Diffuse reflectance IR study of molecular hydrogen and deuterium adsorbed at 77 K on NaA zeolite Part 1.—Fundamentals, combination and vibrational–rotational modes

Abstract

The adsorption of hydrogen and deuterium on the sodium form of zeolite A at 77 K has been investigated by diffuse reflectance FTIR (DRIFT) spectroscopy. The spectra exhibited bands of stretching vibrations which appeared because of the disturbance of the homonuclear diatomic molecules in the electric field of the adsorption sites (cations). These bands 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 equilibrium pressure under isothermal conditions was found. This indicates that, at low pressures, only the interaction with the cations is relevant for H ₂ or D ₂ adsorption in NaA at 77 K and non-specific physical adsorption is negligible. Moreover, satellites of the main bands and weaker high-frequency features were observed. The positions of both the main stretching vibration bands and the satellites depended on the pressure of the adsorbate. It is suggested that, at lower pressures, sodium cations located in the eight-membered rings were involved as sites of higher adsorption potential, whereas at higher pressures weaker adsorption occurred on sodium cations residing in the six-membered rings of the framework. Stronger adsorption was indicated by a higher separation 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 appear owing to a combination of the stretching vibration mode of the molecular bonds of H ₂ or D ₂ with an oscillation of the whole molecule against the adsorption site. Several weaker signals were observed at higher 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 interpreted as features arising from vibrational–rotational transitions of the ortho- and para-forms of H ₂ or D ₂ . Comparison of the above frequency data with the simple model for rotational energy levels of diatomic molecules suggests that the rotation of H ₂ and D ₂ molecules adsorbed on NaA zeolite is to some extent hindered.