In the oxidation reaction of volatile organic compounds
catalyzed
by metal oxides, distinguishing the role of active metal sites and
oxygen mobility at specific preferentially exposed crystal planes
and diverse temperatures is challenging. Herein, Co3O4 catalysts with four different preferentially exposed crystal
planes [(220), (222), (311), and (422)] and oxygen vacancy formation
energies were synthesized and evaluated in styrene complete oxidation.
It is demonstrated that the Co3O4 sheet (Co3O4–I) presents the highest C8H8 catalytic oxidation activity (R
250 °C = 8.26 μmol g–1 s–1 and WHSV = 120,000 mL h–1 g–1). Density functional theory studies reveal that it
is difficult for the (311) and (222) crystal planes to form oxygen
vacancies, but the (222) crystal plane is the most favorable for C8H8 adsorption regardless of the presence of oxygen
vacancies. The combined analysis of temperature-programmed desorption
and temperature-programmed surface reaction of C8H8 proves that Co3O4–I possesses
the best C8H8 oxidation ability. It is proposed
that specific surface area is vital at low temperature (below 250
°C) because it is related to the amount of surface-adsorbed oxygen
species and low-temperature reducibility, while the ratio of surface
Co3+/Co2+ plays a decisive role at higher temperature
because of facile lattice oxygen mobility. In situ diffuse reflectance
infrared Fourier spectroscopy and the 18O2 isotope
experiment demonstrate that C8H8 oxidation over
Co3O4–I, Co3O4–S,
Co3O4–C, and Co3O4–F is mainly dominated by the Mars–van Krevelen mechanism.
Furthermore, Co3O4–I shows superior thermal
stability (57 h) and water resistance (1, 3, and 5 vol % H2O), which has the potential to be conducted in the actual industrial
application.