Ab initio study of hydrogen interaction with pure and nitrogen-doped carbon nanotubes

Detailed studies of mechanisms for hydrogen dissociative adsorption and diffusion on pure and nitrogen-doped (8, 0) carbon nanotubes are carried out using the first-principles density functional theory method. (1) For pure carbon nanotubes, we have identified the energetically most favorable dissociative pathway for hydrogen adsorption, with a barrier height of 1.3 eV. We also found that the adsorbed hydrogen atoms can act as an autocatalyst for further dissociative adsorption of hydrogen molecules. (2) It is found that on pure carbon nanotubes the diffusion of hydrogen atoms is constrained by interaction with neighboring adsorbed hydrogen atoms. The diffusion barrier is around 0.7 eV for an isolated hydrogen atom, but becomes substantially higher at around 1.4 eV in the presence of adsorbed hydrogen in neighboring positions. (3) Doping the nanotube with nitrogen considerably alters the catalytic effects of the carbon nanotube for hydrogen dissociative adsorption. The dissociative adsorption of hydrogen on the carbon nanotube is greatly enhanced, with the barrier substantially reduced to ca. 0.9 eV. The differences in the barrier heights are explained through analysis of the electronic structure changes of the nanotube.