Cluster Formation in Sodium-Doped Zeolite Y: Ab-Initio Simulation Study

Abstract

The species which arise when sodium atoms are absorbed by a zeolite (Na-Y) have been studied using ab-initio molecular dynamics methods. Calculations pertinent to a wide range of doping levels have been performed, and the factors which determine the preferred arrangements of the added electrons and ions were identified. At low levels of up to one added sodium atom per sodalite unit, the added electron is transferred to the interior of the sodalite cage to form highly ionized molecular species (predominantly Nq3+ at the zeolite Stoichiometry studied) leaving the added ion coordinated to a nearby oxide site in the supercage. The interactions between Nq3+ species were studied and are discussed with reference to magnetic resonance and susceptibility data. At higher doping levels, the added electrons form ionized molecular clusters in the supercage rather than the small metal particles whose presence had been postulated. The character of these clusters is determined by electronic structure effects, analogous to those which influence the structure of gas-phase clusters, and by their desire to remain coordinated to oxide ions at the windows between supercages. Parallels between the behavior of metal atoms absorbed by zeolites and by liquid ammonia, crown ethers, and other solvents are examined.

Insulating state of Na clusters and their metallic transition in low-silica X zeolite

“…As pointed out also in the ab-initio simulation of Na/Na-Y by Ursenbach et al [10], when the Na 4 3 + clusters are generated in b cages at low loading densities, the Na + ions of guest Na atoms should stay in supercages although the s-electrons are trapped in b cages because there is no unoccupied cation-site in b cages in Na-LSX. Therefore, with increasing the number of guest Na atoms, the number of Na + ions in supercages gradually increases.…”

“…The drastic change of the electronic states may be caused by the successive connection between adjacent clusters in supercages due to some special arrangement of Na + ions as well as the delocalized wave function of electrons at high Na-loading density. Ursenbach et al [10] also predicted the Na clusters connected each other formed in the supercage network at high loading density. Careful structural analysis in the vicinity of the I-M transition will give more detailed information for the abrupt change of the electronic properties of Na/Na-LSX.…”

“…The insulating state of this material was, however, confirmed by optical studies, and hence, the origin was assigned to the exchange narrowing [9]. Ursenbach et al [10] investigated the structure of Na clusters in Na-Y by ab-initio calculation for various Na-loading densities. According to their calculation, Na 4 3 + clusters are formed in the b cages only at low loading density such as nr1, where n means the average number of Na atoms per b cage.…”

Ferromagnetism of Na–K Alloy Clusters Incorporated in Zeolite Low-Silica X

“…The s-electron interaction through D6Rs is very weak, because the D6R acts as the high electronic barrier between adjacent β-cages. 22) The s-electrons in adjacent supercages can have a mutual overlapping through 12Rs. As the location of cations are adjusted by the interaction with s-electrons, and vice versa, the wave function of s-electrons are determined by the distribution of cations and mutual correlation of s-electrons as well as the repulsion for framework.…”

“…The direct exchange interaction between the adjacent β-cages is scarcely expected because the β-cages are well separated by D6Rs. 22) β-cage clusters can have an interaction with super-cage clusters through S6Rs. Actually in sodalite which has a body centered cubic structure of β-cages by the sharing of S6Rs, an antiferromagnetic ordering is observed below Néel temperature 48 K. 4,5) A finite transfer energy of s-electrons is calculated between Na 3þ 4 clusters in adjacent β-cages.…”