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carbon nanotubes nanocomposite prepared via a precursor route and enhanced catalytic property
Journal of Solid State Chemistry 197 (2013) 14–22
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Co–Al mixed metal oxides/carbon nanotubes nanocomposite prepared via a precursor route and enhanced catalytic property Guoli Fan, Hui Wang, Xu Xiang, Feng Li n State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, PO Box 98, Beijing 100029, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2012 Received in revised form 17 July 2012 Accepted 5 August 2012 Available online 14 August 2012
The present work reported the synthesis of Co–Al mixed metal oxides/carbon nanotubes (CoAl-MMO/ CNT) nanocomposite from Co–Al layered double hydroxide/CNTs composite precursor (CoAl-LDH/CNT). The materials were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), low temperature nitrogen adsorption–desorption experiments, thermogravimetric and differential thermal analyses (TG–DTA), Raman spectra and X-ray photoelectron spectroscopy (XPS). The results revealed that in CoAl-MMO/CNT nanocomposite, the nanoparticles of cobalt oxide (CoO) and Co-containing spinel-type complex metal oxides could be well-dispersed on the surface of CNTs, thus forming the heterostructure of CoAl-MMO and CNTs. Furthermore, as-synthesized CoAl-MMO/CNT nanocomposite was utilized as additives for catalytic thermal decomposition of ammonium perchlorate (AP). Compared to those for pure AP and CoAl-MMO, the peak temperature of AP decomposition for CoAl-MMO/CNT was significantly decreased, which is attributed to the novel heterostructure and synergistic effect of multi-component metal oxides of nanocomposite. & 2012 Elsevier Inc. All rights reserved.
Keywords: Nanocomposite Multi-walled carbon nanotubes Metal oxide Thermal decomposition
1. Introduction In the past two decades, carbon nanotubes (CNTs) have attracted worldwide attention because of their unique structural, mechanical and electronic conducting properties, superior chemical stability and promising applications in transistors, field-emission tips, sensors, supercapacitors, catalyst supports and storage materials for hydrogen [1–5]. More interestingly, its flexible sp2-hybridized graphitic structure with high carrier mobility and good electron-accepting property affords an excellent electrical conductivity for storing and shuttling electrons. Recently, the assembly of CNTs-based heterosturctures or hybrids with the desired nanoscale guests including metals [6,7], semiconductors [8], and metal oxides (e.g. TiO2 [9], MnO2 [10], WO3 [11], Co3O4 [12], CuO [13], a-Fe2O3 [14], ZnO [15]), as an emerging and quickly developing field, has been stimulated for enhancing the properties of resulting versatile materials and achieving a broad range of practical applications. On the other hand, ammonium perchlorate (NH4ClO4, AP) has extensively been used as oxidizer in composite solid propellants (CSP) for rocket propulsion, due to the fact that it is cheap and contains a large amount of oxygen [16]. The performance of CSP, strongly dependant on the thermal decomposition of AP, is greatly
n
Corresponding author. Fax: þ86 10 64425385. E-mail addresses: [email protected], [email protected] (F. Li).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.08.016
improved by solid additives or catalysts [17]. Indeed, numerous works have been done on the catalytic decomposition of AP, and metal oxides (e.g. NiO, CuO, ZnO, CoO and Fe2O3) and complex metal oxides (e.g. LaMO3, Cu–Cr–O and CuFe2O4) are found to be good catalysts and can further enhance the energy-generating characteristics of energetic materials [14,18–27]. Although extensive studies suggest the dependence of AP decomposition on surface microstructures of metal oxide catalysts, the catalytic activity is less directly associated with synergistic effect of diversified active components of catalysts, which could be a remarkable factor on thermal decomposition of AP. xþ Layered double hydroxides (LDHs, [MII1 xMIII (An )x/n x (OH)2] mH2O), also known as a family of highly ordered two-dimensional layered materials, consist of positively charged brucite-like layer xþ ([MII1 xMIII ) and charge-balancing anion (An ) in the x (OH)2] hydrated interlayer galleries [28]. The identities of the divalent and trivalent cations (MII and MIII, respectively) and the interlayer chargecompensating anion (An ) together with the value of the stoichiometric coefficient (x) may be varied over a wide range, giving rise to a large class of isostructural materials with different physicochemical properties. In recent years, LDHs have attracted increasing interests for their potential applications in catalysis, adsorption, photochemistry, electrochemistry and other areas [29,30]. One of the most interesting features of LDHs is their extraordinary capability as catalyst precursors. Upon calcination at intermediate temperatures (450–600 1C), LDHs undergo decomposition and structural transformation, thus giving poorly crystallized mixed metal oxides with large
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specific surface areas, high thermal and chemical stability, and homogeneous dispersion of the metal species [31]. Considerable attention has been paid to Co-containing LDHs due to high catalytic activity of the deriving homogeneous metal oxides for various oxidation reactions [32–34]. Recently, the catalytic activity of Cu–Co metal oxides from LDH precursor for thermal decomposition of AP also has been explored [35]. In this study, for the purpose of combing the unique properties of CNTs with LDHs to achieve excellent performance, we established a novel approach to synthesis Co–Al mixed metal oxides/ carbon nanotubes (CoAl-MMO/CNT) nanocomposite from CoAlLDH/CNT nanocomposite precursor. Furthermore, the catalytic behavior of CoAl-MMO/CNT was investigated for the decomposition of AP. To the best of our knowledge, detailed studies of the synthesis of CoAl-MMO/CNT nanocomposite and its catalytic performance have not been reported before. The results showed that the nanoparticles including cobalt oxide and Co-containing spinel-type complex metal oxides could be well-dispersed on the surface of nitric acid modified CNTs. As-synthesized CoAl-MMO/ CNT nanocomposite exhibited much higher catalytic activity towards the decomposition of AP, as compared to either pristine CoAl-MMO or CNTs, which is closely related to the novel heterostructure and synergistic effect of multi-component Co-containing metal oxides of such nanocomposite.
