[American Institute of Aeronautics and Astronautics 43rd AIAA/ASME/SAE/ASEE Joint Propulsion...

American Institute of Aeronautics and Astronautics

1

Performance of Different Catalysts Supported on Alumina

Spheres for Hydrogen Peroxide Decomposition

Luca Romeo1, Lucio Torre

2, Angelo Pasini

3, Angelo Cervone

4, Luca d’Agostino

5

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

Fausto Calderazzo6

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

With the financial support of the European Space Agency, ALTA S.p.A. is conducting an

extensive experimental campaign for the development of advanced catalytic beds for

hydrogen peroxide monopropellant thrusters in collaboration with the Chemistry and

Industrial Chemistry Department of the University of Pisa. Manganese oxides, palladium,

platinum, ruthenium and silver have been implanted on γ-Al2O3 and Si-doped Al2O3 spheres,

starting from different precursors during the impregnation phase. Metal loadings on the

ceramic substrates were evaluated by Scanning Electron Microscope analysis (SEM). Tests

have been carried out in a specifically-designed experimental set-up in order to characterize

and compare the chemical activity of different catalysts and for investigating the influence of

the relevant operational parameters.

Nomenclature

tmax = Time needed for the temperature of the liquid mixture to reach its maximum value

k = Arrhenius constant

I. Introduction

N recent years low toxicity (or “green”) liquid rocket propellants have become attractive as possible substitutes

for hydrazines and nitrogen oxides because of the significant cost saving associated with the drastic simplification

of the health and safety precautions required in the production, storage and handling of these propellants. These

advantages have a special relevance to low or medium thrust rocket engines, where the above costs do not scale

down proportionally to the engine size.

The most promising high-energy green propellants, like ADN, HAN and HNF (Wucherer et al.1, Schoyer et al.

2),

are based on complex organic molecules and compensate the comparatively higher molecular weight of their

decomposition products with proportionally higher operational temperatures of the exhaust gases. As a consequence,

the operational life of the catalytic beds is drastically reduced and extremely expensive materials and manufacturing

processes are necessary for the realization of radiatively cooled chambers typical of low thrust propulsion systems.

Hydrogen peroxide which does not suffer from these disadvantages, is now being reconsidered as a promising

green propellant for low and medium thrust applications. It is relatively easy to handle with respect to other common

rocket propellant liquid oxidizers like dinitrogen tetroxide, nitric acid and liquid oxygen (Ventura and Muellens 3).

The nominal propulsive performance of hydrogen peroxide as a monopropellant is about 20% lower than hydrazine,

but the volume specific impulse attainable with 90% H2O2 is higher than for most other propellants. This is

particularly useful for systems with significant aerodynamic drag losses or stringent volume constraints, as is often

the case for small satellites. Finally, when used in bipropellant and hybrid rocket engines, hydrogen peroxide yields

1 Project Engineer, Alta S.p.A.; [email protected]

2 Project Manager, Alta S.p.A., AIAA Member; [email protected]

3 Project Engineer, Alta S.p.A., AIAA Member; [email protected]

4 Project Manager, Alta S.p.A., AIAA Member; [email protected]

5 Professor, Department of Aerospace Engineering , University of Pisa, AIAA Member; [email protected]

6 Professor, Department of Chemistry and Industrial Chemistry, University of Pisa , [email protected]

I

43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 8 - 11 July 2007, Cincinnati, OH

AIAA 2007-5466

Copyright © 2007 by ALTA S.p.A. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


2

a specific impulse comparable to other liquid oxidizers like dinitrogen tetroxide, nitric acid and even liquid oxygen

(Wernimont and Muellens 4, Wernimont and Garboden

5).

The most significant technology challenge in the realization of hydrogen peroxide monopropellant thrusters is

the development of effective, reliable and durable catalytic beds providing fast and reproducible performance,

insensitive to poisoning by stabilizers and impurities contained in the propellant. In addition, such thrusters need to

be capable of sustaining the large number of thermal cycles imposed by typical mission profiles in small satellite

applications and preferably should not require pre-heating for efficient operation. Nowadays the most used catalyst

materials for H2O2 are metallic silver (Runckel, Willis, and Salters 6; Morlan et al.

