Application of thermo-Raman spectroscopy to study dehydration of CaSO₄·2H₂O and CaSO₄·0.5H₂O

Abstract

In this work, dehydration of CaSO₄·2H₂O and CaSO₄·0.5H₂O in stagnant air was studied by thermo-Raman spectroscopy from 25 to 300°C. The composition could be identified directly from the Raman spectra by comparing with the known Raman spectra of CaSO₄·2H₂O, CaSO₄·0.5H₂O and CaSO₄. Thermo-Raman spectra indicated that dehydration of CaSO₄·2H₂O was a two step consecutive reaction and CaSO₄·0.5H₂O was the intermediate in the experimental conditions. The band intensities illustrated the variation in the relative amount of each species. The derivatives of the band intensities showed the maximum rate in dehydration of that species. The shifts in band positions also revealed the transformation.

Keywords：Raman spectroscopy; Thermal analysis; Dehydration; CaSO₄·2H₂O; CaSO₄·0.5H₂O

1. Introduction

The transformation in composition and phase of a solid as the temperature varies can be obtained by thermogravimetric analysis (TGA), differential thermal analysis (DTA), etc. for thermal analysis [1]. However, no detailed information on the composition or phase is available. Raman spectroscopy has the advantage in identification of the composition and phase of a solid from its vibrational bands [2]. Therefore, it can be used to monitor in situ the transformation in composition and phase of a solid in a thermal process. The changes in Raman spectra during a thermal process should correspond to the peaks in the thermograms from TGA and DTA. Raman spectra were measured in a small temperature range for the information on phase transformation in many solids 3, 4 and 5. Sometimes, the spectra during the transformation were measured continually to show the transformation 6, 7 and 8. In fact, more information could be obtained from the Raman data than those from TGA and DTA in thermal analysis. Calcium oxalate monohydrate CaC₂O₄·H₂O, a calibration sample for TGA and DTA, was studied in situ by taking the Raman spectra continually in a thermal process [9]. The composition and phase could be identified by the spectra and the amount of each species can be measured from the band intensities during the phase transformation or composition change (chemical reaction). The formation of anatase and rutile from TiO₂ gel was also studied by this method 10 and 11. The structure of TiO₂ is important for its usage and has been studied by Raman spectroscopy for the transformation [12].

Dehydration in minerals or crystals is important and has been studied for many crystals 13 and 14. The transformations in compositions and phases during the dehydration processes of calcium sulfate dihydrate (CaSO₄·2H₂O) and hemihydrate (CaSO₄·0.5H₂O) to anhydrite (CaSO₄), are interesting. It is known that CaSO₄·2H₂O transforms to CaSO₄·0.5H₂O, then to CaSO₄15 and 16. That system has been studied by Raman spectroscopy [17], IR spectroscopy 17, 18, 19 and 20and TGA and DTA 21, 22, 23, 24, 25, 26 and 27. The kinetics of dehydration has been studied by TGA 21, 22, 23, 24, 25, 26 and 27. In this work, dehydration of CaSO₄·2H₂O and CaSO₄·0.5H₂O were studied by thermo-Raman spectroscopy and compared with the results from TGA and DTA. The variation in Raman spectra showed distinctly the disappearance of the old phase and the appearance of a new one. The amounts of CaSO₄·2H₂O, CaSO₄·0.5H₂O and CaSO₄ could be measured individually from the intensities of the characteristic bands. The peaks and the dips of the derivatives of the band intensities indicated the maximum rates of dehydration. The shifts in band positions also indicated the transformation. Even the background revealed some clue for transformation. Dehydration of CaSO₄·2H₂O was found to be a two step consecutive reaction with the intermediate CaSO₄·0.5H₂O in 50% humid air (11.9 Torr of water vapor pressure) as the thermo-Raman spectra showed.

2. Experimental

The laser light with a wavelength of 514.5 nm at 20 mW from an argon ion laser (Coherent, Innova 100-15) was focused onto the sample. The sample was filled into a sample holder and placed in a homemade oven with a glass window. The Rayleigh scattered light was reduced by a Notch filter. The scattered light was collected at right angle, dispersed by a single spectrometer (Spex, 0.5 m) with a resolution of 5 cm^minus;1 and detected by a CCD camera (Princeton Instruments, 1024 times; 1024 pixels). One spectrum was taken every 1°C over a temperature range of 25–300°C, the temperature increasing at the rate

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应用热拉曼光谱研究CaSO4·2H2O和CaSO4·0.5H2O脱水

摘要

在这项工作中，研究了热拉曼光谱从25到300℃,CaSO₄·2H₂O和CaSO₄·0.5H₂O在静止的空气中脱水。 成分组成可以直接从拉曼光谱与已知的CaSO₄·2H₂O、CaSO₄·0.5H₂O和CaSO₄ 的拉曼光谱进行比较来确定。 热拉曼光谱表明，CaSO₄·2H₂O的脱水是一个两步骤的连续反应和CaSO₄·0.5H₂O是本实验条件下的中间产物。谱带强度的变化显示出各物质的相对量。谱带强度的衍生物表现出物质脱水的最大速率。谱带的位置变化也揭示了转型。

