O2 Reduction Process Research Articles

A metal-free cascaded process for efficient H2O2 photoproduction using conjugated carbonyl sites

Carbon-based metal-free catalysts are promising green catalysts for photocatalysis and electrocatalysis due to their low cost and environmental friendliness. A key challenge in utilizing these catalysts is identifying their active sites, given their poor crystallinity and complex structures. Here we demonstrate the key structure of the double-bonded conjugated carbon group as a metal-free active site, enabling efficient O2 photoreduction to H2O2 through a cascaded water oxidation − O2 reduction process. Using ethylenediaminetetraacetic acid as a precursor, we synthesized various carbon-based photocatalysts and analyzed their structural evolution. Under the polymerization conditions of 260 °C to 400 °C, an N-ethyl-2-piperazinone-like structure was formed on the surface of the catalyst, resulting in high photocatalytic H2O2 photoproduction (2884.7 μmol g−1h−1) under visible light. A series of control experiments and theoretical calculations further confirm that the double-bond conjugated carbonyl structure is the key and universal feature of the active site of metal-free photocatalysts.

BiOBr/COF S-scheme photocatalyst for H2O2 production via concerted two-electron pathway

Constructing step-scheme (S-scheme) heterojunctions has become a popular strategy for efficient photocatalytic H2O2 generation. Herein, we in situ grew BiOBr nanosheets (NSs) on a Schiff-base covalent organic framework (COF) with large π-conjugated structures to prepare S-scheme BiOBr/COF photocatalysts for H2O2 synthesis. The highest photocatalytic H2O2 production performance of the composite sample constituting the S-scheme heterojunction is 3749 µmol g−1 h−1, which was 1.85 and 27 times the rates of COF and BiOBr, respectively. The construction of S-scheme heterojunction contributed to efficient carrier transfer and separation in space and enhanced redox power. Moreover, the lying-down O2-adsorption configuration on the COF surface favors the concerted two-electron O2 reduction process, which greatly reduced the reduction potential requirement for O2-to-H2O2 conversion. The synergy between the S-scheme heterojunction and the unique O2–COF interaction boosted photocatalytic H2O2 production activity.

Rational design of three-in-one Pt/MnO2/GO hybrid nanozyme with enhanced catalytic performance for colorimetric detection of ascorbic acid and cysteine

In this work, a novel three-in-one Pt/MnO2/GO hybrid nanozyme with wide pH and temperature working range was rational prepared using simple hydrothermal and reduction strategy. The prepared Pt/MnO2/GO displayed enhanced catalytic activity than single active component due to the excellent conductivity of GO, the increased active sites, the increased electron transfer capacity, the synergistic effect between each component and the decreased binding energy for adsorbed intermediates. Combing chemical characterization and theoretical simulation calculations, the O2 reduction process on the Pt/MnO2/GO nanozymes and the generated reactive oxygen species in the nanozyme-TMB system were thoroughly illustrated. Based on the excellent catalytic activity of Pt/MnO2/GO nanozymes, a colorimetric strategy was proposed to detect the ascorbic acid (AA) and cysteine (Cys), and the experimental results indicated that detection range of AA was 0.35–56 μM with a LOD of 0.075 μM and detection range of Cys was 0.5–32 μM with a LOD of 0.12 μM. Good recoveries were achieved in the human serum and fresh fruit juice detection procedures, demonstrating the potential applications of Pt/MnO2/GO-based colorimetric strategy in complex biological and food samples.

Evidence of Sulfur Non-Innocence in [CoII (dithiacyclam)]2+ -Mediated Catalytic Oxygen Reduction Reactions.

