ausblenden:
Schlagwörter:
Auger electron spectroscopy, carbonaceous deposits, catalysis, deactivation, dehydrogenation, ethylbenzene, iron oxide, low energy electron diffraction, potassium, promotor, single crystal, styrene, oxygen
19: Iron oxide model catalysts: Adsorption and catalysis
Zusammenfassung:
An in-depth model catalysis study on a complex system is reviewed. Both 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. In order to bridge the pressure and material gap for the catalytic dehydrogenation of ethylbenzene to styrene in presence of steam, this reaction is studied at reactive gas pressures between 10-6 and 36 mbar. The samples are transferred under vacuum into an stagnation point micro-flow reactor where the reaction is studied, followed by post-reaction 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. Potassium has basically the same functions as O2. It does not seem to be involved in the catalytic dehydrogenation step but rather to block active sites if its concentration is high. Long-term deactivation occurs mainly by potassium removal in form of volatile KOH. Regeneration by “steaming” in pure H2O accelerates this process while ethylbenzene in the feed stabilizes potassium. This is ascribed to the formation of non-volatile K2CO3 which is an intermediate in potassium catalysed coke removal. The addition of O2 instead of K-promotion may be an alternative reaction route.
