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Abstract

Crystalline Co₃O₄ nanoparticles with a uniform size of 9 nm as shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM) were synthesized by thermal decomposition of cobalt acetylacetonate in oleyl amine and applied in the oxidation of 2-propanol after calcination. The catalytic properties were derived under continuous flow conditions as function of temperature up to 573 K in a fixed-bed reactor at atmospheric pressure. Temperature-programmed oxidation, desorption (TPD), surface reaction (TPSR) and 2-propanol decomposition experiments were performed to study the interaction of 2-propanol and O₂ with the exposed spinel surfaces. Co₃O₄ selectively catalyzes the oxidative dehydrogenation of 2-propanol yielding acetone and H₂O and only to a minor extent the total oxidation to CO₂ and H₂O at higher temperatures. The superior activity of Co₃O₄ reaching nearly full conversion with 100% selectivity to acetone at 440 K is attributed to the high amount of active Co³⁺ species at the catalyst surface as well as surface-bound reactive oxygen species observed in the O₂ TPD, 2-propanol TPD, TPSR, and 2-propanol decomposition experiments. Density functional theory calculations with a Hubbard U term support the identification of fivefold coordinated octahedral surface Co_5c³⁺ as the active site, and oxidative dehydrogenation involving adsorbed atomic oxygen was found to be the energetically most favored pathway. The consumption of surface oxygen and reduction of Co³⁺ to Co²⁺ during 2-propanol oxidation derived from X-ray absorption spectroscopy and X-ray photoelectron spectroscopy measurements before and after reaction as well as poisoning by strongly bound carbonaceous species result in the loss of the low-temperature activity, while the high-temperature reaction pathway remained unaffected.