Thermal Decomposition of AP with Nanometer α-Fe2O3
Yi YANG, Feng-Sheng LI, Hong-Ying LIU
(National Special Superfine Powder Research and Engineering Center, Nanjing
University of Sci. & Tech, Nanjing 210094, P. R. China)
Abstract: Nanometer α-Fe2O3 catalysts are prepared by hydrolyzation in high
temperature. Three kinds of precipitators, NaOH, (NH4)2CO3 and urea, are used to
compare the effect in the process of hydrolyzation. Nanometer sizer, TEM and XRD
are employed to test the profiles and diameters of the product particles. The test
results indicated that the production is nanometer α-Fe2O3 with narrow particle size
distribution (PSD) and good dispersibility. The catalysts are mixed with ammonia
perchlorate (AP) in 1.0%wt. And the composite particles of catalysts with AP are
prepared by new-style solvent-nonsolvent method. DTA is employed to analysis the
thermal decomposition of the composite particles and pure AP sample. The results
imply that the thermal decomposition curve peaks of the samples added nanometer αFe2O3 catalysts are comparatively more ahead than that of pure AP sample. Among
these mixtures added nanometer material, the smaller the particle diameter of catalyst,
the more ahead the thermal decomposition curve peaks of AP. The high and low
temperature thermal decomposition curve peaks of AP mixed with the catalyst
deposed by urea are more ahead of 77.8
and 9.7
than that of pure AP, respectively.
The mechanism of the catalyst deposed by urea of smaller size in diameter and the
distinct catalysis of the particles on the thermal decomposition of AP are discussed.
Keyword: nanometer material, α-Fe2O3, catalyst, thermal decomposition, ammonia
perchlorate
Monodispersed α-Fe2O3 is currently the subject of intense research work due to its
novel properties, which could has promising applications in technology, such as gas
sensitivity materials, high quality pigment and catalytic materials. Monodispersed
nanometer α-Fe2O3 can be prepared by many ways
[1-3]
, such as Sol-Gel,
microemulsion, precipitation and hydrolyzation. Many properties of nanometer-size
α-Fe2O3 are enabled its excellent catalysis,
[4-6]
because of its high special surface
area, large numbers of surface atoms, oxygen vacancies on its surface and so on. Keu
H. K. [4] and Bruce C. F. [5] have previously studied the catalytic oxidation of sulfur
dioxide in the presence of α-Fe2O3. And the kinetics and mechanisms of the catalytic
oxidation of carbon monoxide on α-Fe2O3 has been investigated amply [6]. However,
reports on nanometer-size catalysts applied to improve the performance of
propellants, gunpowder and explosive are few in the literatures we have.
Ammonia perchlorate (AP) is the main composition in many propellants. The
application of superfine AP can improve the performance of propellants to some
extent. However, the preparation of superfine AP is very dangerous and difficult,
because the material of AP is a kind of strong oxidant. The performances of
propellants can be improved further more by added a small amount of catalyst in
superfine AP. Nanometer-size catalytic materials are studied prosperously in this days.
And nanometer α-Fe2O3 is one of the most promising candidates as catalyst for new
type propellants. In this article, hydrolyzation in high temperature Fe(NO3)3 solution
is applied to prepare nanometer α-Fe2O3. Three kinds of precipitators, NaOH,
(NH4)2CO3 and urea, are used to compare the effect in the process of hydrolyzation.
Differential thermal analyzer (DTA) is employed to test the thermal decomposition of
AP in the presence of nanometer α-Fe2O3. It indicated that the thermal decomposition
curve peaks of AP containing nanometer α-Fe2O3 are much more ahead than that of
pure AP.
1 Experimental
Known amounts of AR grade Fe(NO3)3·9H2O solution in 1.0M is added to the
40 solution dissolved precipitator (NaOH, (NH4)2CO3 or CO(NH2)2) under violent
stirring. Then the above aqueous solution are stirred for 22h at 90
in oil bath. When
the time is up, the precipitates formed in the solution are centrifuged at 12,000 rpm
and washed thoroughly with distilled water and absolute alcohol, and then dried at
100
in vacuum oven, and subsequently calcined in air at 450~500
for 2h. (In this
article, nanometer catalyst sample deposed by the precipitators of CO(NH2)2,
(NH4)2CO3 and NaOH would be named with UR, AC and SH, respectively.)
