AIAA 2010-7128
46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit
25 - 28 July 2010, Nashville, TN
Development and Testing of a High Temperature N2O
Decomposition Catalyst
D. T. Wickham*, B.D. Hitch†, and B.W. Logsdon‡
Reaction Systems LLC, Golden, CO 80401
Present designs for scramjet-powered hypersonic missiles employ simple rocket boosters
to bring them up to minimum operating speeds where a dual-mode ram/scram engine can
take over. However, the low air pressures and temperatures and the very short residence
times make scramjet ignition at altitude difficult. Various methods to improve ignition and
flame holding have been used with some success. However, all methods have limitations
and therefore improved technologies are still needed. One way to improve scramjet ignition
and performance would be to utilize the mixture of 33% O2 and 66% N2 produced from N2O
decomposition. N2O decomposition to N2 and O2 is a very exothermic reaction, and the heat
produced is sufficient to generate product temperatures of 1300°C (2400°F). Unfortunately,
N2O is a relatively stable compound and it needs to be heated to about 800°C (1470°F) to
begin decomposing in the gas phase. In addition, N2O can decompose into NO and N2,
which is an endothermic reaction and therefore this process would not be beneficial for
ignition. However, employing a catalyst could solve both of these problems. Catalysts can
reduce the temperature required for reaction and they can also direct the reaction along the
desired pathway. Therefore, in this SBIR Phase I project, Reaction Systems’ objectives
were to identify catalyst formulations that are active for N2O decomposition under
representative conditions, characterize their activity and thermal stability, and produce a
kinetic model that can be used to predict rate as a function of N2O partial pressure and
temperature. The results obtained in this project showed that our catalysts can meet the
demanding criteria needed to take this technology from the laboratory to a vehicle. We
demonstrated that our catalysts were extremely active for the reaction at low temperatures.
They reduced the temperature required for reaction to occur by over 600°C compared to
results obtained without catalyst. In addition we found that our catalysts were highly
selective for N2 and O2. On the other hand without catalyst, we obtained N2 but very little
O2 in the products, suggesting that gas phase N2O decomposition follows the endothermic
pathway, producing N2 and NO. Finally, we generated a kinetic model, which accurately
predicted N2O decomposition rates over a wide range of temperatures and pressures.
Nomenclature
atm
BET
cm3
D
Ea
FW
g
GC
GHSV
h-1
HX/R
m2/g
*
†
‡
atmospheres
Brunauer Emmett and Teller
cubic centimeter
tube diameter (m)
activation energy (kcal/mole/K)
Formula weight
gram
gas chromatograph
gas hourly space velocity
per hour
heat exchanger reactor
square meters per gram
President and Member AIAA
Chief Engineer and Senior Member AIAA
Engineer
1
American Institute of Aeronautics and Astronautics
Copyright © 2010 by Reaction Systems, LLC. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
min
ml
ms
PN2O
psi
R
SLPM
T
TCD
ν
minutes
milliliter
millisecond
partial pressure of N2O (atm)
pound per square inch
ideal gas constant (1.98 cal/mole/K)
standard liters per minute
absolute temperature (K)
thermal conductivity detector
preexponential factor [moles N2O/(g cat h atm N2O)]
I.
Introduction
Aircraft capable of rapid global strike and reconnaissance as well as long range, rapid time to target missiles
require hypersonic (Mach >5) flight to achieve their performance goals. While rockets routinely achieve these
speeds, the payload fraction of rockets is relatively small due to the requirement that they carry along all of the
oxidizer needed for their fuel load. On the other hand, employing air breathing engines instead of rocket engines
can allow the same overall payload delivery using vehicles that are smaller, lighter, and less expensive.
Unfortunately, high speed flight in the atmosphere presents a number of challenges, such as very high air
recovery temperatures and aerodynamic heating loads, high aerodynamic drag, and large internal flow total pressure
losses. In addition, no single air breathing engine cycle performs well over all speed ranges, and therefore present
designs for scramjet-powered hypersonic missiles anticipate employing simple rocket boosters to bring them up to a
minimum operating speed where a dual-mode ram/scram engine can take over. These missiles would probably be
launched from platforms such as the F-22 or F-35 and therefore could spend a considerable amount of time at
altitude before they are used. Unfortunately, igniting scramjet engines at altitude is difficult. Low air pressure,
low air temperature, short residence time all combine to make reliable ignition and stable flame holding a
challenging problem. Making matters worse, the fuel and engine components will all be cold soaked prior to
ignition. At high altitudes the temperatures expected during capture carry are well below 0°C. All of these
conditions can profoundly affect the ability of the scramjet engine to ignite properly and operate reliably after the
burnout of the rocket booster.
Previous work has been carried out to identify effective ways to improve ignition and flame holding in scramjet
engines. Smaller fuel droplets decrease ignition delay, therefore improved atomization through the use of
effervescent or barbotage atomization helps. Plasma igniters1,2,3, hydrogen pilot flames, or pyrophoric ignition aids
such as silane4, and “Sugar Scoop” inlets5 have all been used with some success. However none of these measures
represents an acceptable solution because they would require that large, heavy power supplies or reactive and
perhaps toxic chemicals be carried on board the vehicle.
Another way to improve scramjet performance is to introduce a mixture that is very hot, about 1300°C
(2400°F), and contains a high concentration of oxygen. This mixture could be used in two ways. First if it were
mixed with fuel, it would ignite immediately and at the proper flows could be used as a pilot flame for ignition of
the main engine. In this case, the hot oxygen flow would only be needed for a short time, 10 seconds or less.
