Styrene synthesis over iron oxide catalysts: from single crystal model system to real catalystsw Achim Schu¨le, a Ulrich Nieken,* a Osama Shekhah, b Wolfgang Ranke, b Robert Schlo¨gl b and Grigorios Kolios c Received 12th January 2007, Accepted 16th March 2007 First published as an Advance Article on the web 4th May 2007 DOI: 10.1039/b700483d Surface science methods originating from analysis of noble metal catalysts are increasingly applied to metal oxides. These methods provide direct access to fundamental structural properties and phase equilibria governing the catalytic properties of metal oxide surfaces. However, no systematic way existed so far for transferring this knowledge to technical catalysts. The aim of this paper is to combine surface science with chemical engineering methods to bridge this gap. Styrene synthesis over pure and K-doped iron oxides is used as an example to develop and to explain the methodology. Single crystal films (SCF), grown epitaxially on a Pt-carrier are considered as ideal model surfaces. Comprehensive UHV analyses yield the structural properties of SCF as well as their interaction with relevant components of the reaction mixture. Their results are combined with conversion experiments to derive a mechanistic catalyst model along with quantitative information on the reaction rates. The activity of SCF as well as their phase transitions under reactive conditions can be described with a continuum model depending on the macroscopic properties of the system. This model forms the crucial link towards technical catalysts. It is shown that the behaviour of a powder catalyst can be described as a superposition of the above kinetic model and an appropriate porous model. In this paper we review the developed methodology and conclude with the evaluation of the concept. 1. Introduction Metal oxides constitute an important class of heterogeneous catalysts. They are used for synthesizing many organic com- pounds via selective oxidation, dehydrogenation, isomeriza- tion and other chemical processes. 1 Despite the technical importance of metal oxides, our understanding of their basic surface chemistry greatly lags behind that of semiconductors and metals. Particularly, very few is known of the atomic scale mechanisms of catalytic reactions on metal oxides. Conse- quently, the mechanism and the kinetics of catalytic reactions over metal oxides are described by empirically developed models. These models are valid only within a restricted window of operating parameters and conditions. This impli- cates grave limitations regarding process design and optimiza- tion. Empirical models usually regard catalysts as invariant to their environment and do not account for long-term structural changes such as transition of oxidation state or deposition of carbonaceous species. Instead, huge experimental effort is taken to optimize catalyst properties within the mentioned window of operating parameters. This way, catalysts become increasingly complex. A fundamental understanding of the reaction mechanism and of the structure–reactivity relation- ship of metal oxide catalysts would greatly promote model- based engineering of catalytic processes by extending their range of reliability. This fundamental approach has been attained for metal catalysts. Several reactions performed over metal catalysts were analyzed successfully by integrated sur- face science approaches, the most prominent being the ammo- nia synthesis. 2 In the surface science approach, the elementary steps constituting the reaction mechanism are studied under ultrahigh vacuum (UHV) conditions. In a second step, the problem of the pressure–material gap has to be addressed: does the mechanism change when going to realistic operation conditions at high gas pressures and temperatures? One can approach this question by combining surface science experi- ments with kinetic studies at high pressures. 3 In a final step, the model system activities may be compared to those of real polycrystalline catalyst samples. The structural complexity of oxide catalysts constitutes the main problem when conducting catalysis research in an inte- grated surface science approach. Technical oxide catalysts are polycrystalline materials and contain several components and promoter additives. Compared to metals, more complex reac- tion mechanisms may exist. The catalyst surface may change its structure or composition under reaction conditions, e.g. through donating or accepting oxygen within the redox cycle of the Mars-van-Krevelen mechanism 4 or through phase segregation in the course of a long-term deactivation process. 5 Besides, various obstacles occurred to the application of a Institute of Chemical Processes Engineering, University of Stuttgart, Bo ¨blinger Str. 72, D-70199 Stuttgart, Germany. E-mail: ulrich.nieken@icvt.uni-stuttgart.de b Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG, Faradayweg 4-6, D-14195 Berlin, Germany c Christ Pharma & Life Science AG, Hauptstr. 192, CH-4147 Aesch, Switzerland w The HTML version of this article has been enhanced with colour images. This journal is  c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 3619–3634 | 3619 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics