Bera Mrinal K, Luo Guangming, Schlossman Mark L, Soderholm L, Lee Sungsik, Antonio Mark R
§Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, United States.
J Phys Chem B. 2015 Jul 16;119(28):8734-45. doi: 10.1021/acs.jpcb.5b02958. Epub 2015 May 28.
Grazing-incidence (GI) X-ray absorption spectroscopy (XAS) under conditions of total external reflection is used to explore the coordination environment of the trivalent erbium ion, Er(3+), at an electrolyte-vapor interface. A parallel study of the bulk aqueous electrolyte (1 M ErCl3 in HCl at pH = 1.54) shows that the Er(3+) ions have a simple hydration shell with an average Er-OH2 bond distance of 2.33(1) Å, consistent with previous descriptions of the aquated cation, Er(OH2)8. No other correlations are observed in the electrolyte EXAFS (extended X-ray absorption fine structure) data acquired at room temperature. In contrast, the coordination of the Er(3+) ions at the electrolyte-helium interface, as interrogated by use of electron-yield detection, reveal correlations beyond the Er-OH2 hydration shell that are unexpectedly well-defined. Analyses show an environment that consists of a first coordination sphere of 6-7 O atoms at 2.36(1) Å and a second one of 3 Cl atoms at 2.89(2) Å, suggesting the formation of a neutral [(H2O)6-7ErCl3] entity at the surface of the electrolyte. The presence of a third, distant peak in the Fourier transform data is attributed to Er-Er correlations (in possible combination with contributions from distant Er-O and Er-Cl interactions). The best-Z and -integer fits reveal 3 Er atoms at 3.20(2) Å, confirming the near-surface-enrichment of Er(3+) as revealed previously by use of X-ray reflectivity measurements (J. Phys. Chem. C 2013, 117, 19082). Here, the strong associations between the Er-aqua-chloro entities at the electrolyte-vapor interface are shown to be consistent with the formation of domains of polynuclear cluster motifs, such as would arise through hydrolysis reactions of the aquated Er(3+) cations. The local structural results and the calculated surface coverage are of relevance to understand the myriad reactions involved in the hydrometallurgical process of solvent extraction (SX) for metal purification, which involves the transfer of a selected metal ion, like Er, across an interface from an aqueous electrolyte to an organic phase.
在全外反射条件下，掠入射（GI）X射线吸收光谱（XAS）被用于探究三价铒离子（Er(3+)）在电解质 - 气相界面处的配位环境。对本体水性电解质（pH = 1.54的1 M HCl中的ErCl3）的平行研究表明，Er(3+)离子具有简单的水合壳层，平均Er - OH2键距为2.33(1) Å，这与先前对水合阳离子Er(OH2)8的描述一致。在室温下获取的电解质扩展X射线吸收精细结构（EXAFS）数据中未观察到其他相关性。相比之下，通过电子产率检测所探究的电解质 - 氦气界面处Er(3+)离子的配位情况显示，除了Er - OH2水合壳层之外，还存在定义明确的其他相关性。分析表明，其环境由第一配位层组成，包含6 - 7个O原子，键距为2.36(1) Å，第二配位层包含3个Cl原子，键距为2.89(2) Å，这表明在电解质表面形成了中性的[(H2O)6 - 7ErCl3]实体。傅里叶变换数据中第三个远距离峰的出现归因于Er - Er相关性（可能与远距离Er - O和Er - Cl相互作用的贡献相结合）。最佳Z值和整数拟合显示在3.20(2) Å处有3个Er原子，证实了如先前通过X射线反射率测量（《物理化学杂志C》2013年，117卷，19082页）所揭示的Er(3+)在近表面的富集。在此，电解质 - 气相界面处的Er - 水 - 氯实体之间的强关联表明与多核簇基序域的形成一致，例如通过水合Er(3+)阳离子的水解反应可能产生的情况。局部结构结果和计算出的表面覆盖率对于理解金属提纯的溶剂萃取（SX）湿法冶金过程中涉及的众多反应具有重要意义，该过程涉及将选定的金属离子（如Er）从水性电解质跨界面转移到有机相中。
