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Thermal decomposition of ammonium perchlorate in the presence of Cu(OH)2·2Cr(OH)3 nanoparticles
Thermal Decomposition of Ammonium Perchlorate in the Presence of Cu(OH)2 · 2Cr(OH)3 nanoparticles Xiaodan Zheng, Ping Li, Sili Zheng, Yi Zhang PII: DOI: Reference:
S0032-5910(14)00738-4 doi: 10.1016/j.powtec.2014.08.038 PTEC 10483
To appear in:
Powder Technology
Received date: Accepted date:
17 May 2014 15 August 2014
Please cite this article as: Xiaodan Zheng, Ping Li, Sili Zheng, Yi Zhang, Thermal Decomposition of Ammonium Perchlorate in the Presence of Cu(OH)2 · 2Cr(OH)3 nanoparticles, Powder Technology (2014), doi: 10.1016/j.powtec.2014.08.038
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Thermal Decomposition of Ammonium Perchlorate in the
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Presence of Cu(OH)2·2Cr(OH)3 nanoparticles
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
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Xiaodan Zheng1,2, Ping Li1,2, Sili Zheng1,2, Yi Zhang1,2
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National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
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Email: [email protected]
Abstract
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The Cu(OH)2·2Cr(OH)3 nanoparticles preparation procedure and its accelerating effect and accelerating mechanism on thermal decomposition of ammonium
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perchlorate (AP) were investigated using transmission electron microscopy (TEM),
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energy dispersive spectroscopy (EDS), selected area electron diffraction (SAED), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR),
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thermogravimetric analysis and differential scanning calorimetry (TG-DSC), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis and mass spectroscopy (TG-MS). In the preparation procedure, TEM, SAED, and EDS showed
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that the Cu(OH)2·2Cr(OH)3 particles were amorphous with nanometer size under the controllable preparation conditions. When the amorphous Cu(OH)2·2Cr(OH)3 nanoparticles were used as additives for the thermal decomposition of AP, the TG-DSC results showed that the addition of Cu(OH)2·2Cr(OH)3 nanoparticles to AP remarkably decreased the onset temperature of AP decomposition from approximately 450 oC to 259 oC. The XRD and FT-IR results confirmed that the Cu(OH)2·2Cr(OH)3 nanoparticles were transformed from amorphous form to Cr2O3 and CuCr2O4 phases after used as additives for the AP thermal decomposition. The XPS results indicated that Cu2+ of the Cu(OH)2·2Cr(OH)3 surface gained the electron from the perchlorate ion of AP, and the Cu ions were enriched on the Cu(OH)2·2Cr(OH)3 surface in the AP decomposition. The electron transfer process plays a major role in the decrease the 1
ACCEPTED MANUSCRIPT onset temperature of AP decomposition, while the nano-effect is beneficial to easier enrichment of Cu ions on the surface of Cu(OH)2·2Cr(OH)3 nanoparticles.
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Key words
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Cu(OH)2·2Cr(OH)3 nanoparticles, Ammonium perchlorate, Thermal decomposition,
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Electron transfer, Nano-effect
1. Introduction
Ammonium perchlorate (AP) is the most common oxidizer used in composite
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solid propellants[1,2]. The characteristics of its thermal decomposition influence the combustion of these propellants: lower high-temperature thermal decomposition
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temperature results in a shorter ignition delay time and a higher burning rate of these propellants[3-5].
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A specific feature of the AP thermal decomposition is its extreme sensitivity to
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metal oxide additives[6,7]. The most commonly used additives are p-type metal oxides, such as CuO, MnO2, Nd2O3 and CuCr2O4 etc.. The CuCr2O4, as the best metal oxide
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additive, decreases the onset temperature of the AP decomposition from about 450 oC to 273℃[8-10]. The thermal decomposition of AP catalyzed by the p-type metal oxides can be widely interpreted by the electron transfer process. In the electron transfer
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process, the rate-controlling step of the thermal decomposition of AP is the transfer of an electron from the perchlorate ion to the positive hole in p-type metal oxides additives. The annihilation of the positive hole in the valence band of a metal oxide enhances the thermal decomposition of AP. Recent investigations have shown that understanding of the behavior of AP thermal decomposition accelerated by nanoparticles is changing due to the particles’ nano-effect. Nansized Fe2O3, CuO nanorod, NiO nanoparticle and Cu-Cr-O nanocomposite[1,11-13], have been shown to be efficient additives for the thermal decomposition of AP. However, interpretation of the relative accelerating behavior is still confusing, for example, accelerating behavior of the CuO nanorod additive in AP decomposition is associated with a proton transfer process, while the accelerating 2
ACCEPTED MANUSCRIPT behavior of the nanosized Fe2O3 and Cu-Cr-O nanocomposite additives are explained by the electron transfer process. Nanosized Cr(OH)3[14] and Cr(OH)3·Al(OH)3[15]
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additives prepared by this laboratory are benefit to accelerating the oxidation of NH3
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in the AP decomposition due to their surface hydroxyl and amorphous form.
be explained by the proton transfer process.
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Accelerating behavior of the nanosized Cr(OH)3 and Cr(OH)3·Al(OH)3 additives can
This study thus aimed to study thermal decompositon of AP in the presence of
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prepared nanosized Cu(OH)2·2Cr(OH)3 which possessed electron transfer and nano-effect characteristics. The preparation procedure of Cu(OH)2·2Cr(OH)3
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nanoparticles was studied. The effect of the prepared Cu(OH)2·2Cr(OH)3 nanoparticles used as additives on the thermal decomposition of AP was studied in
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2. Experimental Section
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detail, and its related accelerating mechanism was also discussed.
