Recent progress on transition metal oxides and carbon-supported transition metal oxides as catalysts for thermal decomposition of ammonium perchlorate

Teng Chen ,Yi-wen Hu ,Ci Zhng ,Zho-jin Go

a School of Material and Chemical Engineering,Xuzhou University of Technology,Xuzhou,Jiangsu,PR China

b Xi’an Modern Chemistry Research Institute,Xi’an,Shaanxi,PR China

c Xuzhou University of Technology,Xuzhou,Jiangsu,PR China

Keywords: Transition metal oxides Carbon-supported transition metal oxides Catalyst Ammonium perchlorate Thermal decomposition

ABSTRACT As a main oxidizer in solid composite propellants,ammonium perchlorate(AP)plays an important role because its thermal decomposition behavior has a direct in fluence on the characteristic of solid composite propellants.To improve the performance of solid composite propellant,it is necessary to take measures to modify the thermal decomposition behavior of AP.In recent years,transition metal oxides and carbon-supported transition metal oxides have drawn considerable attention due to their extraordinary catalytic activity.In this review,we highlight strategies to enhance the thermal decomposition of AP by tuning morphology,varying the types of metal ion,and coupling with carbon analogue.The enhanced catalytic performance can be ascribed to synergistic effect,increased surface area,more exposed active sites,and accelerated electron transportation and so on.The mechanism of AP decomposition mixed with catalyst has also been brie fly summarized.Finally,a conclusive outlook and possible research directions are suggested to address challenges such as lacking practical application in actual formulation of solid composite propellant and batch manufacturing.

1.Introduction

Composite solid propellants are extensively used as one of the most important propulsion energy sources in the field of rocket launching and space vehicles carrying[1-3].With the rapid development of aerospace technology and the increasing competition among different nations,higher requirements have been put forward for the performance of composite solid propellants.Developing composite solid propellants with high energy characteristics,high firing range and high survivability have become the mainstream research directions.It is known that composite solid propellants are mainly composed of fuel,oxidizing agent,polymer binder and other functional components.Ammonium perchlorate(AP),which is a kind of strong oxidizer[4,5],presents some unique characteristics,including high density,high oxygen content,high heat generation,large gas production rate and high stability.Owing to these excellent features,AP has been widely used as oxidizing agent in composite solid propellants[6].In addition,AP accounts for 65-70 wt percent of the overall propellant,and in some formulations,the content can even be as high as 90%.It can be seen that the characteristics of AP have a decisive impact on the property of composite solid propellant[7-9].Burning rate and energy performance,which can directly dominate the ballistic property of missiles and rockets,are two key factors in evaluating the property of composite solid propellant.The improvement of burning rate and energy performance of propellant can be gained by enhancing the thermal decomposition of AP.

Therefore,it is necessary to take feasible technical measures to modify the thermal decomposition behavior of AP.Generally,there are two major methods to modify the thermal decomposition of AP,including physical method(super-re fining treatment of AP),and chemical method(utilizing various catalysts).Super-fining treatment of AP is one of measures currently taken to promote the thermal decomposition of AP,which can be ascribed to the increased speci fic surface area and active contact sites by decreasing the particle size of AP.Nevertheless,the super fine particles tend to aggregate,which will reduce the effectiveness in practical use.Besides,the super-fining process of AP should be carried out under severe conditions to ensure safety[10,11].Hence,many research works are concentrated on different catalysts on AP decomposition and the thermal decomposition of AP can be accelerated by adding a small amount of catalysts[12-16].Utilizing a reasonable catalyst can reduce the thermal decomposition temperature of AP,and increase the thermal decomposition rate and the amount of heat release,which are bene ficial to shortening the ignition time and increasing the combustion rate of propellant.Moreover,the pressure index of propellant can also be adjusted by rational design of catalyst.Consequently,designing and constructing of different catalytic materials with complex microarchitectures have raised a wide concern in recent years.

In the last few decades,a variety of catalysts,such as metal powders[17],metal alloys[18],metal oxides[19-24],metal hydroxide [25-27],metal-organic chelates [28-31],carbonsupported composites[32-35],and so forth,have been extensively researched and demonstrated to be effective in modifying thermal decomposition behavior of AP.In the past five years,most research work has focused on transition metal oxides and carbonsupported transition metal oxides,due to their high reactivity,versatile structure,low cost and natural abundance.Although extensive research on the catalytic effect for thermal decomposition of AP in the presence of transition metal oxide and carbonsupported transition metal oxides has been performed,the high thermal decomposition(HTD)temperature,the amount of heat release,and kinetic parameters,remain as key factors to evaluate the catalytic activity.This paper provides a comprehensive summary on transition metal oxides and carbon-supported transition metal oxides as catalysts for thermal decomposition of AP in recent five years.

2.Transition metal oxide catalysts

It is well known that transition metal oxides(TMOs)can serve as active catalysts for AP decomposition[36-38].When a small amount of TMOs are introduced,the thermal decomposition performance of AP can be regulated.Up to now,a variety of TMOs with different morphologies and versatile composition have been explored to catalyze AP[39].Here,we classify TMO catalysts into three categories,including single transition metal oxide catalysts,binary transition metal oxide catalysts and composite transition metal oxide catalysts.The related reports on catalytic performance for the thermal decomposition of AP are summarized.

2.1.Single transition metal oxide catalysts

Nowadays,single transition metal oxide catalysts,such as ferric oxides,cobalt oxides,nickel oxides,zinc oxides,and copper oxides and so on,have been extensively researched,due to their facile fabrication,tunable structure and high catalytic activity.

2.1.1.Ferric oxide

The catalytic performance of ferric oxide is closely related to its morphology and average particle size.When the particle size decreases to nano size,the catalytic activity of ferric oxide will be greatly improved.Cao et al.[40]investigated the catalytic performance of nano-sizedα-Fe2O3with four different particle sizes(127 nm,115 nm,86 nm and 84 nm)using differential scanning calorimetric(DSC)method.DSC tests indicated that the temperature for high-temperature decomposition(HTD)of AP decreased by 40.7°C,42.9°C,50.6°C and 53.4°C with the addition of 2 wt%of four differentα-Fe2O3,implying the catalytic activity ofα-Fe2O3on the thermolysis of AP increased when the average particle size ofα-Fe2O3decreased.For pure AP,the released heat(ΔH)during the process of thermal decomposition was calculated to be 864 J/g.When AP was mixed with 2 wt%of 127 nm and 84 nmα-Fe2O3,the values of released heat were increased to 984 J/g and 1235 J/g,which indicated that the thermal decomposition of AP could be improved by nano-sizedα-Fe2O3.The authors also studied the kinetic analysis and the results further illustrated the decreased particle size ofα-Fe2O3could increase the ef ficiency of catalytic action.They thought that more active sites were exposed on the surface of smaller particles,which would result in higher catalytic activity.Mechanism was not proposed here.