Another way to use the hot gas would be in a barbotage fuel injector. Since it is hot, it would heat the fuel, which
will improve atomization. At gas to liquid ratios typically used in barbotage systems, the hot flow would heat the
fuel by 33°C (60°F) which will reduce fuel viscosity and result in smaller droplets. An alternative configuration
would be to first mix the gas with a stoichiometric flow of fuel, causing combustion and then adding the balance of
the fuel for injection. This would produce a temperature increase of about 90°C (160°F) increase in temperature.
Perhaps most importantly, generating a hot flow of oxygen on board a vehicle is relatively straightforward. In the
following section, we show this high temperature oxygen flow can be generated by catalytic decomposition of
nitrous oxide.
Nitrous oxide (N2O) is a non-toxic, near-critical liquid at ambient temperature6. In addition, it can decompose
exothermically, producing a mixture of 33% O2 and 67% N2 as shown in Reaction 1 below:
Rxn. 1
N2O Æ N2 + ½ O2
∆H = -802 Btu/lb
This is a very exothermic process and will produce a mixture of 33% O2 in N2 at a temperature of approximately
1300°C, which is easily hot enough to ignite fuel. For combustion purposes, N2O is currently used in hybrid rocket
systems like the SpaceDev engine7 that was used on Scaled Composites’ X-Prize winning Space Ship One as well as in
high-performance racing engines8 to increase the inlet fuel/air charge density and volumetric heat release. Liquid
2
American Institute of Aeronautics and Astronautics
N2O also has some attractive physical properties. First, it has a relatively high density, over 60 lb/ft3 at
temperatures expected at high altitude, and it is therefore possible to store a large quantity of N2O in a small tank
and expel it at high flow rates. In addition, N2O is capable of providing self-pressurization across the expected
operational temperature range.
However, there are two problems that must be overcome in order to employ N2O decomposition as an ignition
aid. First, N2O is relatively stable up to temperatures up to 800°C. Therefore high temperatures would be
required to take advantage of the energy available from this compound, which may not be practical on board a
vehicle. The second problem is that N2O can decompose into N2 and NO as shown below:
Rxn. 2
N2O Æ ½ N2 + NO
∆H = +8.24 Btu/lb
The problem with Reaction 2 is that it is endothermic and therefore absorbs heat. If this reaction occurs to a
significant degree it will substantially reduce the temperature of the gas flow, making the process useless for this
application. Thus, there are two potential problems associated with the utilization of the chemical energy
contained in N2O: it requires a high temperature for decomposition to occur and it potentially can produce NO,
which would not release heat.
However both of these problems can potentially be solved with a catalyst. The catalysts can provide an
alternative, low activation energy pathway for decomposition, which will substantially reduce the temperature
required for the reaction to begin. In addition if the catalyst has the proper selectivity, it will direct the reaction
along the exothermic pathway, producing N2 and O2, and minimize the the undesirable formation of NO.
Therefore a catalyst has the potential to solve both problems associated with harnessing the chemical energy in N2O.
In order to be effective, the catalyst must have excellent activity, have good selectivity for N2 and O2 production,
and finally remain active at the temperatures expected, up to 1300°C.
Unfortunately, most catalysts will not tolerate temperatures of 1300°C without quickly losing their activity.
Catalysts consist of active metal compounds dispersed on materials that have an extensive pore structure giving
them very high surface areas, over 200 m2/g. However, when these materials are exposed to high temperatures the
material will sinter, burying the active metal particles and causing the catalyst to become inactive. However,
Reaction Systems is developing thermally stable catalysts that can survive at these temperatures and therefore they
could be quite useful in this application. Thus, in this project our objective was to demonstrate the feasibility of
catalytically decomposing N2O to improve ignition, combustion stability, and combustion efficiency in a scramjet
engine. In the following sections, we describe the technical approach we used along with the results, which clearly
demonstrate the feasibility of this technology.
Recently, new catalyst supports known as hexaaluminates have been developed which resist surface area loss at
high temperatures. This material consists of alumina (Al2O3) in which a heteroatom such as barium, lanthanum, or
manganese has been substituted for an aluminum atom as seen in Figure 1. A typical chemical formula is
MAl11O18, where M is the heteroatom. When the hexaaluminate is heated to high temperatures, the heteroatom
prevents the oxide from undergoing a phase change
into the low surface area form of alumina by forming a
structure consisting of spinel-like blocks that are
separated by mirror planes containing the alkali or
alkaline earth cations9,10,11,12. The crystal growth in the
direction normal to the plane (the [001] direction in
Figure 1) is much slower than the rate of growth in the
directions parallel to the plane even at temperatures of
up to 1300°F. As a result, the crystals do not sinter
even at very high temperatures.
Other approaches to preparing high surface area
materials have also been developed. For example,
Shigapov et al.13 used a cellulose templating approach
to prepare high surface area, mixed metal oxide
supports that retain their surface area at temperatures in
excess of 1000°C. After calcination at 1050°C, the
authors report surface areas up to 30 and 140 m2/g for
Ce-Zr and La-Al mixed metal oxides. Moreover, the
process produces a high fraction of the larger meso
pores, which would facilitate rapid diffusion of N2O to
Figure 1: Structure of a hexaaluminate compound
3
American Institute of Aeronautics and Astronautics
the catalyst surface. Therefore, there are several potential methods that could be used to prepare thermally stable
materials with high surface areas, which would serve as suitable supports for the active catalytic metal for N2O
decomposition in a high temperature application.
Once a stable support material has been identified, the active catalytic metal can be added to its surface.
