Synthesis And Structure Control Of Nano ZIF-67,ZnO And Their Composites To Promote AP Thermal Decomposition

Rocket propellant technology is a key role of national defense industry and aerospace field.In its complex component,oxidizer is very important,on the one hand,due to its decomposition providing abundant oxygen to the combustion reactions of metal incendiary agent and other components as the power of rocket or missile,on the other hand,also providing much heat to transfer to be the power.Ammonium perchlorate(AP)is a traditional and most used oxidizer,whose decomposition property indexes including decomposition temperature,heat release and decomposition energy activation are highly influenced combustion properties of rocket propellant.Therefore,researchers use various methods to promote its decomposition properties.Among these methods,the most efficient way is to add catalyst or promoter,and the most widely applied catalyst is transition metal oxides.There is a electron or proton transfer process in AP thermal decomposition,and transition metal oxides can efficiently promote this electron or proton transfer process.Among these transition metal oxides,ZnO is the most extensive used in AP thermal decomposition,due to its synthesis with low cost,abundant prepared methods,low toxicity,environment-friendly and its excellent reactivity and thermal stability,and shows outstanding catalytic property.Moreover,metal-organic frameworks(MOFs)with tremendous specific surface area and exceptional gaseous adsorption property can promote the adsorptions and reactions of the intermediate HClO4 and NH3 gases in AP thermal decomposition.Furthermore,to synthesize Zn O/MOF composite promotor can further promote AP thermal decomposition property,due to combine the advantages of both materials and synergistic effect.Firstly,in AP thermal decomposition the effect of surface lattice oxygen atom on ZnO(100)facets as the most common exposed had been investigated by the comparative study of the catalyses of mesoporous ZnO and ZnS nanosheets with the same wurtzite structure and(100)exposed crystal facets,which were topological transformation from precursors under different heat treatments.ZnO nanosheets with smaller surface area showed better catalytic activity due to the adsorbed NH3 gas was oxidized by the surface lattice oxygen to generate nitrogen-containing oxides including N2 O,NO and NO2 with oxygen vacancy formation.And these oxygen vacancies were replenished by active oxygen subsequently to recover the ZnO(100)facets.these works had made bedding for the follow-up investigations about catalytic activity enhancement through exosed facet adjustment.Secondly,ZnO hierarchical microspheres assembled by nanosheets should be manufactured to avoid the agglomerate and stacking between their layers.Moreover,low active(100)exposed facets were adjusted into high active(001)facets in order to optimize ZnO catalytic activity.It was proposed that in the topological transformation,their(001)and(100)exposed facets originated from the(001)and(100),(010)exposed facets of hydrozincite precursors,respectively.All these works laid the root for follow-up works about ZnO/MOF composite promotor.Thirdly,mechanism about MOFs promoting AP thermal decomposition is of great significance to investigate ZnO/MOF composite promotor.ZIF-67 nanopolyhedral particles,which were prepared by liquid phase method as a kind of typical MOFs,showed a promoting effect for AP thermal decomposition.The effect mechanism had been investigated.Co-O bonds generated when AP was absorbed on ZIF-67 surface at 250 °C could weaken the adjacent Co-N bonds due to the electronegativity difference between O and N,leading to Co-N bonds decomposed at ultralow temperature 268 °C rather than their normal decomposition temperature 360 °C to generate Co3O4.And then the concentration of surface adsorbed oxygen was highly increased at high-temperature decomposition 330 °C,indicated that Co3O4 was promoting HClO4 decomposition to generate more active oxygen to oxidize NH3 as a catalytic process.These works had made bedding for optimizing ZIF-67 to promote AP thermal decomposition.Fourthly,the effect mechanism of ZIF-67 promoting AP thermal decomposition was highly related to the catalyst of its derivate Co3O4 for AP thermal decomposition,but there was a difference between them.ZIF-67 nanopolyhedral particles with different sizes had been prepared by adjusting concentration of reactant and Co3O4 particles with corresponding size were obtained under heat treatment.ZIF-67 showed a size effect which was that the smaller the size was,the better the catalytic activity displayed,while undersized Co3O4 nanopolyhedral particles showed worse catalytic activity due to their agglomerate.Decreasing sizes of ZIF-67 nanopolyhedral particles was a way of optimizing their property for promoting AP thermal decomposition.All these works established the foundation for ZnO/MOF composite promotor.Finally,based on the optimized ZnO hierarchical microspheres and the optimized ZIF-67 nanopolyhedral particles,ZnO/MOF composite promotor had been synthesized through hydrothermal method on account of heterogeneous nucleation,and its structure had been adjusted as well.All the catalytic property indexes of ZnO/MOF composite promotor were enhanced compared to ZnO hierarchical microspheres and ZIF-67 nanopolyhedral particles,due to synergistic effect combining the advantages of ZnO hierarchical structure avoiding agglomerate,high active(001)exposed facets and advantages of ZIF-67 possessing tremendous specific surface area and exceptional gases adsorption property.