Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 112, Issue 15, Pages 4600-4605Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1421018112
Keywords
manganese oxide; photoreduction; band-gap excitation; pump-probe spectroscopy; water oxidation
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Funding
- Swiss National Science Foundation [200021_143742]
- Sandoz Family Foundation
- Office of Science, Office of Basic Energy Sciences, of the US Department of Energy (BES-DOE) [DE-AC02-05CH11231]
- BES-DOE [DE-AC02-05CH11231]
- BES-DOE by Argonne National Laboratory [DE-AC02-06CH11357]
- Swiss National Science Foundation (SNF) [200021_143742] Funding Source: Swiss National Science Foundation (SNF)
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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 delta-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 Jahn-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.
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