Marafatto Francesco Femi, Strader Matthew L, Gonzalez-Holguera Julia, Schwartzberg Adam, Gilbert Benjamin, Peña Jasquelin
Institute of Earth Surface Dynamics, University of Lausanne, CH-1015 Lausanne, Switzerland; and.
Chemical Sciences Division.
Proc Natl Acad Sci U S A. 2015 Apr 14;112(15):4600-5. doi: 10.1073/pnas.1421018112. Epub 2015 Mar 30.
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 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 Ca(2+) identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidation.
在阳光照射的水生环境中，氧化锰矿物的光还原溶解将锰循环与有机物质的氧化以及与锰氧化物相关的微量元素的归宿联系起来，但在没有有机电子供体的情况下矿物溶解的内在速率和机制尚不清楚。我们研究了在400纳米光照射下，以钠或钙作为层间阳离子的δ-MnO₂纳米片在pH 6.5时的光还原反应，并对Mn(III)生成的产率和时间尺度进行了量化。我们使用时间分辨光学和X射线吸收光谱对瞬态中间态进行的研究表明，化学性质不同的Mn(III)物种起着关键作用。反应途径包括：(i) 在光激发后0.6皮秒内，在八面体片中形成 Jahn-Teller 畸变的Mn(III)位点；(ii) 在600皮秒内，Mn(III)迁移到层间；(iii) 纳米片堆积增加。我们提出，不可逆的锰还原与表面水分子或羟基的空穴清除以及相关自由基的形成有关。这项工作证明了二氧化锰直接光还原在环境过程中的重要性，并提供了一个框架来测试关于有机分子和金属物种在与氧化锰相的光化学反应中作用的新假设。从光谱测量中确定的Mn(III)物种产生和演化的时间尺度以及层间Ca(2+)的催化作用，也可以指导用于水氧化的高效锰基催化剂的设计。
