Facile Tailoring of the Electronic Structure and the d-Band Center of Copper-Doped Cobaltate for Efficient Nitrate Electrochemical Hydrogenation

Cu-Co Dual Sites Tandem Synergistic Effect Boosting Neutral Low Concentration Nitrate Electroreduction to Ammonia

Electrochemical nitrate reduction reaction (NO3 -RR) has emerged as an alternative strategy for wastewater treatment and ammonia production in neutral low-concentration nitrate. However, the electrocatalyst faces the challenge of limited NO3 - distribution and deficient active hydrogen (Hads) on the catalyst surface resulting from the low concentration of NO3 - and the difficulty of water splitting under neutral conditions. Here, a Cu-Co dual sites tandem synergistic catalysis mechanism has been proposed by doping Cu into CoP to facilitate the adsorption and conversion of NO3 - on Cu and to accelerate the water splitting on CoP leading to the significantly high NO3 -RR performance. The designed Cu-CoP catalyst exhibits an ammonia yield of 7.65 mg h-1 cm-2 and a Faraday efficiency of 85.1% at -1.0 V under neutral low-concentration nitrate (10 m M), which is the highest ammonia yield in the reported data. In situ characterization and theoretical calculations confirm the tandem synergistic effect, in which the Cu site favors the adsorption and activation of NO3 - to form NO2 -, and concurrently modulates the electronic structure of the Co site with optimized Hads adsorption resulting in the significantly enhanced NO3 -RR at neutral low concentration nitrate.

Effective Nitrate Electroconversion to Ammonia Using an Entangled Co3O4/Graphene Nanoribbon Catalyst

There has been huge interest among chemical scientists in the electrochemical reduction of nitrate (NO3-) to ammonia (NH4+) due to the useful application of NH4+ in nitrogen fertilizers and fuel. To conduct such a complex reduction reaction, which involves eight electrons and eight protons, one needs to develop high-performance (and stable) electrocatalysts that favor the formation of reaction intermediates that are selective toward ammonia production. In the present study, we developed and applied Co3O4/graphene nanoribbon (GNR) electrocatalysts with excellent properties for the effective reduction of NO3- to NH4+, where NH4+ yield rate of 42.11 mg h-1 mgcat-1, FE of 98.7%, NO3- conversion efficiency of 14.71%, and NH4+ selectivity of 100% were obtained, with the application of only 37.5 μg cm-2 of the catalysts (for the best catalyst ─Co3O4(Cowt %55)GNR, only 20.6 μg cm-2 of Co was applied), confirmed by loadings ranging from 19-150 μg cm-2. The highly satisfactory results obtained from the application of the proposed catalysts were favored by high average values of electrochemically active surface area (ECSA) and low Rct values, along with the presence of several planes in Co3O4 entangled with GNR and the occurrence of a kind of "(Co3(Co(CN)6)2(H2O)12)1.333 complex" structure on the catalyst surface, in addition to the effective migration of NO3- from the cell cathodic branch to the anodic branch, which was confirmed by the experiment conducted using a H-cell separated by a Nafion 117 membrane. The in situ FTIR and Raman spectroscopy results helped identify the adsorbed intermediates, namely, NO3-, NO2-, NO, and NH2OH, and the final product NH4+, which are compatible with the proposed NO3- electroreduction mechanism. The Density Functional Theory (DFT) calculations helped confirm that the Co3O4(Cowt %55)GNR catalyst exhibited a better performance in terms of nitrate electroreduction in comparison with Co3O4(Cowt %75), considering the intermediates identified by the in situ FTIR and Raman spectroscopy results and the rate-determining step (RDS) observed for the transition of *NO to *NHO (0.43 eV).

Boosting Electrochemical Nitrate Reduction to Ammonia by Fe Doped CuO/Co3O4 Nanosheet/Nanowire Heterostructures

The electrochemical nitrate reduction reaction (NO3 -RR) is a novel green method for ammonia synthesis. The development of outstanding NO3 -RR performance is based on reasonable catalyst. Metal oxides have garnered significant attention due to their exceptional electrical conductivity and catalytic properties. Doping serves as an effective strategy for enhancing catalyst performance due to its ability to change the electron cloud distribution and energy levels. In this study, we develop a heterojunction catalyst Fe doped copper oxide nanosheet and cobalt tetroxide nanowire growing on carbon cloth simultaneously (Fe-CuO@Co3O4/CC) via hydrothermal method. The well-designed Fe-CuO@Co3O4/CC has excellent NH3 yield (470.9 μmol h-1 cm-2) and Faraday efficiency (FE: 84.4 %) at -1.2 V versus reversible hydrogen electrode (vs. RHE). The heterostructure increases the specific surface area of the catalyst, and the possibility of contact between the catalyst and NO3 - ions, enhances the catalytic efficiency. In addition, the catalyst has excellent stability and can stably carry out the electrocatalytic nitrate reduction reaction (NO3 -RR), which provides a way for further research on the high-efficiency reduction of nitrate.

