Mechanistic Study on the Production of Hydrogen Peroxide in the Anthraquinone Process

Oxidation of Flavin by Molecular Oxygen: Computational Insights into a Possible Radical Mechanism

As a highly electrophilic moiety capable of oxidizing a variety of small organic molecules and biomolecules, flavin is an important prosthetic group in many enzymes. Upon oxidation of the substrate, flavin is converted into its reduced (dihydrogenated) form. The catalytic cycle is completed through oxidation back to the oxidized form, thus restoring the enzyme's oxidizing capability. While it has been firmly established that oxidation of the reduced form of flavin is cast by molecular oxygen, yielding oxidized flavin and hydrogen peroxide, the mechanism of this process is still poorly understood. Herein, we investigate the radical mechanism, which is one of the possible reaction mechanisms, by quantum chemical calculations. Because molecular oxygen exists as a triplet in its electronic ground state, whereas the products are singlets, the reaction is accompanied by hopping between electronic surfaces. We find that the rate-limiting factor of flavin oxidation is likely associated with the change in the spin state of the system. By considering several possible reactions involving flavin and its derivatives in the radical form and by examining the corresponding parts of the potential energy surface in various spin states, we estimate the effective barrier of the kinetically and thermodynamically preferred variant of flavin oxidation to be about 15 kcal/mol in the gas phase and about 7 kcal/mol in a polar (aqueous) environment. This is in agreement with kinetic studies of the corresponding monoamine oxidase enzymes, confirming the radical mechanism as a viable option for flavin regeneration in enzymes.

A Solar to Chemical Strategy: Green Hydrogen as a Means, Not an End

Green hydrogen is the key to the chemical industry achieving net zero emissions. The chemical industry is responsible for almost 2% of all CO2 emissions, with half of it coming from the production of simple commodity chemicals, such as NH3, H2O2, methanol, and aniline. Despite electrolysis driven by renewable power sources emerging as the most promising way to supply all the green hydrogen required in the production chain of these chemicals, in this review, it is worth noting that the photocatalytic route may be underestimated and can hold a bright future for this topic. In fact, the production of H2 by photocatalysis still faces important challenges in terms of activity, engineering, and economic feasibility. However, photocatalytic systems can be tailored to directly convert sunlight and water (or other renewable proton sources) directly into chemicals, enabling a solar-to-chemical strategy. Here, a series of recent examples are presented, demonstrating that photocatalysis can be successfully employed to produce the most important commodity chemicals, especially on NH3, H2O2, and chemicals produced by reduction reactions. The replacement of fossil-derived H2 in the synthesis of these chemicals can be disruptive, essentially safeguarding the transition of the chemical industry to a low-carbon economy.

Enzymatic Oxy- and Amino-Functionalization in Biocatalytic Cascade Synthesis: Recent Advances and Future Perspectives

Biocatalytic cascades are a powerful tool for building complex molecules containing oxygen and nitrogen functionalities. Moreover, the combination of multiple enzymes in one pot offers the possibility to minimize downstream processing and waste production. In this review, we illustrate various recent efforts in the development of multi-step syntheses involving C-O and C-N bond-forming enzymes to produce high value-added compounds, such as pharmaceuticals and polymer precursors. Both in vitro and in vivo examples are discussed, revealing the respective advantages and drawbacks. The use of engineered enzymes to boost the cascades outcome is also addressed and current co-substrate and cofactor recycling strategies are presented, highlighting the importance of atom economy. Finally, tools to overcome current challenges for multi-enzymatic oxy- and amino-functionalization reactions are discussed, including flow systems with immobilized biocatalysts and cascades in confined nanomaterials.