2. Experimental 2.1. Materials As-received multi-walled CNTs (1.0 g) from Shenzhen Nanotech Port Co., Ltd. were added into 100 mL concentrated nitric acid (68 wt%) at 85 1C for 6 h. After cooling, the suspension was filtered and washed with deionized water to neutral pH, and then dried under vacuum at 60 1C for 12 h. L-Cysteine (98.5%) and other chemicals (analytical grade) were used as received without further purification. 2.2. Synthesis of CoAl-MMO/CNT nanocomposite
carried out on a JEOL JEM-2010 electron microscope at an accelerating voltage of 200 kV. 27 Al solid-state magic-angle spinning nuclear magnetic resonance (NMR) spectrum was measured on a Bruker AV300 spectrometer operating at 78.20 MHz with a pulse width of 0.5 s and spinning rate of 8000 Hz. The specific surface area determination and pore size and volume analysis were performed by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) from the N2 adsorption– desorption measurements at 77 K using a static volumetric Quantachrome Autosorb-1C-VP Analyzer. Raman spectra were conducted on a Jobin Yvon Horiba HR800 spectrometer using a 532 nm line of Ar þ ion laser as excitation source at room temperature. The power to the sample was filtered down to 0.3 mW. X-ray photoelectron spectra (XPS) was recorded on a Thermo VG ESCALAB2201-XL X-ray photoelectron spectrometer at a base pressure of 2 10 9 Pa using Al Ka X-ray (1486.6 eV) as the excitation source. The binding energy (BE) calibration of the spectra has been referred to carbon 1s peak, located at 284.8 eV. Thermogravimetric and differential thermal analysis (TG–DTA) and the mass spectrometer (MS) profiles of the gas evolution were performed on a PerkinElmer Diamond TG/DTA connected to a MS (Pfeiffer ThermoStar) under N2 flow at the ramping rate of 5 1C/min. 2.4. Thermal decomposition of AP The thermal decomposition behaviors of pure AP and the mixture of AP and catalyst (5 wt%) were investigated by TG– DTA with a heating rate of 5 1C/min from room temperature to 550 1C under air atmosphere.
3. Results and discussion 3.1. Synthesis and structure of CoAl-MMO/CNT nanocomposite Fig. 1 shows the powder XRD patterns of CoAl-LDH and CoAlLDH/CNT samples. Two samples display the intensive characteristic
CoAl-LDH/CNT was prepared by a coprecipitation method. Firstly, Co(NO3)2 6H2O (15 mmol) and Al(NO3)3 9H2O (5 mmol) were dissolved in 100 mL of deionized water. Then modified CNTs (1.5 g) along with L-cysteine (15 mmol) were dispersed into the solution in an ultrasonic bath for 30 min under nitrogen atmosphere. Subsequently, the solution was titrated with an alkali solution of NaOH (0.4 M) and Na2CO3 (0.1 M) under vigorous stirring at room temperature until pH¼10.5. The resulting suspension was aged at 60 1C with further stirring for 6 h. The solid was then recovered and washed in deionized water and finally dried under vacuum at 60 1C for 12 h. For comparison, pure CoAl-LDH sample was also prepared under identical reaction conditions without addition of CNTs and L-cysteine. As-prepared CoAl-LDH/CNT and CoAl-LDH samples were calcined under nitrogen gas flow (flow rate: 60 standard-state cm3 min 1) at 500 1C for 4 h at a heating rate of 5 1C/min, and the obtained products were denoted as CoAl-MMO/CNT and CoAl-MMO, respectively. 2.3. Characterization Powder X-ray diffraction (XRD) data were collected at room temperature on Shimadzu XRD-6000 diffractometer with graphitefiltered Cu Ka source (l ¼0.15418 nm), 40 kV, 30 mA. The samples were step-scanned in steps of 0.04 (2y) using a count time of 10 s/step. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were
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Fig. 1. XRD patterns of CoAl-LDH (a) and CoAl-LDH/CNT (b).
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reflections corresponding to two-dimensional hydrotalcite-like LDH family, i.e. (003), (006), (012), (015), (110) and (113) [35], which reveals the high-crystalline nature of layered LDH phase. In each case, the basal spacing value (d003) of LDH phase is approximately 0.75 nm, consistent with that of carbonate-intercalated LDH materials [29,36]. Meanwhile, as for CoAl-LDH/CNT sample, the reflection at about 26.4o is assigned to that (002) plane of graphitic carbon
with an interplaner distance of about 0.33 nm (JCPDS No.41-1487), confirming the existence of CNTs. The morphology and microstructure of the starting CNTs and asprepared CoAl-LDH/CNT sample was characterized by TEM. It can be clearly observed from Fig. 2 that modified CNTs by nitric acid are multi-walled nanotubes with inner diameter of about 15 nm and outer diameter of about 20–50 nm. As for CoAl-LDH/CNT sample,
Fig. 2. TEM and HRTEM micrographs of CNTs (a and b).
Fig. 3. TEM micrographs of CoAl-LDH/CNT (a and b) and CoAl-MMO/CNT (c), and a HRTEM micrograph of CoAl-MMO/CNT (d).
G. Fan et al. / Journal of Solid State Chemistry 197 (2013) 14–22
the surface combination with CoAl-LDH did not change the morphology and structure of nanotubes. Obviously, note from Fig. 3a that besides a few aggregates of self-nucleated particles, most of nanoparticles are adjacent to the surface of CNTs, suggestive of the affinity between CoAl-LDH nanocrystallites and CNTs matrix. A high-magnification TEM micrograph of two nanotubes with the different outer diameters of about 35 and 100 nm (Fig. 3b) illustrates more clearly the hybrid nanostructure of such nanocomposite, where some nanoparticles with uniform size of about 15 nm are dispersed on the surface of two nanotubes. In the course of synthesis for CoAl-LDH/CNT, L-cysteine with amino and carboxyl groups mainly exists in the ionic form of –(COO )NH3þ [37], when initially mixed with the acidic salt solutions. Therefore, L-cysteine can be immobilized onto the surface of modified CNTs through the electrostatic interaction between surface groups of L-cysteine and CNTs. Moreover, –COO groups of immobilized L-cysteine can coordinate selectively and/or interact electrostatically with Co2 þ and Al3 þ cations in the solution. Subsequently, CoAl-LDH nucleates in situ onto the surface of CNTs through hydrogen bonding between carboxyl groups of L-cysteine and hydroxyl groups of brucite-like layers, which facilitates the adhesion of CoAl-LDH nuclei onto CNTs. It suggests that the dispersion of CoAl-LDH nanocrystallites can be effectively improved by L-cysteine as interfacial bridging linker. XPS analyses were performed for pristine CoAl-LDH and CoAlLDH/CNT sample (Fig. 4). Core levels of Co 2p, Al 2p, C 1s and O 1s can be identified from the survey of XPS. As for CoAl-LDH sample, the peaks with the binding energy (BE) of 782.0 and 786.6 eV are assigned to Co 2p3/2 and its shake-up satellite for Co2 þ ions,
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respectively [38]. The fine spectrum of Al 2p shows a peak at 75.1 eV, which is related to the Al3 þ species in the form of Al–OH [39]. In the O 1s spectrum, a single peak at 532.2 eV originates from lattice oxygen species of hydroxyl groups [40]. The BE values of Co 2p, Al 2p and O 1s for CoAl-LDH/CNT shift to higher values (around 1.4–2.5 eV), as compared to those for CoAl-LDH. Such large positive shift of BE for metal and oxygen elements has been found in other CNT-based composites reported in the literature [12]. It is known that the BE shift of elements can be associated with the chemical circumstances of elements and the BE value increases as decreasing electron density. The present finding may be explained by the charge transfer from CoAl-LDH crystallites to the surface of CNTs due to the hybridized nanostructure of assynthesized CoAl-LDH/CNT composite involving the strong interaction between the positively charged layers of LDH and the negatively charged functional groups of modified CNTs. In addition, the hydrogen bonding among the carboxylic and/or hydroxyl groups on CoAl-LDH and CNTs could be responsible for the positive shift of BE observed here to some extent. Fig. 5 shows the XRD patterns of calcined CoAl-LDH and CoAlLDH/CNT samples at 500 1C. Obviously, calcination has destroyed the layered structure of the LDH phase, as no characteristic reflections of LDH are present in the XRD pattern for CoAl-MMO sample, and the newly appearing characteristic reflections confirm the presence of cobalt-containing spinel-type complex metal oxide phases, such as Co3O4 and/or CoAl2O4. No characteristic reflections corresponding to Al2O3 phase are observed in the XRD patterns of samples. However, it is rather difficult to distinguish between these mixed oxide phases due to their almost
Fig. 4. XPS spectra of CoAl-LDH/CNT survey, Co 2p, Al 2p, and O 1s of CoAl-LDH (a) and CoAl-LDH/CNT (b).
