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Platinum supported on doped alumina catalysts for propulsion applications. Xerogels versus aerogels
Journal of Non-Crystalline Solids 350 (2004) 113–119 www.elsevier.com/locate/jnoncrysol
Platinum supported on doped alumina catalysts for propulsion applications. Xerogels versus aerogels q Laurence Courthe´oux a, Florin Popa b, Eric Gautron a, Sylvie Rossignol a, Charles Kappenstein a,* a
UMR CNRS 6503, Laboratory of Catalysis in Organic Chemistry (LACCO), Department of Chemistry, University of Poitiers, 40 Avenue du Recteur Pineau, France b Department of Inorganic Chemistry, University Al.I.Cuza Iasi, Romania
Abstract Different supports and catalysts have been prepared for the decomposition of aqueous 79 wt% HAN solutions (HAN = hydroxylammonium nitrate NH3OHNO3). The supports (silica-doped alumina) were prepared by sol–gel synthesis using subcritical drying for xerogel samples or CO2 supercritical drying for aerogel samples, then calcined at 1200 C for 5 h. The active phase was introduced by impregnation from aqueous platinum precursor solutions or by one-step addition of the precursor before the sol formation. Characterization (X-ray diffraction, BET surface area, transmission electron microscopy) and evaluation of the catalysts (thermal analysis apparatus, constant volume reactor) were performed. Aerogels present much better thermal stability at high temperature and are more homogeneously dispersed than xerogels. Nevertheless, aerogels lead to less efficient catalysts for the decomposition of HAN solutions at low temperatures. The one-step procedure leads to an increase of the thermal stability for the aerogel samples. The impregnated catalysts always display the best activity, which correlates with the smaller Pt crystallite size that they support. The possibility to decompose aqueous HAN solutions catalytically at low temperatures (<40 C) with a short ignition delay (less than 1 s) has been demonstrated. 2004 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 81.07.Bc; 81.20.Fw; 81.70.Pg; 82.33.Lr; 82.33.Vx
1. Introduction The orbit and attitude control of satellites is currently obtained through small on-board engines using the catalytic decomposition of a monopropellant. Fig. 1 displays the scheme of a current engine with pure q Originally published for a part as ‘‘Improvement of catalysts for the decomposition of HAN-based monopropellant comparison between aerogels and xerogels’’, AIAA paper no. 2003-4645. Copyright 2003 by the American Institute of Aeronautics and Astronautics, Inc. Reprinted with permission. * Corresponding author. Tel.: +33 5 49 45 38 60; fax: +33 5 49 45 40 20. E-mail address: [email protected] (C. Kappenstein).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.06.051
hydrazine as propellant and alumina-supported iridium as catalyst [1]. The injection of hydrazine on the catalyst bed through a remotely controlled valve leads to a strongly exothermic decomposition giving hot gases (900 C) according to the following equation: 3N2 H4 ðlÞ ! 4ð1 xÞNH3 ðgÞ þ 6xH2 ðgÞ þ ð1 þ 2xÞN2 ðgÞ The high vapor pressure of hydrazine and its high toxicity, however, induce high storage and handling costs. The replacement of hydrazine by non-toxic monopropellants (also called green propellants) is a current objective [2] and the most studied hydrazine substitutes are aqueous ionic liquids containing hydroxylammonium nitrate [NH3OH]+[NO3] (or HAN) [3–6] or
L. Courthe´oux et al. / Journal of Non-Crystalline Solids 350 (2004) 113–119
114
2. Experimental
catalyst Ir/Al2O3 tank thrust N2H4 20 bars
valve
hot gases N2, H2, NH3 injector
nozzle
Fig. 1. Scheme of a hydrazine engine.
ammonium dinitramide [NH4]+[N(NO2)2] (or ADN) [7] as oxidizer in association with a fuel (methanol, glycine, glycerol, etc.). Unlike hydrazine, the decomposition of such mixtures releases very hot gases whose temperature can exceed 1400 C. The current challenge is to find new catalysts able to: (i) trigger the decomposition of the monopropellant mixture at low temperature (much less than the temperature of thermal decomposition); (ii) limit the formation of nitrogen oxides during the thermal decomposition; (iii) maintain good thermal stability and specific surface area at high temperatures; and (iv) prevent the loss of active phase under propellant flow. Recently, we demonstrated that sol–gel alumina samples doped with silicon (IV) atoms are able to sustain thermal treatments at high temperatures (1200 C or more) over several hours [8]. Therefore, a promising way to answer this challenge is to use catalysts containing an active phase (generally a platinum-group metal) supported on this thermally stable alumina. The first results concerning the thermal and catalytic decomposition of binary HAN–water solutions of various concentrations were presented in a previous proceeding [9]. The use of adequate catalysts can decrease the decomposition temperature to <40 C whereas thermal decomposition requires temperatures >130 C. The catalytic decomposition of HAN is thus possible at very low temperatures, in the presence of water, and in the absence of any fuel. Moreover the catalytic decomposition rate is one order of magnitude higher than that of thermal decomposition [9]. Thermally stable supports (doped alumina) have been prepared via sol–gel routes leading to xerogel or aerogel materials, depending on the drying conditions [10–12]. The use of transition metal-doped xerogels and aerogels has been pursued for different purposes, e.g., for the preparation of materials for lithium batteries (vanadia doped with copper or zinc) [13] or, of more relevance for this work, the preparation of platinum metal-doped sol–gel supports (i.e., Pt/SiO2) for catalytic applications [14]. The catalytic materials were obtained using two procedures for the introduction of the active phase precursor: one-step synthesis or impregnation. We present here preliminary results concerning the impact on the thermal stability and the catalytic activity of the prepared samples of the drying procedure (subcritical for xerogel or supercritical for aerogel) and the method to introduce the active phase.
