzyl alcohol (BA) to benzaldehyde (BAD). The photocatalytic performance of NS is about 8 times higher than that of bulk Bi₄Ti₃O₁₂. In-situ FTIR of pyridine adsorption and NH₃-TPD reveal that NS has more surface Lewis acid sites (Ti⁴⁺) for the adsorption and activation of BA. The photogenerated electrons (e^-) and holes (h⁺) of NS can be fully used to produce the superoxide radicals and carbon-centered radicals, respectively. The monolayer nanosheet structure of NS not only greatly promotes the separation of photogenerated carriers, but also achieves the efficient activation of BA molecules via the CO⋯Ti coordination. This work successfully reveals the surface/interface interactions between the surface active sites of a photocatalyst and the reactive molecules via using ultrathin nanosheet as a molecular platform.

In-situ synthesis of N-doped TiO2 onto Ti3C2 MXene with enhanced photocatalytic activity in selective reduction of nitrate to N2

Photocatalysis exhibited promises in the reduction of nitrates into harmless dinitrogen. Herein, the synthesis of N-doped TiO2/Ti3C2 at low calcination temperature using NH4Cl and Ti3C2 was reported for the first...

Ambient ammonia production via electrocatalytic nitrate reduction catalyzed by a flower-like CuCo2O4 electrocatalyst

The flower-shaped CuCo2O4 spinel catalyst was used for electrochemical nitrate reduction in an electrolyte. Theoretical calculation shows that NO3− can be adsorbed on CuCo2O4 to form the *NOH intermediate, so as to obtain a good yield rate and FE.

Amorphization and defect engineering in constructing ternary composite Ag/PW10V2/am-TiO2−x for enhanced photocatalytic nitrogen fixation

Ag/PW10V2/am-TiO2−x was designed by decorating OVs-enriched am-TiO2−x with Ag NPs and PW10V2. The formed Z-scheme heterojunction, Ag–am-TiO2−x interface and plentiful surface OVs account for its high photocatalytic efficiency in nitrogen fixation.

A review on water treatment technologies for the management of oxoanions: prospects and challenges

Oxoanions are a class of contaminants that are easily released into the aquatic systems either through natural or anthropogenic activities. Depending on their oxidation states, they are highly mobile, resulting in the contamination of underground water. Above the permissible level in groundwater, they pose as threats to mammals when the contaminated water is consumed. Some of the health challenges caused are cancer, neurological, cardiac, gastrointestinal, and skin disorders. Several treatment technologies have been adopted over the years for the management of these oxoanions present in the aquatic systems. However interesting these treatment technologies might be, they also have their limitations such as cost-effectiveness, the complexity of the process, and generation of secondary pollutants. This work focused on some of the water treatment technologies applied for the removal of oxoanions. Some of the advantages and disadvantages of these treatment technologies are also highlighted. Amongst all the treatment technologies, adsorption is the most applied method for the removal of oxoanions. However, photocatalysis has a higher prospect since it is non-selective and secondary pollutants are not generated after the treatment process. Also, photocatalysis can simultaneously reduce and oxidise oxoanions as well as organic pollutants respectively.

Electrostatic self-assembly to form unique LiNbO3/ZnS core-shell structure for photocatalytic nitrate reduction enhancement

Photocatalytic NO₃^- reduction in water has been regarded as a promising route due to its high efficiency and green feature. Several limiting factors, such as lack of catalytic sites, insufficient light collection, and spatial charge separation capacity photocatalytic denitrification, still need to be overcome for the practical applications. Herein, an innovative LiNbO₃/ZnS heterojunction with a unilateral opening core-shell structure was constructed. ZnS was tightly anchored on the surface of LiNbO₃ by modified electrostatic self-assembly method. High nitrate removal rate (98.84%) and N₂ selectivity (98.92%) were achieved with a molar ratio of LiNbO₃ and ZnS of 1:5 (1:5L-ZS) using formic acid as a hole scavenge. The LiNbO₃/ZnS degradation kinetics of NO₃^- was corresponding to the first-order kinetics equation. The nitrate removal rate and N₂ selectivity remained stable after three cycles in such photocatalytic NO₃^- reduction. The outstanding photocatalyst performance can be ascribed to the improved surface active sites, the well-matched band structure, and the unique core-shell structure. It provides an effective strategy for controllable fabrication of core-shell photocatalyst with strong light-harvesting ability and charge separation efficiency to enhance the removal rate of nitrate in water.

