Exothermic catalytic decomposition of energetic ionic liquids on iridium based catalysts

Abstract

Hydrazine (N2H4) is the most commonly used propellant for in-orbit spacecraft propulsion. However, utilization of hydrazine in space missions has challenges associated with health, environment and safety risks. Energetic ionic liquids (EILs) such as ammonium dinitramide (ADN) present themselves as environmentally friendly alternative fuels to hydrazine. EILs can be decomposed efficiently and safely in the presence of a heterogenous catalyst. In this context, monometallic catalysts containing Ir and Al2O3 were synthesized using both wetness impregnation and incipient to wetness impregnation methods, and the structural properties of these catalysts were investigated. Furthermore, the effects of the Al2O3 support material on Ir dispersion and catalytic performance of anaerobic ADN decomposition were studied. In order to improve the Ir active site dispersion on the Al2O3 support material, promoters such as La and Ce were added to the catalyst systems and different pretreatment conditions were applied to the synthesized catalysts. Furthermore, LaMnO3 (perovskite) promoted alumina catalysts with Ir active sites were also studied. Catalysts with high performance, 5Ir/TH100 (5Ir/Al2O3), 5Ir/L3 (5Ir/La-Al2O3), and 5Ir/Sir10 (5Ir/Si-Al2O3) were investigated with in-situ X-ray Absorption Near Edge Spectroscopy (XANES), in-situ Extended X-ray Absorption Fine Structure (EXAFS), in-situ Fourier Transform Infrared Spectroscopy (in-situ FTIR), Temperature Programmed Desorption (TPD), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX), Pyridine adsorption via FTIR, CO Chemisorption, X-ray Photoelectron Spectroscopy (XPS) and X-ray Fluorescence (XRF) analysis techniques. Our findings revealed that 5Ir/TH100 and 5Ir/L3 catalysts favorably lowered the onset temperature of the ADN decomposition reaction, whereas 5Ir/Sir10 boosted the pressure generation during the reaction. The formation of mostly metallic Ir nanoparticles on 5Ir/TH100 and 5Ir/L3 enables the lowering of the activation energy of the reaction. On the other hand, enhancement in the pressure generation for 5Ir/Sir10 catalyst is associated with the generation of small oxidic Irn x+ clusters which are strongly interacting with the SiOx-AlOx surface domains of the support material. The fundamental structure-functionality relationships unraveled in the current work may allow design of novel catalytic systems for aerospace monopropellant propulsion systems with higher performance by simultaneous exploitation of Ir active sites with different electronic properties.