Deciphering NH3 Adsorption Kinetics in Ternary Ni-Cu-Fe Oxyhydroxide toward Efficient Ammonia Oxidation Reaction

Asymmetric Coordination of Single-Atom Ru Sites Achieves Efficient N(sp3)-H Dehydrogenation Catalysis for Ammonia Oxidation

Ruthenium single-atom catalysts have great potential in ammonia-selective catalytic oxidation (NH3-SCO); however, the stable sp3 hybrid orbital of NH3 molecules makes N(sp3)-H dissociation a challenge for conventional symmetrical metallic oxide catalysts. Herein, we propose a heterogeneous interface reverse atom capture strategy to construct Ru with unique asymmetric Ru1N2O1 coordination. Ru1N2O1/CeO2 exhibits intrinsic low-temperature conversion (T100 at 160 °C) compared to symmetric coordinated Ru-based (280 °C), Ir-based (220 °C), and Pt-based (200 °C) catalysts, and the TOF is 65.4 times that of Ag-based catalysts. The experimental and theoretical studies show that there is a strong d-p orbital interaction between Ru and N atoms, which not only enhances the adsorption of ammonia at the Ru1N2O1 position but also optimizes the electronic configuration of Ru. Furthermore, the affinity of Ru1N2O1/CeO2 to water is significantly weaker than that of conventional catalysts (the binding energy of the Pd3Au1 catalyst is -1.19 eV, but it is -0.39 eV for our material), so it has excellent water resistance. Finally, the N(sp3)-H activation of NH3 requires the assistance of surface reactive oxygen species, but we found that asymmetric Ru1N2O1 can directly activate the N(sp3)-H bond without the involvement of surface reactive oxygen species. This study provides a novel principle for the rational design of the proximal coordination of active sites to achieve its optimal catalytic activity in single-atom catalysis.

The Enhanced Performance of NiCuOOH/NiCu(OH)2 Electrode Using Pre-Conversion Treatment for the Electrochemical Oxidation of Ammonia

Electrochemical oxidation of ammonia is an attractive process for wastewater treatment, hydrogen production, and ammonia fuel cells. However, the sluggish kinetics of the anode reaction has limited its applications, leading to a high demand for novel electrocatalysts. Herein, the electrode with the in situ growth of NiCu(OH)2 was partially transformed into the NiCuOOH phase by a pre-treatment using highly oxidative solutions. As revealed by SEM, XPS, and electrochemical analysis, such a strategy maintained the 3D structure, while inducing more active sites before the in situ generation of oxyhydroxide sites during the electrochemical reaction. The optimized NiCuOOH-1 sample exhibited the current density of 6.06 mA cm-2 at 0.5 V, which is 1.67 times higher than that of NiCu(OH)2 (3.63 mA cm-2). Moreover, the sample with a higher crystalline degree of the NiCuOOH phase exhibited lower performance, demonstrating the importance of a moderate treatment condition. In addition, the NiCuOOH-1 sample presented low selectivity (<20%) towards NO2- and stable activity during the long-term operation. The findings of this study would provide valuable insights into the development of transition metal electrocatalysts for ammonia oxidation.

Local tensile strain boosts the electrocatalytic ammonia oxidation reaction

The introduction of local tensile strain in Ni(OH)2 nanosheets accelerates the Ni(OH)2-to-NiOOH transition and boosts the electrocatalytic ammonia oxidation reaction (EAOR), i.e., reducing the onset potential by 80 mV, doubling both the current density and N2 faradaic efficiency, and enabling 1000 hours of operation at 160 mA cm-2.

Highly Selective Ammonia Oxidation on BiVO4 Photoanodes Co-catalyzed by Trace Amounts of Copper Ions

High-efficient photoelectrocatalytic direct ammonia oxidation reaction (AOR) conducted on semiconductor photoanodes remains a substantial challenge. Herein, we develop a strategy of simply introducing ppm levels of Cu ions (0.5-10 mg/L) into NH3 solutions to significantly improve the AOR photocurrent of bare BiVO4 photoanodes from 3.4 to 6.3 mA cm-2 at 1.23 VRHE , being close to the theoretical maximum photocurrent of BiVO4 (7.5 mA cm-2 ). The surface charge-separation efficiency has reached 90 % under a low bias of 0.8 VRHE . This AOR exhibits a high Faradaic efficiency (FE) of 93.8 % with the water oxidation reaction (WOR) being greatly suppressed. N2 is the main AOR product with FEs of 71.1 % in aqueous solutions and FEs of 100 % in non-aqueous solutions. Through mechanistic studies, we find that the formation of Cu-NH3 complexes possesses preferential adsorption on BiVO4 surfaces and efficiently competes with WOR. Meanwhile, the cooperation of BiVO4 surface effect and Cu-induced coordination effect activates N-H bonds and accelerates the first rate-limiting proton-coupled electron transfer for AOR. This simple strategy is further extended to other photoanodes and electrocatalysts.

