AVS 61st International Symposium & Exhibition
    Surface Science Tuesday Sessions
       Session SS+AS+EN-TuM

Paper SS+AS+EN-TuM2
Oxidation and Chemical Reactivity of TbOx Thin Films on Pt(111)

Tuesday, November 11, 2014, 8:20 am, Room 309

Session: Synthesis, Structure and Characterization of Oxides
Presenter: Jason Weaver, University of Florida
Authors: W. Cartas, University of Florida
R. Rai, University of Florida
A. Sathe, University of Florida
A. Schaefer, University of Bremen, Germany
J.F. Weaver, University of Florida
Correspondent: Click to Email
Rare earth oxides (REOs) exhibit favorable catalytic performance for a diverse set of chemical transformations, including both partial and complete oxidation reactions. In this talk, I will discuss our recent investigations of the growth, oxidation and chemical reactivity of TbOx(111) thin films on Pt(111), and make comparisons with results for Sm2O3(111) films grown on the same substrate. Bulk terbia and samaria represent examples of REOs that are reducible vs. effectively irreducible, respectively. From low energy electron diffraction and scanning tunneling microscopy, we find that samaria and terbia grow as high quality thin films on Pt(111) during deposition in ultrahigh vacuum. Both oxides develop in the Ln2O3 stoichiometry and adopt an oxygen-deficient fluorite structure wherein the metal cations form a hexagonal lattice in registry with the Pt(111) substrate, while oxygen vacancies are randomly distributed within the films. We find that plasma-generated O-atom beams are highly effective in transforming the Tb2O3(111) films to higher Tb oxides. Based on results of X-ray photoelectron spectroscopy and O2 temperature programmed desorption (TPD), we show that exposure to O-atom beams completely oxidizes the Tb2O3(111) films to TbO2 at 300 K, for film thicknesses up to at least seven layers. Heating to ~1000 K in UHV restores the films to the Tb2O3(111) stoichiometry, and produces O2 desorption in two distinct TPD features centered at ~370 K and 660 K which we attribute to oxygen release from lattice sites located in the surface vs. bulk layers, respectively. We also find that O-atom adsorption at 90 K produces a weakly-bound state of oxygen on the TbOx films which desorbs between ~100 and 270 K. This state of oxygen may correspond to a form of chemisorbed oxygen on the TbOx film. Consistent with this interpretation, TPD experiments performed after oxidizing a Tb218O3 film with 16O-atoms demonstrate that oxygen desorption below about 500 K originates only from the oxygen that is “added” to the Tb2O3 film, while all isotopic combinations of O2 desorb from the bulk above 500 K. Lastly, I will present results which show that the oxidized TbOx films exhibit high activity and selectivity for the dehydrogenation of methanol to formaldehyde, whereas the initial Tb2O3 films have limited reactivity toward methanol.