Phase stability of ceria-hafnia and zirconia-hafnia nanoparticles and catalysis applications with copper-ceria

Chapter 1. Despite the technological importance of cerium oxide (ceria), zirconium oxide (zirconia), and hafnium oxide (hafnia), not enough is known about their mixed oxides, ceria-hafnia and zirconia-hafnia, when prepared as nanoparticles. This knowledge is critical as resources become scarcer.;Chapter 2. By aqueous co-precipitation, amorphous zirconia, hafnia, and their mixed oxides were prepared as nanoparticles. Electron microscopy of amorphous hafnia showed that the particle size is smaller if the hafnium cation concentration in the precursor solution is 0.01 M rather than 0.04 M. Annealing the pure oxides (or mixtures) prepared with the lower concentration in a reducing environment allowed for the cubic fluorite structure to be stabilized, as determined by x-ray diffraction, when the particle size is 6 nm or less.;Chapter 3. By aqueous co-precipitation and annealing, nanoparticles of ceria-hafnia, 17 nm or larger in size, as determined by the Scherrer equation, were prepared. For hafnia contents of 17% or less, x-ray diffraction patterns are consistent with the cubic structure, but Raman spectroscopy shows that tetragonal distortion in the oxygen sublattice increases with increasing hafnia content. The oxygen vacancy concentration increases with x as well.;Chapter 4. Nanoparticles of ceria-hafnia, prepared as above, were annealed in a reducing environment resulting in solubility of hafnia in ceria up to 74% hafnia as a single phase, cubic structure. Such particles were characterized by high levels of cerium in the +3 oxidation state and inhomogeneous strain.;Chapter 5. Ceria nanoparticles with copper contents of 2--20% have been prepared by co-precipitation, resulting in 6--13 nm crystallites exhibiting a defective microstructure and partial solid solution formation. The critical copper content for water-gas shift catalysis in these samples is 6--8%. Leaching experiments suggest copper migration during testing, but copper's oxidation state, before and after testing, is the same: +2.;Chapter 6. In situ x-ray diffraction of 1% and 8% copper-ceria shows that water-gas shift activity correlates well with inhomogeneous strain, ranging from 0--1% when calculated using the Williamson-Hall method. Various other in situ methods are considered as well.;Chapter 7. Conclusions are drawn based on the work presented in the preceding chapters.