High-Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single-Atom Catalysts with Tetradentate N2O2 Coordination in a Three-Phase Flow Cell

Carbon-supported Ni^II single-atom catalysts with a tetradentate Ni-N₂O₂ coordination formed by a Schiff base ligand-mediated pyrolysis strategy are presented. A Ni^II complex of the Schiff base ligand (R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine was adsorbed onto a carbon black support, followed by pyrolysis of the modified carbon material at 300 °C in Ar. The Ni-N₂O₂/C catalyst showed excellent performance for the electrocatalytic reduction of O₂ to H₂O₂ through a two-electron transfer process in alkaline conditions, with a H₂O₂ selectivity of 96 %. At a current density of 70 mA cm⁻², a H₂O₂ production rate of 5.9 mol g_cat.⁻¹ h⁻¹ was achieved using a three-phase flow cell, with good catalyst stability maintained over 8 h of testing. The Ni-N₂O₂/C catalyst could electrocatalytically reduce O₂ in air to H₂O₂ at a high current density, still affording a high H₂O₂ selectivity (>90 %). A precise Ni-N₂O₂ coordination was key to the performance.     

High‐Yield Electrosynthesis of Hydrogen Peroxide from Oxygen Reduction by Hierarchically Porous Carbon

H₂O₂ production by electroreduction of O₂ is an attractive alternative to the current anthraquinone process, which is highly desirable for chemical industries and environmental remediation. However, it remains a great challenge to develop cost‐effective electrocatalysts for H₂O₂ synthesis. Here, hierarchically porous carbon (HPC) was proposed for the electrosynthesis of H₂O₂ from O₂ reduction. It exhibited high activity for O₂ reduction and good H₂O₂ selectivity (95.0–70.2 %, most of them >90.0 % at pH 1–4 and >80.0 % at pH 7). High‐yield H₂O₂ generation has been achieved on HPC with H₂O₂ concentrations of 222.6–62.0 mmol L^?1 (2.5 h) and corresponding H₂O₂ production rates of 395.7–110.2 mmol h^?1 g^?1 at pH 1–7 and ?0.5 V. Moreover, HPC was energy‐efficient for H₂O₂ production with current efficiency of 81.8–70.8 %. The exceptional performance of HPC for electrosynthesis of H₂O₂ could be attributed to its high content of sp³‐C and defects, large surface area and fast mass transfer.     

Coordination Tunes Selectivity: Two‐Electron Oxygen Reduction on High‐Loading Molybdenum Single‐Atom Catalysts

Single‐atom catalysts (SACs) have great potential in electrocatalysis. Their performance can be rationally optimized by tailoring the metal atoms, adjacent coordinative dopants, and metal loading. However, doing so is still a great challenge because of the limited synthesis approach and insufficient understanding of the structure–property relationships. Herein, we report a new kind of Mo SAC with a unique O,S coordination and a high metal loading over 10 wt %. The isolation and local environment was identified by high‐angle annular dark‐field scanning transmission electron microscopy and extended X‐ray absorption fine structure. The SACs catalyze the oxygen reduction reaction (ORR) via a 2 e^? pathway with a high H₂O₂ selectivity of over 95 % in 0.10 m KOH. The critical role of the Mo single atoms and the coordination structure was revealed by both electrochemical tests and theoretical calculations.     

Regulating the Coordination Environment of MOF‐Templated Single‐Atom Nickel Electrocatalysts for Boosting CO2 Reduction

The general synthesis and control of the coordination environment of single‐atom catalysts (SACs) remains a great challenge. Herein, a general host–guest cooperative protection strategy has been developed to construct SACs by introducing polypyrrole (PPy) into a bimetallic metal–organic framework. As an example, the introduction of Mg²⁺ in MgNi‐MOF‐74 extends the distance between adjacent Ni atoms; the PPy guests serve as N source to stabilize the isolated Ni atoms during pyrolysis. As a result, a series of single‐atom Ni catalysts (named Ni_SA‐N_x‐C) with different N coordination numbers have been fabricated by controlling the pyrolysis temperature. Significantly, the Ni_SA‐N₂‐C catalyst, with the lowest N coordination number, achieves high CO Faradaic efficiency (98 %) and turnover frequency (1622 h^?1), far superior to those of Ni_SA‐N₃‐C and Ni_SA‐N₄‐C, in electrocatalytic CO₂ reduction. Theoretical calculations reveal that the low N coordination number of single‐atom Ni sites in Ni_SA‐N₂‐C is favorable to the formation of COOH* intermediate and thus accounts for its superior activity.     

