The introduction of hydroxylammonium nitrate (HAN) ionic liquid as a replacement for hydrazine as a spacecraft monopropellant has been of great interest recently due to the reduced toxicity and improved thrust performance afforded by HAN over hydrazine. The understanding of thermal and catalytic decomposition mechanisms of next-generation HAN-based monopropellant formulations has not been sufficient to accurately model their ignition properties using chemical kinetics models. In this work, the reactivity of HAN aerosols on heated copper and iridium targets was investigated using tunable vacuum ultraviolet photoionization time of flight aerosol mass spectrometry. The reaction products were identified by their mass-to-charge ratios and their ionization energies. Products include NH3, H2O, NO, hydroxylamine (HA), HNO3 and a small amount of NO2 at high temperature. No N2O was detected under these experimental conditions, despite the fact that N2O is one of the expected products according to the generally accepted thermal decomposition mechanism of HAN. Upon introduction of iridium catalyst, a significant enhancement of the NO/HA ratio was observed. This observation indicates that the formation of NO via decomposition of HA is an important pathway in the catalytic decomposition of HAN. It is our belief that the iridium plays a crucial role in stripping hydrogens from the HA intermediate, a concept that has not yet been introduced into the chemical kinetics model for HAN ignition and combustion. To support our experimental observations, ab initio quantum chemical calculations on a model iridium catalyst represented by small Irn (n=4, 6, 8…) clusters were performed using the M11-L functional with the LANL2DZ basis set. Results obtained on the mapping of hydrogen migrations from HA to Ir atoms in the Irn-HA complexes will also be discussed.

Decomposition of hydroxylammonium nitrate ionic liquid aerosols on catalytically active metal surfaces

254th National Meeting (2017)

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