Oxygen abstraction together with CO adsorption and oxidation over palladium/platinum-doped cerium (IV) oxide and gold catalyst supported on iron (III) oxide were studied employing density functional theory with the inclusion of on-site Coulomb interaction (DFT+U). Hybrid functionals employing DFT method are able to re-produce structural properties for CeO2 that agrees well with experimental data. The localisation of two excess electrons upon the removal of an oxygen atom from the CeO2 lattice is well described by DFT+U and is found to be most favourable on two next nearest neighbour cerium sites from the vacancy site. This defective bulk structure gave an oxygen vacancy formation energy (Evac) of 2.45 eV using PW91+U (2.43 eV using PBE+U). The surface defect formation energies are calculated to be lower than that of the bulk structure. Other structures with different pair of Ce3+ sites at higher Evac are also present. At higher temperature, it is predicted that the energy gained from thermal heating will allow the defect structure to end up at one of the higher energy defective structures obtained. Both the CeO2 and α-Fe2O3 support are reduced more easily in the presence of transition metal atoms or clusters. Supported gold nanoparticle is found to affect the Evac on the α-Fe2O3(0001) surface only to a certain limited influential area around the nanoparticle. The Evac is reduced further when the Au atoms at the periphery sites are oxidised to give Au10O6 cluster. CO has weak interaction with the CeO2(111) surface. However, by doping the surface with Pd2+ and Pt2+ ions, CO is found to adsorb strongly at the three coordinated metal dopant that has a vacancy coordination site exposed on the surface. Weak adsorptions are also observed at the perimeter sites of Au10O6/α-Fe2O3(0001). Overall, it is predicted that CO oxidation, which follows the Mars-van Krevelen type mechanism can be enhanced by the presence of transition metal dopants or clusters. The continuous effort of researchers to reduce CO emission and the curiosity on where the excess electrons from the removed oxygen localised in the CeO2 system have been the motivation of this project. This work will provide insight on catalyst design and the understanding of the electronic structure of the systems studied.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:633601 |
| Date | January 2014 |
| Creators | Hoh, Soon Wen |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/69756/ |
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