Atomistic and quantum mechanical computational modelling of the bulk and surface structures of the catalytically active perovskite LaCoC>3 have been undertaken to develop a better understanding of the processes involved during catalytic oxidation. Two catalytically important effects have been studied firstly the creation of oxygen vacancies resulting from Sr2+ doping at the La3+ site or reduction of Co3+ and secondly substitution by Ce4+at the La3+ site. Bulk atomistic calculations were carried out using the GULP code to compare the Mott Littleton and supercell models. Results for the former, which represent the dilute defect case, suggested that, generally, for the various defect reactions studied, interactions between nearest neighbours determines structure stability as anticipated from simple electrostatic considerations. In contrast, the supercell calculations, which represent a higher concentration of defects, showed that interaction between defect clusters is also important. In addition, calculations for Ce doping confirmed that Ce has only limited solubility in bulk LaCoC>3. However, since catalysis is a surface process the above defect calculations were repeated at the surface of LaCoC>3 using the MARVIN code. For the oxygen vacancy creation reactions, there is a general tendency that the smaller the cation defect- vacancy separation, the lower the energy of the cluster. 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. For cerium doping, computational modelling has shown that Ce is more soluble at the surface of LaCoC>3 compared to the bulk. This result has been discussed in terms of its impact both on low temperature, suprafacial and high temperature, intrafacial oxidation catalysis. Quantum mechanical supercell simulations were carried out on bulk stoichiometric and oxygen vacancy containing LaCoC>3 using DMol3 to study electronic effects. These bulk calculations showed that the low spin is favoured compared to the high spin. For the defect calculations the high spin ferromagnetic state was the most favoured. These simulations have also shown that, in the presence of oxygen vacancies, electrons are localised on cobalt.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:639532 |
| Date | January 2007 |
| Creators | Khan, Saira |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1444820/ |
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