The high efficient catalytic properties for thermal decomposition of ammonium perchlorate using mesoporous ZnCo2O4 rods synthesized by oxalate co-precipitation method

Mesoporous ZnCo2O4 rods have been successfully prepared via oxalate co-precipitation method without any template. The nano-sized spinel crystallites connected together to form mesoporous structure by annealing homogeneous complex oxalates precursor at a low rate of heating. It is found that the low anneal rate plays an important role for the formation of mesoporous ZnCo2O4 rods. The effects of the heat temperature on the phase, morphology and catalytic properties of the products were studied. The XRD, SEM TEM, and N2 absorption/desorption have been done to obtain compositional and morphological information as well as BET surface area of the as-prepared sample. Catalytic activities of mesoporous ZnCo2O4 rods toward the thermal decomposition of ammonium perchlorate (AP) were investigated with differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques. The results show that the addition of ZnCo2O4 rods to AP dramatically reduces the decomposition temperature. The ZnCo2O4 rods annealed at 250 °C possesses much larger specific area and exhibits excellent catalytic activity (decrease the high decomposition temperature of AP by 162.2 °C). The obtained mesoporous ZnCo2O4 rods are promising as excellent catalyst for the thermal decomposition of AP.


Preparation of mesoporous ZnCo 2 O 4 rod.
The typical procedure adopted for the preparation of mesoporous ZnCo 2 O 4 rod is as follows: 16 mM Co(NO 3 ) 2 ·6H 2 O and 8 mM Zn(NO 3 ) 2 ·6H 2 O were successively dissolved in 80 mL deionized water followed by magnetic stirring for 30 min to obtain a homogeneous solution. Subsequently, the 24 mL saturated sodium oxalate solutions was added slowly. After continuous stirring for 2 h, the precipitate was collected by centrifugation and washed with deionized water and absolute ethanol several times and dried in air at 60 °C overnight.
Afterward, the pink precursor was annealed at various temperature (250 °C, 300 °C, 350 °C and 400 °C) for 2 h at a lower heating rate (1 °C/min) in temperature programming furnace. After that, the product was cooled inside the furnace to room temperature. The resultant black powder was collected and directly subjected to the various characterizations.
Characterization of mesoporous ZnMCo 2 O 4 rod. X-ray powder diffraction (XRD) patterns of the product was carried out on a Rigaku D/max-3B diffractometer with an incident X-ray wavelength of 1.540 Å (Cu Kα line), operated at 40 kV, 100 mA. The morphology was observed using field emission scanning electron microscopy (FESEM) taken on FEI nova nanosem 450 with microscope operating at 30 kV. Detailed studies of the microstructure were also carried out by transmission electron microscopy (TEM) (JEOL JEM-2100) at an acceleration voltage of 200 kV. XPS was carried out at room temperature in a PHI 5500 spectrometer with polychromatic Al/Mg-Kα X-ray source. During XPS analysis, Al Kα X-ray beam was adopted as the excitation source and power was set to 250 W. Vacuum pressure of the instrument chamber was 1 × 10 −7 Pa as read on the panel. Measured spectra were decomposed into Gaussian components by a least-square fitting method. Bonding energy was calibrated with reference to C1s peak (285.0 eV). The pore size distributions and the BET surface areas were measured by nitrogen adsorption/desorption using a NOVA2200e gas sorption analyzer (Quantachrome Corp.). Prior to the measurements, the sample was degassed at 300 °C in vacuum for 3 h. The functional groups and coordination of the samples were studied via the FT-IR analysis performed on the Nicolet iS 10 (Semerel technology Corp.), the frequency is range from 4000 to 400 cm −1 . Thermogravimetric and differential thermal analysis (TG-DTA) were performed on a HCT-3 thermal analyzer (Bei-jing) at a heating rate of 10 °C/min from 25 °C to 400 °C.