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Fig. 6.
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Al NMR spectra of CoAl-MMO/CNT sample.
Fig. 5. XRD patterns of CoAl-MMO (a) and CoAl-MMO/CNT (b).
identical positions of the characteristic reflections. Furthermore, Pe´rez-Ramı´rez et al. [41] have reported that a part of Co3 þ could be substituted by Al3 þ in the Co3O4 phase, and thus another kind of stable spinel-type Co2 þ (Co3 þ , Al)2O4 phase consequently presented in calcined product of CoAl-LDH. Although, Co and Al mixed oxides usually lead to normal spinels, CoAl2O4-type, where Co2 þ occupies tetrahedral positions and Al3 þ fills the octahedral positions. Compared to those for CoAl-MMO, the characteristic reflections of spinel phases for CoAl-MMO/CNT composite are greatly lowered. More interestingly, its XRD pattern shows a series of intense characteristic (111), (200) and (220) reflections of CoO phase (JCPDS no. 431004), except for those corresponding to spinel phases. This should result from the strong interaction between CoAl-LDH nanocrystallites and CNTs in the hybrid nanostructure, leading to the formation of simple CoO. Additionally, the broad characteristic reflections suggest the existence of amorphous alumina phase in sample [42]. In the 27Al NMR spectrum of CoAl-MMO/CNT (Fig. 6), an intensive resonance centered at 9 ppm corresponds to 6-fold coordinated aluminum ions [43]. In addition, the presence of 4-fold and 5-fold coordinated aluminum ions with chemical shifts at around 65 and 45 ppm is demonstrated in sample [44]. This is consistent with the XRD diffraction data described above. The results confirm the collapse of layered structure and the formation of small amount of amorphous Al2O3 domain and appreciable amount of Alcontaining spinel phases after the calcination. A typical TEM micrograph of CoAl-MMO/CNT (Fig. 3c) shows that most of solid nanoparticles are distributed on the surface of nanotubes, indicative of the good dispersion of resulting CoO and Co-containing spinel-type complex metal oxides. In addition, detailed HRTEM observation (Fig. 3d) reveals the presence of interplanar distance of about 0.25 nm that is characteristic of (111) plane for CoO phase, and that CoO nanocrystallites and CNTs are with distinguished and coherent interfaces, indicative of the high affinity between them. Meanwhile, some dark zones distributed over the surface should be assigned to a small amount of amorphous Al2O3 domain. The kind of amorphous Al2O3 phase as a dispersant agent may further inhibit the aggregation of Cocontaining metal oxide particles formed.
Fig. 7. Raman spectra of CNTs (a) and CoAl-MMO/CNT (b).
Raman spectroscopy was utilized to determine the microstructural characteristics of modified CNTs and CoAl-MMO/CNT samples. As seen in Fig. 7, two Raman bands are observed at around 1350 and 1583 cm 1 in each case, which are assigned to D and G modes of nanotubes, respectively, associated with the vibrations of carbon atoms with dangling bonds in terminations of disordered graphite and the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice [45]. Usually, the value of R (ID/IG) is considered for evaluating the degree of graphitization of carbon materials [46]. Correspondingly, the R value calculated using maximum peak intensities increases from 0.61 for modified CNTs to 1.09 for CoAl-MMO/CNT, indicative of more structural defects of CNTs in the composite. The increase in the R value suggests the anchoring of metal oxide nanoparticles onto the surface of CNTs involving the formation of some chemical bonds between them [47]. Additionally, as for CoAl-MMO/CNT, two peaks, located at 484 and 693 cm 1, correspond to the Raman active modes Eg and A1g of CoO phase,
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respectively [48], while a broad band located between 500 and 650 cm 1 is assigned to anharmonic interactions in the CoO structure. XPS analyses for CoAl-MMO and CoAl-MMO/CNT samples were performed (Fig. 8), in order to further obtain an insight into the surface microstructural character. Core levels of Co 2p, Al 2p, C 1s and O 1s can be identified from the survey of XPS for CoAlMMO/CNT, and no other contaminant species are detectable. As for CoAl-MMO sample, the XPS of Co 2p region can be fitted into four contributions. The first two peaks with the binding energy (BE) values of about 780.6 and 786.4 eV are assigned to Co 2p3/2 and its shake-up satellite, while the higher BE peaks around 795.8 and 803.4 eV correspond to Co 2p1/2 and its shake-up satellite, respectively [49]. In the case of high-spin Co2 þ -containing compounds, the BE values of intense Co 2p shake-up satellites are about 5–6 eV higher than those of Co 2p transitions [38], and lowspin Co3 þ species only indicate weak Co 2p shake-up satellites. Therefore, the medium intense Co 2p shake-up satellites confirm the presence of both Co2 þ and Co3 þ species in CoAl-MMO sample. The fine spectrum of Al 2p shows a peak at around 73.8 eV, which is related to the Al3 þ species bonded to oxygen
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[50]. From the O 1s spectrum of CoAl-MMO, it is seen that there are two fitted peaks, representing two different kinds of surface oxygen species. There is general agreement between the literature and the present results such that the peak with the lower BE around 530.6 eV is the characteristic of lattice oxygen bound to metal cations of the structure, while the peak with the higher BE at 532.6 eV belongs most likely to surface oxygen [51], including mainly oxygen species of hydroxyl groups [40]. As compared with those for CoAl-MMO, the BE values of Co 2p, Al 2p, and O 1s for CoAl-MMO/CNT are increased greatly, which should be ascribed to the charge transfer from metal oxides to the surface of CNTs due to the strong interaction between CoAl-MMO and CNTs matrix. Fig. 9 delineates the nitrogen sorption isotherms of CNTs, CoAl-MMO and CoAl-MMO/CNT samples. It can be found that the samples have a type IV isotherm with a H3 type hysteresis loop that does not exhibit any limiting adsorption at high P/P0, indicating the presence of capillary condensation in the mesoporous structure [52]. CNTs show a narrow pore size distribution from 2 to 8 nm (inset in Fig. 9a), which may be the result of the exposure of the inside pores of nanotubes. The pore size
Fig. 8. XPS spectra of CoAl-MMO/CNT survey, Co 2p, Al 2p, and O 1s of CoAl-MMO (a) and CoAl-MMO/CNT (b).
