【作者】 李元元；

【导师】 严有为；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2010， 博士

【摘要】 环境污染和能源问题制约了人类的可持续发展。半导体光催化技术能直接利用太阳能降解有机污染物或者分解水获取氢能,有望成为解决环境和能源问题的有效途径。半导体光催化技术的核心是获取高活性的可见光响应的半导体光催化材料。可见光响应是充分利用太阳能的前提,高活性是半导体光催化技术应用于实际的基础。本文以高效可见光活性催化剂的开发为目标,结合材料的表征、密度泛函理论的计算,研究了钙钛矿型可见光活性光催化剂的合成过程及其光催化活性的调控机制。采用溶胶凝胶法制备了系列钙钛矿氧化物LaBO3(B=Cr、Mn、Fe、Co和Ni)和TiO2粉末。分别在紫外线和可见光下,比较了合成粉末光催化降解甲基橙溶液的活性。结果表明,LaCoO3和LaNiO3在紫外光下比TiO2光催化活性更高,LaFeO3、LaCoO3和LaNiO3具备可见光催化活性且活性依次增强。采用密度泛函理论计算了五种钙钛矿结构氧化物的电子结构和能带结构,结果发现,LaCrO3的价带导带离散性好,但能隙较大；LaMnO3的能带结构显示金属特性,光生电子空穴对容易复合；LaFeO3和LaCoO3的导带离散性不好,不利于光生电子的迁移；LaNiO3的价带导带离散性好,且连续的价带位置更负,光生空穴的氧化能力更强。对五种钙钛矿结构氧化物的晶体结构研究表明,菱形结构的LaCoO3和LaNiO3较正交结构的LaCrO3, LaMnO3和LaFeO3的BO6氧八面体扭曲变形更严重,更有利于光生电子对的分离。因此,在上述5种钙钛矿结构的氧化物中,LaNiO3的电子结构、能带结构和晶体结构最有利于可见光激发和光生电子空穴对的分离,具有最高的可见光催化活性。研究了制备方法及制备工艺对LaNiO3的合成及光催化活性的影响。利用DTA-TG,XRD,SEM,TEM和FT-IR等对催化剂的合成过程及产物进行了表征。通过甲基橙溶液的可见光降解实验,研究了煅烧温度、煅烧时间等对LaNiO3的结构和催化活性的影响。结果表明,溶液燃烧法制备的LaNiO3粉末在可见光下的催化活性最高。600℃是LaNiO3相形成的最低温度,当在600℃下煅烧4h所得催化剂的光催化活性最高,在可见光下照射5h对10mg/L的甲基橙溶液的降解率达到74.9%,且催化剂具有很好的稳定性和重复使用性。XPS分析结果表明,在LaNiO3中存在大量氧空位,能有效降低光生电子空穴对复合几率。由氧空位产生的表面吸附氧在光照下能生成高活性的羟基自由基和超氧离子自由基。甲基橙分子在羟基自由基、超氧离子自由基和光生空穴的作用下被降解成二氧化碳和水等无害物质。为了进一步提高LaNiO3的光催化性能,研究了Ti元素掺杂对LaNiO3催化活性的影响,并采用溶液燃烧合成技术制备了LaNiO3光催化剂。结果表明,Ti掺杂减缓了溶液燃烧反应的剧烈程度；在650℃煅烧4 h能获得结晶完好,粒径小的LaNi1-xTixO3粉末。以甲基橙为模型化合物研究了Ti掺杂量对LaNi1-xTixO3粉末的合成及光催化活性的影响,结果表明,随着Ti掺杂量的增加,LaNi1-xTixO3粉末的光生电子空穴对复合几率先降低后升高；当Ti掺杂量为3.0 wt%时,电子空穴对复合几率达到最低值；由于LaNi1-xTixO3存在导致催化剂表面吸附氧量随掺杂量增加而减小。因此,LaNi1-xTixO3的催化效果随着Ti掺杂量的增加呈先增大后减小趋势,当Ti掺杂量为3.0wt%时,所得催化剂的催化效果最好。在可见光下照射5h后,LaNi1-xTixO3对甲基橙溶液(10 mg/L)的降解率高达92.2%。建立了光催化反应动力学方程,分别研究了LaNi0.97Ti0.03O3用量、甲基橙溶液的初始浓度对光催化反应过程的影响。结果表明,2g/L的LaNi0.97Ti0.03O3对初始浓度为5mg/L的甲基橙溶液浓度降解满足一级反应方程,当甲基橙溶液初始浓度高于10mg/L时,降解动力学方程为零级反应方程,且降解速率常数随甲基橙溶液初始浓度的增加而减小。当LaNi0.97Ti0.03O3投加量高于2g/L时,甲基橙溶液(10 mg/L)降解动力学方程满足一级反应动力学方程。研究了LaNiO3复合TiO2光催化材料的制备、结构表征和光催化性能。以分析纯TiO2粉末为原料,通过溶胶凝胶法制备了系列LaNiO3-TiO2复合氧化物,采用DTA-TG、XRD、TEM和BET等测试方法对催化剂的合成过程及其结构进行表征。结果表明,采用溶胶凝胶法在700℃能合成LaNiO3-TiO2复合氧化物,且产物结晶良好。TEM结果显示,纳米级的LaNiO3颗粒分散在不规则的微米级TiO2表面；BET结果显示,复合光催化剂比表面积高于TiO2比表面积。甲基橙溶液降解的光催化性能测试显示,TiO2复合LaNiO3后具备可见光催化活性,10 wt%LaNiO3含量的LaNiO3-TiO2具有最高的光催化活性,6 h甲基橙溶液降解率达55%,远远高于相同量LaNiO3对甲基橙溶液的降解率(10%)。根据能带结构计算和绝对电负性估算了LaNiO3和TiO2的价带和导带组成及位置,认为复合光催化剂活性提高的本质是P-N异质结的内电场及导带电势差促使了光生载流子的分离,降低了光生电子空穴对的复合几率。更多还原

