A novel hydrolysis method to synthesize chromium hydroxide nanoparticles and its catalytic effect in the thermal decomposition of ammonium perchlorate

Article abstract of DOI:10.1016/j.tca.2012.06.021

A procedure for the preparation of spherical Cr(OH)₃ nanoparticles was developed based on the aging of chromium nitrate aqueous solutions in the presence of sodium fluoride, urea, and polyvinylpyrrolidone. Using scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy, the morphological characteristics of Cr(OH) ₃ were controlled by altering the molar ratio of fluoride ion to chromium ion, as well as the initial pH and chromium ion concentration. The prepared nanosized Cr(OH)₃ decreased the temperature required to decompose ammonium perchlorate from 450 °C to about 250 °C as the catalyst. The possible catalytic mechanism of the thermal decomposition of ammonium perchlorate was also discussed.

Full text of DOI:10.1016/j.tca.2012.06.021

Thermochimica Acta 544 (2012) 71–76
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Thermochimica Acta
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A novel hydrolysis method to synthesize chromium hydroxide nanoparticles and
its catalytic effect in the thermal decomposition of ammonium perchlorate
Ping Lia,b, Zhen Zhoua, Hongbin Xua,b,∗, Yi Zhanga,b
a Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
b National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Chinese Academy of Science, Beijing 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A procedure for the preparation of spherical Cr(OH)3 nanoparticles was developed based on the aging of
chromium nitrate aqueous solutions in the presence of sodium fluoride, urea, and polyvinylpyrrolidone.
Using scanning electron microscopy, transmission electron microscopy, and energy dispersive spec-
troscopy, the morphological characteristics of Cr(OH)3 were controlled by altering the molar ratio of
fluoride ion to chromium ion, as well as the initial pH and chromium ion concentration. The prepared
nanosized Cr(OH)3 decreased the temperature required to decompose ammonium perchlorate from
450 ◦C to about 250 ◦C as the catalyst. The possible catalytic mechanism of the thermal decomposition of
ammonium perchlorate was also discussed.
Received 22 February 2012
Received in revised form 16 June 2012
Accepted 19 June 2012
Available online 28 June 2012
Keywords:
Chromium hydroxide
Fluoride ion
Morphological characteristics
Ammonium perchlorate
Thermal decomposition
Catalytic effect
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Ammonium perchlorate (AP) is the most common oxidizer used
in composite solid propellants. The characteristics of its thermal
Chromium hydroxide (Cr(OH)3) nanoparticles have found
potential technological applications, i.e., in nitrogen fertilizer, and
petrochemical engineering catalysis, as a pigment, or as a model
system in colloid science [1–3].
decomposition influence the combustion of these propellants. A
specific feature of the thermal decomposition of AP is its extreme
sensitivity to metal oxide additives, which accelerate the low-
temperature thermal decomposition of AP [10]. The best metal
oxide catalyst is CuCr2O4, which decreases thermal decomposition
temperature of AP from about 450 ◦C to 273 ◦C [11].
The thermal decomposition of AP initially produces NH3 and
HClO4 as intermediates which are catalyzed by metal oxides to
further decompose to NH3, H2O, N2O, NO, N2, Cl2, and so on [12].
Some p-type metal oxides, such as CuO and NiO which catalyze AP
thermal decomposition at 275.8 ◦C and 357 ◦C, respectively, have
been shown to be efficient catalysts of the thermal decomposition
of AP. In addition, recent investigations have shown that nano-
sized particles of metal oxides can increase the burning rate of AP.
The efficiency of this catalytic process increases dramatically when
nanosized oxide particles are used rather than microscale and bulk
particles [13].
According to the literature, the most reactive catalyst for the
decomposition of HClO4 is Cr2O3 [14]. Cr2O3 also serves as an
excellent catalyst for the decomposition of NH3 by catalyzing the
oxidation of NH3 to form N2 and H2O at 240 ◦C [15]. However, Cr2O3
is proved to be a less efficient catalyst of the thermal decomposi-
tion of AP [16] due to its n-type semiconducting structure which is
believed to accept an electron from the perchlorate ion hardly.
