Investigation into the Enhanced Catalytic Oxidation of o -Xylene over MOF-Derived Co 3 O 4 with Different Shapes: The Role of Surface Twofold-Coordinate Lattice Oxygen (O 2f )

In the removal of volatile organic compounds (VOCs), Co 3 O 4 catalysts with various morphologies are highly efficient due to rich active sites such as Co 3+, 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 Co 3 O 4 catalysts with different shapes for the catalytic oxidation of o -xylene: rod-like Co 3 O 4 -R exhibited a lower T 90% of 270 °C, superior stability, and water resistance compared with spherical Co 3 O 4 -S. The enhanced catalytic performance of Co 3 O 4 -R probably originated from its more active surface lattice oxygen, namely, twofold-coordinate lattice oxygen (O 2f ), on exposed (220) planes, which gave rise to larger CO 2 generation as revealed by HR-TEM, O 2 -TPD, and o -xylene-TPD. In situ DRIFTS study also showed that Co 3 O 4 -R adsorbed and oxidized more o -xylene via surface O 2f, 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 Co 3 O 4 -R owing to the good oxygen mobility. The lower formation energy of oxygen vacancy ( E Ov, 2.61 eV) and higher adsorption energy of oxygen ( E ads,O2, −1.71 eV) on Co 3 O 4 -R theoretically confirmed the easier oxygen vacancy formation and gaseous oxygen replenishment on Co 3 O 4 -R due to the existence of active O 2f on the surface. Therefore, the role of surface O 2f in oxygen vacancy formation and gaseous oxygen replenishment crucially contributed to the enhanced catalytic oxidation of o -xylene over MOF-derived Co 3 O 4 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.