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distribution of CoAl-MMO/CNT is similar to that of CNTs but obviously different from that of CoAl-MMO, due to the fact that CoO or spinel particles have been formed densely on the surface of CNTs. Moreover, the BET surface area of CoAl-MMO/CNT (73 m2/g) is larger than that of CNTs (56 m2/g) and smaller than that of CoAl-MMO (105 m2/g), which should be attributed to the high dispersion of metal oxide particles on CNTs. 3.2. Catalytic performance of CoAl-MMO/CNT nanocomposite As-synthesized CoAl-MMO/CNT nanocomposite was studied as an additive in promotion of the thermal decomposition of AP in order to understand the effects of chemical composition and microstructure on the catalytic performance of materials. Fig. 10A presents TG curves for pure AP with and without additive CNTs, CoAl-MMO and CoAl-MMO/CNT. In the case of pure AP, the thermal decomposition proceeds in two stages, as obviously observed from its DTG curve (Fig. 10B).The first low-temperature decomposition centered at about 330 1C corresponds to a heterogeneous process which includes the proton transfer in the
Fig. 9. Low temperature nitrogen adsorption–desorption isotherms and pore size distribution curves (inset) of CNTs (a), CoAl-MMO (b), and CoAl-MMO/CNT (c).
AP subsurface to yield NH3 and HClO4, the adsorption of NH3 and HClO4 in the porous structure, and the decomposition of HClO4 and reaction with NH3 [17,19]. The second high-temperature decomposition with a peak of about 420 1C is assigned to the simultaneous dissociation and sublimation of AP to HClO4(g) and NH3(g) [53]. It is found that when AP was mixed with CNTs, CoAlMMO and CoAl-MMO/CNT, the high-temperature decomposition process disappears completely, which should be associated with the concentration reduction of HClO4(g) and NH3(g). As for CNTs, the peak temperature for low-temperature decomposition is closer to that observed for pure AP (355 1C). In the case of the additive of CoAl-MMO, its peak temperature for low-temperature decomposition (295 1C) is much lower. More surprisingly, when involving CoAl-MMO/CNT as an additive, the peak temperature for lowtemperature decomposition is significantly reduced to 274 1C. As a consequence, CoAl-MMO/CNT nanocomposite could be used as an effective additive to accelerate the AP decomposition. On the other side, extensive works on the decompositions of AP have revealed that the formed HClO4(g) and NH3(g) could be further decomposed [54–57]. In order to determine the catalytic mechanism of CoAl-MMO/CNT, TG-MS measurements for the decomposition of AP with and without CoAl-MMO/CNT were carried out. The TG-MS results indicate that the products of thermal decomposition of pure AP mainly consist of NH3, H2O, HCl, O2, NO, NO2, N2O, and small amounts of ClO and ClO3 species. In the presence of CoAl-MMO/CNT, the decomposition products are almost the same as those for the decomposition of pure AP. As a result, in our case, the possible main chemical reactions involved in the thermal decomposition of AP are proposed as follows (Eqs. (1)–(5)) NH4ClO4-HClO4(g)þNH3(g)
(1)
HClO4(g)-2O2 þHCl
(2)
4NH3(g)þ5O2-NOþNO2 þN2Oþ6H2O
(3)
2ClO4 -2ClO3 þO2
(4)
ClO3 -ClO þ O2
(5)
Fig. 11 shows the DTA curves of AP decomposition with and without additives. For pure AP, there are three events occurring during the decomposition. The first endothermic peak at about 245 1C is ascribed to a phase transition of AP crystals from orthorhombic form to cubic form, accompanied by no weight change. The second exothermic peak at about 329 1C corresponds
Fig. 10. TG (A) and DTG (B) curves measured using aluminum pans for pure AP (a) and mixtures of AP with 5 wt% additives CNTs (b), CoAl-MMO (c) and CoAl-MMO/ CNT (d).
G. Fan et al. / Journal of Solid State Chemistry 197 (2013) 14–22
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through composite precursor of CoAl-LDH and modified CNTs. The resulting CoAl-MMO/CNT showed the enhanced catalytic activity in the decomposition of AP than pristine CoAl-MMO. The dispersion of CoAl-MMO onto the surface of CNTs facilitates the effective adsorption of intermediate species and electron transfer from CoO to Co-containing spinels in composite. Our present study provides an excellent method for assembling metal oxide nanoparticles on CNTs. The as-synthesized CNTs-based composites can be regarded as a sort of promising materials for further application in CSP for rocket propulsion.
Acknowledgments This work is financially supported by the National Basic Research Program of China (Grant No. 2011CBA00506) and the National Natural Science Foundation of China. References Fig. 11. DTA curves for the decomposition of pure AP (a) and mixtures of AP with 5 wt% additives CNTs (b), CoAl-MMO (c) and CoAl-MMO/CNT (d).
to the low-temperature partial decomposition of AP process, while the third exothermic peak at about 444 1C is associated with the high-temperature complete decomposition of the intermediate products into final volatile products. With the addition of CoAl-MMO and CoAl-MMO/CNT to AP, it can be found that the DTA patterns for the decomposition of AP at both low and high temperatures are significantly changed: high-temperature decomposition process disappears and low-temperature decomposition process of AP only presents a single exothermic process. In the case of CoAl-MMO, AP shows a maximum decomposition temperature of 297 1C. Surprisingly, as for CoAl-MMO/CNT as additive, the maximum decomposition temperature of AP is significantly decreased to 271 1C. However, the decomposition peak temperature does not present a close dependence on the surface area of the additives. These aforementioned results clearly indicate that CoAl-MMO/CNT nanocomposite is the most efficient additive to promote the decomposition of the AP in the aspect of decreasing the decomposition temperature. It is well-documented that the thermal decomposition of AP is closely depended on the chemical nature of the additives [58,59], although the detailed mechanism still cannot be definitely described. Several assumptions have been proposed to demonstrate AP decomposition mechanism with oxide additives, which involves ammonia oxidation and dissociation of ClO4 species into ClO3 and O2 species [22,23,60]. As for CoAl-MMO/CNT additive, both CoO and Cocontaining spinels presenting here can exhibit high catalytic activity toward ammonia oxidation [61,62]. Meanwhile, the decomposition of ClO4 can be initiated in the presence of donator (CoO) and accepter (spinels) of electrons, thus promoting the AP decomposition. Furthermore, the relatively high dispersion of metal oxide nanoparticles onto the surface of CNTs promotes the adsorption of intermediate species (NH3, ClO4 , ClO3 , etc.) from AP decomposition, and further donates or entraps more surface electrons to involve the reactions, thus promoting the AP decomposition. The result indicates that the unique hybrid nanostructure and synergistic effect of multi-component metal oxides of CoAl-MMO/CNT nanocomposite play key roles for the enhanced decomposition performance for AP.