In this work, a novel hydrolytic method, using F− instead of
SO42−, was developed to prepare nanosized Cr(OH)3. The spherical
Cr(OH)3
nanoparticles
are
traditionally
synthesized
by hydrolysis of very diluted Cr3+ aqueous2−solutions
(2 × 10−4–1 × 10−3 mol dm−3) in the presence of SO4
at mod-
erate temperatures (60–90 ◦C) for long durations (18 h to several
days) [4,5]. Strengthening methods are also adopted to synthesize
monodispersed Cr(OH)3 particles such as forced hydrolysis and
microwave dielectric heating. For example, Ocan˜a prepared nano-
sized Cr(OH)3 spherical particles by forced hydrolyzing aqueous
solution of Cr(NO3)3 and Na2SO4 in the presence of urea and
polyvinyl pyrrolidone K-30 (PVP) [6]. Gómez et al. synthesized
Cr(OH)3 sub-micro- and nanoparticles by microwave dielectric
heating both chrome alum (KCr(SO4)2·12H2O) solutions and
solutions of Cr(NO3)3–K2SO4 with different initial [Cr3+]/[SO4
]
2−
2−
ratios, at the boiling temperature [7]. The role of SO4
has been
proved to be crucial in the formation of Cr(OH)3 nanoparticles
because the SO42−, acting as a counter ion, forms complexes such
as [Cr2(OH)2SO4]2+ and Cr(OH)SO4 and others that are believed to
constitute units of precursor complexes [8,9].
∗
Corresponding author. Tel.: +86 10 8254 4808; fax: +86 10 8254 4808.
E-mail address: lipinggnipil@home.ipe.ac.cn (H. Xu).
0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.tca.2012.06.021

72
P. Li et al. / Thermochimica Acta 544 (2012) 71–76
Table 1
Composition and morphological characteristics of the prepared precipitates by aging for different initial pH, solutions containing variable amounts of NaF, Cr(NO3)3, urea,
and PVP.
Precipitates
Cr(NO3)3 (M)
F−/Cr3+ mole ratio
Initial pH
PVP (g dm−3
)
Urea (M)
Composition
MorphologicalCharacteristics
Shape
Mean size
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
0.005
0.005
0.005
0.005
0.005
0.02
0
5
20
50
5
5
5
5
5
5
5.0
5.0
5.0
5.0
3.0
5.0
1.0
1.0
5.0
5.0
20
20
20
20
20
20
20
20
–
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
–
Cr, O
Cr, O
Cr, O
Na, Cr, F, O
Na, Cr, F
Cr, O
Na, Cr, F
Na, Cr, F
Cr, O
Aggregated
Spherical
Spherical
Cubic-like
Spherical
Spherical
Spherical
Spherical
Aggregated
Spherical
–
50 nm
500 nm
>2 ␮m
>2 ␮m
500 nm
10 ␮m
>2 ␮m
–
0.02
0.005
0.005
0.005
20
Cr, O
500 nm
Cr(OH)3 nanoparticles were synthesized by altering the molar ratio
of F−/Cr3+, initial pH, and Cr3+ concentration. The catalytic effect of
the prepared Cr(OH)3 nanoparticles on the thermal decomposition
of AP was studied in detail.
solution (pH 10.14), filtered and freeze-dried at −50 ◦C for 24 h. The
concentration of reagents and initial pH in the starting solutions
was systematically varied to analyze morphological characteristics
(shape and size) and the F−-dependence of the particles with regard
to the particle size.
In order to study catalytic effect of the prepared precipitates on
the thermal decomposition of AP, different amounts of the precip-
itates were mixed with saturated aqueous solution of AP; and then
freeze-dried in 10 Pa at −50 ◦C for 24 h. The obtained samples were
sintered in argon atmosphere in a pipe furnace with 10 ◦C min−1
from atmospheric temperature to 500 ◦C.
