SINOPSIS

The reaction mechanism of catalytic decomposition of hydroxylammonium nitrate (HAN) aqueous solutions was investigated to understand the catalytic decomposition and to develop the thruster using a HAN-based monopropellant. It is important to understand the catalytic decomposition because the chemical reaction of the monopropellant is activated by a catalyst in the existing monopropellant thruster. Previously, several researchers investigated the thermal decomposition and the combustion characteristics of HAN-based solutions. However, catalytic decomposition of HAN-based monopropellant has not been well understood. In this study, in order to clarify the reaction mechanisms of catalytic and thermal decompositions, differential thermal analysis and thermogravimetric analysis of HAN aqueous solution and product gas analysis were conducted in the cases of catalytic and thermal decompositions. These analyses show that the Ir-based catalyst was effective to reduce the onset temperature of HAN decomposition and that the difference in product gas species was not obvious between catalytic and thermal decompositions. Based upon these results, a reaction mechanism of catalytic decomposition was discussed. It was found that the reaction mechanism of catalytic decomposition is almost the same as that of thermal decomposition. Furthermore, nitric acid (HNO₃) was generated at lower temperature in the case of catalytic decomposition than that of thermal decomposition. Therefore, Ir-based catalyst cannot change the reaction mechanism but can activate the reaction path of HNO₃ generation.