4. Conclusions In summary, we have developed a facile and effective approach for synthesizing hybrid CoAl-MMO/CNT nanocomposite
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Co–Al mixed metal oxides/carbon nanotubes nanocomposite prepared via a precursor route and enhanced catalytic property Guoli Fan, Hui Wang, Xu Xiang, Feng Li n State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, PO Box 98, Beijing 100029, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2012 Received in revised form 17 July 2012 Accepted 5 August 2012 Available online 14 August 2012
The present work reported the synthesis of Co–Al mixed metal oxides/carbon nanotubes (CoAl-MMO/ CNT) nanocomposite from Co–Al layered double hydroxide/CNTs composite precursor (CoAl-LDH/CNT). The materials were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), low temperature nitrogen adsorption–desorption experiments, thermogravimetric and differential thermal analyses (TG–DTA), Raman spectra and X-ray photoelectron spectroscopy (XPS). The results revealed that in CoAl-MMO/CNT nanocomposite, the nanoparticles of cobalt oxide (CoO) and Co-containing spinel-type complex metal oxides could be well-dispersed on the surface of CNTs, thus forming the heterostructure of CoAl-MMO and CNTs. Furthermore, as-synthesized CoAl-MMO/CNT nanocomposite was utilized as additives for catalytic thermal decomposition of ammonium perchlorate (AP). Compared to those for pure AP and CoAl-MMO, the peak temperature of AP decomposition for CoAl-MMO/CNT was significantly decreased, which is attributed to the novel heterostructure and synergistic effect of multi-component metal oxides of nanocomposite. & 2012 Elsevier Inc. All rights reserved.
Keywords: Nanocomposite Multi-walled carbon nanotubes Metal oxide Thermal decomposition
1. Introduction In the past two decades, carbon nanotubes (CNTs) have attracted worldwide attention because of their unique structural, mechanical and electronic conducting properties, superior chemical stability and promising applications in transistors, field-emission tips, sensors, supercapacitors, catalyst supports and storage materials for hydrogen [1–5]. More interestingly, its flexible sp2-hybridized graphitic structure with high carrier mobility and good electron-accepting property affords an excellent electrical conductivity for storing and shuttling electrons. Recently, the assembly of CNTs-based heterosturctures or hybrids with the desired nanoscale guests including metals [6,7], semiconductors [8], and metal oxides (e.g. TiO2 [9], MnO2 [10], WO3 [11], Co3O4 [12], CuO [13], a-Fe2O3 [14], ZnO [15]), as an emerging and quickly developing field, has been stimulated for enhancing the properties of resulting versatile materials and achieving a broad range of practical applications. On the other hand, ammonium perchlorate (NH4ClO4, AP) has extensively been used as oxidizer in composite solid propellants (CSP) for rocket propulsion, due to the fact that it is cheap and contains a large amount of oxygen [16]. The performance of CSP, strongly dependant on the thermal decomposition of AP, is greatly
n
Corresponding author. Fax: þ86 10 64425385. E-mail addresses: [email protected], [email protected] (F. Li).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.08.016
improved by solid additives or catalysts [17]. Indeed, numerous works have been done on the catalytic decomposition of AP, and metal oxides (e.g. NiO, CuO, ZnO, CoO and Fe2O3) and complex metal oxides (e.g. LaMO3, Cu–Cr–O and CuFe2O4) are found to be good catalysts and can further enhance the energy-generating characteristics of energetic materials [14,18–27]. Although extensive studies suggest the dependence of AP decomposition on surface microstructures of metal oxide catalysts, the catalytic activity is less directly associated with synergistic effect of diversified active components of catalysts, which could be a remarkable factor on thermal decomposition of AP. xþ Layered double hydroxides (LDHs, [MII1 xMIII (An )x/n x (OH)2] mH2O), also known as a family of highly ordered two-dimensional layered materials, consist of positively charged brucite-like layer xþ ([MII1 xMIII ) and charge-balancing anion (An ) in the x (OH)2] hydrated interlayer galleries [28]. The identities of the divalent and trivalent cations (MII and MIII, respectively) and the interlayer chargecompensating anion (An ) together with the value of the stoichiometric coefficient (x) may be varied over a wide range, giving rise to a large class of isostructural materials with different physicochemical properties. In recent years, LDHs have attracted increasing interests for their potential applications in catalysis, adsorption, photochemistry, electrochemistry and other areas [29,30]. One of the most interesting features of LDHs is their extraordinary capability as catalyst precursors. Upon calcination at intermediate temperatures (450–600 1C), LDHs undergo decomposition and structural transformation, thus giving poorly crystallized mixed metal oxides with large
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specific surface areas, high thermal and chemical stability, and homogeneous dispersion of the metal species [31]. Considerable attention has been paid to Co-containing LDHs due to high catalytic activity of the deriving homogeneous metal oxides for various oxidation reactions [32–34]. Recently, the catalytic activity of Cu–Co metal oxides from LDH precursor for thermal decomposition of AP also has been explored [35]. In this study, for the purpose of combing the unique properties of CNTs with LDHs to achieve excellent performance, we established a novel approach to synthesis Co–Al mixed metal oxides/ carbon nanotubes (CoAl-MMO/CNT) nanocomposite from CoAlLDH/CNT nanocomposite precursor. Furthermore, the catalytic behavior of CoAl-MMO/CNT was investigated for the decomposition of AP. To the best of our knowledge, detailed studies of the synthesis of CoAl-MMO/CNT nanocomposite and its catalytic performance have not been reported before. The results showed that the nanoparticles including cobalt oxide and Co-containing spinel-type complex metal oxides could be well-dispersed on the surface of nitric acid modified CNTs. As-synthesized CoAl-MMO/ CNT nanocomposite exhibited much higher catalytic activity towards the decomposition of AP, as compared to either pristine CoAl-MMO or CNTs, which is closely related to the novel heterostructure and synergistic effect of multi-component Co-containing metal oxides of such nanocomposite.
2. Experimental 2.1. Materials As-received multi-walled CNTs (1.0 g) from Shenzhen Nanotech Port Co., Ltd. were added into 100 mL concentrated nitric acid (68 wt%) at 85 1C for 6 h. After cooling, the suspension was filtered and washed with deionized water to neutral pH, and then dried under vacuum at 60 1C for 12 h. L-Cysteine (98.5%) and other chemicals (analytical grade) were used as received without further purification. 2.2. Synthesis of CoAl-MMO/CNT nanocomposite
carried out on a JEOL JEM-2010 electron microscope at an accelerating voltage of 200 kV. 27 Al solid-state magic-angle spinning nuclear magnetic resonance (NMR) spectrum was measured on a Bruker AV300 spectrometer operating at 78.20 MHz with a pulse width of 0.5 s and spinning rate of 8000 Hz. The specific surface area determination and pore size and volume analysis were performed by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) from the N2 adsorption– desorption measurements at 77 K using a static volumetric Quantachrome Autosorb-1C-VP Analyzer. Raman spectra were conducted on a Jobin Yvon Horiba HR800 spectrometer using a 532 nm line of Ar þ ion laser as excitation source at room temperature. The power to the sample was filtered down to 0.3 mW. X-ray photoelectron spectra (XPS) was recorded on a Thermo VG ESCALAB2201-XL X-ray photoelectron spectrometer at a base pressure of 2 10 9 Pa using Al Ka X-ray (1486.6 eV) as the excitation source. The binding energy (BE) calibration of the spectra has been referred to carbon 1s peak, located at 284.8 eV. Thermogravimetric and differential thermal analysis (TG–DTA) and the mass spectrometer (MS) profiles of the gas evolution were performed on a PerkinElmer Diamond TG/DTA connected to a MS (Pfeiffer ThermoStar) under N2 flow at the ramping rate of 5 1C/min. 2.4. Thermal decomposition of AP The thermal decomposition behaviors of pure AP and the mixture of AP and catalyst (5 wt%) were investigated by TG– DTA with a heating rate of 5 1C/min from room temperature to 550 1C under air atmosphere.
