Frabrication Of Modified-TiO₂ Photocatalyst And Mechanism Of Photocatalytic Degradation For Toluene

With rapid development of energy and chemical industry,the content of VOCs in the atmosphere increases year by year,which is a serious threat to ecological safety and human health.Photocatalytic oxidation technology（PCO）based on titanium dioxide（TiO₂）is considered to be a promising VOCs treatment method.However,most TiO₂-based catalysts still suffer from some drawbacks such as the poor light utilization efficiency.To the best of our knowledge,the research on VOCs treatment using TiO₂-based catalysts is very limited.Thus,with toluene as the target pollutant,this paper developed a method to regulate the crystal surface,and further doping TiO₂ with Ag/Ag₂O and nitrogen-doped quantum dot（NCDs）to further improve its photocatalytic activity.The main findings were as follows:（1）The optimization of titanium dioxide（D-TiO₂）.The decahedral particle（D-TiO₂）with（001）crystal planes was fribracated by hydrofluoric acid（HF）hydrothermal method.The characterization and photocatalytic degradation results indicated that when the hydrothermal time is 24 h,the hydrothermal temperature is 200℃ and the amount of HF is 3.4 mL,the ratio of（001）crystal surface is the largest（60.3%）,and the degradation efficiency of toluene is the highest（94.9%）.The formation of（001）crystal planes was conducive to the separation of photogenerated electrons-holes,which in turn promoted the degradation of toluene.（2）The fabrication of Ag/Ag₂O@TiO₂ by chemical reduction precipitation method and the photocatalytic performance of gaseous toluene.Transmission electron microscope（TEM）and X-ray photoelectron energy spectrum（XPS）results showed that TiO₂ was composed of（001）/（101）crystal planes and silver mainly existed in the surface of Ag and Ag₂O.The UV-visible absorption spectra（UV-vis）,Photoluminescence Spectroscopy（PL）,Electrochemical Impedance Spectroscopy（EIS）and photocurrent curve indicated that the addition of Ag/Ag₂O contributed to the UV-visible light absorption and the separation of photo-generated carriers.When the concentration of toluene ranged from 222～469 ppm,the residence time of the gas was 2.5 s,the relative humidity was 55～75±5%,and the dosage of photocatalyst was 0.02 g·cm-2,the best degradation performance was achieved for Ag/Ag₂O@TiO₂.It showed that Ag/Ag₂O@TiO₂ could maintain high and stable degradation efficiency of 99.3%or 48.3%for toluene even for 400 min with UV light or simulated sunlight irradiation.With the natural sunlight,toluene also had a degradation efficiency of 28.5%,which has practical application potential.A possible Ag/Ag₂O@TiO₂ photocatalytic degradation mechanism was also proposed.The synergeitic effect of excellent conductivity of Ag and low-energy band embedding of Ag₂O could promote the photo-electrons separation and transfer as well as the expansion of visible light absorption so as to prevent Ag₂O photodecomposition and to promote oxygen free radicals（·O₂-）generation.（3）Preparation and visible photocatalytic activity tests for NCDs@TiO₂ and Ag₂O-NCDs@TiO₂.NCDs@TiO₂ was prepared by impregnation-calcination method and Ag₂O-NCDs@TiO₂ was prepared by chemical reduction precipitation method.TEM,XRD,and XPS indicated that the catalyst consisted of TiO₂,loaded with Ag,Ag₂O,and NCDs,and FTIR,UV-vis,and PL indicated that there was a large amount of-OH on its surface which could significantly improved visible light absorption and photo-electrons separation efficiency.Under the optimum conditions（220 ppm of toluene concentration,3.2s of gas residence time,0.01 g·cm-2 of photocatalyst and 37～63±±5%of relative humidity）,the photocatalytic activity of Ag/Ag₂O-NCDs@TiO₂ for toluene under visible light was significantly higher than that of NCDs@TiO₂,and the degradation efficiency of toluene was increased from 28.0%to 49.1%,and 78.8%of toluene degradation efficiency was achieved for Ag/Ag₂O-NCDs@TiO₂ under simulated sunlight.The free radical quenching experiments results showed that the main active substances in photocatalytic degradation were ·O₂-、holes and·OH.The photocatalytic mechasim of Ag/Ag₂O-NCDs@TiO₂ was speculated as follows,the SPR effect of Ag and the up-conversion effect of NCDs contributed to enhanced light absorption for Ag/Ag₂O-NCDs@TiO₂.With the excellent electrical conductivity of Ag and NCDs,the transfer of photogenerated electrons was accelerated,thus preventing the photodecomposition of Ag₂O and promoting the generation of ·O₂-、holes and·OH,which degraded tolueneas into small substance.