Hydrogen spillover mechanism on covalent organic frameworks as investigated by ab initio density functional calculation

The hydrogen spillover mechanism, including the H chemisorption, diffusion, and H-2 associative desorption on the surface of COFs and H atoms migration from metal catalyst to COFs, have been studied via density functional theory (DFT) calculation. The results described herein show that each sp(2) C atom on COFs' surface can adsorb one H atom with the bond length d(C-H) between 1.11 and 1.14 angstrom, and the up-down arrangement of the adsorbed H atoms is the most stable configuration. By counting the chemisorption binding sites for these COFs, we can predict the saturation storage densities. High hydrogen storage densities show that the gravimetric uptakes of COFs are in the range of 5.13-6.06 wt%. The CI-NEB calculations reveal that one H atom diffusing along the C-C path on HHTP surface should overcome the 1.41-2.16 eV energy barrier. We chose tetrahedral Pt-4 cluster and HHTP as the representative catalyst and substrate, respectively, to study the H migration from metal cluster to COFs. At most, two H atoms can migrate from Pt-4 cluster to HHTP substrate. The migration reaction is an endothermic process, undergoing an activation barrier of 1.87 eV and 0.57 eV for the first and second H migration process, respectively. Three types of H-2 associative desorption from hydrogenated COFs were studied: (I) the two H adatoms recombining to one H-2 molecule with a recombination barrier of 4.28 eV, (II) the abstraction of adsorbed H atoms by gas-phase hydrogen atoms through ER type recombination reactions with a recombination barrier of 1.05 eV, (III) the H-2 desorption through the reverse spillover mechanism with an energy barrier of 2.90 eV.