School of Engineering and Science, Jacobs University Bremen Bremen, Germany.
Department of Physics, AlbaNova University Center, Stockholm University Stockholm, Sweden.
Front Chem. 2015 Feb 2;3:2. doi: 10.3389/fchem.2015.00002. eCollection 2015.
The spillover mechanism of molecular hydrogen on carbon nanotubes in the presence of catalytically active platinum clusters was critically and systematically investigated by using density-functional theory. Our simulation model includes a Pt4 cluster for the catalyst nanoparticle and curved and planar circumcoronene for two exemplary single-walled carbon nanotubes (CNT), the (10,10) CNT and one of large diameter, respectively. Our results show that the H2 molecule dissociates spontaneously on the Pt4 cluster. However, the dissociated H atoms have to overcome a barrier of more than 2 eV to migrate from the catalyst to the CNT, even if the Pt4 cluster is at full saturation with six adsorbed and dissociated hydrogen molecules. Previous investigations have shown that the mobility of hydrogen atoms on the CNT surface is hindered by a barrier. We find that instead the Pt4 catalyst may move along the outer surface of the CNT with activation energy of only 0.16 eV, and that this effect offers the possibility of full hydrogenation of the CNT. Thus, although we have not found a low-energy pathway to spillover onto the CNT, we suggest, based on our calculations and calculated data reported in the literature, that in the hydrogen-spillover process the observed saturation of the CNT at hydrogen background pressure occurs through mobile Pt nanoclusters, which move on the substrate more easily than the substrate-chemisorbed hydrogens, and deposit or reattach hydrogens in the process. Initial hydrogenation of the carbon substrate, however, is thermodynamically unfavoured, suggesting that defects should play a significant role.
使用密度泛函理论，我们对含催化活性铂团簇的碳纳米管中分子氢的溢出机制进行了严格而系统的研究。我们的模拟模型包括一个 Pt4 团簇作为催化剂纳米颗粒，以及两个示例性的单壁碳纳米管（CNT）的弯曲和平面 coronene，分别为（10,10）CNT 和一个大直径 CNT。我们的结果表明，H2 分子在 Pt4 团簇上自发解离。然而，即使 Pt4 团簇完全饱和有六个吸附和离解的氢分子，离解的 H 原子也必须克服超过 2eV 的势垒才能从催化剂迁移到 CNT。先前的研究表明，氢原子在 CNT 表面的迁移性受到势垒的阻碍。我们发现，相反，Pt4 催化剂可能沿着 CNT 的外表面以仅 0.16eV 的活化能移动，并且这种效应提供了 CNT 完全氢化的可能性。因此，尽管我们没有找到一种低能量途径使氢溢出到 CNT 上，但根据我们的计算和文献中报道的计算数据，我们建议在氢溢出过程中，观察到 CNT 在氢气背景压力下达到饱和是通过可移动的 Pt 纳米团簇实现的，这些纳米团簇比底物化学吸附的氢更容易在基底上移动，并在这个过程中沉积或重新附着氢。然而，碳基底的初始氢化在热力学上是不利的，这表明缺陷应该发挥重要作用。
