N掺杂TiO₂的结构及可见光催化降解有机物研究

【作者】 唐玉朝；

【导师】 俞汉青；

【作者基本信息】 中国科学技术大学 ， 环境科学， 2008， 博士

【摘要】 本论文研究了以几种不同方法制备氮掺杂改性的TiO₂光催化剂,包括水解方法（N/TiO₂-H）、氨热还原方法（N/TiO₂-A）、机械化学方法（N/TiO₂-M）和尿素混合热处理方法（N/TiO₂-T）。对氮掺杂催化剂的结构以XRD,XPS,UV-VIS-DRS等方法进行了表征,探讨了氮掺杂的方式和机理,研究了N/TiO₂对可见光的激发机制,并对N/TiO₂光催化剂的对降解有机污染物的反应活性和机理进行了初步研究。UV-VIS-DRS分析掺杂N/TiO₂结果表明,N/TiO₂-H和N/TiO₂-T两种催化剂在490nm处有吸收带边,可见光激发途径是掺杂的N以填隙方式形成的杂质能级吸收电子发生的跃迁引起的;而N/TiO₂-A和N/TiO₂-M两种方法得到的N/TiO₂在整个可见光区域内具有可见光吸收,没有显著的可见光吸收带边,其对可见光的激发途径是掺杂N和氧空缺共同作用的结果。理论计算的N杂质能级位于价带上0.75eV,与实验观察到的吸收带边结果十分吻合。掺杂浓度越大,可见光吸收能力越强。掺杂的N在TiO₂内不稳定,高温煅烧会破坏掺杂,光催化反应过程中的高活性自由基也会导致N被氧化而脱离TiO₂母体。XPS分析结果支持了间隙掺杂的结论,除N/TiO₂-H的N1s电子结合能在398.4eV外,其他三种N/TiO₂催化剂的N1s电子结合能都在399.4-399.7eV之间,属于以填隙N原子存在形式的间隙掺杂。此外,几种掺杂N/TiO₂的Ti 2P电子结合能与纯TiO₂相比增加了0.3-0.6eV,O1s电子结合能也增加了0.2-0.5eV,这些结果也说明了N掺杂是间隙形式的掺杂。N掺杂的机理是掺杂过程中形成的NH₃或掺杂前驱物的N原子首先与Ti原子连接,进一步在热处理时掺杂进入晶格间隙。N/TiO₂-H和N/TiO₂-T具有较好的可见光催化降解有机物的活性,500℃煅烧样品,前者在太阳光下对甲基橙,4.5h反应脱色率最高可以达到69.6%;后者在太阳光下对甲基橙经过290min反应可脱色70.4%。而N/TiO₂-M和N/TiO₂-A活性一般,除少数样品具有较好可见光催化反应活性,其他样品几乎没有显著可见光活性。N/TiO₂-M和N/TiO₂-A活性较低的原因是掺杂过程形成了过多的氧空位。不同掺杂浓度对活性影响研究结果表明,光催化反应存在最佳的N掺杂浓度,如N/TiO₂-T催化剂制备条件为0.024g尿素/3.24gTi（OH）₄的样品可见光下对苯酚降解活性最高。对有机污染物的降解途径进行了分析,发现各种有机物经过较长时间的可见光反应,都可以被氧化分解。经过5h的可见光催化反应,苯酚、亚甲基蓝和水杨酸等降解产物在UV-Vis光谱上都无吸收,说明有机物的发色基团遭到破坏,同时对这三种有机物的总有机碳（TOC）测定结果说明,三者TOC降低分别达到78.9%、62.8%和96.0%,特别是水杨酸几乎达到彻底矿化的程度,证明了可见光N/TiO₂光催化反应是高级氧化过程。自由基或空穴俘获剂对可见光下有机物降解存在影响,除草酸对亚甲基蓝由于能够促进吸附而促进了光催化降解速率,俘获剂CH₃OH和NaHCO₃对污染物的降解都存在显著的抑制作用。如可见光对甲基橙的降解在有CH₃OH和无CH₃OH的条件下,经过90min反应,脱色率分别为63.3%和4.2%。同样的,对苯酚的经过180min的可见光反应降解为26.7%;而相同条件下存在CH₃OH时仅降解了1.0%,由此可见,CH₃OH对可见光催化反应抑制作用非常显著,说明了可见光下N/TiO₂主要是形成HO·自由基的反应机理。进一步以二甲基亚砜和水杨酸为分子探针进行了系统中HO·自由基的测定,结果也证明了N/TiO₂的反应过程中形成HO·自由基是可见光催化反应的主要途径。更多还原

【Abstract】 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.更多还原