Advances in the direct synthesis of hydrogen peroxide from hydrogen and oxygen

High-Efficient Generation of H2O2 by Aluminum-Graphite Composite through Selective Oxygen Reduction for Degradation of Organic Contaminants

Hydrogen peroxide (H₂O₂) is an effective green oxidant, which has been widely applied for environmental remediation. Here, we prepared a novel aluminum-graphite (Al-Gr) composite, which was capable of high-efficient production of H₂O₂ through selective O₂ reduction via a two-electron pathway. We discovered the production of H₂O₂ at a wide pH range, which could be enhanced by optimizing Al-Gr synthesis conditions. Poly(ethylene glycol) (PEG) addition could promote the formation of a welding interface and porous structure between Al and Gr in the Al-Gr composite, which enhanced the galvanic corrosion of Al⁰, the selectivity of oxygen reduction via the two-electron pathway, and the mass transfer of O₂ in the Al-Gr/O₂ system. The formation of Al₄C₃ could be regulated by sintering temperature and sintering time, which could promote the intergranular corrosion of Al⁰ and enhance the mass transfer of O₂ by reaction with water to generate the porous structure in the Al-Gr composite. The concentration of H₂O₂ reached 777.5 mg/L at an initial pH of 9.0, an Al-Gr dosage of 8 g/L, and an O₂ gas flow rate of 400 mL/min. The possible mechanisms of Al-Gr synthesis and H₂O₂ production in the Al-Gr/O₂ system were proposed. The Al-Gr composite was effective for the in situ production of H₂O₂, which could be further decomposed into a hydroxyl radical (^•OH) by Al⁰ in the Al-Gr composite. This composite could be used not only to decolorize the Rhodamine B dye but also to degrade various organic contaminants in different water matrices, indicating its environmental significance.

Carbon-based materials for photo- and electrocatalytic synthesis of hydrogen peroxide

The high demand for hydrogen peroxide (H₂O₂) has been dominantly supplied by the anthraquinone process for various applications globally, including chemical synthesis and wastewater treatment. However, the centralized manufacturing and intensive energy input and waste output are significant challenges associated with this process. Accordingly, the on-site production of H₂O₂via electro- and photocatalytic water oxidation and oxygen reduction partially is greener and easier to handle and has recently emerged with extensive research aiming to seek active, selective and stable catalysts. Herein, we review the current status and future perspectives in this field focused on carbon-based catalysts and their hybrids, since they are relatively inexpensive, bio-friendly and flexible for structural modulation. We present state-of-the-art progress, typical strategies for catalyst engineering towards selective and active H₂O₂ production, discussion on electro- and photochemical mechanisms and H₂O₂ formation through both reductive and oxidative reaction pathways, and conclude with the key challenges to be overcome. We expect promising developments would be inspired in the near future towards practical decentralized H₂O₂ production and its direct use.

Palladium-Based Bimetallic Nanocrystal Catalysts for the Direct Synthesis of Hydrogen Peroxide

The direct synthesis of H2 O2 from H2 and O2 is a strongly desired reaction for green processes and a promising alternative to the commercialized anthraquinone process. The design of efficient catalysts with high activity and H2 O2 selectivity is highly desirable and yet challenging. Metal dopants enhance the performance of the active phase by increasing reaction rates, stability, and/or selectivity. The identification of efficient dopants relies mostly on catalysts prepared with a random and non-uniform deposition of active and promoter phases. To study the promotional effects of metal doping on Pd catalysts, we employ colloidal, bimetallic nanocrystals (NCs) to produce catalysts in which the active and doping metals are colocalized to a fine extent. In the absence of any acid and halide promotors, PdSn and PdGa NCs supported on acid-pretreated TiO2 (PdSn/s-TiO2 , PdGa/s-TiO2 ) were highly efficient and outperformed the monometallic Pd catalyst (Pd/s-TiO2 ), whereas in the presence of an acid promotor, the overall H2 O2 productivity was also further enhanced for the Ni-, Ga-, In-, and Sn-doped catalysts with respect to Pd/s-TiO2 .

