Modeling the framework stability and catalytic activity of pure and transition metal-doped zeotypes

We present a thorough computational study of transition metal-doped zeolite and aluminophosphate (AlPO) frameworks. The structural and electronic chemistry of the dopants is examined with ab initio quantum mechanical calculations, and the results correlated with the Brønsted and Lewis acid strength, and with the redox potential of the dopant ions in the framework. The energetics of doping is provided, and is employed to analyze the mode of dopant incorporation, and its site ordering in the microporous framework. In total, 23 dopant ions are examined in the isostructural framework of chabasite and AlPO-34. These cover most of the isomorphous framework replacements known to occur experimentally, but also framework replacements that have not yet been achieved. In this case, ab initio modeling techniques are employed in a predictive way. Finally, we present a computational study of the alkene epoxidation on titanosilicates, that covers the whole catalytic cycle.     

The cubo-octahedral cluster in the fluorite-type lattice: A theoretical approach

A cubo-octahedral cluster within the anionic sublattice is proposed as an extended defect in the AM₃F₁₀ phases (A = K, Rb; M = Y, Bi). A computer simulation technique is used starting from crystallographic data to determine their validity and to evaluate some physical properties. A simulation study of new point defects in alkaline-earth fluorides doped with tetravalent cations is developed. The cubo-octahedral cluster is shown to be stable when it is introduced within the fluorite lattice. A mechanism is proposed for its formation on the basis of the aggregation of simpler defects.     

The defect structure of anion excess CaF2

Neutron scattering and computer simulation techniques have been used to investigate the defect cluster structure of CaF₂ doped with 5% La^3?. The results strongly support the formation of small discrete clusters rather than the superstructures that have been suggested in recent studies of anion excess fluorites. The type of cluster that emerges as dominant comprises an interstitial-dopant dimer (of the 2:2:2 type) which has captured an additional F^? interstitial. The formation of such clusters is supported by recent ITC studies.     

Theory of fission gas migration in UO2

This contribution aims to summarize the present theoretical methods used in treating gas migration in UO₂, and to review briefly the extent to which predictions of such theories for the case of Xe in UO₂, are compatible with experiment. The theory has two components: first the statistical mechanical treatment of gas trapping and migration; and secondly the calculation by computational methods of those energy terms which are shown by the statistical mechanics to be significant. The applicability of the results to the analysis of the experimental data depends upon certain crucial assumptions concerning the extent of thermodynamic control of the system. These problems will be described after our account of the theory which we present in the next section.     

The defect structure of calcia stabilized zirconia

Theoretical techniques have been used to examine the two models proposed for the defect structure of stabilized zirconias. The first model is based on simple clusters of dopant ions and the charge compensating oxygen vacancies, while the second suggests that fluorite-related microdomains form within the host lattice.

We show that the energies of formation of the clusters and microdomains are almost equal, the microdomain being favoured by 0.09 eV per dopant ion. Thus the calculations suggest that both point defects and microdomains may be present, with point defects predominating at low dopant concentration and higher temperatures.     

The effects of ligand variation on enantioselective hydrogenation catalysed by RuH2(diphosphine)(diamine) complexes

Density functional theory calculations have been used to investigate the hydrogenation of acetophenone (ACP) catalysed by the RuH(2)(diphosphine)(diamine) complexes with emphasis on the effect of the structure of the diphosphine and diamine ligands on the enantioselectivity. The computed reaction coordinate diagrams of RuH(2)(diphosphine)[(S,S)-DPEN] catalysed reactions with different (S)-diphosphine ligands (XylBINAP, TolBINAP, and BINAP) show that the presence of two methyl groups in the meta position is critical to obtaining a high difference in activation energy for the reaction pathways associated with the (R)- and (S)-alcohols, and consequently high enantioselectivity. The effect of the diamine structure while keeping the TolBINAP and XylBINAP fixed has also been analysed. To enhance the enantioselectivity of the TolBINAP system, the addition of two methyl groups and the removal of a phenyl group of the diamine (DMAPEN) offer the necessary steric interactions. We conclude by reporting a correlation between the enantiomeric excess and the difference in the computed activation energies of the two most favourable (S) and (R) reaction pathways, which shows that the computational procedure adopted could be used to predict the enantiomeric excess of ketone hydrogenation reactions catalysed by the Noyori-type catalysts, and assist in the choice of ligand when optimising the enantiomeric excess.     

Microscopic origins of electron and hole stability in ZnO

We present a fundamental method to assess the doping limits of hetero-polar materials; applied to the case of ZnO, we show clearly that electrons are stable and holes are unstable under the limits of thermodynamic control.