2. Experimental
All chemicals were analytical grade reagents and used as
received without further purification. Cr(NO3)3, NaF, urea and PVP
were dissolved in distilled water to prepare an aqueous solution. In
a typical synthesis, 100 mL deionized water, 20 mL 0.2 M Cr(NO3)3,
20 mL 0.8 M urea, 20 mL 200 g dm−3 PVP, 40 mL 0.5 M NaF aqueous
solution were added to a Pyrex test tubes (250 cm3) which were
tightly sealed with Teflon caps and placed in a preheated oven at
100 ◦C for 24 h. 0.1 M HNO3 was used to adjust initial pH. After
aging, the dispersions were cooled naturally and then centrifuged at
7000 rpm. The precipitates were washed several times with buffer
The morphological characteristics, quantitative analysis, and
microstructure of precipitates were investigated by field-emission
scanning electron microscopy (SEM), using
a FEI 200/INCA
Oxford (USA/UK), energy dispersive spectroscopy (EDX) and
transmission electron microscopy (TEM) on a JEOL JEM 2010 instru-
ment subjected to an acceleration voltage of 200 kV. Phases of
Fig. 1. SEM and TEM micrograph of precipitates S2, S3, and S4; SAED pattern of precipitate S2; EDX mapping of precipitates S2 and S4 (Cu and Au were attributed to Cu TEM
grid in the TEM detection and high vacuum gold jetting in the SEM measurement, respectively).

P. Li et al. / Thermochimica Acta 544 (2012) 71–76
73
1
(002)
a
a
Nanosized Cr(OH)3
330 oC
(112)
100
-17%
(011)
(004)
0
90
80
252 oC
-11.7%
(-312)
(-211)
(222)
-1
-2
(111)
S4
-1.8%
70
100 oC
S2
60
50
100 150 200 250 300 350 400 450 500
Temperature(ºC)
10
20
30
40
50
60
70
80
90
2θ
500
400
300
200
b
Cr2O3
550
b
(110)
(104)
293
(113)
S4
540
534
(300)
S3
S2
10
20
30
40
50
60
70
80
90
2 θ
200
400
600
800
1000 1200 1400 1600 1800 2000
c
Raman shift (cm-1)
542 cm-1
Fig. 2. XRD patterns and Raman spectra of precipitates S2, S3, S4.
precipitates and samples were characterized by powder X-ray
diffraction (XRD). Differential thermal analysis (TGA) and differen-
tial scanning calorimetry (DSC) of precipitates and samples were
300 cm-1
343 cm-1
1359 cm-1
602 cm-1
performed in argon atmosphere at a heating rate of 10 ◦C min−1
.
Raman spectra of the precipitates were obtained with a laser Raman
apparatus with the 514.5 nm line of an Ar+ laser as the excitation
source. The elemental composition of the precipitate was carried on
an Optima 5300DVPerkin-Elmer inductively coupled plasma (ICP-
AES).
0
200 400 600 800 1000 1200 1400 1600 1800 200
Wavenumber (cm-1)
3. Results and discussion
Fig. 3. (a) TG and DSC curves of precipitate S2. (b and c) XRD pattern and Raman
Table 1 summarizes the effects of the F−/Cr3+ molar ratio, initial
pH, urea, PVP, and Cr(NO3)3 concentrations, on the morpholog-
ical characteristics and composition of the particles precipitated
via hydrolysis route. Low Cr(NO3)3 concentration, suitable F−/Cr3+
molar ratio, and high initial pH are beneficial to preparing nano-
sized particles. Through the aging of solutions with the F−/Cr3+
molar ratio at 5, the Cr(NO3)3 concentration maintained at 0.005 M,
and the initial pH (5.0) constant, nanoparticles with a spherical
50 nm were synthesized (S2). The morphological characteristics
of the particles were as follows: spherical, 500 nm and cubic-like,
2 ␮m, under the condition of the F−/Cr3+ molar ratio of 20 (S3) and
50 (S4). The spherical particle size growth resulted from the ini-
tial pH decreased from 5.0 to 1.0 and the Cr(NO3)3 concentrations
spectra of sample calcined by precipitate S2 at 360 ◦C.
increased from 0.005 M to 0.2 M, implying that size of the particles
was sensitive to the reaction conditions. It should be noted that the
solutions without NaF or PVP addition yielded aggregated particles
(S1 and S9) whereas for without urea addition spherical particles
with 500 nm (S10) were obtained. PVP as capping agent and urea
as release precipitation agent, have been used to prepare a lot of
nanomaterials such as Fe2O3 microcages and BaCO3 nanoparticles
[17,18].
