The temperature-dependent composition of suspension during citric acid mediated crystallization of tungsten trioxide (WO3) from sodium tungstate was studied by in situ Raman spectroscopy. Additionally, microwave-assisted hydrothermal synthesis experiments combined with ex situ analysis by X-ray diffraction and SEM were performed to analyze the effect of pH on the eventually, isothermally, obtained crystal phase and morphology. The Raman results suggest that WO3·2H2O precipitates from the tungstate solution upon acidification to pH 0.5 at room temperature. This is first transformed to WO3·H2O initiating at T = 70 °C. At temperatures above 170 °C, the crystallization of phase-pure monoclinic WO3 with well-defined plate-like morphology was observed at pH 0.5. Using the microwave-assisted hydrothermal synthesis procedure shows that increasing the pH to values of 1.5 or 2 results in significant or dominant formation of hexagonal WO3, respectively. Comparing the activity of selected samples in photocatalytic oxidation of propane using visible light, demonstrates the presence of hydrate phases or hexagonal WO3 is detrimental to performance.

中文翻译：

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温度和 pH 值对柠檬酸介导的氧化钨水热生长相变的影响

通过原位拉曼光谱研究了柠檬酸介导的从钨酸钠中结晶出三氧化钨 (WO3) 过程中悬浮液的温度依赖性组成。此外，还进行了微波辅助水热合成实验，结合 X 射线衍射和 SEM 的非原位分析，以分析 pH 值对最终等温获得的晶相和形态的影响。拉曼结果表明，在室温下酸化至 pH 值为 0.5 时，WO3·2H2O 从钨酸盐溶液中沉淀出来。这首先在 T = 70 °C 时转化为 WO3·H2O。在高于 170 °C 的温度下，在 pH 0.5 下观察到具有明确定义的板状形态的纯相单斜晶 WO3 的结晶。使用微波辅助水热合成程序表明，将 pH 值增加到 1.5 或 2 分别导致显着或主要形成六方 WO3。比较所选样品在使用可见光光催化氧化丙烷中的活性，表明水合物相或六方 WO3 的存在对性能有害。