Synthesis and catalytic activity of Cu – Cr – O – TiO 2 composites for the thermal decomposition of ammonium per-chlorate: enhanced decomposition rate of fuel for solid rocket motors

This study presents the sol – gel synthesis of Cu – Cr – O $ n TiO 2 particles calcined at di ﬀ erent temperatures and their catalytic e ﬀ ects on thermal decompostion of AP. The study focuses on the impact of crystallite size, shape and concentration of TiO 2 in the catalyst composition on the thermal decomposition behaviour of ammonium per-chlorate (AP). During synthesis, the molar ratio of Cu/Cr was kept to 0.7 and TiO 2 nanoparticles were added into Cu – Cr – O – citric acid solution at di ﬀ erent molar ratios to form three di ﬀ erent compositions of the catalyst Cu – Cr – O $ n TiO 2 ( n ¼ 0.5, 0.7 and 0.9 mol%). The e ﬀ ect of temperature on the thermal, structural and spectroscopic properties of the di ﬀ erent Cu – Cr – O compositions was also studied by calcining them at two di ﬀ erent temperatures, 300 and 1050 (cid:2) C. Post synthesis characterizations of the prepared catalysts were carried out by using XRD, FT-IR, SEM, EDAX and TEM (with SAED pattern) techniques. The desired quali ﬁ cation of Cu – Cr – O – citric acid (the precursor of the catalyst) and the ﬁ nal compositions of the catalysts were carried out by using thermogravimetric and di ﬀ erential thermal analysis (TG-DTA) techniques. The e ﬃ ciency of the synthesized catalysts was evaluated on thermal decomposition behaviour of AP using TG-DTA techniques. The Cu – Cr – O $ n TiO 2 composition with the molar ratio of n ¼ 0.7 was found to be the most e ﬃ cient catalyst for decomposition of AP; it was much better than other laboratory prepared samples ( n ¼ 0.5 and 0.9) as well as the industrial catalyst ( i.e. activated copper chromite (ACR); Cu – Cr – O). Further experimental work showed that addition of 10 wt% Cu – Cr – O $ 0.7TiO 2 into AP signi ﬁ cantly lowered the AP decomposition temperature to 306 (cid:2) C from 385 (cid:2) C and was accompanied by a very sharp exothermic peak indicating a single stage decomposition. The excellent ﬁ nding of the study was also veri ﬁ ed by heat of reaction ( i.e. calibrated delta H ) values. This study ﬁ nds potential application due to the remarkable enhancement in the thermal decomposition rate of the AP used as oxidizer in propellant of solid rocket motors (SRMs) and space vehicles (SVs) at lower decomposition temperature. The fast decomposition rate of oxidizer at lower decomposition temperature enhances the e ﬃ ciency of fuel which ultimately will enhance the e ﬃ ciency of SRMs and SVs.


