Insights into Heteroatom-Doped Graphene for Catalytic Ozonation: Active Centers, Reactive Oxygen Species Evolution, and Catalytic Mechanism

Abstract

To guide the design of novel graphene-based catalysts in catalytic ozonation for micropollutant degradation, the mechanism of catalytic ozonation with heteroatom-doped graphene was clarified. Reduced graphene oxide doped with nitrogen, phosphorus, boron, and sulfur atoms (N-, P-, B-, and S-rGO) were synthesized, and their catalytic ozonation performances were evaluated in the degradation of refractory organics and bromate elimination simultaneously. Doping with heteroatoms, except sulfur, significantly improved the catalytic ozonation activity of graphene. Introducing sulfur atoms destroyed the stability of graphene during ozonation, with the observed partial performance improvement caused by surface adsorption. Degradation pathways for selected refractory organics were proposed based on the intermediates identified using high-resolution Orbitrap mass spectroscopy and gas chromatographic–mass spectroscopy. Three and six new unopened intermediates were identified in benzotriazole and p-chlorobenzoic acid degradation, respectively. Roles of chemical functional groups, doped atoms, free electron, and delocalized π electron of heteroatom-doped graphene in catalytic ozonation were identified, and contributions of these active centers to the formation of reactive oxygen species (ROS), including hydroxyl radicals, superoxide radicals, singlet oxygen, and H2O2, were evaluated. A mechanism for catalytic ozonation by heteroatom-doped graphene was proposed for the first time.

Graphene-Based Catalysts for Ozone Processes to Decontaminate Water

“…The better performance of doped catalysts could be attributed to the catalytic sites created by atom doping. Similar results were found by Song et al [70], who employed rGO doped with different heteroatoms (N, P, B, and S), their performances being evaluated in terms of the degradation of refractory organics (p-CBA and benzotriazole) and bromate elimination simultaneously. Doping with heteroatoms except sulfur significantly improved catalytic ozonation activity of rGO, the catalytic activity ranking being as follows: N-rGO > P-rGO > B-rGO > rGO > S-rGO.…”

“…In this respect, negligible adsorption of some low molecular weight carboxylic acids (i.e., oxalic, acetic, and formic acids) as well as acetyl salycilyc acid, p-HBA and 4-nitrophenol onto a prepared rGO (362 m 2 g −1 ) has also been reported [60]. Doping GO and rGO with heteroatoms did not enhance adsorption of organic compounds from aqueous solution, as observed by various researchers [67,68,70,71]. Regarding graphene-based composites, different adsorption capacities have been reported.…”

“…Most of the reviewed works on graphene-based catalyst for ozone reactions in water make use of XPS, especially when the graphene material is doped or used in a nanostructure with other components. Thus, XPS survey spectra have been used to confirm the presence of foreign atoms on the graphene (doped graphene) and element spectra provide information about the surface functional group, which may be involved in the catalytic mechanism [46,47,48,49,53,59,60,61,64,68,70,71,72,73].…”

“…In order to improve the catalytic properties of graphene, the designed incorporation of heteroatoms into the carbon framework has been reported [69]. In this sense, rGO has been doped with nitrogen, phosphorous, sulfur, and boron aimed to enhance its catalytic activity in ozone reactions [49,67,70,71]. In all of these works, the preparation of doped rGO was accomplished by thermal treatment of a mixture of GO and the dopant agent.…”

Reduced graphene oxide–metal nanoparticle composite membranes for environmental separation and chloro-organic remediation

“…Active sites involved in electron transfer reactions are believed to be the edge defects present in the basal plane of rGO. [14][15][16] Ketonic groups present on the basal plane has higher electronic density, which enhances the tendency of these materials to get involved in the electron transfer reactions. 15 Doping of GO with nitrogen and sulfur have also been reported to enhance the capability of GO to improve reaction efficiency because of the redistribution of charge densities and creation of more active sites.…”