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04/24/2015

MnO2 Nanosheets Need Only Water, Sunlight To React

Source:  Ben Gilbert and Dan Hawkes

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Ben Gilbert

Manganese (Mn) oxides are some of the most redox reactive minerals in natural waters, with a strong tendency to grab electrons from organic molecules, thereby breaking down natural organic matter into carbon dioxide. In surface waters, including oceans, rivers and lakes, sunlight is well known to drive manganese oxide redox chemistry. It had long been assumed that electron-rich organic molecules were required to serve as electron donors for Mn reduction driven by light. A recent theoretical prediction, however, suggested that photoexcitation of birnessite, a common Mn(IV) oxide that forms a lamellar nanosheet structure, could lead to net reduction in water in the absence of any organic molecule.

Confirmation of this prediction, and additional insights into the mechanisms of the redox reaction, have now been revealed by a collaboration involving Benjamin Gilbert, Geochemistry Department Head in the Earth Sciences Division (ESD) at LBNL; Jackie Peña, a former ESD postdoc who is now an associate professor at the University of Lausanne (UniL), Switzerland; and UniL graduate student Francesco Marafatto, who was the lead researcher on the work. Their findings, which were published late last month (March 30, 2015) in the Proceedings of the National Academy of Sciences (PNAS), demonstrate the importance of direct MnO2 photoreduction in environmental processes, and provides a framework to test new hypotheses regarding the role of organic molecules and metal species in photochemical reactions with Mn oxide phases.

The team used laboratory experiments to show that visible light, in the absence of any organic molecule, was sufficient to cause the permanent reduction of Mn. This finding extended the depth of the water column in which light-driven Mn cycling could occur, and strongly indicated that Mn reduction must be coupled to the oxidation of water. The researchers then used ultrafast spectroscopy facilities available at the Advanced Light Source and the Molecular Foundry to develop a new mechanism for the photoreduction of birnessite that considered electronic and structural transitions that occurred on the picosecond, nanosecond, and much longer timescales (see figure). The result is one of the most detailed depictions of a mineral redox reaction ever established experimentally.

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A step-wise mechanism for the light-driven reduction of birnessite, a manganese(IV) oxide mineral forms nanosheets. From left to right, the absorption of a visible photon causes the creation of a Mn(III) site in the nanosheet, the displacement of the atom to an adsorption site above the nanosheet, and eventually to more ordered stacking of nanosheets.

To read the paper, go to: http://www.pnas.org/content/112/15/4600.full

Citation:

Marafatto, F.F., M.L. Strader, J. Gonzalez-Holguera, A. Schwartzberg, B. Gilbert, and J. Pena (2015), Rate and mechanism of the photoreduction of birnessite (MnO2) nanosheets.  Proceedings of the National Academy of Sciences (PNAS), 112 (15), 4600–4605; DOI: 10.1073/pnas.1421018112.

Funding Source: BES