Two-electron Reduction Reaction Research Articles

Schottky Junction Enhanced Photosynthesis of Hydrogen Peroxide by Ultrathin Porous Carbon Nitride Supported Ni Nanoparticles.

Artificial photosynthesis for high-value hydrogen peroxide (H2O2) through a two-electron reduction reaction is a green and sustainable strategy. However, the development of highly active H2O2 photocatalysts is impeded by severe carrier recombination, ineffective active sites, and low surface reaction efficiency. We developed a dual optimization strategy to load dense Ni nanoparticles onto ultrathin porous graphitic carbon nitride (Ni-UPGCN). In the absence and presence of sacrificial agents, Ni-UPGCN achieved H2O2 production rates of 169 and 4116 μmol g-1 h-1 with AQY (apparent quantum efficiency) at 420 nm of 3.14% and 17.71%. Forming a Schottky junction, the surface-modified Ni nanoparticles broaden the light absorption boundary and facilitate charge separation, which act as active sites, promoting O2 adsorption and reducing the formation energy of *OOH (reaction intermediate). This results in a substantial improvement in both H2O2 generation activity and selectivity. The Schottky junction of dual modulation strategy provides novel insights into the advancement of highly effective photocatalytic agents for the photosynthesis of H2O2.

Redox Properties of Bacillus subtilis Ferredoxin:NADP+ Oxidoreductase: Potentiometric Characteristics and Reactions with Pro-Oxidant Xenobiotics.

Bacillus subtilis ferredoxin:NADP+ oxidoreductase (BsFNR) is a thioredoxin reductase-type FNR whose redox properties and reactivity with nonphysiological electron acceptors have been scarcely characterized. On the basis of redox reactions with 3-acetylpyridine adenine dinucleotide phosphate, the two-electron reduction midpoint potential of the flavin adenine dinucleotide (FAD) cofactor was estimated to be -0.240 V. Photoreduction using 5-deazaflavin mononucleotide (5-deazaFMN) as a photosensitizer revealed that the difference in the redox potentials between the first and second single-electron transfer steps was 0.024 V. We examined the mechanisms of the reduction of several different groups of non-physiological electron acceptors catalyzed by BsFNR. The reactivity of quinones and aromatic N-oxides toward BsFNR increased when increasing their single-electron reduction midpoint redox potentials. The reactivity of nitroaromatic compounds was lower due to their lower electron self-exchange rate, but it exhibited the same trend. A mixed single- and two-electron reduction reaction was characteristic of quinones, whereas reactions involving nitroaromatics proceeded exclusively via the one-electron reduction reaction. The oxidation of FADH• to FAD is the rate-limiting step during the oxidation of fully reduced FAD. The calculated electron transfer distances in the reaction with nitroaromatics were close to those of other FNRs including the plant-type enzymes, thus demonstrating their similar active site accessibility to low-molecular-weight oxidants despite the fundamental differences in their structures.

Phosphorylated g-C3N4/sulfur self-doped g-C3N4 homojunction carboxymethyl cellulose beads: An efficient photocatalyst for H2O2 production

The development of highly reusable, affordable, and durable photocatalysts for the production of hydrogen peroxide (H2O2) remained a challenge. In this study, a homojunction photocatalyst (SPGCN) is constructed between phosphorylated g-C3N4 (PCN) and sulfur self-doped g-C3N4 (SCN) using a simple wet impregnation method. Later, the obtained SPGCN homojunction is transformed into hydrogel beads using carboxymethyl cellulose via an effective cross-linking strategy (SPGCN/CMC). The photocatalytic beads displayed a phenomenal H2O2 production of 3.5 mM under visible light illumination for 60 min. The SPGCN/CMC hydrogel beads showed a maximum reusability of 10 cycles with a decline of 1.5 mM H2O2 production. The improved photocatalytic efficiency is indicated by strengthened utilization of visible light via tuning of the band gap, suppressed recombination of electron-hole pairs, and higher separation efficiency through the effective construction of Z-scheme between the phosphorylated carbon nitride and the sulfur-self-doped carbon nitride present in the SPGCN/CMC beads. The mechanistic studies affirmed the dominant role of superoxide radicals in H2O2 production. The photocatalytic H2O2 production followed a highly selective two-electron reduction reaction. Overall, this study highlights the efficient engineering of carbon nitride-based materials towards artificial photosynthesis.

