Defect structure & catalytic activity of yttria-stabilised zircona

Yttria-stabilised zirconia (YSZ) is an important technological material for a number of wide- ranging applications. Of interest to this thesis is the experimental finding that YSZ is an active catalyst for methane partial oxidation. Within this thesis computer simulations are performed in order to investigate the reaction pathway for this catalytic partial oxidation of methane (CPOM), as well as to understand better the defect arrangements and ordering present in YSZ. In the first instance water adsorption on YSZ is investigated. Here the surface models used throughout this thesis are developed and the study of surface hydration is not only important when considering CPOM but also of fundamental importance and interest to solid oxide fuel cell applications. It is found that a very strong dissociative chemisorption of water with one hydroxyl occupying a stoichiometric oxygen vacancy can occur due to local structure rearrangement in YSZ. Surface reduction and re-oxidation calculations reveal that oxygen ion mobility within YSZ lowers the reduction energy, giving rise to a plausible mechanism for generating partially reduced molecular oxygen species on the YSZ surface presenting an active surface for oxidation catalysis. CPOM calculations on this activated and oxidised surface reveal the Mars- van Krevelen nature of catalysis on YSZ and demonstrate that methane oxidation occurs via a formaldehyde intermediate, which in agreement with experiment will further oxidise to a stable surface formate species. Finally a solid-solution investigation for yttria doping into zirconia over a range of dopant concentrations provides not only a method for effectively sampling the extremely large configuration space of YSZ, reproducing a number of macroscopic experimental observables about the system, but also yields local structure information about the yttrium and vacancy distribution of YSZ. In particular the energetic favourability of the yttrium-vacancy second nearest neighbour separation is established and its effect on vacancy separation revealed