Study On Oxidation Mechanism Of Active Oxygen Species In Photocatalytic Hydrogen Peroxide Production

As an environmentally friendly and high-efficiency oxidant,hydrogen peroxide（H₂O₂）has been widely used in the fields of organic synthesis,wastewater treatment,disinfection and pulp and paper industry.The anthraquinone method,currently the most widely used method for preparing H₂O₂,has a complex production process,requires high energy input,and produces a large amount of waste water.In contrast,the process of photocatalytic H₂O₂ production is expected to be an ideal technology to replace the anthraquinone method due to its low cost and environmental friendliness.Therefore,photocatalytic H₂O₂ production has received extensive attention in recent years.The use of organic electron donors for photocatalytic H₂O₂ production can not only greatly increase the photocatalytic yield of H₂O₂,but also obtain value-added chemical.This"kill two birds with one stone"approach has also recently become a research hotspot.However,it is generally believed that hole is the only driving force for oxidizing electron donors to generate H⁺,and reactive oxygen species（ROS）can only combine with the generated H⁺to form H₂O₂.Based on this mechanism,the ultimate goal is to promote the two-electron reduction of oxygen.However,the existing mechanism may not be complete.In addition to hole,photocatalytic reactions can also produce ROS with oxidizing ability,such as superoxide radical（·O₂^-）and singlet oxygen（¹O₂）,which may also react with organic electron donors to form H₂O₂.Exploring the oxidative role of ROS in the photocatalytic production of H₂O₂ is of great significance for improving the relevant reaction mechanism and the design principles of photocatalysts.Therefore,we systematically explored the specific roles of photocatalyst,ROS and water molecule in photocatalytic H₂O₂production,especially the oxidative role of ROS,by combining experiments and theoretical calculations.The production,interconversion and corresponding active sites of each ROS were discussed in detail.Meanwhile,the reaction barriers corresponding to different reactive species as the oxidation driving force were compared,and the essential reasons for the most favorable reaction pathway were analyzed in detail.As a result,two novel reaction mechanisms were discovered successively.Subsequently,these novel reaction mechanisms were used for the design of photocatalysts and a new mechanism was discovered again,realizing a virtuous circle of"revealing new mechanisms,thereby guiding the development of materials and exploring deeper mechanisms from new materials".The main research results are as follows:（1）Nitrogen vacancies and cyano groups were introduced into carbon nitride（g-C₃N₄）by Na OH-assisted heat treatment.Meanwhile,it leads to the reduction of N-H.The introduction of nitrogen vacancies expanded the photoresponse range of the photocatalyst and improved the separation efficiency of electrons and holes.The as-prepared nitrogen-deficient g-C₃N₄ exhibited an extremely high H₂O₂ yield,which was9.7 times that of the pristine g-C₃N₄.·O₂^-may be an intermediate in all reaction pathways during the photocatalytic H₂O₂ production.A part of·O₂^-can be directly converted into H₂O₂,and the other part of·O₂^-becomes ¹O₂ before being converted into H₂O₂.¹O₂may have a major contribution to the generation of H₂O₂.（2）The introduced nitrogen vacancies in nitrogen-deficient had effects of oxidation,reduction and charge recombination,which promote the generation of h⁺,·O₂^-and ¹O₂,respectively.Under the catalysis of nitrogen vacancies,there are five different pathways for H₂O₂ generation.Surface h⁺,surface·O₂^-,surface ¹O₂,¹O₂assisted by water molecules and free ¹O₂ can all be used as oxidative driving forces in the H₂O₂ generation reaction.The ¹O₂ on photocatalyst surface can also synergize with the water bridge to generate H₂O₂ through an indirect attack mechanism:One hydrogen of the organic electron donor was directly transferred to ¹O₂,while the other hydrogen was first combined with a water molecule to form a hydrated proton,which then transfered one hydrogen to ¹O₂ to generate H₂O₂.Compared with the ROS-driven H₂O₂generation pathway,the hole-driven reaction pathway had a higher energy barrier and thus could not provide a major contribution to the generation of H₂O₂.