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题名: 化学发光法研究纳米TiO2光催化体系产生的活性氧
作者: 王大彬
学位类别: 博士
答辩日期: 2015-05
授予单位: 中国科学院研究生院
授予地点: 北京
导师: 郭良宏
关键词: TiO2光催化 ; ROS ; 化学发光 ; 在线测定 ; 表面吸附O2•- ; 定量 ; 动力学研究,TiO2 photocatalysis, ROS, chemiluminescence, online detection, surface-adsorbed O2•-, quantification, kinetic study
其他题名: Research on the reactive oxygen species produced in photo-irradiated nano-TiO2 suspensions by chemiluminescence
学位专业: 环境科学
中文摘要:     纳米材料光催化氧化技术是一种新型的高效、节能、环保的高级氧化技术。活性氧(Reactive Oxygen Species, ROS)是光催化反应过程中产生的一类高活性物种,它的种类、数量、形态以及寿命等因素对污染物光降解反应的机制有重要影响,并最终决定了污染物的光降解效率。因此,对于光催化反应中产生的ROS进行种类和形态鉴别、数量实时动态测定以及生成和衰减动力学过程的研究,可以加深对光催化降解污染物机理的认知。然而,光催化反应中生成的ROS具有活性高、寿命短、浓度低且相互转化等特点,传统的分析测定技术(电子顺磁共振、光谱法、色谱法等)已无法满足上述研究要求,因此开发一种实时、在线、灵敏度高、特异性好的检测技术是探索以上科学问题的技术保证。
    化学发光(Chemiluminescence,CL)具有灵敏度高、反应瞬时性以及操作方便等优点,非常适合于不稳定活性氧自由基的测定;而连续流动技术具有测定速度快、精度高,易于实现在线监测等特点。因此我们将化学发光技术和连续流动技术相结合,开展了以下几方面的工作:
    构建了三种连续流动化学发光体系,实现了TiO2纳米颗粒光催化反应中生成的三种ROS(•OH、O2•-和H2O2)的在线测定。对于O2•-,选择鲁米诺作为化学发光探针,针对O2•-的特点,通过优化体系的流速、管路长度等参数,实现了O2•-在线动态测定;对于•OH,选择邻苯二甲酰肼作为探针捕获•OH,然后与氧化剂H2O2/K5Cu(HIO6)2混合产生化学发光信号,实现了•OH的在线动态测定;对于H2O2,将受光激发的TiO2胶体放置在黑暗处30 min消除O2•-的干扰,然后与luminol/K3Fe(CN)6混合,实现了H2O2的在线测定。在此基础上,对不同浓度、不同晶型的TiO2在光催化过程中产生的ROS进行了半定量比较研究,结果与传统方法一致,证明了该方法的可靠性。但是该方法操作简单、测定速度快且具有更高的灵敏度,在测定O2•-、•OH和H2O2时灵敏度分别比传统方法高3倍、1200倍和5倍。以罗丹明B作为模型研究其降解过程中起主要作用的活性氧,发现•OH是主要的反应物种,与已有报道一致,再次证明了方法的可靠性。因此该方法可以作为快速评价新型纳米材料光催化性能的技术手段;此外,该方法在测定•OH和O2•-时还具有准实时性的特点,这也为我们提供了一种研究光催化反应中活性氧动力学的方法。
    首次通过实验证实了光催化反应中生成的O2•-吸附在TiO2表面,从而延长了它的寿命,并研究了表面吸附O2•-的生成和衰减机制。在本工作中,对于光照前和光照后的TiO2胶体,我们通过鲁米诺静态注射化学发光实验分别测定了过滤得到的TiO2颗粒和滤液的化学发光强度,发现化学发光信号来自于光照后的TiO2纳米颗粒表面,表明TiO2光催化反应生成的O2•-倾向于吸附在其表面,而不是解离到溶液中,该过程延长了O2•-的寿命。理论计算(DFT)也证实了O2•-吸附在TiO2表面具有更低的能量状态。基于此,我们建立了O2•-在TiO2表面的生成和衰减模型,并推导了它的生成和衰减动力学公式。基于O2•-生成模型预测,通过连续流动化学发光法考察了不同TiO2浓度、光照强度、溶解氧浓度条件下O2•-的生成动力学过程,与公式拟合的生成趋势高度一致,证明了该模型能够准确反映O2•-在TiO2表面的生成机制。根据O2•-衰减模型预测,我们运用连续流动化学发光法考察了溶液pH值、TiO2晶型(锐钛矿、P25和金红石)以及金属氧化物纳米颗粒(TiO2、ZnO、SnO2、CeO2和Fe2O3)对O2•-衰减的影响,并通过动力学公式对它们进行拟合,得到了O2•-衰减的动力学参数(半衰期(t1/2)、表观解离速率常数(Kd’))。以上动力学计算结果进一步证明了O2•-倾向于吸附在纳米材料表面。该工作首次在常温常压条件下实现了纳米材料表面吸附O2•-的测定,为其它自由基的表面测定工作提供了方法借鉴和思路启发。
    运用连续流动化学发光技术在线定量测定了TiO2光催化反应生成的O2•-和H2O2,并研究了两者的生成动力学过程。在本工作中,使用鲁米诺作为化学发光探针,针对两者寿命不同,建立了连续流动化学发光在线定量检测方法。对于O2•-,由于其寿命短,且标准品不易得到,将光照后TiO2样品在10 s内与鲁米诺混合产生化学发光信号,根据鲁米诺与O2•-化学计量关系,通过鲁米诺浓度与化学发光强度的标准曲线将鲁米诺与O2•-的化学发光强度转换成O2•-的浓度,实现了O2•-间接定量;对于H2O2,将光照后的TiO2胶体放置在黑暗处30分钟后进行定量。该方法测得TiO2光催化产生O2•-和H2O2的浓度范围分别为7.5-30 nM和0.60-3.0 μM,检测限分别为1.95 nM和18.0 nM。根据生成模型研究O2•-和H2O2生成动力学过程发现,两者的生成均符合指数衰减函数增长,通过拟合计算,其生成速率常数(kf)分别为0.0653 nmol s-1和15.0 nmol s-1,表明在TiO2光催化反应中H2O2的生成速率高于O2•-,揭示了在TiO2光催化还原反应中氧气分子更倾向于得到两个光生电子生成H2O2。
    综上所述,本工作首先通过建立连续流动化学发光方法成功实现了TiO2纳米材料光催化反应生成的三种ROS的高灵敏度和高选择性的在线测定,为半导体纳米材料光催化性能评价提供了一种可靠的技术手段;其次证实了光催化反应生成的O2•-吸附在TiO2表面,从而使得其寿命延长,研究了不同实验条件对它的生成和衰减动力学过程的影响,从分子水平揭示了O2•-的生成和衰减机制;最后运用连续流动化学发光法在线定量测定了TiO2光催化生成的O2•-和H2O2,并定量比较了两者的生成动力学过程,揭示了TiO2光催化反应中光生电子与氧气分子的反应机制。
