The nature of the iron oxide-based catalyst for dehydrogenation of ethylbenzene to styrene 2. Surface chemistry of the active phase

The combination of an XPS/UPS surface analysis instrument with a microreactor allowed the investigation of the surface composition of catalysts characterized by varying activities and selectivities. The active surface is a potassium iron oxide with a 1 : I atomic ratio of K : Fe, whereby iron is only in its trivalent state. Conversion of oxidic oxygen to OH groups is detrimental to the activity. No significant amount of promotor additives is present in the active surface. The process of regeneration with steam removes carbonaceous deposits but cannot reoxidize iron from Fe 2+ to Fe 3+ . A constant but small amount of potassium carbonate that cannot be increased by addition of CO 2 to the feed of the working catalyst is present at the surface. Catalysts are precursors, active materials, and irreversibly deactivated samples were studied by SEM and TEM. The surface morphology as well as the microstructure clearly indicates a solid as the active phase. This phase is generated and maintained through solid-state reactions during operation. A potassium-rich liquid film with a thickness exceeding one monolayer can be ruled out for the catalyst performance. Formation of droplets of KOH in certain regions of the catalyst signals bulk structural desintegration of the active material.