All chemicals were analytical-grade reagents and used as received without further
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purification. Cr(NO3)3, CuSO4, Na2SO4, urea, and polyvinylpyrrolidone (PVP) were dissolved in distilled water to prepare aqueous solutions. In a typical synthesis, 20 mL 0.0025 M CuSO4 and 0.005 M Cr(NO3)3, 40 mL
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0.02 M urea and 40 mL 20 g dm−3 PVP, 100 mL 0.18 M Na2SO4 were added to Pyrex test tubes (250 cm3), which were tightly sealed with Teflon caps and placed in a preheated oven at 100 oC for 20 h. After aging, the dispersions were cooled naturally and then centrifuged at 8000 rpm. The obtained precipitates were washed several times with distilled water, filtered, and freeze-dried at –50 oC for 20 h. The concentrations of reagents and Na2SO4 in the starting solutions were systematically varied to analyze morphological characteristics (shape and size) of the precipitates. To study accelerating effect of the prepared precipitates on the thermal decomposition of AP, different amounts of the precipitates were mixed with an ethanol solution of AP, and then dried in an oven at 40 oC for 10 h. The obtained samples were sintered in a nitrogen atmosphere in a furnace at 10 oC min-1 from 3
ACCEPTED MANUSCRIPT ambient temperature to 500 oC. The crystal structure of the precipitates and samples was characterized by powder
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X-ray diffraction (XRD) at diffraction angles (2θ) ranging from 5 to 90◦ and Fourier
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Transform Infrared Spectroscopy (FT-IR) on a Spectrum GX spectrophotometer. The morphological characterization and quantitative analysis of the powders were
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performed by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS) on a JEOL JEM 2100
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(UHR) instrument operated at an acceleration voltage of 200 KV. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the precipitates and
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samples were performed at a heating rate of 10 oC min-1 with Al2O3 as the reference on a Mettler- Toledo TGA/DSC 1 instrument. X-ray photoelectron spectroscopy (XPS) data of the powders were obtained with an ESCALab220i-XL electron spectrometer
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from VG Scientific using 300-W Al Kα radiation. TG-MS analyses were performed in
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a nitrogen atmosphere at a heating rate of 10 oC min-1 from 50 to 500 oC, using a NETZSCH STA449C instrument.
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3. Results and Discussion
3.1 Preparation of Cu(OH)2·2Cr(OH)3 nanoparticles
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Table 1 summarizes the effects of the reagent concentrations, SO42-/Cr3+ molar ratios, urea and PVP on the morphological characteristics of the particles precipitated after aging at 100 oC for 20 h. As observed, low SO42-/Cr3+ molar ratio and suitable reagent concentrations contributed to preparing spherical particles of different size. Typically, spherical nanoparticles (S3) with a diameter of 70 nm were synthesized through the aging of solutions with a SO42-/Cr3+ molar ratio of 1:2, the CuSO4 concentration maintained at 0.0025 M, and the Cr(NO3)3 concentration maintained at 0.005 M. The SO42-/Cr3+ molar ratio required for the formation of such uniform dispersions was even more restrictive, since for values higher than 0.5 either inhomogeneous precipitates (SO42-/Cr3+ molar ratio=2, 5) or ill-defined precipitates (SO42-/Cr3+ molar ratio=20) were observed. The addition of urea to the starting 4
ACCEPTED MANUSCRIPT solutions is observed to be essential in producing precipitates in the above-described systems. No precipitates were obtained without this compound addition. When the
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above solutions were aged in the absence of PVP, ill-defined precipitates were formed,
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indicating that the PVP addition was also needed to form the regular particles.
Precipitate
CuSO4
Cr(NO3)3
PVP -3
Urea
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Table 1 Effects of the reagent concentrations and SO42-/Cr3+ molar ratios on the morphological characteristics of the precipitates prepared by aging solutions of CuSO4 and Cr(NO3)3 at 100 oC for 20 h in the presence of sulfate, urea, and PVP. Mean
SO42-/Cr3+
Particle shape
0.5
Spheres
100-300nm
(M)
(gdm )
(M)
S1
0.0005
0.001
20
0.02
S2
0.0015
0.003
20
0.02
0.5
Spheres
150-600nm
S3
0.0025
0.005
20
0.02
0.5
Spheres
70nm
S4
0.01
0.02
20
0.02
0.5
Agglomerates
—
S5
0.0025
0.005
—
0.02
0.5
Agglomerates
—
S6
0.0025
0.005
20
—
0.5
No precipitation
—
S7
0.0025
0.005
20
0.02
2
Spheres
120-350nm
S8
0.0025
0.005
S9
0.0025
0.005
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(M)
size
0.02
5
Spheres
130-400nm
20
0.02
20
Agglomerates
—
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20
Figure 1 TEM image of S3, EDS mapping and SAED pattern of S3.
The composition of S3 prepared in this study was confirmed by EDS, as shown in the attached EDS of Fig. 1. The nanoparticles (S3) consisted of Cr, Cu, O, and S. Atomic contents of Cu and Cr were 8.54 % and 16.98 %, respectively, and the atomic ratios of Cu and Cr were 1:1.99, which was close to theoretical molar ratio of Cu(OH)2·2Cr(OH)3. The C was also observed in the EDS mapping, which was attributed to the residual carbon on the grid during TEM detection. In addition, the selected area electron diffraction (SAED) pattern of S3 was provided in Fig. 1, 5
ACCEPTED MANUSCRIPT showing that the isolated nanoparticles possessed a completely disordered structure. Overall, the TEM, EDS and SAED indicate that the precipitates are amorphous Cu(OH)2·2Cr(OH)3 particles with a spherical morphology in the size range of 70 to
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600 nm.
3.2 Thermal decomposition of AP in the presence of Cu(OH)3·2Cr(OH)3
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nanoparticles
Different amounts of Cu(OH)2·2Cr(OH)3 nanoparticles are mixed as additives with AP to study the accelerating behavior of the nanoparticles on the thermal
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decomposition of AP. In a typical experiment, 50 g Cu(OH)2·2Cr(OH)3 nanoparticles (S3) with a Cr and Cu mole ratio of 1.99 were mixed with 50 g pure AP to produce
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what was referred to as a 50 % addition.
The TG-DSC curves of pure AP and AP in the presence of different amounts of
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S3 are shown in Figure. 2. For pure AP (A7), an endothermic peak was observed at
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approximately 245 oC, which was assigned to the phase transition of AP from orthorhombic to cubic form due to the rotation of perchlorate ion[16]. AP underwent
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two complicated decomposition processes, a low-temperature stage at 332 oC attributed to the partial decomposition of AP and a high-temperature stage at 450 oC
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caused by the complete decomposition of AP to volatile products.
