Identification of Superoxide O2- during Thermal Decomposition of Molten KNO3-NaNO2-NaNO3 Salt by Electron Paramagnetic Resonance and UV-Vis Absorption Spectroscopy

On account of excellent thermal physical properties, molten nitrates/nitrites salt has been widely employed in heat transfer and thermal storage industry, especially in concentrated solar power system. The thermal stability study of molten nitrate/nitrite salt is of great importance for this system, and the decomposition mechanism is the most complicated part of it. The oxide species O-2(2-) and O-2(-) were considered as intermediates in molten KNO3-NaNO3 while hard to been detected in high temperature molten salt due to their trace concentration and low stability. In this work, the homemade in situ high temperature UV-Vis instrument and a commercial electron paramagnetic resonance were utilized to supply evidence for the formation of superoxide during a slow decomposition process of heat transfer salt (HTS, 53 wt% KNO3/40 wt% NaNO2/7 wt% NaNO3). It is found that the superoxide is more easily generated from molten NaNO2 compared to NaNO3, and it has an absorption band at 420-440 nm in HTS which red shifts as temperature increases. The band is assigned to charge-transfer transition in NaO2 or KO2, responsible for the yellow color of the molten nitrate/nitrite salt. Furthermore, the UV absorption bands of molten NaNO2 and NaNO3 are also obtained and compared with that of HTS.