Recent reports have also indicated that noble metals are active for N2O decomposition at temperatures as low as
350°C (598°F) 14,15, and therefore catalyst consisting of these metals on thermally stable supports were evaluated in
this project.
II.
Experimental Methods
In this section we describe the methods and materials we used to carry out the experimental portion of the Phase
I work. We describe the methods we used to prepare and screen the catalysts for N2O decomposition activity and
also discuss the methods we used prepare and test the wall mounted catalyst at high pressure.
A. Catalyst Preparation
To synthesize the hexaaluminate supports, aqueous solutions of aluminum and the heteroatom were first
prepared by dissolving the nitrate salts of each compound in DI water. We then coprecipitated the hydroxides of
each metal by slowly adding ammonium hydroxide until the pH was about 8.5. After settling overnight the
supernatent liquid was poured off and the samples were dried and then calcined for two hours at 1000°C, resulting in
approximately four grams hexaaluminate, which appear as white powders. We also prepared two samples of the
Al2O3/La2O3 mixed metal oxide, using two different types of cellulosic templating agents: a dense low porosity slow
filter paper and a coarse, fast filter paper. We first prepared an aqueous solution of aluminum nitrate and a second
metal nitrate, dripped the solution onto two pieces of each of the filter papers, then dried the samples overnight, and
calcined them at 1000°C for one hour.
After preparation, all of the catalyst supports exist as a fine powder, which unfortunately is not a suitable form
for a heterogeneous catalyst in a packed bed reactor. In order to test these materials in our test section, we coated
the material on cylindrical-shaped sections of porous ceramic foam at thicknesses of between 25 and 50 microns.
We then impregnated the support with an aqueous solution of catalytic metal by dissolving the nitrate salt in water
and adding just enough solution to the support to wet the entire length of the tube. This technique, referred to as
incipient wetness, assures maximum dispersion of the metal over the entire surface of the support. We then
calcined it a final time to convert the dry metal salt to the metal oxide. A photograph of the foam section before
and after coating with a hexaaluminate catalyst is shown in Figure 2. The dark color on the coated structure in the
right photograph appears relatively even, suggesting that the catalyst is well-distributed in the foam structure.
B. Description of the Automated Catalyst Testing Rig
A schematic of the automated test rig we used to carry out the catalysts tests is shown in Figure 3. The system
consists of a manifold where a feed gas mixture is generated using a combination of Porter mass flow controllers for
gases. Although in this test we only need to supply N2O, the manifold gives us the ability to add a diluent such as
helium in order to carry out kinetic measurements when we want to test the catalyst at lower N2O partial pressures
or carry out tests under more isothermal conditions. After mixing, the feed gas is heated in a preheater and then
directed into the test section consisting of a quartz tube three feet in length by approximately ¾-in in diameter.
Thermocouples are inserted
from the top and bottom of the
tube so we have a good
characterization
of
the
temperature above and below
the catalyst. The test section is
enclosed and heated by a
Lindberg split furnace, capable
of generating temperatures up to
1000°C. After exiting the test
section, a portion of the product
gas stream is directed into an
SRI
gas
chromatograph
equipped with a thermal
conductivity detector (TCD) and
Figure 2: Ceramic foam structure prior to catalyst addition (left) and
a 15-ft x 1/8-in OD SS column
coated with catalyst (right)
4
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Heated lines
PT-1
TC-1
Lindberg Split Furnace
Computerized
Process Control
Catalyst coated foam
S
0.75-in diameter quartz tube
N 2O
PV-1
MFC-1
2 SLPM
CV-1
MFC-2
10 SLPM
CV-2
S
He
PV-2
TC-2
Pressure
Control Valve
Liquid Fuel
HPLC
Pump
SRI Gas Chromatograph
for product analysis
CV-3
Figure 3:
Schematic of the catalyst testing rig
packed with Carboxen 1000. The test rig is fully automated using National Instruments DAQ data acquisition and
control hardware and LabVIEW software.
C. Catalyst Screening Procedure
In order to measure the activity of the catalyst for N2O decomposition, we first wrapped the catalyst coated
foam section with a thin layer of quartz wool blanket insulation, which allowed us to seal the foam sample tightly
inside the inner wall of the quartz tube test section. We then began flowing N2O at a rate of 0.5 SLPM, which
represents a space velocity of 200,000 standard cm3 of feed per cm3 catalyst per hour. These units are commonly
referred to as gas hourly space velocity (GHSV), and therefore our N2O feed flow rate was 200,000 h-1. We then
heated the oven to 275°C, and maintained these conditions long enough to obtain two GC samples. In this
configuration, we use a thermocouple located in the oven, outside of the test section for control feedback because
this temperature changed rapidly with oven power and results in very stable control. As pointed out previously, we
also monitored the temperatures above and below the catalyst with separate thermocouples located inside the test
section. After obtaining data at 275°C, we raised the oven temperature in 25°C increments until we obtained high
levels of N2O conversion which occurred at an oven temperature of 350°C. At each test point, we used GC data to
calculate the percentage of N2O that was converted at each test point. We used a nitrogen basis to calculate activity
as shown below:
%Activity = mole % N2 /(mole N2 + mole% N2O) * 100
(Equation 1)
D. Wall-Mounted Catalysts
Although catalyst screening at ambient pressure allowed us to identify active catalyst formulations, it is also
important to carry out tests at higher pressures to verify that the rate expression can be extended to pressures that
might be encountered in an application. To accommodate tests at higher pressures, we prepared a test section
where the catalyst was coated on the inside wall of a six inch length of a ¼-in OD stainless steel tube. We
accomplished this by preparing a solution of the hexaaluminate support powder and an alumina based binder and
then alternately filling the tube with liquid and draining the tube and letting it dry. We repeated the process several
times until we obtained a layer thickness of about 25 microns. At the conclusion of this process, the tube had a
very well-adhered layer of hexaaluminate that appeared as a very white, uniform layer. We then added the metal
using the incipient wetness technique described earlier and finally calcined the tube at 550°C for one hour. We
then attached 1-ft length of ¼-in OD SS tubing to both ends of the test section and installed the assembly into our
test rig, replacing the quartz tube.
