Surface chemistry and catalysis on well-defined epitaxial iron-oxide layers

Metal-oxide-based catalysts are used for many important synthesis reactions in the chemical industry. A better understanding of the catalyst operation can be achieved by studying elementary reaction steps on well-defined model catalyst systems. For the dehydrogenation of ethylbenzene to styrene in the presence of steam both unpromoted and potassium promoted iron-oxide catalysts are active. Here we review the work done over unpromoted single-crystalline FeO(1 1 1), Fe3O4(1 1 1) and alpha-Fe2O3(000 1) films grown epitaxially on Pt(1 1 1) substrates. Their geometric and electronic Surface structures were characterized by STM, LEED. electron microscopy and electron spectroscopic techniques. In an integrative approach, the interaction of water, ethylbenzene and styrene with these films was investigated mainly by thermal desorption and photoelectron emission spectroscopy. The adsorption-desorption energetics and kinetics depend on the oxide Surface terminations and are correlated to the electronic Structures and acid-base properties of the corresponding oxide phases, which reveal insight into the nature of the active sites and into the catalytic function of semiconducting oxides in general. Catalytic Studies, using a batch-reactor arrangement at high gas pressures and post-reaction surface analysis, showed that only alpha-Fe2O3(0001) containing surface defects is catalytically active, whereas Fe3O4(1 1 1) is always inactive. This can be related to the elementary adsorption and desorption properties observed in ultrahigh vacuum. which indicates that the surface chemical properties of the iron-oxide films do not change significantly across the "pressure-gap". A model is proposed according to which the active site involves a regular acidic Surface site and a defect site next to it. The results on metal-oxide surface chemistry also have implications for other fields such as environmental science, biophysics and chemical sensors. (C) 2002 Published by Elsevier Science Ltd.