3. Results and discussion 3.1. Synthesis and structure of CoAl-MMO/CNT nanocomposite Fig. 1 shows the powder XRD patterns of CoAl-LDH and CoAlLDH/CNT samples. Two samples display the intensive characteristic
CoAl-LDH/CNT was prepared by a coprecipitation method. Firstly, Co(NO3)2 6H2O (15 mmol) and Al(NO3)3 9H2O (5 mmol) were dissolved in 100 mL of deionized water. Then modified CNTs (1.5 g) along with L-cysteine (15 mmol) were dispersed into the solution in an ultrasonic bath for 30 min under nitrogen atmosphere. Subsequently, the solution was titrated with an alkali solution of NaOH (0.4 M) and Na2CO3 (0.1 M) under vigorous stirring at room temperature until pH¼10.5. The resulting suspension was aged at 60 1C with further stirring for 6 h. The solid was then recovered and washed in deionized water and finally dried under vacuum at 60 1C for 12 h. For comparison, pure CoAl-LDH sample was also prepared under identical reaction conditions without addition of CNTs and L-cysteine. As-prepared CoAl-LDH/CNT and CoAl-LDH samples were calcined under nitrogen gas flow (flow rate: 60 standard-state cm3 min 1) at 500 1C for 4 h at a heating rate of 5 1C/min, and the obtained products were denoted as CoAl-MMO/CNT and CoAl-MMO, respectively. 2.3. Characterization Powder X-ray diffraction (XRD) data were collected at room temperature on Shimadzu XRD-6000 diffractometer with graphitefiltered Cu Ka source (l ¼0.15418 nm), 40 kV, 30 mA. The samples were step-scanned in steps of 0.04 (2y) using a count time of 10 s/step. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were
15
Fig. 1. XRD patterns of CoAl-LDH (a) and CoAl-LDH/CNT (b).
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reflections corresponding to two-dimensional hydrotalcite-like LDH family, i.e. (003), (006), (012), (015), (110) and (113) [35], which reveals the high-crystalline nature of layered LDH phase. In each case, the basal spacing value (d003) of LDH phase is approximately 0.75 nm, consistent with that of carbonate-intercalated LDH materials [29,36]. Meanwhile, as for CoAl-LDH/CNT sample, the reflection at about 26.4o is assigned to that (002) plane of graphitic carbon
with an interplaner distance of about 0.33 nm (JCPDS No.41-1487), confirming the existence of CNTs. The morphology and microstructure of the starting CNTs and asprepared CoAl-LDH/CNT sample was characterized by TEM. It can be clearly observed from Fig. 2 that modified CNTs by nitric acid are multi-walled nanotubes with inner diameter of about 15 nm and outer diameter of about 20–50 nm. As for CoAl-LDH/CNT sample,
Fig. 2. TEM and HRTEM micrographs of CNTs (a and b).
Fig. 3. TEM micrographs of CoAl-LDH/CNT (a and b) and CoAl-MMO/CNT (c), and a HRTEM micrograph of CoAl-MMO/CNT (d).
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the surface combination with CoAl-LDH did not change the morphology and structure of nanotubes. Obviously, note from Fig. 3a that besides a few aggregates of self-nucleated particles, most of nanoparticles are adjacent to the surface of CNTs, suggestive of the affinity between CoAl-LDH nanocrystallites and CNTs matrix. A high-magnification TEM micrograph of two nanotubes with the different outer diameters of about 35 and 100 nm (Fig. 3b) illustrates more clearly the hybrid nanostructure of such nanocomposite, where some nanoparticles with uniform size of about 15 nm are dispersed on the surface of two nanotubes. In the course of synthesis for CoAl-LDH/CNT, L-cysteine with amino and carboxyl groups mainly exists in the ionic form of –(COO )NH3þ [37], when initially mixed with the acidic salt solutions. Therefore, L-cysteine can be immobilized onto the surface of modified CNTs through the electrostatic interaction between surface groups of L-cysteine and CNTs. Moreover, –COO groups of immobilized L-cysteine can coordinate selectively and/or interact electrostatically with Co2 þ and Al3 þ cations in the solution. Subsequently, CoAl-LDH nucleates in situ onto the surface of CNTs through hydrogen bonding between carboxyl groups of L-cysteine and hydroxyl groups of brucite-like layers, which facilitates the adhesion of CoAl-LDH nuclei onto CNTs. It suggests that the dispersion of CoAl-LDH nanocrystallites can be effectively improved by L-cysteine as interfacial bridging linker. XPS analyses were performed for pristine CoAl-LDH and CoAlLDH/CNT sample (Fig. 4). Core levels of Co 2p, Al 2p, C 1s and O 1s can be identified from the survey of XPS. As for CoAl-LDH sample, the peaks with the binding energy (BE) of 782.0 and 786.6 eV are assigned to Co 2p3/2 and its shake-up satellite for Co2 þ ions,
17
respectively [38]. The fine spectrum of Al 2p shows a peak at 75.1 eV, which is related to the Al3 þ species in the form of Al–OH [39]. In the O 1s spectrum, a single peak at 532.2 eV originates from lattice oxygen species of hydroxyl groups [40]. The BE values of Co 2p, Al 2p and O 1s for CoAl-LDH/CNT shift to higher values (around 1.4–2.5 eV), as compared to those for CoAl-LDH. Such large positive shift of BE for metal and oxygen elements has been found in other CNT-based composites reported in the literature [12]. It is known that the BE shift of elements can be associated with the chemical circumstances of elements and the BE value increases as decreasing electron density. The present finding may be explained by the charge transfer from CoAl-LDH crystallites to the surface of CNTs due to the hybridized nanostructure of assynthesized CoAl-LDH/CNT composite involving the strong interaction between the positively charged layers of LDH and the negatively charged functional groups of modified CNTs. In addition, the hydrogen bonding among the carboxylic and/or hydroxyl groups on CoAl-LDH and CNTs could be responsible for the positive shift of BE observed here to some extent. Fig. 5 shows the XRD patterns of calcined CoAl-LDH and CoAlLDH/CNT samples at 500 1C. Obviously, calcination has destroyed the layered structure of the LDH phase, as no characteristic reflections of LDH are present in the XRD pattern for CoAl-MMO sample, and the newly appearing characteristic reflections confirm the presence of cobalt-containing spinel-type complex metal oxide phases, such as Co3O4 and/or CoAl2O4. No characteristic reflections corresponding to Al2O3 phase are observed in the XRD patterns of samples. However, it is rather difficult to distinguish between these mixed oxide phases due to their almost
Fig. 4. XPS spectra of CoAl-LDH/CNT survey, Co 2p, Al 2p, and O 1s of CoAl-LDH (a) and CoAl-LDH/CNT (b).
