The Catalyst Development for Space Propulsion Applications

Monopropellant rocket systems work by generating a combination of high-pressure hot gases through an exothermic decomposition reaction of a propellant compound. Afterward, the products are accelerated through a converging–diverging nozzle to provide the desired thrust. These systems can generate thrust in the range of 0.1 to 500 N, with a moderate specific impulse of up to 250 s. In small missiles and satellite engines that require low thrust, monopropellant engines are preferred. This thermal decomposition process has a large activation energy, meaning that the compound does not decompose spontaneously at ambient temperatures and must be suitably heated. A special catalyst can lower the activation threshold to speed up the reaction. As a monopropellant, hydrazine (N2H4) is most often used because the successful development of the Shell S405 catalyst provides high efficiency, with a specific impulse (Isp) value of 237 s. However, its carcinogenic and toxic nature causes transportation and handling problems. Therefore, green alternatives are receiving increasing attention nowadays. Their lower toxicity and safety precautions result in lower costs for manufacturing, handling, and storage. One of the most promising choices among the green monopropellants is highly concentrated hydrogen peroxide (H2O2). Although highly concentrated hydrogen peroxide has a lower Isp value (179s) than hydrazine, H2O2 is ecologically friendly because its decomposition products include oxygen and steam. The development of an appropriate catalyst and a reliable system for green monopropellants are key aspects of catalytic decomposition in thruster systems. The main aim of this thesis is to demonstrate the utilization of an effective and reliable monopropellant thruster. As a chemical approach, the importance of the support and active material of the catalyst is studied, and their properties are analyzed with characterization techniques and reaction kinetic experiments. Then, they are used in the monopropellant thruster as an engineering approach; the catalyst bed is analyzed based on temperature and pressure changes over time during decomposition in the thruster system. In this thesis, the aluminum oxide-supported manganese oxide catalyst is synthesized and tested. A microcrystalline cellulose-templated alumina catalyst support was prepared, and a parametric study was conducted to determine the optimum preparation conditions. The chosen micro cellulose-templated aluminum oxide was 15 wt% of micro cellulose to alumina at a calcination temperature of 900 °C. Macropores were formed on the MnOx/alumina catalyst surface. Thanks to these pores, hot gas was allowed to discharge from the pores without breaking the catalyst. In addition, by increasing the calcination temperature, catalysts with a longer life have been designed for thruster operations. Therefore, the unstable performance of thrusters caused by cracking of the pellet catalyst can be solved by adjusting the porosity of the catalyst. The primary goal of previous studies was to explore methods to improve the mechanical stability of the catalyst within the thruster, which resulted in increased strength but only moderate catalytic activity. As a secondary aim, efforts have now shifted toward enhancing the catalyst's reaction activity while maintaining its improved mechanical properties. The research continued by improving the loading of active material through double impregnation techniques and intermediate heating at 325 °C. According to the BET analysis, high-concentration precursor solutions cause more material to accumulate on the support, leading to pore blockage and a decrease in specific surface area. This observation was also correlated with the diminution of visible pores in SEM images. Increasing the molarity of the manganese precursor and adjusting the synthesis process resulted in significant improvements in catalyst performance, as evidenced by higher MnOx loading and enhanced apparent reaction rates in kinetic and thruster tests at first however, after a while, all catalysts show similar performance. Beyond catalyst synthesis, the study explored various factors affecting thruster performance, including catalyst mass, particle size, the concentration of hydrogen peroxide and the amount of stabilizer in hydrogen peroxide and radiation in space environments. The decomposition of H2O2 using a manganese oxide catalyst exhibits first-order kinetics; however, external mass transfer limitations affect the observed reaction rate. Testing various catalyst amounts (250 mg to 1500 mg) demonstrated a proportional increase in reaction rate, emphasizing the significance of catalyst concentration. The particle size of the catalyst was also crucial in the catalytic bed of the thruster because size can also cause flow instabilities during thruster tests. According to the test results, larger particles (700 µm) resulted in higher pressure drops compared to smaller particles (300 µm), which provided more stable performance with lower pressure loss and greater thrust, indicating their superior effectiveness in thruster applications. Moreover, the presence of stabilizers, such as sodium pyrophosphate, was found to negatively impact catalyst efficiency by inhibiting H2O2 decomposition. Tests with lower-phosphate peroxide grades showed improved catalytic activity, emphasizing the importance of propellant purity in thruster performance. Also, from 87.5wt to 78wt% hydrogen peroxide concentrations are examined, and the catalysts are shown to breakdown them efficiently. Although lower decomposition temperatures resulted in minor thrust losses, these are unlikely to impair the overall performance of the propulsion system. Experiments also examined the effects of cold starts and preheating on thruster operation. Preheating the catalyst bed to 150 °C was found to provide optimum thrust performance by reducing thrust response times. Subsequently, gamma radiation was applied to both hydrogen peroxide and the catalyst to examine the effects of space radiation exposure. Ionizing radiation can cause hydrogen peroxide to decompose into oxygen, reducing its concentration and impacting thruster performance. However, tests on 88% hydrogen peroxide exposed to 158.4 Gray showed no significant change in concentration. While radiation can degrade stabilizers such as sodium pyrophosphate, the changes were minimal. Catalysts were also tested for radiation effects, showing some shifts in manganese oxidation states (increase in Mn²⁺, decrease in Mn⁴⁺); however, the catalyst's structure and functionality remained stable in thruster and kinetic tests, with only a slight reduction in apparent reaction rates. In summary, this thesis presents a comprehensive study of catalyst development for hydrogen peroxide decomposition in monopropellant thrusters. The findings demonstrate significant advancements in catalyst stability, efficiency, and performance, providing valuable insights for future applications in space propulsion systems