Rate Of H2O2 Formation Research Articles

Efficient Catalytic Production of Reactive Oxygen Species through Piezoelectricity in Bismuth Sulfide Rich in Sulfur Vacancies.

Sulfur (S) vacancies in metal sulfides are of interest in electrocatalysis and photoelectronics, but their effect on the generation of reactive oxygen species (ROS) during mechanical catalysis is unclear. This study investigates the impact of S-vacancies in defective bismuth sulfide (Bi2S3-x) on ROS production under ultrasonic irradiation and organic contaminant decomposition. S-vacancies disrupt the centrosymmetric structure of intrinsic Bi2S3, inducing piezoelectric effects and enhancing the electrical energy in Bi2S3-x. The positively charged S-vacancies in Bi2S3-x promote the separation of ultrasound-activated electron-hole pairs by capturing electrons. As a result, the optimal rate of H2O2 formation and the reaction rate constant for degrading Rhodamine B dye on Bi2S3-x are found to be 1.9 and 37 times higher, respectively, than those on Bi2S3 under ultrasonic irradiation. The nonzero catalytic efficiency in centrosymmetric Bi2S3 is due to the flexoelectric catalytic effect from nonuniform strain. These results guide the piezocatalyst design and elucidate mechanical catalysis mechanisms.

Highly efficient electrosynthesis of hydrogen peroxide through reversible transformation between catechol and o-benzoquinone on polydopamine modified carbon black

Carbon-based materials are more promising catalysts for H2O2 electrochemical production. However, the common method for modifying carbon-based materials is surface functionalization with harsh and uncontrollable reaction conditions, hindering the precise regulation of the active sites. Herein, we proposed a carbon-based material modified by polydopamine (CB-PDA) that was easily prepared and used in air diffusion electrode system for H2O2 production. The rapid and reversible transformation between catechol and o-benzoquinone on PDA could improve the H2O2 selectivity from 75.5 % to 97.0 % during the electrocatalytic process. The detection of adsorbed *HOOH and *OOH in oxygen reduction reaction proved the preference of CB-PDA for the two-electron pathway. The H2O2 formation rate constant of CB-PDA increased significantly from 25.85 to 60.82 mM h−1. The highest cumulative H2O2 concentration could reach 10200 mg L−1 in 9 h. Long-term operation tests proved the good operational stability. Furthermore, this system was cost-effective without additional aeration energy consumption and could achieve rapid disinfection in 10 min, which would have great potential to be used in environmental remediation.

Ru–modified graphitic carbon nitride for the solar light–driven photocatalytic H2O2 synthesis

Hydrogen peroxide (H2O2) is an efficient and environmentally friendly oxidant as well as a promising energy-carrier alternative to hydrogen. Its solar light-driven photocatalytic synthesis from H2O and O2 is a high-prospect sustainable alternative to the industrial anthraquinone process. Hybrid Ru-modified graphitic carbon nitride (g-C3N4) catalysts were prepared by thermal polymerisation of melamine/Ru(III) acetylacetonate mixtures, and subsequent thermal exfoliation. Simultaneous Ru incorporation and thermal exfoliation impacted the morphology and the structure of the g-C3N4 sheets, and low-atomicity Ru species with small nanoclusters and single atoms were observed only in the Ru-modified exfoliated g-C3N4 photocatalysts. We demonstrated the potential of a joint thermal exfoliation and modification with Ru to enhance the H2O2 synthesis efficiency of the g-C3N4 photocatalyst under simulated sunlight. A volcano-type behavior was observed with increasing the Ru content, and the best performance was obtained for exfoliated g-C3N4 with an ultra-low Ru content of 0.019 wt.%, that outperformed both its bulk counterpart and the pristine exfoliated reference in terms of initial H2O2 synthesis rate and H2O2 formation rate constant. The enhanced performance was attributed to the presence of highly dispersed low atomicity Ru as well as to more accessible active sites and carrier migration channels. Quenching experiments revealed a mixed reactional pathway involving both two-step one-electron and one-step two-electron O2 reduction in the photocatalytic H2O2 production.

