Enhancements of hydrogen adsorption energy in M-MOF-525 (M= Ti, V, Zr and Hf): A DFT study

Electric dipole moment and hydrogen adsorption properties of four different Metal–Organic Framework-525, namely M-MOF-525 (M= Ti, V, Zr and Hf) were computed via the first-principles technique. Our calculation results revealed that the bond length between M–O (M= Ti, V, Zr and Hf) of the metal-oxide cluster is proportional to the atomic radius of M in metal-oxide cluster. The increasing of the M–O distance results in enhancement of electric dipole moment of the M-MOF-525. Moreover, it was found that the averaged M-O distances are 2.10, 2.05, 2.24 and 2.25 Å for Ti-, V-, Zr- and Hf-MOF-525, respectively. To investigate the hydrogen adsorption property, all possible hydrogen adsorption sites in the M-MOF-525 must be firstly searched. According to our calculation results, five stable hydrogen adsorption sites were found. For each adsorption site, two orientations of the HH bonding, namely parallel and perpendicular directions, were considered. Based on our calculation results, it was found that the Ti- and V-MOF-525 are unable to bind a hydrogen molecule on their surfaces, whereas a hydrogen molecule can trap on the surface of the Zr- and Hf-MOF-525. This is owing to stronger electric dipole moments of Zr- and Hf-MOF-525 in comparison to Ti- and V-MOF-525. For the Zr-MOF-525, the hydrogen molecule can trap on this structure with hydrogen adsorption energy ranging from 0.04–0.15 eV/H2. Additionally, the hydrogen binding energies of the Hf-MOF-525 range from 0.06 to 0.16 eV/H2. According to our computational results, it was obviously seen that hydrogen adsorption energy of the site near metal oxide cluster is always larger than that of the TpCPP linker. Lastly, we also found that the interaction between the hydrogen molecule and the M-MOF-525 host is mainly governed by a weak dispersive interaction.