【作者】 孙旭辉；

【导师】 马军；

【作者基本信息】 哈尔滨工业大学 ， 城市水资源， 2010， 博士

【摘要】 利用光催化作用进行水处理的核心任务是寻找性能优良的光催化剂,光催化剂的可见光响应程度是光催化技术实现太阳能的充分利用,走向实用化的关键,所以具备可见光响应的高效光催化剂的筛选及制备是光催化研究的核心课题。Bi和Ti都因廉价无毒被称为“绿色金属”,Bi₂O₃和TiO₂复合可形成具有多种晶相结构的复合氧化物,钛酸铋晶相的一种Bi₂₀TiO₃₂尚未被系统研究过,本文制备并表征了钛酸铋Bi₂₀TiO₃₂,研究了其光催化降解有机物的性能和机理。首先用溶胶凝胶法制备光催化剂Bi₂₀TiO₃₂。实验结果表明,在前躯体Bi/Ti比例为M时,300℃焙烧30分钟制备的催化剂—Bi₂₀TiO₃₂主峰（201晶面）成长较好,其它各个晶面均发育良好,且杂质峰较少。焙烧温度过高或保温时间过长都会导致氧丢失,引起晶相的转变;其它Bi/Ti前躯体比例则会使晶体发育不全或杂质过多。经表征该催化剂Bi₂₀TiO₃₂为锥形,颗粒大小约为90nm,比较容易聚结,BET比表面积为7.96m2/g,等电点pH值为7.9。紫外可见漫反射光谱和光电压光谱检测显示,所制备的Bi₂₀TiO₃₂具有高度可见光响应,响应波长达530nm,带隙能较窄为2.34eV。经计算其价带约为2.58V,价带空穴电位足以氧化有机物,导带0.24V,经测试费米能级在0.622V处,比较接近导带,电子逸出功小,催化剂给电子能力强。催化剂活性比较实验表明,所制备的钛酸铋Bi₂₀TiO₃₂和混晶钛酸铋相比,Bi₂₀TiO₃₂吸附能力较强,催化降解有机污染物性能也优于混晶催化剂。在相同催化剂质量的前提下,Bi₂₀TiO₃₂与P-25相比,在氙灯照射下降解甲基橙、苯酚时,P-25表观速率常数较大,由于Bi₂₀TiO₃₂比表面积较小,它的比活性大于P-25。而在波长大于400nm的可见光照射下,Bi₂₀TiO₃₂光催化降解苯酚、甲基橙的速度远远大于P-25。Bi₂₀TiO₃₂降解苯酚的表观速率常数为0.0133min^-1,是P-25(0.0004min^-1)的33.25倍,降解甲基橙的表观速率常数为0.0119min^-1,是P-25(0.0038min^-1)的3.13倍。说明钛酸铋Bi₂₀TiO₃₂具有良好的可见光催化降解有机物的性能。催化剂的组成、晶体结构对光催化剂性能影响很大,外界因素也影响着催化剂的活性,优化这些条件可以更好地发挥催化剂的性能。实验发现短波长光照降解效果更好;提高光强度,有助于提高降解速率。pH值对降解效果的影响相当显著,对催化剂的影响也是多方面的,其中最重要的一方面是不同pH值下催化剂表面电荷不同,影响了对目标有机物的吸附,因而针对不同物质的光降解存在不同的最佳pH值。Bi₂₀TiO₃₂降解甲基橙在酸性条件下效果最佳,降解苯酚则是在中性条件较好。污染物浓度越低反应速率越快,说明光催化反应只适合于处理微污染水。向反应体系中曝气不利于降解强吸附的甲基橙、苯酚,因为气泡阻隔了反应物与催化剂的接触。反应温度在40℃时比20℃和60℃反应速度快,温度过低不利于分子运动和中间产物从催化剂表面尽快脱附,而高温又影响了物理吸附。外加过氧化氢量低于4%（V/V）时,抑制了对甲基橙的降解;加入过氧化氢量超过6%（V/V）时,浓度越大越能促进光催化降解反应的进行。自来水和江水中的离子抑制了对甲基橙的降解,却促进了亚甲基蓝的光催化降解,这可能与光催化降解机理有关。研究光催化作用机理的一个重要方面就是反应活性物种,这关系到反应位点和进攻有机物的位置,以及相应的不同的降解路径、降解产物。经质谱分析,对羟基苯乙醇（4-hydroxybenzylalcohol , HBA）的降解产物为对羟基苯乙醛（4-hydroxybenzaldehyde HBZ）,说明HBA被Bi₂₀TiO₃₂光催化降解是空穴和羟基自由基共同作用的结果。为考察Bi₂₀TiO₃₂光催化降解不同类型污染物活性物种的异同,目标污染物选择了阴离子有机物甲基橙、阳离子型染料亚甲基蓝和中性物苯酚,实验主要通过加入活性物种抑制剂的方法来进行。实验结果表明在去离子水中,强吸附的阴离子有机物甲基橙的光催化降解主要是光生空穴的直接氧化造成的,因而反应位点应在催化剂表面。超氧阴离子自由基对甲基橙的光催化降解也有贡献,但不是主要的活性物种。苯酚的光催化降解规律与甲基橙基本相同。弱吸附的阳离子型有机物亚甲基蓝的光催化降解中,羟基自由基是主要活性物种,反应可能发生在催化剂表面或是其附近的溶液中。在自来水和江水中,甲基橙和亚甲基蓝的降解速度变化规律不同,这与主要活性物种的变化有直接关系。由于自来水和江水中阴离子的竞争吸附,甲基橙对空穴的利用率迅速降低,甲基橙的降解速度也急剧下降,这时主要活性物种不再是空穴,而是次生的羟基自由基。亚甲基蓝则由于水中阴离子的媒介作用,能更好地吸附于催化剂表面,从而更有效地利用空穴的直接氧化作用,降解速度反而提高了,这时主要活性物种不再是羟基自由基,而是空穴。空穴的量子化效率高即浓度较高,所以降解速度显著加快。光催化技术要得到大规模工业化应用所需要克服的另一关键问题就是催化剂的失活。研究发现Bi₂₀TiO₃₂光催化剂在使用之后晶型未变,说明其结构比较稳定。采用乙醇在超声波中振荡清洗催化剂能较好地恢复其活性,说明失活主要是由于中间产物吸附所致,清除催化剂表面吸附的有机物即能较好地恢复活性。高温焙烧既去除了表面吸附的有机物,又使得晶型结构得以保持,是一种有效的再生方法。更多还原

【Abstract】 The important mission in using photocatalysis for the water treatment is to find photocatalysts with excellent performance. Preparation of visible light responsive catalyst is the key to make the best use of sunlight and put it into application, so the synthesis of high performance photocatalysts which is visible light responsive is the main subject.Bi and Ti are both cheap and non-poisonous metal which are called“green metal”. Compound of Bi₂O₃ and TiO₂ can form many crystalline phase. One of the bismuth titanate is Bi₂₀TiO₃₂, which has not been studied systematically yet. In this subject, bismuth titanate Bi₂₀TiO₃₂ was prepared and characterized. Its performance and mechanism for photocatalytic degradation of organic pollutants was discussed.First of all, Bi₂₀TiO₃₂ was prepared by sol-gel method. Optimum experiments demonstrate that when the precursor ratio of Bi/Ti is M, the catalyst crystal can be made by being calcined at 300℃for 30 minutes. Under this condition, the main crystalface of