Catalytic performance to thermal decomposition of AP. The as-prepared sample was mixed with AP to reach certain mass ratios of 2%, 5%, 7% and 10%, respectively. The mixture was fully grinded in the presence of a certain amount of anhydrous ethanol until the ethanol volatilize. Afterwards, the catalytic activities of mesoporous ZnCo 2 O 4 rod in the thermal decomposition of AP were performed using a HCT-3 thermal analyzer (Bei-jing) at a heating rate of 20 °C/min in nitrogen atmosphere over the range of 25-500 °C.

Results and Discussion
Researches show that the pyrolytic process of precipitated precursors has a strong effect on the crystallite phase and the morphology features (crystallite size, surface area, etc.) of the product 23 . Therefore, the pyrolytic behavior of homogeneous complex oxalates precursor was first explored by TG-DTA analysis in air atmosphere, the results Scientific RepORts | (2018) 8:7571 | DOI:10.1038/s41598-018-26022-2 of which is showed in Fig. 1. From the DTA curve, it can be seen that the endothermic process (the coordinated water elimination) happens at about 177 °C, and the precipitated precursor lost 12% of its original weight in the endothermic step as revealed by the TG curve. With increasing temperature, there is a rapid decline in mass (approximately 48% of its original) shown in the TG curve, which indicates the decomposition of oxalic groups and oxidation of the precursor into crystalline ZnCo 2 O 4 . This transitional process is accompanied by a strong exothermic peak at approximately 285 °C in the DTA curve. The single-phase feature of cobalt-zinc complex oxalate is further confirmed by the phenomenon that only one dehydration and one decomposition stage can be observed, as the endothermic and exothermic peaks of zinc oxalate do not accord with those of cobalt. Thus, it can be concluded that the zinc and cobalt atoms distribute on a molecular level in the resulting oxide lattice 19 . The decomposition temperature has huge effect on the crystalline size and the surface area of the resulting sample, and the anneal temperature can be reduced finely with the heating rate decreasing 22,23 . Hence, in this paper, in order to unveil the effects of the heat temperature on the phase, morphology and catalytic properties of the products, the pyrolysis temperature was set at various values from 250 °C to 400 °C on the basis of the above TG-DTA analysis.
XRD patterns of final products obtained by sintering the precursor at 250 °C, 300 °C, 350 °C and 400 °C are shown in Fig. 2(a-d) correspondingly. All the diffraction peaks of the samples are in good consistency with the  that of spinel ZnCo 2 O 4 (JCPDS card no. 80-1543). It also can be seen that the diffraction peaks become increasingly sharp as the calcination temperature elevating, which confirms better crystallization and bigger grain size of the decomposed products with the temperature elevating. But there are diffraction peaks originating from the other phases besides the pure ZnCo 2 O 4 phase (curve (c,d) in Fig. 2) in the XRD spectrum when the heat temperature increases. The peak positions of the additional phases agree with the standard values of the wurtzite ZnO (JCPDS No. . Surface morphologies of the as-synthesized ZnCo 2 O 4 calcined at 250 °C and 400 °C are studied with FESEM, and the obtained results are shown in Fig. 3. Figure 3(a,b) reveals the LRSEM and HRSEM images of the product calcined at 250 °C, and Fig. 3(c,d) corresponds to 400 °C. From the Fig. 3, it can be seen that the morphology of the pyrolysis products are nano-sized crystallites connected together to form mesoporous rod structure, and no obvious changes of the integral structure are observed as the heating temperature increases.
The more information about the shape and crystallite size of the calcined products were further analysed by TEM (Fig. 4). Figure 4(a,b) display the TEM images of the sample calcined at 250 °C with low and high magnification, respectively. From these images it can be seen that the as-synthesized ZnCo 2 O 4 nano-crystals interconnected together to form porous structure. The low and high magnification TEM images of the ZnCo 2 O 4 annealed at 400 °C are exhibited in Fig. 4(c,d). These images demonstrate that the overall porous structure did not change at all, while enlargement of the crystalline size can be observed. In addition, lattice spacing between adjacent planes measured in the high-resolution TEM images ( Fig. 4(b,d)) is 0.470 nm, and 0.462 nm, showing agreements with the distance between (111) crystal planes of cubic spinel ZnCo 2 O 4 . The porous structure might be formed by large amounts of gases slowly released from the micrometer sized oxalate particles leaving over plenty of space during the pyrolysis process 19 .
