Selective charge transfer to dioxygen on KPF6-modified carbon nitride for photocatalytic synthesis of H2O2 under visible light

A Review on Photocatalytic Hydrogen Peroxide Production from Oxygen: Material Design, Mechanisms, and Applications

Hydrogen peroxide (H2O2) finds extensive applications in various industries, particularly in the environmental field. The photocatalytic production of H2O2 through the oxygen reduction reaction (ORR) or the water oxidation reaction (WOR) offers a promising approach. However, several challenges hinder effective on-site production, such as the rapid electron-hole pair recombination, inefficient visible light utilization, and limited selectivity in H2O2 formation. Thus, developing efficient photocatalysts to overcome these challenges is crucial. This review comprehensively outlines the development of photocatalysts and their modification techniques. It also summarizes and compares the H2O2 yield and apparent quantum yield among various photocatalysts with and without the use of organic sacrificial reagents. Density functional theory (DFT) calculations propose the band structure of photocatalysts and the mechanisms underlying oxygen reduction to H2O2. Finally, this review explores the potential environmental applications of photocatalytically produced H2O2. This review guides the design and optimization of photocatalysts, facilitating the continued advancement and application of photocatalysts in environmental contexts.

Photocatalytic Achmatowicz Rearrangement on Triphenylbenzene-Dimethoxyterephthaldehyde-Covalent Organic Framework-Mo for Converting Biomass-Derived Furfuryl Alcohol to Hydropyranone

The conversion of biomass-based feedstocks into high-value platform compounds remains a fundamental but challenging topic. Studies on such transformations are sparse due to the complex nature of biomass and limited upgrading strategies. Photocatalytic aerobic oxidation is a promising pathway for making pharmaceutical precursors from biomass-derived chemicals. In this study, we demonstrate the transformation of furfuryl alcohol into dihydropyranone acetyl by using a molybdenum-incorporated triphenylbenzene-dimethoxyterephthaldehyde-covalent organic framework via a singlet oxygen (1O2)-mediated photocatalytic Achmatowicz reaction. The COF-Mo photocatalyst exhibits exceptional selectivity up to 98% in producing hydroxy-2H-pyran-3(6H)-one, a complex synthone crucial for synthesizing anti-AIDS drugs. Mo incorporation enhances the generation rate by 3.9 times, primarily due to the synergistic effect between Mo active sites and the COF backbone. More importantly, Mo clusters create a superfast charge tunnel for photogenerated electron transfer from COF to adsorbed O2. The metallic state and hexavalent of Mo facilitate the generation of superoxide anions and 1O2, respectively, facilitating photocatalytic reactions.

Photosynthesis of Hydrogen Peroxide Based on g-C3 N4 : The Road of a Cost-Effective Clean Fuel Production

Emerging artificial photosynthesis promises to offer a competitive means for solar energy conversion and further solves the energy crisis facing the world. Hydrogen peroxide (H2 O2 ), which is considered as a benign oxidant and a prospective liquid fuel, has received worldwide attention in the field of artificial photosynthesis on account of the source materials are just oxygen, water, and sunlight. Graphitic carbon nitride (g-C3 N4 )-based photocatalysts for H2 O2 generation have attracted extensive research interest due to the intrinsic properties of g-C3 N4 . In this review, research processes for H2 O2 generation on the basis of g-C3 N4 , including development, fabrication, merits, and disadvantages, and the state-of-the-art methods to enhance the performance are summarized after a brief introduction and the mechanism analysis of an efficient catalytic system. Also, recent applications of g-C3 N4 -based photocatalysts for H2 O2 production are reviewed, and the significance of active sites and synthetic pathways are highlighted from the view of reducing barriers. Finally, this paper ends with some concluding remarks to reveal the issues and opportunities of g-C3 N4 -based photocatalysts for producing H2 O2 in a high yield.

