Synthesis and characterization of ceria-zirconia mixed oxides based catalysts.

摘要

In the last five years, the thermal durability of automotive catalysts became a very important issue because now these catalysts will have to efficiently remove pollutants even after 100,000 miles of operation in a car. One of the approaches to obtain more durable catalysts is to use more resistant oxygen storage materials. This project was a part of this effort and consisted on investigating ceria and ceria-zirconia mixed oxides as oxygen storage materials.; Two low temperature methods to synthesize ceria-zirconia mixed oxides were studied. The addition of zirconia clearly enhances the reducibility and the oxygen storage properties of ceria. However, the benefit of the addition of zirconia is larger when ceria and zirconia form a solid solution, which is obtained by the precipitation method. The Zr concentration that optimized the oxygen storage properties of these materials is 25 mol%.; This study demonstrated that the diffusion of O{dollar}sp{lcub}2-{rcub}{dollar} from the bulk of the ceria particles is not what limits the reducibility of these materials. It was proposed that the energetics of incorporation of a new vacancy is the rate limiting step of the reduction process. In addition, it was shown that ceria-zirconia is more active for the oxygen release reaction on a surface area basis than a pure ceria support. For Pt promoted samples, it was found that the Pt/CeO{dollar}sb2{dollar} is extremely sensitive to the degree of reduction of ceria, while Pt/CeZrO{dollar}sb2{dollar} is somewhat insensitive.; The aging studies demonstrated that the noble metal area, not BET surface area or noble metal loading, is the crucial parameter in determining the oxygen storage capacities of a ceria-zirconia catalyst. Overall, this thesis showed that ceria-zirconia mixed oxides are remarkably durable oxygen storage materials. The preparation method used is very important on determining the properties of these materials.