Discovering the Mechanism of H_2 Adsorption on Aromatic Carbon Nanostructures to Develop Adsorbents for Vehicular Applications

摘要

Hydrogen adsorption has been observed with a binding energy of ～50 kJ/mol on as-synthesized carbon multi-wall nanotubes (MWNTs). The MWNTs are virtually free of non-nanotube carbon impurities but contain residual iron catalyst particles. The MWNTs are also highly graphitic. No hydrogen adsorption is observed at near ambient temperatures for purified MWNTs that are free of iron particles. However, hydrogen adsorption is also not observed on bare iron particles even following reduction in the presence of hydrogen at 775 K. These results imply that a special synergy occurs when small iron particles or atoms are in intimate contact with sp~2-hybridized aromatic carbon. Interestingly, reducing the as-synthesized MWNTs in H_2 at 573 K results in an increased hydrogen capacity. Understanding this hydrogen storage mechanism could facilitate the economical engineering of a hydrogen storage material that meets the United States Department of Energy targets for vehicular fuel cell applications. Recent theoretical studies have shown that an iron ad atom forms a complex with a C_(36) fullerene and shares charge with four carbon atoms of a bent five-membered ring. Three H_2 ligands then coordinate with the iron forming a stable 18-electron organometallic complex. Here the binding energy of the molecular hydrogen ligands is ～43 kJ/mol. These theoretical results could possibly explain the unique hydrogen storage properties of MWNTs that are grown with an iron catalyst.