Photocatalytic and Electrocatalytic Generation of Hydrogen Peroxide: Principles, Catalyst Design and Performance

Hydrogen peroxide (H2O2) is a high-demand organic chemical reagent and has been widely used in various modern industrial applications. Currently, the prominent method for the preparation of H2O2 is the anthraquinone oxidation. Unfortunately, it is not conducive to economic and sustainable development since it is a complex process and involves unfriendly environment and potential hazards. In this context, numerous approaches have been developed to synthesize H2O2. Among them, photo/electro-catalytic ones are considered as two of the most promising manners for on-site synthesis of H2O2. These alternatives are sustainable in that only water or O2 is required. Namely, water oxidation (WOR) or oxygen reduction (ORR) reactions can be further coupled with clean and sustainable energy. For photo/electro-catalytic reactions for H2O2 generation, the design of the catalysts is extremely important and has been extensively conducted with an aim to obtain ultimate catalytic performance. This article overviews the basic principles of WOR and ORR, followed by the summary of recent progresses and achievements on the design and performance of various photo/electro-catalysts for H2O2 generation. The related mechanisms for these approaches are highlighted from theoretical and experimental aspects. Scientific challenges and opportunities of engineering photo/electro-catalysts for H2O2 generation are also outlined and discussed.

Recent progress in the design of photocatalytic H2O2 synthesis system

Photocatalytic synthesis of hydrogen peroxide under mild reaction conditions is a promising technology. This article will review the recent research progress in the design of photocatalytic H2O2 synthesis systems. A comprehensive discussion of the strategies that could solve two essential issues related to H2O2 synthesis. That is, how to improve the reaction kinetics of H2O2 formation via 2e- oxygen reduction reaction and inhibit the H2O2 decomposition through a variety of surface functionalization methods. The photocatalyst design and the reaction mechanism will be especially stressed in this work which will be concluded with an outlook to show the possible ways for synthesizing high-concentration H2O2 solution in the future.

Protonated g-C3N4 coated Co9S8 heterojunction for photocatalytic H2O2 production

Photocatalytic H₂O₂ production is an eco-friendly technique because only H₂O, molecular O₂ and light are involved. However, it still confronts the challenges of the unsatisfactory productivity of H₂O₂ and the dependence on organic electron donors or high purity O₂, which restrict the practical application. Herein, we construct a type-II heterojunction of the protonated g-C₃N₄ coated Co₉S₈ semiconductor for photocatalytic H₂O₂ production. The ultrathin g-C₃N₄ uniformly spreads on the surface of the dispersed Co₉S₈ nanosheets by a two-step method of protonation and dip-coating, and exhibits improved photogenerated electrons transportability and e^--h⁺ pairs separation ability. The photocatalytic system can achieve a considerable productivity of H₂O₂ to 2.17 mM for 5 h in alkaline medium in the absence of the organic electron donors and pure O₂. The optimal photocatalyst also obtains the highest apparent quantum yield (AQY) of 18.10% under 450 nm of light irradiation, as well as a good reusability. The contribution of the type-II heterojunction is that the migrations of electrons and holes within the interface between g-C₃N₄ and Co₉S₈ matrix promote the separation of photocarriers, and another channel is also opened for H₂O₂ generation. The accumulated electrons in conduction band (CB) of Co₉S₈ contribute to the major channel of two-electron reduction of O₂ for H₂O₂ production. Meanwhile, the electrons in CB of g-C₃N₄ participate in the single electron reduction of O₂ as an auxiliary channel to enhance the H₂O₂ production.

Highly crystalline sulfur and oxygen co-doped g-C3N4 nanosheets as an advanced photocatalyst for efficient hydrogen generation

Heteroatom doping has become a promising strategy to tailor the band structure and physicochemical properties of graphitic carbon nitride (g-C3N4). However, doping heteroatoms usually lead to decreased crystallinity, thereby an...

