Stability of molten nitrate salts containing light absorbing additives as solar flux absorbers

Abstract

The decomposition of molten sodium nitrite-sodium nitrate-potassium nitrate ( 40:7:53 wt% ) and sodium nitrate-potassium nitrate ( 46:54 wt% ) with release of nitrogen dioxide was studied at 320°C, 440°C and 500°C, in the absence and presence of suspensions of various additives, without or with illumination by UV and visible light. In the NaNO 2NaNO 3KNO 3 melt, TiO 2, Fe 2O 3, CuO, Co 3O 4, graphite and active charcoal formed quite stable suspensions, while MoS 2 decomposed, and Fe 3O 4 and carbon black failed to disperse. The photoinduced release of nitrogen dioxide was enhanced in the presence of TiO 2, Co 3O 4 and Fe 2O 3, but was much smaller in the presence of CuO. At 440°C, in the presence of graphite powder, the release of NO 2 in the dark and under illumination with a 150 W xenon lamp were 18 and 35 μ mol h −1, respectively. In the NaNO 3KNO 3 melt, Fe 2O 3, Co 3O 4 and CuO formed stable suspensions. With this melt in the presence of Co 3O 4 low rates of decomposition were observed, which at 440°C in the absence and presence of illumination with a 150 W Xe lamp were 0.07 and 0.1 μ mol h −1 per 50 g of the melt. In electrochemical experiments with stainless steel electrodes at 300°C–306°C, the release of nitrogen dioxide increased with applied potential. At 1.6 V cell potential and 21 mA current, the rate of NO 2 release was 42 μmol h −1. Using a polycrystalline TiO 2 electrode under illumination with a 150 W Xe lamp, versus a stainless steel cathode in a NaNO 3KNO 3 (46–54 wt%) melt at 286°C, the photocurrent amounted to 0.2 mA at an applied potential of 1.0 V.