Figure 2 TG and DSC curves for pure AP and AP mixed with different amounts of S3: A7, A6, A5, A4, A3, A2, A1 represent pure AP and AP mixed with 2%, 5%, 10%, 20%, 50%, 80% S3, respectively.
The decomposition patterns of AP with the addition of S3 show significant 6
ACCEPTED MANUSCRIPT differences. Fig. 2a shows the DSC curves for the thermal decomposition of AP in the presence of different amounts of the S3. The addition of 2 %, 5 %, 10 %, 20 %, 50 %, and 80 % S3 lowered the onset temperature of AP decomposition by 75 oC, 85 oC, 137 C, 154 oC, 189 oC, and 191 oC, respectively. As the amount of S3 increased, the
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exothermic peak that was attributed to the complete decomposition of AP and
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formation of some intermediates gradually shifted towards lower temperatures. In particular, only one exothermic peak was obtained at approximately 259 ℃ with the 80 % S3 addition in the thermal decomposition of AP.
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Fig. 2b shows the TG curves for A1, A2, A4, A6, and A7. Each curve exhibits a weight loss step corresponding to the exothermic peak of the DSC curve. As observed,
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the S3 used as additives for AP decomposition had significantly effect on steps of high-temperature decomposition (HTD) and low-temperature decomposition (LTD) of
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AP decomposition. The weight loss of HTD (LTD) was 35.10% (4.90%), 18.09%
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(41.91%), 90.05% (0), 72.98% (22.05%), 94.58% (0), respectively, with the addition of 0, 2 %, 10 %, 50 %, 80 % S3 for the AP decomposition. A higher content of the S3
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was also observed to accelerate the thermal decomposition rate of AP.
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3.3 Accelerating mechanism
Figure 3 XRD patterns of S3 and R1 which was residues of thermal decomposition of A1 (AP mixed with 80% S3).
The phase, microstructure, and surface characteristics of S3 and R1 which is obtained by residues of A1 (AP mixed with 80% S3) decomposition, are explored to 7
ACCEPTED MANUSCRIPT investigate the accelerating mechanism of the additives. The X-ray diffraction (XRD) patterns of S3 and R1 (Fig. 3) are obtained to
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investigate the phase formation of the samples. The patterns revealed that the S3 was
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amorphous; and the R1 showed peaks representing Cr2O3 (JCPDS no: 38-1479) and CuCr2O4 (JCPDS no: 26-0509) phases. The product of pure Cu(OH)2 or Cr(OH)3
[17-19]
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thermal decomposition at approximately 400-500 oC is CuO or Cr2O3, respectively . In this study, the phase transition of nanosized Cu(OH)2·2Cr(OH)3 from
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amorphous form to CuCr2O4 and Cr2O3 was observed during the indirect thermal decomposition of the nanosized Cu(OH)2·2Cr(OH)3.
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The FT-IR spectra of S3 and R1 are shown in Fig. 4. As observed, the bands at 1636 and 1574 cm-1 of S3 spectrum could be ascribed to the bending modes of the nondissociated water molecules[20]. The bands at 1045 and 1162 cm-1 indicated the
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presence of a small amount of sulfate anions in the S3, which were confirmed by EDS
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analyses. The bands at 1382, 1350 and 833 cm−1 could be attributed to infrared adsorption of carbonate anions. The bands at 524 and 614 cm-1 of R1 spectrum could
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be attributed to the stretching vibrations of Cu-O [21,22] or Cr-O [23], in accordance with
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the XRD results (CuCr2O4 and Cr2O3 phase).
Figure 4 FT-IR spectra of S3 and R1 which was residues of thermal decomposition of A1.
X-ray photoelectron spectroscopy is widely used for studies of surface chemistry because it provides a range of useful information depths, reasonable quantification, and chemically specific information for each element detected through chemical shifts. 8
ACCEPTED MANUSCRIPT To investigate the surface chemical states of the most active additives, a typical wide-energy scan of S3 and R1 provide the XPS spectra with clearly resolved Cu 2p,
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Cr 2p, O 1s, N 1s, and C 1s peaks. As shown in Fig. 5a, the C 1s XPS spectra
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indicated that carbon deposition occurred during charging correction, which resulted in a shift in all other transitions. The presence of nitrogen could be attributed to the
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existence of a nitrogenous ligand bonding with metal or NO3-, derived from the preparation process. The Cr 2p peaks occurred at 575.9- 576.7 eV and 585.8- 586.5
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eV, corresponding to Cr3+, and Cr3+ was the only observed oxidation state (Fig. 5c). The surface of S3 and R1 also exhibited the Cu 2p3/2 and Cu 2p1/2 peaks at binding
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energy of 934.5 and 932.5 eV, respectively, together with intense satellite peaks at 940.2, 942.4, 955 and 962.9 eV (Fig. 5b). All of the Cu 2p spectra of S3 and R1 were fitted using a Gaussian-Lorentzian peak shape and satisfactory fitting results were
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obtained as shown in Fig. 6a and Table 2.
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Figure 5 (a) XPS spectra of S3 and R1; (b) Cu 2p XPS spectra of S3 and R1; (c) Cr 2p spectra of S3 and R1.
