Investigation into the Enhanced Catalytic Oxidation of o-Xylene over MOF-Derived Co3O4 with Different Shapes: The Role of Surface Twofold-Coordinate Lattice Oxygen (O2f)

Abstract

In the removal of volatile organic compounds (VOCs), Co3O4 catalysts with various morphologies are highly efficient due to rich active sites such as Co3+, adsorbed oxygen, and surface lattice oxygen species. It is worthwhile further disclosing the crucial roles and reaction mechanism of these active sites. The facile and uniform metal–organic framework (MOF) derivation method was selected to synthesize Co3O4 catalysts with different shapes for the catalytic oxidation of o-xylene: rod-like Co3O4-R exhibited a lower T90% of 270 °C, superior stability, and water resistance compared with spherical Co3O4-S. The enhanced catalytic performance of Co3O4-R probably originated from its more active surface lattice oxygen, namely, twofold-coordinate lattice oxygen (O2f), on exposed (220) planes, which gave rise to larger CO2 generation as revealed by HR-TEM, O2-TPD, and o-xylene-TPD. In situ DRIFTS study also showed that Co3O4-R adsorbed and oxidized more o-xylene via surface O2f, forming intermediates including alkoxide, carboxylate, and anhydride species and leaving oxygen vacancy. After the introduction of gaseous oxygen, the disappearance of intermediates occurred more rapidly on Co3O4-R owing to the good oxygen mobility. The lower formation energy of oxygen vacancy (EOv, 2.61 eV) and higher adsorption energy of oxygen (Eads,O2, −1.71 eV) on Co3O4-R theoretically confirmed the easier oxygen vacancy formation and gaseous oxygen replenishment on Co3O4-R due to the existence of active O2f on the surface. Therefore, the role of surface O2f in oxygen vacancy formation and gaseous oxygen replenishment crucially contributed to the enhanced catalytic oxidation of o-xylene over MOF-derived Co3O4 catalysts with different shapes. This study might shed light on the thorough understanding of active oxygen species in VOC catalytic oxidation and the preparation of efficient catalysts with various morphologies.