Boosting H2O2 photosynthesis by accumulating photo-electrons on carbonyl active site of polyimide covalent organic frameworks

The low trapping efficiency of photogenerated electrons by the targeted reduction site seriously restricts the kinetics of H2O2 photosynthesis via two-electron oxygen reduction reaction. Here, two polyimide covalent organic frameworks (PT-COF and PB-COF) possessing carbonyl groups with different electron-trapping capacity were elaborately designed. PB-COF with electron-rich carbonyl site presented 4.22 times improvement in H2O2 formation rate up to 2044 μmol g−1 h−1, and exhibited outstanding photostability after 120 h continuous opearting. Meanwhile, solar to-chemical energy efficiency was 0.68%, representing one of advanced polymer based photocatalysts. We demonstrated electronic structure of the carbonyl active center was modulated by tuning electron attraction capability of the donor unit via increased the built-in electric field to accelerate charge separation and directional transfer. The electron-rich carbonyl site is identified to boosted H2O2 photosynthesis activity via reducing the *OOH binding energy. Our work offers a tailoring electron density of pre-designable active site strategy for enhanced solar-to-chemical energy conversion.

Indirect H2O2 synthesis without H2

Industrial hydrogen peroxide (H2O2) is synthesized using carbon-intensive H2 gas production and purification, anthraquinone hydrogenation, and anthrahydroquinone oxidation. Electrochemical hydrogenation (ECH) of anthraquinones offers a carbon-neutral alternative for generating H2O2 using renewable electricity and water instead of H2 gas. However, the H2O2 formation rates associated with ECH are too low for commercialization. We report here that a membrane reactor enabled us to electrochemically hydrogenate anthraquinone (0.25 molar) with a current efficiency of 70% at current densities of 100 milliamperes per square centimeter. We also demonstrate continuous H2O2 synthesis from the hydrogenated anthraquinones over the course of 48 h. This study presents a fast rate of electrochemically-driven anthraquinone hydrogenation (1.32 ± 0.14 millimoles per hour normalized per centimeter squared of geometric surface of electrode), and provides a pathway toward carbon-neutral H2O2 synthesis.

Employing controlled periodic illumination in identifying limiting factors of heterogeneous catalytic processes: A case study of photocatalytic generation of H2O2 and H2

Semiconductor photocatalysis is a versatile process that has been utilised in a wide range of applications from water treatment to sustainable energy generation and storage. In a heterogeneous photocatalytic reaction, identifying the limiting factor in a reaction is crucial in optimising operating parameters such as catalyst loading or irradiation intensity. With the application of Controlled Periodic Illumination (CPI), it is possible to control the intensity of photons entering the system. In this paper, two processes were evaluated and monitored by employing CPI to identify limiting factors: photocatalytic generation of H2O2 and H2. The relationship between catalyst loading and light intensity on H2/H2O2 generation constant rate was explored using CPI. In photocatalytic generation of H2O2, two commercial catalysts, TiO2 and ZnO were used under identical experimental conditions and the limiting factors on them differed. The effect of CPI varied greatly between both catalysts with TiO2 demonstrating that up to 50% less light is required to achieve comparable H2O2 formation rates at lower catalyst loadings (0.1 g L−1). In contrast, for ZnO photocatalysis, the CPI experiments highlighted the impact of adsorbed O2 levels, catalyst loading, and light intensity on H2O2 yields. The H2O2 formation rate constant plateaued beyond a duty cycle of 0.6 regardless of the tested catalyst loading. For photocatalytic generation of H2 on Pt/TiO2 from glucose, the generation rate constant increased linearly at low duty cycles and plateaued with different catalyst loading, which helped to identify different limiting factors for each condition. Therefore, exploring H2O2 and H2 generation via CPI is reported for the first time and the results show the importance of monitoring the interdependency of photocatalytic parameters and prove the validity of CPI on determining the limiting factor.

