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 elemantary 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(111), Fe 3 O 4 (111) and α-Fe 2 O 3 (0001) films grown epitaxially on Pt(111) 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 adsorptiondesorption 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 α-Fe 2 O 3 (0001) containing surface defects is catalytically active, whereas Fe 3 O 4 (111) 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 sites 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.-2 -"2kyEi9FYSQjBVrZxIPQ.FHIAC_WRa02_review.doc", Datum: 19.02.03

Using spin-density functional theory we investigated various possible structures of the hematite (0001) surface. Depending on the ambient oxygen partial pressure, two geometries are found to be particularly stable under thermal equilibrium: one being terminated by iron and the other by oxygen. Both exhibit huge surface relaxations (−57% for the Fe-and −79% for the O-termination) with important consequences for the surface electronic and magnetic properties. With scanning tunneling microscopy we observe two different surface terminations coexisting on single crystalline α-Fe2O3 (0001) films, which were prepared in high oxygen pressures.PACS numbers:68.35. Bs,61.16.Ch,68.35.Md,71.15.Ap Although metal-oxide surfaces play a crucial role for several profitable processes, good quality experimental and theoretical studies of their atomic structure and electronic properties are scarce. For example, α-Fe 2 O 3 appears to be the active catalytic material for producing styrene, 1 which was substantiated by recent reactivity studies performed over single crystalline hematite model catalyst films. 2 Other candidate applications are photoelectrodes 3 and non-linear optics materials 4 . Nevertheless, the surface properties of α-Fe 2 O 3 are basically unknown, and also for other metal oxides an understanding is developed only badly. The reason is the difficult preparation of clean surfaces with defined structures and stoichiometries, which, as in the case of hematite, can require high oxygen pressures not suitable in standard ultrahigh vacuum systems. Furthermore, electron spectroscopy techniques and scanning tunneling microscopy (STM) are hampered by the insulating nature of the material. We also note that surface-science techniques often do not probe a thermal equilibrium geometry but a frozen-in metastable state. Theoretical studies, on the other hand, have to deal with 3d electrons, oxygen with very localized wave functions, a rather open structure, unusual hybridization of wave functions, huge atomic relaxations, big super cells, and magnetism. This renders an ab initio study of α-Fe 2 O 3 surfaces a most challenging investigation. Some theoretical studies of the geometry of α-Fe 2 O 3 (0001) had been performed using empirical (classical) potentials 5,6 , and Armelao et al. 7 studied the electronic structure employing a cluster approach. In this paper we report spin-density functional theory (SDFT) calculations for a slab geometry (see Fig. 1). We use the generalized gradient approximation (GGA) 8 for the exchange-correlation functional and the full-potential linearized augmented plane wave (FP-LAPW) method 9,10 to solve the Kohn-Sham equations. The STM study was performed on a thin hematite film grown epitaxially onto a Pt (111) substrate.The identification of thermal equilibrium structures of surfaces is a prerequisite for an understanding of the endurance, electronic, magnetic, and chemical properties of the material. Upper half of the slab for the unrelaxed O3-terminated surface. The cross section of the upper half ...

The growth of iron-oxide films on Pt͑111͒ prepared by iron deposition and subsequent oxidation was studied by scanning tunneling microscopy ͑STM͒ and high-resolution low-energy electron diffraction ͑LEED͒. Despite a 10% lattice mismatch to the substrate, an epitaxial growth of well-ordered films is observed. The oxide starts to grow layer by layer in a ͑111͒ orientation of the metastable cubic FeO structure up to a thickness of about 2.2 monolayers ͑ML͒. The completion of the second and third FeO layer depends on the precise oxidation temperature, and at coverages of approximately 2 ML three-dimensional Fe 3 O 4 (111) islands start to grow. The FeO͑111͒ layers consist of hexagonal close-packed iron-oxygen bilayers that are laterally expanded when compared to bulk FeO and slightly rotated against the platinum substrate. They all exhibit oxygen-terminated unreconstructed (1ϫ1) surface structures. With increasing coverage several structural film changes occur, and four coincidence structures with slightly different lateral lattice constants and rotation misfit angles against the platinum substrate are formed. In the submonolayer regime an FeO͑111͒ bilayer with a lattice constant of 3.11 Å and rotated by 1.3°against the platinum substrate is observed. Upon completion of the first layer the film gets compressed leading to a lattice constant of 3.09 Å and a rotation misfit angle of 0.6°. Between 1.5 and 2 ML a coincidence structure rotated by 30°against the platinum substrate forms, and at 2 ML a nonrotated coincidence structure with a lattice constant of 3.15 Å evolves. All these coincidence structures exhibit large periodicities between approximately 22 and 38 Å that are visible in the STM images up to the third FeO layer surface. The LEED patterns exhibit characteristic multiple scattering satellite spots. The different coincidence structures reflect lowest-total-energy arrangements, balancing the contributions of substrate-overlayer interface energies and elastic energies within the strained oxide overlayer for each coverage. ͓S0163-1829͑98͒02311-X͔

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The adsorption of water on ordered epitaxial FeO(111) and Fe 3 O 4 (111) films was investigated by thermal desorption spectroscopy (TDS) and photoelectron spectroscopy (UPS, XPS) under adsorption-desorption equilibrium conditions. On the purely oxygen-terminated FeO(111) surface, water monomers get physisorbed first, followed by the formation of a hydrogen-bonded bilayer with an ice-like structure and condensation of ice multilayers as the coverage is increased. On the Fe 3 O 4 (111) surface exposing both iron and oxygen atoms, water dissociates resulting in adsorbed hydroxyl groups, followed by coadsorption of water monomers and condensation of ice multilayers. A quantitative comparison between the hydroxyl saturation coverage and the defect concentrations deduced from LEED and STM measurements rules out a purely defect related dissociation of water. It is proposed that OHgroups are bound to iron cations and the H + species to oxygen anions exposed in the topmost layer of the regular Fe 3 O 4 (111) surface. The comparison between the FeO(111) and Fe 3 O 4 (111) surface chemistry demonstrates that the chemical reactivity of metal oxides is related to surface metal sites. The saturation coverages, isosteric heats of adsorption, preexponential frequency factors, and initial dipole moments of the different species were determined quantitatively. On the basis of these data, structural models for the adsorbed phases on the iron oxide surfaces are proposed.