In situ synchrotron X-ray diffraction of Fe oxide transformations in aqueous solutions

水溶液中 Fe 氧化物转变的原位同步加速器 X 射线衍射

• 批准号: 1552211
• 负责人: Peter Heaney
• 金额: $ 40.86万
• 依托单位: Pennsylvania State Univ University Park
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-02-15 至 2020-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1552211&HistoricalAwards=false
• 关键词: situ; synchrotron; ray; diffraction; Fe

The mineral hematite (Fe2O3) is of critical economic importance, as it serves as the primary mineral reserve for the extraction of iron, which accounts for 90% of worldwide metal consumption. In addition, hematite formation in soils is of the highest environmental significance, because hematite nanoparticles can attach to the surfaces of soil particles and modify their ability to sequester harmful metals. Hematite also is used technologically in applications ranging from magnetic data storage to pigmentation, and it is being tested as an agent for water-splitting in solar cells. However, despite the ubiquity of hematite at Earth's surface, the chemical reactions by which hematite crystallizes are still poorly understood. This study will explore a novel hypothesis that the first crystals of hematite to nucleate from aqueous solutions are chemically and physically distinct from ideal hematite, not only because of their nanometer-scale particle size but because up to one-third of the iron atoms are missing from the crystal structure. Previous studies have shown that these iron defects can dramatically alter the magnetic properties, color saturation, and chemical reactivity of hematite. In this study, the investigators will explore the relation between iron defects and the physical properties of hematite, and they will determine the connection between the abundance of defects and the temperature, pH, and oxygen content of the groundwaters from which natural hematite crystallizes.The investigators hypothesize that when hematite (Fe2O3) forms from ferrihydrite and goethite in aqueous solutions, the hematite first nucleates as highly iron-deficient, hydrous nanocrystals. These defective precursor phases are metastable and may anneal to endmember hematite with time. The investigators base these inferences on their recent observations of crystallization sequences in ferric chloride solutions. That work revealed incipient phases of "hydrohematite" with a chemical formula close to FeOOH forming from precursor akaganéite. The investigators will determine whether the more common crystallization pathways for natural hematite - from antecedent ferrihydrite and goethite - experience an analogous multi-step reaction route. In contrast to approaches based on dry heating of powders or on "before-and-after" characterization of batch-reacted mixtures, the investigators will exploit novel synchrotron-based techniques that allow them to capture X-ray diffraction patterns of mineral-fluid mixtures from 25 to 200 degree C with very high time resolution. The diffraction data are amenable to Rietveld analysis and will yield high-quality cell parameters, atom positions, and occupancies. In addition to pinpointing the evolution of hematite growth at the atomic scale, the experiments will yield kinetic models from which they can extract robust activation energies for the nucleation and growth of iron (hydr)oxides as a function of pH. The investigators will couple these experiments with detailed microscopic and spectroscopic examinations of natural and historic specimens of "hydrohematite", and they will perform density functional calculations to gauge the relative stabilities of hydrous hematite phases and to relate chemical properties to particle size.

矿物赤铁矿（Fe 2 O3）具有重要的经济意义，因为它是提取铁的主要矿物储备，占全球金属消费量的90%。 此外，土壤中的赤铁矿形成具有最高的环境意义，因为赤铁矿纳米颗粒可以附着在土壤颗粒的表面，并改变其螯合有害金属的能力。 赤铁矿在技术上也被用于从磁性数据存储到着色的各种应用，它正在被测试作为太阳能电池中水分解的试剂。 然而，尽管赤铁矿在地球表面无处不在，但对赤铁矿结晶的化学反应仍然知之甚少。 这项研究将探索一个新的假设，即从水溶液中成核的第一个赤铁矿晶体在化学和物理上与理想的赤铁矿不同，不仅因为它们的纳米级颗粒大小，而且因为多达三分之一的铁原子从晶体结构中缺失。 以前的研究表明，这些铁缺陷可以显着改变赤铁矿的磁性，颜色饱和度和化学反应性。 在这项研究中，研究人员将探索铁缺陷与赤铁矿物理性质之间的关系，并确定缺陷丰度与天然赤铁矿结晶的地下水的温度、pH值和含氧量之间的关系。研究人员假设，当赤铁矿（Fe 2 O3）在水溶液中由水铁矿和针铁矿形成时，赤铁矿首先成核为高度缺铁的含水纳米晶体。 这些有缺陷的前体相是亚稳的，并且可以随时间退火成端元赤铁矿。 研究人员根据他们最近对氯化铁溶液结晶顺序的观察得出了这些推论。这项工作揭示了“水赤铁矿”的初期阶段，其化学式接近于由前体赤霞岩形成的FeOOH。 研究人员将确定天然赤铁矿更常见的结晶途径-从前驱水铁矿和针铁矿-是否经历类似的多步反应途径。 与基于粉末干燥加热或间歇反应混合物的“前后”表征的方法相反，研究人员将利用新型同步加速器技术，使他们能够以非常高的时间分辨率捕获25至200摄氏度的矿物流体混合物的X射线衍射图案。 衍射数据服从Rietveld分析，并将产生高质量的单元参数，原子位置和占位。 除了在原子尺度上精确定位赤铁矿生长的演变外，这些实验还将产生动力学模型，从中可以提取铁（氢）氧化物成核和生长的强大活化能作为pH值的函数。研究人员将把这些实验与“水赤铁矿”的自然和历史标本的详细显微镜和光谱检查结合起来，他们将进行密度泛函计算，以测量含水赤铁矿相的相对稳定性，并将化学性质与粒度联系起来。