Nitrogen-rich carbon nitride (C3N5) coupled with oxygen vacancy TiO2 arrays for efficient photocatalytic H2O2 production

Developing efficient and facilitated recycling photocatalysts for H2O2 formation is an ideal strategy for solar-to-chemical energy conversion. In this work, we synthesized ultrathin C3N5 nanosheets through the process of thermal polymerization and polyvinylpyrrolidone (PVP)-assisted solvent exfoliation. Subsequently, the obtained ultrathin C3N5 nanosheets were tightly attached to the surface of TiO2-x arrays, resulting in an enhanced photocatalytic H2O2 production rate. The density functional theory (DFT) calculations demonstrate that an internal electric field (IEF) is generated between the TiO2-x array and the ultrathin C3N5 due to the different work functions. The presence of IEF provides an additional driving force for carrier separation and transfer in the heterointerface. Benefitting from this unique strategy, the optimal heterojunction obtains the highest H2O2 formation rate (2.93 μmol/L/min), which is about 4.1 times than that of TiO2-x arrays. The rotating disk electrode (RDE) analysis manifests H2O2 formation through 2e--dominated oxygen reduction reaction (ORR). This research provides an innovative strategy for assembling a type-II heterojunction with a useful IEF for efficient photocatalytic H2O2 production.

Simplified synthesis of direct Z-scheme Bi2WO6/PhC2Cu heterojunction that shows enhanced photocatalytic degradation of 2,4,6-TCP: Kinetic study and mechanistic insights

For this work, we employed n-type Bi2WO6 and p-type PhC2Cu to formulate a direct Z-scheme Bi2WO6/PhC2Cu (PCBW) photocatalyst via simplified ultrasonic stirring technique. An optimal 0.6PCBW composite exhibited the capacity to rapidly photodegrade 2,4,6-TCP (98.6% in 120 min) under low-power blue LED light, which was 8.53 times and 12.53 times faster than for pristine PhC2Cu and Bi2WO6, respectively. Moreover, electron spin resonance (ESR), time-resolved PL spectra, and quantitative ROS tests indicated that the PCBW enhanced the separation capacity of photocarriers. It also more readily associated with dissolved oxygen in water to generate reactive oxygen species (ROS). Among them, the ability of PCBW to produce ·O2- in one hour was 12.07 times faster than for pure PhC2Cu. In addition, the H2O2 formation rate and apparent quantum efficiency of PCBW are 10.73 times that of PhC2Cu, which indicates that PCBW not only has excellent photocatalytic performance, but also has outstanding ROS production ability. Furthermore, Ag photodeposition, in situ X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations were utilized to determine the photogenerated electron migration paths in the PCBW, which systematically confirmed that Z-scheme heterojunction were successfully formed. Finally, based on the intermediate products, three potential 2,4,6-TCP degradation pathways were proposed.

Suppression of Membrane Degradation Accompanied with Increased Output Performance in Fuel Cells by Use of Silica-Containing Anode Catalyst Layers.

Polymer electrolyte membranes (PEMs) for fuel cells are chemically degraded by the attack of ·OH radicals generated from the decomposition of H2O2, which is predominantly produced at the Pt/C hydrogen anode. The incorporation of conventional radical scavengers into the PEM suffers from a decrease in the output performance. We, for the first time, demonstrate that the addition of hygroscopic silica nanoparticles (NPs) to the Pt/C anode catalyst layer provides a remarkably prolonged (ca. 4 times) lifetime of a Nafion membrane in an accelerated stress test and open circuit voltage (OCV) holding at 90 °C, accompanied by improved output (I-E) performances at low relative humidity. It has been found that the use of silica NPs decreases H2O2 formation rate from the OCV to a practical H2 oxidation potential in a half-cell using 0.1 M HClO4 at 90 °C and provides reduced ohmic resistance (increase in water content) and effective utilization of Pt cathode catalyst in a single cell, by which the improvement of the durability of the PEM and increased output performance are explained rationally.

