American Institute of Aeronautics and Astronautics

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Comparative Characterization of Advanced Catalytic Beds

for Hydrogen Peroxide Thrusters

L. Romeo1, L. Torre

2, A. Pasini

3, L. d’Agostino

4

ALTA S.p.A. - Via Gherardesca, 5 - 56121 Ospedaletto, Pisa, Italy

Fausto Calderazzo5

Department of Chemistry and Industrial Chemistry , University of Pisa, Via Risorgimento 35- 56126 Pisa, Italy

In the last two years Alta S.p.A., Pisa, Italy, has been engaged in the development of

advanced catalytic beds for hydrogen peroxide (HP) decomposition in collaboration with the

Department of Chemistry and Industrial Chemistry of Pisa University. A series of chemical

activity tests at atmospheric pressure on a number of catalyst-substrate preparations

indicated a platinum catalyst supported on αααα−−−−alumina and named FC-LR-87 as the most

promising candidate for characterization under more realistic conditions in a suitably

designed HP monopropellant thruster prototype. Scanning Electron Microscopy (SEM) and

X-Ray diffractometry (XRD) analyses have been carried out on FC-LR-87 catalyst samples

before and after continuous and pulsed firing tests. These analyses showed that the

decomposition of 2 kg of high grade HP did not cause any measurable loss of catalyst from

the support, confirmed the stability of the small and well dispersed platinum particles on the

alumina surface, and indicated the absence of oxidation of the active phase.

Nomenclature

tmax = time need for the liquid mixture to reach the maximum value of temperature

x = mean value

s = sample variance

I. Introduction

ONOPROPELLANT propulsion systems are attractive for orbit maintenance and attitude control due to their

simplicity, which translates into cost reductions and partially counterbalances their lower specific impulse

compared to bipropellant systems. The use of hydrogen peroxide (HP), a non toxic or green monopropellant, offers

increased safety and cost-effectiveness in the operation of space propulsion systems owing to the drastic

simplification of the health and safety protection procedures.

High concentration or “rocket grade” HP has a long heritage in aerospace propulsion. A wide variety of

applications on both manned and unmanned systems can be cited from the 1930’s to the present time (Ventura and

Garboden1). Up to the 1960s a significant amount of work has been carried out by NASA laboratories on HP

1 Ph.D. Student, Aerospace Engineering Department, Pisa University - Project Engineer, ALTA S.p.A., AIAA

Member; l.romeo@alta-space.com 2 Project Manager, ALTA S.p.A., AIAA Member; l.torre@alta-space.com

3 Ph.D. Student, Aerospace Engineering Department, Pisa University - Project Engineer, ALTA S.p.A., AIAA

Member; a.pasini@alta-space.com 4 Professor, Aerospace Engineering Department, Pisa University, AIAA Member; luca.dagostino@ing.unipi.it

5 Professor, Department of Chemistry and Industrial Chemistry, University of Pisa , facal@dcci.unipi.it

M

44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit21 - 23 July 2008, Hartford, CT

AIAA 2008-5027

Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

American Institute of Aeronautics and Astronautics

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decomposition and its application to monopropellant rockets (Runckel and Willis2, Willis

3). The development of the

Shell 405 catalyst and higher purity hydrazine led to a decreased use of HP due to the superior performance and

long-term stability characteristics of hydrazine (Wucherer and Cook4). In the last decade HP has been receiving a

renewed interest for application to cost and safety driven systems (Wernimont and Ventura5, Scharlemann et al.

6).

Effective operation of HP monopropellant thrusters and gas generators is closely related to the availability of long-

lived and reliable catalytic beds, able to provide repetitive performances both in continuous and intermittent

operations. The attainment of high decomposition efficiency with reduced bed volumes is a crucial design

requirement for HP catalytic reactors in space applications.