The doped alumina samples are prepared by sol–gel procedure using aluminum tri-sec-butoxide Al(OCH(CH3)C2H5)3 as precursor (H2O:Al molar ratio 100:1), following the work of Yoldas [15]. The doping element (Si(OC2H5)4 precursor) is introduced as described previously [10] (Al:Si atomic ratio 94:6): after 1 h at 60 C, the corresponding amount of Si-precursor is slowly added, followed by a small quantity of HCl (Al + Si:HCl = 1:0.07). The mixture is heated to 80 C for 2 h in a covered beaker; the beaker is then left uncovered for several hours at the same temperature and gelling occurs. The gel of the resulting doped-boehmite AlO(OH) Æ xH2O is dried under subcritical conditions (1 bar, 120 C, 12 h) to lead to xerogel samples. The aerogel samples are obtained, after water–acetone exchange, by drying under CO2 supercritical conditions (120 bars, 40 C, 2 h), then ground to a powder. The (Al2O3)0.88(SiO2)0.12-doped aluminas are finally obtained in powder form by heating the dried xerogels or aerogels at 1200 C under air for 5 h [16]; Fig. 2 summarizes the different preparation steps. Two procedures have been used to modify the materials with the active phase: impregnation or a one-step synthesis (Fig. 2). For the impregnation method, the precursor of the active phase (H2PtCl6) is introduced from aqueous solutions onto the surface of the support (doped alumina already stabilized at 1200 C), followed by drying and reduction under H2 flow at 400 C for 3 h. The corresponding samples are called impregnated catalysts. To obtain a one-step catalyst, a solution of the platinum precursor is added directly into the sol during the preparation of the boehmite, followed by drying and calcination at 1200 C for 5 h. The supports and the catalysts were characterized by the following techniques: powder X-ray diffraction (XRD, Bruker-Siemens D-5005 hh apparatus), specific BET surface area (Brunauer–Emmett–Teller,
Fig. 2. Scheme for the preparation of the supports and the catalysts.
L. Courthe´oux et al. / Journal of Non-Crystalline Solids 350 (2004) 113–119
Micromeritics ASAP 2000), and transmission electron microscopy (TEM, Philips CM120). For TEM examination, the samples were grinded in ethanol and dispersed ultrasonically. A drop of the suspension was deposited on a Cu grid previously covered with a carbon film, and then the solvent was evaporated. The evaluation of sample homogeneity was performed by X-ray mapping obtained using local energy-dispersive X-ray analysis (EDX analysis, SiKa = 1739 eV, AlKa = 1486 eV, PtLa = 9441 eV). [16]. The catalysts were also evaluated for their ability to decompose a HAN–water mixture (79 wt% HAN), using thermal analysis equipment and a constant volume batch reactor. For the thermal analysis (TA instrument), 16 mg of catalyst is put into an aluminum crucible, 10 ll of monopropellant is added at room temperature, and the oven is closed and flushed with an argon flow for 5 min at 25 C. The crucible is then heated at 10 K min1. The mass and temperature of the sample are recorded versus time; an example of the results is shown in Fig. 3. The constant volume batch reactor has already been described [17]. The pressure and the temperature of the gas phase and catalyst are recorded versus time. Two measurement modes are used:
1400
Pressure /mbar
(A)
100
Weight /%
Temperature /°C
150 60 T(catalyst) 40
0 5
100 50
∆ T * 1000
20
0
10
time /min
15
(B)
120
1200
80
T(catalyst)
1100 T(gas) 1000 650
40 750
850
650
time /s
750
850
time /s
Fig. 4. Pressure (A) and gas phase and catalyst temperatures (B) obtained in the batch reactor during the catalytic decomposition of HAN–water mixture (increase-temperature mode).
120 Temperature /°C Catalyst
100 80 60
Gas phase
20 Pressure /mbar
2500
2000
1500
1000
0
2000
4000
6000
time /s Fig. 5. Pressure and gas phase and catalyst temperatures measured in the batch reactor in isothermal mode for the decomposition of HAN– water mixture on a catalyst prepared by the impregnation method.
250 200
80
T /°C
1300
40
• Temperature increase mode: the monopropellant (100 ll) is added to the catalyst (160 mg) at room temperature with a microsyringe through a septum, and then heated with a ramp of 10 K min1. An example is given in Fig. 4. The pressure profile displays a spike followed by a step due to the formation of gaseous products, whereas the temperature shows a peak and then decreases as a result of the thermal transfer to the reactor. • Isothermal mode: The catalyst (160 mg) is first heated at a predefined temperature (40 C) and then injections of monopropellant (100 ll) are successively added with a microsyringe. A typical example is given in Fig. 5, which shows successive temperature peaks and pressure steps preceded by spikes.
115
-50 20
Fig. 3. Thermogravimetric and differential thermal analysis during the catalytic decomposition of a HAN–water mixture.
