【作者】 姜芳婷；

【导师】 路勇；

【作者基本信息】 华东师范大学 ， 物理化学， 2010， 博士

【摘要】 碳纳米管由于具有大比表面积、独特的结构、化学稳定性和微结构及表面物化性质的多变性和可控性,在多相催化、电催化、电极材料等方面有着巨大的应用前景。然而,由于制备的碳纳米管多是粉体状的,在实际应用中存在成型问题,需要加入高分子粘接剂高压压铸成型,随后高温碳化处理。此过程不但产生NO、CO等有毒气体,还造成碳纳米管比表面积有效利用率的下降。制备整体式结构碳纳米管复合材料是解决这一问题的最佳选择。本论文提出了一种以具有三维网络结构的整体式烧结金属微米纤维（SMF_M）为催化剂、高温下裂解含碳化合物制备整体式跨尺度结构复合材料CNTs/SMF_M的方法。利用N₂吸附-脱附等温线（N₂ adsorption-desorption isothemal）、扫描电镜（SEM）、投射电镜（TEM）、拉曼光谱（Raman）、热重分析（TG）等表征手段对复合材料的性质进行了考察,并采用循环伏安、恒电流充放电和交流阻抗谱法对复合材料的电化学性质进行了研究。在此基础上,考察了复合材料CNTs/SMF_Ni在以下几个方面的应用:双电层电容器电极脱除水中金属离子Na⁺和Cr³⁺的能力;电催化氧化对甲氧基甲苯（茴香醚）合成对甲氧基苯甲醛（茴香醛）反应的活性,即对茴香醚转化率、茴香醛选择性和反应电流效率的影响;催化乙苯氧化脱氢合成苯乙烯反应的活性。第一部分内容是以具有开放的三维网络结构、空隙率任意可调、大表面积、可裁剪成任意形状而不影响其物化性质的烧结金属微纤（SMF_M）为催化剂,高温裂解含碳化合物制备整体式跨尺度结构复合材料CNTs/SMF_M及其性质表征。此材料具有尺度跨越微观、介观和纳米级的整体式结构特点,使碳纳米管在应用过程中的成型问题得以解决;复合材料仍具有SMF_M的三维网络结构特性,提高了碳纳米管表面的可接近性;复合材料具有良好的传质、传热和导电性能,为其在电极材料方面的应用提供了不可或缺的保证;大比表面积和特殊的表面化学性质使其作为催化剂时具有较高的催化活性。通过SEM、TEM的表征信息,发现催化裂解不同碳源制备的复合材料CNTs/SMF_Ni的结构存在一定的差异。在本文实验条件下,SMF_Ni催化裂解乙烯制备的复合材料整体式结构牢固性最好,在甲醇中超声处理10 min,复合材料的损失率低于5 wt%,厚度为3～4μm的碳纳米管层均匀、牢固地根植于SMF_Ni,其孔径主要分布在40 nm左右,比表面积在100 m²/g以上,顶端为开口状,管壁石墨碳层取向和管轴线呈15～20°角,管壁厚度为10～15 nm;甲烷制备的复合材料中碳纳米管管壁较厚,顶端闭口状,并含有大量的颗粒碳,整体式结构牢固性较好;而醇类制备的复合材料中含有大量热解产生的颗粒碳,整体式结构牢固性非常差,甲醇中超声处理10 min,复合材料的损失率高于20 wt%。SMF_SS催化裂解乙烯制备的复合材料CNTs/SMF_SS的整体式结构牢固性差,甲醇中超声处理10 min,复合材料的损失率为20 wt%,易破碎,材料中碳纳米管管壁石墨碳层取向和管轴线平行,管壁厚度大约10 nm,管顶端为封闭状。第二部分研究了复合材料CNTs/SMF_Ni作为双电层电容器电极用于脱除水中金属离子Na⁺和Cr³⁺的能力。复合材料的整体式结构使其作为电极时,避免了碳纳米管成型过程造成的诸多问题,如比表面积利用率低、过程烦琐和产生有毒气体等;其三维网络结构、大孔隙率、中孔结构极大地提高了碳纳米管可接近性和比表面积的有效利用率;因不同碳源制备的复合材料具有不同的结构特点和表面性质,造成其脱除离子能力存在一定的差异。通过循环伏安、恒电流充放电和交流阻抗谱法对不同复合材料电化学性质的研究,发现裂解乙烯制备的复合材料具有最好的电容特性（比电容量80 F/g左右）,其循环伏安曲线具有良好的矩形特征,可瞬间完成充放电,且电容性质稳定,离子在其表面迁移阻力小。第三部分考察了复合材料CNTs/SMF_Ni在电化学催化氧化茴香醚合成茴香醛反应中的应用。与其他电极材料相比,复合材料CNTs/SMF_Ni作为阴极时,合成反应转化率、选择性和电流效率均达最佳,分别为96.9%、86.8%和82.8%。这主要是由于在此电催化氧化反应中,O₂电还原为HO₂^-的反应发生在阴极上,复合材料大比表面积、良好的吸附性能可以吸附大量的O₂分子,并且其表面上的缺陷、暴露的碳原子以及悬空的碳碳键均是O₂电还原活性中心,而其他材料如碳纤维、石墨碳均不具备此性质。不同气氛下的反应结果证明了复合材料CNTs/SMF_Ni电极电催化氧化反应是由有氧参与的反应历程控制的反应。第四部分研究了复合材料CNTs/SMF_Ni催化乙苯氧化脱氢合成苯乙烯的活性,实验结果表明复合材料表现出强催化活性,即使在较低温度下反应也具有很高的选择性。在400℃、空速1.25 h^-1、O₂/EB=1/1条件下,反应转化率可达42.1%,选择性高达99.7%。且整体式结构复合材料催化剂的使用避免了颗粒催化剂床层存在的堆积、堵塞问题。通过TPD研究发现材料的表面性质如含氧基团的类型和强度是影响其催化活性的一个重要因素。更多还原

