Catalytic oxidation of acetaminophen through pristine and surface-modified nitrogen-doped carbon-nanotube-catalyzed peroxydisulfate activation

Abstract

Carbon nanotube (CNT) surfaces with different oxygen contents have been fabricated through an N-doping/chemical oxidation to investigate the relationship between structural features (defects and nitrogen and oxygen contents) and catalytic degradation of acetaminophen (AAP) via potassium peroxydisulfate (PDS) activation. Scavenging experiments and EPR analysis indicated that PDS activation was initiated by radical (•OH, SO4• , and O2• ) and non-radical (1O2, surface-bound radicals and free electrons) mechanisms for AAP degradation. The surface-area-normalized AAP degradation rate constant (kN) at 0.6 mM PDS increased from 0.30 × 10−2 m−2 s−1 for pristine CNT to 0.82 × 10−2 m−2 s−1 for Ox-NCNT. The kN was well-correlated with defects (ID/IG), the fraction of N + O (hybrid sp3/sp2 configuration), and oxygen groups (C-OH, CO and COOH). Correlation analysis indicates that the electron-donating C-OH group dominantly governs the PDS activation and AAP oxidation. Similarly, among N-groups, a good correlation between kN and graphitic N also substantiates the idea that substitutional N plays on key active sites for PDS activation rather than boundary Ns (pyridinic N and pyrrolic N). The above findings were also validated through the quantitative relationship between PDS adsorption ability (QPDS) and structural features of the catalyst (defects, N + O fractions, graphitic N and oxygen groups). Pyridinic N and pyrrolic N exhibited a weak linear relationship with QPDS as well as kN, indicating that the PDS adsorption and AAP oxidation is not directly controlled by these boundary N-groups.