3. Results Regardless of the preparation procedure, all samples contain platinum crystallites dispersed on Si(IV)-doped h-alumina support (XRD data, not shown). No a-alumina has been observed, despite the high thermal treatment at 1200 C. The specific surface areas (Fig. 6) corroborate the excellent thermal stability of the aerogel samples (108–120 m2 g1) in contrast to the xerogel samples (67–40 m2 g1). The difference is due to the method of drying and can be related to the higher porosity for the supercritically dried aerogel samples, which is maintained even after impregnation. The impregnated aerogel- and
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Fig. 6. Specific surface area SBET (m2 g1) of supports and catalysts after calcination at 1200 C for 5 h.
xerogel-based samples have pore volumes of 0.58 and 0.14 ml g1, respectively, and average pore diameters of 19.3 and 8.5 nm, respectively. The transmission electron microscopy results are displayed in Figs. 7–9. Fig. 7 (from Ref. [10]) shows representative data to compare xerogel and aerogel supports before impregnation: electron micrographs, Al and Si X-ray mapping, and EDX spectra. The micrographs of Pt/Al2O3–Si(IV) aerogel and xerogel catalysts prepared by the impregnation method and reduced are given in Fig. 8: small metallic particles (2–5 nm) are observed. Fig. 9 displays a micrographic comparison between the one-step and impregnation procedures for xerogel catalysts; the-one-step method leads to much larger platinum crystallites (>50 nm). The catalytic results are displayed in Figs. 10–12. Fig. 10 shows the onset decomposition temperature of HAN–water solutions obtained by thermal analysis on different support and catalyst samples, as shown in Fig. 3; this temperature corresponds to the strong
Fig. 8. Transmission electron micrographs of catalysts prepared by the impregnation method. Comparison of aerogel and xerogel.
exothermic event. To obtain more information on temperature and pressure evolutions, the thermal analyses were supplemented by using the batch reactor. In temperature-increase mode, similar results were obtained concerning the temperature-time evolution (see Fig. 4),
Fig. 7. Transmission electron micrographs, Al and Si X-ray mapping, and EDX spectra for aerogel and xerogel supports (from Ref. [10]).
L. Courthe´oux et al. / Journal of Non-Crystalline Solids 350 (2004) 113–119
117
Pressure /mbar 1 step-xero
1400
1 step-aero Imp-aero
Imp-xero
1200 105 °C
88 °C 90 °C
60 °C
time /s 1000 0
200
400
600
800
1000
1200
Fig. 11. Variation of the pressure in the batch reactor during HAN decomposition in increase-temperature mode, on one-step (1 step) and impregnated (Imp) catalysts prepared with xerogel and aerogel supports. The decomposition temperatures are indicated.
1200
Pressure /mbar
1100 Temperature /°C
Fig. 9. Transmission electron micrographs of xerogel catalysts. Comparison of the one-step procedure and the impregnation procedure.
80
2nd injection
1st injection
100
T(catalyst)
60 40
1000
0
400
800
1200
time /s Fig. 12. Pressure and temperature evolutions obtained in the batch reactor in isothermal mode at 40 C for the decomposition of HAN. The catalyst was prepared by one-step method.
Fig. 10. Onset decomposition temperature of 79 wt% HAN–water mixtures on supports and catalysts. Results obtained from thermal analysis.
but the onset temperatures are systematically lower, due to the different reaction conditions. The variations in pressure after injection of the monopropellant are reported in Fig. 11. The degree of pressure increase is
similar for the different samples and related to the amount of injected propellant. Isothermal measurements were carried out at 40 C, to determine the ignition delay and the stability of the catalyst after successive monopropellant injections. Catalysts prepared by the impregnation procedure lead to results similar to those presented in Fig. 5. In contrast, Fig. 12 shows the results obtained under similar conditions, but with a catalyst prepared by the one-step method: the first propellant injection leads to a rapid decomposition after an 7 min ignition delay, whereas the second injection is not followed by decomposition even after 30 min. We only observe a slight and slow pressure increase.
4. Discussion The doping of the alumina support strongly improves the thermal resistance at 1200 C in comparison with pure
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alumina [8]. The effect of the drying conditions is clearly demonstrated (Fig. 6), leading to an obvious interest in the aerogel preparation. Moreover, the presence of the active phase increases the stability of the aerogel samples, but a reverse influence is seen for the xerogel samples (Fig. 6). In agreement with the BET results, the TEM measurements reveal a much more homogeneous distribution of aluminum and silicon atoms for the aerogel samples than for the xerogel samples, as observed by the Al and Si mapping images (Fig. 7). For the xerogel, silica particles are clearly present in heterogeneous areas and the Al:Si atomic ratio (from EDX spectra) indicates higher silicon concentrations for these areas (Al:Si = 68:32 in Fig. 7). For the aerogel samples, this ratio is in agreement with the calculated value (Al:Si = 94:9). The final difference in surface area after the thermal treatment at very high temperature can be explained by the different behavior and fate of the silicon(IV) dopant. In the case of the xerogel samples, we observe mainly the formation of silica with little incorporation of silicon into the structure of the alumina. For the aerogel samples, the incorporation of silicon into the alumina network was demonstrated in a previous paper [10], leading to a more homogeneous material with better resistance to sintering and transformation into a-alumina. Homogeneous multicomponent systems are known, e.g., tetrahedrally