Direct H2O2 Synthesis, without H2 Gas

Electrochemical Pilot H2O2 Production by Solid-State Electrolyte Reactor: Insights From a Hybrid Catalyst for 2-Electron Oxygen Reduction Reaction

The electrochemical oxygen reduction reaction (ORR) offers an alluring and sustainable alternative to the traditional anthraquinone process for hydrogen peroxide (H₂O₂) synthesis. However, challenges remain in developing scalable electrocatalysts and cost-effective reactors for high-purity H₂O₂ production. This study introduces a simple yet effective mechanical mixing method to fabricate a hybrid electrocatalyst from oxidized carbon nanotubes and layered double hydroxides (LDHs). This easily accessible and low-cost catalyst achieves near-perfect Faradaic efficiency (∼100%) with low overpotentials of 73 mV at 10 mA cm⁻2 and 588 mV at 400 mA cm⁻2 in a solid electrolyte cell. Through theoretical calculations and in-situ analyses, we uncover the pivotal role played by the LDH co-catalyst in fine-tuning the local pH at the catalyst/solid-electrolyte interface that drives both the activity and selectivity. We also design a low-cost solid-state reactor using cation-exchange resin (CER) as both a proton conductor and a microchannel for efficient mass transfer, achieving a production rate of 5.29 mmol cm⁻2 h⁻¹ and continuous output concentrations of 11.8 wt.% H₂O₂. Scaled to an industrial area of 2 × 100 cm2, the pilot reactor achieves an impressive H₂O₂ production rate of approximately 127.0 mmol h⁻¹ at 15 A, marking a significant advancement in sustainable H₂O₂ production.

A ZnO-based Catalytic System for the Synthesis of Hydrogen Peroxide from Air

Hydrogen peroxide (H2O2) has a wide range of applications as an eco-friendly and sustainable oxidant. However, the clean, efficient and convenient synthesis of this compound remains challenging. This work demonstrates a rationally designed electron-self-supplied catalytic system capable of generating H2O2 from water and atmospheric oxygen without extra energy input. This catalytic system is made of a ZnO coating containing oxygen vacancies and a Zn substrate. The ZnO catalyst layer obtains electrons from the Zn substrate to synthesize H2O2. The H2O2 concentration produced by this catalytic system is up to 17.9 mM without any secondary processing. This remarkably high concentration is attributed to the formation of a liquid film on the hydrophilic ZnO surface that promotes the oxygen reduction reaction by accelerating the transfer of oxygen from the ambient air to the catalyst surface. By integrating with atmospheric fog collection, this system can continuously collect H2O2 directly from the air.

Water Spillover to Expedite Two-Electron Oxygen Reduction

Limited by the activity-selectivity trade-off relationship, the electrochemical activation of small molecules (like O2, N2, and CO2) rapidly diminishes Faradaic efficiencies with elevated current densities (particularly at ampere levels). Nevertheless, some catalysts can circumvent this restriction in a two-electron oxygen reduction reaction (2e- ORR), a sustainable pathway for activating O2 to hydrogen peroxide (H2O2). Here we report 2e- ORR expedited in a fluorine-bridged copper metal-organic framework catalyst, arising from the water spillover effect. Through operando spectroscopies, kinetic and theoretical characterizations, it demonstrates that under neutral conditions, water spillover plays a dual role in accelerating water dissociation and stabilizing the key *OOH intermediate. Benefiting from water spillover, the catalyst can expedite 2e- ORR in the current density range of 0.1-2.0 A cm-2 with both high Faradaic efficiencies (99-84.9%) and H2O2 yield rates (63.17-1082.26 mg h-1 cm-2). Further, the feasibility of the present system has been demonstrated by scaling up to a unit module cell of 25 cm2, in combination with techno-economics simulations showing H2O2 production cost strongly dependent on current densities, giving the lowest H2O2 price of $0.50 kg-1 at 2.0 A cm-2. This work is expected to provide an additional dimension to leverage systems independent oftraditional rules.

Recent Progress in Situ Application of H2O2 Produced via Catalytic Synthesis

Industrial production of H2O2 requires lots of energy and causes environmental pollution. Moreover, in subsequent applications, much economic loss could be produced during the transportation process of H2O2 and its dilution process. Therefore, it is highly desirable for in situ application of H2O2. In recent years, catalytic synthesis of H2O2, e. g., direct catalytic synthesis, electrocatalytic synthesis, and photocatalytic synthesis, has attracted more and more attention because the continuous and low-concentration H2O2 produced by catalytic synthesis can be directly used for the oxidation of organic compounds, effectively avoiding the shortcomings of the current industrial route. Here, we briefly reviewed the latest processes for the catalytic production of H2O2 via various routes. On this basis, we summarized and discussed the in situ application of H2O2 in typical organic conversion reactions, including the ammoximation of ketones, the oxidation of alcohols, the oxidation of C-H bonds, and the oxidation of olefins. Some in situ coupling reactions have shown excellent performance with high conversion and selectivity, and the economic cost has been significantly reduced. Finally, the shortcomings of the in situ utilization of H2O2 in coupling reactions were analyzed, and some strategies for promoting the efficiency of the H2O2 application in organic synthesis were proposed.

