Role of extraframework metal sites for hydrogen adsorption into the pores of a zeolite: FT-IR study

In order to compare the adsorptive properties of nanoporous zeolites containing extraframework cations of different nature, we have studied the interaction of H2 with Na-A, Ca- A, and Co,Na-A zeolites. Low temperature Fourier transform infrared (FT-IR) spectroscopy was used for the investigation, as this technique is highly sensitive and responsive to the nature of the gas/surface interaction and can in addition allow for the estimation of the adsorption enthalpy. In all cases the spectra of adsorbed H2 have complex structure due to ortho/para splitting as well as to surface structural disorder. Naþ and divalent Ca2þ were found to induce almost similar perturbation on H2 molecule, resulting in the shift of the HeH vibrational frequency of 86 cm1 and 76 cm1 respectively (as compared to the Raman frequency of gaseous H2). The enthalpy of adsorption, estimated by the Variable Temperature Infrared (VTIR) method, is 13 1 kJ mol1 for the strongest adsorptive sites in Na-A and Ca-A samples. In the case of Co,Na-A the shift of the HeH frequency due to the formation of H2/Co2þ complexes is larger (ca. 180 cm1) suggesting that the interaction can involve some, although small, chemical contribution.