High Electron Affinity as a Determinant of Oxidizing Strength in Layered Birnessite

Abstract

Birnessite, a layered manganese oxide, has strong oxidation ability, but the main factor that controls this activity is still unclear. In this study, twelve natural and synthetic birnessite samples with different interlayer cations and Mn (III)/Mn (IV) ratios were prepared to examine how electron affinity affects oxidation. The electron affinity measured by ultraviolet photoelectron spectroscopy ranged from 5.6 to 6.1 eV and changed with interlayer composition. Oxidation tests using Fe (II) and As (III) followed first-order kinetics, with rate constants between 0.013 and 0.072 min⁻¹ for Fe (II) and between 0.006 and 0.041 min⁻¹ for As (III). A clear linear relation (R² > 0.85) was found between electron affinity and oxidation rate, showing that higher electron affinity increases oxidation ability. X-ray photoelectron spectroscopy showed that part of Mn (IV) was reduced to Mn (II) and that surface hydroxyl groups increased after reaction, indicating surface changes that reduced activity over time. Density functional theory calculations supported these results by linking conduction-band position with electron affinity. The findings show that electron affinity is a useful parameter for predicting the oxidation behavior of birnessite and can help in developing manganese oxide catalysts for environmental oxidation and pollutant control.