Rouff Ashaki A, Rabe Stefan, Nachtegaal Maarten, Vogel Frédéric
General Energy Research (ENE), Laboratory for Energy and Materials Cycles, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.
J Phys Chem A. 2009 Jun 25;113(25):6895-903. doi: 10.1021/jp811276t.
Phosphorus K-edge X-ray absorption fine structure (XAFS) was explored as a means to distinguish between aqueous and solid phosphates and to detect changes in phosphate protonation state. Data were collected for H(3)PO(4), KH(2)PO(4), K(2)HPO(4) and K(3)PO(4) solids and solutions and for the more complex phosphates, hydroxylapatite (HAP) and struvite (MAP). The X-ray absorption near-edge structure (XANES) spectra for solid samples are distinguishable from those of solutions by a shoulder at approximately 4.5 eV above the edge, caused by scattering from cation sites. For phosphate species, the intensity of the white line peak increased for solid and decreased for aqueous samples, respectively, with phosphate deprotonation. This was assigned to increasing charge delocalization in solid samples, and the effect of solvating water molecules on charge for aqueous samples. In the extended X-ray absorption fine structure (EXAFS), backscattering from first-shell O atoms dominated the chi(k) spectra. Multiple scattering (MS) via a four-legged P-O(1)-P-O(1)-P collinear path was localized in the lower k region at approximately 3.5 A(-1) and contributed significantly to the beat pattern of the first oscillation. For EXAFS analysis, increasing Debye-Waller factors suggest more disorder in the P-O shell with addition of protons to the crystal structure due to the lengthening effects of P-OH bonds. This disorder produces splitting in the hybridized P 3p-O 2p band in the density of states. For aqueous samples, however, increased protonation reduced the structural disorder within this shell. This was linked to a change from kosmotropic to chaotropic behavior of the phosphate species, with reduced effects of H bonding on structural distortion. The intensity of MS is correlated to the degree of disorder in the P-O shell, with more ordered structures exhibiting enhanced MS. The observed trends in the XAFS data can be used to distinguish between phosphate species in both solid and aqueous samples. This is applicable to many chemical, geochemical and biological systems, and may be an important tool for determining the behavior of phosphate during the hydrothermal gasification of biomass.
磷 K 边 X 射线吸收精细结构（XAFS）被用作区分水相和固相磷酸盐以及检测磷酸根质子化状态变化的一种手段。收集了 H₃PO₄、KH₂PO₄、K₂HPO₄ 和 K₃PO₄ 固体及溶液以及更复杂的磷酸盐——羟基磷灰石（HAP）和鸟粪石（MAP）的数据。固体样品的 X 射线吸收近边结构（XANES）光谱与溶液的光谱可通过边上方约 4.5 eV 处的一个肩峰区分开来，该肩峰是由阳离子位点的散射引起的。对于磷酸盐物种，随着磷酸根去质子化，固体样品的白线峰强度增加而水相样品的白线峰强度降低。这归因于固体样品中电荷离域增加，以及水相样品中溶剂化水分子对电荷的影响。在扩展 X 射线吸收精细结构（EXAFS）中，第一壳层 O 原子的背散射主导了 χ(k) 光谱。通过四足 P - O(1) - P - O(1) - P 共线路径的多重散射（MS）位于约 3.5 Å⁻¹ 的较低 k 区域，并对第一次振荡的拍频模式有显著贡献。对于 EXAFS 分析，德拜 - 瓦勒因子增加表明随着质子添加到晶体结构中，P - O 壳层无序度增加，这是由于 P - OH 键的延长效应。这种无序在态密度中导致杂化的 P 3p - O 2p 能带分裂。然而，对于水相样品，质子化增加减少了该壳层内的结构无序。这与磷酸盐物种从促晶型到离液型行为的变化有关，氢键对结构畸变的影响减小。MS 的强度与 P - O 壳层的无序度相关，结构越有序，MS 越强。XAFS 数据中观察到的趋势可用于区分固体和水相样品中的磷酸盐物种。这适用于许多化学、地球化学和生物系统，并且可能是确定生物质水热气化过程中磷酸盐行为的重要工具。