The pore size distribution and surface area of the as-synthesized ZnCo 2 O 4 obtained at different calcination temperature were measured by nitrogen adsorption/desorption method at 77 K. Figure 5 are N 2 adsorption-desorption isotherms and corresponding pore size distribution plots (plotted by using the BJH calculation model) of the as-synthesized ZnCo 2 O 4 . As can be seen in Fig. 5, all of the porous ZnCo 2 O 4 calcined at different temperatures exhibit a type IV isotherms and H3 hysteresis loop according to the IUPAC classification, which suggest that the mesoporous structure is formed by slit-like mesopore. Those pores were produced form the decomposition of cobalt -zinc oxalate hydrate crystallization during the pyrolysis process. The structural parameters and BET specific surface areas of the as-prepared ZnCo 2 O 4 are derived from the isotherms and tabulated in Table 1. The BET surface areas of ZnCo 2 O 4 calcined at 250 °C, 300 °C, 350 °C, 400 °C are determined to be 102.34, 101.36, 68.02 and 43.25 m 2 ·g −1 , respectively. The above data show that the calcination temperature has a significant impact on the specific surface areas of the products, i.e., the high calcination temperature will induce extensive growth of the spinel ZnCo 2 O 4 and the collapse of the pore network. Thus, the specific surface area of ZnCo 2 O 4 calcined at 400 °C dropped to 43.25 m 2 ·g −1 . From the pore size distribution curve (embedding figure in Fig. 5), it can be seen the size of pores exhibits a strong peak between 2.0 nm to 50 nm for the ZnCo 2 O 4 calcined at different temperature, further verifying the presence of the mesoporous structure in the as-synthesized ZnCo 2 O 4 . The BJH pore volume of ZnCo 2 O 4 calcined at 250 °C, 300 °C, 350 °C, 400 °C is 0.256, 0.294, 0.275 and 0.261 cm 3 ·g −1 based on the calculation, respectively.
The chemical states and surface properties of as-synthesized ZnCo 2 O 4 calcined at 250 °C were analyzed via X-ray photoelectron spectroscopy (XPS), as shown in Fig. 6. Figure 6(a) shows a full survey spectrum of the ZnCo 2 O 4 rod. Characteristic peaks for Zn, Co, O and C elements can be observed in the obtained curve. The binding energy values of the major peaks are 780.5 and 795.5 eV in the Co 2p spectrum ( Fig. 6(b)), corresponding to Co 2p3/2 and Co 2p1/2, respectively. Additionally, the spine-orbit splitting of the mentioned two peaks is 15.0 eV, in accordance with data reported in the literatures 24,25 . Two accompanied weak satellite peaks located at 790.1 and 805.4 eV can also be observed, and the energy gap between the main peak and the satellite peaks is around 9.6 eV. This suggests that Co cation should be trivalent 26 . The strong resolution Zn 2p spectrum is presented in (Fig. 6(c)), in which two strong peaks at 1021.50 and 1044.50 eV can be clearly seen, corresponding to the binding energy of Zn 2p 3/2 and Zn 2p 1/2, respectively, indicating the presence of Zn 2+ in the ZnCo 2 O 4 structure. It is observed that there is an energy separation of 23 eV between the Zn 2p 3/2 and Zn 2p 1/2 peaks, which is in agreement with an earlier report on ZnCo 2 O 4 27 . From the O1s spectrum ( Fig. 6(d)), it can be seen that the spectrum can be fitted to two gauss peaks at 529.8 eV and 531.7 eV are attributed to the lattice oxygen from the ZnCo 2 O 4 rod and the oxygen from hydroxide ions 28 .
