Showing papers on "Redox published in 2019"

Promoting nitrogen electroreduction to ammonia with bismuth nanocrystals and potassium cations in water

Abstract: The electrochemical nitrogen reduction reaction (ENRR) can allow the production of ammonia from nitrogen and water under ambient conditions and is regarded as a sustainable alternative to the industrial Haber–Bosch process. However, electrocatalytic systems that selectively and efficiently catalyse nitrogen reduction remain elusive due to the strong competition with the hydrogen evolution reaction. Here, we report a strategy to simultaneously promote ENRR selectivity and activity using bismuth nanocrystals and potassium cations. Bismuth exhibits higher intrinsic ENRR activity than transition metals due to the strong interaction between the Bi 6p band and the N 2p orbitals. Potassium cations stabilize key nitrogen-reduction intermediates and regulate proton transfer to increase the selectivity. A high Faradaic efficiency of 66% and ammonia yield of 200 mmol g–1 h–1 (0.052 mmol cm–2 h–1) are obtained in aqueous electrolyte under ambient conditions. This strategy represents a general method to expand the library of catalysts and promoters for the selective electrochemical reduction of stable molecules. The electrochemical reduction of nitrogen to ammonia represents a challenge of major interest that would substantially decrease the burden of the energy-consuming Haber–Bosch process. Now, Yin, Yan, Zhang, Si and colleagues achieve high ammonia yield and Faradaic efficiency over 66% using bismuth nanocatalysts promoted by alkali cations.

Activation of Peroxydisulfate on Carbon Nanotubes: Electron-Transfer Mechanism.

Abstract: This study proposed an electrochemical technique for investigating the mechanism of nonradical oxidation of organics with peroxydisulfate (PDS) activated by carbon nanotubes (CNT). The electrochemical property of twelve phenolic compounds (PCs) was evaluated by their half-wave potentials, which were then correlated to their kinetic rate constants in the PDS/CNT system. Integrated with quantitative structure-activity relationships (QSARs), electron paramagnetic resonance (EPR), and radical scavenging tests, the nature of nonradical pathways of phenolic compound oxidation was unveiled to be an electron-transfer regime other than a singlet oxygenation process. The QSARs were established according to their standard electrode potentials, activation energy, and pre-exponential factor. A facile electrochemical analysis method (chronopotentiometry combined with chronoamperometry) was also employed to probe the mechanism, suggesting that PDS was catalyzed initially by CNT to form a CNT surface-confined and -activated PDS (CNT-PDS*) complex with a high redox potential. Then, the CNT-PDS* complex selectively abstracted electrons from the co-adsorbed PCs to initiate the oxidation. Finally, a comparison of PDS/CNT and graphite anodic oxidation under constant potentials was comprehensively analyzed to unveil the relative activity of the nonradical CNT-PDS* complex toward the oxidation of different PCs, which was found to be dependent on the oxidative potentials of the CNT-PDS* complex and the adsorbed organics.

Singlet Oxygen Triggered by Superoxide Radicals in a Molybdenum Cocatalytic Fenton Reaction with Enhanced REDOX Activity in the Environment

Abstract: As an important reactive oxygen species (ROS) with selective oxidation, singlet oxygen (1O2) has wide application prospects in biology and the environment. However, the mechanism of 1O2 formation, especially the conversion of superoxide radicals (·O2-) to 1O2, has been a great controversy. This process is often disturbed by hydroxyl radicals (·OH). Here, we develop a molybdenum cocatalytic Fenton system, which can realize the transformation from ·O2- to 1O2 on the premise of minimizing ·OH. The Mo0 exposed on the surface of molybdenum powder can significantly improve the Fe3+/Fe2+ cycling efficiency and weaken the production of ·OH, leading to the generation of ·O2-. Meanwhile, the exposed Mo6+ can realize the transformation of ·O2- to 1O2. The molybdenum cocatalytic effect makes the conventional Fenton reaction have high oxidation activity for the remediation of organic pollutants and prompts the inactivation of Staphylococcus aureus, as well as the adsorption and reduction of heavy metal ions (Cu2+, Ni2+, and Cr6+). Compared with iron powder, molybdenum powder is more likely to promote the conversion from Fe3+ to Fe2+ during the Fenton reaction, resulting in a higher Fe2+/Fe3+ ratio and better activity regarding the remediation of organics. Our findings clarify the transformation mechanism from ·O2- to 1O2 during the Fenton-like reaction and provide a promising REDOX Fenton-like system for water treatment.

