【作者】 卢文新；

【导师】 周兴贵； 隋志军；

【作者基本信息】 华东理工大学 ， 化学工程， 2010， 博士

【摘要】 纳米碳纤维(CNFs)是一种具有广阔应用前景的新型碳材料。结构调控和连续化低成本制备是CNFs实现工业应用的技术关键。本文针对CO在Fe催化剂上歧化制备CNFs反应体系,研究了合成工艺条件及催化剂颗粒性质对CNFs生长过程速率的影响,采用TEM、SEM、XRD和N2物理吸附等方法对合成的CNFs进行了较为系统的结构表征,探索CNFs生长动力学特征与其形貌之间的关系,并通过合成过程中催化剂颗粒性质的变化讨论了这种关系的根源。最后对适用于CNFs连续合成的回转窑反应器进行了冷模实验研究和概念设计。本文主要研究结果简述如下：(1) H2在Fe催化CO歧化反应制备CNFs过程中具有非常重要的作用。TPD-MS表征以及理论分析结果表明反应气体中加入H2会改变CO在Fe催化剂表面的反应历程,从而促进CO在Fe催化剂上的歧化反应。另外,增加反应气体中的H2浓度,将使CNFs的最大生长速率先升高后下降,可以缩短CNFs生长的诱导期和催化剂的使用寿命,并对CNFs的形貌结构、织构性质和石墨化程度产生显著影响。(2)催化剂还原条件一定时,制备的CNFs形貌结构与其最大生长速率存在密切对应关系。随着CNFs最大生长速率的不断增加,CNFs的形貌结构先是从螺旋状到弹簧状,然后再到较直的CNFs,最后到无定形炭。通过改变反应气体中CO浓度、反应温度等操作条件可以控制CNFs生长速率,进而控制CNFs的形貌结构。(3) CNFs生长过程中,Fe催化剂颗粒首先破碎形成小的颗粒并形成碳化铁,并在不同的生长条件下形成不同的形貌。这一系列变化决定了生长过程诱导期的长短、最大速率的大小和失活速率的快慢,并同时决定了合成所得CNFs的形貌。(4)经过不同还原条件处理后,催化剂颗粒具有不同粒径和相态。即使在最大生长速率相同条件下,CNFs的形貌结构仍具有很大差别,这同样说明催化剂的颗粒性质是CNFs形貌结构决定性因素。适宜的还原条件对可控结构CNFs的合成具有重要作用。(5)通过BOC理论和过渡态理论计算,研究了Fe催化CO歧化制备CNFs的反应历程。利用微观反应动力学方法,建立了CO在Fe催化剂上歧化反应制备CNFs的动力学模型。建立的动力学模型可以预测不同反应条件下的CNFs的生长速率,并以此为基础控制和预测CNFs的形貌结构。模型计算结果表明,在Fe催化CO歧化制CNFs的过程中,CH4和H20生成的能垒较大,说明其难以生成,这与反应尾气中没有检测到CH4和H20的结果一致。(6)不同回转窑转速下,催化剂平均停留时间都随倾斜高度线性递减,并且转速较快时递减速率较快,而转速较低时递减速率较慢；不同回转窑倾斜高度时,催化剂平均停留时间与转速成反比关系,且当回转窑转速达到9 r/min以上后,转速对催化剂平均停留时间的影响可以忽略。催化剂停留时间分布的分散程度随回转窑反应器转速和倾斜高度的增加而减小。CNFs物料的动态休止角和流型基本不受回转窑转速和倾斜高度的影响,仅与CNFs物料在回转窑反应器内填充率的有关。更多还原

【Abstract】 Carbon nanofibers (CNFs) are novel structural carbon materials with good application potentials in many fields. A key factor to the successful application of this material relies on its large scale production with well defined structrures and low cost. In this thesis, the carbon nanofibers sytheiszed by CO disproportionation on iron catalyst are investigated. The influences of synthetic conditions and catalyst properties on the growth rate are explored. TEM, SEM, XRD and nitrogen physical adsorption are used to characterize the structural properties of the produced CNFs and a relationship between its kinetic characteristics and the morphology of CNFs is established. The underlying causes of the relationship are also discussed by characterizing the changes of the catalyst particles’structural properties. Finally, cold mold experiments and conceptual design are carried out on a rotary kiln reactor, which is suitable for continous growth of CNFs. The main results of this work are summarized as follows:(1) H2 concentration is a prominent factor that influences the syntheis of carbon nanofibers by CO disproportionation on iron catalyst. TPD-MS characterization and theoretical analysis results indicate that the presence of H2 in the feed gas may change the reaction shechme and can accelerate CO disproportionation on iron catalyst. The CNFs preparation experiment results show that the maximal growth rate first increases and then decreases with the increase of H2 concentration and the induction period and life time of the catalyst are also shortened at elevated H2 concentration. The morphology, texture and graphitization degree of as-synthesized CNFs are also found to be influenced by H2 concentration.(2) The morphology of CNFs is strongly related to the maximal growth rate in case of adoption of identical reduction conditions. As the maximal growth rate increases, the morphology changed from twist to helical, then to straight and tight helical and finally to amorphous. The maximal growth rate of CNFs can be controlled by adjusting the H2 concentration, CO concentration and temperature. And so is the morphology of CNFs.(3) During the process of CNFs synthesis, Fe particles first would be fragmentated to small particles and transformed to iron carbide. The morphology of the particles are determined by the synthetic conditions, which can affect the induction period, maximal reaction rate, deactivation rate and finally the morphology of the CNFs.(4) The catalyst particles have different particle sizes and composition after reduction under various conditions. As a result, CNFs have different morphologies and structural properties even though the maximal growth rates are identical. This result also reveals that the morphology and structure of CNFs are determined by the properties of catalyst particles. To preparation of CNFs with well defined structures, a proper reduction of the catalyst is very important.(5) The reaction scheme of CO disproportionation on iron catalyst is studied by using BOC and transition state theory. A kinetic model is developed using microkinetic method. The modeling results are found to fit the experiment data well and can be used for controlling and monitoring the morphology of produced CNFs. The modeling results also show the energy barriers for the formation of CH4 and H2O are very high and it is hard for CH4 and H2O to form in the reaction system, which is consistent with the experimental results.(6) The average residence time of catalyst in the rotary kiln decreases linearly with the increase of inclination height at both high and low rotating rates. The average residence time decreases faster at high rotating rate then slower at a low rotaing rate. At different inclination heights, the average residence time of catalyst is inversely proportional to the rotating rate until it reaches 9 r/min, above which the effect of rotating rate on the average residence time of catalyst can be neglected. The distribution density of residence time becomes more concentrated as the rotate speed and the inclination height increase.更多还原