Mechanism for Generating H2 O2 at Water-Solid Interface by Contact-Electrification

The recent intensification of the study of contact-electrification at water-solid interfaces and its role in physicochemical processes lead to the realization that electron transfers during water-solid contact-electrification can drive chemical reactions. This mechanism, named contact-electro-catalysis (CEC), allows chemically inert fluorinated polymers to act like single electrode electrochemical systems. This study shows hydrogen peroxide (H2 O2 ) is generated from air and deionized water, by ultrasound driven CEC, using fluorinated ethylene propylene (FEP) as the catalyst. For a mass ratio of catalyst to solution of 1:10000, at 20 °C, the kinetic rate of H2 O2 evolution reaches 58.87 mmol L-1  gcat -1  h-1 . Electron paramagnetic resonance (EPR) shows electrons are emitted in the solution by the charged FEP, during ultrasonication. EPR and isotope labelling experiments show H2 O2 is formed from hydroxyl radicals (HO• ) or two superoxide radicals (O2 •- ) generated by CEC. Finally, it is traditionally believed such radicals migrate in the solution by Brownian diffusion prior to reactions. However, ab-initio molecular dynamic calculations reveal the radicals can react by exchanging protons and electrons through the hydrogen bonds network of water, i.e., owing to the Grotthuss mechanism. This mechanism can be relevant to other systems, artificial or natural, generating H2 O2 from air and water.

Recent Advances of Electrocatalyst and Cell Design for Hydrogen Peroxide Production

Electrochemical synthesis of H2O2 via a selective two-electron oxygen reduction reaction has emerged as an attractive alternative to the current energy-consuming anthraquinone process. Herein, the progress on electrocatalysts for H2O2 generation, including noble metal, transition metal-based, and carbon-based materials, is summarized. At first, the design strategies employed to obtain electrocatalysts with high electroactivity and high selectivity are highlighted. Then, the critical roles of the geometry of the electrodes and the type of reactor in striking a balance to boost the H2O2 selectivity and reaction rate are systematically discussed. After that, a potential strategy to combine the complementary properties of the catalysts and the reactor for optimal selectivity and overall yield is illustrated. Finally, the remaining challenges and promising opportunities for high-efficient H2O2 electrochemical production are highlighted for future studies.

Organo-photocatalytic C-H bond oxidation: an operationally simple and scalable method to prepare ketones with ambient air

Oxidative C-H functionalization with O₂ is a sustainable strategy to convert feedstock-like chemicals into valuable products. Nevertheless, eco-friendly O₂-utilizing chemical processes, which are scalable yet operationally simple, are challenging to develop. Here, we report our efforts, via organo-photocatalysis, in devising such protocols for catalytic C-H bond oxidation of alcohols and alkylbenzenes to ketones using ambient air as the oxidant. The protocols employed tetrabutylammonium anthraquinone-2-sulfonate as the organic photocatalyst which is readily available from a scalable ion exchange of inexpensive salts and is easy to separate from neutral organic products. Cobalt(ii) acetylacetonate was found to be greatly instrumental to oxidation of alcohols and therefore was included as an additive in evaluating the alcohol scope. The protocols employed a nontoxic solvent, could accommodate a variety of functional groups, and were readily scaled to 500 mmol scale in a simple batch setting using round-bottom flasks and ambient air. A preliminary mechanistic study of C-H bond oxidation of alcohols supported the validity of one possible mechanistic pathway, nested in a more complex network of potential pathways, in which the anthraquinone form - the oxidized form - of the photocatalyst activates alcohols and the anthrahydroquinone form - the relevant reduced form of the photocatalyst - activates O₂. A detailed mechanism, which reflected such a pathway and was consistent with previously accepted mechanisms, was proposed to account for formation of ketones from aerobic C-H bond oxidation of both alcohols and alkylbenzenes.

The Diverse Modes of Oxygen Reactivity in Life & Chemistry

Oxygen is a molecule of utmost importance in our lives. Beside its vital role for the respiration and sustaining of organisms, oxygen is involved in numerous chemical and physical processes. Upon combination of the different forms of molecular oxygen species with various activation modes, substrates, and reaction conditions an extremely wide chemical space can be covered that enables rich applications of diverse oxygenation processes. This Review provides an instructive overview of the individual properties and reactivities of oxygen species and illustrates their importance in nature, everyday life, and in the context of chemical synthesis.