7), alkali permanganates

8 and

manganese oxides (typically MnO2 and Mn2O3). Some data are also available with alumina-deposited platinum,

ruthenium dioxide, divanadium pentoxide and lead oxide9. None of these solutions is free from drawbacks, the most

important being: temperature limitations and poisoning for metallic silver (Wernimont and Mullens10

; Ventura and

Wernimont11

), powdering and thermomechanical resistance for ceramic-deposited catalysts, excessive flow

resistance for pellet beds, and flow stratification for channel−matrix supported catalysts (Beutien et al.12

).

Kappenstein et al.13

have recently investigated the thermal decomposition and the hydrothermal reduction of

different permanganate precursors of manganese oxide-based catalysts, finding that higher reaction surface area and

activity are exhibited when using potassium permanganate rather than sodium permanganate. An extensive

experimental study carried out by Rusek9 indicated that catalysts based on MnO2 or Mn2O3 on different ceramic

pellets lead to an activity about one order of magnitude higher than obtained with silver. Other catalysts, like

ruthenium dioxide, displayed activities about three times higher than manganese oxides. Moreover, a series of

thermal tests in the same study showed that the activity of platinum on alumina is one order of magnitude higher

than exhibited by manganese oxides-based catalysts. These findings, however, are not fully consistent with those by

Pirault-Roy et al.14

who investigated the activity of platinum supported on silica, silver, iridium and platinum-tin or

that of manganese oxides supported on alumina, observing that silver on alumina yielded the highest activity,

followed by manganese oxides on alumina and by platinum on silica.

Resistance to poisoning is clearly of special importance in rocket propellant catalysts in order to attain the

necessary levels of control and repeatability of the engine thrust. Typical stabilizers used to prevent self-

decomposition of hydrogen peroxide are pyrophosphate ions, which generally tend to poison the catalytic bed. On

the other hand, other stabilizers, like stannate ions, seem to have an opposite effect on silver-based catalysts, leading

to an increase of the reaction rate at higher stabilizer contents (Pirault-Roy et al.14

).

As a direct consequence of the renewed interest in the use of hydrogen peroxide shown by the rocket propulsion

community, ALTA S.p.A. in Italy and DELTACAT Ltd. in the United Kingdom jointly undertook the development

of advanced catalytic beds for hydrogen peroxide monopropellant thrusters (Cervone et al.15

). This activity was

funded by the European Space Agency in the framework of the LET-SME program, supporting innovative projects

of small and medium size European companies. Specifically, the objective of the activity consisted in the

development of advanced deposition techniques of different catalysts/substrate combinations and in the design,

realization and testing of two engine prototypes (5 and 25 N thrust). The present paper illustrates the results of

recent experiments on the developed catalysts, with particular attention to the impregnation technique.

II. Test Bench

A specific test bench has been designed and realized by ALTA for the characterization of the catalysts. Figure 1

shows a schematic drawing and a picture of the test bench. It consists of a 100 ml reaction flask contained in a glass

vessel of about 2 liters. The upper part of the vessel is closed by a sealing lid with:

− the connection for the H2O2 supply funnel;

− two thermocouple taps, one for the measurement of the temperature of liquid H2O2 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.

Before each test, a known mass of catalyst is placed in the reaction flask. A given quantity of hydrogen peroxide

solution is then added by opening the tap of the H2O2 vessel. 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 hydrogen peroxide 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 hydrogen peroxide vapors in a liquid separator.


3

As a consequence, under regime conditions, the gas flow at the exit of the heat exchanger practically contains

pure oxygen at known pressure and temperature conditions; in this way the flow rate, measured by means of a flow-

meter, can be directly related to the rate of H2O2 decomposition16

. 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 thermocouples are

mounted in a stainless steel shield in order to provide protection against oxidation and avoid significant interference

with the catalytic reaction. The analytical model used for the design of the test bench was presented in a previous

paper16

.

III. Catalysts

Catalysts deposited on ceramic supporting spheres have been prepared by means of different implantation

techniques developed by ALTA S.p.A. in collaboration with the Chemistry and Industrial Chemistry Department of

the University of Pisa. 0.6 mm diameter spheres made of γ−Al2O3 and Si−doped Al2O3 were used as catalyst

ceramic support. The ceramic spheres were supplied by SASOL (see Table 1).