coordinated silicon atoms are substituted by vanadium atoms (with reduction from VV to VIV) in vanadia–silica sol–gel-derived materials prepared by co-condensation of the respective alkoxides, [18–21]. The similar coordination environments favor the formation of a homogeneous xerogel, which is not the case for the Si-doped boehmite sol–gel in which Si is in a tetrahedral environment and Al an octahedral one. A more recent example of mixed-oxide materials that are uniform on the nanoscale is given by iron(III) oxide and silica dispersion (Fe:Si atomic ratio 1–5), obtained after gelation using organic epoxide [22]. For the platinum-embedded catalysts prepared by the impregnation method (Fig. 8), the size of the metal crystallite remains small (2–5 nm); the aerogel samples are more homogeneously dispersed than the xerogel samples and very few platinum particles reach 10 nm. The onestep procedure of the active phase introduction leads to very large Pt particles for both xerogel and aerogel samples (Fig. 9); this difference is obviously due to sintering effects during the high-temperature treatment. The catalytic activity is based on the decomposition of HAN in aqueous solution. The complete stoichiometric decomposition of the propellant corresponds to the following equation: NH3 OHNO3 ðaqÞ ! N2 ðgÞ þ 2H2 Oðg or lÞ þ O2 ðgÞ For all the thermal analyses, the following steps are observed (Fig. 3): (i) evaporation of water (4–21% weight loss, depending on the catalyst) characterized
by a broad endothermic event on the differential thermal analysis (DTA) curve; (ii) decomposition leading to a sudden decrease in weight accompanied by a strong exothermic peak (at about 90 C in Fig. 3). As expected, all samples display a lower decomposition temperature i.e., a better catalytic activity in comparison with thermal decomposition at our reaction temperature (165 C, Fig. 10). The supports alone (i.e., without platinum) are also catalytic, probably derived from the activity at surface acidic centers. The catalysts prepared by the impregnation procedure lead to lower decomposition temperature than the one-step catalysts, in agreement with the smaller Pt crystallite size. The aerogel materials, however, are less efficient than the xerogel samples, and require a higher decomposition temperature (>20 C higher), possibly in relation to the different insulating character of the aerogel versus xerogel. The use of the batch reactor in temperature-increase mode (Figs. 4 and 11) leads to results in very good agreement with the above data. The xerogels display a lower onset temperature and thus are better catalysts than aerogels (temperature differences >20 C). Moreover, a higher efficiency of the impregnated materials is observed in comparison with the one-step samples (temperature differences >15 C), in agreement with the larger platinum crystallite size and the related lower metal dispersion. The difference between catalysts prepared by the impregnation procedure and catalysts prepared by the one-step procedure is much more apparent for reactions run in the isothermal mode (Figs. 5 and 12) as they completely different catalytic behavior results, regardless of the nature of the support, xerogel or aerogel. The HAN decomposition is very rapid at Pt-impregnated xerogel catalyst with an ignition delay of <1 s (Fig. 5) and the catalyst remains active with the same efficiency after 15 monopropellant injections, despite the presence of liquid water in the sample holder. The decrease of the maximum of temperature peak with successive injections arises from the increasing amount of remaining liquid water. The gas-phase temperature tracks the successive injections, but with limited amplitude. The catalytic activity of the one-step catalysts is very poor, as demonstrated by the results given in Fig. 12, and is again related to the poor platinum dispersion. On the basis of all these experiments, the best proposed catalyst is a xerogel sample prepared by impregnation. Nevertheless, this catalyst has to be checked in real conditions.
5. Conclusion In this paper, we present the first results concerning the thermal stability and the evaluation of different supports and catalysts for the decomposition of binary HAN–water solutions. The drying technique and the
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platinum introduction procedure display an important influence on the properties and activity of the corresponding supported catalysts. The most important result is the possibility to catalytically decompose aqueous HAN solutions at very low temperature (<40 C) with a short ignition delay (less than 1 s). For the preparation of the catalyst supports (silicadoped alumina), two drying procedures have been used, leading to xerogel samples (subcritical drying) and aerogel samples (supercritical drying). Aerogels exhibit much better thermal stability at high temperature (1200 C) and are more homogeneously dispersed than xerogels. Nevertheless, aerogels are less efficient for the decomposition of HAN solutions and the onset temperature is higher in comparison with xerogels. The active phase was introduced on the support surface in two ways: (i) by impregnation of thermally stabilized supports from aqueous metal precursor solutions or (ii) by one-step addition of the precursor before the sol formation. The one-step procedure leads to an increase of the thermal stability for the aerogel samples, but the catalytic activity is less efficient at low temperature in comparison with xerogel samples. The impregnated catalysts always display higher activity, which correlates with the smaller crystallite size of the supported metallic particles. Acknowledgments We thank the French Space Agency CNES (Centre National dEtudes Spatiales, Toulouse) for funding this study as well as the Snecma Company (A. Melchior) for helpful discussions and constant interest.