Modified Poly(Heptazine Imides): Minimizing H2O2 Decomposition to Maximize Oxygen Reduction

Photocatalysis provides a sustainable pathway to produce the consumer chemical H2O2 from atmospheric O2 via an oxygen reduction reaction (ORR). Such an alternative is attractive to replace the cumbersome traditional anthraquinone method for H2O2 synthesis on a large scale. Carbon nitrides have shown very interesting results as heterogeneous photocatalysts in ORR because their covalent two-dimensional (2D) structure is believed to increase selectivity toward the two-electron process. However, an efficient and scalable application of carbon nitrides for this reaction is far from being achieved. Poly(heptazine imides) (PHIs) are a more powerful subgroup of carbon nitrides whose structure provides high crystallinity and a scaffold to host transition-metal single atoms. Herein, we show that PHIs functionalized with sodium and the recently reported fully protonated PHI exhibit high activity in two-electron ORR under visible light. The latter converted O2 to up to 1556 mmol L-1 h-1 g-1 H2O2 under 410 nm irradiation using inexpensive but otherwise chemically demanding glycerin as a sacrificial electron donor. We also prove that functionalization with transition metals is not beneficial for H2O2 synthesis, as the metal also catalyzes its decomposition. Transient photoluminescence spectroscopy suggests that H-PHIs exhibit higher activity due to their longer excited-state lifetime. Overall, this work highlights the high photocatalytic activity of the rarely examined fully protonated PHI and represents a step forward in the application of inexpensive covalent materials for photocatalytic H2O2 synthesis.

Enhanced H2O2 Production via Photocatalytic O2 Reduction over Structurally-Modified Poly(heptazine imide)

Solar H₂O₂ produced by O₂ reduction provides a green, efficient, and ecological alternative to the industrial anthraquinone process and H₂/O₂ direct-synthesis. We report efficient photocatalytic H₂O₂ production at a rate of 73.4 mM h^-1 in the presence of a sacrificial donor on a structurally engineered catalyst, alkali metal-halide modulated poly(heptazine imide) (MX → PHI). The reported H₂O₂ production is nearly 150 and >4250 times higher than triazine structured pristine carbon nitride under UV-visible and visible light (≥400 nm) irradiation, respectively. Furthermore, the solar H₂O₂ production rate on MX → PHI is higher than most of the previously reported carbon nitride (triazine, tri-s-triazine), metal oxides, metal sulfides, and other metal-organic photocatalysts. A record high AQY of 96% at 365 nm and 21% at 450 nm was observed. We find that structural modulation by alkali metal-halides results in a highly photoactive MX → PHI catalyst which has a broader light absorption range, enhanced light absorption ability, tailored bandgap, and a tunable band edge position. Moreover, this material has a different polymeric structure, high O₂ trapping ability, interlayer intercalation, as well as surface decoration of alkali metals. The specific C≡N groups and surface defects, generated by intercalated MX, were also considered as potential contributors to the separation of photoinduced electron-hole pairs, leading to enhanced photocatalytic activity. A synergy of all these factors contributes to a higher H₂O₂ production rate. Spectroscopic data help us to rationalize the exceptional photochemical performance and structural characteristics of MX → PHI.

Highly efficient photosynthesis of H2O2 via two-channel pathway photocatalytic water splitting

To overcome the problems of high energy consumption, heavy pollution and unsafety in the current H2O2 production process, extensive researches have been carried out on the low-cost, green and safety...