A. Catalyst Preparation According to open literature, materials for H2O2 decomposition are: manganese oxides, palladium, platinum,

ruthenium oxides and silver. The catalyst systems tested in the context of the present work are listed in Table 2. For

simplicity, the catalysts are denoted by the chemical formulas of both the active species and the supporting material.

Different catalyst “types” identify the use of different precursors during the impregnation phase. The data in

additional columns refer to the ALTA ID code, the deposition procedure and the nominal molar loading of the active

metal present in the impregnating solution.

The catalyst samples developed in the framework of the present activity have been developed using different

preparation techniques (PT), which can be summarized as follows:

− PT1: impregnation at 20 °C, washing with pure solvent, filtration, drying in vacuo (10-4

atm) at room

temperature.

Figure 1. A schematic drawing (left) and a picture (right) of the test bench.

Table 1. Properties of ceramic supports

Type BET Surface Area (m2/g) Pore Volume (ml/g) Al2O3 (%) SiO2 (%) Side crushing

strength (N)

0.6/170 170 0.53 96.1 0 14

SIRALOX-30 401 0.79 69.3 30.7 15


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atm) at room

temperature, atmospheric calcination at 400 °C for 4 h.


atm) at room

temperature, atmospheric calcination at 400 °C for 4 h and at 800 °C for 2 h.


atm) at room

temperature, reduction under hydrogen at 130 °C for 2 h.


atm) at room

temperature.


atm) at room

temperature, atmospheric calcination at 400 °C for 4 h.

Before impregnation, both ceramic supports, 0.6/170 and Syralox-30 spheres, were dried in vacuo (10-4

atm) at

140 °C. Both platinum and manganese oxide catalysts have been obtained from three different precursors.

Ruthenium oxide catalysts have been prepared using two different precursors. Only one precursor of palladium or

silver has been employed to generate the catalyst.

B. Scanning Electron Microscopy (SEM) Scanning electron microscopy analysis (SEM) was used for investigating the impurities introduced in the

impregnation, the metal loading and the potential existence of metal particle distribution. Scanning electron

micrographs of LR-59 Pt/Al2O3 2nd

type, LR-123 Pt/Al2O3 3rd

type, LR-63 Pd/Al2O3 and LR-155 Ru/Syralox-30 1st

type at magnifications 161×, 1293×, 161× and 621×, respectively, are in Fig. 2.

Table 2. Summary of the catalysts studied.

ALTA ID

code

Catalyst Preparation

Technique

Nominal metal

loading (mol××××103)

SEM metal

loading (At%)

LR-17 MnxOy/Al2O3 1st type PT2 3.32 0.71

LR-27 MnxOy/Al2O3 1st type PT3 3.32 0.71

LR-175 MnxOy/Al2O3 3rd

type PT6 7.60 0.71

LR-181 MnxOy/Al2O3 2nd

type PT6 8.70 0.35

LR-63 Pd /Al2O3 PT4 3.70 0.53

LR-63-400 Pd /Al2O3 PT2 3.70 0.53

LR-57 Pt/Al2O3 1st type PT4 3.99 0.39

LR-59 Pt/Al2O3 2nd

type PT4 3.82 0.47

LR-123 Pt/Al2O3 3rd

type PT4 8.60 0.81

LR-147 Pt/Al2O3 3rd

type PT4 0.11 0.49

LR-121 Ag/Al2O3 PT4 4.60 9.54

LR-75 RuxOy/Al2O3 1st type PT1 4.16 0.31

LR-75-H2 Ru/Al2O3 1st type PT4 4.16 0.31

LR-79 RuxOy/Al2O3 1st type PT1 n.d. 0.06

LR79-115 Ru/Al2O3 1st type PT4 n.d. 0.06

LR-159 Ru/Al2O3 2nd

type PT5 8.14 0.15

LR-159-400 RuxOy/Al2O3 2nd

type PT6 8.14 0.15

LR-155 Ru/SYRALOX-30 1st type PT1 4.30 0.35

LR-155-400 RuxOy/ SYRALOX-30 1st type PT2 4.30 0.35

LR-189 RuxOy/ SYRALOX-30 2nd

type PT6 8.00 0.19


5

(a) (b)

(c) (d)

Figure 2. SEM images of (a) LR-59 Pt/Al2O3 2nd

type, 161××××; (b) LR-123 Pt/Al2O3 3rd

type, 1293××××; (c)

LR-63 Pd/Al2O3, 40××××; (d) LR-155 Ru/Syralox-30 1st type, 621××××.

