Hydrogen adsorption in a copper ZSM5 zeolite. An inelastic neutron scattering study

Currently microporous oxidic materials including zeolites are attracting interest as potential hydrogen storage materials. Understanding how molecular hydrogen interacts with these materials is important in the rational development of hydrogen storage materials and is also challenging theoretically. In this paper, we present an incoherent inelastic neutron scattering (INS) study of the adsorption of molecular hydrogen and hydrogen deuteride (HD) in a copper substituted ZSM5 zeolite varying the hydrogen dosage and temperature. We have demonstrated how inelastic neutron scattering can help us understand the interaction of H₂ molecules with a binding site in a particular microporous material, Cu ZSM5, and by implication of other similar materials. The H₂ molecule is bound as a single species lying parallel with the surface. As H₂ dosing increases, lateral interactions between the adsorbed H₂ molecules become apparent. With rising temperature of measurement up to 70 K (the limit of our experiments), H₂ molecules remain bound to the surface equivalent to a liquid or solid H₂ phase. The implication is that hydrogen is bound rather strongly in Cu ZSM5. Using the simple model for the anisotropic interaction to calculate the energy levels splitting, we found that the measured rotational constant of the hydrogen molecule is reduced as a consequence of adsorption by the Cu ZSM5. From the decrease in total signal intensity with increasing temperature, we were able to observe the conversion of para-hydrogen into ortho-hydrogen at paramagnetic centres and so determine the fraction of paramagnetic sites occupied by hydrogen molecules, ca. 60%.