Preparation of g-C3N4 Nanosheets/CuO with Enhanced Catalytic Activity on the Thermal Decomposition of Ammonium Perchlorate

Article abstract of DOI:10.1002/ejic.202001068

The thermal oxidation etching assisted g-C₃N₄ nanosheets/CuO was prepared through a facile co-precipitation strategy. In this work, the structure, morphology, and composition of g-C₃N₄ (UCN, prepared by urea), g-C₃N₄ nanosheets (TCN, prepared by thermal oxidation etching of UCN), g-C₃N₄/CuO (UCN/CuO), g-C₃N₄ nanosheets/CuO (TCN/CuO) were characterized via X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Furthermore, the catalytic effect of the obtained samples on the thermal decomposition of ammonium perchlorate (AP) was examined by thermal gravimetric analysis (TGA). As a result, in the case of 5 wt% TCN/CuO, the high decomposition temperature of AP decreased by 120.6 °C, which is much lower than that of UCN, TCN, CuO and UCN/CuO. In addition, the exothermic heat released from the decomposition of AP increased from 430.64 J g^?1 to 2856.08 J g^?1. This evident catalytic activity may be related to the synergistic effect of CuO and TCN. This work provides a novel strategy for the construction of composite catalyst for the thermal decomposition of AP, which is supposed to possess significant potential in the solid propellant field.

Full text of DOI:10.1002/ejic.202001068

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doi.org/10.1002/ejic.202001068
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Preparation of g-C N Nanosheets/CuO with Enhanced
3 4
Catalytic Activity on the Thermal Decomposition of
Ammonium Perchlorate
Dan Na Ma, Xiao Meng Li,* Xiao Qing Wang, and Yun Jun Luo
[a]
[a]
[a]
[a]
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The thermal oxidation etching assisted g-C N nanosheets/CuO
metric analysis (TGA). As a result, in the case of 5 wt% TCN/CuO,
the high decomposition temperature of AP decreased by
120.6°C, which is much lower than that of UCN, TCN, CuO and
3
4
was prepared through a facile co-precipitation strategy. In this
work, the structure, morphology, and composition of g-C N
3
4
(
UCN, prepared by urea), g-C N nanosheets (TCN, prepared by
UCN/CuO. In addition, the exothermic heat released from the
decomposition of AP increased from 430.64 Jg
3
4
À 1
thermal oxidation etching of UCN), g-C N /CuO (UCN/CuO), g-
to
3
4
À 1
C N nanosheets/CuO (TCN/CuO) were characterized via X-ray
2856.08 Jg . This evident catalytic activity may be related to
the synergistic effect of CuO and TCN. This work provides a
novel strategy for the construction of composite catalyst for the
thermal decomposition of AP, which is supposed to possess
significant potential in the solid propellant field.
3
4
diffraction (XRD), Fourier transform infrared spectroscopy (FT-
IR), X-ray photoelectron spectroscopy (XPS) and transmission
electron microscopy (TEM). Furthermore, the catalytic effect of
the obtained samples on the thermal decomposition of
ammonium perchlorate (AP) was examined by thermal gravi-
Introduction
exothermic heat, which is not beneficial to the operation of
[9]
propellant. Taking this into consideration, the introduction of
catalysts could decrease the high-temperature decomposition
temperature (HTD) of AP effectively. Furthermore, lower HTD
and more exothermic heat may result in a higher burning rate
to some extent.
Solid propellant is an important source of energy for the
propulsion systems of missiles and rockets, which is capable of
converting chemical energy into heat energy by combustion.
The combustion behavior, a main indicator to evaluate the
performance of propellants, determines the rate of energy
release, efficiency, and stability of propellants, which is closely
[
1–2]
To date, numerous catalysts have been developed and
employed in AP and AP-based propellants. Transition metal
[3–4]
[10–15]
[16–17]
related to the operational effectiveness of rocket weapons.
oxides (TMOs),
materials,
carbon materials (CNT and GO),
MOFs
[
18–19]
[20–21]
AP is the main constituent part of propellant formulation and
the most commonly used oxidant, and its thermal decomposi-
tion parameters, especially decomposition temperature and
exothermic heat, are directly connected with the combustion
characteristics of AP-based propellants.
