Computational modelling of yttrium stabilised zirconia in catalysis

Abstract

This 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.