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Fig. 6.
27
Al NMR spectra of CoAl-MMO/CNT sample.
Fig. 5. XRD patterns of CoAl-MMO (a) and CoAl-MMO/CNT (b).
identical positions of the characteristic reflections. Furthermore, Pe´rez-Ramı´rez et al. [41] have reported that a part of Co3 þ could be substituted by Al3 þ in the Co3O4 phase, and thus another kind of stable spinel-type Co2 þ (Co3 þ , Al)2O4 phase consequently presented in calcined product of CoAl-LDH. Although, Co and Al mixed oxides usually lead to normal spinels, CoAl2O4-type, where Co2 þ occupies tetrahedral positions and Al3 þ fills the octahedral positions. Compared to those for CoAl-MMO, the characteristic reflections of spinel phases for CoAl-MMO/CNT composite are greatly lowered. More interestingly, its XRD pattern shows a series of intense characteristic (111), (200) and (220) reflections of CoO phase (JCPDS no. 431004), except for those corresponding to spinel phases. This should result from the strong interaction between CoAl-LDH nanocrystallites and CNTs in the hybrid nanostructure, leading to the formation of simple CoO. Additionally, the broad characteristic reflections suggest the existence of amorphous alumina phase in sample [42]. In the 27Al NMR spectrum of CoAl-MMO/CNT (Fig. 6), an intensive resonance centered at 9 ppm corresponds to 6-fold coordinated aluminum ions [43]. In addition, the presence of 4-fold and 5-fold coordinated aluminum ions with chemical shifts at around 65 and 45 ppm is demonstrated in sample [44]. This is consistent with the XRD diffraction data described above. The results confirm the collapse of layered structure and the formation of small amount of amorphous Al2O3 domain and appreciable amount of Alcontaining spinel phases after the calcination. A typical TEM micrograph of CoAl-MMO/CNT (Fig. 3c) shows that most of solid nanoparticles are distributed on the surface of nanotubes, indicative of the good dispersion of resulting CoO and Co-containing spinel-type complex metal oxides. In addition, detailed HRTEM observation (Fig. 3d) reveals the presence of interplanar distance of about 0.25 nm that is characteristic of (111) plane for CoO phase, and that CoO nanocrystallites and CNTs are with distinguished and coherent interfaces, indicative of the high affinity between them. Meanwhile, some dark zones distributed over the surface should be assigned to a small amount of amorphous Al2O3 domain. The kind of amorphous Al2O3 phase as a dispersant agent may further inhibit the aggregation of Cocontaining metal oxide particles formed.
Fig. 7. Raman spectra of CNTs (a) and CoAl-MMO/CNT (b).
Raman spectroscopy was utilized to determine the microstructural characteristics of modified CNTs and CoAl-MMO/CNT samples. As seen in Fig. 7, two Raman bands are observed at around 1350 and 1583 cm 1 in each case, which are assigned to D and G modes of nanotubes, respectively, associated with the vibrations of carbon atoms with dangling bonds in terminations of disordered graphite and the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice [45]. Usually, the value of R (ID/IG) is considered for evaluating the degree of graphitization of carbon materials [46]. Correspondingly, the R value calculated using maximum peak intensities increases from 0.61 for modified CNTs to 1.09 for CoAl-MMO/CNT, indicative of more structural defects of CNTs in the composite. The increase in the R value suggests the anchoring of metal oxide nanoparticles onto the surface of CNTs involving the formation of some chemical bonds between them [47]. Additionally, as for CoAl-MMO/CNT, two peaks, located at 484 and 693 cm 1, correspond to the Raman active modes Eg and A1g of CoO phase,
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respectively [48], while a broad band located between 500 and 650 cm 1 is assigned to anharmonic interactions in the CoO structure. XPS analyses for CoAl-MMO and CoAl-MMO/CNT samples were performed (Fig. 8), in order to further obtain an insight into the surface microstructural character. Core levels of Co 2p, Al 2p, C 1s and O 1s can be identified from the survey of XPS for CoAlMMO/CNT, and no other contaminant species are detectable. As for CoAl-MMO sample, the XPS of Co 2p region can be fitted into four contributions. The first two peaks with the binding energy (BE) values of about 780.6 and 786.4 eV are assigned to Co 2p3/2 and its shake-up satellite, while the higher BE peaks around 795.8 and 803.4 eV correspond to Co 2p1/2 and its shake-up satellite, respectively [49]. In the case of high-spin Co2 þ -containing compounds, the BE values of intense Co 2p shake-up satellites are about 5–6 eV higher than those of Co 2p transitions [38], and lowspin Co3 þ species only indicate weak Co 2p shake-up satellites. Therefore, the medium intense Co 2p shake-up satellites confirm the presence of both Co2 þ and Co3 þ species in CoAl-MMO sample. The fine spectrum of Al 2p shows a peak at around 73.8 eV, which is related to the Al3 þ species bonded to oxygen
19
[50]. From the O 1s spectrum of CoAl-MMO, it is seen that there are two fitted peaks, representing two different kinds of surface oxygen species. There is general agreement between the literature and the present results such that the peak with the lower BE around 530.6 eV is the characteristic of lattice oxygen bound to metal cations of the structure, while the peak with the higher BE at 532.6 eV belongs most likely to surface oxygen [51], including mainly oxygen species of hydroxyl groups [40]. As compared with those for CoAl-MMO, the BE values of Co 2p, Al 2p, and O 1s for CoAl-MMO/CNT are increased greatly, which should be ascribed to the charge transfer from metal oxides to the surface of CNTs due to the strong interaction between CoAl-MMO and CNTs matrix. Fig. 9 delineates the nitrogen sorption isotherms of CNTs, CoAl-MMO and CoAl-MMO/CNT samples. It can be found that the samples have a type IV isotherm with a H3 type hysteresis loop that does not exhibit any limiting adsorption at high P/P0, indicating the presence of capillary condensation in the mesoporous structure [52]. CNTs show a narrow pore size distribution from 2 to 8 nm (inset in Fig. 9a), which may be the result of the exposure of the inside pores of nanotubes. The pore size
Fig. 8. XPS spectra of CoAl-MMO/CNT survey, Co 2p, Al 2p, and O 1s of CoAl-MMO (a) and CoAl-MMO/CNT (b).
