Key Point 1: advantages and applications of metal-free organic semiconductor photocatalytic H 2 O 2 production
This review comprehensively compares the advantages and limitations of methods for H 2 O 2 production, such as the anthraquinone process, electrochemical, photoelectrochemical, piezoelectrochemical, and photochemical routes. It emphasizes the sustainability and safety of metal-free organic semiconductor photocatalytic H 2 O 2 production, proposing from a unique perspective that developing novel surface reactions constitutes one of the most effective strategies for enhancing photocatalyst performance. H 2 O 2 is a versatile oxidant, finding broad applications in pharmaceuticals, environmental remediation, and chemical manufacturing. Industrial H 2 O 2 production predominantly employs the anthraquinone process, which incurs high energy consumption, operational complexity, and precious-metal dependency, leading to substantial economic and environmental burdens. Amid escalating fossil fuel depletion and environmental pollution, metal-free organic semiconductor photocatalysis has emerged as a promising approach for H 2 O 2 generation, providing a robust material platform for green, efficient, and sustainable synthesis. Nevertheless, practical implementation faces significant hurdles, including low energy conversion efficiency, limited H 2 O 2 accumulation, and high photocatalyst costs. To address the above-mentioned challenges, researchers have explored various strategies, among which developing new surface reactions is one of the most effective. Innovative modulation of the reaction pathways is seldom discussed, but it can fundamentally increase the utility of photogenerated excitons by exploiting unexpected chemical processes.
Key Point 2: mechanisms and characterizations of photocatalytic H 2 O 2 production
This review briefly introduces conventional mechanisms for photocatalytic H 2 O 2 production and provides an in-depth discussion on essential characterization techniques for mechanistic investigations. Photocatalytic H 2 O 2 evolution involves fundamental photophysical processes, including light excitation, charge separation, and migration. Additionally, it encompasses diverse surface reactions, where photogenerated charges interact with adsorbed reactants through various pathways to form new chemical products. The generation of H 2 O 2 typically occurs through two key surface reaction processes: the ORR and the WOR. Fig. 1 illustrates a schematic picture of the overall photocatalytic mechanism for H 2 O 2 formation using O 2 and H 2 O. The ORR pathway for H 2 O 2 synthesis can be divided into two categories: a two-step one-electron ORR route (Eqs. 1–2) and a one-step two-electron ORR route with proton-coupled electron transfer (Eq. 3), where the protons primarily derive from H 2 O or organic donors. Eq. 4 illustrates the competing reaction of H 2 O generation from 4e - ORR, which may decrease the utility of O 2 for H 2 O 2 fabrication. It is noteworthy that singlet oxygen ( 1 O 2 ), the excited state of O 2 , does not inherently undergo a two-step single-electron reduction to generate H 2 O 2 . It can also participate in intermediate steps to promote H 2 O 2 formation (Eq. 5). H 2 O 2 can also be obtained via WOR pathways through the direct (Eq. 6) or indirect (Eqs. 7,9) oxidation of H 2 O. It is worth noting that the O 2 molecules produced by 4h + WOR can also produce H 2 O 2 via the 2e - ORR pathway (Eq. 8).
In practical research, a single characterization technique is often insufficient to fully unravel the complex surface reaction mechanisms involved in photocatalytic H 2 O 2 production. Therefore, researchers have increasingly adopted synergistic strategies combining multiple experimental techniques with theoretical simulations to gain deeper mechanistic insights. These methods offer complementary information across various dimensions (Fig. 2). In-situ IR, Raman, and UV-vis spectroscopy allow real-time monitoring of changes in adsorbed intermediates during the reactions. EPR and radical trapping experiments help identify short-lived ROS. Isotopic labeling, combined with MS or NMR analysis, enables precise determination of the O and H sources in products. In addition, surface-sensitive techniques like X-ray photoelectron spectroscopy and X-ray absorption fine structure spectroscopy provide valuable insights into the valence states and coordination environments of active catalytic sites. Meanwhile, density functional theory calculations play a key role in validating experimentally observed intermediates, estimating reaction energy barriers, and elucidating reaction pathways. The integration of experimental and theoretical approaches allows researchers to progressively build predictive reaction models from empirical observations.
Key Point 3: efficient chemical reaction pathways
Compared with the traditional ORR and WOR mechanisms, intermediate-involved reaction pathways and redox dual-channel reactions demonstrate remarkably high efficiencies in solar energy conversion. These pathways generally demonstrate improved light utilization by effectively separating, storing, and utilizing photogenerated charges through designed functional-group reactions, thereby minimizing recombination at material surfaces or interfaces. The redox-dual-channel mechanism features high-efficiency 2e - ORR and 2h + WOR taking place on separated active sites, restricting charge recombination and side reactions like 4h + WOR. For intermediate-involved mechanisms, the formation of intermediates not only bypasses direct oxidation/reduction reactions that require high overpotentials but also fully leverages external energy sources, such as heat (Fig. 3). This section reviews advanced mechanistic pathways including anthraquinone intermediates, peracid intermediates, bipyridine intermediates, and dual-channel synergistic mechanisms, to serve as a valuable reference.
Key Point 4: other strategies to improve photocatalytic performances
Advancing high-performance H 2 O 2 production in photochemical systems requires enhancing photocatalytic efficiency through multiple approaches. Beyond modulating reaction pathways to promote charge separation, reduce energy barriers, and enable controllable reactant conversion, key strategies involve broadening the spectral response range, optimizing photocarrier dynamics, increasing surface reactivity, and improving photostability, as shown in Fig. 4. Concurrently, achieving high efficiency, selectivity, and stability remains central to practical photocatalytic H 2 O 2 synthesis. Effective methodologies include accelerating oxygen mass transfer, enhancing oxygen adsorption, and suppressing hydrogen peroxide self-decomposition.
Key Point 5: perspective on the practical applications of photosynthesized H 2 O 2
Practical implementation of photocatalytic H 2 O 2 production faces considerable challenges, primarily due to the low concentrations typically generated by current photocatalytic methods. Such dilute H 2 O 2 solutions are largely unsuitable for industrial applications requiring higher concentrations. To address this limitation, the research team investigated multiple strategies (Fig. 5), encompassing both utilization approaches for low-concentration H 2 O 2 and methodologies for achieving high-concentration H 2 O 2 production. Low-concentration H 2 O 2 can be directly applied or employed in Fenton/photo-Fenton reactions according to oxidation capacity requirements, effectively inactivating various bacteria and degrading organic pollutants without generating harmful byproducts. Developing reactors that preserve optimal illumination conditions while enhancing gas-liquid mass transfer is crucial for efficient photocatalytic H 2 O 2 generation. Microchannel reactors in continuous-flow photocatalytic systems have gained attention for effectively reducing light penetration distances and liquid layer thickness. Despite these advancements, flow-based photoreactors still confront multiple critical challenges.
Conclusion and outlook
Metal-free photocatalysis technology presents a sustainable alternative to conventional H 2 O 2 synthesis, addressing environmental concerns while providing a cost-effective solution. However, the low energy conversion efficiency and insufficient accumulation concentration impede the practical implementation of this technology. Exploring novel surface reaction mechanisms, such as anthraquinone-intermediate, peroxy-acid-intermediate, bipyridine-intermediate, and redox dual-channel pathways, opens a new revenue to enhance charge separation and utilization, reduce energy barriers, and improve selectivity, leading to substantially increased H 2 O 2 yields. However, the production of H 2 O 2 via these specialized pathways remains at the laboratory scale. To facilitate the practical application of this technology, substantial research efforts are required in several critical areas.
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