Hossein and his co-workers studied the thermal decomposition behavior of AP catalyzed by nano-sizedα-Fe2O3with spherical morphology[41].They found that both the particle size and the content ofα-Fe2O3can affect the decomposition of AP.Small average particle size and high content ofα-Fe2O3can lead to low decomposition temperature and high decomposition enthalpy of AP.Authors also further investigated the variation tendency of kinetic and thermokinetic parameters,the apparent activation energy(Ea)and the activation enthalpy(△H≠)are remarkably decreased in the presence ofα-Fe2O3NPs.Activation energy can be defined as the minimum energy that is required from the reactant molecule to the activated molecule in a chemical reaction.The smaller the activation energy is,the higher the reactivity is.The activation enthalpy(△H≠)represents the reaction heat that the molecules absorbing or releasing from stable state to activated state.The decreased values of△H≠imply less energy is needed during the reaction process.Hence,the reactant activity of AP is improved in the presence ofα-Fe2O3NPs.

Sharma et al.[42]investigated the catalytic performance of hexagonal cones structuralα-Fe2O3with average particle size around 400-500 nm.Adding 2%ofα-Fe2O3to AP can remarkably decrease the LTD and HTD temperature by 20°C and 75°C,respectively.A possible mechanism has been proposed by the authors according to electron transfer mechanism,as shown in Fig.1.

Generally,the decomposition of AP undergoes three primary steps,including endothermic low-temperature crystal transformation,exothermic low temperature decomposition(LTD)and exothermic high-temperature decomposition(HTD).The LTD process acts as a controlling step and electrons transfer from ClO4-to NH4+during this process,while for HTD,the main reaction can be attributed to the transformation from oxygen(O2)to superoxide ion(O2-).Due to the distinct morphology,remarkable photoelectric and conductive performance ofα-Fe2O3HCs,electron movement can be enhanced,which might accelerate electron transmission from ClO4-to NH4+.Besides,the accelerated electron flow would facilitate the translation of O2into O2-.Hence,the thermal decomposition of AP is considerably enhanced.

2.1.2.Cobalt oxide

Researchers are paying great attention to fabricating cobalt oxide(Co3O4)on account of its variety of application in the fields of catalysts[43],sensors[44],lithium sulfur batteries[45],supercapacitors[46]and so on.As an important member of transition metal oxide,Co3O4present outstanding catalytic activity towards the thermal decomposition of AP[47].Li and co-workers[48]introduced Co3O4spherical microspheres to catalyze AP,the thermal decomposition of AP presents a quite different feature in comparison with that of pure AP.There is only one strong exothermic peak located at 325.4°C with the addition of 2 wt%of Co3O4microspheres.The thermal decomposition temperature of AP was 111°C lower than that of pristine AP,indicating Co3O4microspheres show outstanding catalytic activity on thermolysis of AP.Moreover,the heat release of the mixture was estimated to be 1312.9 J/g,which is 3.76 times higher than that of pure AP(349.0 J/g).The activation energy(Ea)and the pre-exponential factor(lnA)of the mixture were calculated to be 121.9±2.87 kJ/mol and 4.40±0.02 min-1,respectively.Whereas,the values ofEaand lnAfor pure AP were calculated to be 280.5±11.8 kJ/mol and 26.40±0.04 min-1,respectively.With the catalytic effect of Co3O4microspheres,the values ofEaand lnAfor AP were signi ficantly decreased,indicating the as-prepared Co3O4microspheres possess highly catalytic ef ficiency in AP thermal decomposition.

Fig.1.Schematic representation of catalytic thermal decomposition process of AP by Fe2O3 hexagonal cones[42].

The performance of nano-sized materials is highly related to their average particle size(APS)and speci fic surface area(SSA).Hossein and his cooperator[49]systematically researched the effect of nano-sized Co3O4with various APS and SSA on thermolysis temperature of AP.Solvent and non-solvent methods were utilized to fabricate AP/Co3O4nanocomposites(2 or 5%of Co3O4NPs in weight percentage).The speci fications of three kinds of commercial Co3O4nanoparticles(marked as A,B and C)with different APS and SSA are summarized in Table 1 and their catalytic performance on AP are listed in Table 2.

Table 1 Properties of different commercial Co3O4 nanoparticles used as catalyst[49].

APS,SSA and the content of Co3O4can directly affect the decomposition behavior of AP according to the results listed in Table.2.With the decrease of APS and the increase of SSA,the catalytic ef ficiency,including decreased decomposition temperature and enhanced decomposition heat,is remarkably improved.Authors have also illustrated that catalytic performance can be improved by increasing the content of Co3O4on the thermal decomposition of AP.

Table 2 Results of the thermal decomposition of AP in the presence of 2 and 5%of different Co3O4 micro-and nanoparticles(μ,A,B and C),(β=10°C/min)[49].

It is well known that the property of materials can be adjusted by tuning their microstructures.It is of great importance to design and construct various micro/nano materials with complex microstructures.Low-dimensional micro/nano structures,including zero-dimensional(0D)nanoparticles[50],one-dimensional(1D)nanowires[51],and two-dimensional(2D)nanosheets[52],have been extensively researched on account of excellent property such as small grainsize,exposed active sites,high speci fic surface area and shortened mass transfer distance.However,the nano structures tend to aggregate due to their high surface energy,which will inhibit practical applications.Some achievements have been made for the preparation of nano transition metal oxides,but the aggregation still remains a challenge for developing catalysts with high activity.In order to inhibit the aggregation of low-dimensional nanomaterials,an ef ficient measure can be taken by designing three-dimensional(3D)hierarchical micro/nanostructure.Investigations by Miao et al.[53]prove that different morphologies of Co3O4have different impact on the thermal decomposition of AP.Different morphological 3D hierarchical Co3O4micro/nanostructures(Fig.2,Sample 1#-5#)are introduced as catalyst for AP decomposition.

