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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 112; no. 15; pp. 4600 - 4605
• Main Authors: Marafatto, Francesco Femi, Strader, Matthew L., Gonzalez-Holguera, Julia, Schwartzberg, Adam, Gilbert, Benjamin, Peña, Jasquelin
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 14.04.2015; National Acad Sciences
• Subjects: Chemical reactions; Dissolution; Minerals; Photochemistry; Physical Sciences; Spectrum analysis; Surface water; Trace elements; manganese oxide; pump–probe spectroscopy; photoreduction; water oxidation; band-gap excitation
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1421018112

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-MnO ₂ 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 MnO ₂ 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 Ca ²⁺ identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidation. Significance The photoreductive dissolution of Mn oxides governs the biogeochemical cycle of Mn and the fate of organic and inorganic species associated with Mn oxides in the euphotic zones of marine and freshwater systems. Mn oxide minerals also have garnered interest as water oxidation catalysts inspired by the Mn ₄CaO ₄ cluster of photosystem II. However, the mechanism of water oxidation by MnO ₂ and the rate limiting steps for this reaction are unknown. In this study, we couple flow-through experiments and ultrafast pump–probe optical and X-ray absorption spectroscopy to develop a photoreduction model that includes the mechanism and timescales for the initial electron transfer steps in the oxidation of water by MnO ₂.