A number of different catalyst substrates for HP, including compressed gauzes, pellets, and high porosity foams7,

have been employed in the past to achieve these targets. In previous applications, mainly grids, pellets or beads

coated with the catalytic substances have been used, since they maximize the active surface-to-volume ratio.

Different metals and metallic oxides deposed on granules or pellets of transition aluminas have been investigated in

search for the best catalyst for HP decomposition (Rusek8, Romeo et al.

9). The results of screening tests recently

carried out at Alta S.p.A., Pisa, Italy, on a number of chemical species indicated platinum as the most promising

catalyst for HP decomposition. The same tests also indicated that thermal stresses induced by the intense release of

heat on the pellet surface often caused the catalyst support to break into fine particles, as commonly observed by a

number of investigators (Pasini et al.10

, Sahara and al.11

). The efforts by Alta S.p.A. and the Department of

Chemistry and Industrial Chemistry of Pisa University, Pisa, Italy, to solve this problem recently led to the

development of the FC-LR-87 catalyst, a Pt/α-Al2O3 catalyst with a surface load of platinum close to 35 % by

weight and adequate thermal shock resistance (Romeo et al.12

).

The impregnation technique presented in (Romeo et al.12

) and a novel advanced method for coating ceramic

spheres with metallic platinum, named PT2, have been implemented on a variety of different catalysts. This paper

illustrates the results of a preliminary series of HP decomposition activity tests at atmospheric pressure on a number

of catalyst-substrate preparations, which led to identify a platinum catalyst supported on α−alumina and named FC-

LR-87 as the most promising candidate for characterization under more realistic conditions in Alta’s Green

Propellant Rocket Tests Facility (GPRTF, Torre et al.13

). Continuous and pulsed firings have been carried out at two

different bed loads in a reconfigurable 5 N monopropellant thruster prototype designed by Alta S.p.A on the FC-LR-

87 catalyst and, for comparison, on a commercial benchmark catalyst for space propulsion applications. The

benchmark catalyst (here indicated as Pt/Al2O3-COM and produced by a major commercial manufacturer) consists

of platinum deposed on the same alumina substrate as the commercial version for hydrazine decomposition.

Scanning Electron Microscope (SEM) and X-Ray Diffractometry (XRD) analyses have been carried out on the

above catalysts before and after the firings in order to assess the possible occurrence of modifications and/or

degradations.

II. Atmospheric Test Bench

A. HP Decomposition

A dedicated test bench has been realized by Alta S.p.A. for the comparative characterization of the activity and

reaction rates of the catalyst formulations in controlled HP decomposition experiments at atmospheric pressure.

Figure 1 shows a schematic drawing of the test bench. It consists of a 100 ml reaction flask contained in a 2 liters

glass vessel closed by a sealing lid with:

• the connection for the HP supply funnel;

• two thermocouple taps, one for the measurement of the temperature of the liquid HP an the other for

measuring the temperature of the gas in the cylinder;

• the connection to the exhaust duct of the hot gas generated by the reaction.

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Before each test, a known mass of catalyst is put in the reaction flask. Next, a given quantity of HP solution is

added by opening the tap of the H2O2 tank. The decomposition reaction promoted by the catalyst generates a hot gas

mixture at nearly atmospheric pressure, where molecular oxygen, steam and a small quantity of gaseous HP are

present. After leaving the vessel, these gases enter a coil heat exchanger, where their temperature is lowered by

means of a cold liquid flow. The heat exchanger has been designed in order to condense most of water and HP

vapors in a liquid separator. As a consequence, the gas flow a the exit of the heat exchanger practically contains pure

oxygen at known pressure and temperature conditions. In this way its flow rate, measured by means of a flow-meter,

can be directly correlated to the rate of HP decomposition (Romeo et al.9). Temperature measurements are carried

out by means of K-type thermocouples with a diameter of 1.5 mm and a length of 250 mm. The reduced order model

used for the design of the test bench and the interpretation of the experimental results has been illustrated in a

previous paper16

.