Introduction
Solid composite propellants are the vital fuel system used in modern rocketry.Solid rocket motors (SRMs) and space vehicles (SVs) derive energy through solid composite propellants. 1The key ingredients of these solid propellants are hydroxyl terminated polybutadiene (HTPB), aluminium (aluminum) powder (Al) and ammonium per-chlorate (AP).The role of HTPB is to provide the structural integrity to the propellant, Al acts as the metallic fuel and AP is the most popular oxidizer used worldwide in the propellant.The burn rate of a conventional propellant is governed by the combustion behavior of the solid composite propellant and is a function of the thermal decomposition of AP. [2][3][4][5][6][7] Transition metals and metal oxides are used as catalysts to enhance the rate of thermal decomposition of AP, eventually modifying the burn rate.The oxides of iron, manganese, copper, nickel and copper chromites are widely used as the burn rate modiers in the solid composite propellant. 8Activated copper chromite (ACR), with chemical formula CuCr 2 O 4 (Cu-Cr-O), is the industrial catalyst used extensively in solid composite propellants.Solid composite propellants with high burn rate and thereby reduced ignition delay and operational time are needed for modern space vehicles.Liu et al. (2004) carried out a comprehensive study to investigate the effect of Ni, Cu, Al, and NiCu on the thermal decomposition of AP.The study reported the shi of the high temperature decomposition (HTD) peak of AP by 130.2, 112.9, 51.8, 35.1 and 140.4 C, respectively. 9Song et al. (2010) studied the thermal decomposition of AP in the presence of synthesized a-Fe 2 O 3 with different particle sizes and morphologies.In this study, the absence of a low temperature decomposition (LTD) peak and the shi of the HTD peak to 344 C from 454 C was observed. 10 particles and observed that a-Fe 2 O 3 shows better catalytic activity. 12,13These individal groups have lowered the HTD of thermal decomposition of AP with remarkable heat release.Chandru et al. (2012) synthesized b-MnO 2 and found that mesoporous b-MnO 2 is an excellent catalyst, lowering the HTD from 426.3 to 301 C. 14 The effect of the shape and size of CuO and CoFe 2 O 4 on the thermal decomposition of AP was also studied and a signicant impact was observed on the decomposition pattern of AP. [15][16][17][18][19][20][21] In previous research, copper chromite (CuO$CuCr 2 O 4 ), [22][23][24] ACR and titanium oxide (TiO 2 ) 25,26 have been the most utilized metal oxide catalysts which have considerably decreased the onset temperature of AP decomposition when used individually as additives.
The lack of available information on the individual use of copper chromites and TiO 2 leads to a tremendous gap requiring further study.In spite of the extensive individual use of copper chromites and TiO 2 in AP, the physics and chemistry of their catalytic roles in the AP decomposition are not fully known.The aim of the present investigation is to achieve the highest burn rate of fuel at the minimum possible temperature.In the present work we investigate the simultaneous effects of copper chromites and TiO 2 catalysts on the thermal decomposition of AP.Several methods such as co-precipitation, solid state synthesis, the ceramic method, the solution combustion method, the hydrothermal method and sol-gel methods have previously been reported for preparation of the catalysts.Due to the better control of homogeneity and the ease of the procedure, we have adopted the sol-gel method, as reported in the literature. 27Thus, aer the successful preparation of different concentrations of TiO 2 -doped copper chromite compositions, i.e.Cu-Cr-O$nTiO 2 (n ¼ 0.5, 0.7 and 0.9), calcined at two different temperatures, 300 and 1050 C, we introduced them into AP to enhance its burn rate.The results are compared with the effect of the industrial catalyst (ACR) on the thermal decomposition of AP.The pronounced effect of TiO 2doped copper chromite modication of AP on the characteristics of thermal decomposition of AP enhanced the burn rate considerably.

Materials and methods
All the starting reagents were of analytical grade and high purity (>99.9%).AP (NH 4 ClO 4 ) with an average particle size of 300 mm was purchased from Pandian Chemicals Cuddalore, India.The average particle size was reduced to 45 mm using an air classifying mill.Copper nitrate tri-hydrate (CuNO 3 $3H 2 O) was purchased from Titan Biotech Ltd., India.Chromium nitrate nonahydrate (Cr(NO 3 ) 3 $9H 2 O) of laboratory grade was purchased from Central Drug House, India.Anhydrous citric acid (C 6 H 8 O 7 ) was purchased from Fisher Scientic, India.Ethanol (C 2 H 5 OH) was purchased from Merck, Germany, and TiO 2 (P-25) of nano-metric size was purchased from Evonik Industries, Germany.The ready-made industrial catalyst (i.e.activated copper chromite (ACR); CuCr 2 O 4 ) was purchased from BLI Kerala, India.

Synthesis of nano-composites by the sol-gel method
In order to prepare Cu-Cr-Ti mixed oxides via the sol-gel technique, the mother solution of Cu-Cr-citric acid was rst prepared.For the preparation of mother solution, 0.021 moles of CuNO 3 $3H 2 O and 0.03 moles of Cr(NO 3 ) 3 $9H 2 O were added into 20 ml of ethanol.The mixture was stirred until it formed a homogenous solution.Citric acid (0.042 moles) was added into the obtained homogeneous mixture with further stirring.The mother solution thus obtained was divided into three equal parts to prepare three different solutions by adding 0.86, 1.172, and 1.516 g of TiO 2 nanoparticles respectively.A further 5 ml of ethanol was added into each solution and stirring was carried out for 4 h to give transparent suspension-cum-solutions.The solutions thus obtained were kept for aging at room temperature (30 C) for 48 h.Aer this initial aging, each suspension was again mixed thoroughly to ensure the homogeneity of the suspended particles and then heated with continuous stirring at 80 C until a paste-like material was obtained.The pastes thus formed were dried for 3 h in an oven maintained at a temperature of 130 C and the obtained materials were individually crushed to ne particles.Each of the three powders thus obtained was divided into two equal parts for calcination at two different temperatures.The rst part of each sample was calcined individually at 300 C (below the crystallization temperature of Cu-Cr-O) in a N 2 atmosphere for 3 h; the second part was calcined individually at 1050 C (above the crystallization temperature of Cu-Cr-O) in the same atmosphere for 3 h.The crystallization temperature of Cu-Cr-O is 700 C.