Nickel-based catalysts for electrolytic decomposition of ammonia towards hydrogen production

Nickel is an attractive metal for electrochemical applications because it is abundant, cheap, chemically resilient, and catalytically active towards many reactions. Nickel-based materials (metallic nickel, its alloys, oxides, hydroxides, and composites) have been also considered as promising electrocatalysts for ammonia oxidation. The electrolysis of ammonia aqueous solution results in evolution of gaseous hydrogen and nitrogen. Up to date studies showed that metallic Ni and Ni (hydro)oxides are not catalytically active unless they are electrochemically converted to NiOOH at ~1.3 V vs. RHE. Then, dehydrogenation of NH3 begins with electron coupled proton transfer to NiOOH resulting in a would-be reversible reduction of the latter to Ni(OH)2. Unlike the water electrolysis process, in which solely oxygen is obtained at the anode, during ammonia electrooxidation apart from release of N2, many undesired oxygenated nitrogen moieties may also turn up. These products appear after at least partial dehydrogenation of ammonia. Studies on NiOOH activity have been conducted for systems containing various modifiers, e.g., Cu, Co, S, P, however, their particular role in catalytic activity has not yet been elucidated. Nowadays research is being conducted in the direction of increasing the activity, selectivity, and stability of NiOOH. In this review, the electroactivity of Ni is analyzed and discussed in accordance with its oxidation states along with the ammonia oxidation mechanism. The main research problems to be solved and challenges for the future industrial use of ammonia are presented.

Self-Terminating Surface Reconstruction Induced by High-Index Facets of Delafossite for Accelerating Ammonia Oxidation Reaction Involving Lattice Oxygen

Ammonia (NH₃ ) is a promising hydrogen (H₂ ) carrier for future carbon-free energy systems, due to its high hydrogen content and easiness to be liquefied. Inexpensive and efficient catalysts for ammonia electro-oxidation reaction (AOR) are desired in whole ammonia-based energy systems. In this work, ultrasmall delafossite (CuFeO₂ ) polyhedrons with exposed high-index facets are prepared by a one-step NH₃ -assisted hydrothermal method, serving as AOR pre-catalysts. The high-index CuFeO₂ facet is revealed to facilitate surface reconstruction into active Cu-doped FeOOH nanolayers during AOR processes in ammonia alkaline solutions, which is driven by the favorable Cu leaching and terminates as the 2p levels of internal lattice oxygen change. The reconstructed heterostructures of CuFeO₂ and Cu-doped FeOOH effectively activate the dehydrogenation steps of NH₃ and exhibit a potential improvement of 260 mV for electrocatalytic AOR at 10 mA cm^-2 compared to the pre-restructured phase. Further, density functional theory (DFT) calculations confirm that a lower energy barrier of the rate-determining step (*NH₃ to *NH₂ ) is presented on high-index CuFeO₂ facets covered with Cu-doped FeOOH nanolayers. Innovatively, lattice oxygen atoms in Fe-based oxides and oxyhydroxide are involved in the dehydrogenation steps of AOR as a proton acceptor, broadening the horizons for rational designs of AOR catalysts.

Tailored Bimetallic Ni-Sn Catalyst for Electrochemical Ammonia Oxidation to Dinitrogen with High Selectivity

Direct electrochemical ammonia oxidation reaction (eAOR) is an efficient and sustainable strategy to process wastewater containing ammonia, and it endures overoxidation and severely competitive oxygen evolution reaction (OER). Herein, we synthesized a Ni(OH)₂/SnO₂ composite catalyst by a multistep strategy and applied it to the eAOR process. Ni(OH)₂/SnO₂ exhibited a N₂-N Faradaic efficiency (FE_N2-N) of 84.2%, with a N₂ partial current density (j_N2-N) of 2.7 mA cm^-2 at 1.55 V vs reversible hydrogen electrode (RHE) in 0.5 M K₂SO₄ with 10 mM NH₃-N (pH 11). The oxophilic Sn promoted NH₃ absorption on Ni sites while suppressing the OER. As the active species, NiOOH accelerated the dimerization of intermediates (*NH₂ or *NH) to form N₂.

A Petal-like Structured NiCuOOH-NF Electrode by a Sonochemical Combined with the Electrochemical Method for Ammonia Oxidation Reaction

Direct electrochemical oxidation, as an economical and efficient method, has recently received increasing attention for ammonia-nitrogen wastewater treatment. Developing a low-cost, efficient catalytic electrode is the key to solve the problem of sluggish ammonia oxidation reaction (AOR) kinetics. In this study, a three-dimensional (3D) Ni foam electrode coated with NiCuOOH petal-like cluster structures was prepared using a simple sonochemical method combined with a surface electrochemical reconstruction strategy. This structure has a large surface area and abundant NiCuOOH active sites, giving a good premise for extraordinary electrocatalytic activity of AOR. The results show that the maximum current density for AOR reaches 97.8 mA cm−2 at 0.60 V vs. saturated calomel electrode (SCE). Additionally, 96.53% of NH4+-N removal efficiency and 63.12% of TN removal efficiency were acquired in the electrolysis system based on the NiCuOOH-NF electrode, as well as a good stability for at least 24 h. It is a promising flow-through anode for the clean treatment of ammonia-nitrogen wastewater.

