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O: Fachverband Oberflächenphysik

O 15: Heterogeneous catalysis I

O 15.1: Vortrag

Montag, 26. März 2012, 16:00–16:15, MA 042

Flow Reactor Investigations of the Ethylene Epoxide Formation over Ag(111) — •Regina Wyrwich¹, Sebastian Böcklein¹, Sebastian Günther², and Joost Wintterlin¹ — ¹Ludwig-Maximilians-Universität, München, Germany — ²Technische Universität München, Germany

The partial oxidation of ethylene to form ethylene epoxide (EtO) over Ag catalysts is a large scale catalytic process in chemical industry. Although intensive studies have been performed in the last decades the mechanism of the reaction is still not clear. Surface science experiments are hardly possible because of the extremely low reaction probability of the ethylene molecules, an example of the "pressure gap" problem. To overcome this problem we used a specially designed flow reactor in which experiments on single crystals up to atmospheric pressure could be realized. Product gas analysis was performed by quadrupole mass spectrometry (QMS). Because of the low reaction probability the QMS data are strongly affected by errors and cross sensitivities, but we show that by a careful analysis EtO formation on a Ag(111) single crystal surface can be unambiguously detected. An initial annealing step to 600 ^∘C reproducibly led to an active state of the sample. Quantitative data on the pressure, temperature and gas composition dependence of the activity and selectivity were obtained. Indications of a participation of oxygen atoms in the near-surface bulk of the Ag sample were found. We also report on experiments in which Ag samples were promoted by Cs, an important component of the industrial EtO catalyst.