Characterization of the catalysts
The phase transformation of the catalyst precursor was conrmed by thermogravimetric analysis (TGA) using a PerkinElmer Thermal Analyzer (Model no.SPA-6000).Crystal structures of the powders were investigated by X-ray diffraction (XRD) using an X-ray diffractometer (Rigaku Miniex-II, Desktop X-ray D) with Cu-K a radiation (l ¼ 1.5418 Å) and a Ni lter operated at 30 kV and 15 mA.The scanning speed and scanning range were kept at 3 min À1 and 20-80 respectively.The surface morphologies and particle sizes were monitored by scanning electron microscopy (SEM) (FEI Quanta 200F).Elemental analysis of the synthesized powders was carried out with the energy dispersive X-ray analysis method (EDAX) inbuilt in the SEM.Chemical bonding and molecular interaction analysis was investigated using Fourier transform infrared (FT-IR) spectra recorded in the wavenumber range from 4000 to 400 cm À1 using a PerkinElmer 577 FT-IR.The catalytic activities of the catalysts were conrmed by thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTA) in a N 2 atmosphere using LABSYSTM Setaram Instrumentation (serial no.1-3347-1 DTA/TGA/DSC) from room temperature (30 AE 2 C) to 600 C with a heating rate of 10 C min À1 .Transmission electron microscopy (TEM) (TECNAI 20 G 2 ) was employed for viewing the interior morphologies/structures of the samples.

Thermogravimetric analysis
Fig. 1 shows the TGA curve of the catalyst precursor (Cu-Cr-Ocitric acid).The curve consists of three regions of weight loss within the temperature range of 30-1000 C.
The rst region (30-180 C) shows a weight loss of 6.6% due to removal of surface moisture.The second region (180-580 C) shows the major weight loss of 54.2% attributed to removal of volatile organics from the decomposition of contained groups.The partial decomposition of nitrate bonds and the formation of intermediate complexes of copper, chromium, TiO 2 and nitrates accounts for the weight loss of 7% in the third segment (580-980 C).Above the temperature of 980 C, the curve looks almost parallel to the temperature axis which shows that no noticeable weight loss occurs.