The thermal decomposition mechanism of AP was put
ferrocene and its derivatives
are effective in
catalyzing AP. Based on numerous investigations, there is a
conclusion that various p-type metal oxides, including CuO,
Mn O , Fe O , Fe O , Ce O , MnO , etc, demonstrate consider-
3
4
2
3
3
4
3
4
2
[5–7]
able effect during decomposition of AP. CuO is one of the most
effective catalysts among multitudinous metal oxides, which
could reduce the HTD of AP magnificently. Nevertheless, pure
CuO particles, especially CuO nanoparticles, are extremely
inclined to agglomerate, which seems to have a negative effect
on the combustion of the propellant.
[8]
forward by Jacobs for the first time, AP is decomposed along
with proton transfer to form NH and HClO during the low
3
4
temperature decomposition, and subsequently evaporated into
the gas phase. However, the reaction between the intermediate
NH and HClO is incomplete, and the residual NH may be
Among various two-dimensional materials, graphitic carbon
nitride (g-C N ) has aroused much concern on account of its
3
4
3
adsorbed on the AP surface. The decomposition reaction of the
AP stops until the surface of the AP is entirely coated with NH3.
3
4
[22–27]
high nitrogen content and peculiar structure,
whose
As the temperature continues to rise, the NH is adsorbed on
applications fields are mainly focused on photocatalytic, electro-
3
[28–33]
the AP surface, reacts with HClO again and finally produces
catalytic and heterogeneous catalytic
fields. In addition, it
4
volatile products such as N O, NO, H O, HCl, etc. However, pure
could be readily obtained from various nitrogen-rich precursors
2
2
[
34–38]
AP usually has higher decomposition temperature and lower
including melamine, urea, thiourea, dicyanamide, etc.
Recently, apart from a promising metal-free catalyst, g-C N has
3
4
also been considered as a carrier for loading nanoparticles that
tend to agglomerate.
For the first time, Peng and coworkers found that g-C N4
could be employed as a novel and effective catalyst for AP and
[
a] D. N. Ma, Prof. Dr. X. M. Li, Prof. Dr. X. Q. Wang, Prof. Dr. Y. J. Luo
School of materials science and engineering,
Beijing Institute of Technology,
Beijing 100081, China
E-mail: 13651278705@139.com
3
a possible mechanism that g-C N is prone to meet the
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202001068
3
4
[
39]
requirement of thermal excitation was illustrated in detail. In
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particular, the HTD of AP was sharply decreased from 454.4°C
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to 384.4°C, and the exothermic heat was increased from
À 1
À 1
5
74.2 Jg to 1362.6 Jg with the existence of 10 wt% obtained
g-C N . Tan et al. prepared g-C N /CeO nanocomposites by
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combination the g-C N and CeO2 via an ordinary mixing-
3
40]
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[
calcination strategy. On the one hand, g-C N is conducive to
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the dispersion of CeO2 nanoparticles by preventing their
agglomeration; on the other hand, the composites can
accelerate the thermal decomposition of AP by a synergistic
effect. After that, Peng et al. successfully prepared g-C N / SnO
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hybrids. SnO , as an excellent semiconductor, was introduced
2
[41]
on g-C N achieving a synergistic effect. Previous research has
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4
demonstrated that higher specific surface area (SBET) is useful for
the dispersion of nanoparticles. Tan et al. prepared g-C N /CuO
3 4
Scheme 1. Schematic illustration of the synthesis procedure for g-C N /CuO.
3
4
composites with melamine as a precursor then the HTD of AP
[42]
was reduced by 105.5°C. Xu et al. used silica as a template,
2
À 1
which increased the
4
S
of g-C N from 3.2 m g
to
as UCN-520, UCN-550, UCN-580, MCN-520, MCN-550 and MCN-
580, respectively.) The specific surface areas of the six
BET
3
4
2
À 1 [43]
9.8 m g . But, in most studies, melamine and dicyandia-
mide are commonly used as precursors, where dicyandiamide is
harmful to human and the environment, and the g-C N
synthesized samples were surveyed by N adsorption-desorp-
2
tion isotherms. The SBET of MCN-520, MCN-550, MCN-580, UCN-
3
4
produced from them usually possess poor specific surface area.