In many metalloenzymes, sulfur-containing ligands participate in catalytic processes, mainly via the involvement in electron transfer reactions. In a biomimetic approach, we now demonstrate the implication of S-ligation in cobalt mediated oxygen reduction reactions (ORR). A comparative study between the catalytic ORR capabilities of the four-nitrogen bound [Co(cyclam)]2+ (1; cyclam=1,5,8,11-tetraaza-cyclotetradecane) and the S-containing analog [Co(S2 N2 -cyclam)]2+ (2; S2 N2 -cyclam=1,8-dithia-5,11-diaza-cyclotetradecane) reveals improved catalytic performance once the chalcogen is introduced in the Co coordination sphere. Trapping and characterization of the intermediates formed upon dioxygen activation at the CoII centers in 1 and 2 point to the involvement of sulfur in the O2 reduction process as the key for the improved catalytic ORR capabilities of 2.

CdS Nanoparticles Supported on Ti3C2Tx MXene for the Efficient Photocatalytic Production of H2O2: Implications for the Photocatalytic Degradation of Emerging Contaminants

Herein, Ti3C2Tx MXene was prepared and used to anchor CdS nanoparticles, and CdS/Ti3C2Tx (CT) photocatalyst with an interfacial Schottky junction was obtained for in situ hydrogen peroxide (H2O2) production to degrade emerging contaminants. CT-0.5 photocatalyst had the highest H2O2 production yield, which was 401 μmol L–1 within 1 h in pure water without organic sacrificial reagents under visible-light irradiation (λ > 420 nm), more than 3 times that of pristine CdS. The excellent photocatalytic performance was attributed to the unique properties of Ti3C2Tx and the formed Schottky junction, which accelerated electron transfer and spatial charge separation. The introduction of Ti3C2Tx improved the selectivity of two-electron O2 reduction, and the direct one-step two-electron O2 reduction route dominated the overall O2 reduction process for H2O2 production. Finally, CT-0.5 was applied to construct a photo-Fenton system, which also exhibited a superior catalytic performance and wide suitability for degrading emerging contaminants, suggesting its environmental significance.

Lewis acid Fe3+ in TiO2 ultra-thin nanosheet boosts oxygen species activation

Breaking the bottleneck of the light absorption, photocarriers transmission and separation (PTS), and surface reactions efficiency of photocatalysis is hot topic. For this, the Fe3+ acted as Lewis acid site is successfully built in TiO2 ultra-thin nanosheet (FT) through hydrothermal method. The detailed effect of Lewis acid site is systematically studied. Based on DFT, in-situ XPS, O2 TPD, CV, hole transfer efficiency (ηtrans) and ESR, Lewis acid of Fe3+ acts as a photo-generated electron trapped site, and the imbalance of charge by the construction of Lewis acid, which makes the O2 reduction process more easily occurs on Ti4+ (adjacent to the Lewis acid site) as well as Lewis acid site itself. Meanwhile, the oxygen defect (OD) near Lewis acid site promotes the adsorption, activation and oxidation of H2O. The photo-redox reaction is simultaneously accomplished around the Lewis acid site, which is the PTS driving force of photocarriers. The photocurrent density of 5% FT is 4 to 5 times higher than intrinsic one (T), and Lewis acid site of Fe3+ leads to the reduction of interfacial resistance due to lower the activation energy of redox process. As result, FT displays higher photocatalytic activity than T.

Post-illumination activity of Bi2WO6 in the dark from the photocatalytic \u201cmemory\u201d effect

Photocatalysts with the photocatalytic “memory” effect could resolve the intrinsic activity loss of traditional photocatalysts when the light illumination is turned off. Due to the dual requirements of light absorption and energy storage/release functions, most previously reported photocatalysts with the photocatalytic “memory” effect were composite photocatalysts of two phase components, which may lose their performance due to gradually deteriorated interface conditions during their applications. In this work, a simple solvothermal process was developed to synthesize Bi2WO6 microspheres constructed by aggregated nanoflakes. The pure phase Bi2WO6 was found to possess the photocatalytic “memory” effect through the trapping and release of photogenerated electrons by the reversible chemical state change of W component in the (WO4)2− layers. When the illumination was switched off, Bi2WO6 microspheres continuously produced H2O2 in the dark as those trapped photogenerated electrons were gradually released to react with O2 through the two-electron O2 reduction process, resulting in the continuous disinfection of Escherichia coli bacteria in the dark through the photocatalytic “memory” effect. No deterioration of their cycling H2O2 production performance in the dark was observed, which verified their stable photocatalytic “memory” effect.

