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 (Npy) and graphitic (Nsp3) 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 (Eads). In the case of Npy-doped SWCNTs, they are increased over a 100%. However, for the internal adsorption, the Eads can be decreased or increased when N atoms are included. For the Nsp3-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.