The FT-IR spectrum of the as-synthesized ZnCo 2 O 4 rod calcined at 250 °C was displayed in the Fig. 7. The characteristic band around 3442 cm −1 is assigned to the stretching vibration mode of H-O-H group, indicating the presence of chemisorbed water molecules. The prominent band of CO 3 2− ions at 1634 cm −1 and the The propellant's burning rate is affected by the AP particle size 31,32 , the particle size of AP is studied through SEM before the thermal decomposition analysis. From the SEM images of pure AP (Fig. S1.), the AP was inhomogeneous bulk structure, and its size was micron level, from tens of micron to few hundreds micron.
The catalytic performance of as-prepared ZnCo 2 O 4 in the thermal decomposition of AP is demonstrated by the DSC analysis. The curves of AP decomposition in the absence and presence of ZnCo 2 O 4 rod calcined at different temperature at a 2% mass basis are shown in Fig. 8. For pure AP (Fig. 8(a)), the curve indicates that the decomposition process of AP consists of three stages. In first stage, the endothermic peak demonstrates that AP undergoes a crystallographic transition from orthorhombic to cubic phase at 243.47 °C 33 , In the subsequent two stages, the following partial decomposition of AP at 336.76 °C is revealed by the low temperature decomposition (LTD) peak. After that, complete decomposition at 471.15 °C is revealed by the high temperature decomposition (HTD) peak 13 . Compared with the pure AP, an obvious difference for AP decomposition in the presence of mesoporous ZnCo 2 O 4 rod with a mass ratio of 2% can be seen in Fig. 8. All of curves show endothermic peak at the almost same temperature, which indicates that the crystallographic phase transition hasn't been affected by the addition of the mesoporous ZnCo 2 O 4 rod, while significant declines in the value of temperature can be  seen from LTD and HTD peaks ( Fig. 8(b-e)). All of the HTD peaks of AP in the presence of the mesoporous ZnCo 2 O 4 rod shifted even to the front of the LTD peaks of pure AP, indicating that the mesoporous ZnCo 2 O 4 rod can immensely promote the thermal decomposition of AP. The specific thermal decomposition temperature data are summarized in   catalytic performance of mesoporous ZnCo 2 O 4 rod calcined at 250 °C. It is reasonable to propose that the relatively highest specific surface area and the lowest pore size of ZnCo 2 O 4 rod calcined at 250 °C might be the origin of its extraordinary performance, as those paramaters are all crucial factors affecting of catalytic efficiency. From the TG curves (Fig. 9), it can be seen that within the scope of 100 °C to 500 °C, two weight loss steps are clearly observed for pure AP. The first weight loss can be attributed to the partial decomposition of AP and formation of some intermediates by dissociation and sublimation. The second weight loss is caused by the complete decomposition of the intermediate to volatile products 34 Table 3. The thermal decomposition of AP is also influenced by the blend ratio of mesoporous ZnCo 2 O 4 rod. DSC curves of AP with a different mass ratio of mesoporous ZnCo 2 O 4 rod calcined at 250 °C and pure AP are shown in Fig. 10. From Fig. 10, obvious changes can be observed in AP with the addition of mesoporous ZnCo 2 O 4 rod (calcined at 250 °C) of different weight ratios. The HTD peaks of AP are lowered with the increasing mass ratio of the mesoporous ZnCo 2 O 4 rod additive (from 308.93 °C at 2% mass ratio to 295.98 °C at 10% mass ratio). The results reveal that better catalytic performance is achieved with mesoporous ZnCo 2 O 4 within the mixture. TG curves of pure AP and mixtures of ZnCo 2 O 4 and AP with different mass ratios are given in Fig. 11. Two weight loss steps are present during the decomposition of AP, whereas only one step can be observed for mixtures according to the TG curves. Moreover, the temperature corresponding to the start of significant weight loss for mixtures decreases with increased the mass ratios. This finding considerably agrees with that derived from Fig. 10.
