Seminars Archive

An approach to solve the pressure gap problem of heterogeneous catalysis - the epoxidation of ethylene over silver as an example

Abstract
The silver-catalyzed reaction of ethylene with oxygen to give ethylene epoxide (EtO), a large-scale industrial process, is not accessible to investigations by standard surface science methods because under ultrahigh vacuum (UHV) conditions the reaction only gives the total oxidation products CO2 and H2O. The synthesis of EtO is thus a prime example of the so-called pressure gap problem of catalysis research that affects all catalytic reactions. We have achieved to monitor the reaction by in-situ scanning tunneling microscopy (STM) on an Ag(111) surface. The STM experiments were performed at elevated temperatures and pressures. Under the same conditions formation of EtO could be detected by quadrupole mass spectrometry, a demanding experiment because of the extremely low reaction probability of ethylene to give EtO and the small surface area of the Ag single crystal. In the STM data an oxygen surface phase was identified that could also be prepared under UHV conditions using NO2 as oxidizing agent. The surface phase, most likely containing the active oxygen species for the reaction, could thus also be characterized by x-ray photoelectron spectroscopy, STM, low-energy electron diffraction, and low-energy electron microscopy.