The objectives of this study were to prepare a high-purity hydroxylammonium nitrate (HAN) solution and evaluate the performance of various types of metal oxide/honeycomb catalysts during the catalytic decomposition of the HAN solution. Hydroxylammonium nitrate was prepared via a neutralization reaction of hydroxylamine and nitric acid. FT-IR was used to analyze the chemical composition, chemical structure, and functional groups of the HAN. The aqueous HAN solution obtained from pH 7.06 showed the highest concentration of HAN of 60% and a density of 1.39 g/mL. The concentration of HAN solution that could be obtained when the solvent was evaporated to the maximum level could not exceed 80%. In this study, catalysts were prepared using a honeycomb structure made of cordierite (5SiO2-2MgO-2Al2O3) as a support, with Mn, Co, Cu, Pt, or Ir impregnated as active metals. The pore structure of the metal oxide/honeycomb catalysts did not significantly depend on the type of metal loaded. The Cu/honeycomb catalyst showed the strongest effect of lowering the decomposition onset temperature in the decomposition of the HAN solution likely due to the intrinsic activity of the Cu metal being superior to that of the other metals. It was confirmed that the effect of the catalyst on the decomposition mechanism of the aqueous HAN solution was negligible. Through a repetitive cycle of HAN decomposition, it was confirmed that the Cu/honeycomb catalyst could be recovered and reused as a catalyst for the decomposition of an aqueous HAN solution.
Keywords: hydroxylammonium nitrate; synthesis; liquid monopropellant; catalytic decomposition; honeycomb catalyst
At present, artificial satellite technology does not allow additional fuel to be supplied in the universe, and thus, fuel is the most important variable during the service lifetime of an artificial satellite. The performance and efficiency of fuel for an artificial satellite can be maximized by utilizing a catalyst in the thruster, which can secure stable posture control and extend the operation period of the artificial satellite [[
Given that hydrazine monopropellants enable decomposition even at a low temperature by means of a catalyst, being able to decrease the ignition temperature drastically is one of the advantages [[
Eco-friendly, low-toxicity liquid propellants that have been topics of research as alternatives to hydrazine include hydrogen peroxide, ammonium dinitramide (ADN), and hydroxyl ammonium nitrate (HAN) [[
The HAN compound synthesis process includes the following methods: a method to cause the reaction of hydroxyl ammonium sulfate [HAS, (NH
In order to overcome such challenges, this study aims to develop a method that minimizes the formation of intermediates with liquid hydroxylamine (HA, NH
NH
Weak-base hydroxylamine was selected as the starting material instead of nitric acid, which is a strong acid that was used in order to reduce the heat-generating reaction in the neutralization process. In addition, in the event that a reaction begins in the reactor with nitric acid as the starting material, it is probable that the nitric acid would cause fumes prior to the proper addition of hydroxylamine, thus decreasing the efficiency of HAN generation. In this study, therefore, hydroxylamine was used as the starting material to form HAN at a consistent density.
In order to apply HAN as an ionic liquid propellant, a high level of stability is required. For this purpose, it must be dissolved in H
HAN + H
NH
NH
2HNO → N
NH
3HONO → 2NO + HNO
HNO + HNO
HONO + HNO
HAN-based liquid propellants are disadvantageous in terms of ignition due to the high moisture content. Since decomposition is essential for ignition of a HAN aqueous solution, heating is necessary to induce a decomposition reaction, and thus, preheating is required. Since energy consumption needs to be minimized due to the spatial limitation inside an artificial satellite, it is necessary to keep the decomposition temperature as low as possible by means of a catalyst. Once a liquid propellant starts decomposition in the artificial satellite thruster, ignition is initiated and the catalyst bed temperature increases intermittently to as high as 1200 °C. Since contact decomposition used in posture control of an artificial satellite is repeated, a catalyst with high heat resistance is essential. Specifically, a catalyst for the decomposition of liquid propellants in the artificial satellite thruster needs to activate decomposition at a low temperature, and at the same time, requirements for high heat resistance and high mechanical intensity need to be met as well [[
Various types of catalysts such as beads, pellets, granules, or honeycombs can be used for the satellite thruster. Honeycomb catalysts offer superior advantages, including high surface areas, low pressure drops, uniform flow distributions, enhanced heat transfer characteristics, good mechanical stability, controlled thicknesses, a compact design, and ease of integration, making them efficient and versatile compared to bead, pellet, or granule catalysts in various chemical processes. With the honeycomb catalyst, a chemical reaction occurs as a reactant passes through a cell on which the catalyst is supported. Therefore, one advantage of a honeycomb catalyst is that the pressure drop is minor because the diffusion of reactants and products is less limited [[
In this study, catalysts were prepared using honeycomb made of cordierite (5SiO
To clarify the objectives of this study, the primary objective is to propose a method with which to prepare HAN solutions using liquid hydroxylamine and nitric acid as starting materials. Secondary objectives are to evaluate the performance capabilities of different types of metal oxide/honeycomb catalysts with regard to the catalytic decomposition of HAN solutions and to verify the reusability using the best catalyst.
