Phase-dependent reactivity of MnO2 catalysts for permanganate activation: Insight into the role of surface-adsorbed MnO4− species

Abstract

The development of green and efficient heterogeneous catalysts for permanganate (Mn(VII)) activation remains unexplored. Given the different roles of crystal structures of MnO2 in many applications, it is of great significance to investigate the reactivity of MnO2 with different phases for Mn(VII) activation. In this study, it was found that the first-order rate constants for phenol degradation in the presence of 100 μM Mn(VII) follow an order of α-MnO2 (0.0884 min−1) > δ-MnO2 (0.0746 min−1) > β-MnO2 (0.0417 min−1) > γ-MnO2 (0.0396 min−1). The fact is in line with Mn(VII) adsorption, which can be related closely to the surface area, pore diameter, and isoelectric points of the catalysts. Other phenolic contaminants such as parachlorophenol and bisphenol A were also degraded efficiently by the Mn(VII)/α-MnO2 system. After coordination with surface Mn species in MnO2, the Mn–O bond of Mn(VII) is stretched with an increase in electron transfer reactivity and a decrease in oxygen atom transfer ability. A surface-promoted electron transfer process is then proposed for phenol degradation by the system. A good adaptability and reusability of the Mn(VII)/α-MnO2 system were observed. The influence of several parameters and comparisons with other catalysts were also studied. This work provides insights into the phase-dependent reactivity of MnO2 catalysts for Mn(VII) activation and pollutant degradation.