Advances in Electrochemical Ammonia Synthesis Beyond the Use of Nitrogen Gas as a Source

Electrocatalytic reduction of dinitrogen has emerged as a new strategy for ammonia synthesis. Despite being environmentally benign and energy-saving, it suffers from low conversion efficiency and short yield of ammonia because of the challenges of activating the inert N≡N bond at room temperature and atmospheric pressure. As a result of this, researchers proposed to reduce the nitrogenous species, one category of air and water pollutants, into valuable ammonia. Although remaining largely underexplored, this alternative approach shows promising efficiency for ammonia synthesis, while achieving high catalytic activity and selectivity remains challenging. In this Minireview, we summarize recent electrocatalytic performances of denitrification with selective formation to ammonia in terms of proposed active sites and reaction mechanisms. Additionally, we discuss the common issues in the state-of-the-art experimental tests and highlight the breakthroughs via computational screening of electrode materials. The aim of this is to steer the future research directions in the field, which is aiming for an optimal catalytic system with higher activity and selectivity for electrocatalytic denitrification.

Phase-selective active sites on ordered/disordered titanium dioxide enable exceptional photocatalytic ammonia synthesis

Photocatalytic N2 fixation to NH3 via defect creation on TiO2 to activate ultra-stable N[triple bond, length as m-dash]N has drawn enormous scientific attention, but poor selectivity and low yield rate are the major bottlenecks. Additionally, whether N2 preferentially adsorbs on phase-selective defect sites on TiO2 in correlation with appropriate band alignment has yet to be explored. Herein, theoretical predictions reveal that the defect sites on disordered anatase (Ad) preferentially exhibit higher N2 adsorption ability with a reduced energy barrier for a potential-determining-step (*N2 to NNH*) than the disordered rutile (Rd) phase of TiO2. Motivated by theoretical simulations, we synthesize a phase-selective disordered-anatase/ordered-rutile TiO2 photocatalyst (Na-Ad/Ro) by sodium-amine treatment of P25-TiO2 under ambient conditions, which exhibits an efficient NH3 formation rate of 432 μmol g-1 h-1, which is superior to that of any other defect-rich disordered TiO2 under solar illumination with a high apparent quantum efficiency of 13.6% at 340 nm. The multi-synergistic effects including selective N2 chemisorption on the defect sites of Na-Ad with enhanced visible-light absorption, suitable band alignment, and rapid interfacial charge separation with Ro enable substantially enhanced N2 fixation.

Constructing Well-Defined and Robust Th-MOF-Supported Single-Site Copper for Production and Storage of Ammonia from Electroreduction of Nitrate

A combined technique of production and storage of ammonia (NH3) from electroreduction of nitrate (NO3 -) through one material is highly desirable but remains a huge challenge. Herein, we proposed a proof-of-concept strategy for combined NH3 production and storage from electroreduction of NO3 - through elaborately designing a single-site CuII-bipyridine-based thorium metal-organic framework (Cu@Th-BPYDC). Noticeably, the single CuII site, anchored by a solid-liquid postsynthetic metalation within Th-BPYDC, shows a novel square coordination structure, as determined by the single-crystal X-ray diffraction. This strongly implies its enormous potential as an open metal site and consequently enables excellent performance in electroreduction of NO3 - for NH3 production, giving 92.5% Faradaic efficiency and 225.3 μmol h-1 cm-2 yield. Impressively, we can further use Cu@Th-BPYDC material to effectively capture the previously produced NH3 from electroreduction of NO3 -, affording an uptake up to 20.55 mmol g-1 at 298 K at 1 bar. The results in this work will outline a new direction toward the combined technique for advanced electrocatalysis such as gas production plus storage/or separation.

Theoretical Exploration of Electrochemical Nitrate Reduction Reaction Activities on Transition-Metal-Doped h-BP

Electrocatalytic conversion of nitrate (NO₃^-) into ammonia can not only eliminate harmful pollutant but also provide a green method for a low-temperature ammonia synthesis. The electrochemical NO₃^- reduction reactions (NO₃RRs) of a series of transition-metal-doped hexagonal boron phosphide (h-BP) monolayers were comprehensively evaluated using density functional theory. The V-doped h-BP monolayer was found to stand near the top of the volcano plot with the limiting potential of -0.22 V versus a reversible hydrogen electrode, exhibiting the lowest overpotential among the investigated systems in this work. Besides, the competing hydrogen evolution reaction is significantly suppressed due to the weak adsorption of the H atom. Importantly, the structure of the V-doped h-BP monolayer can be retained very well until 900 K, illustrating the initial indication of high thermal stability and great promise for synthesis. This study not only offers an eligible NO₃RR electrocatalyst but also provides an atomic understanding of the behind mechanisms of the NO₃RR process.