B. X-Ray Diffractometry

The X-Ray diffraction experiments have been carried out at room temperature (T = 293 K) by means of a Bragg-

Brentano Philips powder XRD instrument.

III. Catalysts

Table 1 summarizes the physical properties of the catalyst supports used in the present study. All of them have

been manufactured by SASOL and consisted in alumina spheres with monodispersed diameters ranging from 0.6 to

0.7 mm. The choice of the substrates has been driven by the aim of validating the effectiveness of the proposed

platinum deposition methods on supports with different values of specific surface area and, at the same time, of

Figure 1. Schematic of the batch reactor.

.

Table 1. Main characteristics of the catalyst support pellets.

Support

Name

BET Surface Area

(m2/g)

Pore Volume

(ml/g)

Al2O3

(%) Al2O3 phase

Diameter

(mm)

0.6/4 4 0.25 99.7 α 0.6

0.6/13 14 0.25 99.7 α 0.6

Alumina δδδδ++++ϑϑϑϑ 104 0.45 95.6 δ and ϑ 0.7

0.6/170 170 0.53 96.1 γ 0.6

Figure 2 Picture of the commercial benchmark

catalyst

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assessing the thermal shock resistance of the different alumina phases. Silicon carbide supports in granules with an

average diameter of 0.55 mm and a specific surface area lower than 1 m2/g have also been used.

A platinum catalyst (here indicated as Pt/Al2O3-COM), produced by a major commercial manufacturer and

supported on alumina granules with monodispersed diameters ranging from 0.71 to 0.85 mm (see Figure 2), has also

been procured for performance comparison with a typical commercial benchmark in atmospheric decomposition

tests at low HP concentration, as well as in more realistic firings in the monopropellant thruster prototype13

. The

Pt/Al2O3-COM catalyst employs the same substrate used for commercial catalysts produced for hydrazine

decomposition space propulsion applications. The catalyst carrier, obtained by means of a suitable sol-gel procedure

capable of yielding nearly spherical granules for more uniform bed packing and reduced pressure drop14

, has a

consistent record of long duration. Up to three hundred thousand firing pulses have been carried out in a 20 N, 4 N

and 1 N hydrazine monopropellant thruster15

using the same substrate as the Pt/Al2O3-COM catalyst, but with a

different active species. The choice of this catalyst support has been motivated by the fact that similar thermo-

mechanical conditions are expected in the present HP decomposition tests in the thruster prototype. The catalyst has

been provided by the manufacturer with a 10 wt% nominal load of platinum on 200 grams of total catalyst weight.

Based on the observed performance of different catalysts on γ-alumina supports (Romeo et al.9), platinum has

been selected as the most promising metallic element for HP decomposition. Two different methods have been used

to depose platinum on the supporting substrate. The first, indicated as PT1, consists in impregnation at 20°C in a

precursor solution with a nominal platinum load of 3.82×10-3

moles, washing with 50 ml of solvent, filtration,

drying by mechanical depressurization at 10-4

atm and room temperature, and final calcination in muffle at 400 °C

for tree hours. The second, indicated as PT2, is an advanced coating technique recently developed in collaboration

with the Chemistry and Industrial Chemistry Department of Pisa University, Pisa, Italy. Before catalyst deposition,

all ceramic supports have been preliminary dried in vacuum (10-4

atm) at 140 °C to eliminate the residual water

bound to the matrix by hydrogen bonds. lists all of the catalysts developed in the context of this work. Samples are

identified with Alta’s identification code and the chemical formulas of the catalyst and the supporting material. The

deposition technique, the nominal molar load of platinum and the metal load determined by SEM spectrographic

analysis are also reported. The following observations are relevant to the catalysts of Table 2:

• for the LR-59 and LR-III-180 catalysts the platinum load detected by SEM analysis resulted to be similar,

despite the different values of surface area of the supports (170 m2/g and 104

m

2/g, respectively);

• the platinum content (determined by SEM analysis) of LR-59 is about tree times higher than for LR-II-122

because of the different values of specific surface area of the supports;

• a platinum load (determined by SEM analysis) similar to the one on LR-59 has been attained on LR-II-108

despite the much lower value of the surface area of the silicon carbide support (less than 1 m2/g);

• a slightly increased platinum load has been attained on LR-III-37 with respect to LR-II-122 because of the

different support, 0.6/13 and 0.6/4 respectively;

Table 2 . Main characteristics of the catalysts.

