LOCAL ENVIRONMENT AROUND GOLD(III) IN AQUEOUS CHLORIDE SOLUTIONS - AN EXAFS SPECTROSCOPY STUDY
LOCAL ENVIRONMENT AROUND GOLD(III) IN AQUEOUS CHLORIDE SOLUTIONS - AN EXAFS SPECTROSCOPY STUDY
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DOI：
10.1016/0016-7037(93)90061-z
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发表时间：
1993-03-01
影响因子：
5
通讯作者：
BROWN, GE
中科院分区：
文献类型：
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作者：
FARGES, F;SHARPS, JA;BROWN, GE
采用x射线吸收光谱法（XAS）测定了常温常压下1 M NaCl水溶液中Au（III）周围环境与pH和Au浓度的关系。研究的溶液中Au的浓度为10（-1）至10(-3)M， pH值为2至9.2。随着Au浓度的增加，Au的形态没有明显变化；在pH = 2时，Au由4个Cl原子配位（平均d[Au-Cl] = 2.28-2.29 +/- 0.01埃），而在pH 7.5和9.2时，Au分别由3个Cl和1个0（或OH）和2个Cl和2个0（或OH）配位（平均d[Au-Cl] = 2.28 +/- 0.02埃）；平均d[Au-0或Au-OH] = 1.97 +/- 0.02埃)，表明随着ph的增加Cl被0（或OH）取代。在所研究的所有溶液中，Au（III）周围的第一近邻数接近4个。XANES分析表明，在所研究的所有pH值下，AuX4 （X = Cl, 0）都存在方形平面几何结构。这些结果与我们之前对金(III) -氯化物溶液的拉曼、共振拉曼和紫外/可见光谱研究（PECK等人，1991年）的结果非常一致，后者发现AuCl4-、AUCl3(OH)-和AuCl2(OH)2-分别是pH范围2-6、6-8.5和8.5-11中的主要物质。我们没有发现在pH为7.5和9.2的溶液中存在Au(I)Cl2-或Au(I)Cl(OH)-络合物的证据，正如PAN和WOOD（1991）最近提出的那样，在温度为100- 100℃的酸性氯化金溶液中存在Au(I)Cl2-或Au(I)Cl(OH)-络合物，尽管我们不能排除这些络合物是少数物种（<溶液中总Au的10%）。我们的EXAFS结果也首次直接证明了在大多数酸性溶液（pH = 2和4.5）中，Cl在AuCl4-络合物周围存在第二近邻。在低Au浓度（10(-3)M）下也检测到这些第二相邻的Cl原子，它们的数量和排列与晶体KAuCl4中观察到的相似。2H2O（两个Cl平均d[Au-Cl(2)] = 4.42 +/- 0.03埃）。没有发现第二个相邻的Au原子的证据，这表明在任何研究的溶液中很少或没有Au聚合物或胶体颗粒。我们的EXAFS结果与早期基于各种化学测量的Au物种形成预测广泛一致。此外，他们直接证实了混合氯-羟基Au（III）配合物比根据热力学估计的稳定性常数预测的更稳定。
The local environment around Au(III) in aqueous solutions containing 1 M NaCl was determined as a function of pH and Au concentration using X-ray absorption spectroscopy (XAS) at ambient temperature and pressure. The solution Au concentrations studied were 10(-1) to 10(-3) M and the pH ranged between 2 and 9.2. No significant changes of Au speciation were detected with increasing Au concentration; however, major speciation changes were caused by variations in pH. At pH = 2, Au is coordinated by four Cl atoms (mean d[Au-Cl] = 2.28-2.29 +/- 0.01 angstrom), whereas at pH 7.5 and 9.2, Au is coordinated by three Cl and one 0 (or OH) and by two Cl and two 0 (or OH), respectively (mean d[ Au-Cl ] = 2.28 +/- 0.02 angstrom; mean d[ Au-0 or Au-OH ] = 1.97 +/- 0.02 angstrom), indicating replacement of Cl by 0 (or OH) with increasing pH. In all solutions studied, the number of first-neighbors around Au(III) is close to four. XANES analysis suggests the presence of a square-planar geometry for AuX4 (X = Cl, 0) at all pH values studied. These results are in excellent agreement with those from our previous Raman, resonance Raman, and UV / visible spectroscopy study of gold (III) -chloride solutions (PECK et al., 1991 ), which found that AuCl4-, AUCl3(OH)-, and AuCl2(OH)2- are the majority species in the pH ranges 2-6, 6-8.5, and 8.5-11, respectively. We did not find evidence for Au(I)Cl2- or Au(I)Cl(OH)- complexes in our pH 7.5 and 9.2 solutions, as was recently suggested by PAN and WOOD (1991 ) for acidic gold chloride solutions at temperatures > 100-degrees-C, although we can't rule these complexes out as minority species ( < 10% of the total Au in solution). Our EXAFS results also provide the first direct evidence for Cl second neighbors around AuCl4- complexes in the most acidic solutions studied (pH = 2 and 4.5). These second-neighbor Cl atoms were also detected at low Au concentrations (10(-3) M) and are similar in number and arrangement to those observed in crystalline KAuCl4 . 2H2O (two Cl at a mean d[Au-Cl(2)] = 4.42 +/- 0.03 angstrom). No evidence was found for second-neighbor Au atoms, which indicates little or no Au polymers or colloidal particles in any of the solutions studied.Our EXAFS results are in broad agreement with earlier predictions of Au speciation based on a variety of chemical measurements. Moreover, they directly confirm that mixed chloro-hydroxo Au(III) complexes are more stable than predicted on the basis of thermodynamically estimated stability constants.