Exposed facet engineering of δ-MnO 2 enhances heterogeneous catalytic ozonation via a nonradical pathway

MnO 2 is one of the most widely used catalysts in heterogeneous catalytic ozonation (HCO), and its crystal structure to enhance ozonation has been largely investigated. However, the fact that different exposed crystal facets increase the oxidizing capacity of HCO has been overlooked. In this study, we synthesized δ-MnO 2 (100) and δ-MnO 2 (-111) with different exposed crystal facets, and analyzed the differences in their catalytic performance in HCO. The results indicated that δ-MnO 2 (-111) exhibited oxidative activity and could not effectively enhance acetaminophen (APAP) degradation via HCO. In contrast, δ-MnO 2 (100) lacks intrinsic oxidizing properties, but can effectively and rapidly oxidize APAP through HCO. The degradation rate constant increase brought about by exposed facets regulation is about 35 times higher than that of conventional crystal phases regulation. Notably, the rapid oxidation of APAP by δ-MnO 2 (100)/O 3 does not rely on conventional reactive oxygen species (ROS), but rather occurs through electron transfer complex (Mn-O 3 * ) generated from surface Mn 3+ sites and O 3 . This unique pathway enhances its potential in practical applications. In contrast, δ-MnO 2 (-111) oxidizes APAP directly, primarily by significantly depleting the surface Mn 3+ , and its enhancing effect in the HCO can be neglected. These findings demonstrate that the exposed facets engineering strategy can selectively regulate the ozonation pathway to a nonradical electron-transfer mechanism, providing a new strategy for designing highly efficient MnO 2 catalysts for HCO.