Photoelectrocatalytic Performance Towards Selective Oxidation Of C-H Bonds In Organics

Selective oxidation of carbon-hydrogen（C–H）bonds present in organic compounds is a crucial process for upgrading and preparing a series of bulk/high-value chemicals.Traditional methods for catalyzing C–H bond activation typically require high temperature and pressure conditions,often employing precious metal catalysts and additional oxidants（such as peroxides,high-valent metal oxides,and O₂）to overcome the barrier of C–H bond cleavage.This results in high energy consumption,elevated costs,and potential safety and environmental hazards.Utilizing water as the reaction medium,photoelectrocatalytic（PEC）systems driven directly by solar energy offer a new approach for achieving green organic synthesis under mild conditions.The core of PEC organic oxidation reactions in aqueous systems lies in the selective oxidation of C–H bonds in organic molecules mediated by active species generated by the activation of water or electrolytes on the surface of the photoanode.Despite significant progress,this field still faces the following issues and challenges:（1）Existing PEC organic synthesis systems mainly involve substrates such as biomass alcohols/aldehydes,while research on the value-added transformation of many important industrial raw materials（such as aromatic hydrocarbons,alkanes,cycloalkanes,etc.）is limited.Expanding the water-mediated PEC synthesis platform to enable selective oxidation of C–H bonds in a wider range of organic substrates remains a significant challenge.（2）Previous studies have shown that active oxygen/halogen species generated by the activation of water molecules/halide ions in photo-driven aqueous solutions can be utilized for C–H bond activation in organic compounds.However,understanding the generation patterns and forms of active species in PEC systems,and matching their oxidation ability with the energy of C–H bonds in organic molecules to achieve selective oxidation,remains one of the core challenges.（3）In addition to understanding the mechanisms of action of key active species（active oxygen/halogen,etc.）in the PEC process,the urgent need in this field is to promote the generation of active species through the rational design of high-performance photoanode materials,and to synergistically regulate the adsorption-activation processes of organic molecules on the surface of the photoanode,thereby enhancing the selective oxidation performance of PEC C–H bonds.This paper addresses the aforementioned issues through research conducted in three main aspects:the construction of the PEC organic C–H bond selective oxidation reaction system in aqueous phase,the elucidation of the generation patterns and reaction mechanisms of active oxygen/active halogen,and the design of high-performance photoanode materials and reaction devices to enhance reaction efficiency.Firstly,based on the utilization of active oxygen/active halogen species,PEC systems for the selective oxidation of aromatic hydrocarbons,cyclic alcohols,and alkane halogenation reactions have been developed,enabling the green synthesis of a series of high-value-added chemicals.Secondly,various in situ characterization techniques combined with theoretical calculation methods have been employed to understand the existence and generation patterns of adsorbed active species,thereby revealing the mechanism of PEC organic oxidation reactions.Finally,surface/interface modifications of photoanode materials have been utilized to regulate the types and concentrations of active species,while promoting the adsorption/activation processes of reactant molecules,thus enhancing the performance of photocatalytic organic C–H bond selective oxidation reactions.Additionally,PEC oxidation reaction devices have been designed for sustainable,continuous synthesis of chemicals.The specific research content and results of this paper are as follows:1.Reactive oxygen species-mediated the PEC selective oxidation of C–H bonds.Achieving the direct activation and oxidation of C–H bonds to prepare oxygen-containing chemicals under mild conditions poses significant challenges.PEC oxidation technology can utilize active oxygen species generated in situ in aqueous solutions to activate C–H bonds,obtaining oxidation products using water as an"oxygen source".However,the efficiency of PEC oxidation of organic C–H bonds has not yet met expectations due to constraints such as low concentrations of active oxygen species and weak interactions between organic molecules and catalysts.In this work,we addressed this issue by focusing on utilizing active oxygen species to catalyze reactions and constructing substrate molecule adsorption sites.Through a dip-coating and calcination method,we prepared Pt O_x-loaded Ti O₂ photoanodes（Pt O_x/Ti O₂）,achieving the selective oxidation of toluene to benzaldehyde（selectivity 83.5%）under mild reaction conditions,with a benzaldehyde production rate approximately four times higher than that of pure Ti O₂.A series of PEC tests demonstrated that the modification of Pt O_x subnanoclusters suppressed the recombination of photo-generated charge carriers and improved hole utilization efficiency.In situ ESR spectroscopy and control experiments confirmed that the reaction process follows an adsorbed active hydroxyl species（OH*）mediated oxidation mechanism.Infrared spectroscopy combined with theoretical calculations revealed that the introduction of Pt O_x promoted the adsorption of toluene molecules on the catalyst surface while lowering the formation energy of OH*,thereby enhancing the toluene oxidation performance.This work provides a new direction for the design of PEC organic oxidation catalysts and improving reaction performance.Furthermore,we combined Pt O_x/Ti O₂ photoanodes with graphene-like materials（r GO）possessing excellent electron transfer and optical properties to prepare ternary r GO/Pt O_x/Ti O₂ photoanodes for the selective oxidation of toluene and its derivatives.The study showed that r GO enhanced light absorption by the photoanode and accelerated charge separation,further improving the efficiency of PEC toluene oxidation.Additionally,by designing a serpentine channel PEC reactor,the flow synthesis of oxygen-containing chemicals was achieved,providing valuable insights for advancing the practical application of PEC technology.2.Reactive halogen species-mediated the PEC selective oxidation of C–H bonds.Activation halogenation of organic C–H bonds can directly yield high-value organic halides or further prepare oxygen-containing chemicals through atomic/group substitution reactions,which are crucial in chemical and chemical engineering production.However,traditional halogenation reactions require hazardous and expensive halogenating agents and additional oxidants,hindering sustainable chemical production.Therefore,in this study,we activated halide ions in an aqueous PEC system under mild conditions to generate halogen radicals,which were utilized for the selective oxidation of the C–H bond of cyclohexanol,achieving a>80%yield of cyclohexanone.Through in situ characterization and a series of control experiments,it was demonstrated that chlorine radicals（Cl·）in the system abstracted theα–H of cyclohexanol,followed by the elimination of chlorocyclohexanol to produce cyclohexanone.This reaction strategy efficiently converts various cyclic alcohols and benzyl alcohols into corresponding carbonyl compounds.Additionally,coupling the cathodic reduction reaction of phenol/NO₂^–with the anodic oxidation reaction of cyclohexanol produced high-value chemicals,further enhancing the economic value of the system.This work provides valuable exploration for expanding the application of active halogen species in PEC organic transformations and designing value-added coupling reaction systems.Based on the halogen radical-mediated PEC halogenation reaction strategy,this study further optimized the Ti O₂ photoanode by constructing oxygen vacancies（Ti O₂-Ov）,achieving the first-ever activation halogenation of the C–H bond of hydrocarbon molecules to prepare high-value organic halides in an aqueous PEC system.Surface photovoltage spectroscopy,electron paramagnetic resonance spectroscopy,ion chromatography,and theoretical calculations demonstrated that oxygen vacancies not only promoted light absorption and separation of photogenerated charge carriers but also facilitated the adsorption and enrichment of halide ions from aqueous solutions,accelerating the generation of halogen radicals and enhancing the selective activation halogenation of C–H bonds,thereby improving reaction activity.Furthermore,a solar-powered,self-driven PEC reaction system was designed,directly using seawater as the electrolyte and chloride source,to achieve the green synthesis of organic chlorides.This work successfully controlled the concentration of active halogen species by constructing oxygen vacancies and provided a new approach for the comprehensive utilization of solar energy and halogen resources in seawater.