Factors Controlling the High Oxidative Activities of Layered Manganese Oxides

Abstract

The two-dimensional layered birnessite mineral is a powerful natural oxidant and an important model compound in the development of bioinspired electrocatalysts for the water oxidation reaction (OER). The aim of this work was to study the factors controlling their unusually high oxidative activities of natural and synthetic samples for both spontaneous and potential-induced oxidation reactions. The results of the study1 of various cation-exchanged birnessites for spontaneous Cr(III) to Cr(VI) oxidation reactions show that the oxidative activity of birnessite, which decreases in 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 sulphides 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.In another study2, the performance of birnessite as an electrocatalytic water oxidation catalyst and the mechanism of key Mn(III) intermediate formation were evaluated through the study of the effects of various electrolyte anions and cations on the catalytic efficiency. In situ spectroelectrochemical measurements show that the activity is controlled by a dynamic dissolution-oxidation process, wherein Mn(III) is formed through the oxidation of labile uncomplexed Mn(II) that shuttles between the crystal lattice and the electrolyte. The role of electrolyte anions is to control the extent of deprotonation of complexed Mn(II) in the lattice while the electrolyte cations control the interlayer spacing. Both in turn govern the shuttling efficiency of uncomplexed Mn(II) and its subsequent electrooxidation to Mn(III). 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. The high electron affinity is another example of the unusual features of Mn compounds, which Nature uses for its many biogeochemical functions. C. Wang; N. Smieszek; V. Chakrapani, Unusually High Electron Affinity Enables the High Oxidising Power of Layered Birnessite Chem. Mater. 2021, 33 (19), 7805-7817.I. Roy; C. Wang; N. Smieszek; X. Li; L. Tsapatsaris; V. Chakrapani, Formation of the Metastable MnIII Water Oxidation Intermediate in Birnessite is Controlled by a Dissolution-Deposition Process Involving Labile MnII. ChemSusChem 2022, 15 (8), e202200062. Figure 1