Cost-efficient microbial electrosynthesis of hydrogen peroxide on a facile-prepared floating electrode by entrapping oxygen

Microbial electrosynthesis of hydrogen peroxide is receiving growing interest for a green substitute for anthraquinone process.However, poor oxygen transmission of electrode remains an obstacle to enhance H₂O₂ production rate without aeration. Here, a superhydrophobic natural air diffusion floating electrode (NADFE), which naturally and efficiently entraps O₂ in the air, was proposed for the first time to improve microbial electrosynthesis of H₂O₂. Furthermore, a one-step calcined electrode preparation method was developed to reduce energy consumption further. In the microbial electrolysis cell with the NADFE, a high H₂O₂ production rate of 39 mg/L/h and current efficiency of 86% were achieved without aeration. The production rate of H₂O₂ was 2.2 times that of a gas diffusion electrode. Importantly, the energy consumption was 34.3 times lower than an electrochemical system. Therefore, the high H₂O₂ production rate and current efficiency, and low energy consumption of the process provide a superior alternative for environmental remediation.

Toward a mechanistic understanding of electrocatalytic nanocarbon

Electrocatalytic nanocarbon (EN) is a class of material receiving intense interest as a potential replacement for expensive, metal-based electrocatalysts for energy conversion and chemical production applications. The further development of EN will require an intricate knowledge of its catalytic behaviors, however, the true nature of their electrocatalytic activity remains elusive. This review highlights work that contributed valuable knowledge in the elucidation of EN catalytic mechanisms. Experimental evidence from spectroscopic studies and well-defined molecular models, along with the survey of computational studies, is summarized to document our current mechanistic understanding of EN-catalyzed oxygen, carbon dioxide and nitrogen electrochemistry. We hope this review will inspire future development of synthetic methods and in situ spectroscopic tools to make and study well-defined EN structures.

A computational study on the reduction of O2 to H2O2 using small polycyclic aromatic molecules

This work presents the complete autoxidation pathway for the anthraquinone process and one alternative catalyst that overcomes its kinetic challenges.

Density Functional Theory Mechanistic Study on H2O2 Production Using an Organic Semiconductor Epindolidione

Organic semiconductors have recently emerged as promising catalytic materials for oxygen reduction to hydrogen peroxide, H₂O₂, a chemical of great importance in industry as well as biology. While examples of organic semiconductor-mediated photocatalytic and electrocatalytic processes for H₂O₂ production become more numerous and improve in performance, fundamental understanding of the reaction mechanisms at play have been explored far less. The aim of the present work is to computationally test hypotheses of how selective oxygen reduction to H₂O₂ generally occurs on carbonyl dyes and pigments. As an example material, we consider epindolidione (EPI), an industrial pigment with demonstrated semiconductor properties, which photocatalytic activity in oxygen reduction reaction (ORR) and thereby producing hydrogen peroxide (H₂O₂) in low pH environment has been recently experimentally demonstrated. In this work, the ability of the reduced form of EPI, viz. EPI-2H (which was formed after a photoinduced 2e^–/2H⁺ process), to reduce molecular triplet oxygen to peroxide and the possible mechanism of this reaction are computationally investigated using density functional theory. In the main reaction pathway, the reduction of O₂ to H₂O₂ reaction occurs via abstraction of one of the hydrogen atoms of EPI-2H by triplet dioxygen to produce an intermediate complex consisting of the radicals of hydrogen peroxide (HOO^•) and EPI-H^• at the initial stage. HOO^• thus released can abstract another hydrogen atom from EPI-H^• to produce H₂O₂ and regenerates EPI; otherwise, it can enter another pathway to abstract hydrogen from a neighboring EPI-2H to form EPI-H^• and H₂O₂. EPI, after reduction, thus plays in ORR the role of hydrogen atom transfer (HAT) agent via its OH group, similar to anthraquinone in the industrial process, while HAT from its amino hydrogen is found unfavorable.

A Mechanistic Dichotomy in Two-Electron Reduction of Dioxygen Catalyzed by N,N'-Dimethylated Porphyrin Isomers

Selective two-electron reduction of dioxygen (O₂ ) to hydrogen peroxide (H₂ O₂ ) has been achieved by two saddle-distorted N,N'-dimethylated porphyrin isomers, an N21,N'22-dimethylated porphyrin (anti-Me₂ P) and an N21,N'23-dimethylated porphyrin (syn-Me₂ P) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved by anti-Me₂ P with higher turnover number (TON=250 for 30 min) than that by syn-Me₂ P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti-Me₂ Iph or syn-Me₂ Iph) by spectroscopic measurements. The rate-determining step in the catalytic ORRs was concluded to be proton-coupled electron-transfer reduction of O₂ with isophlorins based on kinetic analysis. The ORR rate by anti-Me₂ Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn-Me₂ Iph by external protons was observed. The different mechanisms in the O₂ reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O₂ with inner NH protons of the isophlorins.