Fourier transform infrared analysis
Fig. 4(a-c) shows the typical FT-IR spectra of Cu-Cr-O$0.5TiO 2 , Cu-Cr-O$0.7TiO 2 and Cu-Cr-O$0.9TiO 2 calcined at 300 C for 3 h.Fig. 5(a-c) shows the FT-IR spectra of similar compositions calcined at 1050 C for the same duration.The respective spectra of similar compositions calcined at 300 C (see Fig. 4) are quite different from the spectra of the samples calcined at 1050 C (see Fig. 5).It is interesting to note that most of the absorption bands (8 out of 10) disappear dramatically at the higher temperature.Such dramatic vanishing of absorption bands occurred due to the effect of calcination temperature on the spectroscopic properties of these compositions.The crystallization temperature of Cu-Cr-O is 700 C, below which the compositions are in an amorphous state showing similar IR almost patterns for 300 C. No spectroscopic changes are observed due to the varying concentrations of TiO 2 on calcination either at 300 C or at 1050 C. One of the strong shoulders observed at wavenumber 1628 cm À1 is due to the n 2 bending mode of H-O-H in H 2 O. 28,29 The position of this band may vary by AE10 cm À1 depending on the host molecule to which H 2 O is attached.The absorption band appearing at 1414 cm À1 may be attributed to symmetric vibrations of surface hydroxyl groups.A medium shoulder appearing at 1282 cm À1 is attributed to the coupled bending and stretching vibrations of COO À of citric acid.Notably, this band appears in IR patterns of the samples calcined at 300 C but not in the samples calcined at 1050 C.
The dramatic disappearance of this band in the samples red at the higher temperature is due to the fact that citric acid has a melting point of 310 C which is just above the 300 C. The citric acid is not destroyed at 300 C, however, at 1050 C the citric acid is destroyed completely, resulting in the complete absence of the associated bands at this temperature.The peaks at 1058 cm À1 can be attributed to the C-H bending vibration of citric acid.However, a strong absorption band appearing at 1555 cm À1 is assigned to C-C or C-O stretching.These bands also appear at lower temperatures below crystallization Cu-Cr-O but not in the IR of the samples calcined at 1050 C due to the destruction of citric acid at higher temperatures above crystallization.The absorption bands at 721, 793 and 890 cm À1 may be attributed to the different vibrational modes of TiO 2 .TiO 2 exhibits strong absorption in the range 600-890 cm À1 when treated with some additive metal ions as in the present case.The broad band centered at 540 cm À1 in the IR spectra is attributed to the characteristic vibrational band (asymmetric) of Cu-O in CuO.This band also contains the contribution due to the stretching of Cu-O-Cr.These spectra show a strong  shoulder at 469 cm À1 assigned to Ti-O asymmetric stretching vibrations.The amorphous and crystalline natures of the formed compounds are conrmed from the FT-IR spectra shown in Fig. 4(a-c) and Fig. 5(a-c) which are in line with the XRD results.IR spectroscopy conrms the formation of intermediate complex compounds in the amorphous phase as explained by the XRD results.

Scanning electron microscopic analysis
Fig. 6 shows the typical SEM images of the catalyst samples (a) Cu-Cr-O$0.5TiO 2 , (b) Cu-Cr-O$0.7TiO 2 and (c) Cu-Cr-O$0.9TiO 2 calcined at 300 C for 3 h.As shown in the micrographs, the particles have grown in spherical and needle like structures with diameters of up to 180 nm and 50 nm respectively.However, Fig. 7(a-c) exhibits the respective microstructures of the same catalyst samples calcined at 1050 C. Whilst the images shown in Fig. 6 exhibit the formation of nanoparticles of sizes 20-100 nm, the images in Fig. 7(a-c) show that cuboid-like particles of sizes below 1 mm have developed under heat treatment.The shape of the synthesized catalyst particles is more regular and the size is bigger compared with the samples calcined at 300 C.This transformation of surface morphology with the increase in temperature (from 300 to 1050 C) is very interesting.The particle size of the same samples has been changed from nano to micro order due to the diffusion of the particles to each other at the higher temperature (1050 C) minimizing the surface energy.Moreover, the formation of some new phases occurred at 1050 C. The bigger size of the particles leads to a reduction in specic surface area.

Electron diffraction X-ray analysis
Elemental compositions of catalysts calcined at temperatures of 300 and 1050 C for 3 h were carried out using EDAX and are shown in Table 1.This analysis conrmed the presence of copper, chromium, titanium and oxygen.The increasing weight% of oxygen at the increased temperature may occur due to the transformation of intermediate compounds of Cu-Cr-Ocitric acid into metal oxides.Interestingly, CuO has already shown its presence in XRD patterns of the samples calcined at 1050 C but not in the samples calcined at 300 C.

