Crystal Plane Effect of Co 3 O 4 on Styrene Catalytic Oxidation: Insights into the Role of Co 3+ and Oxygen Mobility at Diverse Temperatures.

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, Co 3 O 4 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 Co 3 O 4 sheet (Co 3 O 4 -I) presents the highest C 8 H 8 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 C 8 H 8 adsorption regardless of the presence of oxygen vacancies. The combined analysis of temperature-programmed desorption and temperature-programmed surface reaction of C 8 H 8 proves that Co 3 O 4 -I possesses the best C 8 H 8 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 Co 3+ /Co 2+ plays a decisive role at higher temperature because of facile lattice oxygen mobility. In situ diffuse reflectance infrared Fourier spectroscopy and the 18 O 2 isotope experiment demonstrate that C 8 H 8 oxidation over Co 3 O 4 -I, Co 3 O 4 -S, Co 3 O 4 -C, and Co 3 O 4 -F is mainly dominated by the Mars-van Krevelen mechanism. Furthermore, Co 3 O 4 -I shows superior thermal stability (57 h) and water resistance (1, 3, and 5 vol % H 2 O), which has the potential to be conducted in the actual industrial application.