Oxygen Reduction Pathways in the Li-O2 Battery: Understanding Solvent-Water Interactions

Li-ion and related battery technologies will be important for years to come. However, society needs energy storage that exceeds the capacity of Li-ion batteries. We must explore alternatives to Li-ion if we are to have any hope of meeting the long-term requirements for energy storage. One such alternative technology is the Li-air (O2) battery; its theoretical specific energy exceeds that of Li-ion five-fold, but many obstacles face its realisation.1–4 It is assumed that H2O in the Li-O2 battery will result in irreversible formation of LiOH and thus gas streams must be dried before entering the battery. Projections have suggested that this would sacrifice one third of the pack weight to a gas filtration unit required to bring atmospheric H2O content to functional levels (<1 ppm).5–9 Understanding the specific solvent properties that are associated with H2O tolerance, or distinct lack of, can guide rational selection of solvents able to tolerate wet air streams and reduce the requirements of the gas filtration unit, increasing the practical energy density.Here we consider competition between reversible 2e- O2 reduction and proton coupled irreversible 4e- O2 reduction. The predisposition to favour one of these O2 reduction pathways is largely solvent dependent. We have combined a range of methods including cyclic voltammetry, operando pressure analysis, NMR, FTIR, Raman, UV-vis spectroscopy and SEM to investigate the O2 reduction pathways in four common Li-air electrolytes containing H2O.10 H2O concentrations spanning 0–36% were employed to understand the onset and of the irreversible 4e- O2 reduction process across these solvents, with specific solvent properties being attributed to their performance. The results of these studies will be presented, along with their implications on rational future solvent development for rechargeable Li-O2 battery electrolytes. Reference s : [1] X. Gao, Y. Chen, L. Johnson and P. G. Bruce, Nat. Mater. 15 (2016) 882–888.[2] N. Feng, P. He and H. Zhou, Adv. Energy Mater. 6 (2016) 1502303.[3] L. Ma, T. Yu, E. Tzoganakis, K. Amine, T. Wu, Z. Chen and J. Lu, Adv. Energy Mater. 8 (2018) 1–13.[4] D. Aurbach, B. D. McCloskey, L. F. Nazar and P. G. Bruce, Nat. Energy 1 (2016) 16128.[5] K. G. Gallagher, S. Goebel, T. Greszler, M. Mathias, W. Oelerich, D. Eroglu and V. Srinivasan, Energy Environ. Sci. 7 (2014) 1555-1563.[6] P. Tan, W. Shyy, T. S. Zhao, R. H. Zhang and X. B. Zhu, Appl. Energy 182 (2016) 569–575.[7] C. Shu, J. Wang, J. Long, H. K. Liu and S. X. Dou, Adv. Mater. 31 (2019) 1–43.[8] A. Dai, Q. Li, T. Liu, K. Amine and J. Lu, Adv. Mater. 31 (2018) 1805602.[9] D. G. Kwabi, T. P. Batcho, S. Feng, L. Giordano, C. V. Thompson and Y. Shao-Horn, Phys. Chem. Chem. Phys., 18 (2016) 24944–24953.[10] H. Guo, W. Luo, J. Chen, S. Chou, H. Liu and J. Wang, Adv. Sustain. Syst. 2 (2018) 1700183.