Figure 1 shows the effects of the hydroxylamine/nitric acid ratio on the pH and content of HAN in the HAN synthesis process as high-density nitric acid was added to hydroxylamine. In the initial stage, the pH of hydroxylamine was 9 and that of the nitric acid used in the synthesis was 1. Hence, as the quantity of nitric acid increased, the quantity of nitrogen ions in the nitric acid increased, whereas the pH of the HAN in the aqueous phase decreased. In general, synthesized HAN can generate hydroxylamine and nitric acid in a reversible reaction that proceeds according to Equation (
[NH
With regard to the mechanism of this reaction, it is known that as the number of nitric acid ions is reduced, the amount of nitrous acid (HNO
When HAN-based energetic compounds in an aqueous solution are utilized as a monopropellant, the energy quantity and heat generation characteristics are determined based on the actual HAN content. However, it is difficult to directly measure the concentration of HAN in an ionized form. Hence, the empirical model to estimate the concentration indirectly by measuring the density of an aqueous solution (Equation (
(
Figure 1 shows the concentration of HAN crystallized in an application of Equation (
On the other hand, if the aqueous solution is concentrated under reduced pressure, it changes to a gel state, and here it was possible to obtain a HAN compound content of 80%. In this study, due to the deliquescent properties of HAN, the maximum content that could be obtained when the solvent was evaporated to the maximum level could not exceed 80%.
In order to examine the synthesis characteristics of ionized HAN compounds in a liquid state, the chemical functional groups of the materials were traced by way of infrared spectroscopy. As shown in the spectrum in Figure 3, the peak spectra of the N-H and O-H functional groups in hydroxylamine, which is the initial reactant, were observed at the points of 3244 cm
Figure 5 shows scanning electron microscope (SEM) images before and after the pretreatment of the honeycomb support. As shown in Figure 5a, the surface of the honeycomb support was not smooth and contained a large number of impurities prior to the pretreatment. By following the pretreatment procedures, impurities were removed from the surface by means of alkali, acid, and organic solvents, as well as surfactants, in that order. As a result, the surface of the honeycomb support became smooth, as shown in Figure 5b. Figure 6a,b present SEM images of the Pt/honeycomb and Cu/honeycomb catalysts, respectively. The morphology o the catalyst surface on which Pt or Cu was impregnated differed from that in the image taken prior to the coating step (Figure 5b).
The nitrogen adsorption isotherm of the honeycomb (Figure 7) shows that it corresponded to Type III in the International Union of Pure and Applied Chemistry (IUPAC) classification, meaning that the adsorbed molecules were clustered around the most favorable sites on the surface of a nonporous or macroporous solid and that the micropores were mostly undeveloped [[
Figure 8 shows the results of an X-ray diffraction (XRD) analysis of catalysts on which Mn, Co, Cu, Pt, or Ir metallic oxides were impregnated on the honeycomb. In view of the XRD patterns of the honeycomb support that consisted of MgO-SiO
The thermogravimetric analyzer (TGA) thermogram of the HAN solution is shown in Figure 9. The broad endothermic curve up to 120 °C is attributed to the evaporation of water in the aqueous HAN solution. The corresponding weight loss observed in the TG curve is in good agreement with the water content in the HAN solution, as evaluated via density measurements. The aqueous HAN solution (80%) was found to decompose at approximately 124 °C.
