The rotational and translational dynamics of molecular hydrogen physisorbed in activated carbon: A direct probe of microporosity and hydrogen storage performance

摘要

We have measured Incoherent Inelastic Neutron Scattering (IINS) spectra of H_2 physisorbed in high purity chemically activated carbon (AC) at different surface coverage and at temperatures near the triple point of bulk hydrogen. Our experimental results and DFT calculations show that at low surface coverage, due to the very low corrugation of the adsorption potential, and in the absence of H_2-H_2 lateral interactions, the adsorbed molecules are practically free to translate in the 2D plane parallel to the surface. Model calculations show that a complete mixing between the sub-states of the J = 1 manifold occurs on the free surface. The J = 0-to-1 rotational transition should split if the H_2 molecule is adsorbed in a slit type pore. Rotational splitting of up to 13 meV is found in the narrowest pores of around 6 A investigated. The calculated isosteric heat of adsorption for molecules adsorbed on the free surface, at different sites and molecule orientations, range between -39 and -42 meV/H_2 at 77 K. In the optimum size slit pores, these numbers double up. Micropore volume of 0.34-0.45 ml/g carbon, and an upper limit of 4 wt percent hydrogen storage is anticipated for the investigated material.