Metastable CoCu2O3 Nanocrystals from Combustion-Aerosols for Molecular Sensing and Catalysis

Metastable nanostructures are kinetically trapped in local energy minima featuring intriguing surface and material properties. To unleash their potential, there is a need for non-equilibrium processes capable of stabilizing a large range of crystal phases outside thermodynamic equilibrium conditions by closely and flexibly controlling atomic reactant composition, spatial temperature distribution and residence time. Here, the capture of metastable pseudo-binary metal oxides at room temperature is demonstrated with scalable combustion-aerosol processes. By a combination of X-ray diffraction, electron microscopy and on-line flame characterization, the occurrence of metastable CoCu2O3 is investigated with controlled crystal size (4-16 nm) over thermodynamically stable CuO and Co3O4. Immediate practical impact is demonstrated by exceptional sensing and stable catalytic performance for air pollutant detection (e.g., 15 parts-per-billion benzene) shown for, at least, 21 days. This approach can be extended to various binary, ternary and high entropy oxides with even more components. Also, secondary phases can be loaded on such metastable nanocrystals to access novel materials promising for actuators, energy storage or solar cells.

Defect engineering on electrocatalysts for sustainable nitrate reduction to ammonia: Fundamentals and regulations

Electrocatalytic nitrate (NO3 -) reduction to ammonia (NH3) is a "two birds-one stone" method that targets remediation of NO3 --containing sewage and production of valuable NH3. The exploitation of advanced catalysts with high activity, selectivity, and durability is a key issue for the efficient catalytic performance. Among various strategies for catalyst design, defect engineering has gained increasing attention due to its ability to modulate the electronic properties of electrocatalysts and optimize the adsorption energy of reactive species, thereby enhancing the catalytic performance. Despite previous progress, there remains a lack of mechanistic insights into the regulation of catalyst defects for NO3 - reduction. Herein, this review presents insightful understanding of defect engineering for NO3 - reduction, covering its background, definition, classification, construction, and underlying mechanisms. Moreover, the relationships between regulation of catalyst defects and their catalytic activities are illustrated by investigating the properties of electrocatalysts through the analysis of electronic band structure, charge density distribution, and controllable adsorption energy. Furthermore, challenges and perspectives for future development of defects in NO3RR are also discussed, which can help researchers to better understand the defect engineering in catalysts, and also inspire scientists entering into this promising field.

Electrochemical Nitrate Reduction: Ammonia Synthesis and the Beyond

Natural nitrogen cycle has been severely disrupted by anthropogenic activities. The overuse of N-containing fertilizers induces the increase of nitrate level in surface and ground waters, and substantial emission of nitrogen oxides causes heavy air pollution. Nitrogen gas, as the main component of air, has been used for mass ammonia production for over a century, providing enough nutrition for agriculture to support world population increase. In the last decade, researchers have made great efforts to develop ammonia processes under ambient conditions to combat the intensive energy consumption and high carbon emission associated with the Haber-Bosch process. Among different techniques, electrochemical nitrate reduction reaction (NO3RR) can achieve nitrate removal and ammonia generation simultaneously using renewable electricity as the power, and there is an exponential growth of studies in this research direction. Here, a timely and comprehensive review on the important progresses of electrochemical NO3RR, covering the rational design of electrocatalysts, emerging CN coupling reactions, and advanced energy conversion and storage systems is provided. Moreover, future perspectives are proposed to accelerate the industrialized NH3 production and green synthesis of chemicals, leading to a sustainable nitrogen cycle via prosperous N-based electrochemistry.

Cu doping in FeP enabling efficient electrochemical nitrate reduction to ammonia in neutral media

Electrochemical nitrate reduction to ammonia (NH3) not only provides a promising strategy for green NH3 synthesis, but also removes harmful nitrates from water. Herein, a Cu-doped FeP electrocatalyst was prepared for nitrate reduction, which achieved a high NH3 faradaic efficiency of 92.5% and a high NH3 yield of 0.787 mmol h-1 cm-2 in a neutral electrolyte, greatly surpassing its FeP counterpart.

