Selective 2-Propanol Oxidation over Unsupported Co₃O₄ Spinel Nanoparticles: Mechanistic Insights into Aerobic Oxidation of Alcohols

Abstract

Crystalline Co 3 O 4 nanoparticles with a uniform size of 9 nm as shown by Xray 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 O 2 with the exposed spinel surfaces. Co 3 O 4 selectively catalyzes the oxidative dehydrogenation of 2-propanol yielding acetone and H 2 O and only to a minor extent the total oxidation to CO 2 and H 2 O at higher temperatures. The high catalytic activity of Co 3 O 4 reaching nearly full conversion with 100% selectivity to acetone at 440 K is attributed to the high amount of active Co 3+ species at the catalyst surface as well as surface-bound reactive oxygen species observed in the O 2 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 as the active site, Co 3 + 5c and oxidative dehydrogenation involving adsorbed atomic oxygen was found to be the energetically most favored pathway. The consumption of surface oxygen and reduction of

Identification of Active Sites in the Catalytic Oxidation of 2‐Propanol over Co1+xFe2–xO4 Spinel Oxides at Solid/Liquid and Solid/Gas Interfaces

“…Additionally, the overall oxidation state of the Co cations is lowered with increasing Fe content as observed by XPS and XAS measurements [24] . Co 3+ is assumed to be the most active site in the gas‐phase oxidation of CO and 2‐propanol [15,21] . Similarly, an increased catalytic activity of Co cations in higher oxidation states could also be valid in the liquid phase.…”

“…The temperature corresponding to 50 % CO conversion (T 50 ) decreased from 273 to 188 K by substitution of 15 % Fe and remained similar up to 25 % Fe. Besides total oxidation, Co−Fe‐based spinel catalysts also showed excellent activity in selective oxidation, in particular of 2‐propanol in the gas phase [19–21] . For example, monocrystalline CoFe 2 O 4 synthesized by thermal decomposition of a layered double hydroxide was very active in dehydration yielding propene, while commercial CoFe 2 O 4 primarily catalyzed the oxidative dehydrogenation to acetone [19] .…”

Characterisation and reactivity of oxygen species at the surface of metal oxides

“…It should be noted, however, that such dichotomic division is not rigorous in the light of the latest results. Several examples are provided, which document well selective oxidation reactions involving the O2or Ointermediates that are operating at low temperatures in catalytic gas-solid [84] and catalytic/photocatalytic solid-liquid systems in particular [85] EPR technique has often been applied to monitor surface reactions of oxygen species, but its applicability is limited to studies of radicals in the low temperature processes. Since most of ROS are unstable in nature, it is usually difficult to monitor directly their behavior at higher temperatures where they appear as transient intermediates [65].…”

Impact of Solvation on the Structure and Reactivity of the Co3O4 (001)/H2O Interface: Insights From Molecular Dynamics Simulations

“…Co 3 O 4 is one of them, as it shows electronic, magnetic and redox properties that make it a potential candidate for many catalytic reactions. For example, Co 3 O 4 nanoparticles have been successfully used for water and carbon monoxide oxidation, selective 2-propanol oxidation, or steam reforming of ethanol (Llorca et al, 2004;Deng and Tüysüz, 2014;Anke et al, 2019;Cai et al, 2019).…”