DTA results indicate that different morphologies of Co3O4micro/nano structures present different activity on thermal decomposition of AP.The thermolysis of pure AP undergoes two weight loss procedures,the initial decomposition temperature was about 283°C and the final decomposition temperature was around 443°C.When hierarchical Co3O4micro/nano structures(Sample 1#-5#)are employed,the related initial decomposition temperatures are decreased to 235,233,234,232 and 232°C,and the final decomposition temperatures are decreased to 306,308,315,296 and 301°C,respectively.DSC tests con firm the LTD and HTD process are merged into one exothermic process and the decomposition temperature decreased signi ficantly compared with that of pure AP.Whereas,the exothermic heat is enhanced and the values are increased to 1197,994,1228,933 and 1123 J/g for AP with the addition Co3O4catalyzers(Sample 1#-5#),respectively.These results clearly imply that Co3O4present good catalytic activity and the catalytic performance can be controllably tuned by regulating the morphologies of Co3O4nanoparticles.

Besides,a multiple of Co3O4with different morphologies and particle sizes have been fabricated,and the detailed information including method of synthesis,morphology,particle size,surface area,HTD temperature of AP with and without catalysts,have been summarized in Table 3.

According to the data in Table 3,it can be seen that HTD temperatures for pure AP utilized in different papers are varied.This phenomenon can be ascribed to the difference of the physical properties of AP utilized in different papers,such as particle size,particle size distribution,and morphology of AP.Besides,test conditions during the DSC period,such as types of gaseous condition(nitrogen atmosphere or oxygen atmosphere,gas flow et al.)and types of crucible(aluminum crucible or alumina crucible).All the above mentioned factors can affect the decomposition temperature of AP.Moreover,it can be also observed that morphology,particle size,surface area,and the contents of catalysts have great in fluence on the catalytic performance on Co3O4.

Table 3Summarized performance of various Co3O4.

2.1.3.Nickle oxide

Fig.2.SEMimages of 3D hierarchical Co3O4 micro/nanostructures with different morphology:1#-dandelion flowerlike structure,2#-zephyranthes grandi flora flowerlike structure,3#-sun flower-like structure,4#-chrysanthemum ball-like structure,5#-cotton rose flowerlike structure[53].

Nickel oxide(NiO),as a p-type transparent semiconductor,has been widely used in electronic,magnetic and catalytic aspects.For thermal decomposition of AP,NiO nanoparticles have been drawn great attention due to their apparent catalytic activity.A comparative research on catalytic effects of two different morphologies of NiO was performed by Zhao and his co-workers[66].NiO microflowers present higher catalytic activity than that of NiO nanorods,which could be attributed to the difference of speci fic surface area.Based on the experimental results in this paper,the speci fic surface area for NiO micro flowers is calculated to be 41.725 m2/g,which is higher than that of NiO nanorods(38.077 m2/g).This means more effective and active sites of NiO micro flowers would be exposed on the surface,which is helpful to gas adsorption reaction.Therefore,the catalytic activity of NiO micro flowers is better than NiO nanorods.

Sharma et al.[67]reported a green and eco-friendly biosynthetic strategy to fabricate NiO nanoparticles(NPs)by using leaf extract of plant calotropis gigantea.The as-obtained NiO NPs present spherical morphology with uniformly distributed particle size about 20-50 nm.The catalytic results indicate NiONPs prepared by biosynthetic method possess better catalytic activity than the NPs fabricated by chemical routes.Authors also studied the dependence ofEa on different extent of conversion(α)for AP and mixtures of AP with NiO NPs(Fig.3).According to the results,Eafor pure AP are higher than those for AP mixed with NiO NPs at all values ofα.The variation tendency betweenEaandαmanifests that themolysis of AP is a complicated interaction effect of multiple,competing process and the rate limiting process varies with the extent of conversion.At the initial stage ofα,the high values ofEamay be dominated by nucleation and growth of nuclei.Whereas,the lessening ofEaobserved atα>0.15 is attributed to the transition from kinetically controlled decomposition process to the mass transfer controlled decomposition process[68].

Fig.3.Variation of activation energy(E a)with the extent of conversion(α)for AP with and without biosynthesized NiO NPs[67].

2.1.4.Zinc oxide

Among various transition metal oxides,zinc oxides are also active catalysts in the thermal decomposition of AP.Tian and his coworkers prepared hierarchical porous ZnO hollow microspheres by a facile template-free method in mild experimental conditions[69].The as-obtained ZnO hollow microspheres were assembled by ZnO nanorods and exhibited exposed(001)facets on the external surface.Both ZnO hollow microspheres and ZnO nanorods show catalytic activity towards the thermal decomposition of AP.In the presence of ZnO hollow microspheres,the decomposition temperature of AP is reduced to 308°C and the decomposition heat release can reach up to 1174 J/g.The maximum decomposition temperature and the decomposition heat for AP are estimated to 321°C and 959 J/g,when ZnO dispersed nanorods are added.Kinetic study indicates the values ofEaare remarkably decreased to 63±7 kJ/mol and 90±11 kJ/mol with the catalytic effect of ZnO hollow microspheres and ZnO dispersed nanorods,respectively.Compared with that of ZnO dispersed nanorods,the catalytic activity of ZnO hollow microspheres is prominent in the thermal decomposition of AP.This may be caused by the structural difference between microspheres and nanorods,such as speci fic surface area,crystallinity and exposed facets.ZnO hollow microspheres possess a larger speci fic surface area than ZnO dispersed nanorods,which is bene ficial for the adsorption and diffusion process of gaseous HClO4and NH3(Fig.4(b)).The exposed(001)facets positioned at the external surface of ZnO hollow microspheres can also accelerate the generation of active oxygen species from the adsorbed HClO4which will further oxidize NH3gas.Hence,the absorbed gases will be decomposed.While for ZnO dispersed nanorods,all of the(100)facets,(101)facets and(001)facets are exposed to the gaseous HClO4and NH3(Fig.4(a)).Although most gases were absorbed by the(100)facets,they will not be decomposed[70],which will affect catalytic performance.Therefore,the catalytic activity of ZnO hollow microspheres are enhanced compared with ZnO dispersed nanorods.

