Resumen de Ceria for all seasons
Within this chapter we have tried to highlight the importance, versatility and potential of ceria-based materials. We have seen how it is possible to tailor its properties modifying the synthesis process. Moreover, ceria applications range a variety of fields of high technological impact. From theoretical quantum chemistry to catalysis, or from fuel cell to biomedical applications, ceria can be considered as a material with unique properties. That is why this thesis has as title Ceria for all seasons.
However, a generalized and wide use of ceria in new devices and applications has been hampered by its price. Prices for rare earth oxides have gone up in a surprising way during last twenty years. In fact, prices shot up to historical highs in 2011, even for the two most abundant of the light rare earth elements, lanthanum and cerium. However different analysts think that these prices are projected to collapse. Ceria prices have moved from 3.5 e/kg in 2008 to 139 e/kg in September 2011. However in 2012, the prices have decreased drastically to 58 e/kg and to 9.2 e/kg in 2013. Moreover is predicted this trend can continue until a historical minimum price of 0.8 e/kg in 2017. This trend is due to non-Chinese rare earth projects such as Mountain Pass in California and Mt. Weld in Western Australia that are coming on. As we have shown previously, research related to ceria has intensively increased during last two decades despite of the high prices of the materials. Thus, the collapse of its price is going to stimulate even more not only the research related to ceria but also its commercial applications.
The main concern of this work is to study ceria properties and new applications. Because ceria exhibits different properties and many applications, we have tried to investigate ceria from different points of views. The thesis have been divided in 6 chapters. After presenting a selection of the most important methods and foundations in Chapter 2, we will investigate ceria as a strongly correlated system in Chapter 3. In this chapter, we will explore different methodologies and functionals in order to optimize the description of the material. We will focus on its structure, electronic properties and reactivity. Chapter 4 is dedicated to dynamic processes involved in ceria materials which are also important for their applications. On one hand, electron mobility via polaron hopping determines the behavior of Ce(III) ions that are essential in catalysis. That is why we will study electron transfer (ET) reaction both in bulk and at the surface ceria within the two-state Marcus model. We will evaluate the most important parameters that characterize the ET process. On the other hand, ionic transport via oxygen migration is really important for solid electrolytes in fuel cells. In fact, ionic transport has been intensively studied for ceria and doped ceria bulk systems. However, the vacancy concentration is higher on the surface and the first atomic layers so we will study vacancy formation and migration in surface and subsurface positions. Chapter 5 will be focused on new 1D and 2D ceria nanostructures. For this purpose, we will make use of the CRYSTAL09 code combined with helical-rototranslational symmetry to model mono, bi and tri layer ceria nanotubes, and analyze its stability and properties. We will also investigate 2D YSZ/doped-ceria epitaxial heterostructure and the origin of their huge ionic conductivity. Finally, ceria will be studied as catalyst and photocatalyst in Chapter 6. We will show new advances in a highly active family of water gas shift catalysts based on using TiO2 as substrate and small non-stoichiometric ceria nanoclusters adsorbed with the noble metals. Moreover, ceria/titania heterostructures will be modeled to understand its photocatalytic performance in water splitting reactions.
