1. Introduction

A small satellite is more economical for Low Earth Orbit (LEO) mission than a large one because of less aerodynamic drag and reduced cost of launching [1]. A propulsion system for a small satellite requires high thrust with low power consumption for drag compensation and station keeping. Cold-gas thrusters, electrothermal thrusters, and monopropellant thrusters are available for the propulsion system. However, cold-gas thrusters have low specific impulse. Electrothermal thrusters require a large heating device in order to heat propellant to high temperature; thus, high power consumption is inevitable. Monopropellant thrusters are suitable for a small propulsion system because additional power supply is not required after the startup: propellant decomposition is self-sustained on the catalyst after initiation.

Hydrazine (N₂H₄) has been mostly used as a monopropellant; however, it is very toxic and is of hazard in storage and handling. Recently, eco-friendly monopropellant thrusters have been developed such as nitrous oxide (N₂O) and [2,3] hydrogen peroxide (H₂O₂) [4], which are called “Green propellants.” Among these propellants, non-toxic N₂O propellant has a large storage - temperature range between critical point (-90.8°C) and decomposition temperature (520°C). It shows also good liquefaction characteristics: N₂O exists in a liquid state over 52 atm at 21°C. N₂O is pressurized in a storage tank (over 52 atm); thus, a propellant supply system is not required. Since characteristics of N₂O are relatively safe in storage and handling, development and maintenance expenses of N₂O thruster are significantly reduced. Furthermore, N₂O is very stable and easy to handle at room temperature, and easily available in industrial quantities [2]. Physical characteristics of N₂O are presented in Table 1.

N₂O decomposes into nitrogen and oxygen, generating decomposition heat of 82 kJ/mol as shown in Eq. (1)[5]. N₂O decomposition process generates high temperature gases that are used for various propulsion applications such as monopropellant thrusters, rocket igniters, power generation devices, and auto-ignition for bipropellant and hybrid rockets [6,7].

It is reported that N₂O is thermally decomposed at temperature higher than 520°C [2]. In actual propulsion system, however, the temperature should be higher than 1000°C. Thus, N₂O thermal decomposition requires a big heating device with the high-power consumption. Zakirov et al. [2] reported that N₂O decomposition on the proper catalyst was possible at temperature less than 300°C. Additional heat supply was not required because N₂O decomposition was sustained by the reaction heat after the reaction was initiated. However, adiabatic decomposition temperature of N₂O is 1640°C; thus, the catalyst becomes damaged during N₂O decomposition. Therefore, a heat-resistive and durable catalyst for N₂O decomposition is required for practical applications. Alumina (Al₂O₃) is widely used as a catalyst support for the N₂H₄ and H₂O₂ decompositions [4]. However, Al₂O₃ is sintered at 1000°C; thus, its specific surface area rapidly decreases due to phase transformation, resulting in deactivation of the catalyst active sites [8]. Courtheoux et al. [9] studied catalytic decomposition of hydroxylammonium nitrate (HAN) on platinum (Pt) supported on silicon-doped alumina (Si-Al₂O₃). The Pt/Si-Al₂O₃ catalyst remained active with a fast reaction rate during the HAN decomposition. Zeng et al. [10] studied catalytic decomposition of N₂O on alumina-supported ruthenium (Ru/Al₂O₃) catalysts. The optimum Ru loading was found and the catalytic activity was measured at the various reaction conditions. Boissel et al. [11] studied noble metal-transition metal oxide catalysts supported on cordierite monolith for N₂O decomposition. The effect of combination of transition metal (Cu, Fe, Co, Ni) and noble metal (Ir, Rh) was investigated.

Al₂O₃ has been widely employed as a catalyst support by numerous researchers due to its high specific surface area, excellent thermal stability, and mechanical robustness, which make it well-suited for various catalytic applications. However, Al₂O₃ exhibits a temperature-dependent phase transformation pathway, beginning with the transition from the γ-phase (typically stable below ~800°C), progressing through intermediate phases such as δ-, θ-, and β-Al₂O₃, and ultimately converting to the thermodynamically stable α-phase at temperatures above ~1100–1200°C. These phase transitions are influenced by factors such as heating rate, precursor characteristics, and dwell time during thermal treatment. In the present study, a Ru catalyst supported on Si-doped Al₂O₃ was prepared for N₂O decomposition. Si was doped into the Al₂O₃ support in order to improve heat-resistivity at high temperature. The effect of Si addition on Al₂O₃ surface structure and N₂O decomposition was investigated and a subscale N₂O monopropellant thruster was designed and tested.