III.
Results and Discussion
5
American Institute of Aeronautics and Astronautics
The catalyst development task was divided into two separate parts. The first task was identifying formulations
that were active for N2O decomposition, selective for the formation of O2 and N2, and also had the necessary thermal
stability. The next task was to prepare a wall-mounted catalyst and demonstrate performance under more realistic
flows and pressures. The results of these efforts are presented in the following sections.
A. Thermal Stability of the Catalyst Supports
Surface Area (m2/g)
We characterized the thermal stability
of two hexaaluminate and two templated
90
metal oxide supports by measuring their
As Prepared
80
surface areas after preparation and again
Aged at 1000°C
after aging the materials for eight hours at
70
1000°C. We used the single point BET
60
method to measure surface area in all
cases. The support surface areas before
50
and after aging are shown in Figure 4.
40
The surface areas of the two
hexaaluminate catalyst supports (Hex-1
30
and Hex-2) are 56.3 and 82.1 m2/g after
20
preparation and after aging at 1000°C,
very little surface has been lost in both
10
cases. The post aging surface area for the
0
Hex-1 catalyst support is 56.7 m2/g, within
Hex-1
Hex-2
TMO-FFP
TMO-CFP
the experimental error of the initial value
Catalyst Support
reported for this material. The surface
Figure 4: Surface area measurements made on four catalyst
area for the Hex-2 compound is 77.6 m2/g,
supports after preparation and after aging
which is only a 5.5% reduction from the as
prepared value. Figure 4 also shows that
the surface areas of the templated metal oxide catalyst supports, while substantially lower than the hexaaluminates
after preparation, did not change significantly with aging. We obtained values of 20.7 m2/g for the support
prepared on the fine filter paper (FFP) and 13.5 m2/g on the course filter paper (CFP). After aging, the values were
19.2 and 14.1 m2/g for the FFP and CFP samples respectively. Although these materials had excellent thermal
stability, we conducted the balance of the experimental work with the higher surface area hexaaluminate materials.
Overall, the results indicate that aging at 1000°C did not produce substantial reduction in these catalyst
supports. Even if the aging step were increased in length, these results are encouraging because the sintering
process is most rapid in the initial stages of exposure to high temperature. Thus, we would expect even lower
percent losses in surface area in
subsequent periods of exposure.
Temperature
Temperature
below catalyst
B. Results
of
Experiments
Screening
We carried out separate tests on all
of the catalysts prepared. In addition
we conducted baseline tests to
characterize decomposition without
catalyst. The results obtained with the
first hexaaluminate catalyst we tested,
M1/Hex-1 are shown in Figure 5. The
figure shows the three temperatures
monitored, the oven temperature outside
the test section used for feedback to
control the oven power, along with the
temperatures above and below the
catalyst as a function of time on stream
in minutes. In addition, we have also
included the measured activities at each
above catalyst
Catalyst = M1/Hex -1
N2O Flow = 0.5 SLPM
Pressure = 0.82 atm
GHSV = 200,000 h-1
Furnace
temperature
Catalyst activity
Figure 5: Results obtained for the M1/Hex-1 hexaaluminate
catalyst
6
American Institute of Aeronautics and Astronautics
Temperature (°C)
Percent N2O Conversion
500
100
temperature (solid blue squares - plotted
Catalyst = M2/Hex-1
Catalyst activity
on the secondary y axis).
N2O Flow = 0.5 SLPM
Pressure = 0.82 atm
As
shown,
at
the
lower
450
GHSV = 200,000 h-1
80
temperatures, 275 and 300°C, the N2O
Temperature
Temperature
above catalyst
decomposition activity levels are less
below catalyst
400
than 5% and the temperatures inside the
60
test section are all relatively close to the
value measured by the furnace
Furnace
350
thermocouple outside the reaction zone.
temperature
However when we increased the furnace
40
set point temperature to 325°C, the
300
activity level climbs to about 8% and the
temperatures inside the test section come
20
250
Catalyst activity
to steady state at about 335°C, 10°C
above the furnace temperature. This
difference is likely due to the heat of
0
200
0
50
100
150
200
250
reaction from the N2O decomposition
Run Time (min)
reaction. Finally, when we raised the
temperature to 350°C, the temperature
Figure 6: Results obtained for the M2/Hex-1 catalyst
above the catalyst comes to an initial
semi steady state value of 360°C at a run time of 210 minutes and the activity obtained at that point was
approximately 17%. However, the temperatures above and below the catalyst then began to climb and reached
steady state values of approximately 435 and 445°C respectively. Both of these are well above the oven set point
of 350°C. We obtained three GC measurements at this temperature, and obtained N2O conversions, of 79, 79 and
77%. In addition, we obtained a 1:2 ratio of O2 to N2 in the products, indicating that the catalyst has good
selectivity for the desired exothermic products. Finally at a run time of about 325 minutes, we stopped the N2O
flow and the figure shows that the temperatures inside the reactor dropped rapidly as the heat provided by the
reaction was no longer available.