Alta’s ID code Catalyst Deposition

procedure Support

Nominal

metal load

(mol)

SEM

metal load

(At %)

LR-59 Pt/γ−Al2O3 PT1 0.6/170 3.82×10-3 0.47

LR-II-108 Pt/SiC PT1 SiC granules 3.82×10-3

0.42

LR-II-122 Pt/α−Al2O3 PT1 0.6/4 3.82×10-3

0.13

LR-III-180 Pt//δ+ϑ−Al2O3 PT1 Alumina δ+ϑ 3.82×10-3

0.45

LR-III-37 Pt/α−Al2O3 PT1 0.6/13 3.82×10-3

0.2

LR-III-39 Pt/α−Al2O3 PT2 0.6/13 2.5×10-3 7.49

Pt/Al2O3-COM Pt/Al2O3 Commercial Commercial 10×10-2

1.36

LR-III-89 Pt/α−Al2O3 PT2 0.6/13 2.5×10-3

7.49

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• high load contents, 7.4 At % of Pt (equivalent to 44 wt %), have been obtained using the PT2 procedure in

the LR-III-39 in spite of the relatively low surface area of their α-Al2O3 substrates (4 m2/g);

• LR-III-89 has been obtained by calcinating LR-III-39 in muffle at 400 °C for 5 hours.;

• SEM platinum load on the Pt/Al2O3-COM catalyst is 1.36 At%, corresponding to about the eight percent by

weight; slightly less than the nominal load guaranteed by the manufacturer (10 wt%).

IV. Characterization

A preliminary characterization of the above catalysts (including the commercial benchmark) has been carried out

by conducting a series of atmospheric HP decomposition activity tests (drop tests) in the above experimental set-up

using a 30% HP solution (Riedel-de Haen PERDROGEN

by Sigma-Aldrich) selected for its low contents of

impurities and stabilizers (PO43−

<1 ppm, SO42−

<1 ppm). These tests allowed for:

• ranking the chemical activity, repeatability and susceptibility to poisoning of the various catalysts

preparations;

• establishing quantitative comparisons with the commercial benchmark catalyst;

• identifying the most promising candidate for operation in the HP monopropellant prototype thruster (Torre et

al.13

);

The results of these and previous drop tests by Romeo et al.12

led to the selection of the catalyst preparations and

bed configurations to be fired in the prototype thruster with 87.5% HP (PROPULSE 875 HTP by Degussa) at

elevated pressures (6 to 20 bars, ca.). Since pellet powdering frequently occurred in previous firing tests, an

additional series of drop tests in 87.5% HP has carried out in order to screen out catalyst preparations with

manifestly insufficient thermo-mechanical resistance.

Scanning Electron Microscopy (SEM) analyses were conducted on the catalyst preparations in the attempt to

correlate the chemical composition of the catalyst surface with its catalytic behavior. The platinum load on the

support has been determined by SEM analysis before and after the activity tests in low concentration HP in order to:

• detect possible losses of platinum due to the washing phenomenon induced by the reaction;

• evaluate the difference between the nominal and actual load of platinum on the catalyst carrier.

X-Ray Diffractometry (XRD) analyses have also been carried out on the two catalysts fired in the

monopropellant thruster prototype (Torre et al.13

), with the aim of better investigating the modifications induced by

operation at high pressure and temperature.

