Hydrogen Peroxide Production with Photocatalysts Based on Carbon Nitride: Evaluation Criteria, Reaction Mechanism, Improvement Scheme, and Development Opportunity

Hydrogen peroxide (H2O2), an ecofriendly oxidant, is extensively employed in wastewater treatment, pulp bleaching, and chemical synthesis. Photocatalytic 2e– oxygen reduction reaction (2e– ORR) to H2O2 has emerged as a renewable strategy for solar-to-chemical energy conversion. Carbon nitride (CN) is a promising candidate for producing H2O2 due to its unique optical properties and electronic structure. However, the ambiguity of the reaction mechanism still limits the development of photocatalytic H2O2 production because of the complexity of the reaction active sites. Previous research on the reaction mechanism was not deep enough, leading to the actual role of reaction sites being relatively indistinct in the photocatalytic process. This Review systematically explores the intrinsic mechanism of enhancing the performance of photocatalysts, with a focus on analyzing the action mechanism of multiple types of active sites (such as defect sites, doping sites, surface metal sites, etc.) induced by different modification strategies in 2e– ORR, which provides a theoretical basis for elucidating the structure–activity relationship of different active sites and their key role in selective H2O2 production. Furthermore, the challenges, opportunities, and future research directions of photocatalytic 2e– ORR for H2O2 production have also been emphasized. This work breaks through the conventional research mode of “modification earlier, analysis later” in traditional photocatalytic materials, innovatively explaining the structure–activity relationships of different types of reaction sites in photocatalytic H2O2 production from the perspective of reaction sites. This “mechanism-driven rational design strategy” based on reaction mechanism reverse design of catalytic sites provides a theoretical framework for breaking through the limitations of traditional trial and error approaches in photocatalyst optimization, especially in guiding the functional modification of inefficient catalysts.