Adsorption and Dissociation of H2O on N-Functionalized Carbon Nanotubes (CNTs)

Abstract

Adsorption of one and two water molecules on the surface of pristine, carbamic acid and 2-amino 3-acetly pyridine functionalized CNTs at various positions (center and edges) have been investigated using DFT methods, (with XC of GGA-PBE functional & DZ basis set) predicting accurate water molecules splitting process. Adsorption of two water molecules was found to be energetically more favorable on chiral (2, 4), armchair (5, 5) and zigzag (6, 0) pristine, carbamic acid and 2-amino 3-acetly pyridine functionalized CNTs than the mono-adsorption. Hydrogen storage capacity through percent adsorption & surface coverage of pristine, carbamic acid and 2-amino 3-acetly pyridine functionalized (2, 4), (5, 5) and (6, 0) CNTs was also determined, the surface coverage of (2, 4) CNTs upon one water molecule splitting with stone-wales defect production reaches to 0.057 and in case of two water molecules it reaches to 0.11. The percent adsorption in the case of one water molecule splitting is 5.76 % whereas in case of two molecules splitting, it is 11.53 % respectively. Twenty and thirty water molecules were also adsorbed to reach DOE demand target (7 wt.%) for hydrogen storage capacity for commercial application of hydrogen fuel and 7.15 wt.% capacity had been achieved for pristine and functionalized (2, 4) CNTs. Based on thermokinetic behavior, adsorption constant (Kad) for the adsorption of one and two water molecules was evaluated. For one water molecule splitting on carbamic acid functionalized (6, 0) CNT, value of Kad was found to be 8.84 ×108 /M-1 (which is greater as compared to one water molecule splitting on pristine (6, 0) CNT which is 4.89×107 /M-1 . Pristine (2, 4) and 2-Amino-3-acetlypyridine functionalized (6, 0) CNTs supported the trends that smaller HOMO-LUMO gap (ΔE=0.119eV), indicating the maximum electron transfer due to electronic repulsion of c=c pi bond present in CNTs. Stone-Wales (SW) defects produced in carbon nanotubes by oxygen atom of H2O enhanced the hydrogen production and storage capacity hence providing favorable binding sites to the hydrogen atoms due to the localized donor states induced by these defects. Greater advantage of current research work lies in the production of hydrogen fuel and its storage on same material eliminating the risks of hydrogen transportation.