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References [1] E.W. Schmidt, Hydrazine and its Derivatives: Preparation, Properties, Applications, 2nd Ed., Wiley, 2001, p. 1268. [2] C. Kappenstein, S. Rossignol, L. Pirault-Roy, M. Gue´rin, D. Duprez, N. Pillet, A. Melchior, in: Intl. Conf. on Space Transportation for the XXI Century, 2002, p. 16. [3] Y. Chang, K. Josten, K. Kuo, B. Reed, AIAA Paper 4032, 2002. [4] A.J. Fortini, J.R. Babcock, AIAA Paper 3393, 2001. [5] S. Christofferson, E.J. Wucherer, D.M. Zube, AIAA Paper 3941, 2001. [6] Y.P. Chang, K.K. Kuo, AIAA Paper 3272, 2001. [7] K. Anflo, T. Gro¨nland, AIAA Paper 3847, 2002. [8] S. Rossignol, C. Kappenstein, Intl. J. Inorg. Mater. 3 (2001) 51. [9] L. Courthe´oux, R. Eloirdi, S. Rossignol, C. Kappenstein, D. Duprez, N. Pillet, AIAA Paper 4027, 2002. [10] F. Popa, S. Rossignol, C. Kappenstein, J. Mater. Chem. 12 (2002) 1. [11] Y. Mizushima, M. Hori, J. Non-Cryst. Solids 167 (1994) 1. [12] C.K. Long, J.W. Richardson, M. Ozawa, J. Alloys Comp. 250 (1997) 356. [13] E. Frabetti, G.A. Deluga, W.H. Smyrl, M. Giorgetti, M. Berrettoni, J. Phys. Chem. B 108 (2004) 3765. [14] W. Zou, R.D. Gonzalez, T. Lopez, R. Gomez, Mater. Lett. 24 (1995) 35. [15] E. Yoldas, J. Mater. Sci. 10 (1975) 1856. [16] F. Popa, PhD, University of Poitiers, April 2002. [17] R. Eloirdi, S. Rossignol, C. Kappenstein, D. Duprez, N. Pillet, J. Prop. Power 19 (2003) 213. [18] M.D. Curran, A.E. Stiegman, J. Phys. Chem. B 104 (2000) 8338. [19] M.D. Curran, T.E. Gedris, A.E. Stiegman, G.A. Plett, Chem. Mater. 11 (1999) 1120. [20] M.D. Curran, D.D. Poore, A.E. Stiegman, Chem. Mater. 10 (1998) 3156. [21] A.E. Stiegman, H. Eckert, G. Plett, S.S. Kim, M. Anderson, A. Yavrouian, Chem. Mater. 5 (1993) 1591. [22] B.J. Clapsaddle, A.E. Gash, J.H. Satcher Jr., R.L. Simpson, J. Non-Cryst. Solids 331 (2003) 190.
Platinum supported on doped alumina catalysts for propulsion applications. Xerogels versus aerogels q Laurence Courthe´oux a, Florin Popa b, Eric Gautron a, Sylvie Rossignol a, Charles Kappenstein a,* a
UMR CNRS 6503, Laboratory of Catalysis in Organic Chemistry (LACCO), Department of Chemistry, University of Poitiers, 40 Avenue du Recteur Pineau, France b Department of Inorganic Chemistry, University Al.I.Cuza Iasi, Romania
Abstract Different supports and catalysts have been prepared for the decomposition of aqueous 79 wt% HAN solutions (HAN = hydroxylammonium nitrate NH3OHNO3). The supports (silica-doped alumina) were prepared by sol–gel synthesis using subcritical drying for xerogel samples or CO2 supercritical drying for aerogel samples, then calcined at 1200 C for 5 h. The active phase was introduced by impregnation from aqueous platinum precursor solutions or by one-step addition of the precursor before the sol formation. Characterization (X-ray diffraction, BET surface area, transmission electron microscopy) and evaluation of the catalysts (thermal analysis apparatus, constant volume reactor) were performed. Aerogels present much better thermal stability at high temperature and are more homogeneously dispersed than xerogels. Nevertheless, aerogels lead to less efficient catalysts for the decomposition of HAN solutions at low temperatures. The one-step procedure leads to an increase of the thermal stability for the aerogel samples. The impregnated catalysts always display the best activity, which correlates with the smaller Pt crystallite size that they support. The possibility to decompose aqueous HAN solutions catalytically at low temperatures (<40 C) with a short ignition delay (less than 1 s) has been demonstrated. 2004 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 81.07.Bc; 81.20.Fw; 81.70.Pg; 82.33.Lr; 82.33.Vx
1. Introduction The orbit and attitude control of satellites is currently obtained through small on-board engines using the catalytic decomposition of a monopropellant. Fig. 1 displays the scheme of a current engine with pure q Originally published for a part as ‘‘Improvement of catalysts for the decomposition of HAN-based monopropellant comparison between aerogels and xerogels’’, AIAA paper no. 2003-4645. Copyright 2003 by the American Institute of Aeronautics and Astronautics, Inc. Reprinted with permission. * Corresponding author. Tel.: +33 5 49 45 38 60; fax: +33 5 49 45 40 20. E-mail address: [email protected] (C. Kappenstein).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.06.051
hydrazine as propellant and alumina-supported iridium as catalyst [1]. The injection of hydrazine on the catalyst bed through a remotely controlled valve leads to a strongly exothermic decomposition giving hot gases (900 C) according to the following equation: 3N2 H4 ðlÞ ! 4ð1 xÞNH3 ðgÞ þ 6xH2 ðgÞ þ ð1 þ 2xÞN2 ðgÞ The high vapor pressure of hydrazine and its high toxicity, however, induce high storage and handling costs. The replacement of hydrazine by non-toxic monopropellants (also called green propellants) is a current objective [2] and the most studied hydrazine substitutes are aqueous ionic liquids containing hydroxylammonium nitrate [NH3OH]+[NO3] (or HAN) [3–6] or
L. Courthe´oux et al. / Journal of Non-Crystalline Solids 350 (2004) 113–119
114
2. Experimental
catalyst Ir/Al2O3 tank thrust N2H4 20 bars
valve
hot gases N2, H2, NH3 injector
nozzle
Fig. 1. Scheme of a hydrazine engine.
