Dresden 2009 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 45: Heterogeneous catalysis
O 45.4: Vortrag
Donnerstag, 26. März 2009, 11:15–11:30, SCH A215
XPS, TDS and STM Characterization of Carbonate Species and O-α on Ag(111) — •R. Reichelt, S. Böcklein, S. Günther, and J. Wintterlin — Ludwig-Maximilians-Universität, 81377 München, Germany
The interaction between Ag surfaces and oxygen is extremely complex and characterized by several different oxygen species. For this reason there is no general agreement about the role of these species in the Ag-catalyzed ethylene epoxidation. Carbonate is one of the species, and it is formed on silver at high pressures. Another one is atomic oxygen on reconstructed Ag surfaces, such as the (4×4)O reconstruction on Ag(111). An important role in recent reaction models is played by the so called O-α, O atoms on the non-reconstructed Ag surface.
Using XPS, TDS and STM we have investigated carbonate and other oxygen species on Ag(111). The species obtained sensitively depend on the preparation conditions, especially on very low impurities of COx.
Instead of the formation of O-α by exposure of Ag(111) to oxygen at room temperature we could identify three different species. Besides the (4×4)O we observed carbonate and CO2,ad. Those two can be distinguished in TDS and can be discriminated from the postulated O-α by XPS.
Carbonate could be decomposed by heating to 475 K forming the (4×4)O. Thus it shows the behaviour of O-α described in literature, and the O-α species may have to be reinterpreted. CO2,ad however may be mistaken for carbonate. In STM we followed the decomposition of (4×4)O-patches under exposure of CO2, and CO2,ad is formed instead of carbonate.