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Figure 6 (a) Cu 2p spectra of S3 and R1; the dotted curves represent photo peak contribution from Cu2+, Cu+. (b) O 1s (b) spectra of S3 and R1; the dotted curves represent photo peak contribution from O2-, OH-. Table 2 Cu 2p and O 1s peak parameters for S3 and R1. Peak
Fwhm (eV)
Percent (%)
934.8
2.70
76.96%
Cu+
932.5
2.33
23.04%
OH-
531.2
3.20
72.01%
O2-
529.6
1.10
27.99%
Cu2+
934.5
3.76
100%
Cu+
OH-
531.1
2.00
100%
O2-
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Cu2+
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S3
R1
B.E. (eV)
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Sample
As shown in Fig. 6a, the Cu 2p peaks occurred at 934.5 and 932.5 eV, respectively, which were indicative of the chemical state of Cu2+ and Cu+ [24]. Cu2+ was the main chemical state (100 % and 76.96 %) species, followed by Cu + (0 % and 23.04 %), in S3 and R1, respectively. The Cu+, as the Cu2+ reduced form, proves the electron transfer between Cu2+ and Cu+ of the S3 during the thermal decomposition of AP. Such transfer indicates that the Cu2+ of surface of the Cu(OH)2·2Cr(OH)3 nanoparticles gains the electron from perchlorate ion (HClO4-) in the AP decomposition to form Cu+. Note that the electron transfer plays a key role in the 10
ACCEPTED MANUSCRIPT thermal decomposition of AP in the presence of Cu(OH)2·2Cr(OH)3 nanoparticles. The O 1s peaks are composed of subpeaks with peak energies 530.1-530.3, 531.5-532.0 and 532.8-533.4 (O2-) eV, which are designated as surface O2-, OH- and
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H2O groups, respectively[25]. The O 1s spectra of S3 and R1 were fitted using the above-mentioned Gaussian-Lorentzian peak shape as shown in Fig. 6b and Table 2.
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The O 1s spectra (Fig. 6b) occurred at 529.6 and 531.2 eV, which were indicative of the chemical state of O2- and OH-, respectively. O2- content of the S3 surface increased from 0 to 27.99% after used as additives for the AP thermal decomposition.
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The O2-, representative of oxides, indicates thermal decomposition occurs
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simultaneously between the AP and Cu(OH)2·2Cr(OH)3 nanoparticles, in accordance with the results obtained by XRD and FT-IR.
Cu
S3
5.15
R1
10.02
Cr
O
N
S
10.46
75.75
7.91
0.73
17.6
70.70
1.69
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Atomic ratio %
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Table 3 Data regarding atomic ratios collected from XPS spectra of S3 and R1.
The atomic ratios of Cu, Cr, O, N and S on the surface of S3 before and after the Cu(OH)2·2Cr(OH)3 nanoparticles are used as additives in the AP decomposition are
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summarized in Table 3. The surface of S3 contained 5.15% Cu and 10.46% Cr, whereas the surface of R1 contained 10.02% Cu and 17.6% Cr: the relative Cu and Cr surface atomic ratios of S3 and R1 were 1: 2.03 and 1: 1.76, respectively, demonstrating that the surface atomic content changed a lot before and after used as additives for the AP thermal decomposition. The Cu ions of the S3 surface are enriched in the AP decomposition. Such enrichment of Cu ions not only will be benefit to electron transfer between the Cu2+ and HClO4- of the AP , but also confirms that the accelerating behavior occurs at the surface of nanosized Cu(OH)2·2Cr(OH)3. Unlike the reported accelerating mechanism of AP mixed with nanosized Cr(OH)3 and Cu-Cr-O nanocomposite, the electron transfer is believed to play a major role in the decrease the onset temperature of AP decomposition in the presence of Cu(OH)2·2Cr(OH)3 nanoparticles, while the nano-effect of Cu(OH)2·2Cr(OH)3 11
ACCEPTED MANUSCRIPT nanoparticles such as amorphous form and nanoscale may be beneficial to easier enrichment of Cu on the surface of Cu(OH)2·2Cr(OH)3 nanoparticles. Besides, the N
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and S of S3 surface decreased from 7.91% to 1.69% and from 0.73% to 0%,
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respectively, when the S3 used as additives before and after AP thermal
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decomposition. Note that the SO42-and NO3- were also decomposed.
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Figure 7 Mass spectral analysis of A1.
The mass intensities of molecules that are produced in the thermal
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decomposition of A1 (AP mixed with 80% S3) are also shown in Fig. 7. As observed, the AP decomposed completely within a narrow temperature range, the decomposition
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products were NH3, N2O, NO, HCl, N2 and H2O. The NH3, HCl and other decomposition
products
were
detected
almost
simultaneously in
the
AP
decomposition in the presence of the nanosized Cu(OH)2·2Cr(OH)3. The decomposition behavior further supports the notion that the smooth transfer of electron is the rate-limiting step in the decomposition of AP with the nanosized Cu(OH)2·2Cr(OH)3 addition.
4. Conclusions In summary, a hydrolytic co-precipitation method has been introduced to prepare amorphous Cu(OH)2·2Cr(OH)3 nanoparticles. The prepared Cu(OH)2·2Cr(OH)3 nanoparticles used as additives for the thermal decomposition of AP are observed to a significant reduction of the onset decomposition temperature of AP. In the AP decomposition with the nanosized Cu(OH)2·2Cr(OH)3 addition, the smooth transfer of 12
ACCEPTED MANUSCRIPT electron is the rate-limiting step. The electron transfer plays a major role in decreasing the
onset
temperature
of
AP
decomposition,
while
the
nano-effect
of
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Cu(OH)2·2Cr(OH)3 is beneficial to easier enrichment of Cu on the nanosized
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Cu(OH)2·2Cr(OH)3 surface.
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Acknowledgement
The financial supports from the National Natural Science Foundation of China
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(No. 51204154).
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[24] C. H. Lu, L. M. Qi, J. H. Yang, D. Y. Zhang, N. Z. Wu, J. J. Ma, Simple Template-Free Solution Route for the Controlled Synthesis of Cu(OH)2 and CuO Nanostructures, J. Phys. Chem. B 108 (2004) 17825-17831. [25] Y. Zhang, M. Yang, X. M. Dou, H. He, D. S. Wang, Arsenate Adsorption on an Fe-Ce Bimetal Oxide Adsorbent: Role of Surface Properties, Environ. Sci. Technol. 39 (2005) 7246-7253.
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Graphical abstract
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The onset temperature of AP decomposition remarkably decreased from
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approximately 450 ℃ to 259 ℃ in the AP thermal decomposition with the
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Cu(OH)2·2Cr(OH)3 nanoparticles addition.
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ACCEPTED MANUSCRIPT Highlights Thermal decomposition of AP in the presence of nanosized Cu(OH)2∙2Cr(OH)3.
