Ligand exchange adsorption and coordination structure of Pd on δ-MnO 2 in NaCl solution
Ligand exchange adsorption and coordination structure of Pd on δ-MnO 2 in NaCl solution
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NaCl溶液中δ-MnO 2 上Pd的配体交换吸附及配位结构
DOI：
10.1016/j.chemgeo.2017.04.022
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发表时间：
2017
期刊：
影响因子：
3.9
通讯作者：
Takahashi Yoshio
中科院分区：
文献类型：
--
作者：
Tanaka Kazuya;Tanaka Masato;Watanabe Naoko;Tokunaga Kohei;Takahashi Yoshio
钯在铁锰结核和结壳中相对于其在海水中的浓度而言是高度积累的，但对Pd（II）的结合机制仍然知之甚少，这可能是因为铁锰结核和结壳中的Pd浓度相当低。本文利用X射线吸收精细结构谱研究了Pd（II）在δ-MnO 2上的局部配位结构，以阐明Pd在铁锰结核和结壳中的吸附机制。在0.1和0.5 M NaCl溶液中进行了Pd（II）在δ-MnO 2上的吸附实验，其中带负电荷的氯化物络合物PdCl 42-、PdCl 3-和PdCl 3OH 2-是主要物种。Pd在δ-MnO 2和溶液之间的分配系数Kd（Pd）= [Pd]solid/ [Pd]solution（L/kg）随pH值的增加从2 × 103增加到4 × 106. X射线吸收近边结构（XANES）和扩展X射线吸收精细结构（EXAFS）分析表明，Pd在δ-MnO 2上的吸附是通过配位交换实现的，从Cl配位到O配位.此外，EXAFS光谱的曲线拟合表明，形成两种不同的内球复合物，双齿单核和双齿双核配合物，这一发现得到了密度泛函理论计算的支持。考虑到从吸附实验中获得的相对较大的Kd（Pd）值以及从铁锰结核和结壳以及海水的报告数据中计算出的较大的表观Kd（Pd）值，形成内球络合物是合理的。
Palladium is highly accumulated in ferromanganese (Fe-Mn) nodules and crusts relative to its concentration in seawater but the mechanism by which Pd(II) is incorporated remains poorly understood, probably because the Pd concentration in Fe-Mn nodules and crusts is fairly low. In this study, we investigated the local coordination structure of Pd(II) adsorbed on δ-MnO2, using X-ray absorption fine structure spectroscopy to clarify the mechanism by which Pd is incorporated into Fe-Mn nodules and crusts. Adsorption experiments of Pd(II) on δ-MnO2were conducted in 0.1 and 0.5 M NaCl solutions, in which the negatively charged chloride complexes PdCl42 −, PdCl3−, and PdCl3OH2 −were predominant species. The distribution coefficient of Pd between δ-MnO2and solution, Kd(Pd) = [Pd]solid/ [Pd]solution(L/kg), increased from ~ 2 × 103to ~ 4 × 106with increasing pH. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses indicated that Pd was adsorbed on δ-MnO2through ligand exchange, from Cl coordination to O coordination. Furthermore, curve fitting of EXAFS spectra demonstrated the formation of two different inner-sphere complexes, bidentate-mononuclear and bidentate-binuclear complexes, and this finding was supported by density functional theory calculations. The formation of inner-sphere complexes is reasonable given the relatively large Kd(Pd) values obtained from adsorption experiments and the large apparent Kd(Pd) values calculated from reported data on Fe-Mn nodules and crusts and seawater.