Photocatalytic VOCs Oxidation Enhancement Performance And Reaction Mechnism Of Wide Bandgap Semiconductor Photocatalysts

With the rapid development of industrial economy and the improvement of people’s requirements for the quality of life,clean atmospheric environment is very important for our physical and mental health.At present,air pollutants are mainly composed of particulate pollutants and gaseous pollutants.Particulate pollutants（PM2.5,etc.）can be removed by adsorption.However,the mixture of volatile organic compounds（VOCs）,such as formaldehyde,toluene and halogenated hydrocarbon,is difficult to be removed by simple filtration and interception technology due to its complex composition and difficult degradation.VOCs Pollution mostly comes from the emission of large-scale industrial exhaust gas and vehicle exhaust gas.Secondly,there are a small part of pollution gas released from building furniture materials,coatings,etc.Recently,semiconductor photocatalysis technology is regarded as a green and efficient method for VOCs degradation.It can convert low-density solar energy into high-density chemical and electrical energy efficiently.Formaldehyde and toluene were deeply oxidized to carbon dioxide and water under mild conditions.Currently,some achievements have been made in the study of VOCs degradation on TiO₂ based photocatalyst with wide band gap.However,they are prone to deactivation or poor stability due to the presence of carbon containing intermediates and secondary toxic by-products covering the surface active sites.Therefore,it is necessary to develop a new wide band gap photocatalyst that the stronger redox ability to solve this key scientific problem.In this study,the following three ideas are used to optimize and modify different wide band gap photocatalysts.In order to improve its photocatalytic activity to VOCs pollutants and explore its reaction mechanism:（1）The monoclinic bismuth phosphate（MBPO）and hexagonal bismuth phosphate（HBPO）have different distorted PO₄ tetrahedral and dipole moments.This kind of crystal effect will lead to the separation and migration efficiency of photo-induced carriers and the performance of photocatalytic activity.Combined with the experimental characterization and theoretical calculation results,it is found that the oxidation reaction of formaldehyde on the MBPO interface is carried out according to the path of formaldehyde→formic acid→carbonic acid→carbon dioxide.However,the secondary toxic by-product carbon monoxide appears on the surface of HBPO,and thus the overall degradation path is formaldehyde→formic acid→carbon monoxide→carbonic acid→carbon dioxide.The results indicate that the oxidation process of the active radicals and the reaction intermediates on the MBPO interface will be more smooth.And the target product carbon dioxide conversion is more efficient and inhibit the formation of carbon monoxide.（2）The oxygen defect（OV）can form an intermediate defect level in the band gap of indium hydroxide（InOOH）,which can be reduced the energy barrier of photogenerated electron transition to conduction band,accelerate the transfer of photogenerated charge,enhance the generation and transformation of active oxygen species（·O₂^-,·OH）.It will greatly improve the efficiency of photocatalytic degradation of toluene.Combined with in-situ DRIFTS spectrum and theoretical calculation results,with the increase of oxygen defect concentration（OV_S）in InOOH,the selective conversion of toluene to benzoic acid is more favorable;The energy barrier of ring opening reaction at benzoic acid is lower than that of toluene,benzyl alcohol and benzaldehyde.The results also prove that benzoic acid is the rate determining step in the reaction process of toluene.Therefore,oxygen defect concentration（OV_S）can not only regulate the efficiency of ring opening reaction at benzoic acid,but also promote the conversion of ring opening intermediate to target product carbon dioxide.（3）Hydroxyl radicals（·OH）are considered as the main active radicals in toluene degradation.The Ca₂Sb₂O₅（OH）₂ photocatalyst（CSOH）has the abundant hydroxyl groups on its surface,which can react with holes to facilitate the formation of·OH radicals and then participate in the oxidation process of toluene.The results of（4）in-situ DRIFTS show that the further oxidation ring opening of benzaldehyde is the rate determining step for the reaction process on CSOH that could optimize the benzene ring opening effectively.There was no accumulation of toxic by-products such as epoxides in the reaction process,while the peak intensity of the end product carbon dioxide accumulated significantly.The results indicate that the pathway of toluene ring-opening reaction is regulated on CSOH and the mechanism of photocatalytic reaction is clarified.In conclusion,it provides a new understanding for expanding the reaction mechanism of photocatalytic degradation of VOCs pollutants,improving the effective activation of reaction intermediates,and controlling the formation of toxic intermediates,as well as technical support for the final application of wide band gap photocatalyst.