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O: Fachverband Oberflächenphysik

O 28: Heterogeneous catalysis II

O 28.1: Vortrag

Dienstag, 27. März 2012, 10:30–10:45, MA 042

Ethylene Oxide Formation over Ag: Investigation of the "electrophilic" Oxygen on Ag(111) — •Sebastian Böcklein¹, Miguel À. Niño², Tevfik O. Menteş², Andrea Locatelli², Sebastian Günther³, and Joost Wintterlin¹ — ¹Ludwig-Maximilians-Universität München, Germany — ²Sincrotrone Trieste S.C.p.A., Trieste, Italy — ³Technische Universität München, Germany

The understanding of the ethylene oxide formation crucially depends on the active oxygen species on the Ag catalyst. We have used x-ray photoelectron spectroscopy (XPS), temperature-programmed desorption and reaction (TPD and TPR), low-energy electron- as well as photo electron emission microscopy (LEEM / XPEEM) and scanning tunneling microscopy (STM) to identify and characterize O species on Ag(111). In addition to the (4×4)O phase (O 1s BE 528.3 eV) assigned to "nucleophilic" oxygen, the "electrophilic" oxygen (most likely the active species for the partial oxidation) was prepared under UHV conditions applying a special treatment using NO₂. The latter phase has previously only been prepared by extremely high O₂ exposures. XPS reveals two species (O 1s BE 530.2 and 530.7 eV). STM and LEED show a (7× √3)rect structure, which is connected with a massive change of the surface morphology. STM and TPR experiments show that this phase reacts with ethylene to give ethylene oxide. In situ STM experiments, performed under reaction conditions in the mbar regime, show structures and surface morphology changes that had before been observed in UHV experiments.