Structure Sensitivity of Oxidative Dehydrogenation of Cyclohexane over FeOx and Au/Fe3O4 Nanocrystals

Shape-controlled nanoscale FeOx and Au/Fe3O4 catalysts with an inverse spinet structure were prepared and tested for the oxidative dehydrogenation of cyclohexane. The reaction was studied in situ in SAXS/TPRx mode with isothermal steady-state holds. {111}-Bound Fe3O4 nanooctahedra are highly stable under reaction conditions at 300 degrees C, but {100}-bound nanocubes begin to agglomerate above 250 degrees C. The selectivity to cyclohexene and benzene over CO2 depends strongly on the iron oxide shape and its interaction with the gold. When gold is added onto the iron oxide, the formation of benzene over cyclohexene is favored over both shapes. The highest benzene yield was measured on the Au/Fe3O4 octahedra. A parallel study of a commercial polycrystalline Au/Fe2O3 powder was conducted, and the activity and selectivity of this catalyst were compared with the nanoshapes. After leaching of the gold with a sodium cyanide solution and heat treatment, the atomically dispersed gold on the iron oxide surface selectively catalyzed the ODH of cyclohexane to benzene. Stabilization of cationic gold was found by in situ XANES conducted during the ODH reaction.