Effect of current density on membrane degradation under the combined chemical and mechanical stress test in the PEMFCs

This study elucidates the effects of current density on membrane degradation under combined mechanical and chemical stress tests. Relative humidity (RH) cycling tests using hydrogen gas and air are conducted on a polymer electrolyte membrane fuel cell based membrane NRE211 at the open circuit voltage (OCV), 0.05 and 0.3 Acm−2 conditions. The different current density conditions result in different in-plane membrane stresses and H2O2 formation rates during the test. After every 200 RH cycles, membrane integrity is assessed via the hydrogen crossover rate and OCV. Furthermore, catalytic combustion is analyzed during OCV measurement using a thermal imaging method employing high-transmittance glass at the cathode side. The membrane failed after 1600, 1800, and 2200 RH cycles under the OCV condition, 0.05 of 0.3 Acm−2, respectively. The vigorous membrane degradation under OCV conditions can be attributed to higher mechanical stress and H2O2 formation rate. Hotspots created owing to the combustion between the crossover hydrogen and air were successfully captured, with a maximum temperature rise ranging from 15 to 16 °C compared with a given cell temperature of 80 °C. Moreover, a post-mortem analysis (SEM imaging) revealed the presence of pinholes, through-membrane cracks, and membrane thinning at the hotspot locations.

The Short-Term Opening of Cyclosporin A-Independent Palmitate/Sr2+-Induced Pore Can Underlie Ion Efflux in the Oscillatory Mode of Functioning of Rat Liver Mitochondria.

Mitochondria are capable of synchronized oscillations in many variables, but the underlying mechanisms are still unclear. In this study, we demonstrated that rat liver mitochondria, when exposed to a pulse of Sr2+ ions in the presence of valinomycin (a potassium ionophore) and cyclosporin A (a specific inhibitor of the permeability transition pore complex) under hypotonia, showed prolonged oscillations in K+ and Sr2+ fluxes, membrane potential, pH, matrix volume, rates of oxygen consumption and H2O2 formation. The dynamic changes in the rate of H2O2 production were in a reciprocal relationship with the respiration rate and in a direct relationship with the mitochondrial membrane potential and other indicators studied. The pre-incubation of mitochondria with Ca2+(Sr2+)-dependent phospholipase A2 inhibitors considerably suppressed the accumulation of free fatty acids, including palmitic and stearic acids, and all spontaneous Sr2+-induced cyclic changes. These data suggest that the mechanism of ion efflux from mitochondria is related to the opening of short-living pores, which can be caused by the formation of complexes between Sr2+(Ca2+) and endogenous long-chain saturated fatty acids (mainly, palmitic acid) that accumulate due to the activation of phospholipase A2 by the ions. A possible role for transient palmitate/Ca2+(Sr2+)-induced pores in the maintenance of ion homeostasis and the prevention of calcium overload in mitochondria under pathophysiological conditions is discussed.

Interfacial Schottky junctions modulated by photo-piezoelectric band bending to govern charge carrier migration for selective H2O2 generation

A major challenge faced by most systems is the dissociation of O−O bonding on H2O2 by subsequent electron (e-) reduction. This study investigates interfacial charge transfer by manipulating e- flows through a deflexed band potential on polarized piezoelectric BiFO3 (BFO). The H2O2 accumulation via coupling with the photocatalyst BiOCl/BiVO4 (BCV) was highly effective since the Schottky barrier height (SBH) formed within the heterojunction composite shifted according to the surface polarity of BFO. Additionally, the constant alternating surface charge on BFO, reduced the SBH, forcing the photoexcited e- to flow from BCV→BFO for effective H2O2 production, while restricting decomposition of H2O2 during downshifted band potential (positive surface) as high SBH discontinuing the electrons flow from BCV→BFO. The high interfacial charge transfer resistance (Rct) was also critical for H2O2 accumulation, since it is unfavorable for H2O2 dissociation (H2O2/·OH, +0.39 V) despite the presence of a high band potential (+0.16 eV) on the opposite surface's upshifted band. The formation rate (kf: 1.13 µmol L−1 min−1) of H2O2 was calculated much higher than decomposition rate (kd: 0.01 min−1). Additionally, the RRDE results indicated favorable 2e- transfer with > 90 % selectivity for H2O2. Results from ESR DMPO-·OH abduction and atrazine degradation show an insignificant concentration of ·OH has been produced. This work provides an effective strategy to regulate semiconductors' surface junction by piezoelectric polarization for selective H2O2 generation.

