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Effect of temperature and pH on phase transformations in citric acid-mediated hydrothermal growth of tungsten oxide

MPS-Authors
http://pubman.mpdl.mpg.de/cone/persons/resource/persons71845

Noack,  Johannes
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;
BasCat – UniCat BASF JointLab, Technische Universität Berlin;

http://pubman.mpdl.mpg.de/cone/persons/resource/persons134108

Kärgel,  Anne
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

http://pubman.mpdl.mpg.de/cone/persons/resource/persons22181

Trunschke,  Annette
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

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Citation

Wenderich, K., Noack, J., Kärgel, A., Trunschke, A., & Mul, G. (2017). Effect of temperature and pH on phase transformations in citric acid-mediated hydrothermal growth of tungsten oxide. European Journal of Inorganic Chemistry. doi:10.1002/ejic.201701156.


Cite as: http://hdl.handle.net/11858/00-001M-0000-002E-9CF5-4
Abstract
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 Scanning Electron Microscopy 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.