(a) (b)

Figure 3. Typical SEM diagrams of LR-175 MnxOy/Al2O3 3rd

type and LR-75 RuxOy/ Al2O3 1st type

catalyst spheres. (a) LR-175 sample showing the lines of oxygen (left), aluminum (center) and manganese

(right), corresponding to atomic contents of 55.1%, 43.9% and 1.0%, respectively. (b) LR-75 sample showing

the lines of oxygen (left), aluminum (center) and ruthenium (right), corresponding to atomic contents of 56.9%,

42.8% and 0.3%, respectively.


6

The lighter external layer in the sphere cross-section of the LR-59 catalyst shown in Fig. 2 (a) indicates the depth

of platinum deposition, which extends down to 180 µm below the surface, about half of the pellet radius. The color

of the external surface does not show important solution of continuity thus suggesting that the platinum distribution

is uniform. Conversely, Fig. 2 (b) shows at high magnification the surface of the LR-123 catalyst, obtained by a

different platinum-based precursor. In this case, platinum appears to be structured in small clusters (less than 20 µm

in size). Figure 2 (c) shows a picture of a LR-63 sphere cross section, large agglomerates of metallic palladium

being visible on the outer surface (white spots). The color variation of the cross section reveals that palladium has

penetrated a quarter of the sphere radius below the surface. Finally, a picture of the milled LR-155 catalyst is shown

in Fig 2 (d). The uniform color of catalyst splinters confirms the presence of a highly dispersed ruthenium phase,

both in the inner and outer portions of the catalyst sphere.

SEM analyses were also carried out in order to highlight possible correlations between the chemical composition

of the catalyst surface and its catalytic behavior. Metal loading on the supporting surface has been determined by

SEM analysis for all the catalysts developed during the present work. Two typical SEM diagrams are displayed in

Fig. 3. The spectrum analysis provides an integral measurement of the loading of chemical elements on the sample

surface and its immediate surroundings. Loading is expressed in percent atomic content (At%) and refers to the

average SEM reading over the optical framing window (200×180 µm) down to the typical penetration depth of the

electron beam (about 200 atomic layers). As indicated in Fig. 3, the more intense peaks in the SEM spectra

correspond to the oxygen and aluminum lines of the alumina support (left and center), while the weaker peaks

correspond to the added metal, manganese in Fig. 3 (a) and ruthenium in Fig. 3 (b). In the specific cases illustrated

in Fig. 3, the atomic contents of these metals are 1.0% and 0.3%, respectively. For all the catalysts within the present

activity, SEM analysis confirmed the absence of impurities potentially capable of reducing the catalytic activity. The

last column of Table 2 summarizes the results of the SEM analyses.

IV. Experimental Results

Two different sets of experiments have been conducted in the framework of the present activity using the test

bench described in Section II:

− Tests of catalytic activity, carried out for all the catalysts of Table 2, aimed at also comparing their

repeatability and susceptibility to poisoning.

− Tests of volume variation, carried out on four different catalysts in order to investigate the influence of

the ratio between the catalyst volume and the hydrogen peroxide sample on the catalytic activity.

The results obtained by volume variation tests have also been used for estimating the decomposition reaction rate

as a function of the catalyst volume, as described later.

The hydrogen peroxide solution used for the experiments is the Riedel-de Haen PERDROGEN

(provided by

Sigma-Aldrich), a 30% concentration solution with a particularly low content of impurities and stabilizers like

phosphates and sulphates (PO43−

<1 ppm, SO42−

<1 ppm).

A. Chemical Activity and Poisoning Tests

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

procedure:

− insertion of 0.85 g of catalyst spheres, corresponding to a standard catalyst volume of 1 ml, in the reaction

flask;

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

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

1. temperature of the liquid mixture;

2. temperature of the gas mixture;

3. exhaust mass flow of gaseous oxygen;

− removal of the decomposition products after the reaction;

− return to the initial thermal conditions.

The time histories of the liquid and gas mixtures temperatures and of the mass flow rate during a typical run are

shown in Fig. 4, referring to a test carried out on the LR-59 Pt/Al2O3 2nd

type catalyst. The temperature of the liquid

mixture tends to increase up to about 100 °C, when the decomposition reaction comes to completion; at nearly the

same time an increase of gas mixture temperature is observed. In the experiments exhibiting a fast initial increase of

the liquid mixture temperature, like that shown in Fig. 4 (a), the temperature peaks of the liquid and gas mixtures

tend to occur at nearly the same time.


