Crystal Plane Effect of Co3O4 on Styrene Catalytic Oxidation: Insights into the Role of Co3+ and Oxygen Mobility at Diverse Temperatures

In the oxidation reaction of volatile organic compoundscatalyzedby metal oxides, distinguishing the role of active metal sites andoxygen mobility at specific preferentially exposed crystal planesand diverse temperatures is challenging. Herein, Co3O4 catalysts with four different preferentially exposed crystalplanes [(220), (222), (311), and (422)] and oxygen vacancy formationenergies 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 & DEG;C) = 8.26 & mu;mol g(-1) s(-1) and WHSV = 120,000 mL h(-1) g(-1)). Density functional theory studies reveal that itis difficult for the (311) and (222) crystal planes to form oxygenvacancies, but the (222) crystal plane is the most favorable for C8H8 adsorption regardless of the presence of oxygenvacancies. The combined analysis of temperature-programmed desorptionand temperature-programmed surface reaction of C8H8 proves that Co3O4-I possessesthe best C8H8 oxidation ability. It is proposedthat specific surface area is vital at low temperature (below 250 & DEG;C) because it is related to the amount of surface-adsorbed oxygenspecies and low-temperature reducibility, while the ratio of surfaceCo(3+)/Co2+ plays a decisive role at higher temperaturebecause of facile lattice oxygen mobility. In situ diffuse reflectanceinfrared Fourier spectroscopy and the O-18(2) isotopeexperiment demonstrate that C8H8 oxidation overCo(3)O(4)-I, Co3O4-S,Co3O4-C, and Co3O4-F is mainly dominated by the Mars-van Krevelen mechanism.Furthermore, Co3O4-I shows superior thermalstability (57 h) and water resistance (1, 3, and 5 vol % H2O), which has the potential to be conducted in the actual industrialapplication.

Automated summary

In this study, Co3O4 catalysts with different crystal planes were synthesized and evaluated for styrene complete oxidation. It was found that the Co3O4-I crystal plane exhibited the highest catalytic activity. Furthermore, the specific surface area and the ratio of surface Co(3+)/Co2+ played important roles at low and high temperatures, respectively.