Bi₂₀TiO₃₂ （201） will grow better, and the other peaks of it also develop well, while impurity peaks are very small. Higher calcination temperature or longer retention time will result in the loss of oxygen, which causes the transformation of crystalline phase. Under other precursor ratio of Bi/Ti, Bi₂₀TiO₃₂ can not develop well or more impurity phase will grow up. The prepared catalyst Bi₂₀TiO₃₂ is characterized to be nanocones, with the size around 90nm, and it is easy to coagulate. Its BET surface is 7.96m2/g, and zero point charge pH is tested to be 7.9. The curves of UV-diffuse reflection spectra and surface photovoltage spectra indicates Bi₂₀TiO₃₂ is highly visible light responsive that its sensitive wavelength extends to 530nm, namely the energy gap is 2.34 eV. It was calculated that the valence band is at 2.58V, so the holes’potential there is high enough to oxidize organics. The conduction band is at 0.24V, and the Fermi level was tested to be at 0.622V, which is close to the conduction band. It means that electrons have low work function and the catalyst has strong ability to supply electrons.Experiments on the comparison of the catalytic activity of the prepared catalysts were made. The results show that Bi₂₀TiO₃₂ has stronger adsorption ability, and its catalytic performance is higher than the mingled bismuth titanate. Under the irradiation of xenon lamp and the same weight of catalyst dose, the apparent rate constant of oxidation by P25 is higher than that of Bi₂₀TiO₃₂ for the degradation of phenol and methyl orange. Due to smaller specific surface area of Bi₂₀TiO₃₂, the specific activity of Bi₂₀TiO₃₂ is higher than that of P-25. While under the visible light irradiation （>400nm）, the photodegradation speed of phenol and methyl orange by Bi₂₀TiO₃₂ is much faster than that achieved by P25. The apparent rate constant of degrading phenol by Bi₂₀TiO₃₂ is 0.0133min^-1, which is 33.25 times higher than that achieved by P25 (0.0004min^-1). The apparent rate constant of degrading methyl orange by Bi₂₀TiO₃₂ is 0.0119min^-1, which is 3.13 times higher than that achieved by P25 (0.0038min^-1). The experimental results demonstrate that Bi₂₀TiO₃₂ is qualified to be a favourable photocatalyst.The component and crystal structure of the photocatalyst exert a tremendous influence on the performance, while external factors also affect its activity, and optimization of the conditions will help to give a better place to play its role. It is found that being illuminated by the short wavelength light can get better degradation result. Enhancing the intensity of the light contribute to the degradation rate. pH value has remarkable impact on the degradation and on the catalyst in many aspects, the most important one of which is that the catalyst surface will be directly affected by the charge under different pH. That the catalyst is positively or negatively charged will affect its ability to attract organic pollutants, so for different types of organics, the optimum pH is different. Acidic condition is good for the degradation of methyl orange, while neutral is good for phenol oxidation. Aeration goes against the degradation of methyl orange or phenol, because the air bubbles cut off the contact between reactant and the catalyst. The optimum reaction temperature is 40℃, comparing with the result at 20℃and 