Visible-light-driven H2O2 production from O2 reduction with nitrogen vacancy-rich and porous graphitic carbon nitride

Visible-light-driven H2O2 production from O2 reduction is an eco-friendly strategy and attracts increasing attentions. In this study, KOH modified graphitic carbon nitrides (CNKx) were constructed through the thermolysis of melamine-cyanuric acid supramolecular-complex in the presence of KOH. The BET specific surface area of CNK0.2 with porous structure was 92 m2 g−1. The photocatalyst was characterized by XPS, ESR and element analysis and the analysis suggested that nitrogen vacancy was created in the framework of g-C3N4. Photoluminescence (PL) spectra indicated that the recombination of photogenerated electrons and holes was efficiently suppressed. ζ-potential revealed that more negative charge on the surface of CNK0.2, which promoted the adsorption of O2 and accelerated the photocatalytic H2O2 production. ESR experimental and radical quenching results illustrated that superoxide radical (O2-) was the primary intermediate during the O2 reduction process, suggesting the O2 reduction pathway by CNK0.2 was the sequential two-step single-electron reduction.

Nickel phthalocyanine integrated graphene architecture as bifunctional electrocatalyst for CO2 and O2 reductions

Efficient, durable and low cost electrocatalysts for O2 and CO2 reductions are required as alternate to expensive metal based catalysts in fuel cells and in the CO2 reduction process. Herein, we describe the synthesis of reduced graphene oxide (rGO)-nickel phthalocyanine (NiPc) composite (rGO-NiPc) as a bifunctional electrocatalyst for O2 and CO2 reductions. The composite material, rGO-NiPc is coated on glassy carbon (GC) electrode (GC/rGO-NiPc) and used as a low cost efficient electrocatalyst for O2 and CO2 reductions. The results show that the addition of rGO to NiPc increases the catalytic efficiency and electrochemical charge transfer rate of NiPc. GC/rGO-NiPc provides the low potentials, +0.25 and −0.11 V (vs RHE) for O2 and CO2 reductions, respectively in 0.1 M HClO4. Kinetic interpretation based on rotating disc electrode studies provides the support for the two electron pathway for CO2 reduction to CO in 0.1 M HClO4. Product analysis by 13C NMR confirms CO as the CO2 reduction product. GC/rGO-NiPc shows a facile four electron pathway for O2 reduction which can be exploited in fuel cells, metal-air batteries, chlor-alkali electrolyzers and oxygen sensing. In addition, GC/rGO-NiPc has efficient methanol tolerance capability for O2 reduction process which is suitable to use in fuel cells. Moreover, GC/rGO-NiPc shows the high operational stability more than an hour for both O2 and CO2 reductions in 0.1 M HClO4.

Postillumination Activity in a Single-Phase Photocatalyst of Mo-Doped TiO2 Nanotube Array from Its Photocatalytic “Memory”

Several composite photocatalysts with photocatalytic “memory” effect had recently been developed, which could possess postillumination activity for an extended period of time in the dark for many potential applications. Here, a single-phase photocatalyst of Mo-doped TiO2 nanotube array was developed for the first time with the postillumination photocatalytic memory effect, which could eliminate the requirement of building composite photocatalysts with heterojunctions and largely broaden the material selection for this interesting photocatalytic memory effect. Because of the proper electronic band gap structure and variable valences of Mo dopants, photogenerated electrons could transfer from TiO2 to Mo dopants and be trapped there by reducing Mo6+ to Mo5+ under UV irradiation. When UV irradiation was switched off, these trapped electrons could be released from Mo dopants and react with O2 through the two-electron O2 reduction process to produce H2O2 in the dark. Thus, Mo-doped TiO2 nanotube array could remain active in the dark as demonstrated by its effective disinfection of Escherichia coli cells when UV irradiation was turned off. This work demonstrated that photocatalysts with postillumination photocatalytic memory effect were not limited to composite photocatalysts with heterojunctions. Various single-phase photocatalysts could also possess this interesting photocatalytic memory effect through doping metal elements with variable valences.