V. Results and Discussion

A. Activity

For each of the catalysts listed in Table 2, fifty consecutive tests have been carried out using the following

procedure:

• insertion in the reaction flask of 0.85 gr of catalyst spheres, corresponding to a standard catalyst volume of 1

ml;

• addition of 5 ml of 30% hydrogen peroxide solution;

• continuous acquisition of the following quantities during the decomposition reaction:

– temperature of the liquid mixture;

– temperature of the gas mixture;

– mass flow of developed gaseous oxygen;

• removal of the decomposition products after the reaction;

• return to the initial thermal conditions.

The typical time histories of the liquid and gas mixture temperatures and of the oxygen mass flow rate measured

during an activity test are illustrated in Figure 3 with reference to a LR-III-39 Pt/α−Al2O3 catalyst sample. The

liquid mixture temperature tends to increase rapidly up to about 380 K, corresponding to the boiling point of the

decomposing HP solution, followed by a slower decline as the reaction tends to completion. The evolution of the gas

mixture temperature is similar, apart from the delay caused by the initial presence of cold air in the reaction vessel.

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In the experiments where the increase of liquid mixture temperature is very fast, as shown in Figure 3 (a), the time

separation of the two temperature peaks tends to decrease. The velocity of the reaction is well illustrated by the

parallel behavior of the mass flow of gaseous oxygen (see Figure 3 b). When the temperature of the decomposing

liquid solution approaches its peak value, the reaction becomes very fast and the residual HP decomposes in few

seconds, even if its temperature declines more slowly.

The time needed for the liquid to reach its peak temperature (tmax) and the maximum value of developed oxygen

mass flow rate in each test have been used as quantitative indicators of the decomposition activity and susceptibility

to poisoning of the catalysts, as well as for evaluating the repeatability of the test results. Comparative plots of tmax

for the catalysts developed in the context of the present activity are reported in Figure 4 as functions of the

consecutive test number on the same catalyst sample. The maximum mass flow rates of developed oxygen for all of

the catalysts are presented in Figure 5 as functions of the test number. Analysis of Figure 4 leads to the following

observations:

• LR-II-108, the silicon carbide-based catalyst, exhibited a strong reduction of its chemical activity after only

five tests, as indicated by the rapid increase of tmax from 2 to about 14 seconds, followed by the stabilization

at a value close to 15 seconds observed after the test number 10. Together with the deterioration of chemical

activity, the loss of metallic platinum from the silicon carbide surface has been observed, as confirmed by the

dark color of the reaction products. The lower value of the residual catalytic activity after the test number 10

(a) (b)

Figure 3 Liquid and gas mixture temperatures (a) and flow of gaseous oxygen (b) as functions

of time during a test on the LR-III-39 Pt/αααα−−−−Al2O3 catalyst.

Figure 4. Time needed for the decomposing HP solution to reach the peak temperature

as a function of the test number for all platinum-based catalysts.

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suggests that only a small fraction of the platinum contained in the precursor solution was effectively bonded

to the silicon carbide matrix, probably also as a consequence of its low porosity.

• LR-59, LR-II-180 and LR-II-122 showed an excellent repeatability and no appreciable poisoning over a

sequence of fifty tests (with the exception of LR-II-180, for which only thirty tests have been conducted).

The tmax value remained stable close to 2.5 seconds for the LR-59 catalyst and at about 4.8 seconds for the

LR-II-122 and LR-II-180 catalysts.

• Reducing the specific surface area from 170 m2/g (LR-59) to 4 m

2/g (LR-II-122) and using the same nominal

molar load of platinum in the impregnation solution only increased the value of tmax from 2.5 to about 4.8

seconds. This indicates that the use of the alumina support with lower surface area and excellent thermal

shock resistance, as reported in Romeo et al.12

does not strongly deteriorate the decomposition activity.

Consequently LR-II-122 should be identified as suitable candidates for more realistic experimentation in the

HP monopropellant rocket engine prototype test facility.