Selective electrochemical production of hydrogen peroxide at zigzag edges of exfoliated molybdenum telluride nanoflakes

The two-electron reduction of molecular oxygen represents an effective strategy to enable the green, mild and on-demand synthesis of hydrogen peroxide. Its practical viability, however, hinges on the development of advanced electrocatalysts, preferably composed of non-precious elements, to selectively expedite this reaction, particularly in acidic medium. Our study here introduces 2H-MoTe₂ for the first time as the efficient non-precious-metal-based electrocatalyst for the electrochemical production of hydrogen peroxide in acids. We show that exfoliated 2H-MoTe₂ nanoflakes have high activity (onset overpotential ∼140 mV and large mass activity of 27 A g⁻¹ at 0.4 V versus reversible hydrogen electrode), great selectivity (H₂O₂ percentage up to 93%) and decent stability in 0.5 M H₂SO₄. Theoretical simulations evidence that the high activity and selectivity of 2H-MoTe₂ arise from the proper binding energies of HOO^* and O^* at its zigzag edges that jointly favor the two-electron reduction instead of the four-electron reduction of molecular oxygen.

Microdroplet photofuel cells to harvest high-density energy and dye degradation

In this study, a membraneless photofuel cell, namely, μ-DropFC, was designed and developed to harvest chemical and solar energies simultaneously. The prototypes can also perform environmental remediation to demonstrate their multitasking potential as a sustainable hybrid device in a single embodiment. A hydrogen peroxide (H2O2) microdroplet at optimal pH and salt loading was utilized as a fuel integrated with Al as an anode and zinc phthalocyanine (ZnPC)-coated Cu as a cathode. The presence of n-type semiconductor ZnPC in between the electrolyte and metal enabled the formation of a photo-active Schottky junction suitable for power generation under light. Concurrently, the oxidation and reduction of H2O2 on the electrodes helped in the conversion of chemical energy into the electrical one in the same membraneless setup. The suspension of Au nanoparticles (Au NPs) in the droplet helped in enhancing the overall power density under photonic illumination through the effects of localized surface plasmon resonance (LSPR). Furthermore, the presence of photo-active n-type CdS NPs enabled the catalytic photo-degradation of dyes under light in the same embodiment. A 40 μL μ-DropFC could show a significantly high open circuit potential of ∼0.58 V along with a power density of 0.72 mW cm-2. Under the same condition, the integration of ten such μ-DropFCs could produce a power density of ∼7 mW cm-2 at an efficiency of 3.4%, showing the potential of the prototype for a very large scale integration (VLSI). The μ-DropFC could also degrade ∼85% of an industrial pollutant, rhodamine 6G, in 1 h while generating a power density of ∼0.6 mW cm-2. The performance parameters of μ-DropFCs were found to be either comparable or superior to the existing prototypes. In a way, the affordable, portable, membraneless, and high-performance μ-DropFC could harvest energy from multiple resources while engaging in environmental remediation.

Tailoring the Electrochemical Production of H2 O2 : Strategies for the Rational Design of High-Performance Electrocatalysts

The production of H₂ O₂ via the electrochemical oxygen reduction reaction (ORR) presents an attractive decentralized alternative to the current industry-dominant anthraquinone process. However, in order to achieve viable commercialization of this process, a state-of-the-art electrocatalyst exhibiting high activity, selectivity, and long-term stability is imperative for industrial applications. Herein, an in-depth discussion on the current frontiers in electrocatalyst design is provided, emphasizing the influences of electronic and geometric effects, surface structure, and the effects of heteroatom functionalization on the catalytic performance of commonly studied materials (metals, alloys, carbons). The limitations on the performance of the current catalyst materials are also discussed, together with alternative strategies to overcome the impediments. Finally, directions of future research efforts for the discovery of next-generation ORR electrocatalysts are highlighted.

Recent Progress on Carbonaceous Material Engineering for Electrochemical Hydrogen Peroxide Generation

Electrochemical synthesis of hydrogen peroxide (H2O2) provides a clean and safe technology for large-scale H2O2 production. The core of this project is the development of highly active and highly selective catalysts. Recent studies demonstrate that carbonaceous materials are favorable catalysts because of their low-cost and tunable surface structures. This brief review first summarizes the strategies of carbonaceous material engineering for selective two-electron O2 reduction reaction and discusses potential mechanisms. In addition, several device designs using carbonaceous materials as catalysts for H2O2 production are introduced. Finally, research directions are proposed for practical application and performance improvement.

