Role of extraframework metal sites for hydrogen adsorption into the pores of a zeolite: FT-IR study Olena Zavorotynska*, Jenny G. Vitillo 1 , Giuseppe Spoto 2 , Adriano Zecchina 3 Dipartimento di Chimica IFM, Universita ` di Torino, and NIS center of excellence, Via P.Giuria, 7, 10125 Torino, Italy article info Article history: Received 23 March 2010 Received in revised form 25 October 2010 Accepted 31 January 2011 Available online 4 March 2011 Keywords: Hydrogen adsorption Zeolite A Infrared spectroscopy VTIR abstract 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 þ and divalent Ca 2þ were found to induce almost similar perturbation on H 2 molecule, resulting in the shift of the HeH vibrational frequency of 86 cm 1 and 76 cm 1 respectively (as compared to the Raman frequency of gaseous H 2 ). 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 HeH frequency due to the formation of H 2 /Co 2þ complexes is larger (ca. 180 cm 1 ) suggesting that the interaction can involve some, although small, chemical contribution. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1. Introduction Development of efficient, safe and inexpensive solutions for storage and transportation is among the problems to be solved in view of hydrogen use as an alternative energy carrier. One of the proposed methods involves physical adsorption in high surface area microporous materials [1e8]. In this respect, zeolites have attracted an attention because they offer the advantages of high stability, low cost, high surface area and wide variety of porous structures. It has however become clear that due to the heavy framework constituted by silicon, oxygen and aluminum and due to the low energy of interaction with hydrogen (5e18 kJ mol 1 [9]), the use of these materials for mobile applications is not feasible because of the low hydrogen gravimetric uptake (2.68% by weight on theoretical ground [10]; maximum experimental values 2.19 wt% [10,11]), which stro- ngly depends on temperature e 40e293 K e and pressure e 0.5e60 bar e conditions. Nevertheless, besides their impor- tance as catalysts, zeolites could deserve attention as candi- dates for stationary applications and as adsorbents for hydrogen purification. A crucial role in adsorption process is played by the counterions (cations) balancing the negative charge of the framework. They can interact with hydrogen (acting as Lewis * Corresponding author. Tel.: þ39 011 6707840; fax: þ39 011 6707855. E-mail addresses: olena.zavorotynska@unito.it (O. Zavorotynska), jenny.vitillo@unito.it (J.G. Vitillo), giuseppe.spoto@unito.it (G. Spoto), adriano.zecchina@unito.it (A. Zecchina). 1 Tel.: þ39 011 6707845; fax: þ39 011 6707855. 2 Tel.: þ39 011 6707832; fax: þ39 011 6707855. 3 Tel.: þ39 011 6707860; fax: þ39 011 6707855. Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 36 (2011) 7944 e7950 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.01.179