Construction of bubbles enrichment sites and gas-diffusion-microchannel to assist Cr(VI) electrochemical reduction in a flow-through electrochemical system

Due to characteristics of the forced convection and enhanced mass transfer, graphite felt cathode based flow-through electrochemical system presented high electroreduction efficiency for aqueous Cr(VI). However, disadvantage is that bubbles generated by water electrolysis cannot pass through the liquid filled microchannel in time, which is easy to cause gas accumulation and an unstable electrochemical operation. Herein, we demonstrated graphite felt flow-through cathode designed with hydrophobic polytetrafluoroethylene (PTFE) as bubbles enrichment sites and construction of gas-diffusion-microchannel to assist in Cr(VI) electrochemical reduction. Cr(VI) reduction kinetics constant by graphite felt flow-through cathode increased 4.6 times after loading PTFE and was 2.6 ∼ 5.7 times higher than parallel-plate electrodes system. Typical electrochemical analysis suggested that PTFE can enrich and accelerate the dissolved oxygen mass transfer rate at PTFE-electrode-solution micro-interface, realizing oxygen digestion via two-electron reduction reaction. The unconsumed bubbles were transported out of electrochemical cell along gas-diffusion-microchannel to avoid gas accumulation and maintained a stable operation. Batch experiments indicated the ∼ 99.9% Cr(VI) reduction efficiency when the initial pH 2 and 100 mg/L Cr(VI) feed was treated under a single-pass pattern with flow rate of 1.5 mL/min and current density of 12.74 mA/cm2. Mechanism study revealed that faraday reaction was the main driving force for Cr(VI) reduction, while Cr(VI)-induced activation of in-situ electro-generated H2O2 contributed to formations of •OH and intermediate chromium species. This work also shows the feasibility of remediation for Cr(VI)-organic coexistence environment by a catalyst-free flow-through electrochemical system.

Enhanced photoelectrocatalytic oxidation of hypophosphite and simultaneous recovery of metallic nickel via carbon aerogel cathode

Carbon aerogel (CA) cathode was adopted to an undivided-chamber photoelectrocatalytic system with TiO2 nanotube arrays (TNA) photoanode to enhance the oxidation of hypophosphite (H2PO2−) and simultaneous recovery of metallic nickel (Ni). Both the efficiencies of H2PO2− oxidation and Ni recovery were significantly enhanced after replacing Ti or carbon fiber paper cathode with CA cathode. With 1.0 mM H2PO2− and 1.0 mM Ni2+, the ratio of PO43− production increased from ∼41% or ∼54% to ∼100%, and the ratio of Ni recovery increased from ∼20% or ∼ 37% to ∼93% within 180 min at 3.0 V. H2PO2− was finally oxidized to PO43− by •OH radicals, which was speculated to be generated from UV/H2O2 and bound on TNA photoanode. Meanwhile, Ni2+ was eventually electro-reduced to metallic Ni by a two-electron reduction reaction. The efficiencies of H2PO2− oxidation and Ni recovery were favored at higher cell voltage, faintly acid conditions and larger H2PO2− concentration. The stability of this system exhibited that the ratio of PO43− production increased significantly in each cycle, which was attributed to the increase of H2O2 in-situ-generation via CA cathode caused by deposition of metallic Ni. Finally, the treatment of actual electroless nickel plating effluents was demonstrated.

CO2 and CH2 Adsorption on Copper-Decorated Graphene: Predictions from First Principle Calculations

Single-layer graphene decorated with monodisperse copper nanoparticles can support the size and mass-dependent catalysis of the selective electrochemical reduction of CO2 to ethylene (C2H4). In this study, various active adsorption sites of nanostructured Cu-decorated graphene have been calculated by using density functional theory to provide insight into its catalytic activity toward carbon dioxide electroreduction. Based on the results of our calculations, an enhanced adsorption of the CO2 molecule and CH2 counterpart placed atop of Cu-decorated graphene compared to adsorption at pristine Cu metal surfaces was predicted. This approach explains experimental observations for carbon-based catalysts that were found to be promising for the two-electron reduction reaction of CO2 to CO and, further, to ethylene. Active adsorption sites that lead to a better catalytic activity of Cu-decorated graphene, with respect to general copper catalysts, were identified. The atomic configuration of the most selective CO2 toward the reduction reaction nanostructured catalyst is suggested.