Urea is a familiar, low-cost material befitting for large-scale
manufacture that only contains carbon, nitrogen, oxygen, and
hydrogen elements. Urea provides a self-sufficient gas environ-
ment by generating a large amount of gas which contributes to
the formation of porous and fluffy structure during the heat
treatment process. Therefore, the SBET of g-C N prepared from
520, UCN-550, and UCN-580 were calculated to be 5.5, 9.4, 19.2,
2
À 1
38.2, 55.0, and 90.9 m g (Figure S1), which indicated that the
specific surface area can be remarkably improved by porous
structures when adopting urea as a precursor. As can be seen
from Figure 1, the crystal structures of UCN, TCN, UCN/CuO,
TCN/CuO, and CuO were shown in XRD patterns. The peaks at
13.3° and 27.3° correspond to the plane of (100) and (002) of g-
C N , which were consistent with inter-layer structural packing
3
4
urea should be superior to that fabricated from melamine.
Herein, UCN was prepared after the pyrolysis of urea
without any template and additive assistance at the beginning.
Subsequently, thermal oxidation etching strategy was intro-
duced to prepare TCN with larger area. Eventually, CuO particles
were precipitated on the surface of g-C N nanosheets through
3
4
motif and characteristic inter-planar staking peaks of aromatic
systems, respectively. After thermal oxidation etching, the
characteristic peaks of g-C N have no significant change. In
3
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addition, the XRD patterns of CuO tested in this study was in
accordance with the monoclinic phase of CuO (JCPDS05-0661).
Both the diffraction peaks of g-C N and CuO were found in the
3
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co-precipitation achieving uniform dispersion. We have con-
ducted a systematic investigation on the composite catalyst
and the TGA analysis indicated that this composite material
showed the enhanced catalytic performance for AP. Further-
more, the possible catalytic mechanism was also proposed in
detail.
3
4
X-ray diffraction patterns of UCN/CuO and TCN/CuO nano-
composites, which confirmed the existence of g-C N and CuO
3
4
in hybrids.
Results and Discussion
Fabrication, structure and morphology of samples
The illustration of the preparation of CuO/g-C N4 composite
3
was shown in Scheme 1.
The g-C N /CuO composite was fabricated via a co-precip-
3
4
itation approach. Firstly, cupric nitrate trihydrate was added to
the g-C N suspension. Then, the pH of the suspension was
3
4
adjusted to alkalescence with NaOH after well mixture. Finally,
the powdery composite with pale yellow was obtained by
freeze-drying. And the powder was calcined at 200°C for 2h to
obtain TCN/CuO.
Two precursors, urea and melamine, were used to prepare
g-C N at 520, 550, and 580°C, respectively. (g-C N obtained by
3
4
3
4
Figure 1. XRD patterns of UCN, TCN, CuO, UCN/CuO, and TCN/CuO.
heating urea or melamine at 520, 550 and 580°C was denoted
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FTIR spectroscopy was used to identify the composition
accumulation of hydrogen bonds on the long chains of
polymerized melon units after thermal oxidation etching in the
nanosheet layer. As to pure CuO, the absorption peak centered
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and identification of the five samples. As exhibited in Figure 2,
À 1
the characteristic peak at 811 cm was detected in UCN, which
À 1
was attributed to the vibration of s-triazine ring. The peak at
at 514 cm was a typical signal of CuÀ O vibration. Especially,
À 1
1
641 cm was associated with stretching vibration of C=N
all characteristic peaks appeared at CuO, UCN or TCN can be
also found in the corresponding composite of CuO and g-C N ,
bonds, and due to the NÀ H stretching, there exist wide peaks
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À 1
À 1
À 1
among 3000–3500 cm . The peaks at 1575 cm , 1465 cm ,
which further manifests that the successful fabrication of UCN/
CuO and TCN/CuO hybrids.
