Catalytic reaction mechanism of Mn-doped nanoporous aluminophosphates for the aerobic oxidation of hydrocarbons

In this work we apply state-of-the-art electronic-structure-based computational methods based on hybrid-exchange density functional theory to study the mechanism of the aerobic oxidation of hydrocarbons catalysed by Mn-doped nanoporous aluminophosphates (Mn-AlPOs). We compare our results with available experimental data. We show that the catalytic efficiency of Mn-AlPOs in oxidation reactions is intrinsically linked to 1) the Mn redox activity, in particular between 2+ and 3+ oxidation states, and 2) the coordinative insaturation of tetrahedral Mn embedded in AlPO frameworks, which facilitates the reaction by stabilising oxo-type radicals through the formation of Mn complexes. Our mechanism demonstrates the crucial role of both Mn(III) and Mn(II) in the reaction mechanism: Mn(III) sites undergo an initial reaction cycle that leads to the production of the alkyl hydroperoxide intermediate, which can only be transformed into the oxidative products (alcohol, aldehyde and acid) by Mn(II). A preactivation step is required to yield the reduced Mn(II) sites able to decompose the hydroperoxide intermediates; this step takes place through a transformation of the hydrocarbon into the corresponding peroxo-derivative, stabilised by forming a complex with Mn(III) and yielding at the same time reduced Mn(II) sites. Both species enter a subsequent propagation cycle in which Mn(II) catalyses the dissociation of the hydroperoxide that proceeds until the formation of the oxidative products by two parallel pathways, through alkoxy- or hydroxy-radical-like intermediates, whilst the Mn(III)-peroxo complex enables further production of the hydroperoxide intermediate.     

Modelling hydrogen in silicate materials

The expectation that the hydroxyl analogue state of positive muons implanted in quartz should be stabilized by a suitable charge compensation centre is supported by preliminary data. The diamagnetic fraction correlates with aluminium content over the narrow range investigated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Anomalous changes in CuO2 plane in YBa2Cu4O8 under pressure

We report the results of an atomistic simulation study of the pressure-induced structural changes in YBa₂Cu₄O₈ from 0 to 20 GPa. It is found that the crystal has similar compressibilities in the a- and b-directions over the whole pressure range, whereas in the c-direction there are quite different compressibilities at different pressures. Our results suggest that there exists a correlation between the charge transfer and the change in the Cu(2)---O(1) bond length, but we do not support the suggestion of a direct dependence of T_c on the length of this bond. However, we predict that there are anomalous changes in the CuO₂ plane with pressure, which we propose may lead to a charge carrier redistribution on in-plane copper and in-plane oxygens, which in turn may be related to the optimum carrier concentration in the CuO₂ plane.     

Static lattice simulation of structure and transport in superionic conductors

We describe the scope, methodology and achievements of static lattice simulation methods in the study of fast-ion conductors. We discuss first the type of conduction mechanism to which simulations may be applied; we contrast the information available from static with that from dynamics simulations. The techniques and potentials used by the calculations are then described. Applications to β-Al₂O₃, Li₃N and fluorite structured compounds are then considered.     

Impurity-vacancy interactions in the alkali halides

The interaction energies of vacancies with divalent impurities are calculated using recently developed efficient computer simulation methods. The results show that nearest neighbour and next nearest neighbour complexes are roughly equally bound, thus emphasising the importance of including both types of defect pair in analyses of spin resonance and other experimental data on doped alkali halides. We also find that a simple Coulomb expression for defect interactions gives energies close to those obtained by the full calculation, for all but the nearest neighbour complex. This result encourages the use of Debye-Hückel treatments of defect activities in alkali halide crystals.     

Structural and transport properties of β″-Al2O3

We report results of a molecular dynamics simulation of sodium β″-Al₂O₃. We confirm that the superiotic properties of this material require deviation from the stoichiometric composition. Our results yield values for the transport coefficients of the material that are in good agreement with experiment. Thermal parameters, measured by diffraction studies are well reproduced by the calculations. The simulations suggest a change in the diffusion mechanism from hopping, at low temperatures, to liquid-like at higher temperatures. The change in migration mechanism isaccompanied by corresponding structural changes.     

Simulation of the embryonic stage of ZnS formation from aqueous solution

We investigate the processes of cluster formation and growth of ZnS from aqueous solution using molecular dynamics simulation techniques. The influence of both temperature and concentration is studied. We show that, at lower temperatures, the crucial process is the transformation of an outer-sphere Zn/S complex to an inner-sphere ion pair. Further growth of the latter is fast to generate negatively charged planar clusters. These clusters interact to form more stable, closed structures, which are found to be the global minima configurations in vacuo. At higher temperatures, no outer-sphere ion pairs are formed, and the larger cluster configurations form much more quickly.