Formation of Both Free Hydroxyl Radicals and Surface Oxygen During Catalytic Ozonation by Single-Atom Iron: An Overlooked Pollutant-Dependent Oxidation Mechanism

Single-atom catalysts (SACs) such as iron (Fe) SACs have recently shown great promise for catalytic ozonation, but the major reactive species for pollutant degradation remain unclear. Here, a series of Fe SACs doped in porous nitrogen-doped graphitized carbon (Fe₁@NC, Fe₅@NC, Fe₁₀@NC) were prepared and used as model SACs for catalytic ozonation. It was found that the Fe₅@NC had much greater reactivity for catalytic ozonation than common catalysts, which was ascribed to the abundant catalytic sites including surface oxygen-containing groups and Fe–N₄ moieties. Pretreatment of Fe₅@NC by ozonation for 3 h did not deactivate the material. Accelerated formation of hydroxyl radicals in Fe SACs–O₃ oxidation was verified by electron spin resonance spectroscopy, but quenching tests showed conflicting results. Based on the experimental studies and density functional theory calculations, a pollutant-dependent degradation mechanism involving either free hydroxyl radicals or surface oxygen atoms as oxidizing species was proposed. Surface oxygen atom-dominated oxidation required the pre-adsorption of pollutants onto Fe₅@NC, otherwise, free hydroxyl radical-mediated oxidation occurred. This mechanism is expected to clarify the inconsistency regarding the formation of major reactive species in catalytic ozonation and could deepen our understanding of the catalytic behavior of SACs.