Hollow multishell structures exercise temporal–spatial ordering and dynamic smart behaviour

A hollow multishell structure (HoMS) is an assembly of multiple shells with voids between the individual shells. Accessible through nanopores, these voids represent separate reaction environments in the same assembly, such that HoMSs have unique properties that are applicable to diverse fields. These applications have mostly exploited the large specific surface area, high loading capacity and/or buffering effect of HoMSs, benefiting the mass/energy transmission and effective surface area. In comparison, the temporal-spatial ordering of reactions, as well as the dynamic smart behaviour of HoMSs, have been less explored but are also emphasized in this Perspective. We first describe the synthesis of HoMSs and the thermodynamic and kinetic aspects of their formation. We then consider the composition and structural functionalization of each shell within a HoMS and then highlight how these enable applications based on temporal-spatial ordering and dynamic smart behaviour.

Enhancing the selectivity of Pd/C catalysts for the direct synthesis of H2O2 by HNO3 pretreatment

The acid functional groups of activated carbon improve the H₂O₂ selectivity.

Dynamic structural changes of supported Pd, PdSn, and PdIn nanoparticles during continuous flow high pressure direct H2O2 synthesis

The structure of mono- and bimetallic supported Pd, PdSn, and PdIn NPs was monitored with a combination of techniques during continuous H₂O₂ synthesis with H₂O₂ production rates up to 580 mmol_H2O2 g_cat⁻¹ h⁻¹.

Hydrogen peroxide generation from O2 electroreduction for environmental remediation: A state-of-the-art review

The electrochemical production of hydrogen peroxide (H2O2) by 2-electron oxygen reduction reaction (ORR) is an attractive alternative to the present complex anthraquinone process. The objective of this paper is to provide a state-of-the-arts review of the most important aspects of this process. First, recent advances in H2O2 production are reviewed and the advantages of H2O2 electrogeneration via 2-electron ORR are highlighted. Second, the selectivity of the ORR pathway towards H2O2 formation as well as the development process of H2O2 production are presented. The cathode characteristics are the decisive factors of H2O2 production. Thus the focus is shifted to the introduction of commonly used carbon cathodes and their modification methods, including the introduction of other active carbon materials, hetero-atoms doping (i.e., O, N, F, B, and P) and decoration with metal oxides. Cathode stability is evaluated due to its significance for long-term application. Effects of various operational parameters, such as electrode potential/current density, supporting electrolyte, electrolyte pH, temperature, dissolved oxygen, and current mode on H2O2 production are then discussed. Additionally, the environmental application of electrogenerated H2O2 on aqueous and gaseous contaminants removal, including dyes, pesticides, herbicides, phenolic compounds, drugs, VOCs, SO2, NO, and Hg0, are described. Finally, a brief conclusion about the recent progress achieved in H2O2 electrogeneration via 2-electron ORR and an outlook on future research challenges are proposed.

Looking for the “Dream Catalyst” for Hydrogen Peroxide Production from Hydrogen and Oxygen

The reaction between hydrogen and oxygen is in principle the simplest method to form hydrogen peroxide, but it is still a “dream process”, thus needing a “dream catalyst”. The aim of this review is to analyze critically the different heterogeneous catalysts used for the direct synthesis of H2O2 trying to determine the features that the ideal or “dream catalyst” should possess. This analysis will refer specifically to the following points: (i) the choice of the metal; (ii) the metal promoters used to improve the activity and/or the selectivity; (iii) the role of different supports and their acidic properties; (iv) the addition of halide promoters to inhibit undesired side reactions; (v) the addition of other promoters; (vi) the effects of particle morphology; and (vii) the effects of different synthetic methods on catalyst morphology and performance.

