Thermocatalytic oxidation potential of cobalt(II,III) oxide against gaseous formaldehyde in relation to hydrothermal synthesis temperature

The catalytic activity of various transition metal oxides has been reported to be tuned by controlling their synthesis conditions. In this perspective, the thermocatalytic oxidation performance of cobalt(II,III) oxides (Co₃O₄) against formaldehyde (FA) is studied in relation to the temperature selected for their hydrothermal synthesis (e.g., 30 (as room temperature (RT)), 60, 120, and 250℃) with the corresponding sample codes as Co₃O₄-RT, Co₃O₄-60, Co₃O₄-120, and Co₃O₄-250, respectively. The best performing Co₃O₄-RT records the lowest T₉₀ (temperature required to obtain 90 % conversion) for 50 ppm FA at 72 °C when operated at 0 % relative humidity (RH), 30 mg (catalyst mass: m_cat), and 100,000 mL g^-1h⁻¹ (weight hourly space velocity). The performance of Co₃O₄-RT, if assessed in terms of reaction kinetic rate (r) at X_FA = 10 %, is 2.02E-02 mmol g^-1h⁻¹. According to X-ray photoelectron spectroscopy and temperature programmed characterization analysis, the catalytic activities of Co₃O₄-RT improve greatly through the synergistic combination of diverse factors (e.g., high Co³⁺ content, abundant oxygen vacancies (OVs) and surface adsorbed oxygen (O_β) species, and good surface lattice oxygen (O_α) mobility). The analysis of in-situ diffuse reflectance infrared Fourier transform spectroscopy indicates that the FA should be oxidized into water (H₂O) and carbon dioxide (CO₂) via dioxymethylene (DOM) and formate (HCOO^–) as reaction intermediates. The density functional theory simulations suggest the (1 1 1) surface of Co₃O₄ to be catalytically active against FA. The output of this research is expected to help establish effective strategies for designing high-performance transition metal oxide catalysts for the catalytic oxidation of FA.