Asymmetric structure engineering of polymeric carbon nitride for visible-light-driven reduction reactions

The photocatalytic activity of polymeric carbon nitride (g-C 3 N 4 ) strongly depends on its electronic structure which is highly sensitive to the preparation methods. To design the photocatalysts with efficient charge separation and transfer property, here we report a new ethylenediamine (EDA) induced gas-solid grafting method which enables the asymmetric modification of g-C 3 N 4 featuring the aromatization at the terminal of melon motifs. The obtained terminal-aromatized g-C 3 N 4 (Ar-C 3 N 4 ) exhibits an improved visible-light-driven photocatalytic activity in versatile two-electron reduction reactions, outperforming the pristine g-C 3 N 4 by 15.4 and 6.6 folds respectively in hydrogen evolution and hydrogen peroxide production. Theoretical and experimental results demonstrate the intensified asymmetry of π-electron distribution in the resulting material, which provides significantly improved driving force to guide the efficient separation of photogenerated e-h pairs and enhance the charge carrier mobility compared to its symmetric counterpart. ● Asymmetric modification of g-C 3 N 4 is developed via a new EDA involved gas-solid grafting process. ● The structure demonstrates lopsided arrangement of melon-based motifs featuring aromatization of terminal groups. ● Intensified asymmetric electron distribution is constructed. ● Significantly enhanced photocatalytic two-electron reductions are obtained.

On the path to aqueous organic redox flow batteries: Alizarin red S alkaline negolyte. Performance evaluation and photochemical studies

An aqueous organic redox flow battery (AORFB) based on Alizarin Red S, 3,4-dihydroxy-9,10-anthraquinone-2-sulfonic acid (ARS) and potassium ferrocyanide (PF) was studied in alkaline medium. Charge-discharge processes and potential-current response behaviour in a full cell combining ARS, as the negolyte, and PF, as the posolyte, were investigated. The diffusion coefficient (D) and kinetic rate constant (k0) of ARS were obtained from electrochemical measurements indicating a two-electron reduction reaction of ARS. The electrochemical stability test of the redox pair showed that the negolyte (ARS), even when subjected to extreme potential and current conditions, does not lose its redox activity and exhibits good performance without significant losses of efficiencies in a single cell test. The AORFB was found stable with an average storage capacity of 5.9 mWh L-1 and capacity retention of 89 and 91.5% for a single cell test before and after stability test, respectively. Additionally, the photophysics of ARS was investigated aiming to rationalize the stability of the compound as negolyte. The system showed to have potential as an organic aqueous flow battery closely related to the electrochemical stability and efficiencies associated to ARS.

Metal (Mn, Fe, Co, Ni, Cu, and Zn) Phthalocyanine-Immobilized Mesoporous Carbon Nitride Materials as Durable Electrode Modifiers for the Oxygen Reduction Reaction.

In the search for alternative sources to replace fossil fuels, carbon nitride materials can be used in a variety of ways. In the present work, porosity is introduced to the carbon nitride material using mesoporous silica material, MCM-41, as a hard template, and a mesoporous carbon nitride (MCN) material is synthesized. Further, the MCN is modified by immobilizing metal phthalocyanine (MPc, where M = Mn, Fe, Co, Ni, Cu, and Zn). The resulting MPc-incorporated MCN materials (MPc@MCN) were tested for the electrocatalytic oxygen reduction reaction (ORR) in acidic and basic media. Detailed studies reveal that the FePc@MCN and CoPc@MCN materials exhibit higher ORR activity than the other composites in 0.1 M KOH. FePc@MCN follows a direct four-electron oxygen reduction mechanism and shows ORR onset potential (vs RHE) at 0.93 V (in 0.1 M KOH), which is very close to the onset potential exhibited by the state-of-the-art material, Pt-C (1.0 V), and higher than several similar composites of MPc with carbon supports tested in similar environments. Besides, due to the inherent property of coordination through nitrogen present on the MCN, FePc@MCN shows excellent stability even after 3000 cyclic voltammetry (CV) cycles. FePc@MCN was found to have a better methanol tolerance in comparison to Pt-C in basic medium. CoPc@MCN shows a highly selective two-electron reduction reaction in both acidic and basic media at lower overpotential than many of the reported catalysts for the two-electron oxygen reduction. Therefore, these materials (FePc@MCN and CoPc@MCN) can be used as suitable alternatives to replace Pt and other expensive materials in ORR and related applications.

