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. Na+ and divalent Ca2+ were found to induce almost similar perturbation on H2 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 H2⋯ Co2+ complexes is larger (ca. -180 cm-1) suggesting that the interaction can involve some, although small, chemical contribution.
Bibliographical noteFunding Information:
The authors wish to thank SAES Getters and Enichem for kindly supplying the Na-A and Ca-A zeolites. O.Z. likes to thank Svetoslava Vankova for help in the preparation of the Co-A zeolite. Financial support from “ Regione Piemonte ” through the “Progetto Regionale Materiali Innovativi per lo Stoccaggio di Idrogeno” is gratefully acknowledged.
Copyright 2011 Elsevier B.V., All rights reserved.
- Hydrogen adsorption
- Infrared spectroscopy
- Zeolite A