An ultrafast and facile nondestructive strategy to convert various inefficient commercial nanocarbons to highly active Fenton-like catalysts

The Fenton-like process catalyzed by metal-free materials presents one of the most promising strategies to deal with the ever-growing environmental pollution. However, to develop improved catalysts with adequate activity, complicated preparation/modification processes and harsh conditions are always needed. Herein, we proposed an ultrafast and facile strategy to convert various inefficient commercial nanocarbons into highly active catalysts by noncovalent functionalization with polyethylenimine (PEI). The modified catalysts could be in situ fabricated by direct addition of PEI aqueous solution into the nanocarbon suspensions within 30 s and without any tedious treatment. The unexpectedly high catalytic activity is even superior to that of the single-atom catalyst and could reach as high as 400 times higher than the pristine carbon material. Theoretical and experimental results reveal that PEI creates net negative charge via intermolecular charge transfer, rendering the catalyst higher persulfate activation efficiency.

Due to the rapid pace of urbanization and heavy industrialization, organic pollutants in the aquatic environment have become a serious and ubiquitous problem on a global scale. The Fenton or Fenton-like process, an effective approach to generate active species by activating oxidizing agents for the elimination of a wide range of organic pollutants, has been regarded as a promising strategy to deal with the ever-growing environmental pollution (1, 2). Among the various oxidants, persulfate (S₂O₈²⁻; PS), as an inexpensive, environmentally friendly, and easily handled strong oxidant (E⁰ = 2.1 V), has been widely utilized in various fields, including water oxidation (3, 4), chemical analysis (5, 6), microbial/microfluidic fuel cell (7, 8), organic (molecular/polymer) synthesis (9, 10), and environmental remediation (11, 12), ranging from bench-scale experiments to industrial processes. Since originally introduced for in situ soil and groundwater remediation in the late 1990s to overcome the technical limitations of hydrogen peroxide (H₂O₂) (13), the PS-based Fenton-like system has drawn significant attention as an alternative to the H₂O₂-based Fenton process in water/wastewater treatment, owing to the advantages including high oxidation capacity under circumstance conditions (at neutral pH or with background constituents), high-yield radical production, low cost of storage/transportation, and various activation strategies (11, 12). Over the past few years, various transition-metal-based materials have been widely investigated as Fenton-like catalysts for PS activation (14–16). In terms of sustainable development, metal-based catalysts suffer from prohibitive cost, scarcity in nature, and secondary pollution. In this regard, metal-free carbon materials such as carbon nanotubes (CNTs) and graphene are promising candidates, owing to their unique structures and properties (17–19).In general, the efficiencies of pristine nanocarbons in activation of PS are very low (19, 20). Researchers have successively explored different strategies to boost the PS activation by regulating the local electronic environment of carbocatalysts, such as covalent doping of suitable heteroatoms (e.g., N or S) (17, 21, 22), introduction of intrinsic defects on the edge (23, 24), and construction of specific structure (25). However, the existing strategies usually involve complicated preparation processes (the usage of templates), special equipment (plasma devices), and/or harsh reaction conditions (high-temperature annealing under special atmosphere), which are time-consuming and more laborious, greatly increasing the financial costs for large-scale production and industrialization (22, 25, 26).In principle, PS activation relies mainly on the cleavage of peroxyl bonds induced by electron transfer from catalysts, and the electron-transfer efficiency could be modulated by tuning the electron states (i.e., charge/spin density or density of state) of the catalysts (14, 18, 27). Recently, various polyelectrolytes, including polyethylenimine (PEI), have been utilized to tune the surface electronic structures of electrodes in electronic/electrochemical applications (28–31). Inspired by these observations, herein, we explored an ultrafast and facile approach to transform inefficient commercial nanocarbons into highly active, metal-free catalysts for PS activation by noncovalent functionalization with PEI (SI Appendix, Scheme S1). Density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Kelvin probe force microscopy (KPFM) revealed that PEI could tune the local electronic environment of carbon atoms through the intermolecular electron donation from PEI to nanocarbons. As a result, the activities of the modified catalysts could be greatly enhanced and reach as high as 400 times higher than the pristine carbon material. Moreover, PEI-nanocarbon membranes were further in situ fabricated and applied to treat wastewater in a continuous-flow mode, revealing the feasibility of its practical application.