Electron transfer and nano-effect characteristics of the Cu(OH)2∙2Cr(OH)3.
Decomposition temperature of AP decreased from 450 oC to 259 oC.
The electron transfer and nano-effect accelerated AP decomposition.
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S0032-5910(14)00738-4 doi: 10.1016/j.powtec.2014.08.038 PTEC 10483
To appear in:
Powder Technology
Received date: Accepted date:
17 May 2014 15 August 2014
Please cite this article as: Xiaodan Zheng, Ping Li, Sili Zheng, Yi Zhang, Thermal Decomposition of Ammonium Perchlorate in the Presence of Cu(OH)2 · 2Cr(OH)3 nanoparticles, Powder Technology (2014), doi: 10.1016/j.powtec.2014.08.038
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Thermal Decomposition of Ammonium Perchlorate in the
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Presence of Cu(OH)2·2Cr(OH)3 nanoparticles
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
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1
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Xiaodan Zheng1,2, Ping Li1,2, Sili Zheng1,2, Yi Zhang1,2
2
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
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Email: [email protected]
Abstract
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The Cu(OH)2·2Cr(OH)3 nanoparticles preparation procedure and its accelerating effect and accelerating mechanism on thermal decomposition of ammonium
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perchlorate (AP) were investigated using transmission electron microscopy (TEM),
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energy dispersive spectroscopy (EDS), selected area electron diffraction (SAED), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR),
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thermogravimetric analysis and differential scanning calorimetry (TG-DSC), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis and mass spectroscopy (TG-MS). In the preparation procedure, TEM, SAED, and EDS showed
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that the Cu(OH)2·2Cr(OH)3 particles were amorphous with nanometer size under the controllable preparation conditions. When the amorphous Cu(OH)2·2Cr(OH)3 nanoparticles were used as additives for the thermal decomposition of AP, the TG-DSC results showed that the addition of Cu(OH)2·2Cr(OH)3 nanoparticles to AP remarkably decreased the onset temperature of AP decomposition from approximately 450 oC to 259 oC. The XRD and FT-IR results confirmed that the Cu(OH)2·2Cr(OH)3 nanoparticles were transformed from amorphous form to Cr2O3 and CuCr2O4 phases after used as additives for the AP thermal decomposition. The XPS results indicated that Cu2+ of the Cu(OH)2·2Cr(OH)3 surface gained the electron from the perchlorate ion of AP, and the Cu ions were enriched on the Cu(OH)2·2Cr(OH)3 surface in the AP decomposition. The electron transfer process plays a major role in the decrease the 1
ACCEPTED MANUSCRIPT onset temperature of AP decomposition, while the nano-effect is beneficial to easier enrichment of Cu ions on the surface of Cu(OH)2·2Cr(OH)3 nanoparticles.
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Key words
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Cu(OH)2·2Cr(OH)3 nanoparticles, Ammonium perchlorate, Thermal decomposition,
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Electron transfer, Nano-effect
1. Introduction
Ammonium perchlorate (AP) is the most common oxidizer used in composite
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solid propellants[1,2]. The characteristics of its thermal decomposition influence the combustion of these propellants: lower high-temperature thermal decomposition
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temperature results in a shorter ignition delay time and a higher burning rate of these propellants[3-5].
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A specific feature of the AP thermal decomposition is its extreme sensitivity to
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metal oxide additives[6,7]. The most commonly used additives are p-type metal oxides, such as CuO, MnO2, Nd2O3 and CuCr2O4 etc.. The CuCr2O4, as the best metal oxide
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additive, decreases the onset temperature of the AP decomposition from about 450 oC to 273℃[8-10]. The thermal decomposition of AP catalyzed by the p-type metal oxides can be widely interpreted by the electron transfer process. In the electron transfer
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process, the rate-controlling step of the thermal decomposition of AP is the transfer of an electron from the perchlorate ion to the positive hole in p-type metal oxides additives. The annihilation of the positive hole in the valence band of a metal oxide enhances the thermal decomposition of AP. Recent investigations have shown that understanding of the behavior of AP thermal decomposition accelerated by nanoparticles is changing due to the particles’ nano-effect. Nansized Fe2O3, CuO nanorod, NiO nanoparticle and Cu-Cr-O nanocomposite[1,11-13], have been shown to be efficient additives for the thermal decomposition of AP. However, interpretation of the relative accelerating behavior is still confusing, for example, accelerating behavior of the CuO nanorod additive in AP decomposition is associated with a proton transfer process, while the accelerating 2
ACCEPTED MANUSCRIPT behavior of the nanosized Fe2O3 and Cu-Cr-O nanocomposite additives are explained by the electron transfer process. Nanosized Cr(OH)3[14] and Cr(OH)3·Al(OH)3[15]
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additives prepared by this laboratory are benefit to accelerating the oxidation of NH3
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in the AP decomposition due to their surface hydroxyl and amorphous form.
be explained by the proton transfer process.
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Accelerating behavior of the nanosized Cr(OH)3 and Cr(OH)3·Al(OH)3 additives can
This study thus aimed to study thermal decompositon of AP in the presence of
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prepared nanosized Cu(OH)2·2Cr(OH)3 which possessed electron transfer and nano-effect characteristics. The preparation procedure of Cu(OH)2·2Cr(OH)3
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nanoparticles was studied. The effect of the prepared Cu(OH)2·2Cr(OH)3 nanoparticles used as additives on the thermal decomposition of AP was studied in
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2. Experimental Section
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detail, and its related accelerating mechanism was also discussed.