7

After the two temperatures reach their peak values, the reaction becomes very fast and the residual hydrogen

peroxide decomposes in a few seconds. Together with the increase of the liquid mixture temperature, a parallel

increase of the exhaust gaseous flow is observed. Figure 5 shows the typical appearance of the reactant mixture

during the fast increase of the temperature. The time needed (tmax) for the liquid to reach the peak temperature has

been used as a quantitative assessment of the decomposition activity, the susceptibility to poisoning and the

repeatability of the catalysts. For the various catalysts used during the experimentation, comparative plots of this

quantity are reported in Fig. 6 (palladium and silver), Fig. 7 (manganese oxides), Fig. 8 (platinum) and Fig. 9

(ruthenium) as functions of the test number. In these figures, the points represent the test results and the dashed lines

the corresponding curves of best fit.

From the data of Fig. 6 the following observations are evident:

− the LR-63 catalyst showed a progressive activity

improvement with the test number, with a significant

reduction of tmax from 80 to about 30 seconds. A slight

improvement in the decomposition activity is observed

after the test number 30, with a final tmax value of about

25 seconds after the conclusion of the tests.

− the LR-63-400 catalyst, made of Pd/Al2O3 calcinated at

400 °C in air, showed a fast degradation of the

decomposition activity after 10 tests, exhibiting a tmax of

150 seconds. Further tests revealed the total absence of

chemical activity.

− the Ag/Al2O3 (LR-121) catalyst exhibited a high

decomposition activity within the first 10 tests, showing a

tmax of about 2 s. Suddenly, after the 15th

test, the activity

started to decrease quickly and a significant leaching of

metallic silver from the alumina surface was observed,

suggesting a weak binding between the silver catalyst and

the alumina substrate.

Figure 7, which refers to the manganese−based catalysts, provides the following information:

0 5 10 15 20 25 30300

310

320

330

340

350

360

370

380LR-59

Time [s]

Te

mp

era

ture

[K

]

liquid mixture temperature

gas mixture temperature

(a) (b)

Figure 4. Temperatures of the liquid and gas mixtures (a) and exhaust mass flow rate of gaseous oxygen (b)

as a function of time during a test on a LR-59 Pt/Al2O3 2nd

type catalyst.

Figure 5. The reactant mixture during a

test on a LR-175 MnxOy/Al2O3 3rd

type

catalyst.


8

a) The trends of tmax for both the LR-17 and the LR-27 catalysts showed similar deterioration. Only very slight

evidence of catalytic deterioration for the catalyst with higher calcination temperature has been observed.

Therefore, the different calcination temperatures of the two catalysts did not appreciably influence their

decomposition activity.

b) The LR-161 catalyst, obtained by a 2nd

type manganese−based precursor, displayed the lowest chemical

activity among the MnxOy/Al2O3 catalysts. After 15 tests the value of tmax increased very quickly from 40 to

about 80 s from the beginning of the run.

c) The MnxOy/Al2O3 3rd

type LR-175 catalyst confirmed to be the most reactive of manganese catalysts,

showing a constant activity (tmax = 23 s) and no appreciable poisoning during all of the tests.

Figure 7. Manganese oxide catalysts: time required for the liquid mixture to reach the

peak temperature as a function of the test number.

Figure 6. Silver and palladium catalysts: time required for the liquid mixture to

reach its peak temperature as a function of the test number.


9

Figure 8 illustrates the behavior of tmax measured for platinum catalysts as a function of the test number.

Different considerations can be made:

− All of the platinum catalysts showed values of tmax close to 4 s, about one order of magnitude lower than

palladium and manganese oxide catalysts. Their activity therefore was similar to that of the silver

catalysts.

− The Pt/Al2O3 2nd

type (LR-59) catalyst was found to be the most active, showing an excellent

repeatability and no appreciable poisoning. The value of tmax remained stable at about 2.5 s.

− The Pt/Al2O3 1st type (LR-57) catalyst showed signs of improvement in the catalytic activity after the 15

th

test. From test 15 to 50 the value of tmax remained stable at about 4.5 s, nearly twice as much as for the

LR-59 catalyst.

