α-MnO2 Reactive Lattice Oxygen Promotes Peroxymonosulfate-Activated Sulfamethoxazole Degradation

Activated peroxymonosulfate (PMS) processes have emerged as a highly effective advanced oxidation technique for the removal of emerging organic contaminants in water. This study successfully converted δ-MnO2 into α-MnO2 through a crystal phase transformation method via the application of a mild water bath heating process, enhancing its catalytic properties. α-MnO2 (k = 0.092 ± 0.0059 min−1) exhibited significantly higher activity than δ-MnO2 (k = 0.027 ± 0.0075 min−1) in the PMS-activated degradation of sulfamethoxazole (SMX). Importantly, 1O2 was identified as the primary reactive oxygen species in the α-MnO2 + PMS system for SMX degradation. XPS and O2-TPD characterizations demonstrated that α-MnO2 possesses a higher concentration of active lattice oxygen a and lower concentration of Mn(III) than δ-MnO2. Further analysis reveals that both surface Mn(III) and active lattice oxygen in α-MnO2 are crucial for PMS activation. Notably, 1O2 is predominantly generated through the interaction between PMS and reactive lattice oxygen. Moreover, a heterogeneous PMS activation mechanism toward α-MnO2 was proposed. This research underscores the critical role of active lattice oxygen in MnO2 for PMS activation, providing valuable insight relevant to the design of catalysts aimed at pollutant elimination in environmental applications. To the best of our knowledge, our study is the first to report a pathway for MnO2 crystal phase transition under relatively mild conditions.