All chemicals were analytical-grade reagents and used as received without further
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purification. Cr(NO3)3, CuSO4, Na2SO4, urea, and polyvinylpyrrolidone (PVP) were dissolved in distilled water to prepare aqueous solutions. In a typical synthesis, 20 mL 0.0025 M CuSO4 and 0.005 M Cr(NO3)3, 40 mL
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0.02 M urea and 40 mL 20 g dm−3 PVP, 100 mL 0.18 M Na2SO4 were added to Pyrex test tubes (250 cm3), which were tightly sealed with Teflon caps and placed in a preheated oven at 100 oC for 20 h. After aging, the dispersions were cooled naturally and then centrifuged at 8000 rpm. The obtained precipitates were washed several times with distilled water, filtered, and freeze-dried at –50 oC for 20 h. The concentrations of reagents and Na2SO4 in the starting solutions were systematically varied to analyze morphological characteristics (shape and size) of the precipitates. To study accelerating effect of the prepared precipitates on the thermal decomposition of AP, different amounts of the precipitates were mixed with an ethanol solution of AP, and then dried in an oven at 40 oC for 10 h. The obtained samples were sintered in a nitrogen atmosphere in a furnace at 10 oC min-1 from 3
ACCEPTED MANUSCRIPT ambient temperature to 500 oC. The crystal structure of the precipitates and samples was characterized by powder
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X-ray diffraction (XRD) at diffraction angles (2θ) ranging from 5 to 90◦ and Fourier
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Transform Infrared Spectroscopy (FT-IR) on a Spectrum GX spectrophotometer. The morphological characterization and quantitative analysis of the powders were
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performed by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS) on a JEOL JEM 2100
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(UHR) instrument operated at an acceleration voltage of 200 KV. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the precipitates and
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samples were performed at a heating rate of 10 oC min-1 with Al2O3 as the reference on a Mettler- Toledo TGA/DSC 1 instrument. X-ray photoelectron spectroscopy (XPS) data of the powders were obtained with an ESCALab220i-XL electron spectrometer
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from VG Scientific using 300-W Al Kα radiation. TG-MS analyses were performed in
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a nitrogen atmosphere at a heating rate of 10 oC min-1 from 50 to 500 oC, using a NETZSCH STA449C instrument.
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3. Results and Discussion
3.1 Preparation of Cu(OH)2·2Cr(OH)3 nanoparticles
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Table 1 summarizes the effects of the reagent concentrations, SO42-/Cr3+ molar ratios, urea and PVP on the morphological characteristics of the particles precipitated after aging at 100 oC for 20 h. As observed, low SO42-/Cr3+ molar ratio and suitable reagent concentrations contributed to preparing spherical particles of different size. Typically, spherical nanoparticles (S3) with a diameter of 70 nm were synthesized through the aging of solutions with a SO42-/Cr3+ molar ratio of 1:2, the CuSO4 concentration maintained at 0.0025 M, and the Cr(NO3)3 concentration maintained at 0.005 M. The SO42-/Cr3+ molar ratio required for the formation of such uniform dispersions was even more restrictive, since for values higher than 0.5 either inhomogeneous precipitates (SO42-/Cr3+ molar ratio=2, 5) or ill-defined precipitates (SO42-/Cr3+ molar ratio=20) were observed. The addition of urea to the starting 4
ACCEPTED MANUSCRIPT solutions is observed to be essential in producing precipitates in the above-described systems. No precipitates were obtained without this compound addition. When the
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above solutions were aged in the absence of PVP, ill-defined precipitates were formed,
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indicating that the PVP addition was also needed to form the regular particles.
Precipitate
CuSO4
Cr(NO3)3
PVP -3
Urea
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Table 1 Effects of the reagent concentrations and SO42-/Cr3+ molar ratios on the morphological characteristics of the precipitates prepared by aging solutions of CuSO4 and Cr(NO3)3 at 100 oC for 20 h in the presence of sulfate, urea, and PVP. Mean
SO42-/Cr3+
Particle shape
0.5
Spheres
100-300nm
(M)
(gdm )
(M)
S1
0.0005
0.001
20
0.02
S2
0.0015
0.003
20
0.02
0.5
Spheres
150-600nm
S3
0.0025
0.005
20
0.02
0.5
Spheres
70nm
S4
0.01
0.02
20
0.02
0.5
Agglomerates
—
S5
0.0025
0.005
—
0.02
0.5
Agglomerates
—
S6
0.0025
0.005
20
—
0.5
No precipitation
—
S7
0.0025
0.005
20
0.02
2
Spheres
120-350nm
S8
0.0025
0.005
S9
0.0025
0.005
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(M)
size
0.02
5
Spheres
130-400nm
20
0.02
20
Agglomerates
—
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Figure 1 TEM image of S3, EDS mapping and SAED pattern of S3.
The composition of S3 prepared in this study was confirmed by EDS, as shown in the attached EDS of Fig. 1. The nanoparticles (S3) consisted of Cr, Cu, O, and S. Atomic contents of Cu and Cr were 8.54 % and 16.98 %, respectively, and the atomic ratios of Cu and Cr were 1:1.99, which was close to theoretical molar ratio of Cu(OH)2·2Cr(OH)3. The C was also observed in the EDS mapping, which was attributed to the residual carbon on the grid during TEM detection. In addition, the selected area electron diffraction (SAED) pattern of S3 was provided in Fig. 1, 5
ACCEPTED MANUSCRIPT showing that the isolated nanoparticles possessed a completely disordered structure. Overall, the TEM, EDS and SAED indicate that the precipitates are amorphous Cu(OH)2·2Cr(OH)3 particles with a spherical morphology in the size range of 70 to
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600 nm.
3.2 Thermal decomposition of AP in the presence of Cu(OH)3·2Cr(OH)3
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nanoparticles
Different amounts of Cu(OH)2·2Cr(OH)3 nanoparticles are mixed as additives with AP to study the accelerating behavior of the nanoparticles on the thermal
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decomposition of AP. In a typical experiment, 50 g Cu(OH)2·2Cr(OH)3 nanoparticles (S3) with a Cr and Cu mole ratio of 1.99 were mixed with 50 g pure AP to produce
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what was referred to as a 50 % addition.
The TG-DSC curves of pure AP and AP in the presence of different amounts of
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S3 are shown in Figure. 2. For pure AP (A7), an endothermic peak was observed at
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approximately 245 oC, which was assigned to the phase transition of AP from orthorhombic to cubic form due to the rotation of perchlorate ion[16]. AP underwent
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two complicated decomposition processes, a low-temperature stage at 332 oC attributed to the partial decomposition of AP and a high-temperature stage at 450 oC
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caused by the complete decomposition of AP to volatile products.
