The oxidative dehydrogenation of cyclohexane by cobalt oxide nanoparticles was studied via temperature programmed reaction combined with in situ grazing incidence X-ray absorption spectroscopy and grazing incidence smallangle X-ray scattering and theoretical calculations on model Co 3 O 4 substrates. Both 6 and 12 nm Co 3 O 4 nanoparticles were made through a surfactant-free preparation and dispersed on an Al 2 O 3 surface formed by atomic layer deposition. Under reaction conditions the nanoparticles retained their oxidation state and did not sinter. They instead underwent an assembly/ disassembly process and could reorganize within their assemblies. The selectivity of the catalyst was found to be size-and temperature-dependent, with larger particles preferentially producing cyclohexene at lower temperatures and smaller particles predominantly resulting in benzene at higher temperatures. The mechanistic features thought to control the oxidative dehydrogenation of cyclohexane and other light alkanes on cobalt oxide were established by carrying out density functional theory calculations on the activation of propane, a surrogate model alkane, over model Co 3 O 4 surfaces. The initial activation of the alkane (propane) proceeds via hydrogen abstraction over surface oxygen sites. The subsequent activation of the resulting alkoxide intermediate occurs at a second surface oxygen site to form the alkene (propene) which then desorbs from the surface. Hydroxyl recombination results in the formation of water which desorbs from the surface. Oxygen is necessary to regenerate the surface oxygen sites, catalyze C−H activation steps, and minimize catalyst degradation.

We report the activity of shape-controlled metal oxide (CeO(2), ZnO and Fe(3)O(4)) supported gold catalysts for the steam reforming of methanol (SRM) and the water gas shift (WGS) reactions. Metal oxide nanoshapes, prepared by controlled hydrolysis and thermolysis methods, expose different crystal surfaces, and consequently disperse and stabilize gold differently. We observe that similar to gold supported on CeO(2) shapes exposing the {110} and {111} surfaces, gold supported on the oxygen-rich ZnO {0001} and Fe(3)O(4) {111} surfaces shows higher activity for the SRM and WGS reactions. While the reaction rates vary among the Au-CeO(2), Au-ZnO and Au-Fe(3)O(4) shapes, the apparent activation energies are similar, indicating a common active site. TPR data further indicate that the reaction lightoff coincides with the activation of Au-O-M species on the surface of all three oxide supports evaluated here. Different shapes contain a different number of binding sites for the gold, imparting different overall activity.

Shape-controlled nanoscale FeO x and Au/Fe 3 O 4 catalysts with an inverse spinel 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 Fe 3 O 4 nanooctahedra are highly stable under reaction conditions at 300 °C, but {100}-bound nanocubes begin to agglomerate above 250 °C. The selectivity to cyclohexene and benzene over CO 2 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/Fe 3 O 4 octahedra. A parallel study of a commercial polycrystalline Au/Fe 2 O 3 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.

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