Two-electron Oxidation Of Water Research Articles

Unravelling the surface structure and active site of CaSnO3, as well as H2O2 formation mechanism in two electron water oxidation reaction

The two-electron water oxidation reaction (2e-WOR) is an eco-friendly process to produce H2O2, which attracts much attention due to its low cost and high safety. CaSnO3 is a potential industrial catalyst for 2e-WOR reaction with high performance. However, the surface structure and active site of CaSnO3, as well as H2O2 formation mechanism, are still unknown due to its complex structure and limitation of characterization technique in resolution of surface fine structure and intermediates. To solve this difficult problem, the authors systematically investigated the surface structures of CaSnO3 and the adsorption and reaction behaviors of H2O, OH and H2O2 on CaSnO3 by density functional theory method. The present work indicated that the O4(Sn) surface with 5-coordinated Sn is the most stable termination of CaSnO3(121) by comparing the surface energies of 17 terminations of CaSnO3(121); OH strongly chemisorbs at the top site of Sn on CaSnO3(121) and can diffuse on surface with the free energy barrier of 0.68 ∼ 1.07 eV; the coordination unsaturated Sn atom is the active site for OH adsorption and H2O2 formation. According to the deep understanding of the mechanism of the 2e-WOR reaction to H2O2 on CaSnO3, two practical ways were proposed to design and develop 2e-WOR catalysts with high performance.

Electrochemical Formation and Activation of Hydrogen Peroxide from Water on Fluorinated Tin Oxide for Baeyer–Villiger Oxidation Reactions

The two-electron oxidation of water (2e-WOR) has been studied in the past as a possible method for the alternative preparation of hydrogen peroxide. Often, fluorinated tin oxide (FTO) is used as an anode and FTO itself was found also to be active for 2e-WOR. Because one use of H2O2 is as an oxygen donor for Baeyer–Villiger oxidation of ketones catalyzed by tin compounds and materials, presently we were interested in studying the use of in situ formed H2O2 for these reactions. First, the formation of H2O2 was verified in an acetonitrile/water solvent in a 2e-WOR reaction, which is more efficient than a comparable reaction in water in terms of the H2O2 concentration attained and faradaic efficiency at comparable potentials, that is, ∼3 V vs SHE. Second, initial studies on oxygenation of reactive substrates such as sulfides showed normalized reaction rates (NRRs) for two-electron oxidation reactions that were about 3 times higher than the NRR for H2O2 formation, indicating the formation of an active oxygen-donating or oxidizing species on the electrode surface prior to the formation and release of H2O2 into solution. Third, the Baeyer–Villiger oxygenation of 2-adamantanone at 2.1 V versus SHE in acetonitrile/water showed both the formation of the expected lactone product and hydroxylation at both tertiary and secondary C–H bonds. Hydroxylation is most easily explained by the presence of hydroxyl radical species as supported by the formation of a spin adduct and its identification by electron paramagnetic resonance. However, the potential used, 2.1 V versus SHE, is an underpotential for the formation of a solvated hydroxyl radical in solution, thereby leading to the conclusion that surface-bound hydroxyl species, OH*, are those that are reactive for the apparent one-electron water oxygenation reaction. Fourth, it was shown that although H2O2 can be thermally activated on FTO as a catalyst to a minor degree, electrochemical activation is by far more significant, leading to the use of FTO as an electrochemical catalyst for activation of H2O2 for the Baeyer–Villiger oxygenation and also alkene epoxidation.

Understanding the selectivity trend of water and sulfate (SO42−) oxidation on metal oxides: On-site synthesis of persulfate, H2O2 for wastewater treatment

Herein, H2O2 production activity via two-electron water oxidation reaction is identified on metal oxides (WO3, TiO2, and BiVO4) in acidic SO42− involving electrolyte, in which persulfate (PS) acts as an intermediate. A deep understanding of mechanism of PS production reaction on metal oxides has been realized based on rigorous crystal plane analysis of WO3 and TiO2 by combining density functional theory calculations and zeta potential measurements. It is found that the binding-energy of adsorbed hydroxyl groups on dominant exposed crystal plane of WO3 is lower than that of TiO2, leading to more SO42− adsorption sites at the surface of WO3. Consequently, the formation of PS on WO3 surface is more favorable. Meanwhile, both SO4−· and OH· were detected in the electrolyte by electron paramagnetic resonance measurement, which play a crucial role in organic pollutants wastewater treatment, with a degradation efficiency of 96% in 90 min for a typical pollutant norfloxacin. This work can provide a novel insight for understanding the selectivity of water and SO42− species oxidation on metal oxides, and open a new way for both on-site H2O2 production and advanced oxidation processes application.

