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 extra-framework cations of different nature, we have studied the interaction of H(2) 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 H(2) have complex structure due to ortho/para splitting as well as to surface structural disorder. Na(+) and divalent Ca(2+) were found to induce almost similar perturbation on H(2) molecule, resulting in the shift of the H-H vibrational frequency of -86 cm(-1) and -76 cm(-1) 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 mol(-1) for the strongest adsorptive sites in Na-A and Ca-A samples. In the case of Co,Na-A the shift of the H-H frequency due to the formation of H(2) center dot center dot center dot Co(2+) complexes is larger (ca. -180 cm(-1)) suggesting that the interaction can involve some, although small, chemical contribution. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.