The previous products are grinded absolutely in absolute alcohol. Nanometer α-
Fe2O3 colloid aqueous solutions are prepared by dispersing the absolute alcohol
dispersion into distilled water with strong ultrasonic (600W) for 0.5h. Known amount
of AP is dissolved into the aqueous dispersion containing catalyst in oil bath at 90
under mild stirring. And saturated AP solutions are formed in high temperature
dispersion. Controlled vacuum system is employed to inhale the high temperature
dispersion containing nanometer α-Fe2O3 into ether solution resolved surfactant
bathed with ice-water bath under violently stirring. The precipitates composed by
superfine AP and nanometer α-Fe2O3 are filtered and then dried in vacuum oven at
90 . (In this article, AP containing nanometer catalysts of UR, AC and SH would be
named with AP/UR, AP/AC and AP/SH, respectively.)
The catalysts we prepared are characterized by using nanometer and zeta potential
sizer, X-ray diffraction (XRD), transmission electron microscopy (TEM) and specific
surface area measurement. The diameter and the surface zeta potential of catalysts
particles are measured on the Zetasizer 3000HS nanometer and zeta potential sizer of
Malvern Corp. XRD measurements are carried out on a Bruker D8 X-ray
diffractometer using CuKα radiation. TEM images are taken using a Hitachi H-800
electron microscope. The BET surface areas (SA) of catalysts are measured on a
Coulter SA3100 instrument using N2 adsorption at 77K. Before N2 adsorption
measurement on the catalysts, outgas procedure is conducted at 150
for 1h on the
Coulter instrument. Differential thermal analyzer (DTA, Shimadzu DTA-50) is
employed to test the decomposition of AP composite and pure AP.
2 Results and discussion
2.1 Structure and morphology of α-Fe2O3 catalysts
Fig.1 has combined the particle size distribution (PSD) curves of α-Fe2O3 deposed
by different precipitators. Nanometer in size and narrow PSD of the three samples are
shown in the figure. While the curves peaks of samples at 28.6nm, 41.7nm and
40.9nm are corresponding to the precipitators of CO(NH2)2, (NH4)2CO3 and NaOH,
respectively. The difference in diameter is mainly because the different ionization
degree of the precipitators in water. When the solution of Fe(NO3)3 is adding into the
solution of (NH4)2CO3 or NaOH, most of Fe3+ is deposed by NH4OH or OH- in the
solution immediately. But the origin particles would be grown with the deposition of
the residual Fe3+ in the solution during the subsequent high temperature
hydrolyzation. While CO(NH2)2 solution can releases out NH4OH gradually when the
temperature of the solution is higher than 70 . So the solution pH value is not change
so dramatically before and after the adding of Fe(NO3)3 solution. And the diameter of
most (93% in area) particles is not more than 50nm.
Fig.1 Particle diameter of samples
1-UR 2-AC 3-SH
The
zeta
potentials
of
samples at corresponding solution pH value are listed in Table1. The absolute value of
the zeta potential of the sample particles are much more than 30mV, which is the
critical value evaluating the dispersity of particles in dispersion. It indicated that the
sample particles had good dispersity in the solutions.
Table1 Zeta potential and crystal size of samples
As can be seen from the XRD spectra (are omitted) of the samples, four intense
diffraction peaks appeared at 2θ=33.2˚, 35.7˚, 40.9˚, 54.0˚, which can be assigned to
the diffraction peaks of α-Fe2O3, indicating that the as-prepared Fe2O3 samples have a
hematite structure. The average crystal size of samples are determined from the
broadening of the corresponding X-ray spectral peaks (at 2θ=33.2˚) by the Scherrer
formula: L=0.89λ/βcosθ. And the results are shown in Table1.
Fig.2 is the TEM photo of UR catalyst. (The TEM photos of AC and SH samples
are omitted.) As can be seen from the figure, the particles in the photo have pretty
good dispersity. The particles of UR are about 30nm, while the particles of AC and
SH are both about 40nm. It has a good consistency with the measurement of the
nanometer sizer instrument. But comparing to the mean crystal size in Table1, the two
later results are much more bigger than the results of XRD. It is because that what
TEM and nanometer sizer measured is always the secondary particles of samples,
while the results of XRD are the size of original or crystal particles. Compared the
size of the secondary particles with the original particles we prepared, it indicated that
the as-prepared samples had only a little conglomeration. Particularly, the sample of
UR is hardly conglomeration at all.
