Sub-nanometer characterization of activated carbon by inelastic neutron scattering

Inelastic neutron scattering spectra are calculated for hydrogen molecules adsorbed on activated carbon. The slit-shaped pore model is used to calculate the adsorption potentials of a hydrogen molecule in pores of variable width. The motion of the hydrogen molecules is quantized perpendicular to the plane of the pore and both rotational and vibrational transition energies are found. The perturbation of adjacent hydrogen molecules on the transition energies is discussed. Form factors and Debye–Waller factors are calculated for momentum transfer parallel and perpendicular to the pore; this anisotropy is particularly pronounced in pores where there is nearly enough room for two adsorbed layers. A spectrum composed of a uniform distribution of pore sizes agrees only qualitatively with experimental results for activated carbon. Reasons for this disparity are discussed, including the possibility that transitions in the translational motion parallel to the adsorption plane contribute significantly to the spectra. In addition, the positioning of the center of gravity of the first rotational transition is discussed, with several factors contributing to shift it from the 14.7 meV of the gas phase. Our results suggest that inelastic neutron scattering can be valuable as a complementary sub-nanometer pore characterization technique.