Carbonate-Induced Electrosynthesis of Hydrogen Peroxide via Two-Electron Water Oxidation.

Electrochemical synthesis of hydrogen peroxide (H2 O2 ), via the two-electron water oxidation reaction (2e- WOR), is an attractive method for the sustainable production of valuable chemicals in place of oxygen during water electrolysis. While the majority of 2e- WOR studies have focussed on electrocatalyst design, little research has been carried out on the selection of the supporting electrolyte. In this work, we investigate the impact of potassium carbonate (K2 CO3 ) electrolytes, and their key properties, on H2 O2 production. We found that at electrolyte pH values (>9.5) where the carbonate anion (CO3 2- ) was prevalent in the mixture, a 26.5 % increase in the Faraday efficiency (%FE) for H2 O2 production was achieved, compared to bicarbonate (HCO3 - ) dominant solutions. Utilising boron-doped diamond (BDD) in highly concentrated K2 CO3 solutions, current densities of up to 511 mA cm-2 (in 4 m) and %FEs of 91.5 % (in 5 m) could be attained. The results presented in this work highlight the influence of CO3 2- on electrochemical H2 O2 generation via the 2e- WOR and provide novel pathways to produce desirable commodities at the anode during electrochemical water splitting.

Rational Design of Covalent Heptazine Frameworks with Spatially Separated Redox Centers for High-Efficiency Photocatalytic Hydrogen Peroxide Production.

The redox reaction centers in natural organisms conducting oxygenic photosynthesis are well arranged in a physically separated manner to convert sunlight into chemical energy efficiently. Mimicking natural photosynthesis via precisely constructing oxidative and reductive reaction centers within photocatalysts is ideal for enhancing catalytic performances in artificial photosynthesis. In this study, new covalent heptazine frameworks (CHFs) with spatially separated redox centers are rationally designed for photocatalytic production of H2 O2 from water and oxygen without using any sacrificial agents. Both experimental and computational investigations indicate that the two-electron oxygen reduction reaction occurs on the heptazine moiety, whereas the two-electron water oxidation reaction occurs on the acetylene or diacetylene bond in the CHFs. This unique spatial separation feature is critical for enhancing charge separation and achieving efficient H2 O2 production. Meanwhile, the measured exciton binding energy of the diacetylene-containing polymer is merely 24 meV. Under simulated solar irradiation, the rationally designed CHFs can achieve a solar-to-chemical conversion efficiency of 0.78%, surpassing previously reported photocatalytic materials. This study establishes a molecular engineering approach to construct periodically arranged and spatially separated redox centers in single-component polymer photocatalysts, representing a hallmark to create more exciting polymer structures for photocatalysis moving forward.

ZnO nanoparticles embedded in hollow carbon fiber membrane for electrochemical H2O2 production by two-electron water oxidation reaction

Electrochemical two-electron water oxidation reaction (2e-WOR) provides a promising alternative route for hydrogen peroxide (H2O2) production, where the design of earth abundant and environmentally friendly electrocatalysts with both high selectivity and production rate is crucial. Here we report the synthesis of ZnO nanoparticles embedded in hollow carbon fiber membrane as efficient 2e-WOR electrocatalyst by a metal-organic framework engaged electrospinning-pyrolysis method. The resultant ZnO@carbon composite fiber exhibits a foam-like hierarchical structure composed of interconnected hollow carbon nanocubes encapsulated with oxygen vacancy rich ZnO nanocrystals. Owing to the improved selectivity of ZnO, excellent conductivity of carbon fiber, promoted active site exposure and mass transfer of hollow structure, the free-standing membrane electrode shows superior 2e-WOR performances with high selectivity (83.8% at 2.8 V vs. RHE), H2O2 generation rate (19.7 μmol cm−2 min−1) and robust stability.

The Role of Bicarbonate-Based Electrolytes in H2O2 Production through Two-Electron Water Oxidation

Two-electron water oxidation (2e–WOR) is a promising route toward efficient production of a valuable product in H2O2. Recent attention has focused on developing new electrocatalysts for 2e–WOR, but the role of the electrolyte species in determining water oxidation selectivity and promoting 2e–WOR has not been established. Here, we use electroanalytical experiments to confirm the role of HCO3– as a selective 2e–WOR redox catalyst. We find minimal differences in H2O2 consumption pathways among common electrolytes, indicating that the role of HCO3– is to promote H2O2 production. Mechanistically, our rotating ring disk experiments show that H2O2 is not generated directly at the disk electrode in KHCO3 but formed after a time delay, suggesting electrolyte oxidation and subsequent hydrolysis as the promotional mechanism. Further electrochemical and spectroscopic experiments confirm this hypothesis, demonstrating anodic oxidation of carbonaceous electrolyte species occurs at Faradaic potentials relevant for water oxidation and that these species (likely HCO4– or C2O62–) subsequently oxidize water to H2O2 (t1/2 ≈ 5 min). The H2O2 production rate at 2.5 V vs RHE scales linearly with the concentration of HCO3–, confirming the catalytic role of HCO3– in 2e–WOR and suggesting the electrolyte can be leveraged to enhance electrochemical H2O2 production.

