Nitrogen substitution effect on hydrogen adsorption properties of Ti-decorated benzene

Ab initio calculations are performed to study hydrogen storage properties of Ti-doped benzene and Ti-doped nitrogen-substituted benzene complexes. Two of the carbon atoms in benzene are replaced by two nitrogen atoms. Two nitrogen atoms are substituted either at 1-2, 1-3, or 1-4 positions of a benzene ring and named as BN1-2Ti, BN1-3Ti, and BN1-4Ti, respectively. Maximum four, four, three, and four H-2 molecules get adsorbed on C6H6Ti, BN1-2Ti, BN1-3Ti, and BN1-4Ti complexes respectively with respective H-2 uptake capacity of 6.02, 5.84, 4.45, and 5.84 wt%. The positive Gibbs free energy corrected H-2 adsorption energy values obtained for all these complexes at ambient conditions indicate that the formation of these complexes at room temperature is thermodynamically favorable. Temperature- and pressure-dependent adsorption energy calculations show that the H-2 adsorption on all these complexes is feasible over a wide range of temperature and pressure. The gap between the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbital (LUMO) is found to be greater than 5 eV for all the complexes indicating stability of these complexes. The H-2 molecules interact more strongly with Ti-doped nitrogen-substituted benzene than the Ti-doped benzene that results in higher H-2 desorption temperature obtained using van 't Hoff equation for the former than the latter. The density of states plots have been used to understand the H-2 adsorption mechanism.