Synthesis of carbon nanotubes and catalytic functions for adsorption on carbon nanotube

减少铂的使用是日常生活中使用的燃料电池商业化的关键要求之一，因为其价格高，可用性有限，并且难以找到合适的替代品。廉价的MO_2C具有与Pt相似的催化活性和电子结构。炭黑（CB）被广泛用作Pt纳米颗粒的载体。然而，我们发现，当使用碳纳米管（CNT）而不是CB作为载体时，性能得到改善，特别是低于600 mA/cm ^2。在电流密度低于600 mA/cm ^2时，MO_2C催化剂与碳纳米管的组合可使PEFC的性能提高一半。碳纳米管负载的PtRu催化剂具有很好的抗CO性能。为了阐明CNT的优势的原因，我们使用金属催化剂/HOPG（高取向热解石墨）模型系统进行了表面科学研究。Pt的形状 ...更多信息 粒子是扁平的，只有一个或两个原子高度。这表明Pt的电子结构在CNT载体和CB载体之间应该是不同的。事实上，XPS测量表明，随着HOPG上Pt颗粒尺寸的减小，Pt 4f芯能级向更高能量移动。这是目前归因于从Pt到碳的电荷转移的小Pt颗粒和石墨表面之间的波函数的杂化。H_2的TPD和高压下的H_2-D_2交换反应表明，颗粒尺寸的减小导致氢在Pt上的吸附能降低。吸附能的降低可以解释为降低d带中心诱导的电子转移从Pt到碳的颗粒尺寸减小。即碳的载体效应，这解释了CNT负载催化剂的真实的催化剂的结果。在碳纳米管（CNT）的合成方面，研究了Mo/Co/MgO和Co/MgO催化剂上CH_4分解合成CNT的动力学，以阐明催化剂组分的作用。在不存在Mo组分的情况下，Co/MgO催化剂在较低的反应温度（823-923 K）下在厚CNT（外径为7-27 nm）的合成中是活性的，但不产生薄外径的CNT。Co/MgO催化剂在923 K以上的温度下会因积碳而失活。对于含Mo催化剂（Mo/Co/MgO），薄CNT（2-5壁）形成在高于1000 K时开始而没有失活。Mo的加入降低了碳纳米管催化剂的甲烷分解活性，同时也降低了碳纳米管合成过程中Mo 2C的生成。在Co/MgO和Mo/Co/MgO催化剂上，CNT的合成速率与CH_4压力成正比，表明CH_4的解离是催化剂不失活的速率控制步骤。当石墨烯网络的形成滞后于CH_4沉积成碳时，催化剂的失活发生在动力学上。少

A reduction in Pt usage is one of the key requirements for the commercialization of fuel cells for use in everyday life, because of its high price and limited availability, and the difficulty of finding suitable substitutes. The cheaper MO_2C is known to possess similar catalytic activities and electronic structures to Pt. Carbon black (CB) is widely used as the support for Pt nanoparticles. However, we found that when carbon nanotubes (CNTs) rather than CB are used as the support, the performance is improved, especially below 600 mA/cm^2. We found that a combination of MO_2C catalyst and carbon nanotubes in the anode provides performance as high as half that of the current PEFCs with Pt catalysts below 600 mA/cm^2.The PtRu catalysts supported on CNT were found to be CO tolerant very much. In order to clarify the reason for the advantage of CNT, we have carried out surface science studies using model systems of metal catalysts/HOPG (Highly oriented pyrolytic graphite). The shape of Pt … More particles is flat with one or two atomic heights. This suggests that electronic structures of Pt should be different between CNT support and CB support. In fact, XPS measurements show that Pt 4f core level is shifted to higher energy with decreasing the size of Pt particles on HOPG. This is currently ascribed to charge transfer from Pt to carbon by hybridization of wave functions between the small Pt particles and the graphite surface. The reduction of the particle size leads to a decrease in adsorption energy of hydrogen on Pt, which is shown by TPD of H_2 as well as H_2-D_2 exchange reaction at high pressures. The decrease in the adsorption energy can be explained by lowering d band center induced by electron transfer from Pt to carbon upon reduction of particles size. That is, support effect of carbon, which explains the results of real catalysts of CNT supported catalysts. Concerning CNT synthesis, the kinetics of carbon nanotube (CNT) synthesis by decomposition of CH_4 over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH_4, as well as to the formation of Mo_2C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH_4 pressure, indicating that the dissociation of CH_4 is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation of the grapheme network lags carbon formation by deposition of CH_4. Less