Facet-dependent synthesis of H2O2 from H2 and O2 over single Pt atom-modified Pd nanocrystal catalysts

Hydrogen peroxide (H2O2) is one of the most valuable clean energy sources with a rapidly growing requirement in industry and daily life. The direct synthesis of H2O2 from hydrogen and oxygen is considered to be an economical and environmentally friendly manufacturing route to replace the traditional anthraquinone method, although it remains a formidable challenge owing to low H2O2 selectivity and production. Here, we report a catalyst consisting of Pd(111) nanocrystals on TiO2 modified with single Pt atoms (Pt1Pd(111)/TiO2), which displays outstanding reactivity, producing 1921.3 μmol of H2O2, a H2 conversion of 62.2% and H2O2 selectivity of 80.3% over 30 min. Kinetic and isotope experiments confirm that the extraordinary catalytic properties are due to stronger H2 activation (the rate-determining step). DFT calculations confirm that Pt1Pd(111) exhibits lower energy barriers for H2 dissociation and two-step O2 hydrogenation, but higher energy barriers for side reactions than Pt1Pd(100), demonstrating clear facet dependence and resulting in greater selectivity and amount of H2O2 produced.

Single-zinc vacancy unlocks high-rate H2O2 electrosynthesis from mixed dioxygen beyond Le Chatelier principle

Le Chatelier's principle is a basic rule in textbook defining the correlations of reaction activities and specific system parameters (like concentrations), serving as the guideline for regulating chemical/catalytic systems. Here we report a model system breaking this constraint in O2 electroreduction in mixed dioxygen. We unravel the central role of creating single-zinc vacancies in a crystal structure that leads to enzyme-like binding of the catalyst with enhanced selectivity to O2, shifting the reaction pathway from Langmuir-Hinshelwood to an upgraded triple-phase Eley-Rideal mechanism. The model system shows minute activity alteration of H2O2 yields (25.89~24.99 mol gcat-1 h-1) and Faradaic efficiencies (92.5%~89.3%) in the O2 levels of 100%~21% at the current density of 50~300 mA cm-2, which apparently violate macroscopic Le Chatelier's reaction kinetics. A standalone prototype device is built for high-rate H2O2 production from atmospheric air, achieving the highest Faradaic efficiencies of 87.8% at 320 mA cm-2, overtaking the state-of-the-art catalysts and approaching the theoretical limit for direct air electrolysis (~345.8 mA cm-2). Further techno-economics analyses display the use of atmospheric air feedstock affording 21.7% better economics as comparison to high-purity O2, achieving the lowest H2O2 capital cost of 0.3 $ Kg-1. Given the recent surge of demonstrations on tailoring chemical/catalytic systems based on the Le Chatelier's principle, the present finding would have general implications, allowing for leveraging systems "beyond" this classical rule.

Electrocatalytic Hydrogenation Using Palladium Membrane Reactors

Hydrogenation is a crucial chemical process employed in a myriad of industries, often facilitated by metals such as Pd, Pt, and Ni as catalysts. Traditional thermocatalytic hydrogenation usually necessitates high temperature and elevated pressure, making the process energy intensive. Electrocatalytic hydrogenation offers an alternative but suffers from issues such as competing H2 evolution, electrolyte separation, and limited solvent selection. This Perspective introduces the evolution and advantages of the electrocatalytic Pd membrane reactor (ePMR) as a solution to these challenges. ePMR utilizes a Pd membrane to physically separate the electrochemical chamber from the hydrogenation chamber, permitting the use of water as the hydrogen source and eliminating the need for H2 gas. This setup allows for greater control over reaction conditions, such as solvent and electrolyte selection, while mitigating issues such as low Faradaic efficiency and complex product separation. Several representative hydrogenation reactions (e.g., hydrogenation of C=C, C≡C, C=O, C≡N, and O=O bonds) achieved via ePMR over the past 30 years were concisely discussed to highlight the unique advantages of ePMR. Promising research directions along with the advancement of ePMR for more challenging hydrogenation reactions are also proposed. Finally, we provide a prospect for future development of this distinctive hydrogenation strategy using hydrogen-permeable membrane electrodes.

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.

Catalyst-Free Carbon Dioxide Conversion in Water Facilitated by Pulse Discharges

By inducing CO2-pulsed discharges within microchannel bubbles and regulating thus-forming plasma microbubbles, we observe high-performance, catalyst-free coformation of hydrogen peroxide (H2O2) and oxalate directly from CO2 and water. With isotope-labeled C18O2 as the feedstock, peaks of H218O16O and H216O2 observed by ex situ surface-enhanced Raman spectra indicate that single-atom oxygen (O) from CO2 dissociations and H2O-derived OH radicals both contribute to H2O2 formation. The global plasma chemistry modeling suggests that high-density, energy-intense electron supply enables high-density CO2- (aq) and HCO2- (aq) formation and their subsequent coupling to produce oxalate. The enhanced solvation of CO2, facilitated by the efficient transport of CxOy ionic species and CO, is demonstrated as a crucial benefit of spark discharges interacting with water at the bubble interface. We expect this plasma microbubble approach to provide a novel power-to-chemical avenue to convert CO2 into valuable H2O2 and oxalic acid platform chemicals, thus leveraging renewable energy resources.