Boron containing metal-organic framework for highly selective photocatalytic production of H2O2 by promoting two-electron O2 reduction

A zirconium-based metal-organic framework containing boron (UiO-66-B) is prepared, which displays efficient photocatalytic H₂O₂ production. The H₂O₂ evolution rate is about 1002 μmol g^-1 h^-1, much higher than that of most known photocatalysts. Pristine UiO-66 displays a much lower activity (314 μmol g^-1 h^-1) under the same conditions, suggesting the significant role of boron. Both theoretical calculations and the combined experimental results verify the above conclusion, and the role of boron is ascribed to the following aspects: (1) enhanced O₂ adsorption, (2) highly selective proton-coupled two-electron transfer, (3) faster carrier separation and surface charge transfer, and (4) faster generation but slower decomposition rates of H₂O₂. This work highlights key factors in the two-electron O₂ reduction reaction (ORR), presents a deeper understanding of the role of boron in enhancing H₂O₂ production, and provides a new strategy for designing photocatalysts with excellent H₂O₂ evolution efficiency.

Construction of Porous Tubular In2S3@In2O3 with Plasma Treatment-Derived Oxygen Vacancies for Efficient Photocatalytic H2O2 Production in Pure Water Via Two-Electron Reduction

Tubular In₂O₃ was fabricated by the annealing of In-MIL-68 and further treated by Ar plasma to yield oxygen vacancies (O_v) followed by the growth of In₂S₃ nanoflowers. Unexpectedly, the resulting porous In₂S₃@In₂O₃ composites were discovered to display a broad visible-light response and especially enhanced capacities for efficient photocatalytic production of H₂O₂ in pure water, with a rate of 4.59 μmol·g^-1·min^-1. An apparent quantum yield of 28.9% at 420 nm can also be expected without the use of noble metals or organic scavengers. Herein, the high light utilization might be profited from their porous tubular heterostructure for powerful "light captivity". Moreover, the Ar plasma-derived O_v sites on the composites might tune the H₂O₂ generation route from the single-electron reduction to the two-electron one toward the significantly enhanced photocatalysis, as validated by the Koutecky-Levich plots. This work demonstrates a new perspective of designing porous heterostructures with the advantages of high light harvest and plasma-derived O_v active sites. Importantly, it may provide a promising defect-induced strategy of two-electron reduction triggered by the plasma treatment for the efficient photocatalytic H₂O₂ production under visible light.

Nanocellulose-derived carbon/g-C3N4 heterojunction with a hybrid electron transfer pathway for highly photocatalytic hydrogen peroxide production

Using oxygen reduction for the photocatalytic production of hydrogen peroxide (H₂O₂) has been considered a green and sustainable route. In the present study, to achieve high efficiency, graphitic carbon nitride (g-C₃N₄) was obtained using thermal polymerization from a bi-component precursor and was then assembled with cellulose nanofibers. It was found that a small quantity of cellulose nanofibers that generates carbon fibers upon pyrolysis greatly improves the photocatalytic activity compared with that of g-C₃N₄ alone. The well-defined carbon/g-C₃N₄ heterojunction-type material exhibits as high as 1.10 mmol L^-1h^-1 of photo-production of H₂O₂ under visible light, which is 4.2 times higher than that yielded by pristine g-C₃N₄ from a single precursor. A comprehensive characterization of the photocatalyst enables us to delineate the effect of the carbon nanofiber with respect to porosity, electron-hole separation, band gap regulation, and especially the electron transfer pathway. Our results demonstrate that nanocellulose-derived carbon, when precisely assembled with other functional material such as a photocatalyst, is a promising promoter of their activity.