In situ generation of H2O2 using MWCNT-Al/O2 system and possible application for glyphosate degradation

Hydrogen peroxide (H₂O₂), as a green oxidant, has been widely applied into advanced oxidation processes (AOPs) for the degradation of toxic organic pollutants. The in situ generation of H₂O₂ can not only improve the storage and transportation safety of H₂O₂ but also reduce the capital and operation costs. In the present work, a novel system, i.e., multi-walled carbon nanotube‑aluminum (MWCNT-Al) composite was used to in situ generate H₂O₂ through micro-electrolysis. The MWCNT-Al composite was characterized and optimized. The accumulation concentration of H₂O₂ reached 947 mg/L at the initial pH of 9.0, the MWCNT-Al composite dosage of 8 g/L and oxygen gas flow rate of 400 mL/min after 60 min. The in situ generation of H₂O₂ was achieved by MWCNT-Al/O₂ system, mainly owing to the direct contact between Al⁰ and MWCNT in MWCNT-Al composite, which accelerated the transfer of electrons from Al⁰ to O₂, as well as the excellent electrocatalytic activity of MWCNT toward the two-electron reduction of oxygen. When H₂O₂ in situ generation technology was used in peroxone process (O₃/H₂O₂ process) to degrade glyphosate in aqueous solution, the removal efficiency of TOC and total phosphorus was 68.35% and 73.27%, respectively. Finally, the possible mechanism of in situ generation of H₂O₂ in MWCNT-Al/O₂ system was temporarily proposed.

Direct Synthesis of Hydrogen Peroxide under Semi-Batch Conditions over Un-Promoted Palladium Catalysts Supported by Ion-Exchange Sulfonated Resins: Effects of the Support Morphology

Palladium catalysts supported by a mesoporous form of sulfonated poly-divinylbenzene, Pd/µS-pDVB10 (1%, w/w) and Pd/µS-pDVB35 (3.6% w/w), were applied to the direct synthesis of hydrogen peroxide from dihydrogen and dioxygen. The reaction was carried for 4 h out in a semibatch reactor with continuous feed of the gas mixture (H2/O2 = 1/24, v/v; total flow rate 25 mL·min−1), at 25 °C and 101 kPa. The catalytic performances were compared with those of a commercial egg-shell Pd/C catalyst (1%, w/w) and of a palladium catalyst supported by a macroreticular sulfonated ion-exchange resin, Pd/mS-pSDVB10 (1%, w/w). Pd/µS-pDVB10 and Pd/C showed the highest specific activity (H2 consumption rate of about 75–80 h−1), but the resin supported catalyst was much more selective (ca 50% with no promoters). The nanoparticles (NP) size was somewhat larger in Pd/µS-pDVB10, showing that either the reaction was structure insensitive or diffusion limited to some extent over Pd/C, in which the support is microporous. The open pore structure of Pd/µS-pDVB10, possibly ensuring the fast removal of H2O2 from the catalyst, could also be the cause of the relatively high selectivity of this catalyst. In summary, Pd/µS-pDVB10 was the most productive catalyst, forming ca 375 molH2O2·kgPd−1·h−1, also because it retained a constant selectivity, while the other ones underwent a more or less pronounced loss of selectivity after 80–90 min. Ageing experiments showed that for a palladium catalyst supported on sulfonated mesoporous poly-divinylbenzene storage under oxidative conditions implied some deactivation, but a lower drop in the selectivity; regeneration upon a reductive treatment or storage under strictly anaerobic conditions (dry-box) lead to an increase of the activity but to both a lower initial selectivity and a higher drop of selectivity with time.

Adsorptive Removal of Acetaldehyde from Propylene Oxide Produced by the Hydrogen Peroxide to Propylene Oxide Process

Adsorption method was first introduced into the propene oxide production via hydrogen peroxide process to remove the microimpurity in the propylene oxide (PO) product solution. It could replace the reactive distillation in separating acetaldehyde with less energy consumption and PO loss. A series of adsorbents (e.g., 3A, 4A, 5A, 10X, and Y) are first used to remove the impurity (i.e., acetaldehyde). It is found that 5A molecular sieves shows the best performance due to uniform porous channels with suitable pore size. Various techniques such as X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, Fourier transform infrared, and N₂ physisorption are employed to investigate the structural properties of the adsorbent. Furthermore, effects of space velocity and temperature are also investigated. Cyclic desorption and adsorption tests indicate the PO yield is 92.2%, and 96.3% of acetaldehyde was removed. The acetaldehyde concentration of PO product was 0.0187%, indicating this method can produce industrial-quality PO that meets the first-level technical requirements.