− The Pt/Al2O3 1st type (LR-123) catalyst displayed high chemical performance, similar to that of the LR-59

catalyst, up to the 20th

test. A sudden decrease of activity was observed in the next 4 tests. It was found

that this phenomenon was related to the low platinum dispersion on the alumina surface. This conclusion

was also confirmed by SEM pictures (see Fig. 2 (b)).

Finally, Fig. 9 shows the time needed for the liquid mixture to reach the peak temperature as a function of the

test number for ruthenium/alumina catalysts. A number of aspects of catalyst preparation have been investigated by

changing:

a) the precursor in the impregnation phase;

b) the nominal molar loading of metal in the impregnating solution;

c) the catalyst substrate;

d) the calcination procedure;

e) the hydrogen reduction treatment.

Analysis of Fig. 9 allows one to make the following observations:

− All ruthenium based catalysts, dried under vacuum, showed relatively high decomposition activities,

about three times larger than exhibited by manganese oxides and palladium catalysts. Good repeatability

and low susceptibility to poisoning were observed with the increase of the test number.

− The LR-75-400, LR-155-400 and LR-159-400 catalysts, the versions of the LR-75, LR-155 and LR-159

catalysts calcined at 400 °C for 2 h, showed a rapid deterioration of activity with respect to their

corresponding catalysts dried under vacuum. The value of tmax increased from 30 to 130 s after 10 tests.

Figure 8. Platinum catalysts: time required for the liquid mixture to reach the peak

temperature, as a function of the test number.


10

Table 3. Catalysts used for catalyst volume variation tests.

Alta ID code Catalyst Total catalyst volume (ml)

LR-57 Pt/Al2O3 1st type 2.8

LR-63 Pd /Al2O3 1.6

LR-75 RuxOy/Al2O3 1st type 3.5

LR-175 MnxOy/Al2O3 3rd

type 2.1

− The influence of ruthenium loading on the catalytic activity has been confirmed by the tmax value of

Ru/Al2O3 1st type (LR-75) and Ru/Al2O3 1

st type (LR-79), equal to 15 s for the LR-75 catalyst with

ruthenium loading of 0.31 At% and 35 s for LR-79 with ruthenium loading of 0.06 At%.

− Reduction under H2 at 140 °C for 2 h almost doubled the activity of the LR-79 catalyst (tmax values were

18 and 35 s, respectively).

− Starting from the same metal-containing precursor and changing the substrate from SASOL 0.6/170 to

Syralox-30 (see Table 1), it is possible to highlight the influence of the surface area on the decomposition

activity: this is shown by the trend of tmax for the LR-75 and LR-155 catalysts, which displayed a

reduction of tmax from 18 s to about 10 s.

− The change of the ruthenium precursor from the 1st to the 2

nd type led to a significant improvement of the

catalytic activity, as shown by the results obtained for the LR-75 and LR-159 catalysts, whose tmax

remained stable at about 18 s and 7 s, respectively.

− The Ru/Al2O3 2nd

type LR-159 catalyst confirmed to be the most active ruthenium catalyst, showing a

constant activity (tmax = 7 s) during all of the tests, with no appreciable poisoning and good repeatability.

Its activity therefore resulted to be very close to the Pt/Al2O3 2nd

type (LR-59) catalyst, the most reactive

catalyst developed during the present activity.

B. Volume Variation Tests

During this series of experiments, a selection of the developed catalysts have been tested, as listed in Table 3.

The experiments have been carried out aimed at understanding the influence on the decomposition activity of the

ratio between the catalyst volume and the mass of the hydrogen peroxide reactant. Each catalyst listed in Table 3 has

been subdivided in small volumes of 0.5 ml by means of a graduated cylinder.

Figure 9. Ruthenium catalysts: time required for the liquid mixture to reach the peak

temperature, as a function of the test number.


11

The experiments have been conducted using the following procedure:

− insertion of the first catalyst volume of 0.5 ml in the reaction flask;

− addition of 4 ml of 30% hydrogen peroxide solution;

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

1. temperature of the liquid mixture;

2. temperature of the gas mixture;

3. exhaust mass flow of gaseous oxygen;

− removal of the decomposition products after the reaction;

− return to the initial thermal conditions;

− addition of fresh catalyst (0.5 ml) in the reaction flask.

