Manipulating Selectivity of Hydroxyl Radical Generation by Single- Atom Catalysts in Catalytic Ozonation: Surface or Solution

Hydroxyl radical-dominated oxidation in catalytic ozonation is, in particular, important in water treatment scenarios for removing organic contaminants, but the mechanism about ozone-based radical oxidation processes is still unclear. Here, we prepared a series of transitional metal (Co, Mn, Ni) single-atom catalysts (SACs) anchored on graphitic carbon nitride to accelerate ozone decomposition and produce highly reactive center dot OH for oxidative destruction of a water pollutant, oxalic acid (OA). We experimentally observed that, depending on the metal type, OA oxidation occurred dominantly either in the bulk phase, which was the case for the Mn catalyst, or via a combination of the bulk phase and surface reaction, which was the case for the Co catalyst. We further performed density functional theory simulations and in situ X-ray absorption spectroscopy to propose that the ozone activation pathway differs depending on the oxygen binding energy of metal, primarily due to differential adsorption of O3 onto metal sites and differential coordination configuration of a key intermediate species, *OO, which is collectively responsible for the observed differences in oxidation mechanisms and kinetics.

Automated summary

Hydroxyl radical-dominated oxidation plays an important role in catalytic ozonation, but the mechanism of ozone-based oxidation processes is still unclear. In this study, transitional metal single-atom catalysts were prepared to accelerate ozone decomposition and generate highly reactive hydroxyl radicals for the oxidative destruction of organic pollutants. Experimental and theoretical results showed that the oxidation of oxalic acid occurred dominantly in the bulk phase for the Mn catalyst, while a combination of bulk phase and surface reaction was observed for the Co catalyst. The differential adsorption of O3 and coordination configuration of a key intermediate species, *OO, were proposed to be responsible for the observed differences in oxidation mechanisms and kinetics.