Preparation Of SiO₂/F-TiO₂ Composite Catalysts And Their Thotocatalytic Activity

Dyeing wastewater has the characteristics of large discharge,complex composition and bright color.The treatment technology today has the problems of high cost,difficult chroma removal and low degradation efficiency.Tetracycline wastewater is an antibiotic with stable naphthol ring structure.It i.s pot.entially toxic to humans,and the toxicity of intermediates in the degradation process is greater than that of the parent,which is difficult to remove by conventional methods,Photocatalytic oxidation technology with superior redox capacity,low cost and no secondary pollution has become the most potential wastewater treatment method,However,TiO₂ limits its practical application due to its wide band gap,high electron-hole pair recombination rate and small specific surface area.Modification of TiO₂ to improve its response to visible light,reducing electron-hole pair recombination,and increasing specific surface area are hot topics in photocatalysis.In this thesis,the titanium-based composite photocatalyst with good solar photocatalytic properties was prepared,with butyl titanate as the titanium source,hydroflyoric acd as the fluorine source,tetraethyl orthosilicate and 20%sodium silicate as the silicon source.A series of different Ti:Si of SiO₂/F-TiO₂ composite photocatalysts were prepared using a two-step synthesis method at a low temperature.In order to further enhance the photocatalyst efficiency,the composite photocatalysts were treated with NaOH.The photocatalytic activity of the samples was investigated by treating dyeing wastewater and tetracycline wastewater as the target pollutants,XRD,SEM,TEM,UV-Vis DRS,FT-IR,PL,BET and XPS were used to characterize and analyze the samples.The mechanism of photocatalytic performance improvement was studied by free radical trapping experiments.The results show:?1?30 mg of SiO₂/F-TiO₂ photocalalysts with different Ti:Si is irradiated by sunlight for 2 h,the degradation ratio of 100 mL of 10 mg/L dyeing wastewater is basically complete,wherein the molar ratio of Ti:Si is 8:1,the degradation effect of methylene blue was the best.The other few pollutants had the best degradation effect when the molar ratio of Ti:Si was 4:1.When exposed to light for 3 h,tetracycline wastewater had the best degradation efficiency on tetracycline hydrochloride,and the degradation rate was 65.1%.The degradation of the two types of pollutants is in accordance with the Langmuri-Hinshelwood first-order kinetic model?L-H model?.Combined with SEM and BET analysis,The addition of SiO₂ hinders the growth of F-TiO₂ crystals,resulting in a decrease in sample particle size,a specific surface area increase from 86.32 to 135.99 m2/g which provides more adsorption site and the photocatalytic reaction center that can participate in the contaminant.By UV-Vis DRS,the addition of SiO₂ causes red-shift of the absorption edge,and the forbidden band width decreases from 2.82 eV to 2.73 eV.PL characterization shows that SiO₂/F-TiO₂ has more oxygen vacancies,which is more conducive to the production of active species,and the recombination rate of the electron null--hole is low.In the free radical trapping experiment,superoxide radicals?O₂·-?play a major role in the degradation process.?2?30mg NaOH-SiO₂/F-TiO₂ hydroxylated by NaOH has the best degradation efficiency when the concentration of NaOH is 6mol/L.In the degradation dyeing wastewater,methylene blue showed better removal effect than rhodamine B;the degradation effect in tetracycline wastewater was tetracycline hydrochloride?89.6%?>oxytetracycline?77.1%?>chlortetracycline hydrochloride?73.4%?,both higher than the degradation rate of SiO₂/F-TiO₂ without NaOH treatment,the effect increased by 20.9%-24.5%,both higher than the degradation rate of F-TiO₂,the effect increased by 29.5%-45.1%.Photocatalytic performance combined with evidence analysis shows that the SiO₂/F-TiO₂ were treated with NaOH increases the specific surface area of the catalyst to 166.32 m2/g.The visible light absorption threshold is red-shifted,the forbidden band width is 2.60 eV,and the recombination rate of the electron-hole pair is lowered.The photocatalytic degradation mechanism of NaOH-SiO₂/F-TiO₂ was further explored.The experimental results show that hydroxyl radicals?·OH?,holes?h +?and superoxide radicals?O₂·-?work together,and hydroxyl radicals?·OH?play a leading role.