Structure and Charge Hopping Dynamics in Green Rust

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• 作者：Matthew C. F. Wander ; Kevin M. Rosso ; Martin A. A. Schoonen
• 刊名：Journal of Physical Chemistry C
• 出版年：2007
• 出版时间：August 2, 2007
• 年：2007
• 卷：111
• 期：30
• 页码：11414 - 11423
• 全文大小：583K
• 年卷期：v.111,no.30(August 2, 2007)
• ISSN：1932-7455

文摘

Green rust is a family of mixed-valent iron phases formed by a number of abiotic and biotic processes underalkaline suboxic conditions. Because of its high Fe²⁺ content, green rust is a potentially important phase forpollution remediation by serving as a powerful electron donor for reductive transformation. However,mechanisms of oxidation of this material are poorly understood. An essential component of the green ruststructure is a mixed-valent brucite-like Fe(OH)₂ sheet comprised of a two-dimensional network of edge-sharing iron octahedra. Liquid nitrogen temperature Mössbauer spectra show that any Fe²⁺-Fe³⁺ valenceinterchange reaction must be slower than approximately 10⁷ s^-1. Using Fe(OH)₂ as structural analogue forreduced green rust, we performed Hartree-Fock calculations on periodic slab models and cluster representationsto determine the structure and hopping mobility of Fe³⁺ hole polarons in this material, providing a first principlesassessment of the Fe²⁺-Fe³⁺ valence interchange reaction rate. The calculations show that, among threepossible symmetry unique iron-to-iron hops within a sheet, a hop to next-nearest neighbors at an intermediatedistance of 5.6 Å is the fastest. The predicted rate is on the order of 10¹⁰ s^-1 (at 300 K) and 10³ s^-1 (at 70K), consistent the Mössbauer-based constraint. All other possibilities, including hopping across interlayerspaces, are predicted to be slower than 10⁷ s^-1. Collectively, the findings suggest the possibility of holeself-diffusion along sheets as a mechanism for regeneration of lattice Fe²⁺ sites, consistent with previousexperimental observations of edge-inward progressive oxidation of green rust.