2.1.5.Copper oxide

Oxides of copper,as important transition metal oxides,have been extensively researched in the aspect of thermal decomposition of AP because of their prominent catalytic performance.Ke et al.[71]prepared three-dimensionally ordered microporous(3DOM)CuO and investigated its catalytic performance for the thermal decomposition of AP.DTA results illustrated that with the effort of 2 wt%3DOM CuO,the HTD temperature decreased to 354.9°C,and the heat-release of the apparent decomposition of AP increased from 950 J/g to 1453 J/g.The excellent catalytic activity can be ascribed to large surface area and good mass transfer performance of 3D unique structure.Xie and his co-workers[72]fabricated one-dimensional CuO nano fibers by electrospinning method.They investigated the catalytic performance on the thermal decomposition of AP by TG and DTA.The HTD temperature of AP/CuOnano fibers were decreased by 101.9°C compared with pure AP,indicating CuO nano fibers possess excellent catalytic activity.They ascribed this phenomenon to the higher surface to volume ratio of beaded CuO nano fibers.Hossein et al.[73]prepared uniformly distributed CuO nano particles by calcination of copper carbonate.With the addition of 0.5%,2%and 5%CuO NPs,the HTD temperature of AP was reduced by 69.8,66.9 and 104.5°C.The decomposition heat increased to 1356,1512 and 1588 J/g with the catalytic effect of 0.5%,2%and 5%CuO NPs,whereas,the decomposition heat for pure AP was only 728 J/g.The activation energy(Ea)was also decreased remarkably when CuO NPs were employed.With the addition of 5%CuO NPs,the value ofEadecreases to 178.9 kJ/mol,which is approximately 65%of the value for pure AP(280.3 kJ/mol).Authors explained the catalytic performance by electron transfer mechanism.In their opinion,metal oxides act as a bridge for the transportation of electrons,which speed up the electron transferring from ClO- 4 to NH+ 4,thus,the decomposition behavior of AP was enhanced.

Fig.4.Schematic diagram for the thermolysis procedure of AP catalyzed by(a)ZnO dispersed nanorods and(b)ZnO hollow microspheres[69].

Luo et al.[74]investigated three different morphologies of Cu2O cubes(cubic aggregate,mono-dispersed cube and{100}planes etched cube)for the thermal decomposition of AP.According to the calculated kinetic parameters,the numerical values ofEarrange in an ascending order of{100}planes etched cube(92.6 J/mol),monodispersed cube(103.1 J/mol),cubic aggregate(110.4 J/mol).These results indicate the averageEfor AP mixed with Cu2O cubes are less than half the averageEof pure AP(280.2 J/mol),implying Cu2O cubes possess outstanding performance in catalyzing AP decomposition.Besides,the{100}planes etched cube presents the highest activity in the aspect of decreasing the apparent activation energy.They also investigated the complete decomposition time of AP mixed with Cu2O cubes varies with temperature.The catalytic activity for the three types of Cu2O cubes can be easily distinguished in predicting the isothermal decomposition of AP.The results disclose that{100}planes etched cube shows better catalytic performance in AP isothermal decomposition than the other two.

2.2.Binary transition metal oxide catalysts

Besides single transition metal oxides,binary transition metal oxides with spinel structures have drawn great attention for catalyzing AP decomposition,due to their superb catalytic activity caused by the synergistic effect between two different constituent parts[75,76].Spinel crystal structures usually can be expressed by the formula of AB2O4,where A and B represent di-and trivalent metal cations,respectively.

Xiao et al.prepared mesoporous ZnCo2O4rods through oxalate co-precipitation combined with controlled thermal decomposition method without any template[77].The oxalates precursor was calcined at settled temperature under a slow heating rate and the nano-scaled ZnCo2O4crystallites were automatically gathered to generate mesoporous ZnCo2O4rods.They found that the calcination temperature could not change the ultimate structures of ZnCo2O4rods,but the speci fic surface areas are greatly in fluenced by the calcination temperature.ZnCo2O4nano crystallites will grow rapidly and the pore network will collapse under high calcination temperature.The speci fic surface area of ZnCo2O4will be decreased as calcination temperature arises.Authors have also demonstrated the effect of increasing speci fic surface areas on the thermal decomposition of AP which possessed accelerated catalytic activity by increasing the speci fic surface areas.ZnCo2O4rods calcined at 250°C possess the largest surface area(102.34 m2/g)and highest catalytic performance,which can signi ficantly reduce AP pyrolysis temperature by 162.1°C.The catalytic activity of ZnCo2O4rods can be explained by electrons transferring mechanism.Brie fly,ZnCo2O4rods act as a bridge for electrons transferring from ClO-4 to NH+4 and from O2to O-2.Owing to high speci fic surface area,great adsorption of the mesoporous ZnCo2O4rods and positive synergistic catalytic effect of binary oxide,the decomposition behavior of AP will be enhanced with the addition of ZnCo2O4rods.

A comparative investigation on catalytic performance of spinel MnCo2O4nanoparticles and unclaimed MnCo2O4precursor on the thermal decomposition of AP was done by Juibari and his coworkers[78].The results indicate MnCo2O4NPs present promising catalytic activity in decomposing of AP,while,the unclaimed MnCo2O4precursor has little effect on thermolysis of AP.When 2,3,4 wt%of MnCo2O4NPs are employed,the released heat of AP increase to 1350,1410 and 1480 J/g,meanwhile,the HTD temperature shift downwardly to 308,297 and 293°C,respectively.The results illustrate the catalytic performance can be tuned by changing the content.The kinetic parameters of thermal decomposition of AP further indicate the reaction rate increases with the effort of MnCo2O4NPs.As a p-type semiconductor,MnCo2O4possess activedorbital of Co3+(3d5)and Mn2+(3d5)[79],which can be contemporaneously involved in the process of electron transfer and speed the process by simultaneous exposure toand:

The bivalent cobalt cation(Co2+)are unstable and will transform Mn2+(3d5)into the Mn3+(3d6)during another electron transfer process:

A synergistic effect is probable to take place between Co3+and Mn3+,which in turn promotes the formation of active sites of Mn+and Co4+.The active sites play an important role in accelerating the catalytic process.

Copper chromite is also an active catalyst for modifying thermal decomposition behavior of ammonium perchlorate.Hosseini and his co-workers[80]prepared a pure phase of spinel copper chromite by a sol-gel method.Authors investigated the catalytic performance of different Cu-Cr-O.The results indicated that different samples with various morphologies presented different catalytic activity.With the catalytic effect,all of the exothermic peaks of AP decreased.Among these,the sphere-like CuCr2O4NPs presented the highest catalytic activity in reducing the decomposition temperature of AP.The sphere-like morphology of CuCr2O4can effectively prevent nanostructures from aggregating,resulting in decreased particle size and uniform distribution.Moreover,the more crystallization makes sphere-like CuCr2O4NPs a pure phase.All these factors endow sphere-like CuCr2O4NPs with the highest catalytic activity.

2.3.Composite transition metal oxide catalysts

Nano-structured composite materials(or hybrid materials)with extraordinary physico-chemical performance have been widely researched and applied in versatile fields,ascribing to synergistic effect among different composite components.Inspired by this,extensive studies are focus on designing and synthesis of various nano-sized composite transition metal oxide to enhance catalytic activity toward AP decomposition[81].

