Nanoscale confinement in carbon nanotubes encapsulated zero-valent iron for phenolics degradation by heterogeneous Fenton: Spatial effect and structure–activity relationship

In heterogeneous Fenton, nanostructured iron catalysts with confinement effect greatly improve the effectiveness for organic pollutants removal, however, this effect dependence on the space of substrate remains unclear and thereby limit the application of confined catalysis. Herein, Fe particles encapsulated in carbon nanotubes with inner diameter of 3–30 nm (Fe0@CNTs (x)) were prepared and tested as catalysts to degrade model pollutant of phenol and four phenolic compounds (p-diphenol (p-DP), p-aminophenol (p-AP), p-nitrophenol (p-NP) and p-chlorophenol (p-CP)). With the increasing of CNTs inner diameter from 3 to 30 nm, the encapsulated Fe particles linearly increased from 2.68 to 14.75 nm. The confinement effect was significant in the range of 3–7 nm of CNTs cavity, especially at 5 nm, which was critical to decrease iron leaching and enrich hydroxyl radical generation for efficient pollutants removal by surface heterogeneous reaction (2.13-folds compared with the homogeneous Fenton). In addition, the Fe0@CNTs (3) - (7) with positive charges was conductive to adsorb the negatively charged phenols (p-DP and p-AP) compared with non-spatial confinement of Fe-CNTs with negative charge. According to the experiment and DFT calculations, the quantitative structure–activity relationship about electron transfer was established, indicating that the electron from highest occupied molecular orbital (HOMO) of p-AP could migrate to lowest unoccupied molecular orbital (LUMO) of Fe0@CNTs and further to HOMO of Fe0@CNTs. The electron in HOMO of Fe0@CNTs could migrate into H2O2 to generate ·OH. This finding suggested a new paradigm to fulfill selective oxidation of the pollutants with electron-withdrawing group by suitable heterogeneous catalysts having significant confinement effect with more effective utilization of ·OH in Fenton reaction.