Regulation of Redox Molecular Junctions in Covalent Organic Frameworks for H2 O2 Photosynthesis Coupled with Biomass Valorization

H2 O2 photosynthesis coupled with biomass valorization can not only maximize the energy utilization but also realize the production of value-added products. Here, a series of COFs (i.e. Cu3 -BT-COF, Cu3 -pT-COF and TFP-BT-COF) with regulated redox molecular junctions have been prepared to study H2 O2 photosynthesis coupled with furfuryl alcohol (FFA) photo-oxidation to furoic acid (FA). The FA generation efficiency of Cu3 -BT-COF was found to be 575 mM g-1 (conversion ≈100 % and selectivity >99 %) and the H2 O2 production rate can reach up to 187 000 μM g-1 , which is much higher than Cu3 -pT-COF, TFP-BT-COF and its monomers. As shown by theoretical calculations, the covalent coupling of the Cu cluster and the thiazole group can promote charge transfer, substrate activation and FFA dehydrogenation, thus boosting both the kinetics of H2 O2 production and FFA photo-oxidation to increase the efficiency. This is the first report about COFs for H2 O2 photosynthesis coupled with biomass valorization, which might facilitate the exploration of porous-crystalline catalysts in this field.

Oxidation-Reduction Molecular Junction Covalent Organic Frameworks for Full Reaction Photosynthesis of H2 O2

The full reaction photosynthesis of H₂ O₂ that can combine water-oxidation and oxygen-reduction without sacrificial agents is highly demanded to maximize the light-utilization and overcome the complex reaction-process of anthraquinone-oxidation. Here, a kind of oxidation-reduction molecular junction covalent-organic-framework (TTF-BT-COF) has been synthesized through the covalent-coupling of tetrathiafulvalene (photo-oxidation site) and benzothiazole (photo-reduction site), which presents visible-light-adsorption region, effective electron-hole separation-efficiency and photo-redox sites that enables full reaction generation of H₂ O₂ . Specifically, a record-high yield (TTF-BT-COF, ≈276 000 μM h^-1  g^-1 ) for H₂ O₂ photosynthesis without sacrificial agents has been achieved among porous crystalline photocatalysts. This is the first work that can design oxidation-reduction molecular junction COFs for full reaction photosynthesis of H₂ O₂ , which might extend the scope of COFs in H₂ O₂ production.

The Boundary between Two Modes of Gas Evolution: Oscillatory (H2 and O2) and Conventional Redox (O2 Only), in the Hydrocarbon/H2O2/Cu(II)/CH3CN System

During the oxidation of hydrocarbons using hydrogen peroxide solutions, the evolution of gaseous oxygen is a side and undesirable process, in which the consumption of the oxidizer is not associated with the formation of target products. Therefore, no attention is paid to the systematic study of the chemical composition of the gas and the mechanisms of its formation. Filling this gap, the authors discovered a number of new, previously unidentified, interesting facts concerning both gas evolution and the oxidation of hydrocarbons. In a 33% H2O2/Cu2Cl4·2DMG/CH3CN system, where DMG is dimethylglyoxime (Butane-2,3-dione dioxime), and is at 50 °C, evidence of significant evolution of gaseous hydrogen, along with the evolution of gaseous oxygen was found. In the authors’ opinion, which requires additional verification, the ratio of gaseous hydrogen and oxygen in the discussed catalytic system can reach up to 1:1. The conditions in which only gaseous oxygen is formed are selected. Using a number of oxidizable hydrocarbons with the first adiabatic ionization potentials (AIPs) of a wide range of values, it was found that the first stage of such a process of evolving only gaseous oxygen was the single electron transfer from hydrogen peroxide molecules to trinuclear copper clusters with the formation, respectively, of hydrogen peroxide radical cations H2O2•+ and radical anions Cu3Cl5•− (AIP = 5 eV). When the conditions for the implementation of such a single electron transfer mechanism are exhausted, the channel of decomposition of hydrogen peroxide molecules into gaseous hydrogen and oxygen is switched on, which is accompanied by the transition of the system to an oscillatory mode of gas evolution. In some cases, the formation of additional amounts of gaseous products is provided by the catalytically activated decomposition of water molecules into hydrogen and oxygen after the complete consumption of hydrogen peroxide molecules in the reaction of gaseous oxygen evolution. The adiabatic electron affinity of various forms of copper molecules involved in chemical processes is calculated by the density functional theory method.