Organic semiconductor photocatalyst can bifunctionalize arenes and heteroarenes

Abstract: Photoexcited electron-hole pairs on a semiconductor surface can engage in redox reactions with two different substrates. Similar to conventional electrosynthesis, the primary redox intermediates afford only separate oxidized and reduced products or, more rarely, combine to one addition product. Here, we report that a stable organic semiconductor material, mesoporous graphitic carbon nitride (mpg-CN), can act as a visible-light photoredox catalyst to orchestrate oxidative and reductive interfacial electron transfers to two different substrates in a two- or three-component system for direct twofold carbon-hydrogen functionalization of arenes and heteroarenes. The mpg-CN catalyst tolerates reactive radicals and strong nucleophiles, is straightforwardly recoverable by simple centrifugation of reaction mixtures, and is reusable for at least four catalytic transformations with conserved activity.

(Invited) Evolution of Redox Couples in Li- and Mn-Rich Cathode Materials and Mitigation of Voltage Fade by Reducing Oxygen Release

Abstract: Voltage fade is a major problem in battery applications for high-energy lithium- and manganese-rich (LMR) layered materials. As a result of the complexity of the LMR structure, the voltage fade mechanism is not well understood. Here we conduct both in situ and ex situ studies on a typical LMR material (Li1.2Ni0.15Co0.1Mn0.55O2) during charge–discharge cycling, using multi-length-scale X-ray spectroscopic and three-dimensional electron microscopic imaging techniques. Through probing from the surface to the bulk, and from individual to whole ensembles of particles, we show that the average valence state of each type of transition metal cation is continuously reduced, which is attributed to oxygen release from the LMR material. Such reductions activate the lower-voltage Mn3+/Mn4+ and Co2+/Co3+ redox couples in addition to the original redox couples including Ni2+/Ni3+, Ni3+/Ni4+ and O2−/O−, directly leading to the voltage fade. We also show that the oxygen release causes microstructural defects such as the formation of large pores within particles, which also contributes to the voltage fade. Surface coating and modification methods are suggested to be effective in suppressing the voltage fade through reducing the oxygen release.Voltage decay is a major problem in applications of high-energy Li- and Mn-rich layer-structured battery materials. Here, the authors report the evolution of redox couples as the origin of the voltage decay and discuss strategies to suppress the problem.

Reversible Oxygen Redox Chemistry in Aqueous Zinc-Ion Batteries

Abstract: Rechargeable aqueous zinc-ion batteries (ZIBs) are promising energy-storage devices owing to their low cost and high safety. However, their energy-storage mechanisms are complex and not well established. Recent energy-storage mechanisms of ZIBs usually depend on cationic redox processes. Anionic redox processes have not been observed owing to the limitations of cathodes and electrolytes. Herein, we describe highly reversible aqueous ZIBs based on layered VOPO4 cathodes and a water-in-salt electrolyte. Such batteries display reversible oxygen redox chemistry in a high-voltage region. The oxygen redox process not only provides about 27 % additional capacity, but also increases the average operating voltage to around 1.56 V, thus increasing the energy density by approximately 36 %. Furthermore, the oxygen redox process promotes the reversible crystal-structure evolution of VOPO4 during charge/discharge processes, thus resulting in enhanced rate capability and cycling performance.

Metal-oxygen decoordination stabilizes anion redox in Li-rich oxides.