ammonium dinitramide [NH4]+[N(NO2)2] (or ADN) [7] as oxidizer in association with a fuel (methanol, glycine, glycerol, etc.). Unlike hydrazine, the decomposition of such mixtures releases very hot gases whose temperature can exceed 1400 C. The current challenge is to find new catalysts able to: (i) trigger the decomposition of the monopropellant mixture at low temperature (much less than the temperature of thermal decomposition); (ii) limit the formation of nitrogen oxides during the thermal decomposition; (iii) maintain good thermal stability and specific surface area at high temperatures; and (iv) prevent the loss of active phase under propellant flow. Recently, we demonstrated that sol–gel alumina samples doped with silicon (IV) atoms are able to sustain thermal treatments at high temperatures (1200 C or more) over several hours [8]. Therefore, a promising way to answer this challenge is to use catalysts containing an active phase (generally a platinum-group metal) supported on this thermally stable alumina. The first results concerning the thermal and catalytic decomposition of binary HAN–water solutions of various concentrations were presented in a previous proceeding [9]. The use of adequate catalysts can decrease the decomposition temperature to <40 C whereas thermal decomposition requires temperatures >130 C. The catalytic decomposition of HAN is thus possible at very low temperatures, in the presence of water, and in the absence of any fuel. Moreover the catalytic decomposition rate is one order of magnitude higher than that of thermal decomposition [9]. Thermally stable supports (doped alumina) have been prepared via sol–gel routes leading to xerogel or aerogel materials, depending on the drying conditions [10–12]. The use of transition metal-doped xerogels and aerogels has been pursued for different purposes, e.g., for the preparation of materials for lithium batteries (vanadia doped with copper or zinc) [13] or, of more relevance for this work, the preparation of platinum metal-doped sol–gel supports (i.e., Pt/SiO2) for catalytic applications [14]. The catalytic materials were obtained using two procedures for the introduction of the active phase precursor: one-step synthesis or impregnation. We present here preliminary results concerning the impact on the thermal stability and the catalytic activity of the prepared samples of the drying procedure (subcritical for xerogel or supercritical for aerogel) and the method to introduce the active phase.
The doped alumina samples are prepared by sol–gel procedure using aluminum tri-sec-butoxide Al(OCH(CH3)C2H5)3 as precursor (H2O:Al molar ratio 100:1), following the work of Yoldas [15]. The doping element (Si(OC2H5)4 precursor) is introduced as described previously [10] (Al:Si atomic ratio 94:6): after 1 h at 60 C, the corresponding amount of Si-precursor is slowly added, followed by a small quantity of HCl (Al + Si:HCl = 1:0.07). The mixture is heated to 80 C for 2 h in a covered beaker; the beaker is then left uncovered for several hours at the same temperature and gelling occurs. The gel of the resulting doped-boehmite AlO(OH) Æ xH2O is dried under subcritical conditions (1 bar, 120 C, 12 h) to lead to xerogel samples. The aerogel samples are obtained, after water–acetone exchange, by drying under CO2 supercritical conditions (120 bars, 40 C, 2 h), then ground to a powder. The (Al2O3)0.88(SiO2)0.12-doped aluminas are finally obtained in powder form by heating the dried xerogels or aerogels at 1200 C under air for 5 h [16]; Fig. 2 summarizes the different preparation steps. Two procedures have been used to modify the materials with the active phase: impregnation or a one-step synthesis (Fig. 2). For the impregnation method, the precursor of the active phase (H2PtCl6) is introduced from aqueous solutions onto the surface of the support (doped alumina already stabilized at 1200 C), followed by drying and reduction under H2 flow at 400 C for 3 h. The corresponding samples are called impregnated catalysts. To obtain a one-step catalyst, a solution of the platinum precursor is added directly into the sol during the preparation of the boehmite, followed by drying and calcination at 1200 C for 5 h. The supports and the catalysts were characterized by the following techniques: powder X-ray diffraction (XRD, Bruker-Siemens D-5005 hh apparatus), specific BET surface area (Brunauer–Emmett–Teller,
Fig. 2. Scheme for the preparation of the supports and the catalysts.
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Micromeritics ASAP 2000), and transmission electron microscopy (TEM, Philips CM120). For TEM examination, the samples were grinded in ethanol and dispersed ultrasonically. A drop of the suspension was deposited on a Cu grid previously covered with a carbon film, and then the solvent was evaporated. The evaluation of sample homogeneity was performed by X-ray mapping obtained using local energy-dispersive X-ray analysis (EDX analysis, SiKa = 1739 eV, AlKa = 1486 eV, PtLa = 9441 eV). [16]. The catalysts were also evaluated for their ability to decompose a HAN–water mixture (79 wt% HAN), using thermal analysis equipment and a constant volume batch reactor. For the thermal analysis (TA instrument), 16 mg of catalyst is put into an aluminum crucible, 10 ll of monopropellant is added at room temperature, and the oven is closed and flushed with an argon flow for 5 min at 25 C. The crucible is then heated at 10 K min1. The mass and temperature of the sample are recorded versus time; an example of the results is shown in Fig. 3. The constant volume batch reactor has already been described [17]. The pressure and the temperature of the gas phase and catalyst are recorded versus time. Two measurement modes are used:
1400
Pressure /mbar
(A)
100
Weight /%
Temperature /°C
150 60 T(catalyst) 40
0 5
100 50
∆ T * 1000
20
0
10
time /min
15
(B)
120
1200
80
T(catalyst)
1100 T(gas) 1000 650
40 750
850
650
time /s
750
850
time /s
Fig. 4. Pressure (A) and gas phase and catalyst temperatures (B) obtained in the batch reactor during the catalytic decomposition of HAN–water mixture (increase-temperature mode).
120 Temperature /°C Catalyst
100 80 60
Gas phase
20 Pressure /mbar
2500
2000
1500
1000
0
2000
4000
6000
time /s Fig. 5. Pressure and gas phase and catalyst temperatures measured in the batch reactor in isothermal mode for the decomposition of HAN– water mixture on a catalyst prepared by the impregnation method.
