Oxygen vacancies boost the efficacy of MnO2 nanoparticles in catalyzing hydrolytic degradation of organophosphate esters: Implications for managing plastic additive pollution

Abstract

The widespread plastic pollution has raised significant concerns. The breakdown process of plastic debris during weathering not only generate microplastics and nanoplastics, but also release large quantities of harmful chemical additives such as phthalates and organophosphate esters (OPEs). Metal oxides, particularly those in the form of nanoparticles, play an essential role in mediating the environmental transformation of plastic additives. However, the key structure–activity relationships governing metal oxide-mediated transformation processes remain poorly understood. Here, we demonstrate that oxygen vacancies (OVs), which are common in metal oxide nanomaterials, significantly contribute to the enhanced catalytic performance of α-MnO2 nanoparticles in promoting the hydrolysis of 4-nitrophenyl phosphate (pNPP), a model OPE pollutant. The α-MnO2 nanorods containing different OV concentrations (obtained by calcination under different atmospheres, i.e., N2 versus air) promote pNPP hydrolysis to different degrees, and the α-MnO2 material with a higher OV concentration shows higher catalytic activity. The results from spectroscopic and theoretical investigations reveal that OVs regulate the adsorption affinity to pNPP by adjusting the coordination saturation of the Mn site on the α-MnO2 surface. Additionally, the enhanced Lewis acidity at these sites (as confirmed by pyridine adsorption infrared spectroscopy and temperature-programmed desorption of ammonia) promotes the electron redistribution in pNPP, which decreases the stability of the P–O bond and enhances the reactivity of α-MnO2 towards pNPP. The findings demonstrate that metal oxide nanomaterials can significantly influence the fate and transformation of microplastic additives, and highlight the potential of defect engineering in amplifying metal oxides’ efficacy for environmental cleanup.