A combination of variable-temperature infrared spectroscopy with volumetric gas adsorption measurements was used to study the thermodynamics of hydrogen adsorption, at a low temperature, on calcium-exchanged zeolites X and Y. Two adsorption regimes were considered: (i) localized adsorption of dihydrogen molecules on Ca2+ cation sites, and (ii) delocalized hydrogen adsorption following saturation of the Ca2+ adsorbing centres. For localized adsorption, the corresponding enthalpy change was found to be in the range of −12 to −15 kJ mol−1, while the isosteric heat of delocalized adsorption was found to be in the range of 4.5–5.5 kJ mol−1. These experimental results are discussed in the broader context of corresponding data for other alkaline zeolites, with a focus on correlation between adsorption enthalpy and entropy for the localized adsorption regime.

Financial support from Regione Piemonte (CIPE Project C72), MUR (2004 FISR Project “Vettore Idrogeno”), Spanish MEC and FEDER funds (Project MAT2005- 05350) and from the Conselleria d’Economia, Hisenda e Innovació de les Illes Balears (Project PROGECIB-22A) is gratefully acknowledged.

Peer reviewed