N-doped carbon nanotubes as metal-free catalysts for PAA activation to degrade emerging pollutants: Exploration of reaction mechanisms and prediction of active sites

In this study, we utilized urea as a nitrogen precursor and synthesized a series of nitrogen-doped carbon nanotubes with varying catalytic activities for PAA by adjusting the mass ratio of urea to carbon nanotubes (ranging from 0.01:1 to 2:1) and the preparation temperature (between 400 °C and 1000 °C). The activation mechanism was thoroughly examined through extensive characterization and calculations. During the activation process with PAA, we observed that the removal of contaminants was linearly correlated with the extent of graphitization (R² = 0.873) and the degree of nitrogen doping (R² = 0.951). These findings were further corroborated by density functional theory (DFT) calculations. Different types of nitrogen atoms can reduce the peroxide-breaking energy barrier in PAA to varying degrees, thereby facilitating the conversion of NCNT-PAA∗ complexes into adsorbed hydroxyl radicals. The system achieves an impressive 100 % oxidative removal of 20 μM micropollutants (e.g., bisphenol A) within 60 min, thanks to the synergistic effects of electron transfer and radical adsorption. Furthermore, it maintains a remarkable micropollutant removal efficiency of nearly 80 % after five consecutive uses. Additionally, carbon materials can be effectively integrated with membrane filtration, which not only facilitates the recycling of carbon materials in practical applications but also enhances the catalytic efficiency of nitrogen-doped multi-walled carbon nanotubes (NCNTs) while ensuring the safety of the effluent. These results underscore the extensive application prospects and research potential of carbon-based materials, while also providing a novel approach for the advanced oxidation technology of PAA.