Date of Award

8-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Bruce, David A

Committee Member

Hirt , Douglas

Committee Member

Husson , Scott

Committee Member

Hwu , Shiou-Jyh

Abstract

Ceria-zirconia-yttria (CZY) mixed oxides are used as catalyst supports for three-way catalysts for automotive exhaust emission control and in solid oxide fuel cells. By improving the morphology of CZY mesopores it is possible to reduce the sintering of supported noble metals and enhance overall catalyst lifetime and performance. However, limited studies have been published on the synthetic control of the morphology of CZY materials and the effects that CZY pore geometry has on catalyst operation.
To create optimized CZY catalyst supports, novel mesoporous CZY oxides were synthesized via classical sol-gel and evaporation induced self-assembly (EISA) methods, obtaining mesostructures that exhibited excellent physical and diffusional properties. This dissertation provides a detailed analysis of the factors and mechanisms that promote the creation of ordered mesoporous CZY structures via classical sol-gel and EISA approaches.
Classical sol-gel methods are reaction-limited syntheses in which, for the case of CZY materials, the formation of polyoxides occurs rapidly as a result of the availability of water and hydrolyzing agents in the initial solution, yielding disordered oxide mesostructures. Alternatively, EISA is a diffusion-limited process, where the rate of oxide forming reactions is limited by the slow diffusion of water into the initial ethanolic-metal salts solution. The slow loss of ethanol by evaporation enables the condensation-polymerization of metal oxy-hydroxide species to coincide with the ordering of these oligomers around the self-organized polymer template. Thus, for the templated synthesis of mesoporous CZY materials, it is the rate of metal oxide condensation that determines the nature of the resulting oxide structure.
The choice of templating technique used during CZY synthesis heavily influenced post-calcination morphologies and pore sizes, but to-date no relationships between pore morphology and catalyst performance for CZY materials was presented. For this reason, the diffusion of n-hexane in mesoporous CZY supports synthesized using a variety of templates and synthesis techniques was studied by the Zero Length Column (ZLC) method obtaining each material diffusion coefficients.
Data from the ZLC method proved to be valuable in developing an understanding of the effects of pore morphology on intraparticle diffusion phenomena. The CZY oxides diffusivity values provide means of comparison of diffusional aspects that can significantly impact catalyst performance during the treatment of exhaust gases formed by combustion engines. A higher diffusivity value is expected to bring catalytic advantages for the oxide support.

Share

COinS