Enhancement of oxygen reduction at Co-porphyrin catalyst by supporting onto hybrid multi-layered film of polypyrrole and polyoxometalate-modified gold nanoparticles

Three-dimensional multi-layered films (on glassy carbon) composed of networks of polyoxometallate (PMo12O40 3−)-modified gold nanoparticles linked together through the alternately deposited ultra-thin layers of polypyrrole have served as active supports for Co-porphyrin catalytic centers. The hybrid organic-inorganic films (supports) have been prepared by using the layer-by-layer approach. The fact that polyanionic (phosphomolybdate) adsorbates on gold nanoparticles are attracted by positively charged sites of conducting polymer (polypyrrole) structures leads to the stabilizing effect and facilitates distribution of Au nanostructures. The systems have been characterized using scanning electron microscopy, as well as with chronoamperometric and voltammetric techniques. By supporting Co-porphyrin centers onto the hybrid film of the polymer-linked phosphomolybdate-stabilized gold nanoparticles, significant electrocatalytic enhancement effects (namely voltammetric current increases) have been observed during the electroreduction of oxygen in acid medium relative to a standard response of the simple porphyrin deposit on glassy carbon measured under analogous conditions. Among important issues is the high activity of the hybrid film (support) itself toward the reductive decomposition of hydrogen peroxide to water. When it comes to performance of the Co-porphyrin-containing system, it is reasonable to expect that the O2 reduction process is initiated at Co-porphyrin catalytic sites (two-electron reduction to H2O2) and continued (two-electron reduction to H2O) at the hybrid film containing gold nanoparticles dispersed within the highly porous cauliflower-like structures of polypyrrole multi-layers. While the gold networks facilitate charge distribution within the hybrid electrocatalytic film, non-covalent π-π interactions of porphyrin rings with polypyrrole interlayers and charge transfers between negatively charged (PMo12O40 3− modified) gold nanoparticles and positively charged nitrogen sites of polypyrrole could also cause synergism.

Adiabaticity of the proton-coupled electron-transfer step in the reduction of superoxide effected by nickel-containing superoxide dismutase metallopeptide-based mimics.

Nickel-containing superoxide dismutases (NiSODs) are bacterial metalloenzymes that catalyze the disproportionation of O2(-). These enzymes take advantage of a redox-active nickel cofactor, which cycles between the Ni(II) and Ni(III) oxidation states, to catalytically disprotorptionate O2(-). The Ni(II) center is ligated in a square planar N2S2 coordination environment, which, upon oxidation to Ni(III), becomes five-coordinate following the ligation of an axial imidazole ligand. Previous studies have suggested that metallopeptide-based mimics of NiSOD reduce O2(-) through a proton-coupled electron transfer (PCET) reaction with the electron derived from a reduced Ni(II) center and the proton from a protonated, coordinated Ni(II)-S(H(+))-Cys moiety. The current work focuses on the O2(-) reduction half-reaction of the catalytic cycle. In this study we calculate the vibronic coupling between the reactant and product diabatic surfaces using a semiclassical formalism to determine if the PCET reaction is proceeding through an adiabatic or nonadiabatic proton tunneling process. These results were then used to calculate H/D kinetic isotope effects for the PCET process. We find that as the axial imidazole ligand becomes more strongly associated with the Ni(II) center during the PCET reaction, the reaction becomes more nonadiabatic. This is reflected in the calculated H/D KIEs, which moderately increase as the reaction becomes more nonadiabatic. Furthermore, the results suggest that as the axial ligand becomes less Lewis basic the observed reaction rate constants for O2(-) reduction should become faster because the reaction becomes more adiabatic. These conclusions are in-line with experimental observations. The results thus indicate that variations in the axial donor's ability to coordinate to the nickel center of NiSOD metallopeptide-based mimics will strongly influence the fundamental nature of the O2(-) reduction process.

