Unusually High Electron Affinity Enables the High Oxidizing Power of Layered Birnessite

Two-dimensional layered birnessite is a powerful natural oxidant for many species such as water, metal cations, and organic contaminants and plays an important role in many biogeochemical cycles. Herein, the factors responsible for its high oxidative activity were evaluated through a series of seven birnessites with interlayer cations of H, Li, Na, K, Cs, Ca, and Mg along with other MnOx polymorphs. Results show that the oxidative activity of birnessite, which decreases in the order Mg > Cs = K > Na > Ca > Li > H, is due to its unusually high electron affinity of ∼5.9 eV, which is the highest among all known functional oxides and sulfides in aqueous solution. Both the band gap (Cs > Mg > K > Na > Ca > Li > H) and the electron affinity (Na > Cs > K > Mg > Ca > Li > H) are seen to be strongly affected by the nature of the interlayer cations and their coordinating water molecules. Analysis shows that the band gap scales with the interlayer spacing, while the electron affinity scales with the relative Mn(III)/Mn(II) concentration. The high electron affinity values put the band edges of birnessite-type structures well below the redox potential of most thermodynamically stable cations and water, enabling their spontaneous oxidation. Other tunneled and spinel MnOx polymorphs undergo a slow phase transformation to a layered birnessite structure driven by Mn(II) lattice dissolution. The results here show yet another unusual feature of nature’s exclusively selected Mn compounds with important biogeochemical functions.