Ab Initio Study of Adsorption of Fission Gas Atoms Xe and Kr on MoS2 Monolayer Functionalized with 3d Transition Metals

It is well known that a nuclear reactor generates various fission products including radioactive fission gases made of isotopes of Xe and Kr. The separation of Xe and Kr isotopes and their entrapment are important tasks for efficient operation of nuclear reactors and fuel-reprocessing plants. Two-dimensional materials are known to have a large surface-to-volume ratio; this makes them prospective candidates as gas adsorbent materials. Motivated by this, we carry out ab initio density functional theory-based calculations to explore the reactivity and selectivity of a monolayer of pristine and 3d transition metal (TM)-functionalized MoS2 toward the fission gas atoms Xe and Kr. To this end, we first study and analyze the adsorption of the TM adatoms on the monolayer of MoS2 at different inequivalent crystallographic sites. Further, we calculate partial atomic charges and atom-projected density of states of the functionalized composite systems to understand the bonding mechanism of TM adatoms with the monolayer of MoS2. We predict the coexistence of both ionic and covalent bonding between the TM atoms and the surface atoms. Subsequently, we probe the adsorption of Xe and Kr atoms on both the pristine and the TM-functionalized MoS 2 surfaces. Our calculated results indicate that the adsorption energies for Xe (Kr) gas atoms on the functionalized MoS2 are enhanced up to a maximum of 2.77 (2.86) times of adsorption energy found in case of the pristine surface. We find that the adsorption energy of Xe and Kr gas atoms over different TM atom-functionalized MoS2 follows the order Ti > V > Co > Ni > Fe > Sc > Mn > Cr > Cu. Charge density difference analyses indicate that the polarization of Xe/Kr adatoms is enhanced in the case of functionalized surfaces, which leads to the strengthened interaction between the noble gas (NG) atoms and the functionalized surfaces, compared to the adsorption of NG on the pristine MoS2 surface. This polarization has direct one-to-one correspondence with the adsorption energy of these gas atoms on the surface. Moreover, our calculated results suggest that both the pristine and the functionalized MoS2 surfaces show selectivity for Xe atom over Kr atom. The present study predicts that the TM atom-functionalized MoS2 surfaces may have great potential for adsorption and selective separation of Xe and Kr atoms, which may have important implication in the field of spent nuclear fuel management.