Abstract: Reversible high-voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen-anion redox has garnered intense interest for such applications, particularly lithium-ion batteries, as it offers substantial redox capacity at more than 4 V versus Li/Li+ in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li2−xIr1−ySnyO3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure–redox coupling arises from the local stabilization of short approximately 1.8 A metal–oxygen π bonds and approximately 1.4 A O–O dimers during oxygen redox, which occurs in Li2−xIr1−ySnyO3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighbouring cation sites, driving cation disorder. These insights establish a point-defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry. Reversible high-voltage redox is a key component for electrochemical technologies from electrocatalysts to lithium-ion batteries. A point defect explanation for why anion redox occurs with local structural disordering and voltage hysteresis is proposed.

Improved activity and significant SO2 tolerance of samarium modified CeO2-TiO2 catalyst for NO selective catalytic reduction with NH3

Abstract: The Sm doped CeO2-TiO2 mixed oxide catalyst, which exhibited excellent activity and tolerance to H2O and SO2 in the NH3-SCR reaction, was synthesized. The reasons for the high activity and SO2 resistance of the catalyst were investigated by a series of characterization. The H2-TPR and O2-TPD results suggested that the reducibility and oxygen storage capacity (OSC) of CeTi catalyst were promoted by the addition of Sm species, which was beneficial for improving the activity of catalyst. The in situ DRIFTS results revealed that the adsorptive ability of NOx species and activation ability of NH3 were enhanced by Sm doping, which was also propitious to enhance the activity. XPS combined with DFT calculated results confirmed that the transfer of electron by Sm2++Ce4+⇌Sm3++Ce3+ circles occurred in the SmCeTi catalyst. The redox circles may be the reason of the good SO2 tolerance of the SmCeTi catalyst, for which suppressed the electron transferring from adsorbed SO2 to Ce4+. Through in situ DRIFTS and TG-DSC results, it can be concluded that the sulphation of catalyst was lowered by samarium doping into CeTi catalyst. Consequently, the SmCeTi catalyst exhibited significant SO2 tolerance ability.

Enhanced Superoxide Generation on Defective Surfaces for Selective Photooxidation

Abstract: Photocatalytic selective oxidation reactions hold great promise for the design of high-value-added organic intermediates, but many of these reactions suffer from low conversion efficiency and selectivity due to uncontrollable oxidation processes. In view of using photogenerated reactive oxygen species as the key oxidant in a selective oxidation reaction, we propose that a highly selective oxidation reaction can be achieved by modulating the corresponding photocatalytic molecular oxygen (O2) activation processes. Using cubic indium sulfide (β-In2S3) nanosheets as a model system, we show that the charge carriers involved in O2 activation can be optimized with the introduction of surface S vacancies. Benefiting from the enhanced charge separation and transfer processes, the In2S3 nanosheets with S vacancies could simultaneously activate O2 into superoxide radicals via electron transfer under visible-light irradiation to display outstanding activity for the selective oxidation of alcohols to aldehydes with hi...

Tunable Redox Chemistry and Stability of Radical Intermediates in 2D Covalent Organic Frameworks for High Performance Sodium Ion Batteries.

Abstract: Radicals are inevitable intermediates during the charging and discharging of organic redox electrodes. The increase of the reactivity of the radical intermediates is desirable to maximize the capacity and enhance the rate capability but is detrimental to cycling stability. Therefore, it is a great challenge to controllably balance the redox reactivity and stability of radical intermediates to optimize the electrochemical properties with a good combination of high specific capacity, excellent rate capability, and long-term cycle life. Herein, we reported the redox and tunable stability of radical intermediates in covalent organic frameworks (COFs) considered as high capacity and stable anode for sodium-ion batteries. The comprehensive characterizations combined with theoretical simulation confirmed that the redox of C-O· and α-C radical intermediates play an important role in the sodiation/desodiation process. Specifically, the stacking behavior could be feasibly tuned by the thickness of 2D COFs, essentially determining the redox reactivity and stability of the α-C radical intermediates and their contributive capacity. The modulation of reversible redox chemistry and stabilization mechanism of radical intermediates in COFs offers a novel entry to design novel high performance organic electrode materials for energy storage and conversion.