2. Experiment

2.1 Catalyst preparation

Ru metal element was selected as a catalyst for N₂O decomposition. The catalyst support was γ-Al₂O₃ that provides a large surface area with good thermal-stability [4]. Si was loaded into γ-Al₂O₃ support in order to improve heat-resistivity at high temperature. The detailed procedure of catalyst preparation is as follows. First, pellet-shaped γ-Al₂O₃ support (Alfa Aesar, 1/8” pellets) was used because it is easy to pack the thruster chamber with pellets. Si was doped into γ-Al₂O₃ pellets using wet impregnation method. (C₂H₅O)₄Si (Alfa Aesar) was used as precursor. After drying at 120°C for 12 hours, the pellets were heated at 1200°C at air environment: the temperature was raised up with 10°C/min and maintained for 2 hours. We obtained Si-doped Al₂O₃ (Si-Al₂O₃) with Si content of 5 mol%. Subsequently, Ru was loaded into Si-Al₂O₃ using wet impregnation method. RuCl₃ (KOJIMA) was used as precursor. The prepared catalyst pellets were calcined at 700°C after complete drying. Consequently, Ru/Si-Al₂O₃ catalyst was obtained. Ru loading was 5 wt%.

Fig. 1.

Photograph of the catalysts; (a) bare γ-Al₂O₃, and (b) Ru/Si-Al₂O₃.

All prepared catalysts including Al₂O₃, Si-Al₂O₃, Ru/Al₂O₃, Ru/Si-Al₂O₃, and Ru-Si-Al₂O₃ are listed in Table 2. Fig. 1 shows bare γ-Al₂O₃ and the prepared Ru/Si-Al₂O₃ catalysts. For comparison purposes, bare γ-Al₂O₃ pellets without Si doping were heated at 1200°C and Ru/Al₂O₃ was prepared using conventional method. Another catalyst was prepared using a different method. Ru and Si precursors were simultaneously loaded into γ-Al₂O₃ pellets, while element contents and heat-treatment conditions were the same as with Ru/Si-Al₂O₃ catalyst. The catalyst was named Ru-Si-Al₂O₃. All catalysts were characterized using SEM (Scanning Electron Microscopy), EDS (Energy Dispersive X-ray Spectroscopy), and XRD (X-Ray Diffractometer).

2.2 Experimental setup

Experimental apparatus for N₂O decomposition on the prepared catalysts is shown in Fig. 2(a). The prepared catalyst was fixed into a quartz tubular reactor. The N₂O feed rate was controlled using MFC (BRONKHORST). The reaction temperature was controlled using a programmable electric furnace. A thermocouple was inserted at the middle of the catalyst in order to measure the temperature during the reaction. A subscale monopropellant thruster is shown in Fig. 2(b). The 2-piece flanges were sealed with copper crush gasket. The optimal catalyst selected in the N₂O decomposition testing was packed in the thruster chamber. The thruster chamber volume was 30 ml where 15 g of catalysts were filled. The temperature and pressure ports were welded on the thruster as shown in Fig. 3. 5 thermocouples were inserted through the connecting ports and the tips protruded to the chamber centerline. The catalyst was supported on the up and down sides with porous metal discs [12].

Fig. 2.

(a) Experimental apparatus for N₂O decomposition and (b) subscale monopropellant thruster.

The gas chromatography (YOUNGLIN, YL6100) was used to analyze product gas composition such as N₂O, N₂, and O₂. The N₂O conversion is defined as Eq. (2).

The test facility for evaluating N₂O monopropellant thruster is shown in Fig. 3. The facility contains a N₂O tank, tank pressure regulator, feed line thermocouple, pneumatic valves, turbine flow meter, and electric preheater. The N₂O gas was supplied using a pneumatic valve on the remote site, and the N₂O flow rate was regulated with a sonic orifice. The preheater was installed in the thruster inlet in order to initiate the N₂O decomposition. Both the reactor and the thruster were preheated for a sufficient duration to ensure thermal equilibrium with the catalyst. Considering the difference in heat capacity between the reactor and the thruster, the reactor was preheated for 1 minute, while the thruster was preheated for 3 minutes.

Fig. 3.

Test facility for evaluating N₂O monopropellant thruster.

The N₂O mass flow rate was calculated by measuring the volumetric flow rate, temperature (T_s), and pressure (P_s) at the thruster inlet. Calculation results were in good agreement with 1-D isentropic flow equation given by Eq. (3).

where m˙, γ, A_t, and R denote the N₂O mass flow rate, specific heat ratio, throat area of the thruster nozzle, and the gas constant, respectively.