New insights into the catalytic active-site structure of multicopper oxidases

Structural models determined by X-ray crystallography play a central role in understanding the catalytic mechanism of enzymes. However, X-ray radiation generates hydrated electrons that can cause significant damage to the active sites of metalloenzymes. In the present study, crystal structures of the multicopper oxidases (MCOs) CueO from Escherichia coli and laccase from a metagenome were determined. Diffraction data were obtained from a single crystal under low to high X-ray dose conditions. At low levels of X-ray exposure, unambiguous electron density for an O atom was observed inside the trinuclear copper centre (TNC) in both MCOs. The gradual reduction of copper by hydrated electrons monitored by measurement of the Cu K-edge X-ray absorption spectra led to the disappearance of the electron density for the O atom. In addition, the size of the copper triangle was enlarged by a two-step shift in the location of the type III coppers owing to reduction. Further, binding of O2 to the TNC after its full reduction was observed in the case of the laccase. Based on these novel structural findings, the diverse resting structures of the MCOs and their four-electron O2-reduction process are discussed.

Effects of O2 and H2O2 on TiO2 Photocatalytic Efficiency Quantified by Formaldehyde Formation from Tris(hydroxymethyl)aminomethane

AbstractThe impact of O

Catalytic characteristics of MnO2 nanostructures for the O2 reduction process

Nanorods with an α type MnO2 structure and a diameter ranging from 25 to 40 nm, along with tipped needles with aβ MnO2 structure and a diameter of 100 nm were obtained. The 25 nm diameterα MnO2 nanorods showed the best catalytic activity for dissociation ofHO2 − formed during oxygen reduction in a KOH solution. TheMnO2 nanostructures preferably followed a two-electron oxygen reduction mechanism in a LiOHsolution. The size of the catalyst also affected the specific capacities of the non-aqueousLi/O2 batteries fabricatedusing the MnO2 based air electrode. The highest specific capacity of 1917 mA h g − 1 was obtainedfor an α MnO2 nanorod catalyst having a diameter of 25 nm. The cation present in theMnO2 nanostructures appears to determine the catalytic activity ofMnO2.

Amperometric Flow‐Biosensor for Cyanide Based on an Inhibitory Effect upon Bioelectrocatalytic Reduction of Oxygen by Peroxidase‐Modified Carbon‐Felt

AbstractA novel, simple and relative highly sensitive amperometric flow biosensor for cyanide was developed by using horseradish peroxidase (HRP)‐adsorbed carbon‐felt (CF), based on an inhibitory effect on the HRP‐catalyzed O2 reduction. The HRP‐CF showed a sufficient bioelecrocatalytic activity for O2 reduction in the potential region from 0 to −0.5 V at pH 5.0, due to a direct electron transfer‐based O2 reduction process via ferrous‐HRP and compound III. This HRP‐catalyzed O2 reduction was reversibly inhibited by cyanide, which enabled to fabricate a novel and simple reagentless (i.e., no requirement of the ordinary substrate, H2O2, and the electron transfer mediators) flow‐biosensor for cyanide. When air‐saturated 0.1 M phosphate buffer (pH 5.0) was used as a carrier under the applied potential of −0.2 V vs. Ag/AgCl, the steady‐state base‐current due to the HRP‐catalyzed O2 reduction was reversibly inhibited by the cyanide injection (200 µL), resulting in peak‐shape current responses. The magnitude of the inhibition peak currents linearly increased with increasing concentrations of cyanide up to 1 µM, and the detection limit was found to be 0.04 µM (S/N=2). The apparent inhibition constant Ki′ was estimated to be 0.87 µM.