Acid-base equilibria of axial ligand and peripheral pyridyl group with stepwise formation of nine species of aluminum (III) tera(4-pyridyl) porphyrin

As one of the promising molecular catalysts for the two-electron water oxidation initiated through one-electron oxidation of the catalyst, trans-diaqua-coordinated 5, 10, 15, 20-tetra(4-pyridyl)porphyrinatealuminum (III) (AlTPyP(OH2)2) was studied on the protolytic behavior of the axial ligating water molecules and the redox properties in aqueous methanol solution. The reversible stepwise protolytic reactions of the two axially ligated water molecules, as well as four peripheral pyridyl groups at the meso-position of AlTPyP, were characterized using UV–vis absorption spectroscopy, 1H NMR, fluorescence lifetime measurements, and cyclic voltammetry. The result showed that four-step protolytic equilibrium reactions of the axial ligands proceeded under the basic pH condition and those of meso-substituted pyridyl groups did also under the acidic pH condition. All the nine species are in equilibria in the ground states as characterized with each pKa value. On the other hand, they exist as individual species in the excited states having each single exponential decay time constant. Here we will report the first detailed study on the formation of nine species from a single molecule of AlTPyP through protolytic reactions in the pH range between 0.54 and 12.3.

Effective Hydrogen Peroxide Production from Electrochemical Water Oxidation

The two-electron water oxidation reaction (2e– WOR) is progressively gaining traction as a sustainable approach for in situ electrosynthesis of hydrogen peroxide (H2O2). State-of-the-art 2e– WOR electrocatalysts have shown great promise at low electrical currents yet exhibit diminished electrocatalytic capabilities at larger current densities. Herein, by tailoring the boron doping level of boron-doped diamond (BDD) microfilms, we have fabricated an active, selective, and stable electrocatalyst for the 2e– WOR. Experimentally, we find that our modulated BDD films achieve a peak faradaic efficiency of 87%, as well as a record H2O2 production rate of 76.4 μmol cm–2 min–1, while maintaining a stable electrochemical performance for 10 h at 200 mA cm–2 in carbonate-based solutions. The results reported in this work firmly establish BDD as a primary catalyst candidate for large-scale implementation of the 2e– WOR and synchronously unlock new research avenues for the next-generation design of sp3-structured carbonaceous materials for anodic H2O2 electrosynthesis from water.

Selective hydrogen peroxide conversion tailored by surface, interface, and device engineering

<h2>Summary</h2> Hydrogen peroxide (H₂O₂) is receiving growing interest for energy storage because it can be locally synthesized from renewable energy through the two-electron water oxidation and the two-electron oxygen reduction reactions. Recently, engineering the microenvironment of existing catalysts has become a promising approach to address the activity, selectivity, and stability challenges of H₂O₂ synthesis and fuel cells, reducing the gap between theoretical prediction and experimental observations. We summarize these progresses from a multi-scale perspective, including tailoring the active sites on the catalytic surface, engineering the interface near the reactive sites, and improving the device design to achieve selective H₂O₂ conversion. Such strategies tune the thermodynamic energy barriers and reaction pathways, facilitate mass transfer for reactants and products, and stabilize the products and catalytic surfaces. The discussions here are expected to stimulate further efforts to achieve efficient on-site H₂O₂ production and power generation by H₂O₂ with high round-trip efficiency.

On-Demand Synthesis of H2 O2 by Water Oxidation for Sustainable Resource Production and Organic Pollutant Degradation.

H2 O2 is a versatile and environmentally friendly chemical involved in water treatment, such as advanced oxidation processes. Anthraquinone oxidation is widely used for large-scale production of H2 O2 , which requires significant energy input and periodic replacement of the carrier molecule. H2 O2 production should be customized considering the specific usage scenario. Electrochemical synthesis of H2 O2 can be adopted as alternatives to traditional method, which avoids concentration, transportation, and storage processes. Herein, we identified Bi2 WO6 :Mo as a low-cost and high-selectivity choice from a series of Bi-based oxides for H2 O2 generation via two-electron water oxidation reaction. It can continuously provide H2 O2 for in situ degradation of persistent pollutants in aqueous solution. Clean energy from H2 can also be produced at the cathode. This kind of water splitting producing sustainable resources of H2 O2 and H2 is an advance in environmental treatment and energy science.