3. Results and Discussion

3.1 Characterization of the Catalyst

The γ-Al₂O₃ pellet with the surface area of 220 m²/g was used as the catalyst support. However, the surface area sharply decreased at temperature higher than 1000°C. The γ-phase was then transformed though β-phase to α-phase. Thus, the phase transformation at the high temperature should be prevented in order to maintain the surface area and catalytic activity. Fig. 4(a) shows XRD patterns of the prepared catalysts. In Al₂O₃ pattern, distinct peaks were observed. This indicates that Al₂O₃ changed from γ-phase to α-phase. The α-Al₂O₃ was formed by phase transformation at high temperature. High-intensity peak indicates large particle size; thus, the α-Al₂O₃ surface area was very small. The Si-Al₂O₃ heated at 1200°C shows the result that is different from α-Al₂O₃ pattern. The Si-Al₂O₃ pattern was similar with γ-Al₂O₃ one. Thus, Si addition suppresses phase transformation of γ-Al₂O₃ at high temperature. When Si was loaded into γ-Al₂O₃ pellet, Si particles penetrated into Al₂O₃ lattice. Si particles hindered Al₂O₃ phase transformation at high temperature; thus, Si-Al₂O₃ maintained γ-phase at 1200°C. Ru/Si-Al₂O₃ and Ru-Si-Al₂O₃ contain Ru catalyst particles; however, Ru peaks were not observed in XRD patterns. Ru particles were loaded in γ-Al₂O₃ pores that have a large surface area. Consequently, Ru particles become dispersed over γ-Al₂O₃ surface. Therefore, the peak intensity was very low. We found that Ru/Si-Al₂O₃ and Ru-Si-Al₂O₃ catalysts sustained a large surface area at high temperature.

Fig. 4(b) shows SEM images of the prepared catalysts. The prepared catalysts had different surface structures from a bare γ-Al₂O₃. Al₂O₃ heated at 1200°C had α-Al₂O₃ surface morphology. α-Al₂O₃ contained distinct particles larger than γ-Al₂O₃ because the particles were aggregated at high temperature. Consequently, the surface area was very small. This agrees with XRD results that Al₂O₃ phase was transformed from γ-phase to α-phase at the high temperature. The Si-Al₂O₃ shows a surface structure similar to that of the bare γ-Al₂O₃; thus, Si addition decreases particle aggregation at high temperature. Consequently, Si-Al₂O₃ maintained γ-Al₂O₃ phase; thus, it provided large surface area.

Fig. 4.

(a) XRD patterns, (b) SEM images, and (c) EDS analysis of the prepared catalysts (A: α-Al₂O₃, SA: Si-Al₂O₃, RA: Ru/Al₂O₃, RSA1: Ru/Si-Al₂O₃, RSA2: Ru-Si-Al₂O₃).

Fig. 4(b) shows SEM images of the prepared catalysts. The prepared catalysts had different surface structures from a bare γ-Al₂O₃. Al₂O₃ heated at 1200°C had α-Al₂O₃ surface morphology. α-Al₂O₃ contained distinct particles larger than γ-Al₂O₃ because the particles were aggregated at high temperature. Consequently, the surface area was very small. This agrees with XRD results that Al₂O₃ phase was transformed from γ-phase to α-phase at the high temperature. The Si-Al₂O₃ shows a surface structure similar to that of the bare γ-Al₂O₃; thus, Si addition decreases particle aggregation at high temperature. Consequently, Si-Al₂O₃ maintained γ-Al₂O₃ phase; thus, it provided large surface area.

Different surface morphology was observed in Ru/Si-Al₂O₃ and Ru-Si-Al₂O₃ catalysts. Both catalysts contain the same elements such as Ru and Si, while their preparation method is different. The Ru-Si-Al₂O₃ catalyst was prepared by simultaneously loading Ru and Si in the Al₂O₃. Large Ru particles were observed on the Al₂O₃ surface indicating that Ru particles are not well dispersed in Al₂O₃ pores. The Ru/Si-Al₂O₃ catalyst was prepared by loading Ru in Si-Al₂O₃. Large Ru particles were not seen in the SEM image; that is Ru/Si-Al₂O₃ catalyst has higher Ru loadings and higher surface area than Ru-Si-Al₂O₃ catalyst. Fig. 4(c) shows EDS analysis of the prepared catalysts. In the EDS result of Si-Al₂O₃, Si presence was identified. Si to Al₂O₃ ratio was measured as 5.0. Ru/Al₂O₃ and Ru/Si-Al₂O₃ results identified Ru presence that was not observed in XRD patterns. These results show that Ru and Si were properly loaded into Al₂O₃.

