Laboratory simulation of meteoritic noble gases. I - Sorption of xenon on carbon: Trapping experiments

Abstract We have studied trapping of radioactive 127 Xe in three types of carbon: carbon black (lamp black  LB), pyrolyzed polyvinylidene chloride (PVDC), and pyrolyzed acridine (C 13 H 9 N). A total of 86 samples were exposed to Xe at T between 100 and 1000°C, for times between 5 min and 240 hours, at p xe ~ 5 × 10 −7 atm. Excess gas phase and loosely sorbed Xe were pumped away and the remaining, tightly bound Xe was measured by γ-spectrometry. At 100° C ,× >90% of the Xe desorbs within a few minutes' pumping but a small amount remains even after 4000 min. Distribution coefficients for this tightly bound Xe are ~1 × 10 −2 , 1 and 10 ccSTP/g atm for LB, acridine and PVDC carbons. The tightly bound Xe consists of two components. One occurs over the entire range 100–1000°C, becoming less abundant at high T ; it appears to be physisorbed. The other occurs only at T > 500° C and is probably due to volume diffusion. The adsorbed component in LB has an apparent ΔH between −2.3 and −5.7 kcal/mole. The diffused component, which occurs in LB and possibly in acridine carbon, has an activation energy Q = 27 ± 8 kcal/mole and a diffusion coefficient D = 1.3 × 10 −17 cm 2 /sec at 1000°C. These values are comparable to those found for other types of amorphous carbon ( Morrison et al. , 1963; Nakai et al. , 1960). The low- T component displays two paradoxical features: low ΔH ads , in the range for Xe physisorbed on carbon, but exceedingly long adsorption or desorption times (~10 3 min at 100–400 or 1000°C). Although these long times seem to suggest a high energy process such as chemisorption, our results are best explained by a model that invokes physisorption within a labyrinth of micropores—of atomic dimensions—known to exist in amorphous carbons. The long adsorption/desorption times reflect either the long distances (~5 cm) Xe atoms must migrate by random walk to enter or leave the labyrinth, or the long times needed for Xe atoms to traverse tight spots or constricted pores that connect interior and exterior surfaces of the carbon ( activated entry ). Both variants of this model predict long equilibration times for the observed ΔH ads of −2 to −6 kcal/mole. Apparently, xenon can be tightly trapped in carbon without resorting to high-energy bonding or to exotic mechanisms. These results suggest that “planetary” type noble gases in meteorites, located at or near grain surfaces of amorphous carbon, may be trapped by adsorption in micropores, whereas components such as CCFXe, which are uniformly distributed in their carrier phases, may be trapped by mechanisms such as volume diffusion or ion implantation.