Controlled Synthesis And Catalytic Properties Of Nano-sized Rare Earth Oxides

Rare earth materials have found wide applications in many catalytic areas due to theirunique properties depending on the peculiar electronic structures of rare earth elements,however, there are few studies on rare earth materials in catalytic combustion of compositesolid propellants. Besides their electronic structures, the catalytic properties of rare earthmaterials are intimately related to their composition, size, phase, dispersibility, specificsurface areas and surface state, et al. Therefore, controlled synthesis of rare earth materialshas drawn continuous and worldwide research attention. In this dissertation, valuableexplorations have been carried out on the new chemical synthetic strategies ofnanostructured rare earth oxides and their catalytic properties for combustion of compositesolid propellants.In chapter 2, one-step synthesis of nanocrystalline ceria-based powders via a solutioncombustion route using ethylene glycol as a novel fuel is described. An interpretationbased on an adiabatic flame temperature for different fuel-to-oxidant ratios(EG/NO₃^-) hasbeen proposed for the nature of combustion and its correlation with the powdercharacteristics. Ethylene glycol is nontoxic, cheap, easily available, mild duringcombustion reaction and has broad and potentiao applications in solution combustionsynthesis.To solve the problem of particle agglomeration widely existing in solutioncombustion synthesis and enhance specific surface area of the combustion resultants, Wehave come up with a facile and novel salt-assisted solution combustion synthesis (SSCS)route to high surface area ceria-based nanopowders. It is revealed that the introduction ofsoluble and inert salt in the conventional combustion synthesis process was found to resultin the formation of 4～6 nm well-dispersed ceria nanoparticles and a more than ten-foldincrease in the specific surface area of the products from 14.10 to 156.74 m²/g. In the caseof SSCS of Ce_0.75Zr_0.25O₂ solid solution, the addition of NaCl enhanced the specific surfacearea of the products from 17.34 to 208.17 m²/g and led to the mesoporous structure formedby the loose agglomeration of monodisperse nanoparticles. The effects of such influencingfactors as the fuel-to-oxidant ratio, and the nature and amount of added salt on thecharacteristics of the products were investigated. A mechanism scheme was proposed toillustrate the possible formation processes of highly dispersed ceria-based nanoparticles. Finally, the influnce of nanocrystalline ceria-based powders on thermal decomposition ofammonium perchlorate(AP) was studied.On the basis of the research of chapter 2, chapter 3 deals firstly with preparation ofthe well-dispersed perovskite LaMnO₃ nanoparticles by a solution salt-assisted combustionprocess, using ethylene glycol as a fuel and nitrates as oxidants. By tuning thefuel-to-oxidant ratio and the amount of added NaCl, the phase, dipersibility andmorphology of the resultants can be controlled. It is very significant that the cubic orrehombic LaMnO₃ nanorystal and perovskite nanocubes with narrow size distribution canbe obtained under the proper conditions.In the second part of chapter 3, highly dispersed perovskite NdCoO₃ nanoparticleshave been successfully prepared by calcining the NaCl-containing precursor derived fromsalt-assisted solution combustion process employing glycine as a fuel. The facileintroduction of NaCl in the conventional combustion synthesis process was found toprevent particles from sintering and agglomerating and increase specific surface area of theresultants from 1.70 to 43.22 m²/g. Besides enhancing calcination temperature, thepresence and increase of salt promotes formation of peroskite phase and growth ofcrystallite size in the process of calcination.The TG-DSC results indicate that the combustion-derived nano-sized LaMnO₃ andNdCoO₃ show the intense catalytic activity on thermal decomposition of AP. The formerdecreases the temperature of AP high temperature decomposition peak by 103.9℃andincreases the apparent decomposition heat of AP by 838 J/g at most while the latterdecreases the temperature of AP high temperature decomposition peak by 116.7℃andincreases the apparent decomposition heat of AP by 947 J/g at most. Their catalytic activityis related to their particle size, dispersibility and phase, but is to large degree dependent ontheir specific surface area in the last analysis. Finally, the possible mechanism of catalyticdecomposition