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G. Fan et al. / Journal of Solid State Chemistry 197 (2013) 14–22
distribution of CoAl-MMO/CNT is similar to that of CNTs but obviously different from that of CoAl-MMO, due to the fact that CoO or spinel particles have been formed densely on the surface of CNTs. Moreover, the BET surface area of CoAl-MMO/CNT (73 m2/g) is larger than that of CNTs (56 m2/g) and smaller than that of CoAl-MMO (105 m2/g), which should be attributed to the high dispersion of metal oxide particles on CNTs. 3.2. Catalytic performance of CoAl-MMO/CNT nanocomposite As-synthesized CoAl-MMO/CNT nanocomposite was studied as an additive in promotion of the thermal decomposition of AP in order to understand the effects of chemical composition and microstructure on the catalytic performance of materials. Fig. 10A presents TG curves for pure AP with and without additive CNTs, CoAl-MMO and CoAl-MMO/CNT. In the case of pure AP, the thermal decomposition proceeds in two stages, as obviously observed from its DTG curve (Fig. 10B).The first low-temperature decomposition centered at about 330 1C corresponds to a heterogeneous process which includes the proton transfer in the
Fig. 9. Low temperature nitrogen adsorption–desorption isotherms and pore size distribution curves (inset) of CNTs (a), CoAl-MMO (b), and CoAl-MMO/CNT (c).
AP subsurface to yield NH3 and HClO4, the adsorption of NH3 and HClO4 in the porous structure, and the decomposition of HClO4 and reaction with NH3 [17,19]. The second high-temperature decomposition with a peak of about 420 1C is assigned to the simultaneous dissociation and sublimation of AP to HClO4(g) and NH3(g) [53]. It is found that when AP was mixed with CNTs, CoAlMMO and CoAl-MMO/CNT, the high-temperature decomposition process disappears completely, which should be associated with the concentration reduction of HClO4(g) and NH3(g). As for CNTs, the peak temperature for low-temperature decomposition is closer to that observed for pure AP (355 1C). In the case of the additive of CoAl-MMO, its peak temperature for low-temperature decomposition (295 1C) is much lower. More surprisingly, when involving CoAl-MMO/CNT as an additive, the peak temperature for lowtemperature decomposition is significantly reduced to 274 1C. As a consequence, CoAl-MMO/CNT nanocomposite could be used as an effective additive to accelerate the AP decomposition. On the other side, extensive works on the decompositions of AP have revealed that the formed HClO4(g) and NH3(g) could be further decomposed [54–57]. In order to determine the catalytic mechanism of CoAl-MMO/CNT, TG-MS measurements for the decomposition of AP with and without CoAl-MMO/CNT were carried out. The TG-MS results indicate that the products of thermal decomposition of pure AP mainly consist of NH3, H2O, HCl, O2, NO, NO2, N2O, and small amounts of ClO and ClO3 species. In the presence of CoAl-MMO/CNT, the decomposition products are almost the same as those for the decomposition of pure AP. As a result, in our case, the possible main chemical reactions involved in the thermal decomposition of AP are proposed as follows (Eqs. (1)–(5)) NH4ClO4-HClO4(g)þNH3(g)
(1)
HClO4(g)-2O2 þHCl
(2)
4NH3(g)þ5O2-NOþNO2 þN2Oþ6H2O
(3)
2ClO4 -2ClO3 þO2
(4)
ClO3 -ClO þ O2
(5)
Fig. 11 shows the DTA curves of AP decomposition with and without additives. For pure AP, there are three events occurring during the decomposition. The first endothermic peak at about 245 1C is ascribed to a phase transition of AP crystals from orthorhombic form to cubic form, accompanied by no weight change. The second exothermic peak at about 329 1C corresponds
Fig. 10. TG (A) and DTG (B) curves measured using aluminum pans for pure AP (a) and mixtures of AP with 5 wt% additives CNTs (b), CoAl-MMO (c) and CoAl-MMO/ CNT (d).
G. Fan et al. / Journal of Solid State Chemistry 197 (2013) 14–22
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through composite precursor of CoAl-LDH and modified CNTs. The resulting CoAl-MMO/CNT showed the enhanced catalytic activity in the decomposition of AP than pristine CoAl-MMO. The dispersion of CoAl-MMO onto the surface of CNTs facilitates the effective adsorption of intermediate species and electron transfer from CoO to Co-containing spinels in composite. Our present study provides an excellent method for assembling metal oxide nanoparticles on CNTs. The as-synthesized CNTs-based composites can be regarded as a sort of promising materials for further application in CSP for rocket propulsion.
Acknowledgments This work is financially supported by the National Basic Research Program of China (Grant No. 2011CBA00506) and the National Natural Science Foundation of China. References Fig. 11. DTA curves for the decomposition of pure AP (a) and mixtures of AP with 5 wt% additives CNTs (b), CoAl-MMO (c) and CoAl-MMO/CNT (d).
to the low-temperature partial decomposition of AP process, while the third exothermic peak at about 444 1C is associated with the high-temperature complete decomposition of the intermediate products into final volatile products. With the addition of CoAl-MMO and CoAl-MMO/CNT to AP, it can be found that the DTA patterns for the decomposition of AP at both low and high temperatures are significantly changed: high-temperature decomposition process disappears and low-temperature decomposition process of AP only presents a single exothermic process. In the case of CoAl-MMO, AP shows a maximum decomposition temperature of 297 1C. Surprisingly, as for CoAl-MMO/CNT as additive, the maximum decomposition temperature of AP is significantly decreased to 271 1C. However, the decomposition peak temperature does not present a close dependence on the surface area of the additives. These aforementioned results clearly indicate that CoAl-MMO/CNT nanocomposite is the most efficient additive to promote the decomposition of the AP in the aspect of decreasing the decomposition temperature. It is well-documented that the thermal decomposition of AP is closely depended on the chemical nature of the additives [58,59], although the detailed mechanism still cannot be definitely described. Several assumptions have been proposed to demonstrate AP decomposition mechanism with oxide additives, which involves ammonia oxidation and dissociation of ClO4 species into ClO3 and O2 species [22,23,60]. As for CoAl-MMO/CNT additive, both CoO and Cocontaining spinels presenting here can exhibit high catalytic activity toward ammonia oxidation [61,62]. Meanwhile, the decomposition of ClO4 can be initiated in the presence of donator (CoO) and accepter (spinels) of electrons, thus promoting the AP decomposition. Furthermore, the relatively high dispersion of metal oxide nanoparticles onto the surface of CNTs promotes the adsorption of intermediate species (NH3, ClO4 , ClO3 , etc.) from AP decomposition, and further donates or entraps more surface electrons to involve the reactions, thus promoting the AP decomposition. The result indicates that the unique hybrid nanostructure and synergistic effect of multi-component metal oxides of CoAl-MMO/CNT nanocomposite play key roles for the enhanced decomposition performance for AP.
4. Conclusions In summary, we have developed a facile and effective approach for synthesizing hybrid CoAl-MMO/CNT nanocomposite
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