• LR-III-37, in spite of its higher platinum load with respect to LR-II-122, showed signs of progressive

deterioration, as indicated by the increase of tmax from 2.5 seconds to 10 seconds after fifty tests. Further

investigations must be carried out to understand the reasons of this rather anomalous feature compared to the

other Pt/Al2O3 catalysts.

All catalysts obtained with the PT2 deposition procedure, the LR-II-122 catalyst and the Pt/Al2O3-COM

commercial benchmark catalyst, were subjected to a sequence of 100 activity tests, as shown in Figure 4. In these

tests the LR-III-39 catalyst displayed the highest chemical activity and stability, characterized by a constant value of

tmax of about 1.8 seconds. The additional calcination step in muffle at 400°C for 5 hours carried out on LR-III-39

reduced to a half the catalytic activity, as indicated by the tmax value of about 4 seconds. Compared to the activity of

LR-II-122, the LR-III-39 catalyst exhibited a tmax value about three times lower, probably due to a higher superficial

content of platinum (7.4 At%). Considering that two α-alumina substrates with specific surface areas of 4 m2/g and

14 m2/g have been used for LR-II-122 and LR-III-39, the comparison of the activity of these two catalysts suggests

that the PT2 procedure has been more effective than the PT1 in deposing higher contents of platinum, in spite of the

significant reduction of the surface area of the support. The commercial benchmark catalyst (Pt/Al2O3-COM)

exhibited a tmax value of about 2 seconds, showing an activity very close to LR-III-39. A slight tmax decrease has been

experienced up to test number ten, followed by a stabilization at a value of about 2 seconds thereafter.

From the analysis of oxygen flow rates in Figure 5 the following additional observations can be made:

• The observed trends of tmax and of the maximum oxygen flow rate for LR-III-39 and LR-III-89 are in good

agreement.

Figure 5. Maximum mass flow of oxygen as a function of the test number.

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• LR-III-39 displayed a peak of the oxygen flow at a value of 0.7 gr/sec with strong oscillations of the

experimental data, which remained constant with the increase of test number.

• Compared with LR-III-39, LR-III-89 confirmed the halving of the catalytic activity (0.35 gr/sec). A slight

improvement of the chemical activity after test number 60 has been detected from both the trends of tmax

(decreasing from 4 to about 3.5 seconds) and of the peak values of oxygen flow (increasing from 0.35 to 0.4

gr/sec).

• The different levels of chemical activity between LR-III-39 and LR-II-122 indicated by the tmax values (see

Figure 4) is much less evident when comparing the peak values of the oxygen flow rate, which become

almost comparable in the last 60 tests.

• The commercial benchmark catalyst Pt/Al2O3-COM presented a oscillating value of maximum oxygen flow

rate near to 0.5 gr/sec with slight oscillations with the increase of the test number. When comparing the peak

values of the oxygen flow rate with LR-III-39 catalyst (0.7 gr/sec), the difference in activity showed to be

much more evident.

In conclusion, the LR-III-39 catalyst has been identified as the suitable candidate for more realistic

experimentation in the HP monopropellant rocket engine prototype test facility and the Pt/Al2O3-COM catalyst

proved to be an effective benchmark for comparative performance assessment. As a consequence, a new catalyst,

hereafter indicated as FC-LR-87, has been prepared by means of the PT2 coating technique on the α−Al2O3

substrate with 13 m2/g of BET surface area. Two hundred grams of this carrier were deposed with a total amount of

metallic platinum equivalent to 2 % of the alumina weight.

VI. Post-Firing Characterization

In order to understand the effects on the FC-LR-87 catalyst of the more severe working conditions imposed by

the use in the HP monopropellant thruster prototype, scanning electron microscopy analysis (SEM) and X-ray

diffractometry (XRD) analyses were performed on a 60 grams sample of the FC-LR-87 catalyst that had

decomposed up to 2 kg of 87.5 % HP in steady state and hot pulsed firings at a bed load level of 19.02 kg/s m2. SEM

analysis was also performed on the commercial benchmark catalyst before the firing tests.

