Layer-type manganese oxides (MnOx) are the strongest oxidants among all the solid geochemical phases found in natural systems. However, knowledge on the relationship between structural properties (vacancy sites content, interlayer occupancy, Mn oxidation state etc.) and reducibility of Mn oxides is scarce. This study developed 2,2’-Azino-bis(3-ethylbenzothiazoline-6sulfonic acid) (ABTS) as a redox probe to investigate the effect of these properties on the redox reactivity of layer-type MnOx with Mn(III) content ranging from 3% to 21% (mol Mn(III) mol Mn-1), with unoccupied vacancy sites, Mn(III)-saturated vacancy sites and with no vacancy sites. Working at ABTS:Mn ratios of 0.01 – 0.02, we found that vacancy-free oxides did not oxidize ABTS, but layer-type MnOx containing Mn(III)-saturated vacancy sites led to nearly complete oxidation of ABTS to ABTS°+ at pH 5. In addition, oxides with unoccupied vacancy sites showed not only complete oxidation of ABTS to ABTS°+, but further oxidation of ABTS°+ to ABTS2+ at both pH 5 and 7, which implies a standard reduction potential for these oxides of c.a. 1.6 V vs. SHE. These results shed light on the structural properties driving the redox reactivity of layer-type Mn oxides: while vacancy sites play a critical role in enhancing electron transfer, reducibility is limited by the accumulation of reduced Mn on top of vacancy sites.

Assessing the redox properties of layer-type MnOx using the probe compound 2,2’-Azino-bis(3-ethylbenzothiazoline-6sulfonic acid)

2019 Fall National Meeting

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