  In this work, we studied the hydrogen adsorption-desorption properties and
storage capacities of Ti functionalized [2,2,2]paracyclophane (PCP222) using
density functional theory and molecular dynamic simulation. The Ti atom was
bonded strongly with the benzene ring of PCP222 via Dewar interaction.
Subsequently, the calculation of the diffusion energy barrier revealed a
significantly high energy barrier of 5.97 eV preventing the Ti clustering over
PCP222 surface. On adsorption of hydrogen, the first H2 molecule was
chemisorbed over PCP222 with a binding energy of 1.79 eV with the Ti metals.
Further addition of H2 molecules, however, exhibited their physisorption over
PCP222-Ti through the Kubas type H2 interaction. The charge transfer mechanism
during the hydrogen adsorption was explored by the Hirshfeld charge analysis
and electrostatic potential map, and the PDOS, and Baders topological analysis
revealed the nature of the interaction between Ti and H2. The PCP222
functionalized with three Ti atoms showed a maximum hydrogen uptake capacity of
up to 7.37 wt%, which was fairly above the US-DOE criterion. The practical H2
storage estimation revealed that at ambient conditions, the gravimetric density
of up to 6.06 wt% H2 molecules could be usable, and up to 1.31 wt% of adsorbed
H2 molecules were retained with the host. The ADMP molecular dynamics
simulations assured the reversibility by desorption of adsorbed H2 and the
structural integrity of the host material at sufficiently above the desorption
temperature (300K and 500K). Therefore, the Ti-functionalized PCP222 can be
considered as a thermodynamically viable and potentially reversible H2 storage
material.