A. Scanning Electron Microscopy

The SEM measurements of the catalyst surface compositions in atomic and weight percentages are presented in

Table 3. The columns indicated by progressive roman numbers refer to five different scans obtained using a standard

cross over window of 400 × 250 µm (161× magnification) on five different catalyst granules. The last column

reports the mean value and the sample variance of the measured load, expressed in weight percent, of the chemical

element listed in the second column, for easier comparison of the various catalysts and quantitative assessment of

the measurement accuracy. The average platinum loads of the FC-LR-87 catalyst before and after the firing tests

resulted to be 37.28 and 37.06 wt% respectively, a statistically insignificant difference compared to the dispersion of

the measurements, demonstrating the absence of the washing phenomenon of the active phase away from the

supporting surface and providing convincing evidence of the stable and effective deposition of the platinum on the

alumina substrate obtained by means of the PT2 coating technique. SEM analyses also showed significant residues

of chlorine in the spent Pt/Al2O3-COM catalyst, which initially contained about 3.49% by weight of chlorine. This

high content of chlorine was probably introduced in the alumina carrier during the impregnation with a H2PtCl6

precursor solution and is likely to represent the main reason for the chemical attack of the stainless steel nozzle

during the Pt/Al2O3-COM firing test (Torre et al.13

).

Scanning electron micrographs with magnifications 40× and 35× of the FC-LR-87 and Pt/Al2O3-COM catalyst

samples after the firing tests are shown in Figure 6. In the spent FC-LR-87 catalyst the deposed layer of platinum

remained homogeneous and appeared unchanged, confirming the effectiveness of the active phase deposition on the

substrate in spite of the rather severe temperature and pressure conditions during the firing and the possible

occurrence of friction due to pellet vibration. The irregular grains of the Pt/Al2O3-COM catalyst substrate appears to

be high porous, a property that might contribute to the outstanding performances displayed by this substrate in the

decomposition of N2H4. These performances have been also confirmed by the present firing tests with HP (Torre et

al.13

), where no break-up from internal overpressures or thermal shocking have been experienced.

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Table 3. SEM measurements of the atomic compositions of the catalysts integrated in the thruster.

Catalyst Element

(at.%/wt.%) I II III IV V x ± s

(wt%)

Al 47.68/38.76 47.01/41.71 47.26/39.98 46.46/38.89 48.23/38.74 39.6±1.28

O 45.64/22.00 47.83/25.17 46.76/23.45 47.33/23.49 44.90/21.38 23.09±1.47

Pt 6.68/39.25 5.16/33.12 5.98/36.57 6.22/37.62 6.87/39.88 37.28±2.67

FC-LR-87

(new)

Cl 0.00/0.00 0.00/0.00 0.00/0.00 0.00/0.00 0.00/0.00 0±0

Al 46.93/37.81 41.96/32.44 43.86/36.92 44.13/39.40 45.21/38.58 37.03±2.72

O 46.18/22.06 49.13/22.52 49.48/24.70 50.52/26.74 48.79/24.69 24.14±1.58

Pt 6.89/40.13 7.14/39.89 5.95/36.20 5.09/32.84 5.88/36.25 37.06±3.02

FC-LR-87

(used)

Cl 0.00/0.00 0.00/0.00 0.00/0.00 0.00/0.00 0.00/0.00 0±0

Al 43.46/49.84 39.62/48.14 43.43/49.71 38.09/47.23 40.57/49.42 48.86±1.13

O 52.68/35.83 57.70/41.57 52.68/35.76 59.42/43.68 56.40/40.75 39.51±3.56

Pt 1.26/10.42 0.84/7.34 1.28/10.61 0.68/6.14 0.69/6.09 8.11±2.24

Pt/Al2O3-COM

(new)

Cl 2.60/3.91 1.85/2.95 2.61/3.92 1.81/2.95 2.34/3.74 3.49±0.50

Figure 6. 40×××× and 35×××× scanning electron micrographs of the FC-LR-87 (left) and the Pt/Al2O3-COM (right)

catalysts.

