Preparation Of Halogen Element-doped TiO₂ And Photocatalysis Properites Research

Semiconductor TiO₂ can degrade organic pollutants in environment efficiently, which is difficult to be degraded by biologic methods. In the current work, pure TiO₂, halogen elements (F, Cl, Br and I)doped TiO₂ (HDT) were prepared by hydrothermal method, using cheapness stuff. Halogen elements doped TiO₂ supported on SiO₂ (HDTS) were prepared by in situ hydrothermal method. The crystal phase composition of catalysts was studied by using the X-ray powder diffraction (XRD). The elements component of catalysts was mensurated by using the X-ray photoelectron spectroscopy (XPS) and the X-ray fluorescence spectrometry (XRF). The surface area and pore structure was studied by using BET. The morphology was observed by using TEM. The optical absorbance performance of catalysts was studied by using the UV - visible diffuse reflectance absorption spectra (UV-Vis DRS). The photocatalytic degradation of phenol is used as model reaction to evaluate the photocatalytic activity of the photocatalysts. The effects of preparation and reaction conditions on the photocatalytic performances are studied.The HDT was prepared through a mild hydrothermal route, using Ti(SO₄)₂ as the precursor, containing halogen salt as the halogen source and deionized water as solvent. The photocatalytic activity of samples was influenced greatly by processing parameter. The influence factors were mainly the volume of containing halogen salt and the calcination temperature. Halogen elements, which were doped in TiO₂, can promote anatase phase formation, can inhibit anatase to rutile phase, can broaden the scope of TiO₂ optical absorption. When the amounts of the doping F, Cl, Br and I were moderate, halogen elements can enhance photocatalytic activity of TiO₂. The best doping amounts of F, Cl, Br and I were 30.0%, 15.0%, 40.0% and 100.0%, respectively. Calcined temperature could influence crystal size, phase transformation, light absorption property, distortion and expansion degree of crystal lattice and the ability of doped ions entering into lattice TiO₂, and then influenced the photocatalytic activity of TiO₂. The best calcination temperatures of F-doping, Cl-doping, Br-doping and I-doping TiO₂, were 80℃, 750℃, 700℃and 340℃, respectively.Before calcination at 340℃, the white I-doping TiO₂ showed a much better photocatalytic activity under full-spectrum light irradiation for its drastic photoabsorption in the range of wavelengths from 200 to 325 nm. After calcination at 340℃for two hours, the white I-doping changed into the purple nanoparticles, which were (I2)n encapsulated inside TiO₂. The purple nanoparticles exhibited an obvious photocatalytic activity under visible light illumination for its strong absorption in the range of wavelengths from 384 to 700 nm.HDTS was synthesized by a mild and easy hydrothermal route using Ti(SO4)2, containing halogen salt and commercial silica material. Besides the benefit of nanoparticles catalytic activity enhancement, HDTS can be separated from the suspension solution easier than TiO₂ nanoparticles. The SiO₂ can promote anatase phase formation, inhibit the transformation of anatase to rutile phase, inhibit the growth of TiO₂ nanoparticles, and then enhance the thermal stability of the catalyst. In addition, adding a certain amount of SiO₂ can also change the light absorption property of the photocatalysis, enhance the absorption of visible-light range. The surface area of HDT that is supported by SiO₂ is improved significantly, and then their absorption capacity is enhanced. Organic pollutants could be degraded fleetly because of the dual function of adsorption and photocatalysis.Then research on the process of photocatalytic decomposition of methylene orange, the activity of photocatalysis was compared and the law of photocatalytic decomposition was studied. The results of vacancy study and dark study show that the photocatalysis are noneffective. Under full-spectrum light irradiation, according to the order of FTi(30)080→ClTi(15)750→BrTi(40)700→ITi(100)340→ITi(100)080, the activities of the photocatalysis to decomposed methylene orange are more and more efficient. After silica gel loading, the order of activity does not change, but the activity is enlarged. When the HDT decomposed methylene orange, the model of the reaction is zero order dynamics characteristic. The speed constant K, which is apparent, concerns on the initial concentration of methyl orange. ITi(100)080 and ITiSi(100)080-2 have the better activity in a wide pH value scope. When the pH value is 3 or 7, the other photocatalysis have the best activity. Researching on the process of photocatalytic decomposition of phenol in seawater, the photocatalysis compatibility is inspected. ITi(100)340 and ITiSi(100)340-2 still maintained their high activity, but the other photocatalysis and P-25 did not. The activity of the HDTS is not better than that of HDT.