1,721,333 research outputs found
Atomistic Models and Molecular Dynamics
Full text linkHere we show how atomistic computer simulation can help experiment unravel the rich structuralcomplexity of oxide nanomaterials and, ultimately, aid the fabrication of nanomaterials withimproved, tuneable or indeed new properties. We first explore the simulation methodologies:energy minimisation, monte-carlo, genetic algorithms and molecular dynamics together with thepotential models used to describe the interactions between metal and oxide ions. These tools can beused to generate realistic structures that include all the essential microstructural features observedexperimentally, such as surface structure (morphology, surface energy, faceting, surface steps,corners and edges), grain-boundaries and dislocations, intrinsic and extrinsic point defects andepitaxy. We show how the theoretician is able to capture all these (experimentally observed)structural details by attempting to simulate crystallisation. Equipped with realistic models,important properties can be calculated, including: electronic, chemical (catalytic activity, ionicdiffusion and conductivity) and mechanical (hardness, elastic constants). This is illustrated bycalculating the ease of oxygen extraction from the surface of a CeO2 nanocrystal compared with thebulk parent material with implications for oxidative catalysis. Throughout this chapter weemphasise the importance of molecular graphics - a much maligned and underrated tool - butwithout which, the generation of much of the simulation and experimental data would not havebeen possible
Computational Modelling of Yttrium Stabilised Zirconia in Catalysis
Full text linkThis thesis employs a range of computational modelling techniques to explore the structure, properties and catalytic activity of yttrium stabilised zirconia (YSZ) with a focus on its functions as a catalyst in methane reforming by partial oxidation. The surface and bulk properties of the material are explored, including the use of an exhaustive search of all possible defect configurations at a low yttrium loading in a bulk and a surface system allowing conclusions to be drawn about the relationship between defect configurations and stability. One significant property of YSZ materials is their ability to become oxygen ionic conductors at high temperatures, which is crucial to their use in solid oxide fuel cells and may be significant in catalytic applications. This thesis presents results of calculations designed to explore the effects of surfaces and defects on the ionic conductivity of YSZ materials, presenting evidence that oxygen conduction may be significantly enhanced at the surfaces of the material. Calculations using electronic structure techniques are carried out to examine the catalytic properties of YSZ. Initially potential surface active sites are characterised. The surface model is then shown to strongly adsorb and activate molecular oxygen, carbon dioxide and water from the gas phase. The energetics and electron movements in these surface interactions are described. These results provide the basis for investigations of reforming reactions in subsequent chapters and will be of interest in investigations of other catalytic processes over YSZ materials. A novel mechanism of methane C-H bond activation is reported over YSZ, activated by the presence of an adsorbed partially reduced O2 species. The mechanism is investigated in detail, including the use of two electronic structure techniques to allow mechanistic details to be proposed and activation energy estimated. It may be that this mechanism is more generally applicable to oxidative C – H bond activation over many metal oxide materials
Computational and materials structural science
Full text linkThe themes of materials and computation continue to grow and diversify in IUCrJ, but with a continuing emphasis on unravelling the structural science of complex functional materials and on developing further understanding of structure–property relations. The recent articles in the journal highlight both developments in technique and approach, as well as the exploration of new classes of system and of problems
Editorial: New Editor-in-Chief and the 360th anniversary of Philosophical Transactions
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