The test results obtained for the next catalyst, M2/Hex-1 catalyst are presented in Figure 6. At 300°C, the N2O
decomposition activity levels are relatively low, less than 5% and the temperatures inside the test section are all
relatively close to the values measured by the furnace thermocouple outside the reaction zone. However when we
increased the set point temperature to 325°C, the activity level climbs to about 90% and the temperatures inside the
test section rise rapidly to values over 450°C. Finally, the two subsequent GC analyses show that we are achieving
100% activity in the final hour of the test. These results show that the reaction lit off when the gas temperature was
raised to 325°C and once that occurred, the exotherm from the decomposition reaction provide enough heat to cause
full conversion of the N2O compound.
Figure 7 shows results obtained for
Catalyst = M3/Hex-1
a third metal supported on the
N2O Flow = 0.5 SLPM
hexaaluminate. In this case the figure
Pressure = 0.82 atm
shows that the catalyst is not effective as
GHSV = 200,000 h-1
we did not detect any N2O conversion
with the GC even when the catalyst
temperature was raised to 450°C. In
addition, the figure shows that at all
conditions, the temperatures inside the
furnace are very near the furnace
temperature indicating that no heat is
being generated from the N2O
decomposition process. These results
show that the catalyst does have a
substantial effect on the rate of N2O
Catalyst activity
conversion.
We also carried out tests in a blank
quartz tube with no catalyst to measure
the base line conversion rate for gas
phase N2O decomposition. The results
Figure 7: Results obtained for the M3/Hex-1 catalyst
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American Institute of Aeronautics and Astronautics
of this experiment are shown in Figure 8.
100 % activity but
The figure clearly shows that the N2O
Baseline Run
Low
selectivity for O2
No
Catalyst
decomposition without catalyst requires
N2O Flow = 0.5 SLPM
significantly higher temperatures to
Pressure = 0.82 atm
occur at similar rates.
When the
furnace temperature is at 400°C, the
Less than 100°C difference
Furnace
between furnace and
figure shows that the measured activity
temperature
reaction
zone
temperatures
is less than our detection limit of about
one percent.
In addition, the
temperatures measured inside the reactor
are very similar to the furnace
temperature, confirming that the
exothermic N2O decomposition process
is not occurring at rate anywhere near
Catalyst activity
the rate observed for the M2/Hex-1
catalyst when the furnace temperatures
was at only 325°C. When the furnace
temperature was increased to 800°C, the
measured N2O has increased somewhat,
but it is still less than 10%. Finally,
Figure 8: Baseline results obtained without catalyst
Figure 8 shows that when the furnace
temperature was increased to 1000°C, the N2O conversion increased to 100%. Comparison of data presented in
this figure to that obtained with an active hexaaluminate catalyst (Figure 6) indicates that the catalyst reduces the
temperature required for reaction to occur by 675°C. Since a heater likely would be necessary to start the reaction
in this application, reducing the temperature from 1000°C to 325°C represents enabling technology.
Reducing the temperature required for reaction to occur is not the only benefit offered by the catalyst. Without
catalyst, the N2O decomposition reaction does not produce O2 and N2, which are the desired products responsible for
heat release as shown in Reaction 1. Instead our GC analyses suggest that without catalyst the gas phase reaction
proceeds according to Reaction 2, which is responsible for NO production. Figure 9 shows the products measured
during the N2O decomposition experiments at 100% N2O conversion with and without catalyst. With the M2/Hex1 catalyst at 325°C where we obtained 100% conversion, the chromatogram on the left side of figure shows that the
peak area corresponding to O2 is about one half that of the N2 peak, indicating that oxygen and nitrogen are present
at a 1:2 molar ratio, consistent with the ratio of products produced by Reaction 1. On the other hand, without
catalyst the chromatogram on the right side of the figure shows that the oxygen peak is very small with respect to
nitrogen indicating that very little oxygen has been produced in this reaction. In addition, the chromatogram shows
a peak that likely is NO and therefore the results indicate that without catalyst, the N2O decomposition process is
proceeding along the pathway shown by Reaction 2. Therefore, we conclude that the catalyst in addition to
substantially reducing the temperature required for the N2O reaction to occur, also directs the reaction along the
O2 and N 2 Concentrations
Close to 1:2 Ratio
O2
O2 about 1/10 of N 2
N2
N2
M2/Hex Catalyst
Temperature = 325°C
Activity = 100%
Figure 9:
O2
Empty Quartz Tube
Temperature = 1000°C
Activity = 100%
Additional peak could be NO
GC product distribution obtained at 100% N2O decomposition with catalyst (left) and without
catalyst (right)
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American Institute of Aeronautics and Astronautics
Percent N2O Decomposition
100
pathway that is critically needed to be
M2/Hex‐1
useful for this application.
M1/Hex‐1
GHSV = 200,000 h -1
We screened a number of catalysts
M2/Hex‐2
80
that contained metals that had been
M1/Hex‐2
M3/Hex‐1
reported to be active for N2O
M4/Hex‐1
decomposition. The results of these
Blank Tube
60
tests along with the baseline test without
Selective for N2 and O2
No catalyst
(Exothermic reaction)
catalyst are summarized in Figure 10.
This figure shows percent N2O
40
conversion as a function of furnace
Produces NO and N2
(Endothermic reaction)
temperature for all catalysts tested along
with results obtained in a blank tube
20
when no catalyst is present. The figure
shows that we obtained high
conversions for three catalysts, at
0
furnace temperatures of 350°C or lower.
200
300
400
500
600
700
800
900
1000
1100
We achieved 100% conversions for two
Furnce Temperature (°C)
catalysts containing M2, supported on
two different hexaaluminate materials,
Figure 10: Summary of catalyst testing activity.
