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.