Up to now, the thermal decomposition mechanism of AP is not yet fully understood because the decomposition process of AP is a complex solid-gas multiphase reaction process involving reactions in the solid, absorbed and gaseous phases. Several unsolved issues remain till now 40,41 . At low temperatures decomposition, the AP decomposes leading to formation of a small number of intermediate products, the main pivotal step is that the electrons transfer from perchlorate ion to ammonium ion, which would transfer to NH 3 and HClO 4 by dissociation and sublimation and as follows:       As an electron absorption body, ClO 3 can converted into ClO 3 − which then react with NH 3 in the adsorbed gas to produce various such as NO, N 2 O, H 2 O 15 : 3 3 The intermediate products NH 3 and HClO 4 by dissociation and sublimation of AP are not only absorbed on the surface of perchlorate crystal to react, but also desorbed and sublimed into the gas phase 35 . Because the adsorbed NH 3 can not be completely oxidized by the decomposition products of HClO 4 at low temperature, it overlays continually on the surface of AP. Hence, the NH 3 adsorbed on the surface gets saturated, which causes cessation of the reaction and incomplete transformation of perchlorate. As the temperature rising sequentially, the reaction between NH 3 and HClO 4 absorbed on the surface of AP will be triggered again to produce final volatile products including HCl, H 2 O, NO, N 2 O and so on 43 . In the high-temperature decomposition process, the controlling step is the transformation from O 2 to superoxide ion O 2 − , which can further react with NH 3 to form N 2 O, NO 2 and H 2 O 44 . Therefore, the high electron transfer capacity and large specific surface area of the catalyst have an important effect on the thermal decomposition of AP. According to the electron transfer mechanism, the ZnCo 2 O 4 as p-type semiconductor materials, it has effective sites (positive holes on the surface of catalyst) to accept released electron from perchlorate, accompanied by the abstraction of atomic oxygen from the perchlorate ion 45 .  In the thermal decomposition process of AP, ZnCo 2 O 4 serves as a bridge for transferred electrons from perchlorate ions to the ammonium ions and the other transformation from O 2 to superoxide ion O 2 − , as depicted in Fig. 12. In the end, positive synergistic catalytic effect of ternary oxide may also contribute to AP's decomposition 13 . Due to the high specific surface area and great adsorption of the mesoporous ZnCo 2 O 4 rods, the decomposed intermediate products in the gaseous phase of AP can be facilely adsorbed on the surface of mesoporous ZnCo 2 O 4 rods shown in Fig. 13. Thus, the addition of mesoporous ZnCo 2 O 4 to AP can increase the contact area of the catalytic reaction, augment the number of active sites, which promotes the thermal decomposition of AP. So that the mesoporous ZnCo 2 O 4 rod calcined at 250 °C manifests highest catalytic activity than other products, which is mainly because of its relatively highest specific surface area.

Conclusions
In summary, mesoporous ZnCo 2 O 4 rod has been successfully synthesized via a controlled thermal decomposition of homogeneous complex oxalates precursor, which is no need of the assistance of soft/hard template. XRD, SEM, TEM, XPS and nitrogen adsorption/desorption have been done to systematically characterize the structural and morphological features of the as-prepared products. After calcined at given temperature with a low rate of heating, the nano-sized ZnCo 2 O 4 crystallites connected together to form mesoporous rod. The as-prepared material ZnCo 2 O 4 calcined at 250 °C showed much larger surface area (102.34 m 2 ·g −1 ) and high catalytic activity, shifting the AP high thermal decomposition temperature downwardly to about 162.1 °C. The results suggest that the as-prepared mesoporous ZnCo 2 O 4 rod has great catalytic properties on thermal decomposition of AP, which can be used as a promising additive in the future.