A surface science approach to understanding supported vanadia catalysts

Supported vanadia catalysts are used industrially for a variety of chemical processes including the selective oxidation of alcohols, ammoxidation of aromatic hydrocarbons, and the selective catalytic reduction of NOx with ammonia. Due to their industrial importance, numerous studies on these catalysts have been performed; however, the mechanism by which the support influences the reactivity of the vanadia is still not completely understood. Therefore, in an effort to better understand the structure-activity relationships that govern these catalysts, this thesis describes a study of well-defined model systems that consisted of vanadia films grown on single crystal metal oxide supports. In particular, the supports that were studied included: CeO 2(111), TiO2(110), YSZ(100), and YSZ(110). These model systems allowed the use of various surface sensitive spectroscopic techniques, including AES, HREELS, TPD and XPS. This results of this research project shows that the oxidation of methanol to formaldehyde is a function of the oxidation state of the vanadium cations in the film. In the vanadia/CeO2 (111) system, kinetic isotope effects shows that C-H bond breaking is the rate determining step in the oxidation of methanol to formaldehyde. For the vanadia/YSZ systems, the activity was affected by the crystallographic orientation of the support. In addition, the support influenced the structure of the vanadia species, which impacted the redox properties and activity of the catalyst. When appropriate, the observations from the model systems were compared with results obtained from the powdered systems