Changes in the catalytic activity of oxygen-doped graphitic carbon nitride for the repeated degradation of oxytetracycline

Metal-free carbonaceous catalysts have gained growing interest because of their excellence in organic pollutant degradation. However, most of them suffer from deactivation after use, and the origins have not been investigated or understood. In this study, the changes in the characteristics after multiple uses of a carbonaceous catalyst, i.e., oxygen-doped graphitic carbon nitride (O-gCN), were investigated to identify the key factors affecting its reactivity. The O-gCN was repeatedly used to remove an antibiotic (oxytetracycline, OTC) in the presence of peroxymonosulfate (PMS). OTC removal was significantly reduced as the O-gCN was repeatedly used. The reactivity of O-gCN used five times (O-gCN5) corresponded well with the decreased signals of DMPO-X, DMPO-O2•-, and TEMP-1O2 in electron paramagnetic resonance spectra. These signal changes were accompanied by a shift of the involved reactive species from 1O2 and OH• for O-gCN to 1O2 and SO4•- for O-gCN5. The changes in activity and involved reactive species were attributed to the changes in the properties of O-gCN, considering the negligible OTC adsorption and slight PMS consumption. The results of X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy showed a decrease in the degree of defects, graphene-like layers, and crystallinity in graphitic structures, but an increase in the fractions of N and O, for O-gCN5. However, the OTC degradation pathways and intermediates were not significantly different for O-gCN and O-gCN5. These results provide valuable information for developing strategies for the design, practical use, and regeneration of carbonaceous catalysts.