Direct electrochemical reduction and dyeing properties of CI Vat Yellow 1 using carbon felt electrode

Direct electrochemical reduction of a vat dye used in the dyeing process has been hindered by poor electron transfer between the solid electrode and insoluble dye particles. In this reported work, carbon felt was employed as a cathode for the direct electrochemical reduction of CI Vat Yellow 1 using cyclic voltammetry and electrolytic experiments. By monitoring the reaction products from the electrolysis reaction, the reaction mechanism of the direct electrochemical reduction of CI Vat Yellow 1 was delineated and it was found to be a two-step, two-electron reduction reaction. Electrochemical dyeing experiments at the laboratory scale were also performed. The results showed that carbon felt is an excellent electrode material for the direct electrochemical reduction of CI Vat Yellow 1. Electrochemical dyeing results showed that color shade and washing fastness of the dyed fabric were comparable to traditional dyeing methods.

Mechanism Insight into enhanced photodegradation of pharmaceuticals and personal care products in natural water matrix over crystalline graphitic carbon nitrides

Pharmaceuticals and personal care products (PPCPs), an emerging class of highly recalcitrant water contaminants, have raised considerable concerns in environment community. Graphitic carbon nitride (CN) has shown a great potential towards the photodegradation of water contaminants under visible light irradiation. However, the conventional bulk CN (BCN) presents the amorphous structure, resulting in an inefficient yield of hydroxyl radicals (•OH) for the complete mineralization of PPCPs. This study provides fundamental insights into significantly enhancing the hydroxyl radical generation via improving the crystallinity of the pristine CN materials. Experimental measurements and accompanying density functional theory (DFT) computational analysis suggest that the crystalline carbon nitride (CCN) exhibited an enhanced adsorption ability towards the dissolved O2. Upon the light irradiation, the adsorbed O2 molecules readily undergo a direct two-electron reduction reaction on the CCN surface, instead of the conventional two successive single-electron reduction reactions on the BCN surface, to produce H2O2 subsequently converting into •OH radicals. Along with the improved charge separation efficiency and electron transfer ability, CCN-based materials show superior photocatalytic activity towards PPCPs-type pollutants, compared with the pristine BCN catalysts. Importantly, the catalyst show excellent photodegradation activities under natural sunlight irradiation, at low PPCPs concentration (20 μg/L), in the mixed PPCPs solution or in the real wastewater/water samples, indicating the potential of CCN to enable practical ex situ destructive treatment of PPCPs-contaminated groundwater.

Isolated Pd Sites as Selective Catalysts for Electrochemical and Direct Hydrogen Peroxide Synthesis

Palladium nanoparticles have been studied extensively as catalysts for the direct synthesis of hydrogen peroxide, where selectivity remains a key challenge. Alloying Pd with other metals and using acid and halide promoters are commonly employed to increase H2O2 selectivity; however, the sites that can selectively produce H2O2 have not been identified and the role of these additives remains unclear. Here, we report the synthesis of atomically dispersed PdClx/C as a model catalyst for H2O2 production without the presence of extended Pd surfaces. We show that these isolated cationic Pd sites can form H2O2 with significantly higher selectivity than metallic Pd nanoparticles in both the reaction of H2 and O2 and the electrochemical oxygen reduction reaction. These results demonstrate that catalysts containing high populations of isolated Pd sites are selective catalysts for this two-electron reduction reaction and that the performance of materials in the direct synthesis reaction and electrocatalytic oxygen reduction reaction has many similarities.

