Rate and mechanism of the photoreduction of birnessite (MnO2) nanosheets

Details

Serval ID
serval:BIB_5930D29F2172
Type
Article: article from journal or magazin.
Collection
Publications
Institution
Title
Rate and mechanism of the photoreduction of birnessite (MnO2) nanosheets
Journal
Proceedings of the National Academy of Sciences
Author(s)
Marafatto  Francesco Femi, Strader  Matthew L., Gonzalez-Holguera  Julia, Schwartzberg  Adam, Gilbert  Benjamin, Peña  Jasquelin
ISSN
0027-8424 (Print)
1091-6490 (Electronic)
ISSN-L
0006-2952
Publication state
Published
Issued date
03/2015
Volume
112
Number
15
Pages
4600-4605
Language
english
Notes
Publication types: Journal Article
Publication Status: ppublish
Abstract
The photoreductive dissolution of Mn(IV) oxide minerals in sunlit aquatic environments couples the Mn cycle to the oxidation of organic matter and fate of trace elements associated with Mn oxides, but the intrinsic rate and mechanism of mineral dissolution in the absence of organic electron donors is unknown. We investigated the photoreduction of δ-MnO2 nanosheets at pH 6.5 with Na or Ca as the interlayer cation under 400-nm light irradiation and quantified the yield and timescales of Mn(III) production. Our study of transient intermediate states using time-resolved optical and X-ray absorption spectroscopy showed key roles for chemically distinct Mn(III) species. The reaction pathway involves (i) formation of Jahnâeuro"Teller distorted Mn(III) sites in the octahedral sheet within 0.6 ps of photoexcitation; (ii) Mn(III) migration into the interlayer within 600 ps; and (iii) increased nanosheet stacking. We propose that irreversible Mn reduction is coupled to hole-scavenging by surface water molecules or hydroxyl groups, with associated radical formation. This work demonstrates 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 timescales for the production and evolution of Mn(III) species and a catalytic role for interlayer Ca2+ identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidation.
Keywords
manganese oxide, photoreduction, band-gap excitation, pump-probe spectroscopy, water oxidation
Open Access
Yes
Create date
26/05/2015 21:22
Last modification date
21/08/2019 6:12
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