Figure 2 TG and DSC curves for pure AP and AP mixed with different amounts of S3: A7, A6, A5, A4, A3, A2, A1 represent pure AP and AP mixed with 2%, 5%, 10%, 20%, 50%, 80% S3, respectively.
The decomposition patterns of AP with the addition of S3 show significant 6
ACCEPTED MANUSCRIPT differences. Fig. 2a shows the DSC curves for the thermal decomposition of AP in the presence of different amounts of the S3. The addition of 2 %, 5 %, 10 %, 20 %, 50 %, and 80 % S3 lowered the onset temperature of AP decomposition by 75 oC, 85 oC, 137 C, 154 oC, 189 oC, and 191 oC, respectively. As the amount of S3 increased, the
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exothermic peak that was attributed to the complete decomposition of AP and
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formation of some intermediates gradually shifted towards lower temperatures. In particular, only one exothermic peak was obtained at approximately 259 ℃ with the 80 % S3 addition in the thermal decomposition of AP.
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Fig. 2b shows the TG curves for A1, A2, A4, A6, and A7. Each curve exhibits a weight loss step corresponding to the exothermic peak of the DSC curve. As observed,
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the S3 used as additives for AP decomposition had significantly effect on steps of high-temperature decomposition (HTD) and low-temperature decomposition (LTD) of
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AP decomposition. The weight loss of HTD (LTD) was 35.10% (4.90%), 18.09%
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(41.91%), 90.05% (0), 72.98% (22.05%), 94.58% (0), respectively, with the addition of 0, 2 %, 10 %, 50 %, 80 % S3 for the AP decomposition. A higher content of the S3
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was also observed to accelerate the thermal decomposition rate of AP.
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3.3 Accelerating mechanism
Figure 3 XRD patterns of S3 and R1 which was residues of thermal decomposition of A1 (AP mixed with 80% S3).
The phase, microstructure, and surface characteristics of S3 and R1 which is obtained by residues of A1 (AP mixed with 80% S3) decomposition, are explored to 7
ACCEPTED MANUSCRIPT investigate the accelerating mechanism of the additives. The X-ray diffraction (XRD) patterns of S3 and R1 (Fig. 3) are obtained to
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investigate the phase formation of the samples. The patterns revealed that the S3 was
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amorphous; and the R1 showed peaks representing Cr2O3 (JCPDS no: 38-1479) and CuCr2O4 (JCPDS no: 26-0509) phases. The product of pure Cu(OH)2 or Cr(OH)3
[17-19]
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thermal decomposition at approximately 400-500 oC is CuO or Cr2O3, respectively . In this study, the phase transition of nanosized Cu(OH)2·2Cr(OH)3 from
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amorphous form to CuCr2O4 and Cr2O3 was observed during the indirect thermal decomposition of the nanosized Cu(OH)2·2Cr(OH)3.
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The FT-IR spectra of S3 and R1 are shown in Fig. 4. As observed, the bands at 1636 and 1574 cm-1 of S3 spectrum could be ascribed to the bending modes of the nondissociated water molecules[20]. The bands at 1045 and 1162 cm-1 indicated the
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presence of a small amount of sulfate anions in the S3, which were confirmed by EDS
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analyses. The bands at 1382, 1350 and 833 cm−1 could be attributed to infrared adsorption of carbonate anions. The bands at 524 and 614 cm-1 of R1 spectrum could
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be attributed to the stretching vibrations of Cu-O [21,22] or Cr-O [23], in accordance with
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the XRD results (CuCr2O4 and Cr2O3 phase).
Figure 4 FT-IR spectra of S3 and R1 which was residues of thermal decomposition of A1.
X-ray photoelectron spectroscopy is widely used for studies of surface chemistry because it provides a range of useful information depths, reasonable quantification, and chemically specific information for each element detected through chemical shifts. 8
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Cr 2p, O 1s, N 1s, and C 1s peaks. As shown in Fig. 5a, the C 1s XPS spectra
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indicated that carbon deposition occurred during charging correction, which resulted in a shift in all other transitions. The presence of nitrogen could be attributed to the
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existence of a nitrogenous ligand bonding with metal or NO3-, derived from the preparation process. The Cr 2p peaks occurred at 575.9- 576.7 eV and 585.8- 586.5
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eV, corresponding to Cr3+, and Cr3+ was the only observed oxidation state (Fig. 5c). The surface of S3 and R1 also exhibited the Cu 2p3/2 and Cu 2p1/2 peaks at binding
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energy of 934.5 and 932.5 eV, respectively, together with intense satellite peaks at 940.2, 942.4, 955 and 962.9 eV (Fig. 5b). All of the Cu 2p spectra of S3 and R1 were fitted using a Gaussian-Lorentzian peak shape and satisfactory fitting results were
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obtained as shown in Fig. 6a and Table 2.
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Figure 5 (a) XPS spectra of S3 and R1; (b) Cu 2p XPS spectra of S3 and R1; (c) Cr 2p spectra of S3 and R1.
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Figure 6 (a) Cu 2p spectra of S3 and R1; the dotted curves represent photo peak contribution from Cu2+, Cu+. (b) O 1s (b) spectra of S3 and R1; the dotted curves represent photo peak contribution from O2-, OH-. Table 2 Cu 2p and O 1s peak parameters for S3 and R1. Peak
Fwhm (eV)
Percent (%)
934.8
2.70
76.96%
Cu+
932.5
2.33
23.04%
OH-
531.2
3.20
72.01%
O2-
529.6
1.10
27.99%
Cu2+
934.5
3.76
100%
Cu+
OH-
531.1
2.00
100%
O2-
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Cu2+
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S3
R1
B.E. (eV)
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As shown in Fig. 6a, the Cu 2p peaks occurred at 934.5 and 932.5 eV, respectively, which were indicative of the chemical state of Cu2+ and Cu+ [24]. Cu2+ was the main chemical state (100 % and 76.96 %) species, followed by Cu + (0 % and 23.04 %), in S3 and R1, respectively. The Cu+, as the Cu2+ reduced form, proves the electron transfer between Cu2+ and Cu+ of the S3 during the thermal decomposition of AP. Such transfer indicates that the Cu2+ of surface of the Cu(OH)2·2Cr(OH)3 nanoparticles gains the electron from perchlorate ion (HClO4-) in the AP decomposition to form Cu+. Note that the electron transfer plays a key role in the 10
ACCEPTED MANUSCRIPT thermal decomposition of AP in the presence of Cu(OH)2·2Cr(OH)3 nanoparticles. The O 1s peaks are composed of subpeaks with peak energies 530.1-530.3, 531.5-532.0 and 532.8-533.4 (O2-) eV, which are designated as surface O2-, OH- and
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H2O groups, respectively[25]. The O 1s spectra of S3 and R1 were fitted using the above-mentioned Gaussian-Lorentzian peak shape as shown in Fig. 6b and Table 2.
