AVS 52nd International Symposium
    Surface Science Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM9
Growth and Reactivity of Palladium Oxide Thin Films

Wednesday, November 2, 2005, 11:00 am, Room 203

Session: Reactions on Metals & Oxides
Presenter: J. Wang, Yale University
Authors: J. Wang, Yale University
E.I. Altman, Yale University
Correspondent: Click to Email
Palladium has become the catalyst of choice for the catalytic combustion of hydrocarbons in low NOx gas turbines. Recent studies suggest that oxidized Pd is a more active catalyst for the complete oxidation of hydrocarbons. Our prior work on the oxidation of CO and C3H6 on oxygen-covered Pd(100) showed that on the more oxygen rich surface there existed a direct oxidation route for C3H6 that proceeds with a lower activation energy than dissociation to C followed by oxidation to CO2. It was impossible to increase the oxygen coverage much beyond the oxide-like monolayer by dosing with O2 or NO2 in UHV. Two approaches are carried out to access the regime where bulk PdO forms: oxidizing the Pd(100) using an oxygen plasma in UHV; and direct growth of PdO thin films. In the first approach, XPS showed that exposure to the plasma at 575 K created bulk PdO, while temperature programmed desorption (TPD) showed an order of magnitude increase in the amount of oxygen compared to NO2 exposure. The reactivity of C3H6 on such surfaces is studied using TPD, isothermal kinetic measurements, and LEED. TPD is used to measure adsorption/reaction kinetics; mass spectroscopy to search for potential reaction products. Isothermal oxygen titration studies are performed by loading oxygen and then exposing to the reductant at fixed temperatures, which yields reaction rates versus oxygen coverage at constant temperatures, repeating the experiments at different temperatures yields activation energies. In the second approach, single crystal PdO(001) films are grown on MgO(100) substrate using oxygen plasma assisted molecular beam epitaxy (OPA-MBE). In situ reflection high energy electron diffraction (RHEED) is used to monitor the growth. Ex situ XRD, LEED, XPS and UPS are used to study the bulk and surface orientation, the surface composition and chemical states. The reactivity of the epitaxial films towards C3H6 will be compared with that of the poorly ordered PdO formed by oxidizing Pd(100).