Electrocatalytic methanol oxidation over Cu, Ni and bimetallic Cu-Ni nanoparticles supported on graphitic carbon nitride

Abstract: Ni, Cu and Cu–Ni nanostructures have been fabricated and homogeneously embedded on ultrathin two-dimensional (2D) carbon nitride (g-C3N4), and the surface morphology and composition of the resulting hybrid nanostructures were studied by XRD, TEM, HRTEM-elemental mapping, Raman spectroscopy and XPS. The new hierarchical hetero-structures dropcasted on GC anodes have been visualised by SEM and their catalytic performance have been examined in methanol electrooxidation reaction (MOR) under alkaline conditions. Nanosized Ni particles dispersed finely over g-C3N4 are very active electrocatalysts with MOR onset at potential 0.35 V and charge transfer resistance 0.12 kΩ. The stability of modyfied GC electrodes, examined under chronoamperometric conditions showed that for electrode loading with 4% (wt. %) of NiO the stable current density ca. 36 A g−1 (12 A cm2) was obtained during whole experiment (up to 160 min). For all catalyst studied the curent density obtained during MOR reaction was enhanced when electrode was iluminated by UV light λ∼400 nm, and the highest value were obtained for 4% Ni/CN catalyst ca. 127 A g−1 (22 A cm2). The Cu incorporation in the hybrid material evoke loss of activity mostly due to Cu+ irreversible reduction/oxidation to Cu° and Cu2+, CuO segregation and influencing electron transfer process which results in the increasing in the redox potential. These results represent an important step towards light-enhanced electro-reactive systems and sensors in which heterojunction formation can facilitate electron-hole separation and enable more efficient energy transfer.

Heme and Nonheme High-Valent Iron and Manganese Oxo Cores in Biological and Abiological Oxidation Reactions

Abstract: Utilization of O2 as an abundant and environmentally benign oxidant is of great interest in the design of bioinspired synthetic catalytic oxidation systems. Metalloenzymes activate O2 by employing earth-abundant metals and exhibit diverse reactivities in oxidation reactions, including epoxidation of olefins, functionalization of alkane C–H bonds, arene hydroxylation, and syn-dihydroxylation of arenes. Metal–oxo species are proposed as reactive intermediates in these reactions. A number of biomimetic metal–oxo complexes have been synthesized in recent years by activating O2 or using artificial oxidants at iron and manganese centers supported on heme or nonheme-type ligand environments. Detailed reactivity studies together with spectroscopy and theory have helped us understand how the reactivities of these metal–oxygen intermediates are controlled by the electronic and steric properties of the metal centers. These studies have provided important insights into biological reactions, which have contributed to ...

Development of CuO coated ceramic hollow fiber membrane for peroxymonosulfate activation: a highly efficient singlet oxygen-dominated oxidation process for bisphenol a degradation

Abstract: A CuO coated ceramic hollow fiber membrane with dual functionalities of membrane filtration and peroxymonosulfate (PMS) activation was successfully constructed by applying phase-inversion and dip-coating technologies. The CuO coating condition was investigated, and the optimized CuO coated ceramic hollow fiber membranes (CuO@CHFMs) exhibited excellent catalytic activity for PMS activation to depredate bisphenol A (BPA) in the presence of humic acid (HA), chloride ions (Cl−) and bicarbonate (HCO3−). Based on the scavenger experiments and electron paramagnetic resonance (EPR) analyses, the non-radical reactive oxygen species - singlet oxygen (1O2), rather than sulfate radicals (SO4•-) or hydroxyl radicals (•OH), was elucidated as the primary reactive species responsible for the oxidation of BPA in the system. The redox circles of Cu(II)/Cu(I) on the CuO surface of the CuO@CHFMs are mainly responsible for PMS activation and a possible degradation pathway of BPA was proposed. Moreover, the CuO@CHFMs exhibited excellent stability and reusability without tedious catalyst separation/recovery processes. This study is meaningful for the development of novel catalytic membrane with PMS activation functionality in water treatment.