Oxygen Reduction at Cobalt Phthalocyanine Modified Au(111) Electrodes

Au(111) substrates were modified with cobalt phthalocyanine (CoPc) monolayers. Formation of the adsorbed layer was confirmed by in situ electrochemical scanning tunneling microscopy (STM) while its electrochemical activity was monitored by cyclic voltammetry (CV). The adsorbed molecules have demonstrated a noticeable electrocatalytic activity toward the reduction of oxygen. Significant changes in the STM features of the adsorbed molecules were observed when the measurements were performed in the presence of ca. 1 atm of oxygen at low overpotentials. The reported features were attributed to changes in the electronic structure of the molecule when it forms an intermediate during oxygen reduction. Geometries of two of the possible intermediates, HOOCoPc and O2CoPc, were optimized by density functional theoretical (DFT) method. Frontier orbitals of the hydroperoxo intermediate have electronic density similar to the STM features reported here, which could support that we are probing a reaction intermediate during the O2 reduction process.

Visible Light Sensitive Metal Oxide Nanocluster Photocatalysts: Photo-Induced Charge Transfer from Ce(III) to Keggin-Type Polyoxotungstates

Anchored visible light-absorbing metal oxide nanoclusters, consisting of trivalent cerium ions and Keggin-type polyoxotungstates (PW12O403− and CuIIPW11O397−), have been synthesized on the pore surface of mesoporous silica (MCM-41) as a new class of visible light sensitive photocatalysts. The diffuse reflectance UV−vis, FT-Raman, and Ce LIII-edge X-ray absorption measurements showed that grafting of trivalent cerium ions to the preimmobilized PW12O403− cluster on the silica surface provided a broad absorption tail extending from the UV region to 530 nm assigned to a metal-to-metal charge transfer (MMCT) transition of an oxo-bridged binuclear WVI−O−CeIII unit. Photoexcitation of WVI−O−CeIII → WV−O−CeIV MMCT in the anchored clusters initiated the photooxidation reactions of gaseous isopropanol to CO2, whereby WV and CeIV work as a site-specific reducing and oxidizing centers, respectively. In addition to the PW12O403− cluster, it was also confirmed that the present methods can be extended to metal-substituted polyoxotungstates. Grafting of trivalent cerium ions to the copper-monosubstituted cluster (CuIIPW11O397−) produced an oxo-bridged trinuclear CuII/WVI/CeIII unit and exhibited photoactivity 4 times higher than that for the binuclear WVI/CeIII cluster. CuII-substituted polyoxotungstates are known to initiate a multielectron dioxygen reduction reaction and have a unique redox property in that the two-electron reduction of CuII to Cu0 occurs in a concerted manner. Therefore, the enhanced photoactivity for the trinuclear unit demonstrates that the activation of a single Cu center from CuII to Cu0 is accomplished by sequential photoinduced MMCT through the confined WVI−O−CeIII linkages, which results in acceleration of the photocatalytic two-electron O2 reduction process.

Generation of hydrogen peroxide via the selective reduction of oxygen by hydrazine sulfate over Br-promoted Pd/Al2O3 catalyst in an aqueous medium at ambient conditions

The in situ generation of H2O2 via the selective reduction of O2 by hydrazine sulfate (in an aqueous medium) has been thoroughly investigated over the Br-promoted Pd(or PdO)/Al2O3 catalyst. The influence of the following factors have been addressed in this study: (a) Br concentration (in the reaction medium or incorporated in the catalyst); (b) reaction conditions (viz. reaction time, temperature); (c) concentrations of reducing agent and phosphoric acid in the reaction medium. Decomposition of N2H4 (from N2H4·H2SO4) and its reaction with H2O2 under similar reaction conditions (in the absence of O2) have also been studied. H2O2, which is an intermediate product of the O2 reduction, is involved in further consecutive reactions. It can be converted to water via its decomposition and/or reaction with the unconverted N2H4. In the presence of Br promoter, the later (reaction with unconverted N2H4) is more dominant. Both the consecutive reactions are drastically retarded in the presence of protons and Br promoter. A plausible reaction mechanism has been proposed for illustrating the role of the protons and Br promoter in the selective formation of H2O2 in the O2 reduction process.