Water-Soluble Anthraquinone Photocatalysts Enable Methanol-Driven Enzymatic Halogenation and Hydroxylation Reactions

Peroxyzymes simply use H₂O₂ as a cosubstrate to oxidize a broad range of inert C–H bonds. The lability of many peroxyzymes against H₂O₂ can be addressed by a controlled supply of H₂O₂, ideally in situ. Here, we report a simple, robust, and water-soluble anthraquinone sulfonate (SAS) as a promising organophotocatalyst to drive both haloperoxidase-catalyzed halogenation and peroxygenase-catalyzed oxyfunctionalization reactions. Simple alcohols, methanol in particular, can be used both as a cosolvent and an electron donor for H₂O₂ generation. Very promising turnover numbers for the biocatalysts of up to 318 000 have been achieved.

Visible-Light-Triggered Quantitative Oxidation of 9,10-Dihydroanthracene to Anthraquinone by O2 under Mild Conditions

The development of mild and efficient processes for the selective oxygenation of organic compounds by molecular oxygen (O₂ ) is key for the synthesis of oxygenates. This paper discloses an atom-efficient synthesis protocol for the photo-oxygenation of 9,10-dihydroanthracene (DHA) by O₂ to anthraquinone (AQ), which could achieve quantitative AQ yield (100 %) without any extra catalysts or additives under ambient temperature and pressure. A yield of 86.4 % AQ was obtained even in an air atmosphere. Furthermore, this protocol showed good compatibility for the photo-oxidation of several other compounds with similar structures to DHA. From a series of control experiments, free-radical quenching, and electron paramagnetic resonance spin-trapping results, the photo-oxygenation of DHA was probably initiated by its photoexcited state DHA*, and the latter could activate O₂ to a superoxide anion radical (O₂ ^.- ) through the transfer of its electron. Subsequently, this photo-oxidation was gradually dominated by the oxygenated product AQ as an active photocatalyst obtained from the oxidation of DHA by O₂ ^.- , and was accelerated with the rapid accumulation of AQ. The present photo-oxidation protocol is a good example of selective oxygenation based on the photoexcited substrate self-activated O₂ , which complies well with green chemistry ideals.

Phenolic Hydrogen Transfer by Molecular Oxygen and Hydroperoxyl Radicals. Insights into the Mechanism of the Anthraquinone Process

Hydrogen atom transfer (HAT) by ³O₂ and HO₂^• from arenols (ArOH), aryloxyls (ArO^•), their tautomers (ArH), and auxiliary compounds has been investigated by means of CBS-QB3 computations. With ³O₂, excellent linear correlations have been found between the activation enthalpy and the overall reaction enthalpy. Different pathways have been discerned for HATs involving OH or CH moieties. The results for ArOH + HO₂^• → ArO^• + H₂O₂ neither afford a linear correlation nor agree with the experiment. The precise mechanism for the liquid-phase autoxidation of anthrahydroquinone (AnH₂Q) appears to be not fully understood. A kinetic analysis shows that the HAT by chain-carrying HO₂^• occurs with a high rate constant of ≥6 × 10⁸ M^–1 s^–1 (toluene). The second propagation step pertains to a diffusion-controlled HAT by ³O₂ from the 10-OH-9-anthroxyl radical. Oxanthrone (AnOH) is a more stable tautomer of AnH₂Q with a ratio of 13 (298 K) in non-hydrogen-bonding (HB) solvents, but the reactivity toward ³O₂/HO₂^• is much lower. Combination of the computed free energies and Abrahams’ HB donating (α₂^H) and accepting (β₂^H) parameters has afforded an α₂^H(HO₂^•) of 0.86 and an α₂^H(H₂O₂) of 0.50.