The composition of precipitates synthesized in this work was
confirmed by EDX, as can be seen in the attached EDX of Fig. 1.

74
P. Li et al. / Thermochimica Acta 544 (2012) 71–76
Fig. 4. Morphological evolution of precipitates with different F−/Cr3+ molecular ratios: (a) S1; (b) Cr(NO3)3 = 0.005 M, NaF = 0.01 M (F−/Cr3+ molecular ratio = 2); (c) enlarged
photograph of (b); (d) S2.
The nanoparticles (S2) were consisted of Cr and O, which were
obtained by the F−/Cr3+ molar ratio of 5 and the initial pH of 5.
The microparticles (S5, S7, and S8) were consisted of Na, Cr and F
which were determined by the lower initial pH (≤3). The micropar-
ticles (S4) were found to possess higher content of F and Na, as
well as the presence of O, Cr, which resulted from higher con-
tent of F−/Cr3+ molar ratio (50). Besides, Cu and Au also could
be found in the EDX mappings of Fig. 1S2 and S4, which were
attributed to Cu TEM grid in the TEM detection and high vacuum
gold jetting in the SEM measurement, respectively. Not any Cu or Au
which was sensitive to catalytic ability was added in the precipitate
preparation.
Selected area electron diffraction (SAED) of the S2 is also pro-
vided in Fig. 1. It was shown that the isolated Cr(OH)3 nanoparticle
was a totally disordered structure. Besides, chemical composition
of the S2 was investigated by ICP-AES, Cr content of the nanosized
Cr(OH)3 was about 46%, close to 50.48% of theoretical value; not
other metal such as Cu, Ni, and Au, was detected.
The Raman spectra of these precipitates are shown in Fig. 2b.
One band at 293 cm−1 of the S4 was observed and characteristic
of Na3CrF6 which could be inferred from XRD pattern. One band
each at 534 cm−1 of the S2, 540 cm−1 of the S3, and 540 cm−1 of the
S4 was observed. The Raman band in the 530–550 cm−1 was of Eg
symmetry in the octahedral crystal field of chromium compounds
[20]. This type of band shifted toward higher wavenumbers which
indicated reduction of interatomic distance, increase in the inter-
action force between adjacent atoms, and implied that the higher
F−/Cr3+ molecular ratio contributed to better crystallinity as a result
of chemical bond formation within the synthesized particles.
DSC and TG analysis of the S2 are shown in Fig. 3a. The obtained
DSC curve revealed a weak endothermic peak at about 100 ◦C asso-
ciated with a weight loss of 17% that was attributed to the release
of absorbed water. The endothermic peak (252 ◦C) and its associ-
ated weight loss detected between 250 and 300 ◦C (11.7%) could
be ascribed to removal of crystal water. The endothermic peak
(330 ◦C) observed on the TG curve corresponded to crystalline ␣-
Cr2O3 manifesting the crystallization of the initially amorphous
Cr(OH)3 that the crystallization process normally occurred at least
at 450 ◦C [21]. The TG and DSC curves showed that the S2 had the
lower crystalline formation temperature, which could be attributed
to the nanometer range of Cr(OH)3 particles.
Fig. 2a shows XRD patterns of the S2 (F−/Cr3+ molecular ratio
of 5) and the S4 (F−/Cr3+ molecular ratio of 50) obtained by aging
0.005 M Cr(NO3)3 at the initial pH of 5.0. The S2 showed amor-
phous phase, which coincided with chromium hydroxide (Cr(OH)3)
[19]. Compared to the S2, peaks of the S4 could be clearly indexed
as a cryolite phase, which was consistent with the reported data,
for Na3CrF6 (JCPDS 27-0675). It can be concluded that the F−/Cr3+
molecular ratio increase results in not only particle size growth but
also phase transition.
