Liquid Phase Reduction Method To The Controlled Synthesis Of Cobalt Crystallite And Its Catalytic Behavior

Nano-materials are intensely studied currently and become the most dynamic region in nanotechnology due to their special chemical/physical properties.Cobalt nanocrystals are of great interest over a long time for researchers from a wide range of fields,including catalysis,high density information storage,solar energy absorption,drug delivery,and magnetic sensors,etc,because they have many excellent characters,such as high specific surface area,huge atomicity,high surface tension.There are numerous techniques of preparing ultrafine particles,but different methods have great influences on the microstructure and properties.So far,the investigated methods for preparing Co nanocrystals are pyrolysis of the cobalt carbonyls,microemulsion synthesis,liquid-phase reduction of cobalt salts by hydrazine hydrate,sodium borohydride or metallic lithium,pulse current electro-deposition,etc.Among these methods,the liquid-reduction route is an ambitious method due to its simple and inexpensive nature.Being cheap and with a large amount of oxygen,ammonium perchlorate(AP,NH₄ClO₄),has extensively been used as an oxidizer in composite propellants for rocket propulsion.Indeed,much work has been done on the catalytic decomposition of AP,and the nanosized metals or metal oxides, such as Ni,CuO,and MgO,are found to be good catalysts.Among the investigated additives, metallic cobalt is less studied.In this study,liquid phase reducing method by hydrazine hydrate and solvent-thermal method are used for preparing the nanocrystals cobalt.The effects of preparing factors were studied in details for the purpose of controllable synthesis of metallic Co with different morphologies and crystal phases.Aiming at obtaining the effects of morphologies and contents on the catalytic performance,the cobalt catalysts were used in AP thermal decomposition.In this thesis,our research is mainly focused on the following three parts:1.Different morphologies of cobalt microcrystals are synthesized by the liquid phase reduction process with hydrazine hydrate as a reductive agent.The effects of synthetic conditions such as reaction temperature,concentration of Co(NO₃)₂,solvents,reducing agents,reaction modes,cobalt precursors were investigated.When the concentration of Co(NO₃)₂ was fixed at 0.050g·mL^-1,the volume of hydrazine hydrate was 20mL,and the temperature from 0 to 80℃,we can get full hexagonal close-packed(hcp) cobalt.But the reaction temperatures have significant effects on morphologies.It is found that highly branched 6-fold "snowflakes" were synthesized when the concentration of Co(NO₃)₂ was 0.025g·mL^-1.More regular cobalt balls were obtained in both of ethylene glycol and 1,2-propanediol solutions.Based on these studies,it is concluded that microcrystal cobalts with different shapes and different crystal structures can be obtained by changing the reaction conditions.2.Cobalt salts were reduced to metallic cobalt by 1,2-propanediol under high pressure and high temperature.The reaction parameters such as reaction time,the amount of NaOH,and the temperatures were investigated.The phase composition and morphological structure of the obtained Co evaluated by XRD,SEM and TEM.Results indicate that the optimum conditions for preparing nano cobalt crystals are the NaOH amount of 5mmol,and at least at 120℃for 24h.When ethylene glycol was used as a reductant,reaction products are also metallic Co but with different morphologies as compared with that in 1,2-propanediol.It means that different alcohols have different reducing capacity.3.The catalytic performance of as-prepared cobalt nanocrystals for the thermal decomposition of ammonium perchlorate(AP) was evaluated by the differential scanning calorimetry(DSC).No matter what shapes of the cobalt nanocrystal were added,the decomposition temperature of AP was significantly decreased.The snowflake-like cobalt showed the highest performance in the aspect of decreasing the decomposition temperature.It made the exothermic peaks reduced 157.8℃.The addition of cobalt nanocrystals with ball-like shapes significantly improved the decomposition heat of the AP.The decomposition heat is 1.4470kJ·g^-1 more than that without catalysts.The effects of Co amounts(1,2,5 and 10%) have been studied.We can get the best effects on decreasing the decomposition temperature when the Co amount is 2%by mass additional.The apparent decomposition heat increased with the amounts of Co and the value of the heat increased to more than 6 times when the Co amount is 10%by mass additional.