β-AgVO3/ZnFe2O4nanocomposites were employed as catalyst for thermal decomposition of AP by Abazari and co-workers[82].As a comparison,β-AgVO3and ZnFe2O4were also prepared,respectively.According to the DSC tests for pure AP and AP mixed with 3 wt%ofβ-AgVO3,ZnFe2O4,andβ-AgVO3/ZnFe2O4nanocomposites,the HTD temperatures shift from 432 to 402,367 and 339°C,respectively.Moreover,the heat release(ΔH)for pure AP,AP+ZnFe2O4,and AP+β-AgVO3/ZnFe2O4were estimated to be 764.8,1169,and 1487.3 J/g,respectively.The results indicate thatβ-AgVO3/ZnFe2O4nanocomposites are more active thanβ-AgVO3and ZnFe2O4.

Paulose et al.[83]prepared copper oxide alumina composite by using block copolymer template assisted sol-gel method.Mesoporous copper oxide dispersed on alumina(MCO)with a series of rations of copper oxide and alumina were synthesized.When introducing MCO,the crystallographic phase transition temperature of AP remained unchanged and the LTDtemperatures were not remarkably reduced,indicating mesoporous CuO-Al2O3have a slight impact on the primary decomposition of AP into ammonia and perchloric acid.Whereas,all the samples of as-obtained CuO--Al2O3can signi ficantly in fluent the HTD temperature.The exothermic temperature in HTD process declined,illustrating MCO samples can accelerate the decomposition of AP at a lower temperature.

3.Carbon-supported catalyst

Nanoparticles are easily agglomerated,which remarkably decreases their speci fic surface area and catalytic activity.In order to overcome this problem,carbon materials,such as graphene,nitrogen-doped graphene,graphitic carbon nitride,carbon nanotubes,carbon black and so on,can be employed as a substrate to decorate nano-sized transition metal oxides.Carbon-supported nanocomposites present the combinative merits of nano-sized transition metal oxides and carbon based materials to produce excellent catalytic performance.

3.1.Graphene supported catalyst

The catalytic activity can be remarkably enhanced when the particle size is in nanometer-scale.Bare Fe2O3nanoparticles tend to aggregate and fewer active sites are exposed,resulting in the decrease of catalytic activity.Graphene,owing to its unique structure and performance,can be a promising substrate to disperse and stabilize nanoparticles.With this in mind,the catalytic activities of Fe2O3nanoparticles have been improved considerably by utilizing graphene as substrate.Lan[84]and co-workers synthesized graphene/Fe2O3aerogel via a facile sol-gel and supercritical carbon dioxide drying method,as shown in Fig.5.

Fig.5.Illustration of the synthesis of the graphene/Fe2O3 aerogel[84].

The Fe2O3nanoparticles in graphene/Fe2O3are spherical and well dispersed on the graphene sheets.The speci fic surface area of graphene/Fe2O3aerogel(101 m2/g)is much larger than that of pure Fe2O3nanoparticles(13 m2/g),which con firms that graphene could prohibit the aggregation of Fe2O3particles.The exothermic peaks for low temperature and high-temperature shift to a lower position with the addition of graphene/Fe2O3aerogel.The exothermic heat shows a rising trend with the increased contents of graphene/Fe2O3aerogel.Yuan et al.[85]synthesized Fe2O3/graphene nanocomposite by hydrothermal method.Fe2O3nanoparticles are homogeneously distributed on the wrinkled graphene sheets and the particle sizes are ranged from 50 nm to 80 nm.DSC tests indicate that both Fe2O3/graphene and Fe2O3show high catalytic activity in the thermal decomposition of AP,and Fe2O3/graphene show higher catalytic activity than pure Fe2O3,which is related to the high theoretical surface area and good conductivity of graphene.Graphene could not only prevent the agglomeration of Fe2O3but also provide accelerated electrons to enhance the decomposition of AP.Hence,the catalytic performance of Fe2O3/graphene is superior to pure Fe2O3.So,the support of graphene can effectively improve the catalytic properties of Fe2O3nanoparticles.

To improve the dispersity of CuO nanoplates in the graphene nanosheets,a facile one-step in situ method was employed to fabricate G/CuO nanocomposite according to Fertass and coworker’s report[86].On the basis of G/CuO nanocomposite,Al/G/CuO(Al:G/CuO=82.18:17.82)composite was also obtained by physical mixing of aluminum powder and G/CuO nanocomposite.SEMimages show that some CuO nanocomposites are decorated on the surface of graphene nanosheets,while others are wrapped within the graphene nanosheets.For Al/G/CuO composite,the whole surface of aluminum powder is covered by G/CuO nanocomposites.In the presence of CuO,G/CuO and Al/G/CuO additives,the LTD and HTD peaks of AP all merged into one decomposition peak,which is consistent with the observed result in TGcurves.The high decomposition temperature of AP blended with G,CuO,G/CuO and Al/G/CuO declined from 432°C to 400°C,350°C,325°C and 315°C,respectively.Meanwhile,the activation energy are decreased from 129 kJ/mol to 123.41 kJ/mol,85.12 kJ/mol,71.47 kJ/mol and 56.18 kJ/mol,respectively.The order of catalytic performance for AP thermal decomposition is ranked as Al/G/CuO>G/CuO>CuO>G.The enhancement of AP decomposition is connected with the inherent characteristics of nano additives.Graphene nanosheets present large surface area and high electron transfer,which can accelerate the decomposition of AP.As a transition metal oxide,the d-orbitals of Cu2+cations are partially filled,which can accept electrons generated from AP ions,thus,the electron mobility is promoted and the thermal decomposition of AP is accelerated.The as-prepared G/CuO presents higher catalytic activity than that of pure CuO,which can be attributed to the increased dispersity of CuO nanoplates in graphene nanosheets and more exposed active sites.The substrate of highly conductive graphene decreases the aggregation of CuO,whereas the highly active surface area of graphene remarkably improves the catalytic activity of G/CuO.The catalytic performance of Al/G/CuO is better than G/CuO,indicating the aluminum powder can increase the catalytic activity of G/CuO.The aluminum powder can improve the heat transfer and therefore enhance the chemical reaction process.Moreover,the Al/G/CuO composite can provide a large number of active sites to absorb the gases generated from the initial decomposition process of AP,sequentially,the second decomposition process of AP can be accelerated.Hence,Al/G/CuO shows the best catalytic activity among these additives.