Fe and Cu Double-Doped Co3O4 Nanorod with Abundant Oxygen Vacancies: A High-Rate Electrocatalyst for Tandem Electroreduction of Nitrate to Ammonia

The electrochemical nitrate reduction reaction (NO3RR) is an attractive green alternative to the conventional Haber-Bosch method for the synthesis of NH3. However, this reaction is a tandem process that involves multiple steps of electrons and protons, posing a significant challenge to the efficient synthesis of NH3. Herein, we report a high-rate NO3RR electrocatalyst of Fe and Cu double-doped Co3O4 nanorod (Fe1/Cu2-Co3O4) with abundant oxygen vacancies, where the Cu preferentially catalyzes the rapid conversion of NO3- to NO2- and the oxygen vacancy in the Co3O4 substrate can accelerate NO2- reduction to NH3. In addition, the introduction of Fe can efficiently capture atomic H* that promotes the dynamics of NO2- to NH3, improving Faradaic efficiency of the produced NH3. Controlled experimental results show that the optimal electrocatalyst of Fe1/Cu2-Co3O4 exhibits good performance with high conversion (93.39%), Faradaic efficiency (98.15%), and ammonia selectivity (98.19%), which is significantly better than other Co-based materials. This work provides guidance for the rational design of high-performance NO3RR catalysts.

Rational Design of Cu-Doped Tetrahedron of Spinel Oxide for High-Performance Nitric Oxide Electrochemical Sensor

The real-time detection of nitric oxide (NO) in living cells is essential to reveal its physiological processes. However, the popular electrochemical detection strategy is limited to the utilization of noble metals. The development of new detection candidates without noble metal species still maintaining excellent catalytic performance has become a big challenge. Herein, we propose a spinel oxide doped with heteroatom-Cu-doped Co₃O₄ (Cu-Co₃O₄) for the sensitive and selective detection of NO release from the living cells. The material is strategically designed with Cu occupying the tetrahedral (T_d) center of Co₃O₄ through the formation of a Cu-O bond. The introduced Cu regulates the local coordination environment and optimizes the electronic structure of Co₃O₄, hybridizing with the N 2p orbital to enhance charge transfer. The Cu_Td site can well inhibit the current response to nitrite (NO₂^-), resulting in a high improvement in the electrochemical oxidation of NO. The selectivity of Cu-Co₃O₄ can be markedly improved by the pore size of the molecular sieve and the negative charge on the surface. The rapid transmission of electrons is due to the fact that Cu-Co₃O₄ can be uniformly and densely in situ grown on Ti foil. The rationally designed Cu-Co₃O₄ sensor displays excellent catalytic activity toward NO oxidation with a low limit of detection of 2.0 nM (S/N = 3) and high sensitivity of 1.9 μA nM^-1 cm^-2 in cell culture medium. The Cu-Co₃O₄ sensor also shows good biocompatibility to monitor the real-time NO release from living cells (human umbilical vein endothelial cells: HUVECs; macrophage: RAW 264.7 cells). It was found that a remarkable response to NO was obtained in different living cells when stimulated by l-arginine (l-Arg). Moreover, the developed biosensor could be used for real-time monitoring of NO released from macrophages polarized to a M1/M2 phenotype. This cheap and convenient doping strategy shows universality and can be used for sensor design of other Cu-doped transition metal materials. The Cu-Co₃O₄ sensor presents an excellent example through the design of proper materials to implement unique sensing requirements and sheds light on the promising strategy for electrochemical sensor fabrication.

In situ interface engineered Co/NC derived from ZIF-67 as an efficient electrocatalyst for nitrate reduction to ammonia

Electrocatalytic nitrate (NO₃^-) reduction to ammonia (NH₃) is a promising alternative approach for simultaneous NH₃ green synthesis and NO₃^- contaminants removal. However, the complex eight-electron reaction requires catalysts with superb performance due to the low NH₃ selectivity and yield. In this work, the Co nanoparticles decorated N-doped carbon (NC) by in situ interface engineering were prepared by deriving ZIF-67 at 800 ℃ (Co/NC-800) for the selective NH₃ synthesis. This catalyst exhibits a remarkable performance and excellent cycle stability, achieving a great NH₃ yield of 1352.5 μg h^-1 mg_cat^-1 at -1.7 V vs Ag/AgCl, with a high NH₃ selectivity of up to 98.2 %, and a maximum Faradic efficiency of 81.2 % at -1.2 V vs Ag/AgCl. Moreover, DFT calculation results indicate that the interfacial effect between Co nanoparticle and NC could enhance electron transfer, and the composite Co/NC-800 shows a lower adsorption and conversion free energy, which promotes the production of ammonia.

Enhancing electro-reduction of nitrite to ammonia by loading Co3O4 on CuO to construct elecrocatalytic dual-sites

Improving the performance of CuO in electrocatalytic nitrite reduction to ammonia (NIRA) is the priority for designing efficient NIRA electrocatalysts. The electrocatalytic activity of CuO was enhanced by growing Co₃O₄ nanospheres on it. By comparing Co₃O₄@CuO with the mechanically mixed CuO and Co₃O₄ on a rotating ring-disk electrode, we discovered that the enhancement was attributed to a dual-site catalytic pathway.