B. X-Ray Diffractometry

The X-Ray diffraction pattern of the FC-LR-87 catalyst recorded before the firing test in the thruster prototype is

illustrated in Figure 7. As expected, the analysis of the XRD pattern indicates that the main catalytic component is

metallic platinum surrounded by a well defined corundum matrix. The broad and diffused metal peaks also indicates

that platinum particles are small in size and well dispersed on the Al2O3 surface, further supporting the evidence of

the effectiveness of the PT2 deposition procedure.

XRD analyses on the spent FC-LR-87 catalyst (not shown) confirmed the presence of a well dispersed

distribution of metallic platinum in small size particles. Even more importantly, no traces of platinum oxides (PtO2)

has been detected. Therefore all of the deposed catalyst remained in metallic state (oxidation number equal to zero),

despite the relatively long exposure to a strongly oxidizing atmosphere at elevated temperatures during the

decomposition of about 2 kg of HP in the thruster firing. In a previous work (Tian et al.15

), XRD analysis on a Ir/γ-

Al2O3 catalyst fired under similar conditions in a HP thruster revealed the oxidation of the active metal (specifically

iridium), which was considered as one of the causes of the catalyst deactivation.

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Figure 7. XRD patterns of the fresh FC-LR-87 Pt/αααα-Al2O3 catalyst, (••••) αααα-Al2O3 , (����) Pt .

VII. Conclusions

In summary, after the preliminary characterization tests and the experimental campaign carried out by means of

firing tests in a HP monopropellant thruster prototype, the following conclusions can be drawn:

• At the end of the atmospheric drop tests, the LR-III-39 catalyst displayed the highest activity among the

different preparations, with no appreciable performance degradation and loss of platinum as a consequence of

sustained HP decomposition. In addition, high surface loads of platinum (44 wt%) have been attained on its

α-alumina carrier, in spite of the low value of the specific surface area (4 m2/g).

• The commercial benchmark catalyst Pt/Al2O3-COM, a Pt/Al2O3 catalyst deposed on a very porous substrate

with high mechanical strength and thermal shock resistance and expressly manufactured for hydrazine

decomposition, exhibited a good activity and repeatability during decomposition tests of 30% HP. An

average platinum load of the 8% by weight has been measured by means of SEM analysis, slightly lower

than the nominal 10% value. Significant residues of chlorine (about 3.49 wt%) have also been also detected.

The high content of chlorine was probably introduced on the alumina carrier by the impregnation with a

H2PtCl6 precursor solution, and represented the main reason the observed chemical corrosion of the stainless

steel nozzle during the firing test (Torre et al.13

).

• The comparison of the performance exhibited by the FC-LR-87 (a new catalyst similar to LR-III-39) and the

Pt/Al2O3-COM catalysts during firing tests in the thruster prototype operated with 2 kg. of 87.5 % HP

propellant and the results of the SEM and XRD analyses demonstrated that the FC-LR-87 catalyst is quite

effective for HP decomposition in terms of activity level, chemical and physical stability of the catalytic

species on the supporting surface, resistance to thermal shocking, and low susceptibility to poisoning.

Acknowledgments

The present work has been supported by the Italian Ministry for Production Activities under D. M. 593. The

authors are very grateful to Profs. Mariano Andrenucci, Fabrizio Paganucci and Daniela Belli Dell’Amico of Pisa

University for their constant and friendly support. The contributions of Prof. Marcello Mellini of the University of

Siena, Mr. Franco Colarieti and Prof. Fabio Marchetti of the University of Pisa for the realization of the TEM, SEM

and XRD analyses is gratefully acknowledged.

American Institute of Aeronautics and Astronautics

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