Effect Of Oxygen Defects On Photocatalysis And Its Performance Characterization

At present,with the development of society,more and more volatile organic compounds are produced in industry,and the damage to the environment is becoming more and more serious.In order to solve the problem of volatile organic compounds pollution,people have developed a variety of technologies to solve the above problems,among which photocatalysis technology is one of the most promising catalytic technologies.It cannot cause secondary pollution in the catalytic process,use solar energy photocatalysis to degrade pollutants in the environment,and achieve the purpose of using renewable energy to purify the environment.However,the current photocatalysts have low separation rate of photogenerated electron holes and poor photocatalytic performance.In order to further improve the efficiency of photocatalysts,oxygen defects and doping are introduced into photocatalysts to improve the separation of photogenerated electron holes through the action of oxygen defects and impurity ions,so as to improve the performance of photocatalysts.In this paper,photocatalysis technology is used to solve environmental problems,and the performance of photocatalytic degradation of volatile organic compounds is improved by introducing oxygen defects and doping.The specific contents of the paper are as follows:1.Preparation and photocatalytic properties of ZnMoO₄ containing oxygen defectsIn this experiment,ZnMoO₄ photocatalyst was synthesized by wet chemical method with polyethylene glycol as chelating agent,and oxygen defects were introduced by annealing in hydrogen atmosphere.ZnMoO₄ photocatalyst containing oxygen defects was successfully prepared.XRD test results show that the XRD diffraction pattern of the final sample is consistent with the ZnMoO₄ standard card,indicating that the ZnMoO₄ sample containing oxygen defects has been successfully prepared.The peak intensity hardly changed after hydrogen annealing,indicating that annealing only changed the surface structure of the samples.According to the UV-Vis analysis,the light absorption capacity of the annealed sample is stronger than that of the pure ZnMoO₄ sample,which indicates that the sample with oxygen defect is helpful to enhance the light absorption capacity of the sample,and the band gap of the sample with oxygen defect is significantly smaller than that of the pure ZnMoO₄.The H₂annealing introduces oxygen defect,which reduces the band gap.The photocurrent and electrochemical impedance of the samples were measured by the electrochemical work station.The photocurrent of ZnMoO₄-300-0.5h sample is 1.86 times that of pure ZnMoO₄.The photocurrent results show that the introduction of oxygen defects can improve the separation of photogenerated electron holes.The arc radius of the impedance spectrum of the samples with oxygen defects is smaller than that of the pure ZnMoO₄ samples,and the impedance becomes lower.The impedance spectrum shows that the introduction of oxygen defects is conducive to carrier transport,thereby improving the photocatalytic activity.After annealing for 0.5 h,the specific surface area of the sample reaches 69.9 m~2·g^-1,and the pore diameter of the sample becomes wider after introducing oxygen defects.The results of photocatalytic degradation of isopropanol showed that the acetone formation rate of the sample with oxygen defect was higher than that of the sample with pure ZnMoO₄,which proved once again that oxygen defect could effectively improve the separation of photogenerated electron holes and further improve the photocatalytic performance.The samples with oxygen defects have higher photocatalytic activity,which is closely related to the interaction of their high specific surface area and oxygen defects.The sample surface with high specific surface area has many reactions active points,which provides convenient conditions for the reaction.Oxygen defects can improve the separation of electrons and holes,and further improve the photocatalytic performance.2.Preparation and photocatalytic properties of Cr doped Mn₃O₄In this experiment,Cr doped Mn₃O₄ catalyst was synthesized by sol gel method.The obtained samples are characterized by large specific surface area,small particle size and many defects.The XRD results show that the shift of Mn₃O₄ peak implies that Cr exists in Mn₃O₄ lattice in the form of ions.SEM showed that the morphology of Mn₃O₄ and Cr-Mn₃O₄ showed the state of nanosphere particles,and the doped samples had smaller particle size.EDX and EDS spectra show that the material contains Cr,O and Mn.The specific surface area of Mn₃O₄ is 25.87 m~2·g^-1,while that of Cr doped Mn₃O₄ is 81.49 m~2·g^-1.The specific surface area of Cr doped Mn₃O₄ is 3.15 times that of Mn₃O₄.The photocurrent and electrochemical impedance of the two samples were measured by electrochemical work station.The photocurrent of Cr-Mn₃O₄ and Mn₃O₄were 35.77 and 26.46μA/cm~2 respectively,the photocurrent of Cr-Mn₃O₄ sample is about 1.35 times that of pure Mn₃O_4.The photocurrent results show that doping Cr can improve the separation of photogenerated electron holes.The arc radius of the impedance spectrum of Cr-Mn₃O₄ sample is smaller than that of pure Mn₃O₄,and the impedance becomes lower.The impedance spectrum shows that doping Cr is conducive to carrier transport,thus improving the photocatalytic activity.The results of photocatalytic degradation of isopropanol showed that the degradation rate of isopropanol in Cr-Mn₃O₄ sample at 40 minutes was higher than that of Mn₃O₄,The acetone production rate of Mn₃O₄ sample was 805 ppm·min^-1,that of Cr-Mn₃O₄ sample was 1346 ppm·min^-1,and that of Cr-Mn₃O₄ was also higher than that of Mn₃O₄.The above results show that doping Cr can effectively improve the separation of photogenerated electron holes.In the process of photocatalytic degradation of IPA（isopropanol）,Cr doped Mn₃O₄ showed more excellent photocatalytic activity than Cr undoped Mn₃O₄,which was also related to the special physical and chemical properties of Cr doped Mn₃O₄.In the process of photocatalytic nitrogen fixation,the concentration of NH₄⁺in Mn₃O₄ was 7.07,42.98 and 51.14μmol/L respectively when the irradiation time was 30,60 and 90 min,while the NH₄⁺concentration of Cr-Mn₃O₄ is 18.77,100.10and 106.90μmol/L respectively.When the irradiation time is 30,60 and 90 min,the NH₄⁺concentration of Cr-Mn₃O₄ sample is 2.65,2.33 and 2.09 times that of Mn₃O₄sample,respectively.The experimental results show that Cr doping enhances the effect of light absorption,significantly improves the effect of nitrogen fixation,and improves the photocatalytic performance.The improvement of photocatalytic nitrogen fixation effect mainly has the following two factors:（1）Cr-Mn₃O₄ has a large specific surface area,which can provide more active sites and accelerate the photocatalytic reaction;（2）Mn₃O₄ doped with Cr produces more oxygen defects,which can effectively improve the electron hole separation rate and promote the adsorption of N₂ molecules.