of AP by nano-sized ABO₃ was preliminarily discussed.In chapter 4, we discuss preparation of nano-sized Y₂O₃ and Nd₂O₃ as well as theirdoped resultants via none-aqueous sol-gel process based on hydrated nitrate and ethyleneglycol, auto-propagating combustion and subsequent calcinations (gel combustionmethod).The as-prepared Y₂O₃ particles are spherical in shape and 20 nm in particle sizewhile the resultant Nd₂O₃ crystallites are spherical, well-dispersedand range from 30 nm to40 nm in size. Further, the effects of the molar ratio of ethylene glycol to yttrium ion,calcination temperature on crystallite size of the products were also studied. In summary,the synthetic method is easy to control, shortens the time to form xerogel and avoids segregation when doping.The results in catalysis show that the 2wt.ï¼…Y₂O₃ and Nd₂O₃ respectively reduce thetemperature ofAP exothermic peak by 106.7℃and 96.2℃as well as enhance the apparentdecomposition heat by 645 J/g and 625 J/g. When doping 3.4ï¼…Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺ andMn²⁺ into Y₂O₃ and Nd₂O₃, respectively, the doped resultants further decrease thetemperature of AP high temperature decomposition peak by 1～10℃and increases theapparent decomposition heat of AP. As the amount of doped Co²⁺ increases, the dopedY₂O₃ and Nd₂O₃ increase the apparent decomposition heat of AP, and have no greatinfluence on or reduce the temperature of AP high temperature decomposition peak.In an effort to synthesize nano-sized NdCoO₃ a on a large scale, chapter 5investigates the preparation of nano-sized perovskite NdCoO₃ via a mechanochemicalprocess of NdCl₃·6H₂O+CoCl₂·6H₂O+6NaOH. It is observed that the NaCl derived in-situduring milling and added NaCl can improve the dispersibility and morphology of theas-synthesized particles owing to inhibiting agglomeration and accelerating mass transfer.Moreover, an increasing addition of NaCl not only improves particle dispersibility,increases specific surface area of the product but also promotes the formation of perovskiteNdCoO₃. It is interesting that highly dispersed NdCoO₃ nanocubes or nanorods can beprepared under the proper conditions. The preparative method is characterized byavailability in raw materials, convenient operation, simple process, high yield and potentialindustrialization.In another effort, the well dispersed elliptical NdCoO₃ with particle size in the rangeof 10～23nm was prepared via wet-solid -phase milling and subsequent calcination ofNd₂O₃ and Co₃O₄ mixture, wherein the ZnO diluent was added after some milling period.The results reveal that an increasing addition of ZnO increases the specific surface area ofthe as-prepared NdCoO₃ from 7.39 m²/g in the absence of ZnO to 36.11m²/g and thatincreasing the calcination temperature results in crystallite growth and reduction in specificsurface area However, the synthetic process has such disadvantage as long preparativeperiod, high energy comsumption and difficulty in complete separation of ZnO diluent.In the last part of chapter 5, we investigates catalytic properties of the nano-sizedNdCoO₃ prepared by a mechanochemical route, respectively using oxide and hydratechloride as raw materials, for the thermal decomposition of AP. The catalytic activityinvestigation demonstrates that the former decreases the temperature of AP hightemperature decomposition peak by 144.3℃and increases the apparent decompositionheat of AP by 632 J/g at most while the latter decreases the temperature of AP high temperature decomposition peak by 144.3℃and increases the apparent decompositionheat of AP by 695 J/g at most. As to the nano-sized NdCoO₃ synthesized by the same route,the larger the specific surface area is, the higher the catalytic activity is. The addition of3wt.ï¼…amorphous NdCoO₃ nanoparticles into the AP/HTPB composite solid propellantincorporates two exothermic peaks of the composite solid propellant into one exothermicpeak, decreases the temperature of the exothermic peak by 33℃。The experiments in combustion characteristics show that the addition of 3wt.ï¼…nano-sized NdCoO₃ nanoparticles into the AP/HTPB composite solid propellant,respectively, enhances the burning rate at the pressures of about 4MPa, 7 PMa and 10MPby 40.0ï¼…, 40.6ï¼…and 23.7ï¼…, reduces the pressure exponents in the pressure scopes of4～7 MPa, 7～10MPa and 4～10 MPa by 16.05ï¼…, 44.4ï¼…and 32.1ï¼…. In summary, theamorphous nano-sized NdCoO₃ is expected to be an excellent and promising modifier forcombustion of the AP/HTPB composite solid propellant.