Nanoparticle metal Ni cocatalyst on NiTe2 microsphere for improved photocatalytic hydrogen evolution

Scalable and sustainable photocatalytic hydrogen evolution via water splitting requires highly active, stable and earth-abundant cocatalysts to replace rare and expensive metal Platinum. Herein, two facile and similar thermal injection methods are employed to synthesize 3D NiTe2 microsphere and 0D/3D Ni/NiTe2 Schottky heterojunction photocatalyst. The optimized 6% Ni/NiTe2 composite shows good H2 evolution activity with a rate of 2214.2 μmol g−1·h−1- about 1.55 times higher than that of single NiTe2. Especially, it is the first time to report that NiTe2 and Ni/NiTe2 composite have the activity of producing H2O2 through the two-electron reduction of O2 during photocatalytic water splitting. This enhanced photocatalytic performance can be attributed to the rapid separation and migration of photogenerated charge carriers, low interfacial resistance, abundant surface active sites and two-electron reduction reaction process. This work provides an innovative approach for designing and constructing the potential metal-semiconductor composites with unique electronic structures.

Seed-mediated gold nanoparticle synthesis via photochemical reaction of benzoquinone

In this study, the synthesis of gold nanoparticles via a three-step reaction including a two-step photochemical reaction of 1,4-benzoquinone was demonstrated. The nanoparticles were formed by adding new gold ions to a solution containing seed nanoparticles pre-synthesized with hydroquinone and its oxidized form, 1,4-benzoquinone, by irradiating the solution with visible light. The diameter of the formed nanoparticles depended on the concentration of seed particles, with a higher seed particle concentration yielding smaller nanoparticles. The gold nanoparticles were characterized using UV-visible spectroscopy, transmission and scanning electron microscopy, and dynamic light scattering. Additionally, the synthesis of gold nanoparticles via photoreduction by benzoquinone is suggested to be a two-electron reduction reaction based on the photochemistry of benzoquinone and the pH change of the solution before and after irradiation.

Highly selective two-electron oxygen reduction to generate hydrogen peroxide using graphite felt modified with N-doped graphene in an electro-Fenton system

Preferential promotion of the two-electron reduction reaction of dissolved oxygen by controlling the type and amount of doped nitrogen atoms.

A Bottom-Up Formation Mechanism of Solid Electrolyte Interphase Revealed by Isotope-Assisted Time-of-Flight Secondary Ion Mass Spectrometry.

Understanding the solid electrolyte interphase (SEI) formation mechanism is critically important for the performance and durability of lithium-ion batteries. However, the details of how SEI builds up into a nanometer-thick layer from molecular level reduction reactions on negative electrodes are missing. Here, isotope-assisted time-of-flight secondary ion mass spectrometry analyses were designed to answer this fundamental question. By investigating the isotope ratio profile in SEI during the initial SEI formation cycle, it is discovered that the topmost SEI near the electrolyte formed first and the SEI near the electrode formed later. This new "bottom-up" SEI growth mechanism was then correlated to the electrolyte one-electron and two-electron reduction reaction dynamics, which in turn explains the formation of the two-layered organic-inorganic SEI composite structure.

Chemistry and Antihypertensive Effects of Tempol and Other Nitroxides

Nitroxides can undergo one- or two-electron reduction reactions to hydroxylamines or oxammonium cations, respectively, which themselves are interconvertible, thereby providing redox metabolic actions. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) is the most extensively studied nitroxide. It is a cell membrane-permeable amphilite that dismutates superoxide catalytically, facilitates hydrogen peroxide metabolism by catalase-like actions, and limits formation of toxic hydroxyl radicals produced by Fenton reactions. It is broadly effective in detoxifying these reactive oxygen species in cell and animal studies. When administered intravenously to hypertensive rodent models, tempol caused rapid and reversible dose-dependent reductions in blood pressure in 22 of 26 studies. This was accompanied by vasodilation, increased nitric oxide activity, reduced sympathetic nervous system activity at central and peripheral sites, and enhanced potassium channel conductance in blood vessels and neurons. When administered orally or by infusion over days or weeks to hypertensive rodent models, it reduced blood pressure in 59 of 68 studies. This was accompanied by correction of salt sensitivity and endothelial dysfunction and reduced agonist-evoked oxidative stress and contractility of blood vessels, reduced renal vascular resistance, and increased renal tissue oxygen tension. Thus, tempol is broadly effective in reducing blood pressure, whether given by acute intravenous injection or by prolonged administration, in a wide range of rodent models of hypertension.

