AVS 45th International Symposium
    Surface Science Division Thursday Sessions
       Session SS2-ThA

Paper SS2-ThA6
The Nature, Growth and Stability of Vanadium Oxides on Pd(111)

Thursday, November 5, 1998, 3:40 pm, Room 309

Session: Oxide Growth and Structure
Presenter: F.P. Leisenberger, Karl-Franzens-Universität Graz, Austria
Authors: F.P. Leisenberger, Karl-Franzens-Universität Graz, Austria
M.G. Ramsey, Karl-Franzens-Universität Graz, Austria
S.L. Surnev, Karl-Franzens-Universität Graz, Austria
F.P. Netzer, Karl-Franzens-Universität Graz, Austria
Correspondent: Click to Email
The study of metal - oxide interfaces is at the cutting edge of present day surface science. This particular study is inspired by the fact that the vanadium oxide - palladium phase boundary is an important system in practical heterogeneous catalysis. In this work high resolution XPS and NEXAFS have been used to characterise the growth and stability of vanadium oxide on a Pd(111) single crystal surface as a model system. The vanadium oxides have been prepared by reactive evaporation of vanadium in p@sub O2@ = 2 x 10@super -7@ mbar at 250°C from submonolayer to 15 ML coverages. The oxide morphology has been gauged by the C 1s XPS following the adsorption of CO as a probe molecule for the bare palladium sites. The stoichiometry of the oxide phase varies as a function of coverage, V@sub 2@O@sub 5@ / VO@sub 2@ - like at low coverages to V@sub 2@O@sub 3@ for the thicker oxide layers as indicated by the characteristic NEXAFS fingerprints at both the V 2p and O 1s edges. The thermal stability of the oxides is dependent on the coverage: the decomposition onset temperatures range from 300°C for submonolayer coverage to @>=@600°C for 15 ML. The V 2p XPS data indicate that the oxides disproportionate into higher and lower oxidation state phases. For nominal oxide coverage of 1 ML two different LEED patterns are observed on annealing between 250°C to 350°C. The surface formed below 300°C has a complex pattern and allows the uptake of CO, while on heating to 300 - 350°C a simple p(2x2) structure evolves, on which CO cannot be accommodated. This behaviour suggests that initially ordered oxide island structures are formed with bare palladium in between. At the higher temperature (300 - 350°C) a more homogeneous passive surface layer evolves.