Gradient Li-rich oxide cathode particles immunized against oxygen release by a molten salt treatment

Abstract: Lithium-rich transition metal oxide (Li1+XM1−XO2) cathodes have high energy density above 900 Wh kg−1 due to hybrid anion- and cation-redox (HACR) contributions, but critical issues such as oxygen release and voltage decay during cycling have prevented their application for years. Here we show that a molten molybdate-assisted LiO extraction at 700 °C creates lattice-coherent but depth (r)-dependent Li1+X(r)M1−X(r)O2 particles with a Li-rich (X ≈ 0.2) interior, a Li-poor (X ≈ −0.05) surface and a continuous gradient in between. The gradient Li-rich single crystals eliminate the oxygen release to the electrolyte and, importantly, still allow stable oxygen redox contributions within. Both the metal valence states and the crystal structure are well maintained during cycling. The gradient HACR cathode displays a specific density of 843 Wh kg−1 after 200 cycles at 0.2C and 808 Wh kg−1 after 100 cycles at 1C, with very little oxygen release and consumption of electrolyte. This high-temperature immunization treatment can be generalized to leach other elements to avoid unexpected surface reactions in batteries. Critical issues such as oxygen release during battery cycling plague the development of high-energy Li-rich oxide cathodes. Here the authors report a Li-gradient structure of the oxides, obtained by a selective LiO leaching process via a molten salt treatment, displaying virtually zero oxygen loss.

Revealing molecular-level surface redox sites of controllably oxidized black phosphorus nanosheets

Abstract: Bulk and two-dimensional black phosphorus are considered to be promising battery materials due to their high theoretical capacities of 2,600 mAh g−1. However, their rate and cycling capabilities are limited by the intrinsic (de-)alloying mechanism. Here, we demonstrate a unique surface redox molecular-level mechanism of P sites on oxidized black phosphorus nanosheets that are strongly coupled with graphene via strong interlayer bonding. These redox-active sites of the oxidized black phosphorus are confined at the amorphorized heterointerface, revealing truly reversible pseudocapacitance (99% of total stored charge at 2,000 mV s−1). Moreover, oxidized black-phosphorus-based electrodes exhibit a capacitance of 478 F g–1 (four times greater than black phosphorus) with a rate capability of ~72% (compared to 21.2% for black phosphorus) and retention of ~91% over 50,000 cycles. In situ spectroelectrochemical and theoretical analyses reveal a reversible change in the surface electronic structure and chemical environment of the surface-exposed P redox sites. Black phosphorus is being considered for energy storage but its rate and cycling capabilities are limited by intrinsic (de-)alloying. Molecular-level surface redox sites on oxidized black phosphorus can now be coupled with graphene via strong interlayer bonding.

Non-precious Co3O4-TiO2/Ti cathode based electrocatalytic nitrate reduction: Preparation, performance and mechanism

Abstract: The presence of high nitrate (NO3−) concentration in natural water constitutes a serious issue to the environment and human health. Therefore, the development of low-cost, stable non-precious metal catalysts is imminent for efficient NO3- reduction. In this study, we prepared a Co3O4-TiO2/Ti cathode via combining sol-gel and calcination methods and evaluated its performance for electrocatalytic NO3- reduction. The dispersion of the Co3O4 catalyst particles was improved by the addition of PVP to the coating liquid. The presence of anatase could effectively stabilize Co3O4 and prevent the releasing of toxic Co ions into the solution. The Co3O4-TiO2/Ti cathode with the optimized performance for NO3- reduction could be prepared by four times coating at calcination temperature of 500 °C. The electrocatalytic reduction of NO3- was negligibly impacted by solution pH in the range of 3.0–9.0, while it could be facilitated by elevating the current density from 2.5 to 25 mA cm2. Ammonium ions were the main final NO3- reduction product, and the presence of Cl- was capable to oxidize ammonium ions to N2 due to the electrochemical production of reactive chlorine species. The electrochemical analyses, scavenging experiments and density functional theory calculations collectively confirm that NO3- reduction was mainly induced by the Co2+–Co3+–Co2+ redox process instead of being directly resulted from the electrons generated at the cathode. Unlike noble metal (e.g., Pd and Ag) based catalytic reaction systems, in the present Co3O4 mediated electrocatalytic reaction process, atomic H* would more favorably turn to H2 by Heyrovsky and Tafel routes and therefore contributed marginally to the NO3- reduction. Generally, this study provided a new paradigm for designing the stable and cost-effective cathode for NO3- reduction.