Figure 10 shows representative examples of an aqueous HAN solution decomposition by means of the Pt/honeycomb catalysts. This figure indicates the points of inflection that result from intense heat generation at around 52.9 °C. At such points, intense heat caused the catalyst temperature to increase drastically. The point at which the temperature increases is called the decomposition onset temperature (T
With regard to thermal decomposition without a catalyst during the decomposition reactions of the HAN solution in the batch-type reactor, the decomposition onset temperature was 86.0 °C. In the case of the decomposition of the HAN solution over the cordierite honeycomb without metal deposition, the decomposition onset temperature was 83.0 °C, which means that the cordierite honeycomb without active metal loading played a minor role as a catalyst in lowering the decomposition temperature (Figure 11). The decomposition onset temperatures over the Mn, Pt, Ir, Cu, and Co supported on honeycomb catalysts were measured and found to be 67.8, 52.9, 76.9, 41.0, and 74.7 °C, respectively (Figure 11). To be specific, the Cu/honeycomb catalyst had the effect of lowering the decomposition onset temperature by 45.0 °C compared to during thermal decomposition. The surface area and pore size of the catalysts did not differ significantly. Therefore, the pore structure of the catalyst cannot account for the excellent activity of the Cu/honeycomb. Because the metal-loading amounts did not differ significantly, the greater activity of the Cu/honeycomb catalyst likely arose due to the intrinsic activity of the Cu metal being superior to that of the other metals. It is agreed upon in the literature that Cu oxide has excellent activity in the decomposition reaction of energetic ionic liquid [[
The efficiency of the Cu/Honeycomb catalyst was demonstrated by the decreased decomposition onset temperature of 41 °C instead of 86 °C for the thermal process of the HAN solution. The catalytic effect of the Cu/honeycomb catalyst was clearly evidenced by the decreased decomposition onset temperature in comparison with thermal decomposition. Low decomposition onset temperatures circumvent the preheating of the catalyst bed of the thruster, thus reducing the energy supply [[
The product gases collected from the catalytic decomposition of the HAN solution over the catalysts were analyzed using an infrared spectrometer. As shown in Figure 12, Fourier-transform infrared spectroscopy (FT-IR) absorbance bands associated with HNO
In order to verify the reusability of the Cu/honeycomb catalyst, the decomposition experiments on the aqueous HAN solution were repeated 19 times and the values of T
Once decomposition of a liquid propellant starts in an artificial satellite thruster, ignition is initiated and the catalyst bed temperature increases intermittently to as high as 1200 °C. In order to evaluate the heat resistance of the catalysts, an experiment was conducted in which a thermal shock was applied to the catalysts. After a thermal treatment for the Cu/honeycomb catalysts in a furnace at 1200 °C lasting approximately ten minutes, they were cooled to room temperature and then put into a reactor for the aqueous HAN solution decomposition experiment. To the best of our knowledge, this heat resistance study is the first to evaluate the heat resistance as well as the decomposition activity of a honeycomb-type catalyst during the decomposition of an aqueous HAN solution. After the cycle of thermal shock application and aqueous HAN solution decomposition was repeated five times, there was an insignificant difference in both T
As raw materials for HAN synthesis, this study utilized nitric acid (assay—60%, Daejung Chemical & Metals Co., Ltd.) and hydroxylamine (assay—50%, Sigma-Aldrich, Louis, MO, USA). A total of 95% high-density nitric acid was extracted through a dehydration reaction of 60% nitric acid and high-density sulfuric acid (98%, Sigma-Aldrich, Louis, MO, USA). If the hydroxylamine (assay—50%, NH
4Fe
Since such metallic ions could cause hydroxylamine decomposition, the manufacturing of HAN by means of high-purity hydroxylamine could have been hindered as a result. Accordingly, a small quantity of metallic elements and other impurities were filtered by means of a membrane filter (PTFE, Sigma-Aldrich, Louis, MO, USA)) in this study. For HAN synthesis, high-purity hydroxylamine solution was used with 95% density nitric acid and impurities were removed. Nitric acid was added drop by drop to a 3-neck flask with hydroxylamine in it (1 mL/min), and the temperature in the synthesis reactor was consistently less than 50 °C.
