Catalysis Enhancement of Co₃O₄ through the Epitaxial Growth of Inert ZnO in Peroxymonosulfate Activation: The Catalytic Mechanism of Surface Hydroxyls in Singlet Oxygen Generation

As a prevalent Co-contained catalyst, modified Co₃O₄ has been widely utilized to activate peroxymonosulfate (PMS) for organic wastewater treatment due to its affordability and accessibility. While the catalysis enhancement of Co₃O₄ modified by catalytic inert species is often attributed to the formation of oxygen vacancies (OVs), the underlying mechanism beyond OVs remains unclear. Herein, we designed a one-pot pyrolysis process to synthesize ZnO/Co₃O₄ heterojunctions featuring Zn-Co tetrahedral coordination on their interface. In the PMS-advanced oxidation process for methylene blue (MB), the epitaxial growth of inert ZnO on the surface of Co₃O₄ led to an 88-fold increase in catalytic activity, facilitating the rapid degradation of organic dyes to achieve the deep mineralization of the effluent. Co sites and surface hydroxyls on the ZnO/Co₃O₄ heterojunctions played a crucial role in PMS activation, generating a variety of reactive oxygen species. Among these species, singlet oxygen (¹O₂) was identified as the dominant species responsible for MB degradation, with the assistance of a sulfate radical. Theoretical calculations demonstrated that the Zn-OH group was easier to activate than Co-OH through the polarization during PMS chemisorption. The activated Zn-OH groups served as novel active sites, participating in PMS activation in a nonradical way to generate partial ¹O₂. This study sheds new light on the effect of catalytic inert species (ZnO) on enhancing the catalytic activity of Co₃O₄, refining our understanding of the catalytic generation route of ¹O₂ in PMS activation.