Synergistic effects of Cu2O-decorated CeO2 on photocatalytic CO2 reduction: Surface Lewis acid/base and oxygen defect

Abstract: Ceria (CeO2) with abundant oxygen defects, surface alkalinity, low cost effectiveness and admirable redox ability could be used in the photoreduction of CO2. However, little attention has been paid to the interaction of reactant CO2 molecules on CeO2–based photocatalysts. In this work, Cu2O nanoparticles were applied to the modification of the properties of Lewis acid/base, surface oxygen defect content and visible light adsorption of CeO2, and the adsorption/activation abilities of CO2 reactant on Cu2O/CeO2 and CeO2 photocatalysts were investigated in comparison. The photocatalytic performance showed that Cu2O/CeO2 had better activity than CeO2. And the loading of Cu2O resulted in more oxygen defects and Ce3+ species, which was helpful for available visible light adsorption and higher charge-separation efficiency. Furthermore, CO2-TPD, CO2-adsorption DRIFTS and in-situ ESR results demonstrated that the synergistic effects of Cu2O/CeO2 were beneficial for more generated carboxylate and CO2− radicals, instead of carbonate species which promoted CO2 reduction to CO.

Boosting selective nitrogen reduction to ammonia on electron-deficient copper nanoparticles.

Abstract: Production of ammonia is currently realized by the Haber-Bosch process, while electrochemical N2 fixation under ambient conditions is recognized as a promising green substitution in the near future. A lack of efficient electrocatalysts remains the primary hurdle for the initiation of potential electrocatalytic synthesis of ammonia. For cheaper metals, such as copper, limited progress has been made to date. In this work, we boost the N2 reduction reaction catalytic activity of Cu nanoparticles, which originally exhibited negligible N2 reduction reaction activity, via a local electron depletion effect. The electron-deficient Cu nanoparticles are brought in a Schottky rectifying contact with a polyimide support which retards the hydrogen evolution reaction process in basic electrolytes and facilitates the electrochemical N2 reduction reaction process under ambient aqueous conditions. This strategy of inducing electron deficiency provides new insight into the rational design of inexpensive N2 reduction reaction catalysts with high selectivity and activity.

Hydroxide promotes carbon dioxide electroreduction to ethanol on copper via tuning of adsorbed hydrogen.

Abstract: Producing liquid fuels such as ethanol from CO2, H2O, and renewable electricity offers a route to store sustainable energy. The search for efficient electrocatalysts for the CO2 reduction reaction relies on tuning the adsorption strength of carbonaceous intermediates. Here, we report a complementary approach in which we utilize hydroxide and oxide doping of a catalyst surface to tune the adsorbed hydrogen on Cu. Density functional theory studies indicate that this doping accelerates water dissociation and changes the hydrogen adsorption energy on Cu. We synthesize and investigate a suite of metal-hydroxide-interface-doped-Cu catalysts, and find that the most efficient, Ce(OH)x-doped-Cu, exhibits an ethanol Faradaic efficiency of 43% and a partial current density of 128 mA cm−2. Mechanistic studies, wherein we combine investigation of hydrogen evolution performance with the results of operando Raman spectroscopy, show that adsorbed hydrogen hydrogenates surface *HCCOH, a key intermediate whose fate determines branching to ethanol versus ethylene. Producing ethanol from carbon dioxide, water, and renewable electricity offers a route to sustainable energy. Here, the authors enhance electrocatalytic activity for carbon dioxide reduction by tuning adsorbed hydrogen in a class of copper catalysts with oxide- and hydroxide-modified surfaces.