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The O 1s spectra (Fig. 6b) occurred at 529.6 and 531.2 eV, which were indicative of the chemical state of O2- and OH-, respectively. O2- content of the S3 surface increased from 0 to 27.99% after used as additives for the AP thermal decomposition.
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The O2-, representative of oxides, indicates thermal decomposition occurs
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simultaneously between the AP and Cu(OH)2·2Cr(OH)3 nanoparticles, in accordance with the results obtained by XRD and FT-IR.
Cu
S3
5.15
R1
10.02
Cr
O
N
S
10.46
75.75
7.91
0.73
17.6
70.70
1.69
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Atomic ratio %
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Table 3 Data regarding atomic ratios collected from XPS spectra of S3 and R1.
The atomic ratios of Cu, Cr, O, N and S on the surface of S3 before and after the Cu(OH)2·2Cr(OH)3 nanoparticles are used as additives in the AP decomposition are
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summarized in Table 3. The surface of S3 contained 5.15% Cu and 10.46% Cr, whereas the surface of R1 contained 10.02% Cu and 17.6% Cr: the relative Cu and Cr surface atomic ratios of S3 and R1 were 1: 2.03 and 1: 1.76, respectively, demonstrating that the surface atomic content changed a lot before and after used as additives for the AP thermal decomposition. The Cu ions of the S3 surface are enriched in the AP decomposition. Such enrichment of Cu ions not only will be benefit to electron transfer between the Cu2+ and HClO4- of the AP , but also confirms that the accelerating behavior occurs at the surface of nanosized Cu(OH)2·2Cr(OH)3. Unlike the reported accelerating mechanism of AP mixed with nanosized Cr(OH)3 and Cu-Cr-O nanocomposite, the electron transfer is believed to play a major role in the decrease the onset temperature of AP decomposition in the presence of Cu(OH)2·2Cr(OH)3 nanoparticles, while the nano-effect of Cu(OH)2·2Cr(OH)3 11
ACCEPTED MANUSCRIPT nanoparticles such as amorphous form and nanoscale may be beneficial to easier enrichment of Cu on the surface of Cu(OH)2·2Cr(OH)3 nanoparticles. Besides, the N
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and S of S3 surface decreased from 7.91% to 1.69% and from 0.73% to 0%,
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respectively, when the S3 used as additives before and after AP thermal
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decomposition. Note that the SO42-and NO3- were also decomposed.
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Figure 7 Mass spectral analysis of A1.
The mass intensities of molecules that are produced in the thermal
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decomposition of A1 (AP mixed with 80% S3) are also shown in Fig. 7. As observed, the AP decomposed completely within a narrow temperature range, the decomposition
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products were NH3, N2O, NO, HCl, N2 and H2O. The NH3, HCl and other decomposition
products
were
detected
almost
simultaneously in
the
AP
decomposition in the presence of the nanosized Cu(OH)2·2Cr(OH)3. The decomposition behavior further supports the notion that the smooth transfer of electron is the rate-limiting step in the decomposition of AP with the nanosized Cu(OH)2·2Cr(OH)3 addition.
4. Conclusions In summary, a hydrolytic co-precipitation method has been introduced to prepare amorphous Cu(OH)2·2Cr(OH)3 nanoparticles. The prepared Cu(OH)2·2Cr(OH)3 nanoparticles used as additives for the thermal decomposition of AP are observed to a significant reduction of the onset decomposition temperature of AP. In the AP decomposition with the nanosized Cu(OH)2·2Cr(OH)3 addition, the smooth transfer of 12
ACCEPTED MANUSCRIPT electron is the rate-limiting step. The electron transfer plays a major role in decreasing the
onset
temperature
of
AP
decomposition,
while
the
nano-effect
of
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Cu(OH)2·2Cr(OH)3 is beneficial to easier enrichment of Cu on the nanosized
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Cu(OH)2·2Cr(OH)3 surface.
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Acknowledgement
The financial supports from the National Natural Science Foundation of China
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(No. 51204154).
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[24] C. H. Lu, L. M. Qi, J. H. Yang, D. Y. Zhang, N. Z. Wu, J. J. Ma, Simple Template-Free Solution Route for the Controlled Synthesis of Cu(OH)2 and CuO Nanostructures, J. Phys. Chem. B 108 (2004) 17825-17831. [25] Y. Zhang, M. Yang, X. M. Dou, H. He, D. S. Wang, Arsenate Adsorption on an Fe-Ce Bimetal Oxide Adsorbent: Role of Surface Properties, Environ. Sci. Technol. 39 (2005) 7246-7253.
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Graphical abstract
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The onset temperature of AP decomposition remarkably decreased from
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approximately 450 ℃ to 259 ℃ in the AP thermal decomposition with the
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Cu(OH)2·2Cr(OH)3 nanoparticles addition.
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ACCEPTED MANUSCRIPT Highlights Thermal decomposition of AP in the presence of nanosized Cu(OH)2∙2Cr(OH)3.
Electron transfer and nano-effect characteristics of the Cu(OH)2∙2Cr(OH)3.
Decomposition temperature of AP decreased from 450 oC to 259 oC.
The electron transfer and nano-effect accelerated AP decomposition.
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