In situ investigation of decomposing ammonia and ammonobasic solutions under supercritical conditions via UV/vis and Raman Spectroscopy

• Published in: The Journal of supercritical fluids Vol. 134; pp. 96 - 105
• Main Authors: Steigerwald, Thomas G., Balouschek, Johannes, Hertweck, Benjamin, Kimmel, Anna-Carina L., Alt, Nicolas S.A., Schluecker, Eberhard
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-04-2018
• Subjects: Ammonia decomposition; Ammonothermal crystal growth; Raman spectroscopy; Supercritical ammonia; UV/vis spectroscopy; UV/vis spectroscopy; Supercritical ammonia; Raman spectroscopy; Ammonia decomposition; Ammonothermal crystal growth
• ISSN: 0896-8446; 1872-8162
• DOI: 10.1016/j.supflu.2017.12.028

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•Visualization of temperature dependence of NH3 decomposing to N2 and H2.•Maximum temperature in the autoclave = 563 °C, at heating temperature = 617 °C.•Maximum pressure = 254 MPa.•In situ Raman spectra in the range of 200–4500 cm−1.
An optical cell was used in this work, which allows spectroscopic measurements of fluids up to 600 °C and 300 MPa. The maximum pressure reached in this work was 254 MPa and the internal cell temperature was a maximum of 563 °C at a heating temperature of 617 °C. In the following, a combination of Raman and UV/vis spectroscopy was used for in situ investigation of the breakdown of sodium azide above 300 °C combined with the formation of sodium amide. It was verified that the breakdown of NaN3 occurs before the formation of NaNH2 starts.
In a second step the decomposition of supercritical ammonia under ammonothermal conditions was investigated solely via Raman spectroscopy. The study comprises four different fluid compositions: pure ammonia, ammonia with a ruthenium catalyst, ammonia and mineralizer (sodium azide) and last ammonia, mineralizer and gallium nitride (GaN). By this, it was possible to proof that with pure ammonia and without a proper catalyst the decomposition of ammonia stops quite far before chemical equilibrium is reached.