Unveiling the mechanism of ozone activation in water via singlet oxygen over cyclodextrin derived graphitic carbocatalysts

Abstract

Metal-free heterogeneous ozonation catalysis is recognized as a promising technology for water decontamination. Herein, we report the development of cyclodextrin (CD) derived graphitic (G) carbons through pyrolysis of α-, β- and γ-CDs under Ar atmosphere with fine-tuned activity as ozonation carbocatalysts for the degradation of oxalic acid in water. The G carbon obtained from α-CD showed the highest activity compared to its analogous solids under similar conditions. This solid was reused several times with a slight decrease in its activity, but after deactivation the performance of the fresh material can be restored in a large extent by a thermal pyrolysis treatment. Selective quenching experiments and electron spin resonance measurements with trapping agents revealed that O3 is transformed by interaction with G into 1O2 that becomes the active species. Experimental and computational evidence indicated that 1O2 reacts with the graphitic domains of Gs via endoperoxide intermediate that is the responsible species for oxalic acid degradation. These novel findings underscore the unforeseen importance of carbocatalytic O3 transformation into 1O2 species that further react with graphitic domains of the carbocatalyst leading to the formation of endoperoxide intermediate that is a bis oxyl radical precursor responsible for oxalic acid degradation.