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

减少铂的使用是日常生活中燃料电池商业化的关键要求之一，因为它的价格很高，可获得性有限，而且很难找到合适的替代品。廉价的Mo_2C具有与铂相似的催化活性和电子结构。炭黑(CB)被广泛用作铂纳米颗粒的载体。然而，我们发现当使用碳纳米管(CNTs)而不是CB作为载体时，性能得到了改善，特别是在600 mA/cm2以下。我们发现，在阳极中加入MO_2C催化剂和碳纳米管的组合可以提供比目前铂催化剂低于600 mA/cm2的PEFC高一半的性能。为了阐明碳纳米管具有优势的原因，我们利用金属催化剂/高定向热解石墨(HOPG)模型体系进行了表面科学研究。铂…的形状更多的粒子是平的，有一个或两个原子高度。这表明，碳纳米管载体和CB载体上的铂的电子结构应该不同。事实上，XPS测量表明，随着铂颗粒尺寸的减小，铂4f芯能级向高能方向移动。目前认为这是由于小的铂颗粒与石墨表面之间的波函数杂化导致的从铂到碳的电荷转移。随着粒径的减小，氢在铂上的吸附能降低，表现为高压下H_2的TPD和H_2-D_2交换反应。吸附能的降低可以通过减小颗粒尺寸引起的从铂到碳的电子转移引起的d带中心的降低来解释。也就是说，碳的载体效应，这解释了碳纳米管负载催化剂的真实结果。在碳纳米管合成方面，研究了在Mo/Co/MgO和Co/MgO催化剂上甲烷分解合成碳纳米管(CNT)的动力学，以阐明催化剂组分的作用。在没有Mo组分的情况下，在较低的反应温度(823-923K)下，Co/MgO催化剂对合成粗大的碳纳米管(外径为7-27 nm)具有较高的活性，但没有生成细小的碳纳米管。对于含Mo催化剂(Mo/Co/MgO)，当温度高于1000K时，碳纳米管(2-5壁)的形成没有失活，而Co/MgO催化剂在923K以上的碳沉积使其显著失活。Mo的加入降低了甲烷分解的催化活性，并在高温合成碳纳米管的过程中生成了Mo_2C。在Co/MgO和Mo/Co/MgO催化剂上，碳纳米管的合成速率与CH4压力成正比，说明CH4的解离是催化剂不失活的速率决定步骤。当石墨烯网络的形成滞后于CH4沉积的碳形成时，催化剂因碳沉积而失活。

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