Efficient Hydrogen Peroxide (H2O2) Synthesis by CaSnO3 via Two-Electron Water Oxidation Reaction

Electrochemical in situ hydrogen peroxide (H2O2) generation from a two-electron water oxidation reaction (2e-WOR) is a challenge, not only on catalyst selection but also on electrode making. Herein...

Solar-Assisted eBiorefinery: Photoelectrochemical Pairing of Oxyfunctionalization and Hydrogenation Reactions.

Inspired by natural photosynthesis, biocatalytic photoelectrochemical (PEC) platforms are gaining prominence for the conversion of solar energy into useful chemicals by combining redox biocatalysis and photoelectrocatalysis. Herein, we report a dual biocatalytic PEC platform consisting of a molybdenum (Mo)-doped BiVO4 (Mo:BiVO4 ) photoanode and an inverse opal ITO (IO-ITO) cathode that gives rise to the coupling of peroxygenase and ene-reductase-mediated catalysis, respectively. In the PEC cell, the photoexcited electrons generated from the Mo:BiVO4 are transferred to the IO-ITO and regenerate reduced flavin mononucleotides to drive ene-reductase-catalyzed trans-hydrogenation of ketoisophrone to (R)-levodione. Meanwhile, the photoactivated Mo:BiVO4 evolves H2 O2 in situ via a two-electron water-oxidation process with the aid of an applied bias, which simultaneously supplies peroxygenases to drive selective hydroxylation of ethylbenzene into enantiopure (R)-1-phenyl-1-hydroxyethane. Thus, the deliberate integration of PEC systems with redox biocatalytic reactions can simultaneously produce valuable chemicals on both electrodes using solar-powered electrons and water.

Two-electron oxidation of water to form hydrogen peroxide initiated by one-electron oxidation of Tin (IV)-porphyrins

5, 10, 15, 20-tetra(N-methylpyridinium-4′-yl)porphyrinate tin(IV) (SnTMPyP) was found to exhibit two-electron water oxidation to form hydrogen peroxide as a primary product initiated by electrochemical one-electron oxidation. Dioxygen was also formed by the further two-electron oxidation of hydrogen peroxide on the anode. Isotope labelling experiment using H218O and enzyme catalase that converts H2O2 into O2 revealed an incorporation of 18O into the oxidation products, indicating the two-electron oxidation of water in the electrolysis catalysed by SnTMPyP. The overpotential of forming H2O2 under the practical condition is 370 mV. The Faradaic yield of the water oxidation was almost quantitative both under the basic (pH 10.9) and the nearly neutral conditions (pH 8.5), though the reactivity was much higher under the basic condition.

Protolytic behavior of water-soluble zinc(II) porphyrin and the electrocatalytic two-electron water oxidation to form hydrogen peroxide

Two-electron water oxidation is an energy-efficient way for solar energy conversion and one of the promising candidates to get through the bottleneck of artificial photosynthesis, photon- flux density problem. In light of renewable energy factor (REF) that is defined by Energy output/Energy input, for the realization of practical systems, the energy input for catalyst preparation have much importance. Herein, we report a facile, cost-effective and environment benign synthesis of zinc (II) 5, 10, 15, 20-tetrakis(N-methylpyridinium-4′-yl)porphyrin (ZnTMPyP) in water at room temperature. We observed coordination of water molecules to the central Zn(II) ion of ZnTMPyP and four-step protolytic equilibria among five axially ligated species. Electrochemical and controlled potential electrolysis experiments, as well as theoretical DFT calculation, showed that ZnTMPyP(OH)(O−) exhibits the two-electron water oxidation to form hydrogen peroxide as the primary product initiated by one-electron oxidation process of the catalyst with a moderate turnover frequency (96.4 s-1) and a small overpotential (∼60 mV). Isotope-labeled studies clearly showed that water molecule served as an oxygen atom source in the formation of hydrogen peroxide.

