Stable incorporation of MnOx quantum dots into N-doped hollow carbon: A synergistic peroxymonosulfate activator for enhanced removal of bisphenol A

Abstract

Manganese oxide (MnOx) with earth-abundance and low toxicity have received considerable interest for advanced oxidation processes (AOPs) but suffer from poor catalytic activity and stability. Herein, a unique hybrid hollow structure with ultrasmall size MnOx quantum dots (QDs, ∼3 nm) homogeneously dispersed on N-doped porous carbon shells (denoted as MnOx/N-HPCS) was designed as heterogeneous Fenton-like catalyst. The ultrafine nanoscaling morphology of MnOx QDs enlarges its specific surface area (227.9 m2/g) for providing more active sites. The N-doped carbon (NC) shells can not only greatly improve the conductivity of the hybrid and effectively prevent the aggregation of MnOx QDs, but serve as metal-free active sites for Fenton-like reaction. While the hollow porous structure enables the fast transportation of reactants access to the full-exposure active sites. When evaluated as catalyst for peroxymonosulfate (PMS) activation, the MnOx/N-HPCS hybrid outperformed other benchmark catalysts toward the removal of bisphenol A (BPA) with highest removal rate of 0.29 min−1. Results showed that exceed 99% of BPA (10 mg L−1) was removed within 20 min at the dosage of 0.2 g L−1 PMS and 0.06 g L−1 MnOx/N-HPCS. Moreover, the mechanism of PMS activation by MnOx/N-HPCS was studied. The mixed valence states of Mn, the hydroxyl species induced complexation of Mn(3+/4+)-PMS and the metal-free N-doped sites synergistically enhancing the generation of reactive oxygen species (OH and SO4−). Furthermore, the MnOx/N-HPCS catalyst also displayed good reusability (successive 4 runs) with negligible Mn leaching (less than 2 μg L−1). Together with the investigation of the application in real environment water samples (such as tap water and wastewater), we demonstrated this novel Mn-based AOPs catalyst with unique structure and high activity will hold great potential in environmental remediation.