Interaction of ethylbenzene and styrene with iron oxide model catalyst films at low coverages: A NEXAFS study

The adsorption of ethylbenzene and styrene on well ordered epitaxial iron oxide model catalyst films with different stoichiometries was investigated using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). On the iron-terminated Fe3O4(111) and alpha -Fe2O3(0001) surfaces chemisorption of ethylbenzene and styrene is observed occurring initially on the iron sites via the pi -electron system of the phenyl ring. This forces the molecules into an almost flat configuration (eta (6)-like ring adsorption geometry). In the case of ethylbenzene this adsorption complex is supposed to lead to an activation of the C-H bonds, thus facilitating the dehydrogenation to styrene. The tilt angle of the aromatic ring systems increases to about 40 degrees when approaching monolayer saturation. In contrast, the interaction with the oxygen-terminated FeO(111) surface is weak and of the physisorption type. The adsorbate-adsorbate interaction dominates and causes a tilted adsorption of the molecules from the beginning.