Paul, M; (2008) Investigation of the defect structure and energetics of nickel doped alkaline earth perovskite-type titanates using computational chemistry modelling. Doctoral thesis , UCL (University College London).

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Abstract

Atomistic computational modelling has been used to enhance the understanding of experimental studies into nickel doped alkaline earth titanate materials for use as catalysts for the partial oxidation of methane to synthesis gas. In particular, computational studies have allowed a model to be formulated of the structure of the metastable pre catalytical materials and the release of the dopant cations onto the surface at elevated temperatures prior to reduction to form the catalytically active phases. A rationale is proposed to explain the higher catalytic activity of nickel doped strontium titanate over the other nickel doped alkaline earth titanate systems. Two computational modelling techniques have been employed: interatomic potential based simulations and density functional theory. In the former, the long range and short range potentials that exist between the ions of the lattice are used to model the crystal properties. Density functional theory is an ab initio electronic structure technique which calculates the electron density associated with the lowest energy for a given arrangement of atomic nuclei. The substitution of Ni and Ni cations has been considered at both cation sites of the host perovskite lattices in the bulk and at the surface. The substitution of Ni3+ is shown to be highly unfavourable. A clear correlation between the instability of the doping process at the perovskite lattice A site and the size mismatch of the host and dopant cations has been identified. The dopant cation is also shown to be more stable at the surface than in the bulk and so the materials are expected to be enriched with dopant cations at the surface. For all systems, the dissolution of NiO into the lattice is found to be endothermic and therefore all nickel doped alkaline earth titanate systems would be expected to phase separate on heating. We conclude that the growth of clusters of NiO at the surface of the doped lattices is likely to be seeded from Ni2+ cations at the perovskite lattice A site in CaTi03 and SrTi03 and the perovskite B site in BaTi03. We suggest that the positioning of the proposed seed cation for the growth of NiO clusters suggests that the SrTi03 host will be unique in anchoring the NiO clusters to the surface which will stop them merging to form larger clusters. Therefore, the Ni/SrTi03 system will be unique in forming small, highly dispersed NiO particles on the surface thus enhancing its catalytic activity on reduction.

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