单分散超顺磁核壳型纳米催化剂：制备、表征、催化还原硝酸根性能及机理研究

Nitrate pollution in ground water is a worldwide problem, which is now causing more and more damages to human health with its increasing concentration. Among various physical and chemical processes, the catalytic denitrification has been demonstrated as one of the most promising approaches to remove nitrate pollution from water due to its effectiveness. However, the lack of high nitrogen selectivity and the high cost of effective catalysts restrain the further development and potential application of the catalytic denitrification process despite its encouraging initial results. The development of monodisperse, superparamagnetic core-shell nanocatalysts may contribute to solve these two problems. The monodispersity of catalysts could reduce the mass transfer resistance and improve the buffer capacity of catalytic systems to resist hydroxyl ions to increase the nitrogen selectivity in the catalytic denitrification process, while the creation of superparamagnetic nanocatalysts could make it easier for nanocatalysts to be separated from the treated water bodies by an external magnetic field. In this proposed project, a novel synthesis process will be developed for the creation of monodisperse, superparamagnetic core-shell structure supported Pd catalyst without the traditional tedious preparation process and the use of poisonous modification agents. After the creation of silica shell on superparamagnetic cores, the catalyst precursor will be deposited on etched silica shell uniformly using homogeneous participation, then reduced and meanwhile anchored on the silica shell by cross linked capping agent to create novel catalysts with high selectivity, strong activity, and good recyclability. Furthermore, the catalytic denitrification performance of synthesized samples will be investigated, and their catalytic denitrification mechanism and catalyst deactivation behavior will be carefully examined.

硝酸盐污染已经成一个世界性的问题，随着其在地下水中浓度的增加，给人类健康带来的危害也越来越大。在众多去除硝酸盐的方法中，催化还原是最有应用前景的方法之一。但是，催化还原产物中氮气的选择性不够高和催化剂成本过高的问题却限制了其进一步发展与应用。单分散性超顺磁核壳型纳米催化剂为解决这两个问题提供了很好的思路：单分散有利于降低传质阻力，可以更加有效的中和反应产生的氢氧根，提高氮气选择性；超顺磁性有利于催化剂的回收和再利用。本项目计划从合成方法入手，发展制备磁性核壳型纳米催化剂的新方法，以期解决传统制备方法过程复杂、使用有毒改性剂等缺点。拟采用均相沉淀法在刻蚀后的氧化硅壳层表面均匀沉积催化剂前驱体，通过还原剂的还原和高分子保护剂的交联固定作用制备出选择性高、活性高、循环性能优异的催化剂，对其催化还原硝酸根和亚硝酸根的性能进行测试，并采用多种手段对反应机理和催化剂失活原因进行详尽的研究。

催化还原产物中氮气的选择性不够高和催化剂成本过高是硝酸根催化还原中存在的两个主要问题，本项目的研究针对这两个主要问题展开，取得的主要成果如下：.1.利用新的工艺方法制备了单分散、超顺磁性纳米核壳结构负载的Pd及PdCu催化剂，并将其应用于水相脱硝的反应中。催化剂体系中引入单分散性，可以加快反应物和产物的扩散，使得反应物更快的扩散到催化剂表面，产物更快的从催化剂表面扩散到体相中。单分散性很好的解决了反应过程中氢氧根积累的问题，使得反应产生的氢氧根能够很快的被中和，从而提高了脱硝反应的氮气选择性。磁性的引入使得催化剂更容易回收利用，降低了水相脱硝催化剂的成本。.2.以表面弱碱改性的商业氧化硅为载体，制备了表面含有弱碱性基团的Pd及PdCu催化剂。表面碱性基团可以有效的吸附水中的二氧化碳，将二氧化碳固定在催化剂表面，起到了原位缓冲的作用，更有效的将反应生成的氢氧根中和，增加了反应的选择性。商业氧化硅的大尺寸特点有利于催化剂的回收再利用，进一步降低了催化剂的成本。.3.制备了氨基改性氧化硅负载PdAg催化剂，并将其应用到水相脱硝的反应中。该催化体系以甲酸为还原剂，避免了氢气的使用，增加了水相催化脱硝的安全性。由于Ag较低的功函数，使得Ag的电子更容易转移到Pd上，从而加快了甲酸的分解，加快了硝酸盐的还原速度。另外，在该体系中，加入的甲酸可以完全分解，不会对水质造成二次污染。.4.在前三项工作的基础上，采用改进的喷雾法，以亚毫米级、多孔活性氧化铝颗粒为载体，制备了Pd/Al2O3催化剂，并将其应用于固定床催化体系中，成功实现了天然矿泉水中的溴酸盐的还原。