【Abstract】 Environment and energy issues are still the bottle neck for the continual development of human being. Semiconductor photocatalysis is an advanced technology which can convert solar energy into chemical energy. Pollutants can be removed and hydrogen can be obtained via water-splitting in the presence of converted chemical energy. The focus of semiconductor photocatalysis is to develop visible light induced photocatalysts with high catalytic activity. Visible-light-induced activity is the base of exploiting solar energy. High efficiency makes its application to be possible. In this paper, we focus on developing visible light drived photocatalysts with high catalytic activity and exploring the controllable mechanism of photocatalytic activity by using material characterization and density functional theory (DFT).TiO2 and LaBO3 (B= Cr, Mn, Fe, Co and Ni) powders with perovskite structure have been prepared by sol-gel technique. The photocatalytic activity of the as-prepared samples has been evaluated by degradation of methyl orange solution under UV-light and Vis-Light irradiation. The results showed that the photocatalytic activities increased with the order:TiO2, LaNiO3 and LaCoO3 under UV light irradiation. Compared investigation of photocatalytic activities between the as-synthesized powders under visible light irradiation showed that the LaFeO3, LaCoO3 and LaNiO3 were visible-light-driven photocatalyst. Based on the DFT, the energy band structure of LaBO3 series oxidation has been calculated by CASTEP code. The results showed that LaCrO3 own large band gap and high discreteness valence band (VB) and conduction band (CB). DFT calculations of LaMnO3 showed the metal character, resulting high recombination of photo induced carrier. The splitting of CB for LaFeO3 and LaCoO3 degrade the transportation of electron. LaNiO3 own high discreteness VB and CB. The band potentials of LaNiO3 shift downwards, which indicates that photogenerated holes have stronger oxidative power. In the series oxidation, LaCoO3 and LaNiO3 stabilize in the rhombohedral structure. Compared with the LaMnO3, LaFeO3 and LaCrO3, which stabilize in orthorhombic structure, the BO6 octahedra own higher distortion. Because the internal dipole moments promote the charge separation, the LaCoO3 and LaNiO3 exhibited higher photocatalytic activity. The band structure, crystal structure and the degradation of MO solution indicate that LaNiO3 is the best efficient visible light induced photocatalyst in the series perovskite oxidation.The preparation, characterization and photocatalytic activity of LaNiO3 have been investigated. The results showed that the formation temperature of LaNiO3 phase is 600℃. The sample synthesized by solution combustion synthesis (SCS) and subsequent calcined at 600℃for 4 h own highest photocatalytic activity. The degradation percentage of methyl orange solution (MO,10 mg/L) after 5 h on the sample is 74.9%. The photocatalyst exhibits good stability and precipitation performance in aqueous solution. The XPS spectra of LaNiO3 showed that there are oxygen vacancies existing in the LaNiO3 lattice, which can promote the segregation of electron/hole pairs. In addition, the adsorption oxygen which induced by the oxygen vacancy can be transformed to high-activity substance such as HO·, HO2·,etc under light irradiation. The MO molecule can be degradated into CO2 and H2O in the presence of holes and such high-activity substance under visible light irradiation.The perovskite LaNi1-xTixO3 powder has been prepared by SCS. The synthesis process and structure characterization have been investigated. The results showed that the synthesis process of Ti-doped sample is more persistent and less acute exothermal than undoped sample. Crystalline LaNi1-xTixO3 with fined structure can be prepared after calcined at 650℃for 4 h. MO degradation test showed that the photocatalytic activity of Ti-doped samples first increased and then decreased with the Ti contents. The highest efficiency is observed when the sample calcined at 650℃for 4 h with 3 wt% Ti content. The degradation percentage of MO solution (10 mg/L) on the sample is as high as 92.2% after 5 h visible light irradiation. Combined with the results of PL and XPS spectra, the origin of enhanced performance of the Ti-doped oxidation is the effective separation of charge carriers. The kinetic study on the MO concentration (Co) and the addition concentration of LaNi0.97Ti0.03O3 (C’) have been performed. The results showed that, for 2 g/L LaNi0.97Ti0.03O3, the kinetic equation is a first-order reaction when C0=5 mg/L and zero-order reaction when C0>10 mg/L. The rate constant k decreased with the increasing of Co. For 10 mg/L MO solution, the kinetic equation is zero-order reaction when C’> 2g/L.The preparation, characterization and photocatalytic performance of LaNiO3 modified TiO2 composite have been investigated. The TiO2 is raw material, and LaNiO3-TiO2 composite have been prepared by using sol-gel technique. The crystal, morphology and optical properties have been characterized by DTA-TG, XRD, TEM, DRS and BET. The results showed that the crystal LaNiO3-TiO2 composite can be synthesized by sol-gel technique at 700℃. TEM showed that nano-size LaNiO3 particles dispersed on the surface of TiO2 particles. The surface area of the composites is large than TiO2. MO degradation test showed that the MO solution can be degradated in the presence of LaNiO3-TiO2 composites under visible light irradiation. The highest efficiency sample can be obtained when the LaNiO3 content is 10 wt%. The degradation of MO reaches to 55% after 6 h visible light irradiation. Compared with LaNiO3 (about 10%), the photocatalytic activity increased drasticly. Based on the DFT calculation, The CB and VB of LaNiO3 and TiO2 have been estimated by absoluted electronegativity. The origin of enhanced performance of the composite is the effective separation of electron/hole pairs caused by the P-N heterounction electric field and the electric potential difference between the CB of P-type LaNiO3 and N-type TiO2.更多还原