Role of oxygen vacancies and Mn sites in hierarchical Mn2O3/LaMnO3-δ perovskite composites for aqueous organic pollutants decontamination

Abstract: La-based perovskites are catalytically active owing to the oxygen vacancies, redox metal centers of B sites and surface hydroxyl groups. Nevertheless, the insights into these active centers on environmental catalysis are still insufficient. In this study, hierarchical mixed oxides perovskite microspheres were synthesized for catalytic ozonation over oxalic acid and benzotriazole. LaMn4Ox, with LaMnO3-δ as the dominant crystal phase, demonstrated superior catalytic activity to Mn2O3 and LaMnO3 synthesized from citric acid sol-gel method. Temperature-programmed desorption of NH3 (NH3-TPD) and pyridine-Fourier transform infrared spectroscopy (pyridine-FTIR) tests proved Lewis acid as the main acid type. Temperature-programmed reduction of H2 (H2-TPR), O2-TPD and X-ray photoelectron spectroscopy (XPS) analysis indicated the presence of oxygen vacancies and mixed valences of Mn in the crystal structure facilitated the catalytic process. Moreover, the content of oxygen vacancy was calculated by iodometric titration method. With the aid of theoretical calculations, oxygen vacancies were found to exhibit a strong affinity toward ozone adsorption, where ozone molecules spontaneously dissociated into reactive oxygen species (ROS) such as O2 − and 1O2. The B site of Mn facilitated ozone decomposition by extending the O O bond of ozone due to the electron transfer from Mn3+/Mn4+ redox cycle. In-situ EPR and quenching tests confirmed the contribution of O2 − and 1O2 in benzotriazole degradation along with OH. This study stepped further to unveil the ozone adsorption/decomposition and ROS generation on nanoscale perovskite-based composites.

A Lattice-Oxygen-Involved Reaction Pathway to Boost Urea Oxidation.

Abstract: The electrocatalytic urea oxidation reaction (UOR) provides more economic electrons than water oxidation for various renewable energy-related systems owing to its lower thermodynamic barriers. However, it is limited by sluggish reaction kinetics, especially by CO2 desorption steps, masking its energetic advantage compared with water oxidation. Now, a lattice-oxygen-involved UOR mechanism on Ni4+ active sites is reported that has significantly faster reaction kinetics than the conventional UOR mechanisms. Combined DFT, 18 O isotope-labeling mass spectrometry, and in situ IR spectroscopy show that lattice oxygen is directly involved in transforming *CO to CO2 and accelerating the UOR rate. The resultant Ni4+ catalyst on a glassy carbon electrode exhibits a high current density (264 mA cm-2 at 1.6 V versus RHE), outperforming the state-of-the-art catalysts, and the turnover frequency of Ni4+ active sites towards UOR is 5 times higher than that of Ni3+ active sites.

Micrometer-Sized Water Droplets Induce Spontaneous Reduction.

Abstract: Bulk water serves as an inert solvent for many chemical and biological reactions. Here, we report a striking exception. We observe that in micrometer-sized water droplets (microdroplets), spontaneous reduction of several organic molecules occurs, pyruvate to lactate, lipoic acid to dihydrolipoic acid, fumarate to succinate, and oxaloacetate to malate. This reduction proceeds in microdroplets without any added electron donors or acceptors and without any applied voltage. In three of the four cases, the reduction efficiency is 90% or greater when the concentration of the dissolved organic species is less than 0.1 μM. None of these reactions occurs spontaneously in bulk water. One example demonstrating the possible broad application of reduction in water microdroplets to organic molecules is the reduction of acetophenone to form 1-phenylethanol. Taken together, these results show that microdroplets provide a new foundation for green chemistry by rendering water molecules to be highly electrochemically active without any added reducing agent or applied potential. In this manner, aqueous microdroplets might have provided a route for abiotic reduction reactions in the prebiotic era, thereby providing organic molecules with a reducing power before the advent of biotic reducing machineries.