Outlook on Single Atom Catalysts for Persulfate-Based Advanced Oxidation

Single atom catalysts (SACs) have emerged as a promising catalyst material architecture for energy, chemical, and environmental applications. In the past several years, SACs have been increasingly explored for persulfate-based advanced oxidation processes (AOPs) due to their superior persulfate activation and pollutant degradation performance compared to benchmark dissolved ion and nanoparticle catalysts. However, there still exist uncertainties on the mechanism of persulfate activation by SACs, which involves a complex interplay of sulfate and hydroxyl radicals, singlet oxygen, high-valent metal species, and/or mediated electron transfer. Questions also remain as to how persulfate ions molecularly align on the single atom site, how persulfate ions are converted into reactive species, and what design parameters lead to higher efficiency for persulfate activation and pollutant degradation. In this critical review, we examine past SAC materials employed for persulfate-based AOPs and discuss how they function differently compared to their ion and nanoparticle counterparts. We further our discussion on current limitations, opportunities, and future research needs in (i) filling the knowledge gaps in the mechanisms of persulfate-SAC interactions; (ii) augmenting fundamental research with theoretical simulation and in situ characterization techniques; (iii) improving material design tailored for environmental applications; and (iv) proactively considering the challenges associated with engineering practices and complex water matrixes.