Functional groups of N-H and N-O inside the synthesized HAN compound aqueous solution were detected by means of FT-IR (Spectrum One System, Perkin-Elmer, Waltham, MA, USA). In addition, the ionized HAN was analyzed quantitatively in comparison with ammonium nitrate, hydroxylamine, and hydroxylamine sulfate, which have structures similar to that of HAN. The pyrolysis temperature of the synthesized HAN was measured by means of thermogravimetric analysis (TGA N-1500, Scinco, Seoul, Korea). The specific gravity of the aqueous HAN solution was measured, and the relative content of HAN in the aqueous phase was calculated with Equation (
The honeycomb supports used here were purchased from Ceracomp Co. Ltd (Cheonan, Korea). The chosen material was cordierite (MgO-SiO
The impregnation of metals, in this case, Mn, Co, Cu, Pt, and Ir, was performed by means of the wet impregnation coating method on the honeycomb support. The metallic precursors that were used were manganese nitrate tetrahydrate (Sigma-Aldrich, Louis, MO, USA, 97%), cobalt nitrate hexahydrate (Sigma-Aldrich, Louis, MO, USA, 97%), copper nitrate trihydrate (Sigma-Aldrich, Louis, MO, USA, 78.5%), palladium chloride (Sigma-Aldrich, Louis, MO, USA, 99%), platinum chloride (Kojima Chemicals, Sayama, Japan, 99%), and hydrogen hexachloroiridate hydrate (H
Once a liquid propellant starts to decompose in an artificial satellite thruster, ignition is initiated and the catalyst bed temperature increases intermittently to as high as 1200 °C. Accordingly, the metal oxide/honeycomb used in this study was calcined at 1200 °C during the final step of the catalyst preparation procedure. If calcination was performed at 1200 °C after washcoating Al
The nitrogen adsorption isotherm was measured at −196 °C by means of a BELSORP-mini II device by BEL JAPAN (Osaka, Japan). After treatment of the catalyst specimen in a vacuum at 200 °C for six hours, the quantity of adsorption was measured while nitrogen flowed into the adsorption gas at the temperature of liquid nitrogen. The specific surface area was calculated by applying the BET equation, and the entire volume and the average diameter of the pores were calculated by applying the Barrett–Joyner–Halenda (BJH) equation.
The crystallinity of the catalyst was examined by means of XRD. The XRD used here was the MiniFlex600 model by Rigaku (Tokyo, Japan). The measurement angle was between 3° and 90°, and the angular velocity was 5°/min. XRD pattern data were collected by means of a Rigaku D/teX ultra-diffractometer (Tokyo, Japan) with a Cu tube and a graphite monochromator mounted on it.
In order to determine the composition of the catalyst, the XRF was measured. A Rigaku/ZSX Primus II device (Tokyo, Japan) was utilized for this, and the district target element was Rh. An image of the catalyst particles was analyzed by means of SEM. A TESCAN/MIRA3-LM(Brno, Czech Republic) high-resolution scanning electron microscope (HR FE-SEM) was utilized. The acceleration voltage was 20 kV.
The HAN solution used in this study was composed of 80 wt% HAN and 20 wt% water. The decomposition reaction of the HAN solution was carried out in a custom-made batch-type reactor (Hanwoul Engineering, Gunpo, Korea) (Figure 17) based on earlier work [[
The gases generated upon decomposition were captured by means of a Tedlar bag (Sigma-Aldrich, Louis, MO, USA) injected with heatable gas cells from PerkinElmer in a vacuum and analyzed by means of FT-IR (Spectrum Two FT-IR by PerkinElmer, Waltham, MA, USA).
Hydroxylammonium nitrate was prepared via a neutralization reaction of hydroxylamine and nitric acid. FT-IR was used to analyze the chemical composition, chemical structure, and functional groups of the HAN. The aqueous HAN solution obtained at pH 7.06 showed the highest concentration of HAN at 60% and a density of 1.39 g/mL. The concentration of HAN solution that could be obtained when the solvent was evaporated to the maximum level could not exceed 80%.
