ausblenden:
Schlagwörter:
-
Zusammenfassung:
A model catalysis study on a complex system is reviewed, bridging the gaps to real catalysis. Unpromoted and potassium-promoted iron oxide model catalysts films of single-crystalline quality are prepared and characterized in ultrahigh vacuum (UHV) using surface-science methods. The catalytic dehydrogenation of ethylbenzene to styrene in the presence of steam is studied at reactive gas pressures between 10–6 and 36 mbar. The samples are transferred under vacuum into a microflow reactor where the catalytic performance is studied, followed by postreaction characterization in UHV. Clean hematite Fe2O3 is an excellent catalyst but deactivates quickly by reduction and by coking. Addition of H2O limits reduction to the oxidation state of magnetite Fe3O4 and counteracts coking. Both deactivation mechanisms can be avoided by addition of some O2 to the feed. Comparative measurements on a powder sample reveal that the inner surface is quickly deactivated by coking. Reactivity and deactivation was simulated by microkinetic modeling using physicochemically meaningful parameters, most of which were obtained from independent measurements of adsorption data. Even the properties of porous samples could be simulated after implementation of a pore diffusion model. Potassium as promoter is not involved in the catalytic dehydrogenation step. It prevents reduction and coking and has thus basically the same functions as O2. Long-term deactivation occurs mainly by potassium removal in form of volatile KOH. “Steaming” in pure H2O accelerates this process, while ethylbenzene in the feed stabilizes potassium. This is ascribed to the formation of nonvolatile K2CO3, which is an intermediate in potassium catalyzed coke removal. The addition of O2 instead of K-promotion may be an alternative reaction route.
