Synthesis Of B₅O₁₀-based Photocatalysts And Research Of The Dechlorination Activity Of Chlorophenols

Chlorophenols are one class of organic pollutants,which do great harm to environment.Photocatalytic technique is a useful method for the degradation of many organic pollutants for its high photo-degradation efficiency,non-selectivity,and none secondary pollution.One of the most important problems limiting its applications is the the photo-induced carrier recombination in semiconductor photocatalysts.Recently,a new concept,internal electric fields,which can provide a driving force for the transport of photo-induced charge carrier,was proposed to enhance the separation of photo-induced charge carriers.Nonlinear optical materials have a strong polarization effect with built-in electric field which help effectively separate andtransfer charge carriers.B-O units have long been investigated as nonlinear optical materials due to their rich structural chemistry.However,the photocatalytic properties of these crystals have been barely studied.In the present thesis work,we successfully synthesized inexpensive and stable materials K3ZnB5O10(KZBO),K2NaZnB5O10(KNZBO),Na3ZnBsO10(NZBO)and K3CdB5O10(KCBO),which can function as efficient photocatalystsfor CPs dechlorination.Detailed studied outlined as following:(1)We successfully synthesized B5O10-based materials KZBO,KNZBO,and NZBO by a traditional solid-state reaction technique.Their structures,morphologies,compositions and optical properties have been characterized by XRD,SEM,EDX,and DRS.They exhibited high efficiency in the dechlorination of 2-CP,4-CP,2,4-DCP,and 2,4,6-TCP.The obtained dechlorination efficiency is ten times higher than that of the commercial P25-TiO2 catalyst under the same reaction conditions.The high polarizability of MZBO induces the internal electric fields at the space-charge regions,which facilitates the charge separation of photogenerated electron-hole pairs and thus enhances the photocatalytic activity.The M+ cations with different radius play a significant role in the photocatalytic properties of MZBO,as large cations are prone to be polarized,which influences the cation's surrounding environment.(2)Based on the previous results,KCBO was successfully designed and synthesized by a traditional solid-state reaction technique.Their structures,morphologies,ccompositions and optical properties have been also characterized.They exhibited higher efficiency than KZBO in the dechlorination of 2-CP,4-CP,2,4-DCP,and 2,4,6-TCP.The different activities for CPs dechlorination by the three catalysts are closely related to the substitution of Cd2+ cations.On the one hand,the[MO4]unithas larger polarizationability than[ZnO4].On the other hand,[MO4]tetrahedron shares its four oxygen actoms with four[BO3]units,which influences the polarization of[BO3]and likely affects the electronic structure.Moreover,conduction bands of d10 metal cations consist of sp orbitals,which are dispersed well resulting in high mobility of photogenerated electrons.All above,KCBO showed higher activity than KZBO.(3)In order to optimize the photodechlorination conditions,we studied relevant influencial factors,such as temperature of the photodechlorination reaction,target pollutants concentration,pH value,dosage of catalyst,light source and light intensity.Consequently,elevating temperature and increasing light intensity are helpful to enhance photocatalytic activities,while the photocatalytic activity is weakened with the increasing of the pollutant concentration;there exists a optimum value in the dosage of catalysts;the dechlorinationof 2,4-DCP is more effective in acidic solution.(4)At last,taking 2,4-DCP as an example,a mechanism for CPs degradation is proposed based on the LC-MS and TOC analysis.The photodechlorination pathways outlined both oxidative and reductive dehalogenation.The formation of radical was detected by EPR spin-trapping.technique to further explore the photodechlorination pathway andthe quantitative analysis of active species is carried out with PL measurments.