N Doping Of Tio <sub> 2 </ Sub> Structure And Visible Light Photocatalytic Degradation Of Organics

Nitrogen doped dioxide titanium photocatalysts(N/TiO₂) had been prepared by various methods:hydrolysis of tetrabutyl titanate(N/TiO_2-H),ammonia thermal treatment of titanium dioxide(N/TiO_2-A),mechanochemical treatment of titanium dioxide(N/TiO_2-M) and urea thermal treatment of Ti(OH)₄(N/TiO_2-T).The photo-catalysts were characterized by X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),and UV-Vis diffuse reflection spectra(UV-Vis-DRS).Mechanisms of nitrogen doping,structures and catalytic reaction activities of the catalysts were discussed.Mechanisms of electrons excitation by visible light and pollutants degradation process were researched also.Results of UV-Vis-DRS indicated there had obvious onset of visible light absorption at 490nm of catalysts N/TiO_2-H and N/TiO_2-T,which initiated by impurity energy level located at above valance band formed by doped nitrogen.Excitations from localized states to the conduction band account for the absorption edge shift toward lower energies(visible region) observed in the case of N/TiO₂ with respect to pure TiO₂.Mixture of visible light absorption of nitrogen doping and oxygen defect gave no obvious onset in the whole range between 400 and 800nm were observed of the samples N/TiO_2-A and N/TiO_2-M.Higher concentrations of nitrogen doped gave the stronger visible light absorption,the doped nitrogen was unsteady and it could be destroyed by high temperature process or even by oxidative radicals in the reaction.Results of XPS further proved the doped mechanism of nitrogen was in interstitial form. Binding energy of N ls at 398.4eV for N/TiO_2-H and range of 399.4eV to 399.7eV for N/TiO_2-T,N/TiO_2-A and N/TiO_2-M were observed which all corresponding to interstitial nitrogen.Binding energy of Ti 2p and O ls were increased about 0.3-0.6eV and 0.2-0.5eV respectively of various nitrogen doped TiO₂ with respect to pure TiO₂.These results indicated the doped mechanism of nitrogen in the TiO₂ was interstitial nitrogen but not sub-stitutional.Photocatalytic activities of N/TiO₂ were evaluated with the degradation of model pollutants under UV or visible light irradiation.N/TiO_2-H and N/TiO_2-T series catalysts had obvious visible light activities,for example,69.6%or 70.4%of 10 mg/L methyl orange could be degraded by N/TiO_2-H-500 or by N/TiO_2-T-500 in the presence of visible solar irradiation(light with wavelength above 420nm) respectively.But N/TiO_2-A and N/TiO_2-M series catalysts had very weak visible light activities;some of the catalysts even had no visible light activities although they shown excel UV activities.There had the optimum doping nitrogen concentration on photo-activity was tested,for example,the sample N/TiO_2-T prepared under conditions of 0.024g(NH)₂CO and 3.24gTi(OH)₄ had the most superior visible activity of degradation of phenol.Analysis products of the degradation of model organic pollutants had demonstrated all the tested substances could be mineralize to inorganic compounds if visible photoeatalytic reaction times long enough.After 5 hours visible light reaction,products of the degradation of phenol,methylene blue and salicylic acid had no UV-Visible absorbance demonstrated the chromogen group of their had been destroyed.Removed total organic carbon(TOC) of phenol,methylene blue and salicylic acid after 5 hours visible light reaction by N/TiO_2-T-500 achieved 78.9%,62.8%and 96.0%respectively.Espeeially for salicylic acid,almost thoroughly mineralization had been realization,these results had demonstrated photocatalysis process shown the advanced oxidative potential.Effect of radical or hole scavengers on photocatalytic efficiency had shown rnethanol, bicarbonate or oxalate could decrease the reaction rates in all tested conditions except the system of methylene blue degradation in the presence of oxalate as scavenger.For example, methyl orange could be degrade about 63.3%under solar catalytic reaction within 90rains in the system absent of methanol but only 4.2%when presence of which.Otherwise,26.7% of phenol could be degraded within 180mins reaction but only 1.0%if in the presence of methanol.These results demonstrated hydroxyl radical is the most reactive oxidative substance which initiated photo-degradation of organic compounds in the visible light catalytic reaction systems of N/TiO₂ because the methanol was an effective hydroxyl radical scavenger.Further experiments had detected the hydroxyl radical by dimethyl sulfoxide(DMSO) or salicylic acid as molecular probe,the results also demonstrated hydroxyl radical was the most important reactive oxidative substance in the visible photo-catalytic reaction systems of N/TiO₂.