熔盐辅助微波法制备g-C3N4包覆MgO-Al2O3-Fe2O3异质结催化剂及其光催化制过氧化氢性能

H₂O₂ is industrially produced by the anthraquinone method, in which energy consumption is high because it involves multistep hydrogenation and oxidation reactions. Photocatalytic production of H₂O₂ has received increasing attention as a sustainable and eco-friendly alternative to conventional anthraquinone-based and electrochemical production processes. Herein, we report a novel molten salt-assisted microwave process for the synthesis of a g-C₃N₄-coated MgO-Al₂O₃-Fe₂O₃ (MAFO) heterojunction photocatalyst with outstanding H₂O₂ production ability. The addition of a molten salt during synthesis changes the morphology of the as-prepared catalysts and influences the degree of polycondensation of melamine, leading to a change in the band gap energy. The cladding structure forms the maximum area of the heterojunction, leading to strong electronic coupling between the two components. This strong electronic coupling results in a more effective separation of the photogenerated electron-hole pairs and a faster interfacial charge transfer, leading to higher H₂O₂ formation rate. The equilibrium concentration and formation rate of H₂O₂ over the as-prepared heterojunction catalyst were 6.3 mmol·L^-1 and 1.42 mmol·L^-1·h^-1, which are much higher than that reported for g-C₃N₄ and MAFO individually. In addition, the H₂O₂ decomposition rate also decreases over the as-prepared heterojunction catalysts. A possible mechanism and the electron transfer routes have been proposed based on a free radical trapping experiment.     

H2 Oxidation Electrocatalysis Enabled by Metal‐to‐Metal Hydrogen Atom Transfer: A Homolytic Approach to a Heterolytic Reaction

Oxidation of H₂ in a fuel cell converts the chemical energy of the H?H bond into electricity. Electrocatalytic oxidation of H₂ by molecular catalysts typically requires one metal to perform multiple chemical steps: bind H₂, heterolytically cleave H₂, and then undergo two oxidation and two deprotonation steps. The electrocatalytic oxidation of H₂ by a cooperative system using Cp*Cr(CO)₃H and [Fe(diphosphine)(CO)₃]⁺ has now been invetigated. A key step of the proposed mechanism is a rarely observed metal‐to‐metal hydrogen atom transfer from the Cr?H complex to the Fe, forming an Fe?H complex that is deprotonated and then oxidized electrochemically. This “division of chemical labor” features Cr interacting with H₂ to cleave the H?H bond, while Fe interfaces with the electrode. Neither metal is required to heterolytically cleave H₂, so this system provides a very unusual example of a homolytic reaction being a key step in a molecular electrocatalytic process.     

Synthesis of Terphenyl Derivatives by Pd‐Catalyzed Suzuki‐Miyaura Reaction of Dibromobenzene Using 2N2O‐Salen as a Ligand in Aqueous Solution

A simple and efficient catalytic system for Na₂PdCl₄ catalyzing the Suzuki‐Miyaura reaction of dibromobenzene and arylboronic acid has been developed by using 2N2O‐salen as a ligand in H₂O/EtOH (V:V=4:1) at 100°C. Using this method, the reactions of substrates containing sterically demanding ortho substituents (e.g. dibromobenzene and/or arylboronic acids) proceeded efficiently, with the corresponding terphenyl derivatives being produced in moderate to excellent yields. Furthermore, this method offers interesting features for the multi‐gram scale synthesis of terphenyl compound.     

Kinetic Study of Peroxidase‐Catalyzed Coupling of Benzene‐1,4‐diamine and N‐(2‐Aminoethyl)naphthalen‐1‐amine: Development of Micromolar Hydrogen Peroxide Reaction System

A novel chromogenic method to measure the peroxidase activity using para‐phenylenediamine dihydrochloride (=benzene‐1,4‐diamine hydrochloride; PPDD) and N‐(1‐naphthyl)ethylenediamine dihydrochloride (=N‐(2‐aminoethyl)naphthalen‐1‐amine; NEDA) is presented. The PPDD entraps the free radical and gets oxidized to electrophilic diimine, which couples with NEDA to give an intense red‐colored chromogenic species with maximum absorbance at 490 nm. This assay was adopted for the quantification of H₂O₂ between 20 and 160 μM . Catalytic efficiency and catalytic power of the commercial peroxidase were found to be 4.47×10⁴ M ^?1 min^?1 and 3.38×10^?4 min^?1, respectively. The catalytic constant (k_cat) and specificity constant (k_cat/K_m) at saturated concentration of the co‐substrates were 0.0245×10³ min^?1 and 0.0445 μM ^?1 min^?1, respectively. The chromogenic coupling reaction has a minimum interference from the reducing substances such as ascorbic acid, L ‐cystein, citric acid, and oxalic acid. The method being simple, rapid, precise, and sensitive, its applicability has been tested in the crude vegetable extracts that showed peroxidase activity.     