Recent Strategies for Hydrogen Peroxide Production by Metal-Free Carbon Nitride Photocatalysts

Hydrogen peroxide (H2O2) is a chemical which has gained wide importance in several industrial and research fields. Its mass production is mostly performed by the anthraquinone (AQ) oxidation reaction, leading to high energy consumption and significant generation of wastes. Other methods of synthesis found in the literature include the direct synthesis from oxygen and hydrogen. However, this H2O2 production process is prone to explosion hazard or undesirable by‑product generation. With the growing demand of H2O2, the development of cleaner and economically viable processes has been under intense investigation. Heterogeneous photocatalysis for H2O2 production has appeared as a promising alternative since it requires only an optical semiconductor, water, oxygen, and ideally solar light irradiation. Moreover, employing a metal-free semiconductor minimizes possible toxicity consequences and reinforces the sustainability of the process. The most studied metal‑free catalyst employed for H2O2 production is polymeric carbon nitride (CN). Several chemical and physical modifications over CN have been investigated together with the assessment of different sacrificial agents and light sources. This review shows the recent developments on CN materials design for enhancing the synthesis of H2O2, along with the proposed mechanisms of H2O2 production. Finally, the direct in situ generation of H2O2, when dealing with the photocatalytic synthesis of added-value organic compounds and water treatment, is discussed.

Quinone-amine polymers as metal-free and reductant-free catalysts for hydroxylation of benzene to phenol with molecular oxygen

Quinone-amine polymers can be employed as a metal-free and reductant-free catalyst for the hydroxylation of benzene to phenol and can yield phenol as high as the transition metal catalyst.

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.

Photoirradiation-generated radicals in two-component supramolecular gel for polymerization

While supramolecular gels have been attracting great interest due to their easy design and fabrication, development of new applications based on these gels is always a challenging topic. Here, we report a two-component supramolecular gel that can generate and stabilize radicals through photo-irradiation, which can be subsequently used for polymerization. It has been found that the electrostatic interactions between a cationic amphiphile and anionic sulfonate could afford co-assembly into a two-component supramolecular gel. Upon photo-irradiation, the gel changed colour and produced the radicals, as verified from the EPR measurements. The radical thus formed in the supramolecular gel is relatively stable and could be used to polymerize acrylic acid directly without deoxygenation. In contrast, acrylic acid could not be polymerized in solution under the same conditions. This work expands the application scope of supramolecular gels.

The Effect of Water on Quinone Redox Mediators in Nonaqueous Li-O2 Batteries

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical nonaqueous Li-O₂ batteries. We hereby address the above issues by exploring the synergistic effect of 2,5-di-tert-butyl-1,4-benzoquinone and H₂O on the oxygen chemistry in a nonaqueous Li-O₂ battery. Water stabilizes the quinone monoanion and dianion, shifting the reduction potentials of the quinone and monoanion to more positive values (vs Li/Li⁺). When water and the quinone are used together in a (largely) nonaqueous Li-O₂ battery, the cell discharge operates via a two-electron oxygen reduction reaction to form Li₂O₂, with the battery discharge voltage, rate, and capacity all being considerably increased and fewer side reactions being detected. Li₂O₂ crystals can grow up to 30 μm, more than an order of magnitude larger than cases with the quinone alone or without any additives, suggesting that water is essential to promoting a solution dominated process with the quinone on discharging. The catalytic reduction of O₂ by the quinone monoanion is predominantly responsible for the attractive features mentioned above. Water stabilizes the quinone monoanion via hydrogen-bond formation and by coordination of the Li⁺ ions, and it also helps increase the solvation, concentration, lifetime, and diffusion length of reduced oxygen species that dictate the discharge voltage, rate, and capacity of the battery. When a redox mediator is also used to aid the charging process, a high-power, high energy density, rechargeable Li-O₂ battery is obtained.

