Effect And Mechanism Of Nanometer Composite NiO/MgO On Thermal Decomposition Characteristics Of AP And Hexafluoropropane

Nanometer metal composite oxides have unique performance and are widely used in military and civilian fields. Nanometer NiO/MgO composite particles are used as catalyst of typical solid propellant to promote decomposition of perchlorate. It can also be used as additive of hexafluoropropane of typical halon fire-extinguishing agents, and by discussing the influences of additives on hexafluoropropane pyrolysis. the fire extinguishing performance is improved and the emission of the HF and other toxic gases are reduced.(1) The nanometer NiO/MgO composite particles were prepared via impregnation method. The phase and morphology of NiO/MgO composites were characterized by X-ray diffraction, transmission electron microscopy and energy dispersive X-Ray spectroscopy. The catalysis of nanometer particles (NiO. MgO),NiO+MgO mixture particles and NiO/MgO composite particles the thermal decomposition of ammonium perchlorate (AP) was investigated by DSC and TG-MS. The Results show that NiO particles are highly dispersed on the MgO support. Catalytic performance of the obtained nanometer NiO/MgO composite particles is superior to that of corresponding single ingredient and NiO+MgO mixture particles. Nanometer NiO/MgO can make high-temperature decomposition peak value of AP decrease by92.2℃and apparent decomposition heat increase by0.376kJ-g-1, which indicates good catalytic performance. The supporting effect of the nanometer MgO can effectively prevent the aggregation of NiO particles and increase the active sites. The main gaseous products of thermal decomposition of AP were NH3. H2O. O2, N2O. NO and several chloride gas. The supporting effect of the nanometer MgO can effectively prevent the aggregation of NiO particles and increase the active sites. MgO hydrolyzed to Mg(OH)2and then expanded, which increased the surface of NiO/MgO and made NiO distributed evenly. And there remained vacany in the nanoparticle NiO after the decomposition of Mg(OH)2. hence the activity of NiO was enhanced and reacted with the air instantly. In the meanwhile. superoxide ion (O2) and oxygenic ion (O-. O-2-) on the surface of nanometer metal oxide can accelerate thermal decomposition of AP.(2) The thermal decomposition properties of hexafluoropropane are studied in tubular reactor. The decomposed gas is characterized by gas chromatography-mass spectrometry (GC-MS). gas chromatography (GC). fluoride ion-selective electrode and ion chromatography (IC). The results show that hydrogen fluoride eliminated from hexafluoropropane is the main reaction between600℃to800℃. As reaction temperature and residence time increase, the degree of thermal decomposition of hexafluoropropane becomes high.Five elementary reaction processes were put forwarded to due to the chemical bond type and then the bond dissociation energies (BDE) of hexafluoropropane decomposition processes were calculated respectively with the method of DFT-(U)B3LYP/6-31G*. The results showed that the Ea of H-transfer process ranked the lowest and then the Ea of C-C departure process was followed, and they were282.74kJ·mol-1and381.88kJ·mol-1respectively. It could be inferred that H-transfer was the most possible schedule and then the departure of HF. A little of gas products such as CF、CHF3、CF2=CHF、CF3CF3、CF3CH2F、CF3C＝CCF3、(CF3)3CH were released accompanied with the decompositon of hexafluoropropane. The mechanism was illuminated that H-transfer was the main process of hexafluoropropane decompositon, which accompanied with the C-C bond crack, and the products reacted with free radical to give off the gases.(3) The thermal decomposition characteristics of hexafluoropropane with nanocrystalline MgO or nanocrystalline NiO/MgO were studied in tubular reactor. The contents of decomposed gas were studied by gas chromatography online (GC), gas chromatography-mass spectrometry (GC-MS) and ion chromatography (IC). while nanocrystalline MgO or nanocrystalline NiO/MgO was characterized by X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). The results show that dehydrofluorinationg of hexafluoropropane is the main reaction and pentafluoropropylene is the primary product. It also indicates that the higher reaction temperature can promote hexafluoropropane decomposition in the empty tubular reactor. When nanocrystalline MgO is filled in the reactor, hexafluoropropane decomposition rate increases. As the reaction temperature increases, hexafluoropropane decomposition rate enhances. After nanocrystalline MgO added under700℃, hexafluoropropane decomposition rate raises from23.9%in empty tubular reactor to100%and HF content in thermal gas product decreases by42.4%. The XRD and XPS patterns reveal a composition system of MgF2/MgO is formed. The mechanism of MgO on the thermal decomposition of hexafluoropropane was preliminarily studied. That partial MgO reacted with HF and was transferred to MgF2during the reaction. The resulting MgF2are very active catalysts for HF elimination from hexafluoropropane.The results also show hexafluoropropane decomposition rate increases when nanocrystalline20wt%NiO/MgO is filled in the reactor. Hexafluoropropane decomposition rate increases to37.1%at500℃. which indicates that dehydrofluorinationg of hexafluoropropane is still the main reaction and pentafluoropropylene is the primary product. But hexafluoropropane decomposition rate reaches100%at700℃. There are chromatographic peaks of pentafluoropropylene and many other prouducts, which means that hexafluoropropane has dehydrofluorinated below700℃and pentafluoropropylene continues to decompose. As weight fraction of NiO increases, the degree of thermal decomposition of hexafluoropropane becomes high. Hexafluoropropane decomposition rate reaches79.9%at600℃.The first step involves a reaction between NiO and CF3CH2CF3to form NiF2. In the next step, there are ion exchange reactions by which the surface NiF2exchanges fluoride with oxygen in MgO substrate, thus the regenerated NiO can react with CF3CH2CF3, and the newly formed NiF2can perform another F-/O2-exchange with the unreacted MgO. In this cycle, NiO promoted the transformation of MgO into MgF2, thereby the reactivity of NiO/MgO for CF3CH2CF3decomposition is improved.