Hex-1 and Hex-2. We also obtained
80% conversion on a catalyst consisting of M1/Hex-1. Figure 10 also shows that much lower conversions were
obtained for the other catalysts consisting of M3 and M4 supported on Hex-1. Finally, as discussed previously, the
data show that without catalyst, much higher temperatures are needed to achieve measureable levels of N2O
decomposition and even at high levels of conversion, the production of NO would prevent the reaction from being
useful for this application.
C. Tests to Reduce Light-Off Temperature
In this application a heater is needed to raise the catalyst temperature to values where it will light-off. We
therefore conducted a test to determine if we could reduce the light-off temperature by temporarily reducing the N2O
flow. In this test we used the M2/Hex-1 catalyst and reduced the N2O flow from 0.5 slpm used in the previous
tests to 0.2 slpm. The results of this test are shown in Figure 11. At this lower flow we obtained an activity of
about 5% when the furnace temperature is at only 225°C. This is a higher value than we obtained in the previous
test with this catalyst when the furnace temperature was held at 300°C (Figure 6). When we raised the furnace
temperature to 250°C, we observed that the temperatures above and below the catalyst increased, indicating that the
catalyst was active for N2O at this temperature. GC analyses confirmed that we were converting over 95% of the
N2O to O2 and N2 at this condition. On
the other hand at the higher flow, a
100
350
Catalyst = M2/Hex-1
furnace temperature of 325°C was
0.2 SLPM <N O Flow < 0.5 SLPM
GHSV = 200,000 h
Pressure = 0.82 atm
required for light off to occur. Finally,
at a run time of about 260 minutes, we
80
320
GHSV = 80,000 h
began increasing the N2O flow to the
values used in the previous test and
Temperature
observed corresponding increases in
60
290
below catalyst
temperatures above and below the
catalyst, indicating that more N2O was
Temperature
undergoing reaction.
These results
40
260
above catalyst
show that we can reduce the initial
temperature we need to heat the catalyst
Furnace
temperature
by lowering the N2O flow on a transient
20
230
basis, which in turn reduces the amount
of power that must be carried on board.
2
-1
Temperature (°C)
Percent N2O Conversion
-1
0
200
D. Kinetic Measurements
100
We also carried out kinetic
measurement on the M2/Hex-1 catalyst
in order to generate a rate expression
120
140
160
180
200
220
240
260
280
300
Run Time (min)
Figure 11: Results at reduced N2O flows to demonstrate lower
light off activity
9
American Institute of Aeronautics and Astronautics
from which we can predict reaction rate over a range of temperatures and N2O partial pressures. A simplified
general rate expression for N2O decomposition is shown below:
Rate = ν * exp(Ea/RT) * PmN2O/(1+K1PmN2O)
Equation 2
Where ν is the preexponential term in units of g N2O per gram catalyst per minute per atm N2O, Ea is the
activation energy in cal/mole, R is the ideal gas constant in cal/mole, T is the reaction temperature K, PN2O, is the is
the partial pressure of N2O in the reactant mixture in atm, and m is the dependency of rate on the partial pressure of
N2O and K1 is the equilibrium constant for the adsorption of N2O on the catalyst surface. In this expression, one
can see how the reaction rate might vary with N2O partial pressure. At low pressures, where the product of K1 and
PN2O are small with respect to 1, these terms can be ignored and the rate is simply proportional to PmN2O. In this
case, raising the reactant pressure increases the reaction rate. On the other hand, at higher pressures, the product of
K1 and PmN2O become large with respect to 1 and now the 1 in the denominator term can be ignored. In this case
the PmN2O terms in the denominator and numerator cancel and the rate is no longer dependent on N2O pressure so
raising the pressure has no effect on rate.
In this project, we first carried out kinetic measurements on the M2/Hex-1 catalyst mounted on the alumina
foam in the quartz tube at atmospheric pressure. To carry out the kinetic analysis we measured reaction rate as we
varied the N2O pressure and temperature over as wide a range as possible while maintaining differential conversion
conditions, conversions that were below 25%. Maintaining low conversions reduces the temperature and N2O
concentration gradients through the catalyst bed so that we can assume that the entire length of catalyst is being
exposed to the similar test conditions. We maintained constant partial pressures of N2O and varied the temperature
to obtain the effective activation energy and conversely maintained constant temperature and varied the partial
pressure of N2O to obtain m. Finally, once we solved for Ea and m, we calculated the preexponential factor v,
which is a constant over the range of temperatures and pressures measured.
The results of the kinetic analyses are shown in Figure 12. Under the conditions we used, we found that the
reaction rate was first order with respect to N2O pressure, m=1. Therefore we concluded that the product of K1*
PN2O in the denominator term in the previous expression is small with respect to , which allows us to simplify the
rate expression as shown in
Equation 3:
Rate = ν * exp(Ea/RT) * PN2O
Equation 3
Reaction Rate (g N2O/(g cat min))
The figure includes values for the preexponential factor, ν, activation energy, and finally Ea. As shown we
obtained a value of 7.09E9 g N2O/(g cat min atm) and an activation energy of 29.2 kcal/mole, which is consistent
with many catalyzed reactions. The first order dependence of rate on N2O pressure suggests that the reaction rate
should vary linearly with the
2,000
partial
pressure
of
N2O.
Finally, we used the model to
1,800
Rate = A * exp(‐Ea/RT) * PN2O m
predict reaction rate as a function
A = 7.06 E+09 g N
2O/(g cat min atm)
1,600
of temperature assuming a
Ea = 29.2 kcal/mole
constant N2O pressure of 1 atm.
m = 1
1,400
As shown the rate increases
rapidly, by a factor of ten as the
1,200
temperature is increase from 550
Predicted rate from model
1,000
(PN2O = 1 atm)
to 700°C.
