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

Abstract

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 (O_ads) and lattice oxygen (O_lat) 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 O_ads served as the primary ROS of CeO₂ and that the O_lat played decisive roles in the cases of Co₃O₄ and CuO. Notably, the surface O_lat 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 O_ads on the CeO₂ surface favored the adsorption and cleavage oxidation of the aromatic ring at lower temperature, while the O_lat 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.