Antioxidant potential of pentoxifylline on spermatozoa of small ruminants

Objective: To investigate antioxidant potential of pentoxifylline on spermatozoa of small ruminants including rams and bucks. Methods: The levels of hydrogen peroxide (H2O2) production in ram and buck spermatozoa incubated with 0 (control), 4 and 8 mM of pentoxifylline were measured after 45-min incubation. Then, the motility parameters of ram and buck spermatozoa incubated with 0 (control), 1 mM of H2O2, 1 mM of H2O2 plus 4 mM of pentoxifylline, and 1 mM of H2O2 plus 8 mM of pentoxifylline were analysed. H2O2 was estimated using a fluorometric assay and spermatozoa motility characteristics were analyzed using computer aided sperm analyzer. Results: Pentoxifylline significantly decreased the levels of H2O2 produced from both ram and buck spermatozoa (P<0.05), and significant lower rates of H2O2 formation were noted when 8 mM of pentoxifylline was added (P<0.05). The values of all sperm motility parameters for the two species significantly decreased after incubation with H2O2 (P<0.05). In contrast, when the spermatozoa were incubated with both H2O2 and two concentrations of pentoxifylline, the motility values rose significantly compared to those incubated with H2O2 alone (P<0.05). For both ram and buck sperm samples, the rapid and static subpopulation motility parameters were the most affected categories by pentoxifylline addition compared to the medium and slow categories. Conclusions: Pentoxifylline possesses an antioxidant capacity proved by its ability of reducing H2O2 levels as well as by increasing motility values of stressed spermatozoa. Therefore, pentoxifylline could be recommended as antioxidant additive for spermatozoa of small ruminants under stress conditions.

H2O2 in Liquid Fractions of Hydrothermally Pretreated Biomasses: Implications of Lytic Polysaccharide Monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) are important players in enzyme-aided valorization of lignocellulose. However, the recently discovered dependency of LPMO catalysis on H2O2 along with H2O2-caused inactivation of the enzyme calls for an in-depth understanding of the levels and the dynamics of H2O2 in various streams of the processing of lignocellulosic biomass. Using an LPMO-based sensitive detection, we assessed the dynamics of H2O2 in the liquid fractions (LFs) of hydrothermally pretreated wheat straw (agricultural residue), birch (hardwood), and pine (softwood). Upon contact with air, H2O2 was formed in the LFs of all biomasses. The initially high rate of H2O2 formation decayed with the half-life around 1–2 h to a low but stable plateau value. The rates of H2O2 formation were much higher than the rates of its accumulation, suggesting that H2O2 is an intermediate in aerobic oxidation of the compounds in LFs. Although the general traits were similar, LFs of different biomasses had different rates of H2O2 formation and accumulation. LFs of different biomasses also differed by their effect on the enzymatic degradation of cellulose. Compositional analyses revealed a number of different compounds that were formed and disappeared upon aerobic oxidation of LFs.

Effect of Dapagliflozin on the Functioning of Rat Liver Mitochondria In Vitro.

We studied the effect of a new hypoglycemic compound dapagliflozin on the functioning of rat liver mitochondria. Dapagliflozin in concentrations of 10-20 μM had no effect on the parameters of respiration and oxidative phosphorylation of rat liver mitochondria. Increasing dapagliflozin concentration to 50 μM led to a significant inhibition of mitochondrial respiration in states 3 and 3UDNP. Dapagliflozin in this concentration significantly reduced calcium retention capacity of rat liver mitochondria. These findings indicate a decline in the resistance of rat liver mitochondria to induction of Ca2+-dependent mitochondrial permeability transition pore. In a concentration of 10 μM, dapagliflozin significantly decreases the rate of H2O2 formation in rat liver mitochondria, which attested to an antioxidant effect of this compound. Possible mitochondrion-related mechanisms of the protective action of dapagliflozin on liver cells are discussed.

Unifying Concepts in Electro- and Thermocatalysis toward Hydrogen Peroxide Production.