3.2.Nitrogen-doped graphene supported catalyst

In addition to graphene,nitrogen-doped graphene is also attractive in the field of catalyst due to the combination of the three dimensional frameworks and the prominent performance of graphene.

Hosseini et al.[87]reported a promising catalyst for AP decomposition,which contains CuO nanoparticles and nitrogendoped graphene.CuO nanoparticles are uniformly distributed and directly decorated on three dimensional graphene-based frameworks(3D-GFs)with particle size around 20-30 nm.The catalytic properties of as-obtained CuO@3D-(N)GFs nanocomposite are related to its speci fic surface area.By using nitrogen adsorptiondesorption analysis,the value of speci fic surface area for CuO@3D-(N)GFs nanoparticles is calculated to be 124.6 m2/g,while for CuO,the value is 15.6 m2/g.When 4%3D-(N)GFs are employed,there is only slight effect on the thermal decomposition of AP.Whereas,with the addition of 1,2 and 4%CuO@3D-(N)GFs,remarkably decrease of HTD temperatures of AP can be observed,which may be attributed to large speci fic area and more exposed active sites of CuO nanoparticles.Owing to the synergistic effect between 3D-(N)GFs and CuO,the enhanced exothermic heat for AP mixed with CuO@3D-(N)GFs is signi ficantly improved compared with pure AP.The catalytic mechanisms are proposed according to electron transfer theory and proton transfer theory,respectively.On the basis of electron transfer theory,3D-(N)GFs could provide accelerated electrons to promote the electrons transfer from ClO-4 to NH+4 and the generation of superoxidefrom oxygen(O2).Besides,the positive hole provided by partially filled 3d orbit in Cu2+can act as electron acceptor to decompose AP.Under the combined action of 3D-(N)GFs and CuO,CuO@3D-(N)GFs present excellent catalytic activity.When referring to proton transfer theory(Fig.6),proton transfer happens betweenandthe superoxide ionsgenerated from AP decomposition or located on the surface of CuO nanoparticles can capture protons during the process[88].As depicted in Fig.6,the advantageous performance of 3D-(N)GFs,including large speci fic surface area and high thermal conductivity can facilitate the proton transfer fromtoand adsorb more intermediate gas of HClO4and NH3.As the temperature goes up,the adsorbed NH3and HClO4will desorb and react with each other in the gas phase.Moreover,the graphene based substrate can also participate in combustion reaction with HClO4,which will produce more CO2and more exothermic heat will be produced.Furthermore,the 3D-(N)GFs as a substrate can inhibit the aggregation of CuO NPs,resulting in an increase of speci fic surface area and more active sites,which will further promote the catalytic process.

Fig.6.Thermal decomposition of AP by CuO@3D-(N)GFs according to the proton transfer theory[87].

Ni-Mn bimetallic nanoparticles decorated on three dimensional nitrogen-doped graphene-based frameworks by chemical co-reduction method has been also reported by Hosseini and his group[89].They studied the catalytic performance of molar ratio of Ni:Mn,the weight ratio of Ni1Mn2@3D-(N)GFs,3D-(N)GFs support and synergistic effect of Ni and Mn metals on thermal decomposition of AP in detail.The molar ratio of Ni and Mn contained in NiMn@3D-(N)GFs nanocomposites were 2:1,1:1 and 1:2,respectively.The results indicate Ni and Mn with molar ratio of 1:2 in NiMn@3D-(N)GFs nanocomposites present the best catalytic activity.The effect of the different weight ratio(3,5,and 7 wt%)of Ni1Mn2@3D-(N)GFs nanocomposites toward AP decomposition were also studied.When 3 wt%of Ni1Mn2@3D-(N)GFs was employed,the LTD and HTD peaks shift downwardly from 389 to 430°C to 281 and 335.14°C,respectively,and the heat release increase from 509 J/g to 1411.78 J/g.With the addition of 5 and 7 wt%of Ni1Mn2@3D-(N)GFs nanocomposites,the LTD and HTD exothermic peaks were combined into one peak,which are centered at 329.43 and 287°C,respectively.The overall heat release estimated for samples with 5 and 7 wt%additives were 1744.92 and 1331.17 J/g.The above results indicate the samples of 5 and 7 wt%show better catalytic performance than samples of 3 wt%.Compared with the catalytic effect of Ni1Mn2NPs and Ni1Mn2@3D-(N)GFs nanocomposite,three-dimensional nitrogen-doped graphene act as an ef ficient support in improving the catalytic performance,which may be caused by the synergistic effect between 3D-(N)GFs and Ni1Mn2.Moreover,synergistic effect also exists in Ni and Mn metals.With the addition of Ni@3D-(N)GFs and Mn@3D-(N)GFs nanocomposites,there are two exothermic peaks ascribing to LTD and HTD process,respectively.When adding Ni1Mn2@3D-(N)GFs nanocomposites,there is a single exothermic peak,indicating the synergistic effect between two metals improves the catalytic performance.

3.3.Graphitic carbon nitride catalyst

To date,two dimension graphitic carbon nitride(g-C3N4)as a narrow band gap semiconductor has been extensively concerned owning to its unique physical and chemical performance,such as high nitrogen content,excellence chemical and thermal stability,controllable electronic structure and eco-friendly.All these characteristics make g-C3N4a prospective candidate for catalyst and catalytic substrate[90-92].

Li et al.[93]reported g-C3N4as an ef ficient and eco-friendly catalyst for thermal decomposition of AP by calcining the dicyandiamide.Bulk g-C3N4displays 2Dlayered structures,which consists of several graphitic stacking layers.When g-C3N4was introduced,the LTD and HTD process of AP were combined into a sole procedure with the exothermic temperature ranging from 384.4 to 390.1°C.The result shows that g-C3N4can accelerate thermal decomposition rate of AP.In the presence of 10 wt%g-C3N4,the decomposition temperature and activation energy(Ea)of AP are reduced by 70°C and 119.8 kJ/mol,respectively.With the catalytic effect of 10 wt%g-C3N4,the exothermic heat of AP has a remarkable increase and the value can reach up to 1362.6 J/g,which is much higher than pure AP.The instinct of g-C3N4is made up of triazine units linked by planar amino groups,which can be regarded as a Lewis base.Lewis acid-base interaction will be formed when HClO4is absorbed on the surface of g-C3N4.The activation energy of AP decomposition can be decreased by the Lewis acid-base interaction,resulting in the enhancement of AP decomposition.Moreover,g-C3N4is a kind of polymer semiconductor with a band gap and conduction band potential at 2.7 eV and-1.3 eV vs.RHE,respectively,which can be easily stimulated by external heat.When the energy of external heat surpasses the band gap energy,g-C3N4will be excited to generate conduction-band electrons(e-)and valence band holes(h+)on the surface.In the decomposition process,HClO4could be reduced by the conduction-band electrons to create a superoxide radical anion?O2-.Meanwhile,?O2-and h+would further oxidize NH3to produce H2O,NO2and N2O.Thus,g-C3N4presents catalytic activity on thermal decomposition of AP.

