The dynamics of Fe oxidation in riebeckite: A model for amphiboles
The dynamics of Fe oxidation in riebeckite: A model for amphiboles
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DOI：
10.2138/am-2018-6382
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发表时间：
2018-07
影响因子：
3.1
通讯作者：
G. Ventura;B. Mihailova;U. Susta;M. Guidi;A. Marcelli;J. Schlüter;R. Oberti
中科院分区：
文献类型：
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作者：
G. Ventura;B. Mihailova;U. Susta;M. Guidi;A. Marcelli;J. Schlüter;R. Oberti
摘要在这项工作中，我们用振动FTIR和拉曼光谱研究了近端分子筛Na2(Fe32+Fe23+)$\Begin{ARRAY}{}(Text{Fe}^{2+}_3\Text{Fe}^{3+}_2)\END{ARRAY}$Si8O22(OH)2的氧化行为。将这些结果与以前对同一样品进行的单晶结构修正和穆斯堡尔谱研究相结合，我们得出结论：沸石中铁的氧化是一个多步骤的过程。(1)在~523K<；T<；623K温度范围内，O-H键延长，电子和氢离子都离域。拉曼分析表明，这一过程在冷却到室温时是可逆的。(2)在623K<；T<；723K范围内，动能增加，电子可以从晶体中排出；超过723K，Fe发生不可逆氧化，与SiO4双链中的不可逆变化相结合，导致晶胞体积收缩，即在长程尺度上可检测到的结构变化。(3)超过823K，不可逆氧化完成，H+离子被迫离开晶体。由于这一多步过程，单晶X射线衍射仪对晶胞参数的分析在~700K检测到去质子化过程的开始，但通过对粉末的穆斯堡尔谱分析(以及通过结构修正观察到阳离子分布的变化)，发现去质子化过程在623K开始。拉曼光谱还表明，在FTIR观察到晶体完全去质子化之前，H+从晶体表面开始释放到约100K。因此，Fe的氧化从晶体表面开始，诱导电子和H+从晶体内部向边缘迁移，从而通过晶体本体进行随后的氧化。FTIR分析表明，无论是包埋在KBr中的粉末，还是在氮气气氛中加热的晶体中，都没有观察到去质子化，这意味着H+的释放需要表面(大气)氧来形成H2O分子。Fe2+→Fe3+氧化在整个晶体基质中产生电子通量，从而在角闪石中产生导电性。这项可能在材料科学中有有趣应用的工作的一个重要含义是，在给定的温度范围内，沸石(可能还有其他富铁的硅酸盐)中的铁氧化是可逆的。此外，这项工作表明，如果不使用适当的独立技术组合，就不能完全理解甚至准确地监控复杂的过程。
Abstract In this work, we investigate the oxidation behavior of a nearly end-member riebeckite, ideally Na2 (Fe32+Fe23+) $\begin{array}{} (\text{Fe}^{2+}_3\text{Fe}^{3+}_2) \end{array} $ Si8O22(OH)2, by using vibrational FTIR and Raman spectroscopies. Combining these results with previous studies performed on the same sample by single-crystal structure refinement and Mössbauer spectroscopy, we conclude that iron oxidation in riebeckite is a multi-step process. (1) In the ~523 K < T < 623 K temperature range, the O-H bond lengthens and both the electrons and the hydrogen cations delocalize. Raman analysis shows that this step is reversible upon cooling to room temperature. (2) In the 623 K < T < 723 K range, the kinetic energy increases so that the electrons can be ejected from the crystal; beyond 723 K an irreversible oxidation of Fe occurs that couples with irreversible changes in the SiO4 double-chains leading to a contraction of the unit-cell volume, i.e., to structural changes detectable at the long-range scale. (3) Beyond 823 K, the irreversible oxidation is completed and H+ ions are forced to leave the crystal bulk. Because of this multi-step process, the onset of the deprotonation process is detected at ~700 K by single-crystal XRD analysis of the unit-cell parameters, but starts at 623 K as indicated by Mössbauer spectroscopy on powders (and by changes in the cation distribution observed by structure refinement). Also, Raman scattering shows that the release of H+ from the crystal surface starts ~100 K before the complete deprotonation of the crystal bulk is witnessed by FTIR absorption. Hence, the oxidation of Fe starts at the crystal surface and induces electron and H+ migration from the crystal interior to the rim and thus subsequent oxidation through the crystal bulk. No deprotonation is observed by FTIR either in powders embedded in KBr or in crystals heated in N2 atmosphere, implying that the release of H+ needs surficial (atmospheric) oxygen to form H2O molecules. Fe2+ → Fe3+ oxidation produces a flux of electrons throughout the crystal matrix, which generates electrical conductivity across the amphibole. An important implication of this work, which might have interesting applications in material science, is that iron oxidation in riebeckite (and possibly in other Fe-rich silicates) is reversible in a given range of temperature. Also, this work shows that complex processes cannot be fully understood or even monitored accurately without using a proper combination of independent techniques.