This is very
encouraging as it indicates that
800
the catalyzed reactor for N2O
could be relatively small.
600
As pointed out previously,
400
increasing N2O pressure in the
tube can affect the kinetic rate
200
expression because at some
point, the catalyst surface will
0
300
350
400
450
500
550
600
650
700
750
become saturated with N2O, and
Temperature °C
at this point, the reaction rate is
no longer dependent upon N2O
Figure 12: N2O decomposition rate model and predicted rates as a
partial pressure.
Thus the
function of temperature
10
American Institute of Aeronautics and Astronautics
Temperature (°C)
Percent N2O Conversion
90
650
reaction rate can change from first order
Exit Temperature
in N2O pressure at low pressures to zero
80
600
order in N2O pressure at higher
Temporary
pressures. Therefore it is important to
70
550 reduction of flow
Rate = 26.5 g N2O/(g cat min)
to start reaction
conduct tests at pressures expected in an
60
500
application in order to identify these
behaviors in the kinetic rate expression.
50
450
Unfortunately, the catalyst configuration
used in the screening process, where it
40
400
Oven Temperature
was coated on a ceramic foam and
located inside a quartz tube is not
30
350
Inlet Temperature
designed for tests at high pressures.
20
300
However, once we identified an active
Catalyst = 0.0558 g M2/Hex-2
catalyst formulation in the screening
Pressure = 0.82 atm
10
250
GHSV = 1.5 E6 hr-1
process, we coated it on the inside
Contact Time = 0.043 s
surface of a ¼-in OD x 0.035-in wall
0
200
0
50
100
150
200
250
300
350
thickness 316 stainless steel tube. This
Run Time (min)
test section can handle pressures up to
500 psi even at high temperature, which
Figure 13: Results obtained in the second test with the wallallowed us carry out kinetic analyses of
mounted M2/Hex-2 catalyst
the catalyst under more representative
conditions. These results we obtained with this wall-mounted catalyst configuration are presented in the following
section.
E. Wall-Mounted Catalyst Results - Ambient Pressure
We carried out a series of tests with the wall mounted catalyst initially at atmospheric pressure in order to
compare its rate with the previous catalyst coated on the ceramic foam. Figure 13 shows a run made on the
M2/Hex-2 catalyst coated on the inside of the ¼-in OD 316 SS tube.
Initially, we heated the test section to
425°C and maintained the space velocity at a value of 1,500,000 h-1 and found that the catalyst would not light off.
We then reduced the space velocity to 500,000 h-1 for a short time (between times of 80 and 100 minutes), and
observed that the reaction immediately started, as indicated by the substantial rise in the temperature at the catalyst
exit. Then at 100 minutes, we set the space velocity back to 1,500,000 h-1 and observed no decrease in the
temperature at the catalyst exit, indicating that the catalyst was still very active.
Between runtimes of 150 minutes and 300 minutes, we varied the furnace temperature in order to maximize the
range of conversions we obtained with the catalyst while maintaining a constant space velocity. The figure shows
that we obtained a low conversion of about 41% when the furnace temperature was 325°C and a high value of about
53% when the furnace temperature was set to 450°C. Obtaining a wide range of conversion will allow us to better
verify our kinetic model and identify the parameters that provide best fits. Finally, we have included a reaction rate
at 280 minutes where a conversion of 53% was obtained. As shown, we obtained a rate of 26.5 g N2O/g cat min.
F. Test Results at Elevated Pressure
We then carried out the N2O reaction at pressures greater than atmospheric, with the initial test at a pressure of
90 psig (8.0 atm). Although, this is lower than the ultimate operational pressure, this still represents a factor of 9.7
increase in pressure range compared to the previous tests. The results of this test are shown in Figure 14. The
figure shows that we started the test at a space velocity 710,000 h-1 and then raised the pressure up to about 50 psig.
At this point, we heated the furnace up to 425°C and as shown the temperatures at the reactor inlet and exit reach
values of 650 and about 550°C respectively, indicating that the exothermic N2O decomposition reaction had begun
and was reaching high levels of conversion. At this condition, we obtained two GC analyses, which confirmed the
high level of reaction as we obtained N2O conversion of 93.8 and 94.5%. At a run time of about 110 minutes we
increased the flow rate to achieve a space velocity of 1,500,000 h-1. As shown in the figure, the temperatures at the
catalyst inlet and exit each increase by about 50°C reaching 700 and 600°C respectively. Since there was no
change in furnace temperature, this increase can be attributed to an increase in overall N2O reaction rate. Over the
next three hours we maintained constant reaction conditions and obtained six GC analyses that were all very
consistent. Over this time, we averaged an N2O conversion of about 81%, which can be converted to a reaction
rate of 40 g N2O/g cat min.