Enhanced Oxygen Reduction Activity in Acid by Tin-Oxide Supported Au Nanoparticle Catalysts

Gold nanoparticles supported on hydrous tin-oxide are active for the four-electron oxygen reduction reaction in an acid electrolyte. The unique electrocatalytic of the Au–SnO is confirmed by the low amount of peroxide detected with rotating ring-disk electrode voltammetry and Koutecký-Levich analysis. In comparison, Au supported on Vulcan carbon and catalysts both produce significant peroxide in the acid electrolyte, indicating only a two-electron reduction reaction. Characterization of the catalyst reveals a high-surface area, amorphous support with gold metal particles. The high catalytic activity of the Au–SnO is attributed to metal support interactions. The results demonstrate a possible path to non-Pt catalysts for proton exchange membrane fuel cell cathodes.

Peroxynitrite efficiently mediates the interconversion of redox intermediates of myeloperoxidase

Nitric oxide-derived oxidants (e.g., peroxynitrite) are believed to participate in antimicrobial activities as part of normal host defenses but also in oxidative tissue injury in inflammatory disorders. A similar role is ascribed to the heme enzyme myeloperoxidase (MPO), the most abundant protein of polymorphonuclear leukocytes, which are the terminal phagocytosing effector cells of the innate immune system. Concomitant production of peroxynitrite and release of millimolar MPO are characteristic events during phagocytosis. In order to understand the mode of interaction between MPO and peroxynitrite, we have performed a comprehensive stopped-flow investigation of the reaction between all physiological relevant redox intermediates of MPO and peroxynitrite. Both iron(III) MPO and iron(II) MPO are rapidly converted to compound II by peroxynitrite in monophasic reactions with calculated rate constants of (6.8 ± 0.1) × 10 6 M −1 s −1 and (1.3 ± 0.2) × 10 6 M −1 s −1, respectively (pH 7.0 and 25 °C). Besides these one- and two-electron reduction reactions of peroxynitrite, which produce nitrogen dioxide and nitrite, a one-electron oxidation to the oxoperoxonitrogen radical must occur in the fast monophasic transition of compound I to compound II mediated by peroxynitrite at pH 7.0 [(7.6 ± 0.1) × 10 6 M −1 s −1]. In addition, peroxynitrite induced a steady-state transition from compound III to compound II with a rate of (1.0 ± 0.3) × 10 4 M −1 s −1. Thus, the interconversion among the various oxidation states of MPO that is prompted by peroxynitrite is remarkable. Reaction mechanisms are proposed and the physiological relevance is discussed.

In vitro cytotoxicity of the chlorinated naphthoquinone dichlone to human endothelial ECV304 cells

The cytotoxicity of the pro-oxidant fungicide dichlone (2,3-dichloro-1,4-naphthoquinone), to the human endothelial cell line, ECV304, was evaluated. The sensitivity of these cells to dichlone was intermediate between that of human hepatoblastoma HepG2 cells (least sensitive) and that of human GMO5757 fibroblasts. The midpoint cytotoxicity values for a 24-hr exposure to dichlone was about 0.02 m m when evaluated with the neutral red, acid phosphatase, and XTT tetrazolium assays. Lactic acid dehydrogenase leakage, after a 4-hr exposure, occurred initially at 0.05 m m dichlone. As with other naphthoquinones, cellular metabolism of dichlone presumably could proceed either by a one- or a two-electron reduction reaction. The enhancement of potency of dichlone towards ECV304 cells pretreated with the glutathione-depleting agents, dl-buthionine-[ S,R]-sulfoximine, 1-chloro-2,4-dinitrobenzene, and 1,3-bis(chloroethyl)-1-nitrosourea; the reduction in potency of dichlone to cells pretreated with (−)-2-oxo-4-thiazolidine carboxylic acid; the decrease in intracellular glutathione on exposure to dichlone; the subtle damage to the plasma membrane of dichlone-treated cells (as detected by the leakage of lactate dehydrogenase from these cells); and the lack of potentiation of dichlone toxicity by pretreatment with dicoumarol, are all consistent with the one-electron reduction reaction as the dominant pathway and with the subsequent generation of reactive oxygen molecules. The ECV304 cell line proved to be a useful research tool to study cytotoxic injury to endothelial cells.