À 1
À 1
À 1
1
414 cm , 1311 cm and 1232 cm were on account of the
symbolic stretching vibration of CN heterocycles. After thermal
oxidation, the position of the characteristic peaks had no
significant change between TCN and UCN. However, these
bands of TCN became sharper, possibly due to the more orderly
XPS was carried out to test the valence states and bonding
interactions of UCN, TCN, UCN/CuO, and TCN/CuO. The full scan
XPS spectra of the samples (Figure 3a) manifested that C and N
atoms were detected in UCN and TCN. C, N, Cu as well as O
were found in UCN/CuO and TCN/CuO. In Figure 3b, the Cu 2p
spectra manifested two dominant peaks located at 934.3 eV (Cu
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p 3/2) and 954.1 eV (Cu 2p 1/2), suggesting that the oxidation
state of Cu in the sample is +2. Figure 3c and Figure 3e
revealed the C1s spectra of UCN and TCN/CuO. And the peak at
2
84.6 eV and 288.2 eV might be related to the CÀ C coordination
2
of the surface adventitious carbon and the sp -hybridized C of
the C=NÀ C structure, respectively. The three major peaks of N1s
in Figure 3d at 398.2 eV, 398.6 eV and 400.5 eV can be
attributed to the N atom on (C=NÀ C), (NÀ (C) ), and (C=NÀ H) in
3
the triazine units, separately. Nevertheless, the high-resolution
N1s spectrum of TCN/CuO appeared a significant difference
(
4
Figure 3f). In particular, the peaks at 398.3 eV, 399.2 eV, and
00.7 eV of TCN/CuO showed a positive shift, reflecting the
interaction between CuO and g-C N .
3
4
The reason that the volume of TCN was much larger than
that of UCN on the condition of the same weight might be
explained by the fluffy state of the TCN. Moreover, the
Figure 2. FT-IR spectra of UCN, TCN, CuO, UCN/CuO, and TCN/CuO.
Figure 3. (a) XPS spectra of UCN, TCN, UCN/CuO and TCN/CuO, (b) core-level Cu 2p of TCN/CuO, (c) high resolution C1s spectra of UCN, (d) high resolution
N1s spectra of UCN, (e) high resolution C1s spectra of TCN/CuO, (f) high resolution N1s spectra of TCN/CuO.
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Figure 4. TEM images of (a–b) UCN, (c) TCN, (d) CuO and (e) TCN/CuO.
microstructure and morphology of as-prepared TCN and UCN
were investigated with TEM (Figure 4a–c). UCN with irregular
shapes shown a 2D structure composing of small sheets. The
obtained UCN has an obvious porous structure when urea was
used as the precursor rather than melamine. CuO in the shape
of nanorods were homogeneously dispersed in UCN/CuO and
TCN/CuO composites while pure CuO showed obvious agglom-
eration behavior (Figure 4d). After the process of thermal
oxidation etching, TCN had a larger specific surface area, which
was more favorable to the dispersion of CuO (Figure 4e).
Thermal decomposition of AP in the presence of catalysts.
5
.0 wt% of as-prepared catalysts were mixed well with AP to
investigate their catalytic behavior systematically. TG and DTG
curves of the samples prepared with urea and melamine at
different temperatures were shown in Figure S2 and Figure S3,
Figure 5. DSC curves of pure AP, AP+UCN, AP+TCN, AP+CuO, AP+UCN/
CuO, AP+TCN/CuO.
which could reflect the catalytic effect of g-C N4 on the
3
decomposition of AP. As is shown that the g-C N produced by
3
4
urea displayed an immensely excellent catalytic activity than
was consistent with the crystallographic transition of AP. At this
point, the AP changed from orthorhombic phase to cubic
phase. As the temperature raised further, two exothermic peaks
at 302.3°C and 424.5°C were attributed to the low-temperature
decomposition (LTD) and HTD, respectively. The LTD stage
happened when a little of AP decompose and the formation of
intermedia products. While the HTD means total decomposition
of AP, during which enormous gases were released. The
addition of the catalyst caused significant changes in the high
that by melamine. Among them, g-C N prepared at 550°C had
3
4
the best catalytic effect. Hence, UCN, TCN, UCN/CuO, and TCN/
CuO stand for the samples annealed at 550°C.
