Selective\nDegradation of Organic Pollutants Using\nan Efficient Metal-Free Catalyst Derived from Carbonized Polypyrrole\nvia Peroxymonosulfate Activation

Abstract

Metal-free\ncarbonaceous materials, including nitrogen-doped graphene\nand carbon nanotubes, are emerging as alternative catalysts for peroxymonosulfate\n(PMS) activation to avoid drawbacks of conventional transition metal-containing\ncatalysts, such as the leaching of toxic metal ions. However, these\nnovel carbocatalysts face relatively high cost and complex syntheses,\nand their activation mechanisms have not been well-understood. Herein,\nwe developed a novel nitrogen-doped carbonaceous nanosphere catalyst\nby carbonization of polypyrrole, which was prepared through a scalable\nchemical oxidative polymerization. The defective degree of carbon\nsubstrate and amount of nitrogen dopants (i.e., graphitic nitrogen)\nwere modulated by the calcination temperature. The product carbonized\nat 800 °C (CPPy-F-8) exhibited the best catalytic performance\nfor PMS activation, with 97% phenol degradation efficiency in 120\nmin. The catalytic system was efficient over a wide pH range (2–9),\nand the reaction of phenol degradation had a relatively low activation\nenergy (18.4 ± 2.7 kJ mol^–1). The nitrogen-doped\ncarbocatalyst activated PMS through a nonradical pathway. A two-step\ncatalytic mechanism was extrapolated: the catalyst transfers electrons\nto PMS through active nitrogen species and becomes a metastable state\nof the catalyst (State I); next, organic substrates are oxidized and\ndegraded by serving as electron donors to reduce State I. The catalytic\nprocess was selective toward degradation of various aromatic compounds\nwith different substituents, probably depending on the oxidation state\nof State I and the ionization potential (IP) of the organics; that\nis, only those organics with an IP value lower than ca. 9.0 eV can\nbe oxidized in the CPPy-F-8/PMS system.