We examine relationships between H2O2 and H2O formation on metal nanoparticles by the electrochemical oxygen reduction reaction (ORR) and the thermochemical direct synthesis of H2O2. The similar mechanisms of such reactions suggest that these catalysts should exhibit similar reaction rates and selectivities at equivalent electrochemical potentials (μ̅i), determined by reactant activities, electrode potential, and temperature. We quantitatively compare the kinetic parameters for 12 nanoparticle catalysts obtained in a thermocatalytic fixed-bed reactor and a ring-disk electrode cell. Koutecky-Levich and Butler-Volmer analyses yield electrochemical rate constants and transfer coefficients, which informed mixed-potential models that treat each nanoparticle as a short-circuited electrochemical cell. These models require that the hydrogen oxidation reaction (HOR) and ORR occur at equal rates to conserve the charge on nanoparticles. These kinetic relationships predict that nanoparticle catalysts operate at potentials that depend on reactant activities (H2, O2), H2O2 selectivity, and rate constants for the HOR and ORR, as confirmed by measurements of the operating potential during the direct synthesis of H2O2. The selectivities and rates of H2O2 formation during thermocatalysis and electrocatalysis correlate across all catalysts when operating at equivalent μ̅i values. This analysis provides quantitative relationships that guide the optimization of H2O2 formation rates and selectivities. Catalysts achieve the greatest H2O2 selectivities when they operate at high H atom coverages, low temperatures, and potentials that maximize electron transfer toward stable OOH* and H2O2* while preventing excessive occupation of O-O antibonding states that lead to H2O formation. These findings guide the design and operation of catalysts that maximize H2O2 formation, and these concepts may inform other liquid-phase chemistries.

Particle-Phase Photoreactions of HULIS and TMIs Establish a Strong Source of H2O2 and Particulate Sulfate in the Winter North China Plain.

During haze periods in the North China Plain, extremely high NO concentrations have been observed, commonly exceeding 1 ppbv, preventing the classical gas-phase H2O2 formation through HO2 recombination. Surprisingly, H2O2 mixing ratios of about 1 ppbv were observed repeatedly in winter 2017. Combined field observations and chamber experiments reveal a photochemical in-particle formation of H2O2, driven by transition metal ions (TMIs) and humic-like substances (HULIS). In chamber experiments, steady-state H2O2 mixing ratios of 116±83 pptv were observed upon the irradiation of TMI- and HULIS-containing particles. Correspondingly, H2O2 formation rates of about 0.2 ppbv h-1 during the initial irradiation periods are consistent with the H2O2 rates observed in the field. A novel chemical mechanism was developed explaining the in-particle H2O2 formation through a sequence of elementary photochemical reactions involving HULIS and TMIs. Dedicated box model studies of measurement periods with relative humidity >50% and PM2.5≥75μgm-3 agree with the observed H2O2 concentrations and time courses. The modeling results suggest about 90% of the particulate sulfate to be produced from the SO2 reaction with OH and HSO3- oxidation by H2O2. Overall, under high pollution, the H2O2-caused sulfate formation rate is above 250 ng m-3 h-1, contributing to the sulfate formation by more than 70%.

Effects of bromide adsorption on the direct synthesis of H2O2 on Pd nanoparticles: Formation rates, selectivities, and apparent barriers at steady-state

