Marchbank, HR; (2017) Understanding the reactivity and defect chemistry of ceria based materials using synchrotron and neutron radiation techniques. Doctoral thesis , UCL (University College London).

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Abstract

The work, that has formed the basis of this project, has looked at the developing the understanding of ceria and related ceria materials through the investigation of the short-, medium- and long-range structure. The structure of four samples of ceria was investigated using neutron and X-ray total scattering, and X-ray absorption spectroscopy (XAS). Detailed analysis of the total pair distribution functions (PDFs) revealed that in all the samples the occupancy of both Ce4+ and O2- are very close to the ideal stoichiometry. The main differences observed in the pair correlations from various diffraction techniques were attributed to the particle size of the CeO2. Detailed analysis of the Ce L3- and K-edge EXAFS data support this finding; the decrease in higher shell coordination numbers with respect to the NIST standard, can also be attributed to differences in particle size. A combination of both x-ray and neutron in situ PDF, Ce L3- and K-edge, Pd K-edge and Pt L3-edge XAS were performed on high surface area ceria, 5wt% Pd and 5wt% Pt loaded high surface area ceria samples under reducing conditions. This allowed for a thorough and in depth study of how pure ceria behaves and whether the loading of PGMs onto the support effects its behaviour under reducing conditions. Detailed analysis of PDF and XAS data for the pure ceria support show a reduction under heating in H2:N2 whilst under cooling it ‘reoxidises’ to its original stoichiometry. In contrast both the addition of Pd and Pt to the ceria support shows enhanced reduction of the ceria upon heating and no reoxidation upon cooling under a reducing atmosphere. The effect of Pd loading on ceria was examined by using combined in situ XAS/high resolution x-ray diffraction (HRXRD) as well as in situ PDF studies. This investigated a high surface area ceria support and 1, 2.5 and 5wt% Pd loaded ceria samples respectively. All methods showed that an increase in the Pd loading decreased the reduction temperature for both the Ceria support and PdO to Pd metal formation. Both XAS and HRXRD show the reduction of all samples upon heating, whereas on cooling there is no reoxidation for the Pd loaded samples in comparison to the pure support. The method in which the Pd is incorporated onto ceria is also important, as the reoxidation is seen with a physical mixture of PdO and ceria. The reduction and oxidation of Ceria-Zirconia kappa and fluorite materials was studied using X-ray diffraction and PDF, and how the introduction of Pd into these systems influences the reduction behaviour. The Fluorite based materials show no phase change under heating in either oxidising or reducing atmospheres there are no phase changes observed. In contrast, the heating of the Kappa based materials under reducing conditions show the loss of oxygen from the lattice and the conversion to the pyrochlore structure. Upon cooling and treating with air, these are converted back to the Kappa structure.

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