英文摘要:     Nanomaterial photocatalytic technology, with its advantage of high efficiency, energy saving and environment friendship, is a new promising advanced oxidation technology. Reactive oxygen species (ROS), a sort of highly active species, is produced in photocatalytic reactions. Its sorts, quantity, speciation and lifetime have large impact on the photodegradation mechanism of pollutants, and eventually determining the photocatalytic efficiency. Therefore, the identification of sorts and speciation, determination of quantity and kinetic study of formation and decay mechanism of ROS could give us deeper insights into the photodegradation mechanism of pollutants. However, ROS produced in photocatalytic reactions has the characteristics of high reactivity, short lifetime, low concentration and dynamic change constantly, and thus conventional methods (usually off-line detection) could not meet our demand. Therefore, it is necessary to develop a real-time, online, high-sensitivity and good-selectivity detection method to achieve our research goals.
    Chemiluminescence (CL), with the advantage of high sensitivity, instantaneity and simplicity of operation, is very suitable to detect the unstable ROS. Continuous flow technique (CF) is fast, accurate and easy to online monitor. Therefore, we developed the continuous flow chemiluminescence (CFCL) technique to online study the ROS produced in photo-irradiated nano-TiO2 suspensions. This work mainly includes three parts as follows:
    We developed three CFCL systems to online detect the ROS including •OH, O2•- and H2O2 produced in TiO2 photocatalytic reactions. To detect O2•-, luminol was mixed with TiO2 suspensions under the condition of optimized tubing length and flow rate before it entered the detection cell. For the detection of short-lived •OH, phthalhydrazide was added into the photoreactor to capture •OH, and then mixed with H2O2/K5Cu(HIO6)2 to produce CL. To detect H2O2, an irradiated TiO2 suspension was kept in darkness for 30 min, and then mixed with luminol/K3Fe(CN)6 to produce CL. For a given ROS produced by a series of P25 concentrations and three kinds of TiO2 (anatase, P25 and rutile), a comparison between CFCL and conventional method showed good agreement, confirming the reliability of this method. But the CFCL methods were simpler, faster and more sensitivive than conventional methods (approximately 3-, 1200-, and 5-fold higher than the conventional method for O2•-, •OH and H2O2). Using the Rhodamine B as the model pollutant to unveil which kind of ROS mainly responsible for the photodegradation, it is found that •OH was the main oxidant in the photodegradation of Rhodamine B, in accordance with the previous report. This further confirmed the reliability of our established method. Therefore, it can be used to rapidly evaluate the performance of ROS generation by semiconductor nanomaterials.  In addition, the CFCL method is also almost-real-time, and thus can be used to study the kinetic process of ROS during the photocatalytic reactions.