Fig. 3b and c shows the XRD pattern and Raman spectra of cal-
cination product of the S2 at 360 ◦C. The diffraction peaks of the
XRD pattern were consistent with JCPDS-01-084-1616 of the Cr2O3
with bad crystallization. Bands of the Raman spectra at 300 cm−1
,

P. Li et al. / Thermochimica Acta 544 (2012) 71–76
75
o
256 C
x
X
o
x
250 oC
288 C
a
B7-100 % Cr(OH)3
A6-AP+30% nanosized Cr(OH)3
x
245 oC
318oC
X
X
261 oC
A5-AP+30% sub-micro- Cr(OH)3
B6-50 % Cr(OH)3
B4-20 % Cr(OH)3
x
410oC
347oC
X
298 oC
A4-AP+5% nanosized Cr2O3
x
X
o
x
258 oC
429 C
A3-AP+2%P2
A2-AP+2%P1
332 oC
X
429oC
339oC
B3-10 % Cr(OH)3
309oC
261 oC
X
x
x
x
X
418 oC
450oC
336 oC
X
B2-5 % Cr(OH)3
B1-1 % Cr(OH)3
100
x
X
440 oC
245oC
A1-Pure AP
200
300
400
500
600
50
100
150
200
250
300
350
400
450
500
Temperature (ºC)
Temperature(ºC)
Fig. 5. DSC curves of thermal decomposition of AP as it is catalyzed by different
amounts of Cr2O3 and Cr(OH)3. A1: pure AP; A2: AP mixed with 2% P1 (commercial
Cr2O3 prepared by thermal decomposition of CrO3); A3: AP mixed with 2% P2 (com-
mercial Cr2O3 prepared by reducing Na2CrO7 with (NH4)2SO4); A4: AP mixed with
5% nanosized Cr2O3; A5: AP mixed with 30% sub-micro-Cr(OH)3 (S3); A6: AP mixed
with 30% nanosized Cr(OH)3 (S2).
b
100
80
60
40
20
0
B1
343 cm−1, 542 cm−1, 600 cm−1, and 1359 cm−1 were characteristic
of Cr2O3 [22]. Compared to the bands of Cr2O3 reported by liter-
ature, a low-frequency shift of 5–10 cm−1 was observed, such red
shift effect was due to the greater interatomic distance or nanoef-
fect disappearance.
B6
B7
B3
Fig. 4 shows the morphology evolution of the prepared nanopar-
ticles with different F−/Cr3+ molecular ratios. Without the addition
of F− ion, hydroxylation process of chromium ion occurred, poly-
merization of the precipitated precursors resulted in coagulation
(Fig. 4a). By controlling the F−/Cr3+ molecular ratio in the range of
2–10, spherical particles with size between 40 and 80 nm could be
obtained (Fig. 4b and d). Complex ions of chromium fluoride, such
50
100
150
200
250
300
350
400
450
500
Temperature (ºc)
Fig. 6. DSC and TG curves of thermal decomposition of AP as it is catalyzed by
different amounts of nanosized Cr(OH)3. B1, B2, B3, B4, B6, and B7: AP mixed with
1%, 5%, 10%, 20%, 50%, or 100% nanosized Cr(OH)3, respectively.
as CrF2+, CrF2+, and CrF3 [23,24], were formed and became dom-
0
inating; these complex ions of chromium fluoride contributed to
the separation of coagulated material and obtaining of nanoparti-
cles. Every particle that was approximately 50 nm in size consisted
of several subunits that were approximately 5–10 nm in size, as
shown in Fig. 4c. The F− ion is believed to have a tailoring effect,
literature [10]. With adding 5% commercial nanosized Cr2O3, the
thermal decomposition temperature of AP decreased from 450 to
410 ◦C. It is difficult to further decrease the thermal decomposition
temperature of AP with addition of higher concentration commer-
cial or nanosized Cr2O3. However, when the AP was mixed with the
30% prepared sub-micro-Cr(OH)3 (S3) and the 30% prepared nano-
sized Cr(OH)3 (S2), the thermal decomposition temperature of AP
decreased from 450 to 318 ◦C (A5) and 288 ◦C (A6), respectively,
indicating that the prepared Cr(OH)3 catalyzed thermal decompo-
sition of AP effectively.
The decomposition pattern of AP with the nanosized Cr(OH)3
addition shows noticeable changes. The DSC curves of the thermal
decomposition of AP catalyzed by different amounts of the nano-
sized Cr(OH)3 are shown in Fig. 6a. Addition of 5%, 10%, 20%, 50%,
or 100% nanosized Cr(OH)3 lowered the thermal decomposition
temperature of AP by 32 ◦C, 118 ◦C, 152 ◦C, or 200 ◦C, respectively.
