Comparative insights into the role of oxygen vacancies in α-MnO2 for activating peroxymonosulfate and peroxydisulfate

Abstract

Oxygen vacancies (OVs) play a critical role in enhancing the catalytic performance of transition-metal oxides in advanced oxidation processes (AOPs). However, the effect of OVs on the activation of different persulfates, peroxymonosulfate (PMS) and peroxydisulfate (PDS), is still not well understood. Here, we employ ultrasonic defect engineering to adjust the OV content of α-MnO₂ and systematically benchmark its activation of PMS and PDS. XPS, EPR, and electrochemical analyses revealed that the ultrasound-treated α-MnO₂ exhibited a significantly increased proportion of Mn(III) and a markedly higher OV concentration, thereby creating densely populated redox-active sites for persulfate adsorption and electron transfer. Mechanistic investigations indicated that in the PMS system, the oxidation pathway was closely associated with OVs: at low OV densities, radical oxidation (SO₄˙⁻/˙OH) predominates, whereas at high OV densities, a non-radical route dominated by singlet oxygen (¹O₂) prevails. In contrast, the oxidation pathway in the PDS system exhibited a weak correlation with OV content, implicating distinct adsorption/activation motifs for PMS versus PDS on α-MnO₂. Radical scavenging experiments and EPR analyses provided supportive, semi-quantitative evidence for these mechanistic trends, within the acknowledged limitations of current ROS identification methods. Collectively, this work establishes a clear structure–reactivity correlation and provides a defect-engineering blueprint for tailoring targeted reactive oxygen species in water purification applications.