60℃. Because lower temperature is disadvantageous for the molecular movement and for desorption of intermediate product from the surface. While higher temperature does not facilitate adsorption of the reactant. Adding less than 4% （V/V） of hydrogen peroxide restrained the degradation of methyl orange. When the hydrogen peroxide concentration is larger than 6% （V/V）, the more the hydrogen peroxide, the faster the photocatalyic degradation speed is. The degradation of methyl orange is depressed by the ions in the tap water and river water. On the contrary, the degradation speed of methylene blue is promoted in the tap water and river water,which may be related to the oxidation mechanism. The important aspect in the study of the mechanism of photocatalytic process is the active species generated during the oxidation, because it is related with the reaction site on the catalyst, the attacked position of organics, the corresponding degradation pathways and the degradation products. GC-MS analysis shows the photodegradation product of 4-hydroxybenzylalcohol （HBA） by Bi₂₀TiO₃₂ is 4-hydroxybenzaldehyde （HBZ）, which is the result of the cooperation of h+ and·OH.In order to investigate if there are different main active species for different pollutants in photodegradation by Bi₂₀TiO₃₂, three kinds of organics were chosen as the target pollutants, namely anion methyl orange, cation methylene blue and neutral phenol. The testing was made by adding scavenger of the active species. Experiments indicate that in deionized water, anion methyl orange has strong adsorption ability on the Bi₂₀TiO₃₂ catalyst surface, and holes are the main species, so the reaction site is on the surface of the catalyst. Superoxide anions also help to degrade methyl orange, but they are not the main species. It is basically the same with the case of phenol. Cation methylene blue is not easy to adsorbe onto the catalyst surface, and hydroxyl radicals are the main active species. The reaction site may be on the surface of the catalyst or in the solution near its surface.In tap water or river water, the degradation rate of methyl orange and methylene blue change differently, which has direct relation with the transformation of active species. In tap water or river water, owing to the competitive adsorption of anions, the utilization rate of holes is reduced promptly, so the degradation rate of methyl orange droped sharply. At this time, the main active species are not holes any more, but are secondary hydroxyl radicals. While due to the anions’medium function, methylene blue can adsorb onto the catalyst surface easily, and can be oxidized efficiently by holes. At this time, the main active species are holes instead of hydroxyl radicals. The quantization proportion of holes is higher, namely the concerntration of holes is larger, so the degradation rate speeds up.Another key problem for photocatalyst application is inactivation. It’s found that the crystal phase doesn’t change after reaction, which shows the structure of the catalyst is stable. Using ethanol and ultrasound can recover its activity, as illustrates that inactivation is just due to adsorption of intermediate products. Calcination can remove the intermediates and maintain the structure of the crystal, so it is also an effective method for the regeneration of catalyst.更多还原