Advanced Nanopowder Synthesis

CeO2-based nanomaterials and their rich redox chemistry provide a possible route to pollutant abatement in various heterogeneous processes, and are in particular used as sorbents for flue gases and as oxygen promoters in three-way catalytic converters. The use of CeO2-based materials as an active compound participating in catalytic processes is associated to its function as an oxygen buffer, a key property of many materials central to a variety of economically and environmentally important technologies, such as automotive pollution control. In fact, due to the rapid interconversion between the Ce3+ and Ce4+ states, oxygen vacancies and highly mobile O2– ions, CeO2 shows a superior oxygen storage capacity - a property which can be further tuned and optimized by synthesis methods and the addition of further compounds. Noble metal catalysts supported on CeO2-based materials are key components for these applications and makes the search for a material with an optimal oxygen storage capacity an important task. This thesis deals with the synthesis and characterization of CeO2-based materials, and the investigation of structure-performance relationships for catalytic applications. The one-pot polyol synthesis is introduced for the preparation of nanosized M/CeO2 (M = Pt or Au) CO oxidation catalysts, for which the parameters for single-batch syntheses were defined. A further investigation of the reaction products led to the remarkable formulation of the incorporation-segregation mechanism: Pt is at first ionically dispersed in the support lattice (incorporation, Pt:CeO2) and then exsolutes in strongly anchored subnanometric entities (segregation, Pt/CeO2), whose size can be controlled by this approach depending on the thermal treatment and atmosphere. Secondly, an in-depth investigation of this segregation process and its kinetics with ex- and in-situ/operando characterization techniques employing synchrotron-based diffraction and X-ray absorption spectroscopy were carried out by observing the growth of the Pt entities, the change of oxidation states and the interaction with a provided gas composition. In addition, the synthesis methodology was extended to other material systems, which led to ionically dispersed Pt on ceria-zirconia (M:CeO2-ZrO2) in a broad compositional range and an investigation of the thermal behaviour and interplay between Pt and the CeO2-ZrO2 support composition was carried out. Growth inhibition of the support crystallites was observed upon Pt-doping, because of a pinning effect of Pt2+/4+ ions. A compromise between Zr (thermal stability) content and Pt load (active sites) needs to be found and correlated with the final catalytic performance, since the incorporation of Zr and Pt-doping have an effect on the stability range of the cubic CeO2-ZrO2 phases and on the solubility limit of ionic Pt. To conclude the studies on polyol syntheses of CeO2-based materials, the amount of the products was successfully increased by carrying out the process in a continuous manner ("outscaling") with the Segmented Flow Tubular Reactor. Finally, CeO2-based materials were prepared via hydrothermal synthesis in an increased volume in autoclave reactors ("upscaling") to obtain preferably rod-like particles for sorption applications. Overall, this thesis presents a number of novel findings, such as new synthesis approaches allowing for materials engineering, characterization methods and properties of CeO2-based materials themselves.