Hydrogen adsorption on the zeolite Ca-A: DFT and FT-IR investigation

Adsorption of H2 on the zeolite Ca-A was investigated by a combination of variable temperature IR spectroscopy and calculations at the periodic DFT level. Calculations showed that adsorption complexes can be formed on two types of adsorption sites: Ca2+ in the center of six-member rings and Ca2+ in eight-member rings. Only the former adsorption complexes were experimentally observed in IR spectra; they are characterized by H–H stretching frequency of 4083 cm−1 and ΔH0 = −12 kJ/mol in agreement with calculated results. The most likely explanation for not observing the adsorption complex on Ca2+ in 8-member rings is that the geometry of the complex does not render the H–H stretching mode IR active.