Preparation of Sb2O3‐loaded Activated Carbon Particle Electrode and Its Electrocatalytic Degradation of Methyl Orange in a Three‐dimensional Electrode Cell

Electrocatalysis on the degradation of methyl orange is investigated using Sb₂O₃‐loaded activated carbon (Sb₂O₃/AC), a new particle electrode. The electrode was prepared by an impregnation method. An orthogonal array with four factors and three levels was selected to carry out the experiment. Electrocatalysis on the degradation of methyl orange through Sb₂O₃/AC was characterized by a series of parameters, including the amount of the particle electrode, the concentration of Na₂SO₄, the cell voltage, and the electrolysis time, and the results were compared with those of a conventional AC particle electrode. The results indicate that calcination temperature has the greatest impact on the catalytic activity of the particle electrode. The optimal conditions for preparing the Sb₂O₃/AC electrode include an 8 mL SbCl₃ solution, 90 min hydrolysis time, 400 °C calcination temperature, and 180 min calcination time. As well, the degradation efficiency of the Sb₂O₃/AC electrode is consistently higher than that of the AC electrode under the same electrolysis conditions. The electrochemical oxidation of methyl orange of both electrodes conformed to pseudo first‐order kinetics, but the rate constant of the Sb₂O₃/AC electrode was 2.29 times that of the AC electrode; this is likely due to the high electrocatalytic activity of the experimental electrode. The electrocatalysis results exhibited the synergetic effects of AC and Sb₂O₃ in the new particle electrode.     

Single Si atom supported on defective boron nitride nanosheet as a promising metal‐free catalyst for N2O reduction by CO or SO2 molecule: A computational study

The low‐temperature reduction of N₂O plays a significant role for solving the growing environmental and health issues caused by emission of this greenhouse gas. The aim of this study is to investigate the possible reaction pathways for the reduction of N₂O by CO or SO₂ molecule over Si‐doped boron nitride nanosheet (Si‐BNNS). According to our results, a B or N‐vacancy defect in BN sheet could be able to greatly stabilize the single Si adatom. The relatively large diffusion barrier for the Si atom over the defective BN sheet also indicates Si‐BNNS is stable enough to be utilized in catalytic reduction of N₂O. The large charge‐transfer from the surface to N₂O leads to the spontaneous dissociation of this molecule into N₂ molecule and an activated oxygen atom (O_ads). The O_ads moiety is then eliminated by CO or SO₂ molecule. The calculated activation energies and reaction energies reveal that the Si atom located on top of the B‐vacancy site has a large catalytic activity toward the reduction of N₂O by CO or SO₂.     

Biomimetic Interaction between FeII and O2: Effect of the Second Coordination Sphere on O2 Binding to FeII Complexes: Evidence of Coordination at the Metal Centre by a Dissociative Mechanism in the Formation of μ‐Oxo Diferric Complexes

We report that the formation of μ‐oxo diferric compounds from O₂ and FeCl₂ complexes within the tris(2‐pyridylmethyl)amine series (N. K. Thallaj et al. Chem. Eur. J., 2008 , 14, 6742–6753) involves coordination of O₂ to the metal centre and that this reaction occurs following initial dissociation of the bound equatorial chloride anion. We also report evidence of the formation of a reduced form of dioxygen by an inner‐sphere mechanism, thus leading to modification of the ligand. The solid‐state structures of [FeCl₂L] complexes (L¹=mono(α‐pivalamidopyridylmethyl)bis(2‐pyridylmethyl)amine, L²=mono(α‐pivalesteropyridylmethyl)bis(2‐pyridylmethyl)amine, L³=bis(α‐pivalamidopyridylmethyl)mono(2‐pyridylmethyl)amine are described, and spectroscopic data support the structural retention in solution. In [FeCl₂L³], the two amide hydrogen atoms stabilise the equatorial chloride anion in such a way that its exchange by a weak ligand is impossible: [FeCl₂L³] is perfectly oxygen‐stable. In [FeCl₂L²], the equatorial chloride anion is completely free to move and coordination of O₂ can take place. The reaction product with [FeCl₂L²] is a μ‐oxo diferric complex in which the ester function has been transformed into a phenol group. This conversion can be seen as a hydrolysis reaction in basic medium, hence supporting the initial formation of a reduced form of dioxygen in the medium. Complex [FeCl₂L¹] exhibits a very weak reactivity with O₂, in line with a semistabilised equatorial chloride counteranion.