Gold-Palladium Nanoalloys Supported by Graphene Oxide and Lamellar TiO2 for Direct Synthesis of Hydrogen Peroxide

Hybrid catalysts composed of gold-palladium nanoalloys that are sandwiched between layers of graphene oxide (GO) and lamellar TiO₂ are synthesized via the deposition-reduction method. The resulting AuPd catalysts with different compositions of metal and support are fully characterized by a series of techniques, including X-ray diffraction, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma mass spectrometry. The catalysts are also optimized against Au, Pd, GO, and TiO₂ contents and employed in the direct synthesis of hydrogen peroxide (DSHP) from H₂ and O₂. The sandwich-like AuPd nanoalloy comprising 1 wt % nanoparticle of an equimolar mixture of Au and Pd with 6 wt % GO and 93 wt % TiO₂ supports shows a promising catalytic performance toward the DSHP reaction with H₂O₂ productivity and selectivity of 5.50 mol H₂O₂ g_metal^-1 h^-1 and 64%, respectively. The catalyst is found to be considerably more active than those reported in the literature. Furthermore, the H₂O₂ selectivity of the catalyst is found to improve considerably to 88% when the TiO₂ support is pretreated by HNO₃. It is found that the perimeter sites of the interface of AuPd alloy and TiO₂ are deemed as catalytically active sites for the DSHP reactions and the acidic property of TiO₂ can retard the other overreactions and the decomposition of yielded H₂O₂. Results of the present study may provide a design strategy for partially covered catalysts that are confined by 2D materials for selective reactions.

Experimental Evaluation of a Membrane Micro Channel Reactor for Liquid Phase Direct Synthesis of Hydrogen Peroxide in Continuous Flow Using Nafion® Membranes for Safe Utilization of Undiluted Reactants

In recent years, various modular micro channel reactors have been developed to overcome limitations in challenging chemical reactions. Direct synthesis of hydrogen peroxide from hydrogen and oxygen is a very interesting process in this regard. However, the complex triphasic process (gaseous reactants, reaction in liquid solvent, solid catalyst) still holds challenges regarding safety, selectivity and productivity. The membrane micro reactor system for continuous liquid phase H2O2 direct synthesis was designed to reduce safety issues by separate dosing of the gaseous reactants via a membrane into a liquid-flow channel filled with a catalyst. Productivity is increased by enhanced mass transport, attainable in micro channels and by multiple re-saturation of the liquid with the reactants over the length of the reaction channel. Lastly, selectivity is optimized by controlling the reactant distribution. The influence of crucial technical features of the design, such as micro channel geometry, were studied experimentally in relationship with varying reaction conditions such as residence time, pressure, reactant ratio and solvent flow rate. Successful continuous operation of the reactor at pressures up to 50 bars showed the feasibility of this system. During the experiments, control over the reactant ratio was found to be crucial in order to maximize product yield. Thereby, yields above 80% were achieved. The results obtained are the key elements for future development and optimization of this reactor system, which will hopefully lead to a breakthrough in decentralized H2O2 production.

Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis—A Review

Hydrogen peroxide is an important chemical of increasing demand in today’s world. Currently, the anthraquinone autoxidation process dominates the industrial production of hydrogen peroxide. Herein, hydrogen and oxygen are reacted indirectly in the presence of quinones to yield hydrogen peroxide. Owing to the complexity and multi-step nature of the process, it is advantageous to replace the process with an easier and straightforward one. The direct synthesis of hydrogen peroxide from its constituent reagents is an effective and clean route to achieve this goal. Factors such as water formation due to thermodynamics, explosion risk, and the stability of the hydrogen peroxide produced hinder the applicability of this process at an industrial level. Currently, the catalysis for the direct synthesis reaction is palladium based and the research into finding an effective and active catalyst has been ongoing for more than a century now. Palladium in its pure form, or alloyed with certain metals, are some of the new generation of catalysts that are extensively researched. Additionally, to prevent the decomposition of hydrogen peroxide to water, the process is stabilized by adding certain promoters such as mineral acids and halides. A major part of today’s research in this field focusses on the reactor and the mode of operation required for synthesizing hydrogen peroxide. The emergence of microreactor technology has helped in setting up this synthesis in a continuous mode, which could possibly replace the anthraquinone process in the near future. This review will focus on the recent findings of the scientific community in terms of reaction engineering, catalyst and reactor design in the direct synthesis of hydrogen peroxide.

