液相还原法可控合成钴微晶及其催化行为研究

【作者】 李鑫；

【导师】 刘昭铁；

【作者基本信息】 陕西师范大学 ， 应用化学， 2008， 硕士

【摘要】 纳米材料的物理、化学性质既不同于微观的原子、分子,也不同于宏观物体,它具备很多传统材料不具备的特殊性质,近年来成为纳米科技领域中最有活力、研究内涵最为丰富的学科分支之一。而超细钴粉由于具有大的比表面积、表面原子数、表面能和表面张力,显示出许多优异的性能,在催化、高密度存储材料、太阳能材料、生物抗癌药物和磁性传感器材料等诸多方面具有广泛的应用前景,因而近年来引起广大学者的普遍关注。纳米材料的制备方法很多,不同的制备方法对于控制纳米材料的微观结构和性能具有重要的影响。纳米钴的制备方法主要有:金属有机物高温分解法、高温液相醇解法、脉冲电镀法、微乳液法以及用水合肼、硼氢化钠、金属锂等做还原剂的液相还原法等。在众多制备方法中,液相还原法由于具有工艺流程简单、廉价具有很大的工业应用前景等特点而备受关注。高氯酸铵（AF）是复合固体火箭推进剂中常用的氧化剂利高能组分,在AP系推进剂中占主要成分。AP的性质对固体火箭推进剂的总体性能有重要影响,尤其是其热分解特性与推进剂燃烧特性密切相关,通过研究AP的热分解特性可推测推进剂的燃烧性能。纳米金属及金属氧化物,如Ni、CuO、MgO对AP的热分解表现出很好的催化效果,但是关于Co催化剂在AP热分解中的应用研究较少。本文利用联氨还原法及溶剂热还原法,考察了影响反应过程的诸多因素,渴望实现不同形貌及晶型钴微晶的可控合成。将自制的钴催化剂应用于AP的热分解反应中,获得钴催化剂形貌、含量等对催化性能的影响规律。主要开展了以下三方面的研究:1.利用联氨液相还原法可控合成钴微晶。主要考察了反应温度、Co（NO₃）₂在乙醇中的浓度、反应介质、还原剂用量、反应方式、钴盐前驱体等的影响。反应温度在0到80℃之间,Co（NO₃）₂浓度为0.050 g·mL^-1,还原剂用量20 mL时,制备出具有hcp晶相结构的钴微晶,在不同的温度下产物形貌有所差异。Co（NO₃）₂浓度为0.025 g·mL^-1时能制备出均一雪花状钴微晶。乙二醇和1,2-丙二醇介质中得到球状的钴微晶。根据以上结论可以通过对反应条件的调变可控合成不同形貌及晶型的钴微晶。2.利用1,2-丙二醇作为反应介质及还原剂,采用溶剂热还原法在高温高压下将金属离子还原成单质,通过对反应时间、NaOH用量、反应温度等条件考察,利用XRD、SEM、TEM等表征手段可知:反应时间为24 h,NaOH用量为5 mmol,反应温度为120℃时,1,2-丙二醇即可将钴盐完全彻底还原成钴单质。另一种二元醇,乙二醇做还原剂时,在一定条件下也能将钴盐还原成钴单质,但是所得产物的形貌与1,2-丙二醇作还原剂时有所不同。这说明不同的二元醇的还原能力有所不同。3.利用差示扫描量热法（DSC）初步考察了自制钴微晶催化AP热分解反应的特性。主要研究了雪花状、花菜状和球状钴微晶的催化行为,发现无论钴催化剂的形貌如何对AP分解的过程都有显著的催化效果,但是雪花状钴催化剂在降低分解反应温度方面效果最好,使AP分解温度降低了157.8℃,球状钴微晶催化剂在提高AP分解的表观分解热方面效果最明显,其分解热比未加入催化剂时提高1.4470 kJ·g^-1;对于催化剂的用量也进行了考察,选择催化剂含量分别为1、2、5和10%,当催化剂含量为2%时,对于降低分解温度的效果最佳,而含量越高对其表观分解热的提高越明显,当含量为10%时,其表观分解热比未加入催化剂时提高6倍以上。更多还原

【Abstract】 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.更多还原