Sorption of nitrogen, oxygen and argon in mordenite

Abstract

The sorption of nitrogen, oxygen and argon by well defined large-port mordenite was measured in the pressure region of 3 × 10^–3∼760 Torr at temperatures from –80∼25°C. The sorbed phase is well described by a model of a one-dimensional van der Waals gas. The thermal (nonconfigurational) entropies of adsorbates, s_therm, derived from experimental data, are independent of the coverage, showing the validity of the model. The observed values for s_therm are expressed as s_therm/R=a+b ln T where b has the value of 2.5, 3.5 and 5.2 for argon, oxygen and nitrogen, respectively. It is concluded that adsorbed argon has one translational and two vibrational degrees of freedom, and that adsorbed oxygen has further two degrees of freedom of free rotation other than these. As to adsorbed nitrogen, the temperature region concerned corresponds to a transition region from a seriously hindered rotational state to a free rotational one. Such a hindrance may be attributed to the large electric quadrupole moment of nitrogen which interacts with an inhomogeneous surface field.

By analyzing the value for a, we calculate vibrational frequencies of the adsorbate in the cross-sectional plane of the pore channel, obtaining reasonable values of the order of 10¹² s^–1.