| dc.description.abstract | Monopropellants are widely used for propulsion and gas generation applications. The current state of art monopropellant systems are based on hydrazine propellants. While hydrazine has a strong heritage as a versatile monopropellant, some of the inherent problems associated with hydrazine like toxicity, high vapor pressure and associated storage and handling cost have been a major concern. These concerns led to the exploration of nontoxic and better performing alternate propellants, among which Hydroxylammonium Nitrate (HAN) is a promising candidate and scores over other monopropellants in terms of insensitivity, toxicity and volatility. Some of the other projected advantages of HAN formulations over hydrazine include its lower crystallization point, higher density and volumetric impulse.
The current practice of decomposing HAN is based on available technology used for hydrazine decomposition. However main challenges for HAN thrusters in contrast to hydrazine are material constraints imposed by the higher temperature levels experienced in HAN thrusters and need of high temperature tolerant catalysts. The catalysts under consideration should be both temperature tolerant and resistant to any possible poisoning from the decomposition products of HAN. There have been few reports of late on the development of suitable catalysts for HAN. Besides, unlike hydrazine, HAN propellants are multi-component, undergoes multistage decomposition adding complexity to the decomposition mechanism.
Current work pertains to the development of a high temperature tolerant active catalyst for HAN decomposition. The issues related to catalyst design, catalyst
characterization, reactivity to HAN based monopropellants, performance estimation and its decomposition kinetics are extensively pursued in this work. Special tests were developed to measure catalyst’s capability to sustain extreme chemical and thermal conditions witnessed in HAN thruster. A novel cobalt doped cerium oxide (CeCo) catalyst was prepared via co-precipitation route followed by slip casting for pelletization. Besides, as an improvement over this design, iridium coated bifunctional catalyst was prepared by wet impregnation of CeCo catalyst pellets.
To optimize composition of cobalt doped ceria catalyst, the catalytic activity of different compositions of catalyst was checked and a simultaneous investigation into the possible cause of their reactivity was conducted. The cobalt doped ceria catalyst was compared for reactivity and sustainability against conventional Ir/γ-Al2O3 catalyst. Thermoanalytical methods were employed to determine onset of decomposition temperature, rate of decomposition and exothermicity of the reaction. An inhouse designed batch reactor dedicated to examining durability and sustainability of the catalyst was used. To investigate any change caused by exposure to extreme exothermic decomposition of monopropellant, catalyst samples were subjected to physical and chemical characterization before and after decomposition. The characterization techniques used for the catalyst were Scanning Electron Microscopy/Energy Dispersive Spectroscopy, X-ray Diffraction Spectroscopy and X-ray Photoemission Spectroscopy. The activity of cobalt doped ceria was evident in thermal analysis since it evinced a low temperature and a high exothermic decomposition with HAN. Further, the new catalyst retained its physical integrity and was extremely resistant to catalyst poisoning during HAN decomposition in batch reactor while iridium-based alumina supported catalyst couldn’t sustain its activity either due to poisoning or severe attrition. A specific composition of cobalt was optimized in ceria for optimum performance in terms of lowering of decomposition temperature and endurance in relation to iridium catalyst. The structural failure of Ir/γ-Al2O3 pellets and physical integrity of ceria catalyst during batch reactor studies for multiple trials were demonstrated through SEM studies. The activity of new catalyst in decomposing HAN was found to be a function of Ce3+ presence in ceria matrix as determined from XPS. Thermal poisoning due to high decomposition temperature experienced in HAN thruster was simulated by exposing the catalysts in a high temperature furnace and modifications if any was followed using thermal analysis techniques.
The exceptional ability of iridium metal in initiating decomposition of HAN at a significantly low temperature, leads us to incorporate iridium as a reactive metal over cobalt
doped cerium oxide catalyst to form eventually a bifunctional catalyst. This bifunctional catalyst contained both Lewis acid and base sites to facilitate an optimised redox decomposition of HAN. Thermal analysis and batch reactor studies showed promising features for this catalyst. XPS studies carried out on these bifunctional catalysts show predominantly surface presence of active metal compared to bulk of catalyst. Iridium in bifunctional catalyst showed signs of oxidation during HAN decomposition along with an overall decrease in quantity suggesting bleeding of oxidized iridium due to high decomposition temperatures. However, Ce3+ quantities in bifunctional catalyst followed the pattern similar to the one observed in cobalt doped ceria catalyst for HAN decomposition. Despite deactivation of iridium due to oxidation, the promotional effect of iridium in bifunctional catalyst was indisputably evident in these studies.
Since HAN has a 33% positive oxygen balance it is used in combination with a suitable fuel for higher performance. The stoichiometrically balanced ternary systems obviously produce higher temperatures and extreme conditions to which the catalysts have to be tested. The ternary system tried out in the present study composed to HAN, methanol and water in the ratio 70:15:15 (w/w). The performance analysis of the catalyst was restricted to batch reactor since thermal studies with slow heating rates does not allow a simultaneous decomposition due to the differential boiling points of the components. However, the batch reactor was preheated to facilitate minimum time lag between ternary HAN system injection and its subsequent decomposition. The uncatalyzed ternary HAN decomposition showed lower energy outputs in comparison to binary systems. However, the performance of catalysed decomposition of ternary HAN system was remarkably superior to HAN binary system. Initially, the performance of both the catalysts was promising and comparable. However, Ir/γ-Al2O3 catalyst got disintegrated and deactivated much earlier as demonstrated by XRD and XPS. While the cobalt doped ceria catalyst sustained its activity and provided a consistent performance along at least 50 injections of HAN ternary system. The response of cobalt doped ceria catalyst to HAN ternary system was more or less similar to the binary system. XRD and XPS studies after the decomposition showed retention of Ce3+, the active component of the ceria-based catalyst, even after multiple trials which also point to its resistance to deactivation even in harsh reaction conditions produced by actual propellant system comprising of fuel, HAN and water.
Chemical kinetics provides an important insight into the decomposition mechanism and helps in the performance analysis of propellant compositions. The present work explores
the chemical kinetics involved in the thermal and catalytic decomposition of HAN using established methods in thermal analysis. The kinetic parameters were determined from thermo-gravimetric analysis by using isoconversional methods. Two isothermal methods namely, Popescu-Ortega’s method (integral method) and Friedman’s method (differential method) were used for kinetics estimation. The integral method failed due to its limitations in multi-step reactions when a physical step of water evaporation always preceded HAN decomposition in slow heating thermal analysis studies. Whereas, differential method worked well after smoothing the TGA data. The values obtained from differential method were complemented with the results obtained from EGA (evolved gas analysis) for which a DTA-TG-FTIR hyphenated system was used. Primary product species identified for HAN decomposition were N2O, HNO3 and NO2 for all cases. The reliability of isoconversional method adopted here was initially verified for thermal decomposition for which kinetics parameters are available. The variable activation energy calculated as a function of conversion was used to describe the variation in decomposition mechanisms for both the catalysts tested in the present study. The variation in the concentration of species with change of catalyst was prominent and further demonstrated the superiority of ceria catalyst developed in this work. The EGA profiles further substantiated the proposed decomposition mechanism and suggested reasons for robustness of ceria catalysts over iridium catalyst.
The new class of cerium oxide catalyst developed during this work looks highly promising thanks to its unique features like absence of attrition, resistance to poisoning, high temperature tolerance, propensity to initiate decomposition reaction at low temperature with high exothermicity and preference towards thermodynamic products leading to a low molecular mass product stream. | en_US |
