Berlin 2008 – scientific programme

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O: Fachverband Oberflächenphysik

O 13: Heterogeneous Catalysis

O 13.10: Talk

Monday, February 25, 2008, 16:15–16:30, HE 101

LEEM, XPS, and STM Characterization of Oxygen Species Relevant for the Ethylene Epoxidation on Silver — •R. Reichelt, S. Günther, and J. Wintterlin — Ludwig-Maximilians-Universität, 80337 München, Germany

The epoxidation of ethylene on silver catalysts is an extremely complex reaction because of the complicated nature of the Ag/O interaction. Several oxygen species on Ag surfaces have been identified but until now there is no general agreement about their role for the catalytic reaction. Using LEEM, XPS and STM we have investigated oxygen species on Ag(111) that were prepared by thermal decomposition of NO2. We find that the species obtained sensitively depend on the preparation conditions which may explain part of the conflicting data from literature. Apart from the (4x4)O phase, another oxygen species, the so called O γ could be obtained by applying several preparation cycles of (4x4)O phase followed by flash annealing it slightly above 570 K. Angle-resolved XPS of O γ shows a surface-like behaviour. However XPS data surprisingly show that it is stable up to 800 K, suggesting a strongly bound species. Another species could be prepared by dosing CO2 on the (4x4)O structure which meanwhile vanishes in XPS. In STM the local decomposition of (4x4)-patches and related stripe phases evidenced a reaction of the (4x4)O adsorption layer with CO2. We can propose the formation of two surface carbonate species evidenced by two well separated CO2 desorption peaks in TDS. It is also demonstrated that after using NO2 readsorption of residual gas species can occur, an apparent cause for misinterpretations of spectra.