关键词：拉曼光谱;热分析;脱水;CaSO₄·2H₂O;CaSO₄·0.5H₂O

1 绪论

随着温度的固体成分和相变化的变换可以通过热重分析（TGA），差热分析（DTA）等热分析[1]。然而，成分或相位没有详细的信息可用。拉曼光谱具有的优点从它的振动谱带识别的固体成分和相[2]。因此，它可以被用来监测在一个热过程中的固体成分和相原位转化。拉曼光谱的变化，在一个热过程中的热从热重分析和差热分析（DTA）的峰对应。拉曼光谱测定，在一个小的温度范围内，在许多固体3，4和5上的信息相变。有时，在转换过程中的光谱测定连续到显示变换6，7和8。事实上，更多的信息可以通过以下方式获得比那些从TGA热分析和差热分析（DTA）的拉曼数据。用于校正样品的热重分析和差热分析（DTA）的一水草酸钙，在一个热过程中不断的用拉曼光谱原位研究[9]。组合物和相位可确定的光谱和每个物质的量，可以测量从相变或组合物的变化（化学反应）过程中的谱带强度。形成的锐钛矿和金红石二氧化钛凝胶[10-11]通过该方法也进行了研究。二氧化钛的结构对于用途是重要的，拉曼光谱已经被用来研究变换结构[12]。

脱水的矿物或结晶是重要的，我们已经研究了许多晶体13和14。二水硫酸钙在脱水过程中的组成和阶段转换（CaSO₄·2H₂O）和半水（CaSO₄·0.5H₂O）到石膏（CaSO₄），都很有意思。已知二水石膏变成半水石膏，然后变为无水石膏15和16。该系统已经研究了拉曼光谱研究[17]，红外光谱仪17，18，19和20和TGA和DTA21，22，23，24，25，26和27。脱水的动力学进行了研究TGA21，22，23，24，25，26和27。在这项工作中，CaSO₄·2H₂O和CaSO₄·0.5H₂O的脱水进行了热拉曼光谱研究，热重分析和差热分析（DTA）的结果进行比较。在拉曼光谱的变化表现出明显的老相的消失和一个新的外观。对CaSO₄·2H₂O，CaSO₄·0.5H₂O及CaSO₄可以单独计量特征频段的强度。峰谷的频带强度的衍生物表明最高脱水率。带的位置的转变，也表明了转型。即使背景透露了一些线索进行改型。CaSO₄·2H₂O的脱水被认为是一个两步骤的连续反应的中间体CaSO₄·0.5H₂O在50％的湿空气（11.9乇的水的蒸气压）作为热拉曼光谱显示。

2 实验

用波长为514.5nm的氩离子激光（相干，伊诺100-15）在20毫瓦的激光聚焦在样品上。 将样品填充到样品固定器，并放置在一个玻璃窗口自制的烤箱。 瑞利散射光减少了陷波滤波器。 以正确的角度收集的散射光，由一个单一的光谱仪（Spex的为0.5米）分散，用分辨率为5cm^-1（普林斯顿仪器，1024times;1024像素）的CCD照相机检测。一个谱采取每1℃在25-300℃的温度范围内，温度在5℃min-1的速率增加。 第25光谱都是在25℃拍摄的，因此光谱的序号标明的温度。温度的测定由热电偶连接到样本保持器再由一个可编程的控制器和控制。这个自制的烤箱温度读数的不确定性为约2℃。 该样品是在静止的空气湿度约50％，温度在25℃以下。

一组光谱的测定由排除镜面1403双光谱仪。散射光检测用冷却的RCA C31034A倍增管，并通过固态继电器光子计数系统处理。分辨率分别为5cm^-1。

热谱得自 SEIKO（ISSC 5000）一个热天平分析和差热分析仪。 温度是从25到300℃，空气流中的增加率为5℃或2℃每分钟 。CaSO₄·2H₂O和CaSO₄·0.5H₂O均购自RDH和Merck公司，使用时无需进一步纯化。

3 结果与讨论

CaSO₄·2H₂O和CaSO₄·0.5H₂O的振动光谱带由三部分组成：晶格模式和内部模式，SO₄^2-和H₂O它们分别是在范围50 - 400 ，400 - 1200 ，3300 - 3600cm^{- 1} [2] 。

3.1 CaSO4·2H2O、CaSO4·0.5H2O和CaSO4的拉曼光谱

（a）的CaSO₄·2H₂O，（b）的CaSO₄·0.5H₂O，（c）和（e）的CaSO₄·2H₂O分别加热至200℃和300℃，以及（d ）和（f），在室温下从200和300℃ 冷却后 的拉曼光谱双光谱示于图1。SO₄^2-和H₂O的拉曼光谱2和17是众所周知的。 SO₄^2-水溶液中的四个振动谱带约1000（nu;1），450（nu;2），1150（nu;3）和650（nu;4）cm^-1的。这些条带可能会移位，分裂在不同的对称或组合物的晶体中，如图 1中（a）和（b）即CaSO₄·2H₂O和CaSO₄·0.5H₂O。 H₂O的振动带在3410和3498 cm^-1周围观察到。晶格模式低于400cm^-1处，在相鉴别中也很重要。在200和300℃ 高温下， 光谱与CaSO₄相似， H₂O无带出现示于图1中（c）及（e）。冷却后获得的光谱示于1中（d）及（f）段。光谱（d）相似于CaSO₄·0.5H₂O，暗示CaSO₄可能在冷却过程中吸收水分。谱（f）谱（d）是不同的。这表明，有可能是200和300°C之间的相变 。表1 中列出了该带的位置。 比较这些光谱，1010，1017和1024cm^-1处的吸收峰，可以分别用来表示这三种物质：CaSO₄·2H₂O、CaSO₄·0.5H₂O和CaSO₄。

图 1 拉曼光谱（a）的CaSO₄·2H₂O，（b）的CaSO₄·0.5H₂O（c）及（e）在200和300℃下的CaSO₄·2H₂O ，以及（d）和（f）的CaSO₄·2H₂O加热到200和300℃后冷却至室温。

表1 CaSO₄·2H₂O、CaSO₄·0.5H₂O和CaSO₄光谱带的位置。