【作者】 丁玲；

【导师】 梁长海；

【作者基本信息】 大连理工大学 ， 化学工艺， 2009， 硕士

【摘要】 过渡金属碳化物在许多催化反应中表现出了类贵金属的性质,如涉氢反应,肼分解反应和异构化反应等,被誉为类贵金属化合物,引起了广泛的关注和研究。但是传统的过渡金属碳化物的制备需在高温下进行,反应条件极其苛刻,制备过程繁琐,存在制备得到的碳化物比表面积低,颗粒尺寸大且易团聚,暴露空气前需要通惰性气体钝化等问题。因此我们采用微波辅助的化学气相沉积法/热解法制备了高分散,粒径小的碳纳米管/活性炭负载碳化物催化剂,用微型反应器考察了其催化肼分解的反应性能;又将其作为铂催化剂的载体,引入燃料电池电极反应,研究了其对氧电化学还原的催化性能。主要研究成果如下:以羰基化合物为前体,采用微波辅助的化学气相沉积法制备了碳纳米管负载碳化钨、碳化钼催化剂。微波加热法具有加热温度高、反应速度快和合成的催化剂颗粒小等优点。XRD测试结果表明制备出了碳化钨、碳化钼。透射电镜观察结果表明碳化钨、碳化钼在碳纳米管外壁上分布均匀,颗粒大小为2～5 nm。随着金属担载量的增加,碳化钨颗粒在碳纳米管上的分布更加密集。当金属钨的担载量高达50%时,可以制备出碳化钨包覆碳纳米管复合材料。与传统的直接热解法、碳热氢还原法、程序升温法等方法相比,微波辅助的化学气相沉积法是一种更高效、节能和环保的新方法。把制备得到的碳纳米管负载碳化钨催化剂用于肼分解反应,表现了较好的催化性能。把碳纳米管负载的碳化钨复合材料作为铂催化剂的载体,采用乙二醇还原法制备了Pt@WC_x/CNTs催化剂。XRD表征显示样品中铂和碳化钨共存,高倍透射电镜观察发现,铂和碳化钨均匀的分布在碳纳米管外壁上,粒径小于5 nm。在酸性溶液中,考察了样品的氧还原催化性能,结果显示碳纳米管负载的碳化钨催化剂对氧还原也有活性,而Pt@WC_x/CNTs催化剂对氧还原有良好的催化性能,与传统的Pt/CNTs催化剂相比,初始还原电位正移了50 mV,并且具有很好的氧还原选择性和抗甲醇中毒能力。以七钼酸铵和六次甲基四胺形成的钼盐络合物为前体,采用微波辅助加热,一步热分解制备了活性炭负载的碳化钼催化剂。结果表明,钼盐络合物前体在水溶液中的形成是必要步骤,前体中有过量六次甲基四胺存在时,才制备得到碳化钼。透射电镜观察发现,粒度均一的碳化钼颗粒均匀分散在活性炭上,粒径约2～4 nm。将制备得到的活性炭负载碳化钼和氮化钼催化剂用于萘加氢精制。结果表明,活性炭负载的碳化钼和氮化钼催化剂对萘加氢都具有良好的催化活性和选择性,产物主要为四氢萘。更多还原

【Abstract】 Transition metal carbides as catalytic materials have received considerable attention for their exceptionally high activity,which are similar to those of noble metal catalysts in hydrogen-involving reactions,hydrazine decomposition and isomerization.The preparation of interstitial carbides with nanostructured particles and/or high surface area,however,is very difficult by the conventional methods which have been inherited from the metallurgical industry at high temperature and are energy-intensive,extremely tedious and result in large grains of low surface area that need to be passivated before exposured to the air.Here,evenly distributed tungsten and molybdenum carbides nanoparticles supported on carbon nanotubes (CNTs)/active carbon(AC) have been prepared via microwave-assisted metallo-organic chemical vapor deposition(MOCVD)/thermolysis.The catalytic hydrazine decomposition over the carbides supported on CNTs was performed in a U-shaped quartz microreator.The Pt@WC_x/CNTs electrocatalysts were then prepared by in situ reduction of the metal salt on the uniform WC_x/CNTs hybrid support and used as electrocatalyst for oxygen reduction reaction(ORR).The main results are as follows:The uniform distributed tungsten carbides nanoparticles supportd on CNTs were perared via microwave-assisted MOCVD with tungsten,hexacarbonyl as precursor.The great advantages of microwave heating appear to be the very short time scales involved in the preparation and the selectivity in energy transfer from the microwave field.The results form XRD parttens revealed that the tungsten and molybdenum carbides were successfully synthesized.The TEM images of the tungsten and molybdenum carbides showed that uniform nanostructured tungsten and molybdenum carbides with the particle sizes of 2-5 nm were evenly dispersed on the outer surface of CNTs.The tungsten carbides coated CNTs can be obtained with increasing the tungsten loading to 50%.In contrast to other methods such as direct-thermal decomposition,carbothermal hydrogen reduction and temperature programmed reaction,microwave-assisted MOCVD is a rapid,energy efficient and environmental friendly processing.The as prepared WC_x/CNTs nanocomposites were active catalysts for hydrazine decomposition,and exhibited high selectivity to hydrogen,indicating that nanostructured tungsten carbides on CNTs were an inexpensive and promising alternative to the noble metal catalysts. Pt nanoparticles were then introduced on the WCx/CNTs hybrid support via ethylene glycol reduction method.The results from XRD partterus and TEM images showed that the nanostructured Pt with particle sizes of 2-5nm were evenly deposited on the WC_x/CNTs.The obtained Pt@WC_x/CNTs showed much higher electrocatalytic activity for ORR in sulfuric acid solution than Pt/CNTs catalyst,while the WC_x/CNTs nanocomposite was also active in acid solution for ORR.Meantime.the unique immunity to methanol of the novel electrocatalysts was also observed.Moreover,we have combined the microwave irradiation and thermolvsis processing to prepare molybdenum carbide supported on active carbon with a mixed salt complex containing ammonium heptamolybdate and hexamethylenetetramine(HMT) as precursor.The results revealed that the formation of Mo-HMT complex precursor with abundant HMT in an aqueous solution was a necessary process for the synthesis of molybdenum carbide.TEM images indicated that the uniform molybdenum carbide nanoparticles were obtained in 10 min and evenly distributed on the active carbon.The resultant molybdenum carbide and nitride supported on active carbon exhibited high catalytic activity and selectivity for the hydrogenation of naphthalene.The main product was tetrahydronaphthalene.更多还原