Subsequently, TGA-DSC was employed to evaluate the
catalytic effect of UCN, TCN, CuO, UCN/CuO, and TCN/CuO on
the thermal decomposition of AP. There were three evident
peaks for the decomposition of AP without catalysts (Figure 5).
The single endothermic peak was centered at 243.9°C, which
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and low decomposition temperatures without affecting the
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crystal transition temperature. When treated with TCN/CuO,
UCN/CuO, CuO, TCN and UCN, the HTD temperature of AP
dropped by 120.6°C, 109.3°C, 97.4°C, 22.6°C, 40.3°C and the
exothermic heat was raised to 2856.08, 2570.64, 1853.98,
À 1
1
137.62, 1225.97 Jg (Figure 6), separately, showing that TCN/
CuO displayed tremendous catalytic effect, which was ascribed
to the larger SBET of TCN and well dispersion of CuO. Curiously,
although the specific surface area of carbon nitride increased
after thermal oxidation etching, the catalysis decreased. This
suggests that specific surface area is not the only factor
determining the catalytic effect. In the presence of pure CuO,
the LTD and HTD were combined into a single peak, which was
centered at 327.1°C.
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Furthermore, TGA and DTG analysis of AP and its mixtures
of catalysts were illustrated in Figure 7 and Figure 8, respec-
tively, which was in keeping with the result of DSC. For pure AP,
Figure 8. DTG curves of AP mixed with CuO, UCN, TCN, UCN/CuO, TCN/CuO
and pure AP.
there were two weight loss stages from 30 to 500°C,
corresponding to the LTD and HTD stage. The stage of HTD
moved towards low temperature in the existence of UCN, TCN,
CuO, UCN/CuO and TCN/CuO. However, there was only one
stage appeared in the case of CuO, UCN/CuO and TCN/CuO
because of the coincidence of the LTD and HTD stages. It was
worth noting that a prominent reduction of decomposition
temperature was realized on account of the synergistic effect of
CuO and TCN when TCN/CuO composite was employed to
catalyze AP.
Catalytic mechanism
À
The conduction-band electrons (e ) and valence band holes (h
+
Figure 6. Heat released during the decomposition of AP with and without
catalysts.
) could be produced on the surface of g-C N under photo-
3
4
À
excitation or thermal excitation. Then, the obtained e can
combine with HClO molecules to generate O . Subsequently,
the O2 and h could react with NH4 , promoting the process
of proton transfer. Additionally, g-C N , as a Lewis base, could
*
À
2
4
*
À
+
+
3
4
react with HClO , leading to the promotion of thermal
4
decomposition of AP. Unfortunately, the defection of poor SBET
and high electron-hole recombination rate commonly exist in
À
bulk g-C N , which has a negative effect on the transfer of e .
3
4
CuO, as a transition metal oxide, is quite efficient in catalysis
of AP thermal decomposition. Compared with other metal
oxides, the catalytic superiority of the CuO could be explained
by the fact that the 3d orbital of CuO is partially filled, which
+
2
+
9
prompted Cu (3d ) to transform into the stable full-filled Cu
1
0
(
3d ). However, the CuO particles tend to agglomerate and
have trouble in dispersing adequately in AP.
À
Once combining the CuO with the g-C N , the generated e
3
4
on g-C N would transfer to CuO, which facilitates the
3
4
À
+
separation of electrons e and h . During the decomposition
process of AP, HClO4 could be reduced by e , causing the
production of ClO , HCl and·O . Finally, NH could be oxidized
À
Figure 7. TG curves of AP mixed with CuO, UCN, TCN, UCN/CuO, TCN/CuO
and pure AP.
À
3
2
3
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À
+
by·O2 and h with powerful oxidation ability. The BET surface
areas of UCN and TCN were calculated to be 90.9 and
content of NO2 increased with the rise of temperature,
indicating that part of NH was oxidized to NO gradually, which
signified more violent decomposition of AP.
