Hydrogen adsorption in HKUST-1: a combined inelastic neutron scattering and first-principlesstudy

Abstract

Hydrogen adsorption in high surface area nanoporous coordination polymers has attracteda great deal of interest in recent years due to the potential applications in energy storage. Herewe present combined inelastic neutron scattering measurements and detailed first-principlescalculations aimed at unraveling the nature of hydrogen adsorption in HKUST-1 (Cu3(1,3,5-benzenetricarboxylate)2), a metal–organic framework (MOF) with unsaturated metal centers. We revealthat, in this system, the major contribution to the overall binding comesfrom the classical Coulomb interaction which is not screened due to theopen metal site; this explains the relatively high binding energies and shortH2–metal distances observed in MOFs with exposed metal sites as compared to traditional ones.Despite the short distances, there is no indication of an elongation of the H–H bond for the boundH2 molecule at the metal site. We find that both the phonon and rotational energy levelsof the hydrogen molecule are closely similar, making the interpretation of theinelastic neutron scattering data difficult. Finally, we show that the orientation ofH2 has a surprisingly large effect on the binding potential, reducing the classical bindingenergy by almost 30%. The implication of these results for the development of MOFmaterials for better hydrogen storage is discussed.