Synthesis Of Bi/BiOBr Composite Photocatalyst For Solarlight-driven Photocatalytic Degradation Of Typical VOCs

In recent years,the pollution control of volatile organic compounds?VOCs?has become a hot spot in the research of scientists.VOCs is toxic and complex,and if they are not treated and discharged to the environment directly,it will pollute the atmospheric environme nt seriously and threaten human health.Photocatalytic technology is a treatment technology,which is green,environmental-friendly and high-efficiency.Semiconductor photocatalysis has been proved to be an economical and effective method for VOCs pollution control.Bismut h halide oxide?BiOX?is an important ternary structure semiconductor photocatalyst,because of its unique layered structure,suitable band gap,high chemical stability,catalytic activity and high visible light response,etc.It has become a new direction of photocatalyst research.Therefore,in this study,solvothermal method was used for situ synthesis of Bi-supported BiOBr?Bi/BiOBr?composite photocatalyst.The physical and chemical properties of Bi/BiOBr composite photocatalyst were regulated by changing the solvent type and solvothermal temperature.Using typical VOCs-n-hexane as model compound,the solar photocatalytic degradation of n-hexane over Bi/BiOBr composite photocatalyst was studied.The effects of solvent types,solvothermal temperature and solar light band on the photocatalytic degradation of n-hexane and the conversion mechanism of n-hexane were investigated.The main research contents and results of this paper are as follows:?1?Bi/BiOBr composite photocatalyst was in situ synthesized by solvothermal method.The physical and chemical properties of Bi/BiOBr composite photocatalyst were regulated by changing the solvent type and solvothermal temperature.The effects of solvent types and solvothermal temperature on photocatalytic degradation of n-hexane were further investigated.The results showed that Bi/BiOBr composite photocatalyst was synthesized in situ with Bi salt as precursor,methanol,ethylene glycol and glycerol as solvent at 140 ^oC,160 ^oC,180 ^oC and200 ^oC reaction temperature,respectively.When methanol was used as solvent,Bi/BiOBr composite photocatalyst could not be obtained at all temperatures.For ethylene glycol system,only when the reaction temperature?180 ^o C can the Bi/BiOBr composite photocatalyst be obtained.Under the selected reaction temperature,the Bi/BiOBr composite photocatalyst can be obtained by using glycerol as solvent catalyst,except for 140 ^oC synthesis catalysts,and the content of Bi increases with the increase of temperature.The degradation results of n-hexane showed that Bi/BiOBr composite photocatalyst could achieve complete degradation of n-hexane in 80min.Among them,the photocatalyst synthesized with ethylene glycol as solvent has the highest degradation efficiency?100%?,followed by glycerol and methanol.It may be mainly due to the large specific surface area of the photocatalyst synthesized with ethylene glycol as solvent.The effect of different reaction temperatures shows that:With the increase of reaction temperature,the degradation efficiency of n-hexane was firstly increased and then decreased with the synthesis of methanol as solvent.The degradation efficiency of n-hexane decreased gradually by photocatalysts which were synthesized with ethylene glycol and glycerol as solvents.This is mainly due to the fact that when methanol is used as synthesis solvent,the increase of synthesis temperature may lead to the full growth of the catalyst to form nano-scale.the nano-scale of catalyst collapses at the highest temperature,thus affecting its catalytic activity.When ethylene glycol and glycerol were used as synthesis solvents,the Bi metal was gradually increased with the increase of synthesis temperature,thus the BiOBr decreased and the degradation activity decreased.Therefore,for the synthesis of Bi/BiOBr composite photocatalyst with high activity of n-hexane degradation,the optimum solvent and reaction temperature are ethylene glycol and 140 ^oC.?2?the effects of different wavelengths in solar light on the degradation of n-hexane by Bi/BiOBr composite photocatalyst were investigated by filtration method.The results showed that the degradation efficiency of n-hexane by Bi/BiOBr composite photocatalyst was the highest under full-wavelength irradiation?99.3%and 48.7%for 160/Gl and 200/Gl's degradation efficiency of 40min,respectively?,and then following the filtered infrared light irradiation?83.3%and 3.9%for 160/Gl and 200/Gl's degradation efficiency of 40min,respectively?and filtered ultraviolet light irradiation?32.6%and 6%for 160/Gl and 200/Gl's degradation efficiency of 40min,respectively?.The main reason is that the energy of the whole spectrum is the largest and the destructive power of n-hexane molecule is strong.However,after filtering infrared light or ultraviolet light,there is a lack of some energy,thus the destructive power of n-hexane molecule decreases.The results of free radical,product identification and transformation mechanism showed that:Under sunlight irradiation,a large amount of superoxide radicals can be produced on Bi/BiOBr composite photocatalyst.These free radicals convert n-hexane into 3-methyl-1-pentone,oxetane,2,2,4-trimethyl-,1-penten-3-ol,2-methyl-,3-hexanone,2-hexanone,3-hexanol,2-hexanol,hydroperoxide,1-ethylbut yl.And eventually degraded to CO₂ and H₂O.