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

Abstract

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 MnO x 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 MnO x 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.

Formation of the Metastable MnIII Water Oxidation Intermediate in Birnessite is Controlled by a Dissolution‐Deposition Process Involving Labile MnII

“…Cs‐bir showed Cs peaks without the presence of K. Li‐bir showed a small weak peak indicating that some K may be left behind even after 48 h exchange, which is consistent with prior observations [22] . The XRD patterns of these powders were reported in our previous study, [3] and show that they conform to the standard pattern of a birnessite‐type lattice without the presence of secondary phases. Using the peak at the 2 θ value of approximately 12°, the interlayer d‐spacing of Li‐bir, K‐bir, and Cs‐bir was determined to be 7.17 Å, 7.27 Å, and 7.57 Å, respectively.…”

“…The reason for the spontaneous dissolution of birnessite is likely related to its unusual band structure. Results of our recent study [3] show that many cation‐exchanged birnessites possess unusually large electron affinities of around 6.0 eV in aqueous solution, which puts their CB well below the redox potential of the O 2 /H 2 O reaction. The band structure (Figure 6) was determined by using previously measured values of the work function (flat band potential) and band gap ( E g ) through electrochemical Mott‐Schottky and absorption spectroscopy measurements, respectively, and accounting for the differences in the electrolyte pH by assuming Nernstian behavior for the band edges.…”

“…The closest naturally occurring analog are the members of the birnessite family (“birnessite”), a hydrated MnO 2 mineral group consisting of layers of mostly edge‐sharing MnO 6 octahedrons (a phyllomanganate) intercalated with various alkali cations and water. Results of our recent study [3] show that birnessite, which is one of the strongest naturally occurring oxidants in the environment, is due to its unusually high electron affinity ( χ ≈ 6.0 eV). The high χ , which is the highest among common oxide and sulfide semiconductors in aqueous solution, puts the conduction band (CB) of birnessite well below the redox potential of common metal cations and H 2 O, thereby enabling their spontaneous oxidation.…”

Density Functional Theory Modeling of the Oxidation Mechanism of Co(II) by Birnessite

“…), [82][83][84][125][126][127][128][129][130][131] our results are at odds with that interpretation for the oxidation of Co(II) to Co(III). We found that Co(II) sorbed on a vacancy site (…”

“…According to the predictive thermodynamic models explored in this study, Co­(II) is not oxidized on MnO 2 layer edges but on vacancy sites on basal planes. Vacancy sites are known to form strong TCS complexes with divalent metals and to be involved in electron exchange reaction with Mn­(III) and Mn­(IV). ,,,,,− ,,,,− They facilitate photoconductivity and photoreductive dissolution of the MnO 2 layer. ,, Although Mn­(III) has a strong electron affinity and is usually regarded to mediate the oxidative activity of birnessite, whether it is bonded over a vacancy (Mn TCS 3+ ) or is incorporated in the lattice (Mn Layer 3+ ), − ,− our results are at odds with that interpretation for the oxidation of Co­(II) to Co­(III). We found that Co­(II) sorbed on a vacancy site (Co TCS 2+ ) is oxidized to Co INC 3+ by a layer Mn­(IV) in the vacancy.…”