AVS 54th International Symposium
    Surface Science Tuesday Sessions
       Session SS-TuP

Paper SS-TuP29
Heats of Adsorption and Growth Mode of Lithium on MgO(100)

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Session: Surface Science Poster Session
Presenter: N. Ruzycki, University of Washington
Authors: N. Ruzycki, University of Washington
J. Farmer, University of Washington
J.H. Baricuatro, University of Washington
C.T. Campbell, University of Washington
Correspondent: Click to Email

MgO(100) is a very well studied model oxide surface in model oxide-supported metal catalysis. Lithium-promoted MgO is an active catalyst for the partial oxidation of methane. Inherent surface defects change the surface reactivity and act as anchors for catalytically active metal nanoparticles. This study explored the adsorption energetics and growth mode of lithium atoms on stochiometric and defected MgO(100) surfaces, in a unique UHV chamber designed for sensitive adsorption calorimetry measurements, electron and ion spectroscopies and LEED. The MgO(100) thin films (4 nm thick) were grown on a Mo(100) single crystal. The lithium was vapor deposited onto the MgO(100) film under UHV conditions using a pulsed metal atom source. The sticking probability of lithium was near unity. The heat of adsorption for lithium on MgO(100) was 260 kJ/mol initially, dropping with coverage to reach the heat of sublimation of pure lithium metal (159 kJ/mol) above 0.5 ML. Low energy He+ ion scattering (LEIS) revealed the growth mode for Li adsorption: Li initially wets the surface as adatoms up to ~0.5 ML, after which 3D islands of Li grow. Work function measurements yielded a change with coverage typical for alkali metal adsorption on late transition metals, with a 2.2 eV initial decrease associated with cationic Li adatoms, followed by an increase back to the value for bulk Li metal as the Li depolarizes at higher coverages due to dipole-dipole repulsions in the Li adlayer. Argon ion beam damage created defects which adsorb Li more strongly than MgO(100) terraces (up to 370 kJ/mol or more).