Kinetic Isotope Effect for H₂ and D₂ Quantum Molecular Sieving in Adsorption/Desorption on Porous Carbon Materials.

Abstract

The D2 adsorption and desorption kinetics on porous carbon materials are significantly faster (up to a factor of 1.9) than the corresponding H2 kinetics for specific pressure increments/decrements. This represents the first experimental observation of kinetic isotope quantum molecular sieving in porous materials due to the larger zero-point energy for the lighter H 2 , resulting in slower adsorption/desorption kinetics compared with the heavier D2. -(ZHAO, X.; VILLAR-RODIL, S.; FLETCHER, A. J.; THOMAS*, K. M.; J. Phys. Chem. B 110 (2006) 20, 9947-9955; Sch. Nat. Sci., Univ. Newcastle, Newcastle upon Tyne NE1 7RU, UK; Eng.) -W. Pewestorf 35-021

Quantum effect induced kinetic molecular sieving of hydrogen and deuterium in microporous materials

“…This reversal of kinetic selectivity due to quantum effects, first reported by us in an earlier paper, is supported by the recent experimental work of Zhao et al (Zhao et al 2006). These authors report the experimental verification of this kinetic isotope effect for H 2 and D 2 in porous carbon molecular sieves with micropore dimension of 0.546 and 0.566 nm, similar to the smaller window (0.543 nm) zeolite Rho, wherein H 2 shows a slower adsorption/desorption kinetics than D 2 at 77 K. Based on , we anticipate that the crossover of the diffusivities occurs at about 94 K, so that the faster diffusion of D 2 at 77 K, observed by Zhao et al, is expected given their similar pore size (Zhao et al 2006). These investigations point toward the importance of the pore dimension in achieving the reverse kinetic selectivity.…”

Adsorption dynamics of hydrogen and deuterium in a carbon molecular sieve bed at 77K

“…When the pore size of the adsorbents was small, the successive adsorption stage, after initial surface diffusion and pore diffusion, was governed by quantum sieving stemming from different zero-point energies, and the translational and rotational confinements of the hydrogen isotope molecules. The results in this study agreed well with the quantum sieving effect on adsorbents with small pores, which were reported from various studies [16,20,42,43,[50][51][52]. The quantum effect on the CMS contributed to the successive adsorption stages as discussed in Figs.…”

“…Moreover, the coverage of adsorbates on the adsorbent surface is close to zero at this stage and this stage has little influence on the elongation of breakthrough time in the column. Then, because the adsorption rate of D 2 was higher than that of H 2 at low pressure, deuterium can be preferentially adsorbed on the adsorbent [42,43]. However, the adsorption rates of hydrogen isotopes decreased with an increase in pressure.…”

“…6 and 7. In the studies of hydrogen and deuterium on metal-organic framework and porous carbon, adsorption kinetics were interpreted by a double exponential model using two adsorption rate constants[41,42]. In the study,…”

Identifying and quantifying natural CO2 sequestration processes over geological timescales: The Jackson Dome CO2 Deposit, USA

“…Alternatively, quantum molecular sieving may explain the observed Ne isotopic fractionation (Challa et al, 2001;Zhao et al, 2006). Molecular sieves are microporous materials that can separate components of a mixture based on size, shape, or differences in chemical affinity.…”