【作者】 欧延；

【导师】 廖代伟；

【作者基本信息】 厦门大学 ， 物理化学， 2007， 博士

【摘要】 光催化学科是化学、物理、材料和环境等多学科融合形成的一门新兴交叉学科。从发展现状看,光催化大致可分为能源光催化（光解水制氢,光催化可再生资源制氢）和环境光催化（环境污染治理高级氧化技术）两个主要研究领域。这两个研究领域都与社会和经济的可持续发展密切相关,因而具有广阔的应用前景和巨大的社会、经济效益。通过多种改性手段改善催化剂的活性,提高催化剂的太阳光利用率是将这一技术推向实际应用的重要环节。近年来,一些文献报道了用溶胶-凝胶法将碳纳米管（MWNTs）引入到二氧化钛光催化剂中,发现这种复合光催化剂有着很好的光催化氧化有机物活性。经过详细研究表明,认为是碳纳米管和二氧化钛之间的协同效应促使二氧化钛的光生电子转移到碳纳米管上,延长了光生电子-空穴对的寿命,同时还使该复合催化剂有可见光吸收能力。本文设想类似的碳纳米管和二氧化钛间的相互作用对于光解水制氢体系也同样有效。为了证实假设,本文首先考察了采用通常的溶胶-凝胶法来制备碳纳米管和二氧化钛复合催化剂,然后浸渍负载金属Ni,测量其光催化还原水制氢的活性。实验发现,碳纳米管掺杂会阻止TiO₂的相变和粒径长大;碳纳米管的掺杂量增加,催化剂的比表面积随之增大;当碳纳米管掺杂量为5%并负载1%的金属Ni时,复合催化剂的紫外光催化还原水制氢活性最大,达到122μmol/h,比未掺杂时提高了近一倍。而且掺杂碳纳米管后的TiO₂光催化活性普遍提高,说明碳纳米管与二氧化钛之间确实存在着某种相互作用直接影响了光催化反应的过程。因为这种相互作用是发生在两者的紧密连接处,所以我们推测尽量增加两者间的连接点应该有利于光催化活性的提高。在确定了研究方向后,本文进一步研究了自行设计的聚合络合法-化学气相沉积法来制备复合催化剂。之所以这样设计是巧妙利用Ni既可作为碳纳米管生长点,又可作为助催化剂生成氢气。选用聚合络合法能将Ni均匀地分散在TiO₂基体中,通过高温煅烧生成NiTiO₃,再用氢气可将其从体相中还原出来,到表面成细小的金属Ni作为碳纳米管生长点。由于常规的甲烷裂解化学气相沉积法主要是用来制备碳纳米管的,所以都采用一步生长法希望碳纳米管的产率能尽量高。但是按照前面的设计可以知道碳纳米管并不需要长太长,而是碳纳米管与二氧化钛的接触点应该尽量多些,增加二者的协同效应,提高光催化活性。为了达到该目的,本文研究了利用氢气甲烷化作用原位除去Ni表面的无定形碳,在TiO₂表面反复多次生长碳纳米管的可能性。SEM、Raman表征结果证明改进的化学气相沉积法能够在TiO₂表面生长出一层石墨化结构完整的,平均直径约为28 nm的多壁碳纳米管。在二次生长碳纳米管后,这种复合催化剂的紫外光和可见光催化还原水制氢活性达到最佳,分别高达2076μmol/h和38.1μmol/h。经过反复5次共20 h的稳定性测试,该复合催化剂的活性损失小于5%,无明显失活现象。结合XPS、UV-Vis DRS、PL等多种不同的表征手段,从半导体理论分析了复合催化剂活性高的原因,认为碳纳米管的生长能有效地转移TiO₂导带的光生电子,也使得复合催化剂的紫外光和可见光吸收能力大大增加,同时该制备方法中的氢还原步骤生成的Ti³⁺也有利于光催化活性的提高。在上述研究的基础上,在本论文还试图从分子水平上解释Ni系复合催化剂光催化制氢反应的活化能变化及其表面氢气产生的微观反应过程。通过改变不同的反应温度,计算出紫外光激发下二次生长碳纳米管的复合催化剂表观反应活化能为18.7 KJ/mol,表观量子产率最高为4.8%。动力学实验表明,当用甲醇作为牺牲试剂时,Ni系复合催化剂的催化脱氢活性比以其它醇类作为牺牲试剂高得多,顺序为甲醇＞乙醇＞异丙醇=正丙醇＞正丁醇。甲醇在催化剂表面的离解吸附是反应发生的第一步,与随后的被光生空穴氧化脱H步骤一起作为整个反应的速率控制步骤。同时,认为碳纳米管的生长从动力学上加快了甲醇离解吸附后的脱H步骤,使得复合催化剂的活性大大提高。根据半导体光催化剂的特点,光催化还原和氧化反应都是从一个侧面反映催化剂的性能,所以为了更全面地了解光催化剂的性能,有必要对复合催化剂的光催化还原和氧化两方面性能都进行研究。因此,本论文的后半部分着手从复合催化剂的氧化性能出发去考察其光催化氧化甲基橙活性。实验前必须先将Ni系复合催化剂中的金属Ni用酸洗去,因为金属Ni会与甲基橙抢夺光生的氧化自由基从而大大降低其光催化氧化活性。接下来,进一步的活性测试结果表明,同样二次生长碳纳米管的复合催化剂光催化甲基橙脱色率也是最高,1.5小时的紫外光脱色率达到100%,反应动力学常数k_a为0.066 min^-1;3.5小时的可见光脱色率为99%,反应动力学常数k_a为0.017 min^-1。为了了解该光催化氧化过程的详细机理,即考察究竟是·O₂^-还是·OH引发反应,本文利用在反应体系中加入O₂、CCl₄、t-BuOH等添加剂,对比了它们对活性的影响。实验结果认为在可见光照射下复合催化剂是由碳纳米管作为光敏剂,受激传递光生电子给TiO₂导带,然后进一步生成·O₂^-主导可见光催化氧化甲基橙反应;在紫外光照射下复合催化剂是由TiO₂受激生成价带空穴和导带电子,价带空穴进一步生成·OH,导带电子进一步生成·O2^-,两者共同主导紫外光催化氧化甲基橙反应。更多还原

