ON THE NATURE OF THE ACTIVE-SITE FOR THE ETHYLBENZENE DEHYDROGENATIONOVER IRON-OXIDE CATALYSTS

The dehydrogenation of ethylbenzene to styrene was studied over single -crystalline iron oxide model catalyst films grown epitaxially onto Pt (111) substrates. The role of the iron oxide stoichiometry and of atom ic surface defects for the catalytic activity was investigated by prep aring single-phased Fe3O4(111) and alpha-Fe2O3(0001) films with define d surface structures and varying concentrations of atomic surface defe cts. The structure and composition of the iron oxide films were contro lled by low-energy electron diffraction (LEED) and Auger electron spec troscopy (AES), the surface defect concentrations were determined from the diffuse background intensities in the LEED patterns. These ultrah igh vacuum experiments were combined with batch reactor experiments pe rformed in water-ethylbenzene mixtures with a total gas pressure of 0. 6 mbar. No styrene formation is observed on the Fe3O4 films. The alpha -Fe2O3 films are catalytically active, and the styrene formation rate increases with increasing surface defect concentration on these films. This reveals atomic surface defects as active sites for the ethylbenz ene dehydrogenation over unpromoted alpha-Fe2O3. After 30 min reaction time, the films were deactivated by hydrocarbon surface deposits. The deactivation process was monitored by imaging the surface deposits wi th a photoelectron emission microscope (PEEM). It starts at extended d efects and exhibits a pattern formation after further growth. This ind icates that the deactivation is a site-selective process. Post-reactio n LEED and AES analysis reveals partly reduced Fe2O3 films, which show s that a reduction process takes place during the reaction which also deactivates the Fe2O3 films.