As the amount of nanosized Cr(OH)3 increased, the two exother-
mic peaks that were attributed to the partial decomposition of
AP and formation of some intermediates gradually shifted toward
lower temperatures. Especially, the two exothermic peaks were
overlapped at about 250 ◦C with the 100% Cr(OH)3 addition in the
thermal decomposition of AP, which was close to the crystallo-
graphic transition temperature.
2−
different from the SO4 ion that induces the formation of units of
precursor complexes reported in the literature [8].
When used as catalytic additives, metal oxides mainly acceler-
ate or decelerate the low-temperature thermal decomposition of
AP. Cr2O3 has little catalytic effect on the AP thermal decomposi-
tion, due to its n-type semiconducting structure, compared to other
metal oxides such as CuCr2O4, MnO2, and NiO. The DSC curves for
pure AP, AP with addition of Cr2O3 and nanosized Cr2O3, and AP
with addition of sub-micro-Cr(OH)3 particle (S3) and nanosized
Cr(OH)3 particles (S2) are shown in Fig. 5. The DSC curve of the
thermal decomposition of pure AP (A1) included one endothermic
peak at 245 ◦C (i.e., the crystallographic transition temperature)
and one exothermic peak at approximately 450 ◦C. The endother-
mic DSC peak at 245 ◦C was also accompanied by zero weight loss.
This represents the transition from orthorhombic to cubic AP [25].
The heat of decomposition of pure AP was 546 J g−1. The thermal
decomposition temperature of AP decreased from 450 to 429 ◦C,
with addition of 2% commercial Cr2O3 P1 or P2 which was prepared
by thermal decomposition of CrO3 or by reducing Na2CrO7 with
(NH4)2SO4 (A2 or A3); the exothermic peaks at 339 ◦C and 309 ◦C
were attributed to the partial decomposition of AP and forma-
tion of some intermediates namely NH3 and HClO4 by dissociation
and sublimation [26] which were identical with those reported in
Higher content of nanosized Cr(OH)3 addition is also beneficial
for accelerating thermal decomposition rate of AP. Fig. 6b shows TG

76
P. Li et al. / Thermochimica Acta 544 (2012) 71–76
(3) Catalytic mechanism of thermal decomposition of AP showed
that the Cr(OH)3 nanoparticles were beneficial to catalyzed
oxidation of NH3 so that thermal decomposition of AP was
accelerated.
Acknowledgments
This work was financially supported by the National Nature Sci-
ence Foundation of China (No. 50904058) and the National High
Technology Research and Development Plan of China (863 plan,
No. 2011AA060702).
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2NH3 + 2O2 → N2O + 3H2O
(1)
NO, Cl2 and H2O were then formed simultaneously:
4NH3 + 5O2 → 4NO + 6H2O
(2)
(3)
2HClO4 → H2O + Cl2 +(7/2)O2
Combined with the decomposition products of pure AP and
the AP catalyzed by the nanosized Cr(OH)3, it could be presumed
that the oxidization of NH3 is the rate-limiting step. The cat-
alyzed oxidation of NH3 could be accelerated via the nanosized
Cr(OH)3, forming the three reaction products: N2, N2O, and NO
[27], resulting in decrease of thermal decomposition temperature
of AP.
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4. Conclusions
A novel method to synthesize Cr(OH)3 nanoparticles and
its catalytic effect of AP was studied, leading to the following
conclusions:
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(1) Spherical Cr(OH)3 nanoparticles could be obtained by aging at
100 ◦C aqueous solutions of Cr(NO3)3 (0.005 M) in the pres-
ence of NaF (F−/Cr3+ molar ratio = 5), urea (0.2 M) and PVP
(20 g dm−3) with initial pH of 5. The chromium fluoride com-
plex ions such as CrF2+, CrF2+, and CrF3 which formed with F−
0
and Cr3+, tailored the aggregation to form the Cr(OH)3 nanopar-
ticles.
(2) The prepared Cr(OH)3 nanoparticles were found to decrease
the thermal decomposition temperature of AP from 450 ◦C to
250 ◦C, demonstrating that its catalytic effects were determined
by the size of the nanoparticles.