·O₂^-is an intermediate in all reaction pathways.And ¹O₂is the main active species in the reaction of H₂O₂ generation,which can not only directly attack electron donors on photocatalyst surface or in solution to generate H₂O₂,but also synergize with surrounding water molecules to generate H₂O₂ with the lowest rate-limiting step energy barrier.（3）The results in the previous sections showed that the ROS-driven H₂O₂generation pathways were all more favorable than the hole-driven pathway,while the oxidizability of hole for the used photocatalysts was relatively weak.In order to explore whether the pathways driven by holes with stronger oxidizability can be more favorable than those driven by ROS,i.e.,to investigate whether the previous results were generalizable,both boron doping and nitrogen vacancy sites were introduced in g-C₃N₄,resulting in the distribution of holes on the two modified sites with higher and lower oxidizability.In this way,the contribution of holes with different oxidizability to the photocatalytic production of H₂O₂ could be compared.Compared with nitrogen vacancies,hole with stronger oxidizability on boron sites can oxidize organic electron donors with a lower reaction barrier,but the resulting H⁺anchored on the hole site needed to overcome a larger energy barrier to desorb from the catalyst surface for forming H₂O₂.In contrast,the·O₂^-and ¹O₂ generated on nitrogen defects and boron doping sites can directly attack electron donors or diffuse into solution to react with free electron donors to generate H₂O₂.The corresponding reaction barriers were all lower than the hole-driven reaction pathways.In addition,the ¹O₂ generated at the B site can synergize with the surrounding water molecules to indirectly attack organic electron donors to further reduce the reaction barrier for generation of H₂O₂.Therefore,even if the oxidizability of hole in photocatalysts is enhanced by modification,the reaction pathways driven by holes still cannot play a major role in the generation of H₂O₂.（4）Inspired by the indirect attack mechanism of ¹O₂ and the fact that enhancing the oxidizability of holes fails to increase the production of H₂O₂,promoting the generation of ¹O₂ may be a new breakthrough in enhancing the production of H₂O₂.Therefore,in order to promote the generation of ¹O₂ as much as possible,carbon vacancies and doped boron sites were introduced into g-C₃N₄,which constructed the center of O₂ activation and·O₂^-oxidation,i.e.,the active site for the generation of ¹O₂.Boron-doped g-C₃N₄ with carbon vacancies（BCNC）exhibited a rose-petal-like porous structure and the corresponding specific surface area was 1.8 times that of pristine g-C₃N₄.The introduced modification sites not only promote the separation of photogenerated carriers,but also enhance the ability of the material for activating molecular oxygen.BCNC can activate O₂ to·O₂^-.Subsequently,·O₂^-was immediately oxidized to ¹O₂.The photocatalytic activity of boron-doped g-C₃N₄ with carbon vacancies（BCNC）was 29 times higher than that of pristine g-C₃N₄,and the corresponding apparent quantum efficiency reached 29.5%.（5）With the assistance of water molecules,¹O₂ on the BCNC surface can also indirectly attack organic electron donors to generate H₂O₂,but the corresponding rate-limiting step energy barriers was lower（-0.07 e V）than those in the previous chapters（0.66 and 0.67 e V）.In contrast,the four reaction pathways driven by surface h⁺,surface¹O₂,free ¹O₂,and free ¹O₂ assisted by water bridge do not provide a major contribution to the generation of H₂O₂ due to the higher energy barriers.The H⁺dissociated from the organic electron donor can temporarily combine with water molecule to generate hydrated protons,so that ¹O₂ only needed to break one covalent bond of the organic electron donor instead of two,which indirectly enhanced the oxidation effect of organic electron donor by ¹O₂.However,this also weakened the interaction between ¹O₂and organic electron donor.Fortunately,the spinning electron on BCNC modified the electronic structures of ¹O₂ and organic electron donor on this basis,so that the overlapping degree of molecular orbitals in which they interacted significantly increased,resulting in a significantly enhanced interaction between them.As a result,the rate-limiting step energy barrier for the reaction was lowered to a negative value.