A Robust Approach to In Situ Exsolve Highly Dispersed and Stable Electrocatalysts

Catalysts made of in situ exsolved metal nanoparticles often demonstrate promising activity and high stability in many applications. However, the traditional approach is limited by perovskites as prevailing precursor and requires high temperature typically above 900 K. Here, with the guidance of theoretical calculation, an unprecedented and substantially facile technique is demonstrated for Cu nanoparticles exsolved from interstitially Cu cations doped nickel-based hydroxide, which is accomplished swiftly at room temperature and results in metal nanoparticles with a quasi-uniform size of 4 nm, delivering an exceptional CO faradaic efficiency of 95.6% for the electrochemical reduction of CO₂ with a notable durability. This design principle is further proven to be generally applicable to other metals and foregrounded for guiding the development of advanced catalytic materials.

Facile Fabrication of a Foamed Ag3CuS2 Film as an Efficient Self-Supporting Electrocatalyst for Ammonia Electrolysis Producing Hydrogen

Ammonia (NH₃) is one of the hydrogen carriers that has received extensive attention due to its high hydrogen content and carbon-free nature. The ammonia electro-oxidation reaction (AOR) and the liquid AOR (LAOR) are integral parts of an ammonia-based energy system. The exploration of low-cost and efficient electrocatalysts for the AOR and LAOR is very important but very difficult. In this work, a novel self-supporting AOR and LAOR bifunctional electrocatalyst of a Ag₃CuS₂ film is synthesized by a simple hydrothermal method. The Ag₃CuS₂ film without a substrate shows efficient catalytic activity and enhanced stability for NH₃ electrolysis in both aqueous ammonia solution and liquid ammonia, including an onset potential of 0.7 V for the AOR and an onset potential of 0.4 V for the LAOR. The density functional theory calculations prove that compared to Cu atoms, Ag atoms with appropriate charge density on the surface of Ag₃CuS₂ are more electrocatalytically active for NH₃ splitting, including the low energy barrier in the rate-determining *NH₃ dehydrogenation step and the spontaneous tendency in the N₂ desorption process. Overall, the foamed Ag₃CuS₂ film is one of prospective low-cost and stable electrocatalysts for the AOR and LAOR, and the self-supporting strategy without a substrate provides more perspectives to tailor more meaningful and powerful electrocatalysts.

An Efficient Symmetric Electrolyzer Based On Bifunctional Perovskite Catalyst for Ammonia Electrolysis

Ammonia is a natural pollutant in wastewater and removal technique such as ammonia electro-oxidation is of paramount importance. The development of highly efficient and low-costing electrocatalysts for the ammonia oxidation reaction (AOR) and hydrogen evolution reaction (HER) associated with ammonia removal is subsequently crucial. In this study, for the first time, the authors demonstrate that a perovskite oxide LaNi0.5 Cu0.5 O3-δ after being annealed in Ar (LNCO55-Ar), is an excellent non-noble bifunctional catalyst towards both AOR and HER, making it suitable as a symmetric ammonia electrolyser (SAE) in alkaline medium. In contrast, the LNCO55 sample fired in air (LNCO55-Air) is inactive towards AOR and shows very poor HER activity. Through combined experimental results and theoretical calculations, it is found that the superior AOR and HER activities are attributed to the increased active sites, the introduction of oxygen vacancies, the synergistic effect of B-site cations and the different active sites in LNCO55-Ar. At 1.23 V, the assembled SAE demonstrates ≈100% removal efficiency in 2210 ppm ammonia solution and >70% in real landfill leachate. This work opens the door for developments towards bifunctional catalysts, and also takes a profound step towards the development of low-costing and simple device configuration for ammonia electrolysers.

Ni- and Cu-co-Intercalated Layered Manganese Oxide for Highly Efficient Electro-Oxidation of Ammonia Selective to Nitrogen

We fabricated a thin film of layered MnO₂ whose interlayer space was occupied by hydrated Ni²⁺ and Cu²⁺ ions. The process consisted of electrodeposition of layered MnO₂ intercalated with tetrabutylammonium cations (TBA⁺) by anodic oxidation of aqueous Mn²⁺ ions in the presence of TBA⁺, followed by ion exchange of the initially incorporated bulkier TBA⁺ with the denser transition metals in solution. The resulting layered MnO₂ co-intercalated with Ni²⁺ and Cu²⁺ ions (NiCu/MnO₂) catalyzed the ammonia oxidation reaction (AOR) in an alkaline electrolyte with a much lower overpotential than its Ni²⁺- and Cu²⁺-intercalated single-cation counterparts. Surprisingly, the NiCu/MnO₂ electrode achieved a faradic efficiency as high as nearly 100% (97.4%) for nitrogen evolution at a constant potential of +0.6 V vs Hg/HgO. This can be ascribed to the occurrence of the AOR in the potential region where water is stable and dimerization of the partially dehydrogenated ammonia species is preferred, thereby forming an N-N bond, rather than to be further oxidized into NO_x species.