【Abstract】 Carbon nanotubes （CNTs） have great potential for extensive scopes of applications in heterogeneous, electrocatalysis, energy storage, and electrode materials due to their high specific surface areas, desirable length-to-diameter ratio, good electrical conductivity, chemical inertness, and unique chemical and mechanical properties.However, the practical use of CNTs in the future technologies is particularly challenging, as robust macroscopic/monolithic structure of CNTs that can fully utilize the novel properties of the individual CNTs is required in the real world forms rather than the powders. One approach is to agglutinate CNTs by adding the binders and subsequently pressing at high pressure, yet inconsecutive contact between CNTs and the use of binders unfortunately introduce high internal resistance within the monolithic structure, occlusion of the surface area and low accessibility of CNTs surface that are detrimental for many applications （e.g., in electrochemistry）. An alternative way is to incorporate CNTs into or directly grow CNTs onto the substrates to form composites with well defined sizes/shapes, controlled growth orientations, and large-area structure.The monolithic mico-nano CNTs/SMFm composites are prepared by catalytic decomposition of carboneous compounds on SMF_m (SMF_ni and SMF_SS) with many beneficial properties in terms of large void volume, three-dimensional micro-grade pore structure and large surface area to volume ratio. The composites are characterized by N₂ adsorption-desorption, SEM, TEM, Raman and TG. These noval approaches present substantial potential in many applications such as capacitive deionization for water purification, electro-catalytic synthesis of p-MBA from oxidation of p-MT, and catalyst for oxidative dehydrogenation of ethylbenzene to styrene.In the first part, the CNTs/SMF_m composites, which are formed from various carbon source on different substrates (SMF_ni, SMF_ni modified by Al（NO₃）₃ or Mg（NO₃）₂ and SMF_SS), have shown merits of large specific surface areas and monolithic structure by anchoring of CNTs to the metal fibers. The CNTs/SMF_ni composites, which are formed from catalytic cracking of ethylene on SMF_ni at 750℃ with H₂ as dilute gas, have robust monolithic structure with 3～4μm CNTs layer uniformly anchoring to the SMF_ni surface. The weight is decreased by less than 5 wt% for the composites using ethylene as carbon source within 10-min supersonic treatment in methanol and then almost remains constant with prolonged treatment time （up to 30 min）, indicating that CNTs are rooted firmly on the surface of SMF_ni network. Nevertheless, the weight loss of more than 20 wt% is obverved for the SMF_Ni-composite using ethanol or n-butanol as carbon source and for SMF_SS-composite using ethylene as carbon source. The specific feature of the CNTs within CNTs/SMF_Ni （ethylene as carbon source） is large hollow core （without internal blockage）, entirely opened tip-like tube mouth, thin wall thickness of 8-10 nm, and stacked truncated cone morphology.In the second part, the CNTs/SMF_Ni composites were employed as double electro-layer capacitor electrodes for deionization. Capacitive electrosorption of 82.5μmol/g and 42.4μmol/g can be obtained for 50 ml 100 ppm NaCl solution and 50 ppm CrCl₃ solution at flow rate of 5.0 ml/min at work voltage of 1.2 V. The capacitance of the composites CNTs/SMF_Ni electrode is approximately 80 F/g in 5.0 M KOH at scan rate of 100 mV/s.In the third part, the electrocatalytic activity of the CNTs/SMF_ni composites as electrode for oxidation of p-MT to p-MBA is investigated. In comparison with other material electrodes such as Pt, carbon fiber and graphite, our composites electrode for synthesis of p-MBA in methanol using KF as supporting electrolyte delivers high conversion of 96.9% with selectivity of 86.8% using Pt anode with current of 0.13 A at 20℃in air atmosphere. Good current efficiency of 82.8% can be obtained.In the fourth part, oxidative dehydrogenation of ethylbenzene to styrene using the CNTs/SMF_Ni composites as catalysts is studied. The best conversion and selectivity of 42.1% and 99.7% can be obtained, respectively, using O₂/EB=1/1, WHSV=1.25 h^-1 at 400℃. TPD results reveal that the type and amount of the surface O-containing groups on the CNTs play key roles in selectively converting the ethylbenzene into styrene.更多还原