Insights into the difference in metal-free activation of peroxymonosulfate and peroxydisulfate

Abstract

Abstract Metal-free activation of peroxymonosulfate (PMS) has been reported to be more efficient than peroxydisulfate (PDS) due to the asymmetrical structure of PMS. However, we report for the first time that the nitrogen-doped porous carbon (NC-900) templated from graphitic carbon nitride exhibits unexpectedly higher catalytic activity in the activation of PDS than PMS, as evidenced by 1.74-fold greater rate constant of NC-900/PDS (0.80 min−1) than that of NC-900/PMS (0.46 min−1). Moreover, NC-900 shows superior reactivity and stability to existing nitrogen-doped nanocarbons in persulfate activation. Experimental and theoretical results revealed that PMS activation involves PMS reduction over the electron-rich graphitic N and PMS oxidation around the electron-deficient carbon atom adjacent to graphitic N in NC-900, in which PMS oxidation is the main reaction rendering singlet oxygen (1O2) the major reactive species. By contrast, PDS reduction over the electron-poor carbon atom dominates PDS activation on NC-900 for hydroxyl radical ( OH) generation. The large amount of OH with stronger oxidation capability than 1O2 contributes to the superiority of NC-900/PDS over NC-900/PMS in organic pollutant degradation. This work provides profound insights into the difference in metal-free PMS and PDS activation and presents fundamental frameworks in the design of advanced carbon-based catalysts toward sustainable environmental remediation.