Methane adsorption inside and outside pristine and N-doped single wall carbon nanotubes

The internal and external adsorption of CH4 on zigzag pristine and N-doped SWCNTs has been investigated employing first principles calculations. Two different substitutions were performed to obtain pyridinic (N-py) and graphitic (N-sp3) nitrogen. The adsorption of CH4 inside SWCNTs is expected to be more favorable than on groove sites for open-ended small diameter (0.7-1.1 nm) pure SWCNTs. In general, the introduction of N atoms increases the external adsorption energies (E-ads). In the case of N-py-doped SWCNTs, they are increased over a 100%. However, for the internal adsorption, the E-ads can be decreased or increased when N atoms are included. For the N-sp3-doped (9,0) SWCNT, it is decreased with respect to the undoped tube. Thus, the optimal radius to encapsulate CH4 is increased upon N-doping. The adsorption isotherms of CH4 are going to be dependent on the production method of the SWCNTs, if they have open ends; this fact is in contrast with the experimental measurements on close-ended SWCNT. For undoped SWCNTs, the encapsulation inside small diameter (0.7-1.1 nm) SWCNT is going to be preferred over electric arc-SWCNTs (1.4-1.8 nm diameter). However, for N-doped SWCNTs, the opposite should be true, if enough nitrogen is introduced. (C) 2008 Elsevier B.V. All rights reserved.