Catalytic efficiency of the catalysts for the thermal decomposition of AP
The synthesized nano-composites of Cu-Cr-O$nTiO 2 with varying molar ratios of TiO 2 were explored as additives to inuence the thermal decomposition of AP.Fig. 9 shows the DTA curves of (a) AP 300m and (b) AP 45m , with particle sizes of 300 and 45 mm, respectively.Examination of the curves clearly shows that the decomposition pattern of AP strongly depends on particle size, resulting in step-wise energy release.Fig. 10 exhibits the corresponding % weight loss of the same samples.In Fig. 9, the endothermic peak for both the samples appears at 250 C which may be due to the crystallographic transformation of AP from orthorhombic to cubic structure.Two exothermic peaks coinciding in both the samples are also observed at temperatures of 302 C and 385 C. The decomposition peak at 302 C is attributed to the formation of intermediates such as NH 4 + and HClO 4 À and is reported as LTD.The crystallographic change temperature for any allotropic/ polymorphic material is constant.However, it may be recorded at an increased temperature (2-5 C) in some certain circumstances like a change in pressure or impurity, a measurement error, or calibration fault.The lower shi of 3 C in the crystallographic change temperature of pure AP may be due to one of these reasons.This endothermic peak attracts comparatively less interest than the exothermic peaks in the present study.The onset of thermal decomposition of all these samples was found within the temperature range of 300 C to 313 C. A brief description of decomposition temperature is shown in Table 2.The differential thermograph shown in Fig. 15 clearly depicts the least effect of the catalysts on the thermal decomposition of AP aer calcination at 1050 C.However, results obtained in Fig. 13 show the maximum effect of a catalyst (Cu-Cr-O$0.7TiO 2 calcined at 300 C) on the thermal decomposition of AP when compared with the results obtained in Fig. 11 and 15.The heats of reaction (i.e.calibrated delta (D)H values) are also estimated and are given in Table 3.A careful examination of the table shows the highest calibrated DH value (À1411.2097)for the AP modied with Cu-Cr-O0.7TiO 2 calcined at 300 C. Therefore, the conclusions may be veried by using Table 3.This table also shows the amount of the samples used in the characterization of thermal decomposition behavior.Thus the sample Cu-Cr-O$0.7TiO 2 calcined at 300 C is very clearly the most efficient (best) catalyst compared with other laboratory prepared samples calcined at 1050 C as well as the industrial ACR (Cu-Cr-O).
The decomposition behavior with and without the presence of the synthesized catalyst materials obtained from the DTA technique are shown in Fig. 9   Thus, AP showed the best decomposition behavior when modied with Cu-Cr-O$0.7TiO 2 calcined at 300 C. In general, the modier catalysts calcined at 300 C exhibited better effects on the thermal decomposition of AP than the catalysts calcined at 1050 C. Crystallinity, surface properties, and specic surface area are the three key factors which determine the catalytic activity of the catalyst.In the present case, the most plausible reason behind the superiority of the Cu-Cr-O samples calcined at 300 C over the similar samples calcined at 1050 C may be as follows: during calcination up to 300 C, only about 20% of the precursor sample is decomposed as can be clearly seen in Fig. 1.Further decomposition of the remaining $80% precursor (above 300 C) alters the AP decomposition as shown in Fig. 14, as the crystallization temperature of Cu-Cr-O is 700 C.This is also reaffirmed by Fig. 4 and 5 (FT-IR).The other factors behind this may be that at the higher temperature (1050 C) the particles diffuse to each other and form comparatively larger particles of different surface morphology (see Fig. 6 and 7) which results in the change in surface properties and a reduction in the specic surface area of the catalyst.Due to this reason, the efficiency of the catalysts was found to be reduced ultimately at the higher calcination temperature.However, the catalyst samples prepared at 300 C showed tremendously enhanced thermal decomposition of AP.Among them, the catalyst sample with 0.7 mol% TiO 2 (Cu-Cr-O$0.7TiO 2 ) exhibited a unique decomposition (burn) rate of AP.

Conclusions
Nano sized Cu-Cr-O$nTiO 2 (n ¼ 0.5, 0.7 and 0.9) particles have been synthesized successfully via the sol-gel method.The nano sized composites of Cu-Cr-O$nTiO 2 calcined at 300 C for 3 h showed excellent catalytic activity on thermal decomposition of AP over the Cu-Cr-O$nTiO 2 oxides calcined at 1050 C for 3 h.Among the three compositions of Cu-Cr-O with their varying concentrations (n ¼ 0.5, 0.7 and 0.9) of TiO 2 calcined at 300 C, 0.7TiO 2 showed the best catalytic activity for decomposition of AP of particle size 45 mm.The Cu-Cr-O$0.7TiO 2 composite calcined at 300 C, when used as a catalyst in AP 45m , gave an excellent single step decomposition of AP 45m at 306 C, which is 79 C lower than that for the pure AP 45m , representing a signicant change in HTD.Thus, the said catalyst achieved a remarkable enhancement in the thermal decomposition of AP which is very much larger than that produced by the industrial catalyst (ACR) as well as other laboratory prepared samples.The sharp peak of the decomposition reveals the faster rate of decomposition.This improved characteristic of synthesized Cu-Cr-O$0.7TiO 2 would reduce the ignition delay of solid rocket motors and space vehicles utilizing AP based solid composite propellant.The obtained results of the present study show that the catalytic efficiency/activity of AP depends on the particle size of the AP and the presence of catalyst, and that both the calcination temperature of the catalyst and the molar concentration of TiO 2 in the catalyst play a major role.
Patil et al. (2008) synthesized nano sized particles of nano sized CuO and CuCr 2 O 4 and studied their individual effects on thermal decomposition of AP.They reported the shi of the HTD peak to 360 C for nano CuO and 348 C for nano CuCr 2 O 4 from 467 C with high heat release. 11Xu et al. (2008) and Zhang et al. (2011) synthesized sphere and rod like a-Fe 2 O 3