【Abstract】 Photocatalysis science is a rising cross-disciplinary science,which integrates the multi-disciplinarity of chemistry,physics,materials,environment and so on.From the development status quo,photocatalysis can be broadly divided into two major research fields:one is energy photocatalysis（photocatalytic hydrogen production from splitting water,that is photocatalytic hydrogen production from renewable resources） and the other is environmental photocatalysis（advanced oxidation technology in environmental pollution field）.These two research fields are so closely related to the sustainable development of society and economy that they have broad application prospects and enormous social and economic benefits.Through a variety of means to improve the photocatalytic activity and increase the utilization of sunlight is the important step to make this technology come true.In recent years,some papers reported multi-wall carbon nanotubes（MWNTs） and TiO₂ composite catalyst has a good photocatalytic activity for oxidation organic compounds.MWNTs introduced into TiO₂ matrix by Sol-gel method will produce the synergy effect between MWNTs and TiO₂.The effect is of great benefit to the photoelectron transfer from conduction band of TiO₂ to MWNTs,which can elong the life-span of e^--h⁺ pair.In this paper,we supposed the synergy effect will be equally effective to photocatalytic hydrogen production from splitting water.In order to confirm that idea,we first investigated the photocatalytic activity of hydrogen production for MWNT-TiO₂ composite catalyst,which was prepared by same Sol-gel method as reported.It was found that doped MWNTs will stop phase transition and growing up of TiO₂;With increasing of MWNTs content in composite catalyst,their surface area are also increased;When the doped amount of MWNTs is 5%and co-load amount of metal Ni is 1%,under UV light irradiation,the photocatalytic activity reaches the maximum of 122μmol/h which is almost twice than the activity of raw TiO₂:Ni.Furthermore,the result that activities of MWNT-TiO₂:Ni series are generally increased by doped MWNTs shows there is a certain interaction between MWNTs and TiO₂ directly impacting on the photocatalytic reaction process of hydrogen production.So maximizing the joint between both should help to improve the photocatalytic activity.After determining the research interest,the paper further studied the photocatalytic activity of composite catalyst made by self-designed PCM-CVD method.The role of metal Ni in catalyst is not only as the growing point of MWNTs but also as a cocatalyst for hydrogen generation.The adoptive PCM can make Ni evenly dispersed in TiO₂ matrix and then reduced into fine granules on the surface by hydrogen.On next step,the way of mutli-growth of MWNTs was taken in order to reach our purpose for increasing the joints as more as possible.During each growth period,hydrogen was used to get rid of amorphous carbon by methanation.SEM and Raman results show MWNTs with the average diameter of ca.28 nm and good graphite structure can be grown on the surface of TiO₂ by modified CVD method. After second growth period of MWNTs,the photocatalytic activities of composite catalyst reached the maximum,2076μmol/h and 38.1μmol/h under UV light and visible light irradiation respectively.At the same time,the composite catalyst did not lost much activity after 20 h repeat test.Combined with a lot of means of XPS, UV-Vis DRS and PL,a systematical investigation was conducted to analyse the reason of high activity of MWNT-TiO₂ composite catalyst.The key contributions of grown MWNTs are to transfer photoelectron and increase the photo absorbance.Ti³⁺ appearance in the preparation process is good for improving photoactivity as well.On the basis of above study,this paper further attempts to explain the reaction process of hydrogen production according to kinetic regulation.When methanol acts as sacrifice reagent the activity of composite catalyst is higher than other alcohol’s. The activity sequence of alcohols is methanol＞ethanol＞i-propanol = n-propanol＞butanol.Methanol dissociation adsorption on the catalyst surface is the first step of whole photoreaction.Along with the subsequent oxidation step it control the whole reaction speed.Furthermore,MWNTs growth speeds up the desorpion step of H from methanol dissociation adsorption,which greatly enhances the photocatalytic activity.In the second part of this paper,we took methyl orange（MO） as a model for azo dye and discussed about photocatalytic oxidation performance of MO by composite catalyst.Before the test,metal Ni should be rinsed out by diluted acid because it would snatch the life of free radicals so as to reduce the photocatalytic oxidation activity.Finally,the kinetics constants ka is 0.066 min-1 under UV light irradiation and 0.017 min^-1 under visible light irradiation repectively.Through investigating the effects of dissolved oxygen,CCl₄,t-butyl alcohol on the decolorization of MO,the mechanisms are proposed that the superoxides（·O₂^-） mediated oxidation pathways are dominant for cleavage of azo bond of MO under visible light irradiation,whereas both of·O₂ and·OH mediated oxidation pathways are dominant under UV light irradiation. In the end,charge transfer process on MWNT-TiO₂ composite catalyst and the implication to photocatalytic reaction pathway were discussed.更多还原