Understanding the Different Roles of Adsorbed Oxygen and Lattice Oxygen Species in the Distinct Catalytic Performance of Metal Oxides for o-Xylene Oxidation

Metal oxides have always been considered as promising non-noble metal catalysts for VOC elimination and generally show distinct performance based on their reactive oxygen species (ROS). This work originally investigated the roles of adsorbed oxygen (Oads) and lattice oxygen (Olat) species in the catalytic oxidation of o-xylene. A series of metal oxide catalysts were synthesized through the pyrolysis of MOF precursors. The CeO2 catalyst showed performance superior to that of other metal oxides at lower temperature, while the Co3O4 catalyst had advantages over other metal oxides in the complete oxidation of o-xylene and CO2 generation but also exhibited a larger decrease of o-xylene conversion with the drop of O2 concentration. The o-xylene-TPD and o-xylene-TPSR (18O2/He) profiles of CeO2, Co3O4, and CuO indicated that the Oads served as the primary ROS of CeO2 and that the Olat played decisive roles in the cases of Co3O4 and CuO. Notably, the surface Olat of Co3O4 could be rapidly and completely replenished by gaseous oxygen, relying more on gaseous oxygen compared to CuO. Furthermore, in situ DRIFTS studies and DFT calculations disclosed the interactions of different ROS with o-xylene. The Oads on the CeO2 surface favored the adsorption and cleavage oxidation of the aromatic ring at lower temperature, while the Olat on the Co3O4 and CuO surface preferentially oxidized methyl groups and favored the oxidation of intermediates. Therefore, the different interactions with o-xylene and replenishment of ROS are responsible for the performance differences of CeO2, Co3O4, and CuO in the catalytic oxidation of o-xylene. This work might provide insights into the catalytic mechanism of metal oxides and benefit the design and application of efficient metal oxide catalysts for VOC elimination.