A total of 13.5–17.8 wt% of Mn, Co, Cu, Pt, or Ir metals were impregnated onto a honeycomb support composed of cordierite (MgO-SiO
Graph: Figure 1 Effects of the hydroxylamine/nitric acid ratio on the pH and content of HAN.
Graph: Figure 2 Effects of the pH in the HAN synthesis process on the density of the HAN solution.
Graph: Figure 3 FT-IR spectra of various samples. (a) Synthesized HAN; (b) hydroxylamine; (c) ammonium nitrate; (d) hydroxylamine sulfate.
Graph: Figure 4 FT-IR spectra of various samples. (a) Synthesized HAN; (b) potassium nitrate; (c) ammonium nitrate; (d) nitric acid.
Graph: Figure 5 SEM images of honeycomb support (a) before pretreatment and (b) after pretreatment.
Graph: Figure 6 SEM images of (a) Pt/honeycomb and (b) Cu/honeycomb.
Graph: Figure 7 Nitrogen adsorption isotherms of (a) Mn/honeycomb, (b) Co/honeycomb, (c) Pt/honeycomb, (d) Ir/honeycomb, (e) Cu/honeycomb, and (f) honeycomb support.
Graph: Figure 8 XRD patterns of various catalysts. (a) Mn/honeycomb; (b) Co/honeycomb; (c) Pt/honeycomb; (d) Ir/honeycomb; (e) Cu/honeycomb; (f) honeycomb.
Graph: Figure 9 Thermogravimetric analysis of the HAN solution.
Graph: Figure 10 Decomposition temperature and pressure of the HAN solution over the Pt/honeycomb catalyst.
Graph: Figure 11 Decomposition onset temperature (Tdec) and ∆P in the catalytic decomposition of the HAN solution over various catalysts.
Graph: Figure 12 FT-IR spectra of gas generated during the decomposition of the HAN solution. FT-IR spectra of gas generated during the decomposition of the HAN solution. (a) Thermal; (b) Cu/honeycomb; (c) Pt/honeycomb; (d) Ir/honeycomb.
Graph: Figure 13 Decomposition onset temperature (Tdec) and ∆P in the repeated decomposition of the HAN solution over the Cu/honeycomb catalyst.
Graph: Figure 14 Decomposition onset temperature (Tdec) and ∆P in repeated decomposition of the HAN solution over the Cu/honeycomb catalyst (thermal shock was applied to the catalyst between each run).
Graph: Figure 15 Pretreatment procedure for the honeycomb support.
Graph: Figure 16 Metal oxide catalysts supported on honeycomb. (a) Cordierite honeycomb; (b) Cu; (c) Co; (d) Mn; (e) Pt; (f) Ir; (g) Pd.
DIAGRAM: Figure 17 Schematic diagram of the reactor for the decomposition of the propellant.
Table 1 Surface area and pore volume of various catalysts.
Catalyst BET Surface Area (m2/g) Pore Volume (cm3/g) Honeycomb 1.3 0.005 Cu/honeycomb 0.3 0.004 Ir/honeycomb 0.9 0.010 Pt/honeycomb 0.6 0.010 Co/honeycomb 1.0 0.010 Mn/honeycomb 0.8 0.010
Table 2 Metal loading over honeycomb support determined by XRF.
Catalyst Metal Oxide Loading (wt%) Cu/honeycomb 13.9 Ir/honeycomb 13.5 Pt/honeycomb 14.2 Co/honeycomb 17.8 Mn/honeycomb 15.8
Conceptualization, Y.J. and J.-K.J.; Methodology, Y.J.; Formal analysis, D.Y., M.K. and S.K.; Investigation, D.Y., S.K.O., W.K. and Y.K.; Data curation, D.Y. and M.K.; Writing – original draft, S.H. and J.-K.J.; Project administration, J.-K.J.; Funding acquisition, J.-K.J. All authors have read and agreed to the published version of the manuscript.
Data are contained within the article.
The authors declare no conflict of interest.
By Dalsan Yoo; Munjeong Kim; Seung Kyo Oh; Seoyeon Hwang; Sohee Kim; Wooram Kim; Yoonja Kwon; Youngmin Jo and Jong-Ki Jeon
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