A metal-free photocatalyst for highly efficient hydrogen peroxide photoproduction in real seawater

Artificial photosynthesis of H2O2 from H2O and O2, as a spotless method, has aroused widespread interest. Up to date, most photocatalysts still suffer from serious salt-deactivated effects with huge consumption of photogenerated charges, which severely limit their wide application. Herein, by using a phenolic condensation approach, carbon dots, organic dye molecule procyanidins and 4-methoxybenzaldehyde are composed into a metal-free photocatalyst for the photosynthetic production of H2O2 in seawater. This catalyst exhibits high photocatalytic ability to produce H2O2 with the yield of 1776 μmol g-1h-1 (λ ≥ 420 nm; 34.8 mW cm-2) in real seawater, about 4.8 times higher than the pure polymer. Combining with in-situ photoelectrochemical and transient photovoltage analysis, the active site and the catalytic mechanism of this composite catalyst in seawater are also clearly clarified. This work opens up an avenue for a highly efficient and practical, available catalyst for H2O2 photoproduction in real seawater.

KNO3-Assisted incorporation of K dopants and N defects into g-C3N4 with enhanced visible light driven photocatalytic H2O2 production

Facile KNO3-assisted synthesis of g-C3N4 incorporated with K dopants and N defects for efficient visible light driven photocatalytic H2O2 production.

Recent Progress, Challenges, and Prospects in Two-Dimensional Photo-Catalyst Materials and Environmental Remediation

The successful photo-catalyst library gives significant information on feature that affects photo-catalytic performance and proposes new materials. Competency is considerably significant to form multi-functional photo-catalysts with flexible characteristics. Since recently, two-dimensional materials (2DMs) gained much attention from researchers, due to their unique thickness-dependent uses, mainly for photo-catalytic, outstanding chemical and physical properties. Photo-catalytic water splitting and hydrogen (H2) evolution by plentiful compounds as electron (e-) donors is estimated to participate in constructing clean method for solar H2-formation. Heterogeneous photo-catalysis received much research attention caused by their applications to tackle numerous energy and environmental issues. This broad review explains progress regarding 2DMs, significance in structure, and catalytic results. We will discuss in detail current progresses of approaches for adjusting 2DMs-based photo-catalysts to assess their photo-activity including doping, hetero-structure scheme, and functional formation assembly. Suggested plans, e.g., doping and sensitization of semiconducting 2DMs, increasing electrical conductance, improving catalytic active sites, strengthening interface coupling in semiconductors (SCs) 2DMs, forming nano-structures, building multi-junction nano-composites, increasing photo-stability of SCs, and using combined results of adapted approaches, are summed up. Hence, to further improve 2DMs photo-catalyst properties, hetero-structure design-based 2DMs' photo-catalyst basic mechanism is also reviewed.

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.

Characterization of highly effective plasma-treated g-C3N4 and application to the photocatalytic H2O2 production

Plasma treated g-C₃N₄ (PT-g-C₃N₄) was obtained by a simple and rapid DBD plasma modification process on the pristine g-C₃N₄. Compared with the pristine g-C₃N₄, the grain size of the PT-g-C₃N₄ decreased from 99.2 nm to 57.2 nm, the specific surface area and the pore volume increased by 15% and 33.8%, respectively. Oxygen-containing groups such as -NO₂ and -COOH were observed to form on the surface of PT-g-C₃N₄ so the hydrophilic property of PT-g-C₃N₄ was much higher than that of pristine g-C₃N₄. More importantly, the photocatalytic H₂O₂ production activity of PT-g-C₃N₄ was significantly improved on account of the treatment in plasma atmosphere for only 5 min, the H₂O₂ yield of which was about 13 times that of the pristine g-C₃N₄. Our finding is not only of great significance for effectively promoting the production of H₂O₂ under mild conditions, but also proposes an innovative DBD plasma method to modify the g-C₃N₄ photocatalyst, which effectively promotes the improvement of photocatalytic activity and provides valuable insights for catalyst modification studies.

A comparative perspective of electrochemical and photochemical approaches for catalytic H2O2 production

Recent advances in the design, preparation, and applications of different catalysts for electrochemical and photochemical H₂O₂ production are summarized, and some invigorating perspectives for future developments are also provided.