On this basis of bare g-C3N4,Li also[94]successfully fabricated SnO2/g-C3N4hybrids via one-pot calcining method.The catalytic results indicate SnO2NPs/g-C3N4hybrids display the best catalytic activity compared with SnO2and g-C3N4,which may be ascribed to the synergistic effect between SnO2NPs and g-C3N4.As stated above,e-and h+could be formed on the surface of g-C3N4under heat irradiation.Based on the synergistic effect of SnO2NPs,the generated electrons on g-C3N4would transfer to SnO2(Fig.7),thus increasing the separation ef ficiency and stabilization of the electron-hole pairs.Therefore,the synergistic effect of SnO2NPs and g-C3N4lead to the best catalytic performance among all the counterparts.

Tan et al.[95]reported a direct precipitation method to prepare(g-C3N4/CuO)nanocomposites.The well-dispersed CuO nanorods with length of 200-300 nm and diameter around 5-10 nm were directly anchored on g-C3N4by the ion-dipole interaction between cupric ions and long pair electrons on the nitrogen atoms of g-C3N4.In the presence of different catalysts,including pure g-C3N4,CuO,and g-C3N4/CuO(various content of CuOfrom 5 to 50 wt%versus g-C3N4/CuO),the catalytic performance is varied.The catalytic activities are ranked in ascending sequence of g-C3N4

3.4.Carbon nanotubes supported catalyst

Rice-shaped manganese dioxides(MnO2)nanoparticles with width of around 5-10 nm and 10-30 nm in the length were successfully anchored on the surface of carbon nanotubes(CNTs)using redox reaction between potassium permanganate and CNTs according to Ling’s research[96].The catalytic performance indicates the activity of rice-shaped MnO2/CNTs composite are superior compared with that of the mixture for MnO2nanorods and CNTs,indicating the well distributed rice-shaped MnO2with a larger surface area can expose more catalytic active sites to promote the thermal decomposition reaction of AP.

Cui et al.[97]prepared CNTs/CuO composites by a complexprecipitation method.They also made a comparison between CNTs/CuO composites and the mixture of CuO-CNTs.With the addition of the mixture of CuO-CNTs,the high decomposition temperature decreases by 135°C,while CNTs/CuOcomposites were employed,the high decomposition temperature decreases by 145°C,demonstrating that the catalytic performance can be enhanced when CuO nanoparticles are deposited on CNTs.

3.5.Other carbon-supported catalyst

Fig.7.Schematic of the thermal decomposition process of AP with the additive of SnO2 NPs/g-C3N4 hybrids[94].

Carbon-coated metal nanoparticles possess distinct performance,such as core-shell structure,large speci fic surface area,good adsorption capacity,excellent conduction and heat transfer,which makes it a promising catalyst for the thermal decomposition of AP.An et al.[98]fabricated carbon-coated copper nanoparticles(CCNPs)via a detonating method,by which the mixture of microcrystalline wax,RDX,and copper nitrate hydrate were initiated through an electric detonator under nitrogen atmosphere and normal pressure.The as-obtained detonation products were spherical morphology with particle size ranging from 25 nm to 40 nm.The nanoparticles consist of a darker copper nanocrystal core and a lighter carbonous shell(3-5 nm thickness).With the catalytic effect of CCNPs,the peak at HTD stage decreases from 424.07°C to 327.08°C and the activation energy of AP are decreased by 25%,indicating CCNPs can accelerate the decomposition of AP under thermal stimulus.

It is known that catalytic performance is related to conductionband electrons from the irradiated ZnO.However,the electrons and holes tend to reunite,which will decrease the catalytic activity.Wang and co-workers decorated ZnO on carbon black(CB)via atomic layer deposition(ALD)method[99].When ZnO nanoparticles are decorated on carbon black(CB),the electrons in conduction band will migrate to CB,thus,the recombination of electrons and holes will be prevented,which can further improve the catalytic performance.The ALD ZnO nanoparticles with mean particle size of 10 nm are deposited on the surface of carbon black through stable C-O-Zn bond,which were formed between oxygen-containing functional group on carbon black and Zn2+ion.The catalytic effect of ZnO/CB hybrid was performed and the results indicate ZnO/CB hybrid present outstanding catalytic activity for the thermal decomposition of AP.The exothermic peak of AP catalyzed by ZnO/CB hybrids is located at 295°C,which is lower than that of bare ZnO(311°C).Moreover,the exothermic heat of AP catalyzed by ZnO/CB hybrids is increased from 376 to 1692 J/g,indicating ZnO/CB hybrid possess admirable catalytic performance.This phenomenon is caused by the appearance of C-O-Zn bond,which can promote electron transport from irradiated ZnO to CB,leading to high utilization rate of the electron than bare ZnO.Thus,the catalytic performance of ZnO/CB hybrids is enhanced.

4.Decomposition mechanisms of ammonium perchlorate

In the last few decades,numerous researches have been concentrated on the decomposition mechanisms of ammonium perchlorate and a number of possible decomposition mechanisms of AP have been proposed,but the decomposition mechanism remains a debatable issue.In this review,two possible mechanisms are brie fly summarized.

4.1.Electron transfer mechanism

Bicromshaw and Newman proposed electron transfer mechanism in 1955[100].According to this theory,electrons transfer from ClO-4 to NH+4,the decomposition procure occurs:

After harvesting an electron,decomposition of the ammonium radicals into ammonia and hydrogen atom takes place:

Hydrogen atom transfers over the lattice.Electron moves accurately in the same way over the anion sublattice:

HClO4is generated owing to the reaction between ClO4radical and H.The resultant HClO4may continue reacting with H,hence:

The ClO3radical acts as acceptors of electrons which can capture electrons.After harvesting an electron,the ClO3radical is transformed intoion.Then,chlorite ion and ClO4radical are decomposed and the products can react with NH+4 ions.Accordingly,secondary products including chlorine,nitrogen hemioxide and water are produced.

Fig.8.Illustration of catalytic thermal decomposition process of AP by CoFe2O4/RGO hybrids[101].