250 200
80
T /°C
1300
40
• Temperature increase mode: the monopropellant (100 ll) is added to the catalyst (160 mg) at room temperature with a microsyringe through a septum, and then heated with a ramp of 10 K min1. An example is given in Fig. 4. The pressure profile displays a spike followed by a step due to the formation of gaseous products, whereas the temperature shows a peak and then decreases as a result of the thermal transfer to the reactor. • Isothermal mode: The catalyst (160 mg) is first heated at a predefined temperature (40 C) and then injections of monopropellant (100 ll) are successively added with a microsyringe. A typical example is given in Fig. 5, which shows successive temperature peaks and pressure steps preceded by spikes.
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-50 20
Fig. 3. Thermogravimetric and differential thermal analysis during the catalytic decomposition of a HAN–water mixture.
3. Results Regardless of the preparation procedure, all samples contain platinum crystallites dispersed on Si(IV)-doped h-alumina support (XRD data, not shown). No a-alumina has been observed, despite the high thermal treatment at 1200 C. The specific surface areas (Fig. 6) corroborate the excellent thermal stability of the aerogel samples (108–120 m2 g1) in contrast to the xerogel samples (67–40 m2 g1). The difference is due to the method of drying and can be related to the higher porosity for the supercritically dried aerogel samples, which is maintained even after impregnation. The impregnated aerogel- and
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Fig. 6. Specific surface area SBET (m2 g1) of supports and catalysts after calcination at 1200 C for 5 h.
xerogel-based samples have pore volumes of 0.58 and 0.14 ml g1, respectively, and average pore diameters of 19.3 and 8.5 nm, respectively. The transmission electron microscopy results are displayed in Figs. 7–9. Fig. 7 (from Ref. [10]) shows representative data to compare xerogel and aerogel supports before impregnation: electron micrographs, Al and Si X-ray mapping, and EDX spectra. The micrographs of Pt/Al2O3–Si(IV) aerogel and xerogel catalysts prepared by the impregnation method and reduced are given in Fig. 8: small metallic particles (2–5 nm) are observed. Fig. 9 displays a micrographic comparison between the one-step and impregnation procedures for xerogel catalysts; the-one-step method leads to much larger platinum crystallites (>50 nm). The catalytic results are displayed in Figs. 10–12. Fig. 10 shows the onset decomposition temperature of HAN–water solutions obtained by thermal analysis on different support and catalyst samples, as shown in Fig. 3; this temperature corresponds to the strong
Fig. 8. Transmission electron micrographs of catalysts prepared by the impregnation method. Comparison of aerogel and xerogel.
exothermic event. To obtain more information on temperature and pressure evolutions, the thermal analyses were supplemented by using the batch reactor. In temperature-increase mode, similar results were obtained concerning the temperature-time evolution (see Fig. 4),
Fig. 7. Transmission electron micrographs, Al and Si X-ray mapping, and EDX spectra for aerogel and xerogel supports (from Ref. [10]).
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Pressure /mbar 1 step-xero
1400
1 step-aero Imp-aero
Imp-xero
1200 105 °C
88 °C 90 °C
60 °C
time /s 1000 0
200
400
600
800
1000
1200
Fig. 11. Variation of the pressure in the batch reactor during HAN decomposition in increase-temperature mode, on one-step (1 step) and impregnated (Imp) catalysts prepared with xerogel and aerogel supports. The decomposition temperatures are indicated.
1200
Pressure /mbar
1100 Temperature /°C
Fig. 9. Transmission electron micrographs of xerogel catalysts. Comparison of the one-step procedure and the impregnation procedure.
80
2nd injection
1st injection
100
T(catalyst)
60 40
1000
0
400
800
1200
time /s Fig. 12. Pressure and temperature evolutions obtained in the batch reactor in isothermal mode at 40 C for the decomposition of HAN. The catalyst was prepared by one-step method.
Fig. 10. Onset decomposition temperature of 79 wt% HAN–water mixtures on supports and catalysts. Results obtained from thermal analysis.
but the onset temperatures are systematically lower, due to the different reaction conditions. The variations in pressure after injection of the monopropellant are reported in Fig. 11. The degree of pressure increase is
similar for the different samples and related to the amount of injected propellant. Isothermal measurements were carried out at 40 C, to determine the ignition delay and the stability of the catalyst after successive monopropellant injections. Catalysts prepared by the impregnation procedure lead to results similar to those presented in Fig. 5. In contrast, Fig. 12 shows the results obtained under similar conditions, but with a catalyst prepared by the one-step method: the first propellant injection leads to a rapid decomposition after an 7 min ignition delay, whereas the second injection is not followed by decomposition even after 30 min. We only observe a slight and slow pressure increase.
