Interaction of O2, N2 and He at room temperature with carbon molecular sieves sensed by adsorption measurements

Heats of adsorption of N2 and O2 on carbon molecular sieve fibers (CMSFs) have been determined by flow microcalorimetry (FMC), by static microcalorimetry (SMC), and by temperature programmed desorption mass spectrometry (TPD-MS). The FMC determined the heats of displacement of He, used as a carrier gas, by the more strongly adsorbed O2 and N2. The heats and the amounts of adsorption were determined concurrently and the molar heats of adsorption calculated. Generally, these determinations revealed that the displacement of He by O2 and by N2 is protracted and occurs in two stages. Re-arrangement of the adsorbates occurs in the second stage with little or no adsorption. The heats of adsorption were found to be surprisingly high and He difficult to displace. The determinations of the heats of adsorption on the CMSF were accomplished by two independent methods: microcalorimetry and TPD. Both of these methods produced closely similar heats of adsorption. He adsorption near constrictions increases the adsorption selectivity between O2 and N2 and increases the heat of adsorption of N2 due to a better fit of the N2 molecules to the constriction sites. The heat of adsorption of O2 is decreased by He adsorption because He occupies some of the low energy sites of O2 but not that of N2.