Youshi Zeng

The efficient separation of xenon (Xe) and krypton (Kr) is a critical process in nuclear off-gas treatment and various industrial applications. Metal organic frameworks (MOFs) represent a highly promising class of sorbent materials for the Xe/Kr separation due to their excellent tunable properties. Currently, most studies on Xe/Kr separation using MOFs have been conducted at temperatures near room temperature. However, the effect of cryogenic temperature on the interaction between adsorption thermodynamics and molecular transport is not yet fully understood. This work chooses Ni-MOF-74 and HKUST-1, which exhibit excellent Xe/Kr separation performance, as the adsorbents to study the effect of temperature (183–298 K) on Xe/Kr separation. The static adsorption results showed that Xe uptake and packing density of Ni-MOF-74 increased with cooling down to 213 K, then decreased at lower temperatures, and the ideal adsorbed solution theory (IAST) Xe/Kr selectivity also achieves the maximum at 213 K (21.48). HKUST-1 exhibited a different trend. The adsorption capacities of both Xe and Kr increased as the temperature decreased, and the optimal selectivity of IAST was observed at 195 K. In dynamic breakthrough experiments, cooling delayed Xe breakthrough and enhanced Xe retention for both materials. The dynamic Xe/Kr separation factor of both MOFs reached highest at 195 K (Ni-MOF-74: 33.1, HKUST-1: 30.8). The optimum temperature of Ni-MOF-74 in breakthrough experiments was lower than that from IAST calculations, suggesting that molecular transport affects Xe/Kr separation performance at cryogenic temperatures. HKUST-1 maintained better transport behavior at low temperatures due to their effective pore connectivity, but its higher Kr uptake reduced the difference in Xe and Kr retention compared with Ni-MOF-74. These results show that high Xe packing density can improve equilibrium selectivity, but effective pore connectivity is also needed to maintain gas transport at cryogenic temperatures. Both factors should be considered when designing adsorbents for cryogenic gas separation.

First-principles calculations are utilized to investigate the effect of Ti and Ni co-doping on the anti-disproportionation ability of ZrCo alloy. In Ti and Ni co-doped ZrCoH3, the Ti-H(8e) bond shows a strong ionic feature and the Ni-H(8e) bond exhibits covalent bonding characteristic. Judged by the Zr-H(8e) bond length, the size of the 8e site and H diffusion barrier out of the 8e site in ZrCoH3, it can be concluded that the Ti+Ni co-doping can impair the anti-disproportionation ability of Ti doping in ZrCo alloy, which uncovers the mechanism of the negative effect of Ni on the Ti-doped ZrCo alloy found in the previous experimental results.