4. Discussion The doping of the alumina support strongly improves the thermal resistance at 1200 C in comparison with pure
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alumina [8]. The effect of the drying conditions is clearly demonstrated (Fig. 6), leading to an obvious interest in the aerogel preparation. Moreover, the presence of the active phase increases the stability of the aerogel samples, but a reverse influence is seen for the xerogel samples (Fig. 6). In agreement with the BET results, the TEM measurements reveal a much more homogeneous distribution of aluminum and silicon atoms for the aerogel samples than for the xerogel samples, as observed by the Al and Si mapping images (Fig. 7). For the xerogel, silica particles are clearly present in heterogeneous areas and the Al:Si atomic ratio (from EDX spectra) indicates higher silicon concentrations for these areas (Al:Si = 68:32 in Fig. 7). For the aerogel samples, this ratio is in agreement with the calculated value (Al:Si = 94:9). The final difference in surface area after the thermal treatment at very high temperature can be explained by the different behavior and fate of the silicon(IV) dopant. In the case of the xerogel samples, we observe mainly the formation of silica with little incorporation of silicon into the structure of the alumina. For the aerogel samples, the incorporation of silicon into the alumina network was demonstrated in a previous paper [10], leading to a more homogeneous material with better resistance to sintering and transformation into a-alumina. Homogeneous multicomponent systems are known, e.g., tetrahedrally coordinated silicon atoms are substituted by vanadium atoms (with reduction from VV to VIV) in vanadia–silica sol–gel-derived materials prepared by co-condensation of the respective alkoxides, [18–21]. The similar coordination environments favor the formation of a homogeneous xerogel, which is not the case for the Si-doped boehmite sol–gel in which Si is in a tetrahedral environment and Al an octahedral one. A more recent example of mixed-oxide materials that are uniform on the nanoscale is given by iron(III) oxide and silica dispersion (Fe:Si atomic ratio 1–5), obtained after gelation using organic epoxide [22]. For the platinum-embedded catalysts prepared by the impregnation method (Fig. 8), the size of the metal crystallite remains small (2–5 nm); the aerogel samples are more homogeneously dispersed than the xerogel samples and very few platinum particles reach 10 nm. The onestep procedure of the active phase introduction leads to very large Pt particles for both xerogel and aerogel samples (Fig. 9); this difference is obviously due to sintering effects during the high-temperature treatment. The catalytic activity is based on the decomposition of HAN in aqueous solution. The complete stoichiometric decomposition of the propellant corresponds to the following equation: NH3 OHNO3 ðaqÞ ! N2 ðgÞ þ 2H2 Oðg or lÞ þ O2 ðgÞ For all the thermal analyses, the following steps are observed (Fig. 3): (i) evaporation of water (4–21% weight loss, depending on the catalyst) characterized
by a broad endothermic event on the differential thermal analysis (DTA) curve; (ii) decomposition leading to a sudden decrease in weight accompanied by a strong exothermic peak (at about 90 C in Fig. 3). As expected, all samples display a lower decomposition temperature i.e., a better catalytic activity in comparison with thermal decomposition at our reaction temperature (165 C, Fig. 10). The supports alone (i.e., without platinum) are also catalytic, probably derived from the activity at surface acidic centers. The catalysts prepared by the impregnation procedure lead to lower decomposition temperature than the one-step catalysts, in agreement with the smaller Pt crystallite size. The aerogel materials, however, are less efficient than the xerogel samples, and require a higher decomposition temperature (>20 C higher), possibly in relation to the different insulating character of the aerogel versus xerogel. The use of the batch reactor in temperature-increase mode (Figs. 4 and 11) leads to results in very good agreement with the above data. The xerogels display a lower onset temperature and thus are better catalysts than aerogels (temperature differences >20 C). Moreover, a higher efficiency of the impregnated materials is observed in comparison with the one-step samples (temperature differences >15 C), in agreement with the larger platinum crystallite size and the related lower metal dispersion. The difference between catalysts prepared by the impregnation procedure and catalysts prepared by the one-step procedure is much more apparent for reactions run in the isothermal mode (Figs. 5 and 12) as they completely different catalytic behavior results, regardless of the nature of the support, xerogel or aerogel. The HAN decomposition is very rapid at Pt-impregnated xerogel catalyst with an ignition delay of <1 s (Fig. 5) and the catalyst remains active with the same efficiency after 15 monopropellant injections, despite the presence of liquid water in the sample holder. The decrease of the maximum of temperature peak with successive injections arises from the increasing amount of remaining liquid water. The gas-phase temperature tracks the successive injections, but with limited amplitude. The catalytic activity of the one-step catalysts is very poor, as demonstrated by the results given in Fig. 12, and is again related to the poor platinum dispersion. On the basis of all these experiments, the best proposed catalyst is a xerogel sample prepared by impregnation. Nevertheless, this catalyst has to be checked in real conditions.
5. Conclusion In this paper, we present the first results concerning the thermal stability and the evaluation of different supports and catalysts for the decomposition of binary HAN–water solutions. The drying technique and the
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platinum introduction procedure display an important influence on the properties and activity of the corresponding supported catalysts. The most important result is the possibility to catalytically decompose aqueous HAN solutions at very low temperature (<40 C) with a short ignition delay (less than 1 s). For the preparation of the catalyst supports (silicadoped alumina), two drying procedures have been used, leading to xerogel samples (subcritical drying) and aerogel samples (supercritical drying). Aerogels exhibit much better thermal stability at high temperature (1200 C) and are more homogeneously dispersed than xerogels. Nevertheless, aerogels are less efficient for the decomposition of HAN solutions and the onset temperature is higher in comparison with xerogels. The active phase was introduced on the support surface in two ways: (i) by impregnation of thermally stabilized supports from aqueous metal precursor solutions or (ii) by one-step addition of the precursor before the sol formation. The one-step procedure leads to an increase of the thermal stability for the aerogel samples, but the catalytic activity is less efficient at low temperature in comparison with xerogel samples. The impregnated catalysts always display higher activity, which correlates with the smaller crystallite size of the supported metallic particles. Acknowledgments We thank the French Space Agency CNES (Centre National dEtudes Spatiales, Toulouse) for funding this study as well as the Snecma Company (A. Melchior) for helpful discussions and constant interest.
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