This procedure has been repeated for each catalyst up to consumption of the total available volume (see Table 3).

The results have been reduced by means of the numerical method presented in a previous paper17

in order to obtain a

quantitative evaluation of the catalytic decomposition rates. Figure 10 shows the values of the Arrhenius reaction

rate constant k, as a function of the catalyst volume, for a given temperature of the liquid mixture (350 K).

The following main conclusions have been drawn from these results:

− The Pt/Al2O3 1st type (LR-57) catalyst displayed the highest chemical activity, with a reaction rate one

order of magnitude higher than for the other catalysts under investigation. The reaction rate showed a

linear behavior up to catalyst volumes equal to 1.5 ml, followed by a slight nonlinear decrease at higher

volumes.

− Both the Pd/Al2O3 (LR-63) and the MnxOy/Al2O3 3rd

type (LR-175) catalysts displayed a linear behavior

of the reaction rate as a function of the catalyst volume. The LR-175 catalyst showed a reaction rate

slightly higher than that of the LR-63 catalyst.

− For the Ru/Al2O3 1st type (LR-75) catalyst, the reaction rate increased linearly for catalyst volumes less

than 1.5 ml and remained constant at higher values.

Figure 11 shows the maximum mass flow of exhaust oxygen as a function of the catalyst volume. Its analysis

provides further conclusions about the dependence of the catalytic performance on the catalyst volume:

− The similar results obtained in both sets of data (Fig.s 10 and 11) demonstrate the effectiveness of the

proposed reduction method for comparative evaluation of the various catalysts.

− It is confirmed that the Pt/Al2O3 1st type LR-57 catalyst showed a catalytic activity one order of

magnitude higher than that of the other catalysts. The trend of maximum oxygen flow rate as a function of

the catalyst volume also confirmed the existence of two catalytic regimes for platinum catalysts, the first

Figure 10. Reaction rate constant k as a function of the catalyst volume for different

catalysts at 350 K liquid mixture temperature.


12

for catalyst volumes approximately less than 1.5 ml and the second for higher volumes, showing evidence

of activity saturation.

− The other catalysts exhibited a linear behaviour of the peak value of the oxygen flow rate. An evident gap

in numerical values can be observed by comparison of this group of catalysts with the LR-57 catalyst.

− Comparison between the LR-63 and LR-175 catalysts showed a lower activity for palladium catalysts.

− For high values of the catalyst volume, the LR-75 catalyst showed a nonlinear catalytic regime.

V. Conclusions

In view of the proposed application to hydrogen peroxide monopropellant thrusters, the following main

conclusions can be drawn on the basis of the present experiments:

− SEM analysis generally confirmed the absence of impurities potentially capable of reducing the catalytic

activity of the samples and a very good dispersion of the metallic cations on the alumina support. This

clearly shows the impregnation technique to be very effective.

− The Pt/Al2O3 2nd

type LR-59 catalyst confirmed to be the most active catalyst, showing an excellent

repeatability and no appreciable poisoning.

− The Ag/Al2O3 catalyst showed activity levels comparable to those of platinum-based catalysts, but the

impregnation technique used for its fabrication proved to be unable to adequately bond the silver to the

ceramic support.

− The Ru/Al2O3 2nd

type LR-159 catalyst confirmed to be the most active ruthenium catalyst. Its activity

resulted to be the closest to the Pt/Al2O3 2nd

type (LR-59) catalyst. Furthermore, no appreciable poisoning

and a good repeatability have been attained by optimizing the fabrication technique.

− The Pt/Al2O3 1st type LR-57 catalyst exhibited two different catalytic regimes by increasing the catalyst

volume.

Acknowledgments

The authors gratefully acknowledge the support of the European Space Agency to the present activity under

LET-SME Contract No. 18903/05/NL/DC. The authors would also like to express their gratitude to Profs. Mariano

Andrenucci, Fabrizio Paganucci and Renzo Lazzeretti of the Dipartimento di Ingegneria Aerospaziale, Università di

Pisa, for their constant and friendly encouragement, to Prof. Marcello Mellini, Università di Siena and to Mr. Franco

Colarieti, Università di Pisa, for TEM− and SEM measurements, respectively.

Figure 11 Maximum mass flow of exhaust oxygen as a function of the catalyst volume for

different catalysts.


13

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