The direct synthesis reaction (H2 + O2 → H2O2) may provide a low-cost method to form H2O2 if catalytic systems with sufficient selectivities and stabilities are developed. Here, we examine how sodium bromide, a ubiquitous promoter for direct synthesis, impacts steady-state rates and apparent barriers for H2O2 and H2O formation in water and relate changes to the effect of NaBr on surface and bulk properties of Pd nanoparticle catalysts. Comparisons of turnover rates and selectivities measured over an extended period (>80 h) demonstrate that these systems require more than 10 h to reach steady-state, whereupon, H2O2 selectivities depend strongly upon [NaBr] and increase from 17% in pure water to ~65% at 10−4 M NaBr (55 kPa H2, 200 kPa O2). Contact with these NaBr solutions irreversibly modifies Pd nanoparticles, such that H2O2 selectivities remain at 40% in pure water, due to irreversible uptake of Br*-atoms. Bromide adsorption isotherms measured show that reduced Pd nanoparticles adsorb several monolayers of Br*-atoms, which suggests Br saturates surfaces but also resides within the near-surface region of Pd nanoparticles. Ex situ X-ray photoelectron spectroscopy indicates that Br*-atoms withdraw charge from Pd atoms and yield greater fractions of Pd2+. Comparisons among infrared spectra of adsorbed CO imply that persistent Br atoms preferentially bind to undercoordinated sites. H2O2 and H2O formation rates, both in the presence and absence of Br*-atoms, change with H2 and O2 pressures in ways consistent with elementary steps that involve H2O-mediated proton-electron transfer (PET). Therefore, increased selectivities on Br*-modified surfaces reflect differences in apparent activation enthalpies for H2O2 (ΔH‡H2O2)and H2O (ΔH‡H2O) formation. ΔH‡H2O2and ΔH‡H2O increase systematically with [NaBr], although with different sensitivities. Comparisons of activation enthalpies alongside ex situ characterization provide compelling evidence that Br atoms adsorb onto and intercalate beneath the surfaces of Pd nanoparticles and increase steady-state H2O2 selectivities, which persist without further addition of liquid-phase bromide for at least 15 h. These findings indicate H2O2 selectivities for a continuous catalytic process may be increased by a one-time (or infrequent) addition of halide promoters, which can reduce the complexity of subsequent product purification for many applications of H2O2.

Role of oxygen and superoxide radicals in promoting H2O2 production during VUV/UV radiation of water

As a green and effective water treatment technology, vacuum UV (VUV) radiation could generate reactive species such as hydroxyl radicals (OH) and H2O2 via the homolysis and ionization of water. The formation mechanism of H2O2 in pure water was studied under irradiation of VUV/UV lamp emitting at 185 and 254 nm. The influence of dissolved oxygen was investigated. The production and the formation rate of H2O2 were promoted with enough dissolved oxygen, which demonstrated that the oxygen played an important role in H2O2 evolution. The UV254 radiation could induce the decomposing of H2O2 and had little effect on the synthesis of H2O2. Additionally, the alkaline condition and addition of some anions were unfavorable to the production of H2O2. Besides, 4-chloro-7-nitrobenzo-2-oxa-1,3-dizole (NBD-Cl) was used as the molecular probe to confirm the role of superoxide radicals (O2−/HO2) in the oxygen-containing condition. The possible formation mechanism of H2O2 was proposed. The H2O2 was produced only by the recombination of OH in the oxygen-free system, while related to O2−/HO2 besides the recombination of OH in the oxygen-containing system. This study brought new insights into the process that utilizing the in-situ generated H2O2 in VUV/UV water treatment for organic contaminants.

Effect of Pd Coordination and Isolation on the Catalytic Reduction of O2 to H2O2 over PdAu Bimetallic Nanoparticles.

The direct synthesis of hydrogen peroxide (H2 + O2 → H2O2) may enable low-cost H2O2 production and reduce environmental impacts of chemical oxidations. Here, we synthesize a series of Pd1Aux nanoparticles (where 0 ≤ x ≤ 220, ∼10 nm) and show that, in pure water solvent, H2O2 selectivity increases with the Au to Pd ratio and approaches 100% for Pd1Au220. Analysis of in situ XAS and ex situ FTIR of adsorbed 12CO and 13CO show that materials with Au to Pd ratios of ∼40 and greater expose only monomeric Pd species during catalysis and that the average distance between Pd monomers increases with further dilution. Ab initio quantum chemical simulations and experimental rate measurements indicate that both H2O2 and H2O form by reduction of a common OOH* intermediate by proton-electron transfer steps mediated by water molecules over Pd and Pd1Aux nanoparticles. Measured apparent activation enthalpies and calculated activation barriers for H2O2 and H2O formation both increase as Pd is diluted by Au, even beyond the complete loss of Pd-Pd coordination. These effects impact H2O formation more significantly, indicating preferential destabilization of transition states that cleave O-O bonds reflected by increasing H2O2 selectivities (19% on Pd; 95% on PdAu220) but with only a 3-fold reduction in H2O2 formation rates. The data imply that the transition states for H2O2 and H2O formation pathways differ in their coordination to the metal surface, and such differences in site requirements require that we consider second coordination shells during the design of bimetallic catalysts.