Detection and Origin of Radical Species in the Catalytic Decomposition of Hydroxylammonium Nitrate Aqueous Solution

Hydroxylammonium nitrate (HAN) stands out as a promising green monopropellant; however, the intermediates generated during its catalytic decomposition remain poorly understood. In this work, both liquid-phase and gaseous products generated from HAN decomposition over Rh/C catalysts were systematically investigated. For the first time, direct EPR and HRMS evidence confirmed the formation of hydroxyl (•OH) and nitric oxide (NO•) radicals in aqueous HAN solutions during Rh/C-catalyzed decomposition. Mechanistic analysis suggested that nascent •OH originated from the homolytic cleavage of HNO3, while NO• was subsequently generated through reactions between •OH (or NO2•) and NH3OH+. XPS characterization of both fresh and spent Rh/C catalysts indicated that metallic Rh0 acted as the active site for HAN decomposition. In situ ATR-IR and Raman spectroscopy captured changes in the N–O and N–OH bonds of aqueous HAN during the reaction. This study provided unambiguous evidence for radical involvement in HAN decomposition and proposed a revised radical-involved reaction pathway. These findings advance the fundamental understanding of HAN catalysis and offer guidance for the optimization of HAN-based propulsion systems.