A biomass derived N/C-catalyst for the electrochemical production of hydrogen peroxide

An all-in-one (carbon source, self-template, and heteroatom) biomass precursor to develop an electrocatalyst for highly selective and energy-saving H₂O₂ production.

Base catalysed decomposition of anthracene endoperoxide

Anthracene endoperoxide (EPO) decomposes even under very mild basic conditions to anthraquinone (AQ) and hydrogen peroxide by an interesting mechanism, proposed herein.

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over activated-carbon-supported Pd–Ag alloy catalysts

A series of bimetallic PdAg catalysts on an activated carbon support were prepared for the direct synthesis of hydrogen peroxide from hydrogen and oxygen. The addition of Ag to Pd caused an increase in selectivity due to ensemble and electronic effects.

Combination of redox-active ligand and lewis acid for dioxygen reduction with π-bound molybdenum-quinonoid complexes

A series of π-bound Mo-quinonoid complexes supported by pendant phosphines have been synthesized. Structural characterization revealed strong metal-arene interactions between Mo and the π system of the quinonoid fragment. The Mo-catechol complex (2a) was found to react within minutes with 0.5 equiv of O(2) to yield a Mo-quinone complex (3), H(2)O, and CO. Si- and B-protected Mo-catecholate complexes also react with O(2) to yield 3 along with (R(2)SiO)n and (ArBO)(3) byproducts, respectively. Formally, the Mo-catecholate fragment provides two electrons, while the elements bound to the catecholate moiety act as acceptors for the O(2) oxygens. Unreactive by itself, the Mo-dimethyl catecholate analogue reduces O(2) in the presence of added Lewis acid, B(C(6)F(5))(3), to generate a Mo(I) species and a bis(borane)-supported peroxide dianion, [[(F(5)C(6))(3)B](2)O(2)(2-)], demonstrating single-electron-transfer chemistry from Mo to the O(2) moiety. The intramolecular combination of a molybdenum center, redox-active ligand, and Lewis acid reduces O(2) with pendant acids weaker than B(C(6)F(5))(3). Overall, the π-bound catecholate moiety acts as a two-electron donor. A mechanism is proposed in which O(2) is reduced through an initial one-electron transfer, coupled with transfer of the Lewis acidic moiety bound to the quinonoid oxygen atoms to the reduced O(2) species.

Performance of facet-controlled Pd nanocrystals in 2-ethylanthraquinone hydrogenation

The Pd (100) facet is more active than Pd (111) in CO hydrogenation but less active in saturation of aromatic rings.

Hydrogen Peroxide as a Sustainable Energy Carrier: Electrocatalytic Production of Hydrogen Peroxide and the Fuel Cell

This review describes homogeneous and heterogeneous catalytic reduction of dioxygen with metal complexes focusing on the catalytic two-electron reduction of dioxygen to produce hydrogen peroxide. Whether two-electron reduction of dioxygen to produce hydrogen peroxide or four-electron O2-reduction to produce water occurs depends on the types of metals and ligands that are utilized. Those factors controlling the two processes are discussed in terms of metal-oxygen intermediates involved in the catalysis. Metal complexes acting as catalysts for selective two-electron reduction of oxygen can be utilized as metal complex-modified electrodes in the electrocatalytic reduction to produce hydrogen peroxide. Hydrogen peroxide thus produced can be used as a fuel in a hydrogen peroxide fuel cell. A hydrogen peroxide fuel cell can be operated with a one-compartment structure without a membrane, which is certainly more promising for the development of low-cost fuel cells as compared with two compartment hydrogen fuel cells that require membranes. Hydrogen peroxide is regarded as an environmentally benign energy carrier because it can be produced by the electrocatalytic two-electron reduction of O2, which is abundant in air, using solar cells; the hydrogen peroxide thus produced could then be readily stored and then used as needed to generate electricity through the use of hydrogen peroxide fuel cells.