【作者】 王丽娜；

【导师】 孙爱灵；

【作者基本信息】 郑州大学 ， 物理化学， 2011， 硕士

【摘要】 制备得到纳米级、高分散的金粒子是金催化剂高活性的前提。制备高分散的、稳定的纳米催化剂,可加入表面活性剂,稳定剂；也可以选择带有巯基,氨基,羧基等官能团的载体,这类载体通过与纳米颗粒相互作用,防止纳米粒子团聚长大,失去活性。载体既可以分散活性组分,提高催化剂的机械性和热稳定性,又可以在某些场合提供活性中心位来提高催化剂的活性和选择性,从而达到节约贵金属的目的。所以选择合适的载体至关重要。本文选择富含氨基的壳聚糖(CS)和聚乙烯亚胺(PEI)高分子载体,既是载体,又是稳定剂。采用化学还原法制备出金负载量低、活性较高、粒径分布为30-60nm的Au/CS和粒径分布约为2-7nm的Au/PEI纳米粒子,并且氨基与金相互作用比较强,有效地避免了金纳米粒子之间的团聚,即使放置几个月催化活性依然比较好,较好地实现了预期目标。借助Uv-vis、TEM、HPLC等表征手段研究了纳米金催化剂的大致形貌以及活性。PEI的加入促进成核速率,稳定了金纳米粒子,延缓其进一步长大的速率,从而最终形成粒径较小、粒径分布较窄的纳米粒子。以对硝基苯酚催化还原制备对氨基苯酚为模板反应,考察了Au/PEI纳米粒子制备条件对其催化性能的影响,比如：还原剂的种类、载体、负载量、还原温度、还原时间、Au与还原剂的比例以及反应时不同溶剂及其配比等不同条件对其催化活性的影响,发现制备时还原剂的种类对纳米金的催化效果影响最大,其次是还原时间、Au与PEI的比例、还原温度,Au与还原剂的比例的影响最小。保持金的用量相同,使用负载量不同的同类催化剂时,金负载量高的纳米金催化剂的催化效果相对要好一些。所制得的催化剂在低温避光保存几个月后,仍能表现出良好的催化活性。另外,反应体系中若加入乙醇,会大大降低原来的反应速率,可能是因为乙醇的极性比水低,并且整个反应体系中乙醇的含量越高,反应进行的速度越慢,甚至反应不完全。可能是乙醇的加入降低了NaBH4的水解速率,从而影响整个反应速率。该催化反应的机理有待进一步研究。更多还原

【Abstract】 Preparation of nano-scale, highly dispersed gold particles provides their high catalytic activity as gold catalysts. In order to prepare highly dispersed and stable catalysts, surfactant or stabilizing agent can be added; and there is also another option to select meterials with functional groups such as thiol, amino or carboxyl as the supports, which have some interaction with gold to prevent agglomeration of gold nanoparticles and loss of their activities.Supports have a significant effect on the supported catalysts and it is so important to select proper supports to prepare good catalysts. Not only can supports be used as carriers of active components, dispersing active components, provide suitable pore structure, mechanical supporting and thermal stability for the catalysts, but also they can help active sites to enhance the activity and selectivity of the catalyts, so as to save precious metals and so on.In this paper we select chitosan (CS) and branched polyethylene imine (PEI) that are rich in amino group as the appropriate carrier and stabilizer, and prepare gold nanoparticles by reducing the gold solution with various reductants in the presence of chitosan or polyethylene imine.These obtained Au/CS nanoparticles with low gold loading and high activity are about 30～60nm in diameter and Au/PEI nanoparticles are about 2～7nm in diameter, which both show relatively strong interaction between gold and amino group to avoid agglomeration of gold nanoparticles effectively. Eeven though the catalysts are positioned for several months, their catalytic activities are still relatively good, which achieves the desired objectives.We study the general morphology and activity of nano-gold catalyst with Uv-vis, TEM, HPLC characterization. The addition of PEI can promote the rate of nucleation and stabilize the gold nanoparticles, thereby slow down the rate of its growth further, and ultimately form smaller particle size, narrow particle size distribution of nanoparticles. In this paper we select the catalytic reduction of p-nitrophenol to p-aminophenol as a model reaction to investigate and evaluate the catalytic performance of Au/CS and Au/PEI nanoparticles, which are prepared with different preparation conditions, such as:reducing agents, carriers, loading amounts, reduction temperatures, reduction time, the ratios of Au to reducing agent, reaction time and proportions of different solvents. It was found that the types of reducing agents during the process of preparing gold nanoparticles has the greatest impact on catalytic activity, followed by the reduction time, the ratios of Au to PEI, reduction temperature, yet the ratio of reducing agent to Au has a minimal impact. When maintaining the same total amount of gold, catalysts with different loading amounts were used in the catalytic reacition, and it was found that the catalyst with higher loading amount has a better activity. And when the catalysts are positioned for a few months under the condition of low temperature,the catalyt still show good catalytic activity.In addition, it was detected the original reaction rate will be significantly reduced when ethanol is added into the reaction system, which is probably because that ethanol has lower polarity than water, and the higher the ethanol content is, the slower the reaction is, or even incomplete. The addition of ethanol might reduce the rate of hydrolysis of NaBH4, thus hinder the overall reaction rate. The catalytic reaction mechanism needs to be further studied.更多还原