3.2 Reactivity of catalysts at N₂O decomposition

Fig. 5 shows N₂O conversion on the prepared catalysts depending on reaction temperature. Al₂O₃ and Si-Al₂O₃ had low conversion throughout the entire range because Ru catalyst was not loaded. Si-Al₂O₃ had higher N₂O conversion than Al₂O₃ at 500°C; that is Si had reactivity at N₂O decomposition. The results of comparison of Ru/Al₂O₃ and Ru/Si-Al₂O₃ show the effect of Si addition to N₂O conversion. In both catalysts, N₂O conversion increased with the reaction temperature and 100% conversion was obtained at temperature higher than 500°C. At temperature lower than 500°C, however, the results were different; Ru/Si-Al₂O₃ catalyst had 87.3% conversion, while Ru/Al₂O₃ catalyst had only 48.5% conversion at 400°C. This indicates that Si addition increases N₂O conversion at low temperature. Both Ru/Si-Al₂O₃ and Ru-Si-Al₂O₃ catalysts show 100% conversion at temperature higher than 500°C. At 400°C, however, Ru/Si-Al₂O₃ catalyst had N₂O conversion slightly higher than Ru-Si-Al₂O₃ catalyst. The N₂O conversion was improved due to good Ru dispersion of Ru/Si-Al₂O₃ catalyst as shown in SEM images of Fig. 4(b).

Fig. 5.

N₂O conversion on the prepared catalysts as a function of reaction temperature (A: α-Al₂O₃, SA: Si-Al₂O₃, RA: Ru/Al₂O₃, RSA1: Ru/Si-Al₂O₃, RSA2: Ru-Si-Al₂O₃).

Self-sustained N₂O decomposition on the Ru/Si-Al₂O₃ catalyst was performed as shown in Fig. 6(a). The preheating temperature was 420°C and the N₂O feed rate varied from 1000 to 2000 ml/min. The temperature rapidly increased after N₂O was supplied. Subsequently, the temperature reached a steady value and was maintained at 735°C. The temperature, however, varied greatly with N₂O feed rate. The maximum registered temperature was 930°C at N₂O feed rate of 2000 ml/min. At that time, product gases included 28.7% O₂, 58.51% N₂, and 12.8% N₂O; thus, N₂O conversion was 87.2%. N₂O decomposition was continued for 10 hours in order to evaluate the catalyst durability. N₂O conversion was maintained for 10 hours as shown in Fig. 6(b).

Fig. 6.

(a) Self-sustained N₂O decomposition and (b) durability test for 10 hours of the Ru/Si-Al₂O₃ catalyst.

3.3 Hot-fire test of N₂O monopropellant thruster

Fig. 7(a) shows the N₂O mass flow rate and supply pressure of the monopropellant thruster. The supply pressure ranges from 5.5 to 11 bar. The maximum N₂O flow rate was 1.93 g/s at 11 bar. Temperature distribution in the thruster is shown in Fig. 7(b). The temperatures at the rear part of the thruster (T₃ and T₄) sharply increased within 3 min after N₂O was supplied, reaching 1120°C. Exit temperature (T₅) increased with time, while T₂-T₄ temperature goes down. While the internal temperature of the thruster tended to rise with increasing mass flow rate, a consistent trend was not observed. This phenomenon is presumed to result from non-uniform heat transfer within the thruster, caused by the localized decomposition of N₂O as it passes through the catalyst bed.

Fig. 7.

(a) N₂O mass flow rate and supply pressure, and (b) temperature during the hot-fire test of the monopropellant thruster.

The N₂O feed rate gradually increased 11 min after the startup. T₂-T₄ temperature then increased, while T₅ remained constant. At the maximum N₂O feed rate of 1.93 g/s, the exit temperature and chamber pressure were 1063°C and 9.32 bar, respectively. The c*-efficiency was used to express the degree of the propellant decomposition as Eq. (4).

where, c*, P_c, A_t, m˙, γ, R, and Tc denote the characteristic velocity, chamber pressure, throat area of the thruster nozzle, N₂O mass flow rate, specific heat ratio, gas constant, and the chamber temperature, respectively. The c*-efficiency was calculated with the above measured pressure, temperature, and mass flow rate. The c*-efficiency was 89.05% at 1063°C and 9.32 bar. The hot-fire tests validated that Ru/Si-Al₂O₃ catalyst is feasible for N₂O decomposition for a monopropellant thruster.

4. Conclusion

Heat-resistive Ru catalyst supported on Si-doped Al₂O₃ for N₂O monopropellant decomposition was prepared. The γ-Al₂O₃ was not transformed into α-Al₂O₃ by adding Si element to Al₂O₃ support. It was shown that the amorphous Si-Al₂O₃ was obtained at 1200°C using SEM/EDS and XRD analysis. Ru/Si-Al₂O₃ had better Ru dispersion in Al₂O₃ support. Ru/Si-Al₂O₃ has the highest conversion on N₂O decomposition. N₂O decomposition on Ru/Si-Al₂O₃ was self-sustained and did not require additional heat supply. The hot-fire tests of the subscale N₂O monopropellant thruster containing the Ru/Si-Al₂O₃ catalyst were conducted, resulting in high c*-efficiency as 89.05%.