Highly effective electrosynthesis of hydrogen peroxide from oxygen on a redox-active cationic covalent triazine network

Direct electrosynthesis of hydrogen peroxide (H2O2) by oxygen reduction is a green and safe strategy to replace the traditional anthraquinone process. Herein, we have designed a two-dimensional redox-active cationic covalent triazine network to be used directly as a cost-effective metal-free electrocatalyst for the oxygen reduction reaction (ORR) to form H2O2. Such a dicationic 2D polymer possesses a porous structure with pore diameters of 2-10 nm and a total N content of 13.3 wt%. The electron paramagnetic resonance experiment confirms the reduction of a viologen-based polymer to radical cations and the subsequent generation of superoxygen radicals. The radical characteristics and high N content within this polymer are the essential for the efficient ORR via a two-electron pathway. As a result, the present electrocatalyst exhibits a high ORR activity and excellent H2O2 selectivity (∼85%), thus providing a feasible possibility of designing highly selective metal-free electrocatalysts for electrocatalytic production of H2O2 from O2.

Porous Nanocrystalline Silicon Supported Bimetallic Pd-Au Catalysts: Preparation, Characterization, and Direct Hydrogen Peroxide Synthesis

Bimetallic Pd-Au catalysts were prepared on the porous nanocrystalline silicon (PSi) for the first time. The catalysts were tested in the reaction of direct hydrogen peroxide synthesis and characterized by standard structural and chemical techniques. It was shown that the Pd-Au/PSi catalyst prepared from conventional H₂[PdCl₄] and H[AuCl₄] precursors contains monometallic Pd and a range of different Pd-Au alloy nanoparticles over the oxidized PSi surface. The PdAu₂/PSi catalyst prepared from the [Pd(NH₃)₄][AuCl₄]₂ double complex salt (DCS) single-source precursor predominantly contains bimetallic Pd-Au alloy nanoparticles. For both catalysts the surface of bimetallic nanoparticles is Pd-enriched and contains palladium in Pd⁰ and Pd²⁺ states. Among the catalysts studied, the PdAu₂/PSi catalyst was the most active and selective in the direct H₂O₂ synthesis with H₂O₂ productivity of 0.5 mol gPd-1 h-1 at selectivity of 50% and H₂O₂ concentration of 0.023 M in 0.03 M H₂SO₄-methanol solution after 5 h on stream at −10°C and atmospheric pressure. This performance is due to high activity in the H₂O₂ synthesis reaction and low activities in the undesirable H₂O₂ decomposition and hydrogenation reactions. Good performance of the PdAu₂/PSi catalyst was associated with the major part of Pd in the catalyst being in the form of the bimetallic Pd-Au nanoparticles. Porous silicon was concluded to be a promising catalytic support for direct hydrogen peroxide synthesis due to its inertness with respect to undesirable side reactions, high thermal stability, and conductivity, possibility of safe operation at high temperatures and pressures and a well-established manufacturing process.

Enhancement of catalytic activity of Pd-PVP colloid for direct H2O2 synthesis from H2 and O2 in water with addition of 0.5 atom% Pt or Ir

Atomically dispersed Pt or Ir atoms enhance H₂O₂ and H₂O formation on Pd nano-particles leaving the H₂O₂ hydrogenation rate unchanged, while Ru, Rh, or Au atoms show little effect.

One- or Two-Electron Water Oxidation, Hydroxyl Radical, or H2O2 Evolution

Electrochemical or photoelectrochemcial oxidation of water to form hydrogen peroxide (H₂O₂) or hydroxyl radicals (^•OH) offers a very attractive route to water disinfection, and the first process could be the basis for a clean way to produce hydrogen peroxide. A major obstacle in the development of effective catalysts for these reactions is that the electrocatalyst must suppress the thermodynamically favored four-electron pathway leading to O₂ evolution. We develop a thermochemical picture of the catalyst properties that determine selectivity toward the one, two, and four electron processes leading to ^•OH, H₂O₂, and O₂.