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4
5
6
7
8
9
0
1
2
3
4
5
6
7
3
2
2
À 1
2
70.7 m g
(Figure S4), respectively, manifesting that the
thermal oxidation etching process could separate the layered
structure significantly. The principle of this approach is that the
hydrogen-bond chains (especially the short ones) of polymeric
melon unites were unstable for the oxidation process, and will
be oxidized from bulk g-C N inch by inch, leading to a
significant increase in the specific surface area. After thermal
oxidation, TCN processed a larger SBET, weakening the agglom-
eration of CuO particles to a certain extent.
According to the obtained result, it can be inferred that the
catalyst promoted the thermal decomposition of AP principally
through the HTD step, which could be reflected by the
significant reduction of the decomposition temperature. In a
word, the catalytic mechanism of the thermal decomposition of
AP with g-C N /CuO was illustrated in Figure 11.
3
4
3
4
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
In order to reveal the thermal decomposition of AP, TG/IR, Conclusions
as a real-time instrument, was employed to analyze the gas
products during AP decomposition. Three-dimensional FTIR
spectra of decomposed gas products was shown in Figure 9 to
acquire a visualized view. The FT-IR spectra were also singled
out to analyze the composition of decomposed gas products
In summary, the composite of TCN/CuO was successfully
prepared by a facile strategy without introducing any surfac-
tants or templates. Concretely, we obtained g-C N nanosheet
3
4
by thermal oxidation etching bulk g-C N (UCN), which
3
4
(
Figure 10). For the AP mixed with TCN/CuO, the main gas
endowed the TCN with a higher specific surface area. The larger
specific surface area of TCN can effectively restrain the
aggregation of CuO, and the synergistic effect resulted from the
integration of TCN and CuO excellently enhance the catalytic
activity for AP. It did not only increase the decomposition rate
of AP, but also enhanced the decomposition depth. This work
provides a novel thought to construct composite catalyst with
improved performance for AP and is of great importance for
the combustion of AP-based propellants.
À 1
products were known as NH (3400–3800 cm ), HCl (2644-
3
1
3
À 1
À 1
À 1
093 cm ), H O (1400–1650 cm ), N O (2127–2295 cm ,
2
À 1
2
À 1
220–1340 cm ) and NO (1533–1666 cm ). At HTD stage, the
2
Experimental Section
Preparation of g-C N with urea and melamine at different
3 4
temperatures were illustrated in supporting information.
Preparation of g-C N nanosheets (TCN)
3
4
TCN was fabricated by thermal exfoliation of the bulk g-C N4
3
obtained as follows: 2.0 g of g-C N (UCN-550) was tiled on the
3
4
Figure 9. Three-dimensional FTIR spectra.
bottom of the quartz boat, making it fully contact with the air.
Afterwards, the powder was heated to 550 C for two hours at a
heating rate of 5 k/min. A lighter powder was prepared and
denoted as TCN.
°
Figure 10. FT-IR spectra of decomposed gas products during the decom-
position of AP under the catalysis of TCN/CuO.
Figure 11. Catalytic mechanism of the thermal decomposition of AP with g-
C
3 4
N /CuO hybrids.
Eur. J. Inorg. Chem. 2021, 982–988
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987
© 2021 Wiley-VCH GmbH

Full Papers
doi.org/10.1002/ejic.202001068
Preparation of g-C N /CuO hybrids (TCN/CuO)
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[
X-ray powder diffraction (XRD) patterns were recorded by BRUCKER
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[
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Conflict of Interest
2
The authors declare no conflict of interest.
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Keywords: Thermal
composites Thermal decomposition of ammonium
perchlorate · Catalytic effect · Catalytic mechanism
oxidation
etching
·
g-C N /CuO
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Manuscript received: November 24, 2020
Revised manuscript received: December 31, 2020
Accepted manuscript online: January 4, 2021
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Eur. J. Inorg. Chem. 2021, 982–988
www.eurjic.org
988
© 2021 Wiley-VCH GmbH