In our previous study[101],we investigated the decomposition mechanism of AP catalyzed by CoFe2O4/RGO hybrids on the basis of electron transfer mechanism(Fig.8).The catalytic performance of CoFe2O4/RGO indicates the hybrids can promote both the LTD and HTD process.The lattice of AP is usually assigned to the pair of ions.As a controlling step,electrons transfer fromto4 in LTD process.Meanwhile,oxygen(O2)would transform into superoxidein the HTD process.The partially filled 3d orbits in Fe or Co atom are bene ficial for electron transferring.Moreover,positive hole in Fe or Co atom acts as electron acceptor for AP ion and its intermediate products,which can enhance the thermal decomposition of AP.Owing to the unique structure and excellent performance of graphene,the movement of an electron in graphene is fairly faster than in metal atoms and travel much longer distance without being scattered.CoFe2O4/RGO hybrids could provide accelerated electron flow to promote the controlling step.With the efforts of more active and accelerated electron flow,and4 can be easily transformed to NH3and HClO4.Furthermore,HClO4decompose into O2,which is subsequently transformed into superoxide.The superoxide could help NH3decompose into N2O,O2,Cl2,H2O and a little NO,completely.

4.2.Proton transfer mechanism

Jackobs[102]put forward proton transfer mechanism of AP thermal decomposition and this mechanism can be described as follows:

This mechanism comprises three steps.In step I,the pair of ionsandare involved in NH4ClO4lattice.Step II involves proton transfer from the cationto the anionvia a molecular complex.In Step III,the molecular complex breaks down into ammonia and perchloric acid.The dissociation products of AP,NH3and HClO4molecules,either react in the absorbed layer over the surface of AP or interact by desorption and sublimation in the gas phase.The gaseous phase of NH3and HClO4react quickly,generating O2,N2O,Cl2,NO,and H2O as accessory products at a low temperature(<350°C).

During the reaction happening in the absorbed layer,the perchloric acid desorbed more quickly compared to ammonia,hence,resulting in incomplete oxidation of ammonia and forming a saturated atmosphere of NH3.As a result,the reaction in HTD process will decelerate and transform incompletely,thus forming NO,O2,Cl2,and H2O in the second exothermic decomposition procedure.Both LTD and HTD begin with proton transfer from NH4+to.The difference between LTD and HTD is that the reaction in a low temperature takes place on the defects of AP crystal,whereas the slow reaction in a high temperature occurs in the lattice of the unreacted normal crystal.Ammonia and chloric acid will be absorbed and desorbed in HTD.Thus,proton transfer in the primary process plays an important role.When additives are introduced,the concentration of the protons will be changed,which will further affect ammonia.

Zhang et al.[103]investigated the thermal decomposition behaviors of AP catalyzed m-g-C3N4/CuO and explained the reaction mechanism on the basis of proton transfer mechanism[98].A solidgas heterogeneous reaction happens in the LTD process,during which protons transfer fromto.Subsequently,NH3and HClO4are formed and then HClO4will oxidize NH3in the gas phase.In the primary stages,the adsorbed HClO4located in the surface and pores of AP lattice act as a crucial chain carrier for AP decomposition,which can further accelerate the decomposition of AP.Besides,m-g-C3N4can be classi fied into Lewis base,owing to its distinct morphologies and surface features.According to the Lewis acid-base theory,HClO4absorbed on the surface of g-C3N4would possess decreased activation energy to enhance the thermal decomposition of AP.

As a semiconductor,the band gap for m-g-C3N4is about 2.70 eV,which is bene ficial for thermal decomposition of AP.When the excitation energy surpasses the banding gap energy,the valence band holes(h+)and conduction-band electrons(e-)can be formed on the surface of m-g-C3N4.The unique properties of m-g-C3N4,such as large speci fic surface area,high separation ef ficiency of electrons and pores,are bene ficial for adsorbing HClO4and NH3on the surface.The gaseous HClO4react with e-to form super-oxide radical anion(O2-),which could further react with h+and NH3to generate N2O,H2O and NO2.On the other hand,the low banding gap(1.68 eV)and band potential(0.46 eV)of CuO make it to be easily activated by heating.The excited electrons on CB of m-g-C3N4could transfer to the CB of CuO and the h+on the VB of CuO could transfer to the VB of m-g-C3N4,which inhibited the recombination of charge carrier.The high separation ef ficiency of e-and h+improves the catalytic activity(Fig.9).

Although many efforts have been made to investigate the mechanism of thermal decomposition of AP catalyzed by different catalysts,the primary thermal decomposition mechanism is still not fully understood,so,there is still a long way to further investigate the mechanism by various technologies.

5.Conclusions and outlook

In recent five years,there has been comprehensive research on preparation,modi fications,characterization and performance for various kinds of catalysts in the thermal decomposition of AP.The easily tunable morphology of transition metal oxide and the unique structure of carbon-supported materials,as well as the excellent catalytic performance,makes transition metal oxide and carbonsupported transition metal oxide materials a promising candidate for AP decomposition.This review gives a summary of general strategies and recent process in developing novel and high ef ficient catalyst in thermal decomposition of AP,including transition metal oxide and carbon-supported transition metal oxide.The as-stated materials in this review present tunable catalytic performance by varying metal elements,adjusting morphology and compounding with carbon-supported analogue.This progress has illustrated that transition metal oxide and carbon-supported transition metal oxide are playing and will continue to play a signi ficant part in modifying the decomposition behavior of AP.

Although a remarkable improvement has been made for the development of high catalytic activity of transition metal oxide and carbon-supported transition metal oxide,there are still considerable challenges for further investigation.Most research works focus on the decomposition behavior of AP mixed with versatile catalysts,however,few works are concentrated on the combustion behavior of solid composite propellants when the novel nano-sized catalysts are employed.The combustion process of solid composite propellants is complicated and the internal ballistic property of solid motor related with the mechanisms is still not fully understood.Moreover,batch manufacturing of nano-sized catalysts have not been implemented,which hinders their practical application.So,it’s necessary to do some research about the catalytic effect on solid composite propellants in practical formulation and develop manufacturing technology for large scale production of catalysts.Motivated by the increasing demand of solid composite propellants coupled with critical performance,it can be envisioned that the utilization of novel nano-sized catalysts will be increased in the near future.

Fig.9.Schematic illustration of the thermal decomposition of AP with m-g-C3N4/CuO nanocomposite[98].

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to in fluence the work reported in this paper.

Acknowledgements

This work was financially supported by the Science and Technology project of Jiangsu province(BN2015021,XZ-SZ201819).