Thermoanalytical studies on the thermal and catalytic decomposition of aqueous hydroxylammonium nitrate solution

Green monopropellants based on hydroxylammonium nitrate (HAN) are a promising alternative to hydrazine in space propulsion systems. In the present paper, thermal and catalytic decomposition of aqueous HAN solution was studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and mass spectrometry. The tests were conducted at heating rates of 1, 2.5, 5, and 10 K/min. The values of the apparent activation energy obtained for thermal decomposition using TGA and DSC are 62.2 ± 3.7 kJ/mol and 57.5 ± 3.5 kJ/mol, respectively, and they are in agreement with the literature data for solid HAN and solutions with high concentrations of HAN. The obtained values of the pre-exponential factor, 2.24 × 104 s−1 for TGA and 3.55 × 103 s−1 for DSC, are lower by six to seven orders of magnitude than those reported in the literature for aqueous HAN solutions, apparently because of full vaporization of water from the HAN solution at the beginning of the TGA and DSC tests. The use of an iridium/rhodium foam catalyst decreased the temperature of full decomposition by over 60 °C. The value of the apparent activation energy obtained for the catalytic decomposition using TGA is 63.9 ± 2.5 kJ/mol, while the obtained value of the pre-exponential factor is 3.31 × 105 s−1.