    The surface-adsorbed O2•- (O2,s•-) produced in TiO2 photocatalytic reactions was identified, thereby prolonging its lifetime, and the generation and decay mechanism of O2,s•- was studied. In this work, the TiO2 suspension was separated before and after the UV irradiation, obtaining the nanoparticles and filtrate respectively. Then the CL intensity of these nanoparticles and filtrate was produced by the static-injection CL method with luminol probe. By comparing the CL intensity from the nanoparticles and filtrate before and after UV irradiation, we found that the O2•- produced in TiO2 photocatalytic reactions was preferentially adsorbed on TiO2 surface, not desorbed into the solutions. This increased the stability of O2•- in TiO2 suspension, and thus prolong the lifetime of O2•-. Density function theory (DFT) calculation also confirmed that the O2•- adsorbed on TiO2 surface had lower energy state than in solution. Based on this, the model of formation and decay of O2,s•- was proposed, and their kinetic formulas were deduced. According to the formation model, the effect of TiO2 concentration, irradiation intensity, dissolved oxygen concentration on O2,s•- formation was investigated. The results were in accordance with the model prediction, confirming the model accurately reflecting the formation mechanism of O2,s•-. Based on the decay model of O2,s•-, the decay process of O2,s•- was simulated under different conditions including the solution pH, crystalline structure of TiO2 and the sorts of metal oxides nanoparticles, and thus kinetic parameters of O2,s•-, the half life (t1/2) and apparent dissociation constant (Kd’), were calculated. The calculated results further confirmed the adsorption of O2•- on TiO2 surface. To our best knowledge, this is the first time to identify the surface-adsorbed O2•- in TiO2 suspensions under the condition of normal pressure and temperature.
    The O2•- and H2O2 produced in TiO2 photocatalytic reactions were online quantified by the CFCL methods, and the kinetic process of both them was quantitatively studied. In this work, according to the different lifetime of O2•- and H2O2, online quantification of O2•- and H2O2 was successfully achieved based on the two established continuous flow chemiluminescence (CFCL) methods using luminol as the chemiluminescence probe. For O2•-, the CL intensity was detected by mixing the irradiated TiO2 suspension containing O2•- with luminol within 10 s due to its short lifetime and poor stability, and thus the concentration of O2•- was indirectly quantified according to the linear relationship between the CL intensity and the luminol concentration. For H2O2 detection with the luminol/K3Fe(CN)6 system, the CL intensity was detected after the irradiated TiO2 suspension was placed in darkness for 30 min to ensure the complete disappearance of O2•-, and the concentration of H2O2 produced was directly quantified by the calibration curve between the CL intensity and H2O2 concentration. The concentration of O2•- and H2O2 was determined approximately in the range of 7.5-30 nM and 0.60-3.0 μM, and the detection limit was 1.95 nM and 18.0 nM respectively. Based on the proposed models of O2•- and H2O2 formation, the kinetic process of O2•- and H2O2 formation was studied, and both of them followed the increase of exponential decay function. By simulating the time-dependent concentration change of O2•- and H2O2 with their kinetic formulas, the formation rate constants (kf) of O2•- and H2O2 were calculated to be 0.0653 nmol s-1, and 15.0 nmol s-1 respectively. The formation rate of H2O2 was higher than that of O2•- in TiO2 photocatalytic reactions, indicating that O2 was preferentially reduced to H2O2 by obtaining two photogenerated electrons in TiO2 photocatalytic reactions.
    In general, we firstly developed CFCL technique to achieve the high-sensitivity and good-selectivity online detection of three ROS produced in TiO2 photocatalytic reactions. This provided a convenient tool for the evaluation of photocatalytic performance of semiconductor nanomaterials. Secondly, the O2•- produced in TiO2 photocatalytic reactions was identified to adsorb on TiO2 surface, prolonging its lifetime, and the kinetic process of its formation and decay was systematically studied under different conditions, unveiling the formation and decay mechanism of the O2•- in molecular level. Finally the O2•- and H2O2 produced in TiO2 photocatalytic reactions was online quantified by the CFCL method, and the formation kinetic of both them was quantitatively compared, elucidating the reaction mechanism between the photogenerated electron and oxygen molecule in TiO2 photocatalytic reactions.
   
内容类型: 学位论文
URI标识: http://ir.rcees.ac.cn/handle/311016/34379
Appears in Collections:环境化学与生态毒理学国家重点实验室_学位论文

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王大彬. 化学发光法研究纳米TiO2光催化体系产生的活性氧[D]. 北京. 中国科学院研究生院. 2015.
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