Earlier studies have shown that the most stable structures for (ZnS)n clusters with n = 10-47 are hollow polyhedral clusters ("bubbles"). We report a detailed study of larger clusters, where n = 50, 60, 70, and 80, for which onionlike or "double bubble" structures are predicted. We report calculations of the vibrational spectra and the electronic structure of bubble and double bubble clusters, which may assist in their experimental identification.
Energy minimized structures of (ZnS) n clusters, with n ranging from 10 to 47, have been generated using simulated annealing techniques based on interatomic potentials. The clusters have "bubble" like polyhedral structures, in which all of the atoms are three coordinated, rather than the densely packed bulklike structures. Density functional theory calculations also show that the bubble clusters are more stable than the bulklike clusters.
Following recent studies which showed that the most stable structures for (ZnS)(n) clusters (n= 10-47) are the so-called "bubble clusters", in which all the atoms are three-coordinated, we have used simulated annealing techniques to find the most stable structure for a larger cluster, (ZnS)(60). We find an onion-like structure, with one small cluster enclosed inside a bigger one. The inner cluster has the structure of a sodalite cage. Bonding between the inner and the outer clusters creates a network of four-coordinated atoms.
The ethylene epoxidation cycle in a H2O2/H2O-loaded Ti zeolite has been simulated by a Car-Parrinello approach. Results indicate a process where the zeolitic framework is the active oxygen mediator. The dissociative chemisorption of H2O2 leads, via a transient Ti-hydroperoxo species, to H2O and a Ti-peroxo zeolite intermediate. Transfer of active oxygen to ethylene follows, giving the epoxide and recovering the catalyst. A thorough theoretical characterization indicates that the active oxidizing species is an asymmetric eta2-Ti-peroxo, absorbing in the visible range. The lability of the intermediate is found related to eta2 <--> eta1 interconversions of the Ti-peroxo structure. The interconversions, triggered by water molecules, could account for the experimentally found reduced catalytic activity in aged TS-1 catalysts. The results provide a microscopic picture of the reactivity and dehydration/aging processes of the catalyst fully consistent with experiments and highlight the fundamental role of the Lewis acid character of Ti in the formation, reactivity, and degradation of the active oxidizing species.
We present the results of ab initio molecular dynamics simulations on titanium offretite. The aim of this work is to investigate at a microscopic level the room temperature behavior of titanium inserted into a fully periodic zeolitic framework in dry and hydrated conditions. Structural analysis indicates that titanium, at low water loading, is tetrahedrally coordinated by framework oxygens only. On the other hand, at higher loading, when water molecules are activated via hydrogen bonding, they interact with the Ti site, leading to an increase of the Ti coordination number up to 5. The insertion of a Ti center causes only moderate and local distortions to the silicate framework. Features due to Ti in the calculated vibrational spectra are in line with IR and Raman experiments.
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