C. R. A. Catlow

The Stability of Fission Products in Uranium Dioxide

A review of experimental data concerning the behaviour of fission products in nuclear fuels is used to illustrate the significant variation in solubility exhibited by the different species. To understand the reasons for this variation, it is necessary to obtain a reliable estimate of the solution energies and thus to determine the most stable solution site. This we suggest will be critical in predicting the behaviour of nuclear fuels in both accident and normal operating conditions. We have therefore used the M ott-Littleton simulation technique to calculate solution energies for the fission products Br, Kr, Rb, Sr, Y, Zr, Te, I, Xe, Cs, Ba, La and Ce in UO2. We considered solution at both uranium and oxygen vacancies, the interstitial site and at the di-, tri- and tetra-vacancy complexes. Non-stoichiometry and variable charge state are important components of the model. From these results we conclude that the solubility is significantly affected by non-stoichiometry. In UO2 and UO2-x., products such as Cs, Rb and Ba are thermodynamically more stable as binary oxide precipitates. Conversely, Y, La and Sr are soluble in UO2 and UO2+x, while Cs, Rb, Sr and Ba are only soluble in UO2+x.The behaviour of I, Br and Te is complicated by the fact that these species are most stable as anions in UO2 and UO2-x but as cations in UO2+x. In our model, Zr and the inert gas species Xe and Kr are always predicted to be insoluble, while CeO2 will form a solid solution with UO2.

Computer simulation study of the defect chemistry of rutile TiO2

Abstract We examine the defect chemistry of rutile TiO2 using computer simulation techniques. The stable valencies of dopant transition metal ions in TiO2 are calculated to be Nb(V), V(III), Fe(III) and Cr(III). We find that Cr3+, Al3+, Ga3+, V3+ and Fe3+ dissolve preferentially at substitutional sites in TiO2 with charge compensation by oxygen vacancies rather than Ti4+ interstitials, although experimentally both are observed. Nb5+ is calculated to dissolve preferentially at substitutional sites with charge compensation being facilitated via Ti(III) ions or titanium vacancies. The solution enthalpy of Nb5+ is reduced by the introduction of Al3+ into the lattice. The solution enthalpy is further reduced when we consider bound Al 3+ Nb 5+ clusters.

Molecular dynamics simulations of n-butane and n-hexane diffusion in silicalite

We report molecular dynamics simulations of n-butane and n-hexane adsorbed in the zeolite silicalite. These systems have been modelled within a rigid framework approximation and a Ryckaert-Bellemans model was adopted to describe the adsorbed molecules. The parametrization due to June, Bell and Theodorou has been used to describe the host-guest molecule interactions. Long simulations (1000 ps) have been performed, modelling these systems at a variety of sorbate loadings and temperatures. These have allowed us to investigate how the presence of the zeolite framework affects the thermodynamic properties, confomational distributions and diffusion related properties of the adsorbed molecules, and their response to changes in the loading and temperature. We have obtained estimates of the diffusion coefficients and activation energies which are in good agreement with experimentally measured data.

Assignment of the complex vibrational spectra of the hydrogenated ZnO polar surfaces using QM/MM embedding

Hydrogenated zinc oxide gives rise to complex vibrational spectra with many prominent features that remain unexplained. Our calculations have unambiguously shown that the presence of vacant oxygen and zinc interstitial surface sites is the only way to rationalize the observed spectra, notably the 1710 cm−1 zinc hydride stretching mode. The large number of such sites, which expose low-coordinated surface ions, are inherent at ionically reconstructed polar surfaces. The thermal stability of the sorbed hydrogen and the infrared activity of the resulting species are correlated with site coordination and coverage.

Models of image contrast in scanning force microscopy on insulators

We review the results of theoretical modelling of scanning force microscopy and discuss the possibility of obtaining atomic and chemical resolution in contact and non-contact mode SFM, and a related issue of a working model for interpretation of SFM images. As a prototype system we consider the interactions of hard tips with softer alkali halide surfaces in UHV. We briefly review experimental data, then discuss results of static atomistic simulations and molecular dynamical modelling of contact SFM to test some of the assumptions of intuitive SFM models. Then we illustrate the shortcomings of contact SFM by considering an image of a point defect. The mechanism of resolution in non-contact SFM and the effect of avalanche tip-surface adhesion are discussed next. We conclude by discussing the status of SFM with atomic resolution.

An ab initio Hartree-Fock study of the ilmenite-structured MgSiO3

MgSiO3 ilmenite has been studied using a periodic ab initio Hartree–Fock method in order to characterize the chemical nature of the Si–O bond and to investigate the technical requirements for calculations on minerals, especially concentrating on the effect of basis set size. The unit cell volume has been optimized to assess the reliability of the method and of the computational conditions. Density of states and electron charge density maps have been calculated to investigate the electronic properties. Our calculations stress the importance of using a basis set able to describe the ionicity of O and Mg and the role played by d orbitals on Si.

A computational modelling study of oxygen vacancies at LaCoO3 perovskite surfaces

Atomistic computational modelling of the surface structure of the catalytically-active perovskite LaCoO(3) has been undertaken in order to develop better models of the processes involved during catalytic oxidation processes. In particular, the energetics of creating oxygen ion vacancies at the surface have been investigated for the three low index faces (100), (110) and (111). Two mechanisms for vacancy creation have been considered involving dopant Sr(2+) cations at the La(3+) site and reduction of Co(3+) to Co(2+). For both mechanisms, there is a general tendency that the smaller the cation defect separation, the lower the energy of the cluster, as would be expected from simple electrostatic considerations. In addition, there are clear indications that oxygen vacancies are more easily created at the surface than in the bulk. The results also confirm that the presence of defects strongly influences crystal morphology and surface chemistry. The importance of individual crystal surfaces in catalysis is discussed in terms of the energetics for the creation of oxygen vacancies.

A computational study of the adsorption of the isomers of butanol on silicalite and H-ZSM-5

We have studied the adsorption of the four isomers of butanol on silicalite and on H-ZSM-5 using an energy minimization procedure supplemented by a Monte Carlo/molecular dynamics algorithm to assist in the location of minima. The energetics and the geometries of adsorption of the butanol isomers in the pores of silicalite and H-ZSM-5 are reported. The effect of the relaxation of both the adsorbent framework and of the adsorbate molecule is investigated. Significant changes in the direction of the surface hydroxyl groups at certain crystallographic positions are induced by alcohol physisorption. For both silicalite and H-ZSM-5, similar energy values were obtained for each butanol isomer sorbed at a number of different crystallographic positions. We therefore predict that there are a range of physisorbed states for all butanols at ambient temperatures. The small variations in the adsorption energetics and sites between isomers may be explained in the terms of pore-confinement effects on the adsorption of molecules with dimensions similar to those of pentasil channels.

Solubility of cerium in LaCoO3–influence on catalytic activity

The recent interest in the catalytic properties of lanthanum perovskites for methane combustion and three way catalysis has led to considerable debate as to their structure and defect chemistry. We have investigated the doping of LaCoO3 with the tetravalent cerium cation using atomistic simulation techniques. We have compared three routes for cerium insertion and identified the favoured doping mechanism, which explain experimental observations relating to the effect of cerium on catalytic activity.

Computational Investigation into the Origins of Lewis Acidity in Zeolites

The catalytic abilities of zeolites are attributed largely to their acidic properties. This article presents a computational approach to identifying Lewis acid sites in zeolite frameworks by analyzing the major candidate structures.