Fig. 8
Fig. 8 TEM image of Cu-Cr-O$0.7TiO 2 calcined at 300 C; the selected area electron diffraction (SAED) pattern is shown in the inset image.

Fig. 8
Fig. 8 shows the TEM image of Cu-Cr-O$0.7TiO 2 calcined at 300 C. TEM results are consistent with the previously observed crystallographic, spectroscopic and microstructural characterization results of all the compositions calcined individually at 300 C and 1050 C. The catalyst sample Cu-Cr-O-TiO 2 calcined at 300 C is structurally superb for further application in the desired experiment.The Cu-Cr-O-TiO 2 particles sized around 100 nm can be seen easily in the yellow dotted rings as shown by yellow arrows.The TEM image conrms that the diffusion of particles does not occur at 300 C, however it was happening at the higher temperature of 1050 C as shown in Fig. 7.

2
The peak at 385 C signies the complete decomposition of volatile product into H 2 O, HCl, N 2 , H 2 and O 2 and is termed HTD.Fig. 11 and 12 show the decomposition (DTA) and % weight loss (TGA) patterns, respectively, of AP 45m modied with industrial ACR (Cu-Cr-O).A signicant change can be observed in the thermogram shown in Fig. 11.An endothermic peak appearing at 250 C in pure AP is shied towards lower temperature and appears at 245 C which is the well-known
diffraction peaks corresponding only to TiO 2 (JCPDS le no.46- 1238).This may happen due to the crystallization of TiO 2 in a mixed state at around 300 C, at which temperature the rest of the compounds such as CuCr 2 O 4 , Cu 2 Cr 2 O 4 , Cu 2 O are not found to be crystallized and hence show no diffraction peaks.As the calcination temperature is increased up to 1050 C, the transformation of the composite materials (CuCr 2 O 4 , Cu 2 Cr 2 O 4 , Cu 2 O) from amorphous to crystalline phases takes place due to the well-known crystallization temperature of Cu-Cr-O at 700 C. Fig.3(a-c) shows XRD proles of the same compositions aer calcination at 1050 C for 3 h.The diffraction patterns show the peaks due to CuCr 2 O 4 , Cu 2 Cr 2 O 4 and Cu 2 O along with TiO 2 .The prominent diffraction peaks of CuCr 2 O 4 (JCPDS le no.26-0509) appear at the diffraction angles (2q) of 30.76 and 35.31 .The diffraction peaks of Cu 2 Cr 2 O 4 appear at the diffraction angles of 31.4 , 36.38 , 62.06 , 64.08 and 65.40 (JCPDS le no.05-0668).The presence of Cu 2 O is noticed due to its characteristic peak at 29.78 (JCPDS le no.03-0898).Only three diffraction peaks of TiO 2 , at 27.48 , 54.24 and 56.54 , are observed for the compositions calcined at 1050 C.

Table 2
Decomposition temperatures of pure AP, AP modified with ACR and AP modified with different compositions of Cu-Cr-O-TiO 2 calcined at 300 and 1050 C

Table 3
Calibrated DH values for all the endothermic and exothermic peaks appearing in DTA of pure AP, AP modified with ACR and AP modified with different compositions of Cu-Cr-O-TiO 2 calcined at 300 and 1050 C