11
American Institute of Aeronautics and Astronautics
800
120
We can compare this data to the
GHSV = 710,000 h
GHSV = 1,500,000 h
results presented in Figure 13 to
determine what effect the increased
Inlet Temperature
700
100
pressure has had on the measured
System pressure
reaction rate. Since all of the GC
600
80
measurements shown in Figure 13 were
Exit Temperature
obtained at the space velocity of
Rate = 40.5 g N O/(g cat min)
1,500,000 h-1 we can simply compare the
500
60
N2O conversions in that figure to those
obtained under the same conditions in
Oven Temperature
400
40
Figure 14. Figure 13 shows that when
the furnace temperature was at 425°C,
two GC analyses measured conversions
300
20
Catalyst = 0.0558 g M2/Hex-2
of 48 and 46%. On the other hand, in
Wall-Mounted Catalyst
Pressure
=
90
psig
the test at high pressure at the same
space velocity, we obtained an average
200
0
0
50
100
150
200
250
300
conversion of about 81%, or almost a
Run Time (min)
twofold improvement in rate. Finally
the data show that the reaction rate of 40
Figure 14: Results obtained in a test with the wall-mounted
g N2O/g cat min obtained at high
M2/Hex-2 catalyst at a pressure of 90 psig
pressure is substantially greater than the
maximum value obtained at atmospheric pressure of 26.5 g N2O/g. Moreover, the latter rate at ambient pressure
was obtained at a furnace temperature of 450°C, which is 25°C higher than the furnace temperature used in the tests
at high pressure.
In order to verify the validity of the rate expression, we carried out a series of tests at total pressures up to 500
psi, with N2O pressures ranging from 1.45 to 28.6 atm, exit temperatures from 442 to 705°C, and N2O conversions
ranging from less than 10% up to 78%. In some cases, we diluted the N2O with helium in order to reduce the net
heat produced and have better control of the reaction temperature. For each set of tests, we monitored temperatures
measured by the thermocouples 1.5 and 5 inches through the reactor in addition to the percent N2O converted into
N2 and O2. We then numerically integrated the kinetic rate expression in order to predict the percent N2O
conversions at each condition. In our numerical integration we also included terms for convective and radiative
cooling from the tube and an internal mass transfer term, which in some cases became rate limiting.
In our initial analysis we found that the net conversions predicted by the rate expression shown in Figure 12
were consistently higher than the values we measured with our gas chromatograph. However, we found we could
improve the fit if we simply included a single correction factor of 0.3 in the rate expression. Once we made this
modification, we obtained reasonably good agreement between the N2O conversions measured in the wall-mounted
catalyst to values predicted by
numerically
integrating
our
rate
100
expression. These results are illustrated
1.45 atm < PN2O < 28.6 atm
in Figure 15. Overall the figure shows
442°C < TEXIT < 705°C
80
that we obtain similar agreements over a
very wide range of conversions.
Perhaps more importantly, the rate
60
expression in the figure shows that the
reaction rate is first order with respect to
N2O partial pressure. This indicates
40
that in this pressure range, the K1*PN2O
term in the rate expression shown earlier
is small and therefore it can be ignored,
20
which simplifies the rate expression. In
Rate = 7.06 E9 g N2O/(g cat s atm) *
addition the good agreement over this
exp(-29.2 kcal/mole/RT) P1N2O * 0.3
0
range of temperatures indicates that the
0
20
40
60
80
100
activation energy we measured is
Measured N2O Conversion (%) accurate. Finally, the data suggest that
we should be able to use this rate
Figure 15: Comparison of measured rates obtained in the wall
expression in our conceptual design of
mounted catalytic reactor to the predicted values
-1
Predicted N2O Conversion (%) 2
12
American Institute of Aeronautics and Astronautics
Percent N2O Conversion/ Pressure (psig)
Temperature (°C)
-1
the heat exchanger/rector to provide hot oxygen for a pilot ignition torch. The factor of 0.3 modification we had to
make in the rate expression could be due to the different catalyst configuration we used to generate the original
expression (coated on a foam versus attached to the reactor wall) or a small variation in the individual catalysts
prepared. Nonetheless, the factor is a relatively small change and since the reactor will be operated in a mass
transfer limited mode, small changes in rate expressions will not cause differences in overall N2O conversion or heat
produced.
IV.
Conclusions
Overall the results we have obtained demonstrate that the use of catalytic N2O decomposition on board a
vehicle either as a pilot ignition system or as a source of gas for barbotage fuel atomization is very feasible. We
demonstrated that catalysts consisting of selected metals supported on two different types of hexaaluminate supports
could meet the demanding criteria needed to take this technology from the laboratory to a vehicle. These catalysts
have extremely high activity for the reaction, will withstand the temperatures expected in a heat exchanger/reactor
used to carry out the decomposition reaction, and are highly selective for the production of O2 and N2, which are the
desired exothermic reaction products. With the most active catalysts, we obtained 100% N2O decomposition when
we heated the catalyst to 325°C. On the other hand, without catalyst, a temperature in excess of 850°C was
required to achieve the same level of N2O conversion. The catalyst also substantially improved the selectivity of
the N2O decomposition reaction. With catalyst we obtained effectively 100% selectivity for N2 and O2; on the
other hand, without catalyst, we obtained very little O2 suggesting that gas phase N2O decomposition produces
mostly N2 and NO, which is an undesirable, endothermic reaction. We also showed that a wall-mounted catalyst
adhered very well to metal surfaces that would be used in a heat exchanger reactor, it is also very active, and it is
also selective for the formation of O2 and N2. We performed a kinetic analysis with the most active catalyst in the
quartz tube at less than atmospheric pressures and derived a rate expression that indicates the reaction rate is directly
dependent on N2O pressure. We then conducted tests with a wall mounted catalyst at total pressures up to 29.5 atm
and obtained excellent agreement between predicted and measured N2O conversions after making a small
modification to our original rate expression.
V.
Acknowledgments
The authors gratefully acknowledge the Air Force SBIR office for funding this work under contract number
FA8650-09-M-2956. We also would like to thank our contract monitor, Mr. David Buckwalter at Wright-Patterson
Air Force Base, for his thoughtful comments and direction over the course of this project.
VI.
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14
American Institute of Aeronautics and Astronautics