Analysis Of The Key Role Of Monatomic Pt In Photocatalytic Oxygen Reduction Of Carbon Nitride Nanosheets For Hydrogen Peroxide Production

As an important oxidant,hydrogen peroxide（H₂O₂）is widely used in chemical synthesis,disinfection,medicine,environmental remediation and other fields.At present,the main synthesis methods of H₂O₂are indirect oxidation of anthraquinone,direct reaction of hydrogen and oxygen mixture,electrochemical reaction and so on.However,there are a series of problems in these methods,such as high energy consumption,complex production process and toxic by-products.Therefore,it is urgent to develop efficient,economic and green H₂O₂production technology.Photocatalytic ORR reaction uses solar energy as the driving force and oxygen is reduced to H₂O₂using a photocatalyst,which has attracted much attention due to its green,safe and sustainable advantages.Therefore,the selectivity of photocatalytic preparation of H₂O₂is difficult to regulate.In the process of photocatalytic ORR reaction,the way that four electrons produce water competes with the way that two electrons produce H₂O₂,so H₂O₂is selective and difficult to regulate.In addition,most photocatalysts have problems such as low utilization rate of visible light and serious carrier recombination,which limit the activity of photocatalytic ORR to produce hydrogen peroxide.Carbon nitride photocatalyst has the advantages of non-toxicity,low cost and easy modification.The activity and selectivity of carbon nitride photocatalyst for ORR preparation of H₂O₂can be effectively regulated by morphology regulation,construction defects,heteroatomic doping and monatom modification.However,the following problems still exist in photocatalytic ORR reaction with carbon nitride catalyst:Firstly,the generated H₂O₂is easily decomposed into water on the surface of carbon nitride photocatalyst,which reduces the production rate of H₂O₂photocatalytic preparation.Secondly,the mechanism of photocatalytic ORR reaction to prepare H₂O₂is still unclear.At present,most studies on the preparation of H₂O₂by carbon nitride photocatalytic protooxidant are conducted through traditional detection methods such as UV-vis,fluorescence spectrum,electron para-magnetic resonance and gas chromatography.It is difficult to conduct real-time in-situ monitoring on the photocatalytic ORR reaction process,and direct spectral evidence in the ORR reaction process is lacking.Therefore,it is impossible to accurately identify the specific mechanism of photocatalytic protooxidant reaction.Therefore,the preparation of carbon nitride photocatalyst with high activity and selectivity and the direct in situ detection of ORR reaction process are of great significance to further explore the mechanism of photocatalytic oxidant.In this thesis,we constructed Pt monatom modified carbon nitride nanosheets(Pt-SA/g-C₃N_4-x)to improve the photocatalytic ORR activity and selectivity of carbon nitride by Pt monatom modification,and the reaction process was monitored in situ by surface enhanced Raman technique.The specific content of this thesis is as follows:（1）Three kinds of photocatalysts,namely massive carbon nitride g-C₃N₄,ultra-thin porous carbon nitride nanosheets(g-C₃N_4-x)and ultra-thin porous carbon nitride nanosheets(Pt-SA/g-C₃N_4-x)modified with single atom Pt,were prepared.The activity and selectivity of photocatalytic ORR preparation of H₂O₂were studied respectively.Atomic force microscopy（AFM）image presents that the thickness of g-C₃N_4-xand Pt-SA/g-C₃N_4-xis about 1 nm.Further high-angle annular dark-field scanning transmission electron micros-copy（HAADF-STEM）combined with carbon monoxide（CO）-adsorption infrared spectra verifies atomically dispersed Pt atoms on g-C₃N_4-xnanosheet.The single-atom Pt-SA/g-C₃N_4-xcatalyst exhibits an unexpectedly high H₂O₂generation activity of 40.9 mmol·g^-1·h^-1,which is 3.3 folder of g-C₃N_4-xand 11.0 of pure g-C₃N₄.Electron paramagnetic resonance spectra combined with·O₂^-trapping fluorescence spectra reveal that Pt single atoms can accelerate the dissociation of exciton on g-C₃N_4-x,promoting the separation of electrons and holes.Binding phosphorescence spectra show that the presence of Pt monatom promotes·O₂^-to form singlet oxygen（¹O₂）,which facilitates the formation of the key intermediate*OOH.These results show that the Pt monatomic-supported g-C₃N_4-xphotocatalyst has high 2e^-ORR activity and selectivity.（2）The preparation of H₂O₂from ORR catalyzed by g-C₃N_4-xand Pt-SA/g-C₃N_4-xwas monitored in situ by surface enhanced Raman spectroscopy.Different intermediates were detected in the ORR reaction process catalyzed by g-C₃N_4-xand Pt-SA/g-C₃N_4-x.*OH was detected during the ORR reaction catalyzed by g-C₃N_4-x,and*OOH was detected during the ORR reaction catalyzed by Pt-SA/g-C₃N_4-x,indicating that Pt monatomic loading was favorable for the preparation of H₂O₂.Further calculation results with density functional theory（DFT）show that Pt single atom site can enrich more electrons,which is beneficial to further reduction of adsorbed oxygen.Moreover,the loading of Pt single atom changes the adsorption energy of intermediates in the ORR reaction process.*OOH has lower binding energy at Pt-SA/g-C₃N_4-x.The results show that Pt can inhibit*OOH fracture and promote its further transformation to H₂O₂.The above results show that the loading of Pt single atom improves the activity and selectivity of preparing H₂O₂by inhibiting the breakage of*OOH.In summary,the photocatalytic process and mechanism of photocatalytic ORR reaction of Pt-SA/g-C₃N_4-xnanosheets modified with single atom Pt were studied by UV-visible absorption,fluorescence spectrum and electron para-magnetic resonance.In situ surface-enhanced Raman spectroscopy and DFT theoretical calculation further explain the reaction mechanism of H₂O₂preparation in photocatalytic ORR reaction.It is of great significance to further understand the mechanism and catalytic process of a reaction and develop new and efficient photocatalyst materials.