Boron-Doped Diamond Electrocatalyst for Enhanced Anodic H2O2 Production

Electrochemical production of hydrogen peroxide (H2O2) constitutes a cost-effective and alternative method to the complex and energy-intensive anthraquinone oxidation process. The two-electron water oxidation reaction pathway, while unconventional, is an attractive option for H2O2 generation as it can be combined with suitable reduction reactions to effectuate simultaneous electrosynthesis of valuable chemicals at a large scale. In this work we demonstrate that a carbon-based catalyst, boron-doped diamond (BDD), achieves an H2O2 concentration and production rate of 29.0 mmol dm–3 and 19.7 μmol min–1 cm–2, respectively, illustrating the capability of BDD as a suitable electrocatalyst for H2O2 formation from water.

Confined local oxygen gas promotes electrochemical water oxidation to hydrogen peroxide

Electrochemical two-electron water oxidation is a promising route for renewable and on-site H2O2 generation as an alternative to the anthraquinone process. However, it is currently restricted by low selectivity due to strong competition from the traditional four-electron oxygen evolution reaction, as well as large overpotential and low production rates. Here we report an interfacial engineering approach, where by coating the catalyst with hydrophobic polymers we confine in situ produced O2 gas to tune the water oxidation reaction pathway. Using carbon catalysts as a model system, we show a significant increase of the intrinsic H2O-to-H2O2 selectivity and activity compared to that of the pristine catalyst. The maximal H2O2 Faradaic efficiency was enhanced by sixfold to 66% with an overpotential of 640 mV, under which a H2O2 production rate of 23.4 µmol min−1 cm−2 (75.2 mA cm−2 partial current) was achieved. This approach was successfully extended to nickel metal, demonstrating the wide applicability of our local gas confinement concept. Electrochemical 2e− water oxidation is a promising route for renewable H2O2 production but it suffers from low selectivity due to the competing 4e− process. Here the authors demonstrate an interfacial engineering approach where the catalyst is coated with a hydrophobic polymer to confine in situ produced O2 and promote the 2e− pathway.

Acetylene and Diacetylene Functionalized Covalent Triazine Frameworks as Metal-Free Photocatalysts for Hydrogen Peroxide Production: A New Two-Electron Water Oxidation Pathway.

Metal-free polymer photocatalysts have shown great promise for photocatalytic H2 O2 production via two-electron reduction of molecular O2 . The other half-reaction, which is the two-electron oxidation of water, still remains elusive toward H2 O2 production. However, enabling this water oxidation pathway is critically important to improve the yield and maximize atom utilization efficiency. It is shown that introducing acetylene (CC) or diacetylene (CCCC) moieties into covalent triazine frameworks (CTFs) can remarkably promote photocatalytic H2 O2 production. This enhancement is inherent to the incorporated carbon-carbon triple bonds which are essential in modulating the electronic structures of CTFs and suppressing charge recombinations. Furthermore, the acetylene and diacetylene moieties can significantly reduce the energy associated with OH* formation and thus enable a new two-electron oxidation pathway toward H2 O2 production. The study unveils an important reaction pathway toward photocatalytic H2 O2 production, reflecting that precise control over the chemical structures of polymer photocatalysts is vital to achieve efficient solar-to-chemical energy conversion.

Promotive Effect of Bicarbonate Ion on Two-Electron Water Oxidation to Form H2 O2 Catalyzed by Aluminum Porphyrins.

Two-electron water oxidation initiated by one-electron oxidation of aluminum porphyrins (AlTMPyP) is an alternative water oxidation to the conventional four-electron pathway and could help to avoid the bottleneck subject of photon-flux density in artificial photosynthesis. Here, a dramatic enhancement of the reactivity by bicarbonate ion in the two-electron water oxidation to form H2 O2 is reported. An addition of sodium carbonate (Na2 CO3 ) controlled both catalytic current and product selectivity of the two-electron water oxidation to enhance the activity of AlTMPyP at pH≈10-11. Controlled potential electrolysis experiments at different concentrations of Na2 CO3 (10-100 mm) showed that peroxide selectivity was improved up to approximately 73 % by the increase of [Na2 CO3 ] added to the system. The promotion of the reaction cycle was induced by an enhanced dynamic capturing of H2 O2 from the hydroperoxy complex of AlTMPyP through an attack of a bicarbonate ion. The detailed electrochemical studies and product selectivity indicated that the bicarbonate ion served as a good cofactor for producing H2 O2 from water. At stronger alkaline conditions (pH 12.5), however, a retardative effect of the addition of Na2 CO3 on the catalytic reactivity was observed.

ZnO As an Active and Selective Catalyst for Electrochemical Water Oxidation to Hydrogen Peroxide

Electrochemical synthesis of hydrogen peroxide (H2O2) via two-electron water oxidation reaction (2e-WOR) is an ideal process for delocalized production for water cleaning and other applications. Pr...