Enhanced peroxymonosulfate activation for phenol degradation over MnO₂ at pH 3.5-9.0 via Cu(II) substitution

Cu(II) doped mesoporous MnO₂ (Cu-MnO₂) was prepared to establish an intimate functional link between the structure substitution and catalytic peroxymonosulfate (PMS) activation. Based on the characterization of powder X-ray diffraction (XRD), N₂ adsorption-desorption measurement, scanning electron microscope (FE-SEM) and transmission electron microscope (TEM), Cu-MnO₂ had a typical long range ordered mesoporous structure and Cu was successfully introduced in octahedral framework. It exhibited excellent catalytic activity and stability for the phenol degradation by PMS. Phenol was always efficiently degraded over Cu-MnO₂ at a pH range of 3.5-9.0. For example, the reaction rate constant at pH 7.0 was 0.073 min^-1, which was two times higher than that of MnO₂ (0.039 min^-1). Importantly, ¹O₂ was identified as the primary reactive species in Cu-MnO₂/PMS system. X-ray photoelectron spectroscopy (XPS) confirmed that more exposed surface oxygen defects due to Cu doping were responsible to the enhancement of PMS activation for phenol degradation. The results of PMS decomposition and oxygen evolution indicated that surface oxygen defects lower the reaction energy barrier of PMS decomposition by generating ¹O₂ via the energy trapping by oxygen. Finally, the heterogeneous PMS activation mechanism over Cu-MnO₂ was proposed.