DIFFUSE-REFLECTANCE IR STUDY OF MOLECULAR-HYDROGEN AND DEUTERIUM ADSORBED AT 77 K ON NAA ZEOLITE .1. FUNDAMENTALS, COMBINATION AND VIBRATIONAL-ROTATIONAL MODES

The adsorption of hydrogen and deuterium on the sodium form of zeolite A at 77 K has been investigated by diffuse reflectance FTIR (DRIFT) s pectroscopy. The spectra exhibited bands of stretching vibrations whic h appeared because of the disturbance of the homonuclear diatomic mole cules in the electric field of the adsorption sites (cations). These b ands were detected at 4088 and 2939 cm(-1) for hydrogen and deuterium, respectively. A correlation between the amount of adsorbed molecules and the increase in the intensity of the IR signal as a function of eq uilibrium pressure under isothermal conditions was found. This indicat es that, at low pressures, only the interaction with the cations is re levant for H-2 or D-2 adsorption in NaA at 77 K and non-specific physi cal adsorption is negligible. Moreover, satellites of the main bands a nd weaker high-frequency features were observed. The positions of both the main stretching vibration bands and the satellites depended on th e pressure of the adsorbate. It is suggested that, at lower pressures, sodium cations located in the eight-membered rings were involved as s ites of higher adsorption potential, whereas at higher pressures weake r adsorption occurred on sodium cations residing in the six-membered r ings of the framework. Stronger adsorption was indicated by a higher s eparation of the satellites from the main bands, viz. 93 cm(-1) (H-2) and 75 cm(-1) (D-2) cf. 56 cm(-1) (H-2) and 40 cm(-1) (D-2) for weaker adsorption at higher pressures. It is proposed that the satellites ap pear owing to a combination of the stretching vibration mode of the mo lecular bonds of H-2 or D-2 with an oscillation of the whole molecule against the adsorption site. Several weaker signals were observed at h igher frequencies separated by 288 and 495-570 cm(-1)(H-2) or ca. 173 and ca. 400 cm(-1) (D-2) from the main stretching bands. These were in terpreted as features arising from vibrational-rotational transitions of the ortho- and para-forms of H-2 or D-2. Comparison of the above fr equency data with the simple model for rotational energy levels of dia tomic molecules suggests that the rotation of H-2 and D-2 molecules ad sorbed on NaA zeolite is to some extent hindered.