Fig.2 TEM photo of catalyst deposed by urea (amplified 7.5×104 times)
2.2 Catalysis of α-Fe2O3 on AP
One of the most important properties of nanometer materials is that the materials
can exhibit much more excellent characteristics than bulk materials. While nanometer
catalysts may have better catalysis than normal catalysts. Fig.3 is combined the
thermal decomposition curves of pure AP and AP compounded with different
nanometer α-Fe2O3 catalysts, at the temperature scanning-rate of 20 /min at N2
atmosphere. The figure clearly shows that the curve peaks of thermal decomposition
of AP containing α-Fe2O3 catalysts are much more ahead than that of pure AP. Among
these curves, the high and low temperature exothermal peaks of AP/UR composite is
391.6
and 319.2 , and is more ahead than that of original pure AP up to 77.8
and
9.7 , respectively. Based on ref. [7,8], Arrhenius activation energy (E) of the systems
(listed in Table 2) are estimated by equation (1).
where
d (lg φ )
E
= −0.4567 ⋅
1
R
d( )
Td
φ=temperature program rate Td=thermal
-1
(1)
decomposition
temperature,
-1
E=Arrhenius activation energy, R=8.314 J·mol ·K . These data, to a great extent,
embody the prominent catalysis of as-prepared α-Fe2O3 on AP.
Comparing the thermal decomposition of AP with different catalysts, the effect of
AP/UR is much better than that of AP/AC and AP/SH. It is because that both of the
original and secondary particle size of UR catalyst are smaller than that of AC and SH
catalyst. And the BET SA of UR is much bigger than that of AC and SH (listed in
Table 1). Further more, the incompletely elimination of Na+ is also a key unwanted
reason for the phenomena, during the washing process. While the residual Na+ in
precipitate can lead to agglomeration and growth of the particles in the calcining
process. In addition, the residual Na+ may form acid center in catalyst and result in the
decreasing of catalysis activity during the catalysis reaction.
[9]
As regard to UR and
AC catalyst, the precipitator what they used are urea and (NH4)2CO3 respectively. And the
two kinds of materials can be getting rid of in the form of gas during the thermal treatment of
catalysts.
Meanwhile, the uniformity of AP and catalyst in the composite is very important to
the combustion characterize of propellant. Based on the change of the solubility of AP
with the change of temperature of solvent and the change of the kind of solvent,
solvent-nonsolvent procedure is employed to prepare uniform composite propellant
and superfine AP with small crystal particles. In this article, saturate AP aqueous
solution dispersed nanometer α-Fe2O3 in high temperature is mixed with ether bathed
with water-ice bath. Because the solubility of AP in water at 90
higher than that of at 0
(70g) is much
(12g), while, in ether, AP is dissoluble at all. So with this
procedure, the productivity of AP is very high and the crystal size of AP is very small,
and the nanometer α-Fe2O3 is dispersed very uniformly in the AP composite.
Reference
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(1995)
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(1995)
[4] Keu Hong Kim and Jae Shi Chol, “Kinetics and Mechanism of the Oxidation of
Sulfur Dioxide on α-Fe2O3”, J. Phys. Chem. 85,2447-2450 (1981)
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[7] J Fenerty, J Pearce and S A Jones, “The Catalytic of Iron( ) Oxide, α-Fe2O3,
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[8] Ozawa T. Bull Chem Soc Japan. 38, 1881 (1965)
[9] H F Zhu. “Catalyst Carrier”, Chemistry Industry Press, Beijing 1980, pp.474479
Size distribution(s)
% in class
1
40
1
30
2
20
2
3
3
10
5 10
50 100
500 1000
Diameter (nm)
Fig.1 Particle diameter of samples
1-UR 2-AC 3-SH
Fig.2 TEM photo of catalyst precipitated by urea
(7.5×104 times)
391.6
395.9
402.0
25
DTA(mW)
20
15
319.2
10 1
321.7
320.5
328.9
5 2
0 3
469.4
0
-5
-10
200
250
300
350
400
450
500
T ( ¡æ )
Fig.3 DTA curve of AP added catalyst and original AP
1-AP/UR 2-AP/AC 3-AP/SH 0-original AP
Table1 Zeta potential and crystal size of catalyst samples
Catalyst Sample
UR
AC
SH
pH Value
6.87
6.23
6.31
Zeta Potential (mV)
-54.5
-54.0
-51.5
Crystal Size (nm)
25.1
34.2
36.3
BET SA(m2/g)
161.17
95.95
94.55