A surface science study of model automotive emissions control catalysis: Oxygen storage properties of ceria thin films

Ceria and zirconia are important oxide components of the three-way catalyst (TWC) for automotive emissions control. Simultaneous conversion of harmful exhaust pollutants is enhanced by ceria's ability to store and release oxygen to maintain conditions within a narrow operating window of air-to-fuel ratios. Zirconia increases ceria's effectiveness for oxygen storage, yet a fundamental understanding of this favorable interaction is still lacking. In this thesis, surface-sensitive spectroscopic techniques were used to characterize interactions at ceria-zirconia interfaces and their effect on the activity of model emissions control catalysts. The structure-activity relationships and oxygen transport properties of ceria-based samples were studied primarily using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Thermal desorption of CO and conversion to CO₂ were used as chemical probes of the oxidation and reduction of ceria. NO and CH₃OH reactivity on ceria surfaces was used to monitor oxygen vacancy (Ce3+) sites. XPS was used to directly measure the oxidation state of cerium cations. These chemical and spectroscopic probes demonstrated that interactions at the ceria-zirconia interface are responsible for an enhanced reducibility, and therefore enhanced oxygen transport, throughout the ceria layer. This enhanced reducibility results in an increased oxygen storage capacity in the TWC for emissions control.