【作者】 常苏捷；

【导师】 黄伟新；

【作者基本信息】 中国科学技术大学 ， 物理化学， 2013， 博士

【摘要】 理解催化剂的结构-性能关系对设计和开发高效催化剂具有指导意义,以结构均一规整材料为模型催化剂开展催化表面化学研究是实现上述目标的主要途径,但传统的单晶模型催化剂表面化学研究与真实催化体系之间存在着所谓的“材料鸿沟”和“压力鸿沟”。纳米材料合成技术的快速发展使结构均一的金属和氧化物纳米晶材料的可控制备成为现实。结构均一纳米晶材料的出现为克服“材料鸿沟”和“压力鸿沟”的催化表面化学研究提供了合适的模型催化剂；同时结构均一纳米晶材料也是高活性和高选择性催化剂的理想材料。本篇博士论文以Ag/CeO2催化剂为研究体系,基于结构均一的CeO2纳米晶和Ag纳米晶分别构筑Ag/CeO2模型催化剂和CeO2/Ag模型催化剂,系统研究了Ag/CeO2催化剂体系Ag-CeO2相互作用及结构-性能关系,取得如下创新研究结果：1.以CeO2粉末为载体,利用沉淀沉积法制备不同担载量的Ag/CeO2催化剂,发现焙烧温度对Ag-CeO2相互作用、催化剂结构和催化反应性能有显著影响。200度空气焙烧的Ag/CeO2催化剂中存在溶入Ce02晶格的Ag+离子；500度空气焙烧后该Ag+离子溶出,并导致负载在Ce02表面的Ag纳米粒子的结构重构。非常有意思,该结构重构依赖于Ag的担载量(负载在Ce02表面的Ag纳米粒子)。低担载量Ag/CeO2催化剂中Ag+离子的溶出导致负载在Ce02表面Ag纳米粒子更好的分散,而高担载量Ag/CeO2催化剂中Ag+离子的溶出导致负载在Ce02表面Ag纳米粒子的聚集长大。Ag/CeO2催化剂催化CO氧化反应活性与Ag纳米粒子的分散度成正比,Ag纳米粒子-CeO2界面为Ag/CeO2催化剂催化CO氧化反应的活性结构。2.以暴露特定晶面的Ce02纳米晶为载体,包括形貌均一的暴露{100}面的Ce02立方体和暴露{100}面和{110}面的Ce02纳米棒,利用等体积浸渍法制备Ag/CeO2催化剂,发现晶面依赖的Ce02氧缺陷浓度／类型和Ag-CeO2相互作用及其对Ag/CeO2催化剂结构和催化CO氧化反应性能的影响。具有较高氧缺陷浓度和小尺寸／大尺寸氧缺陷结构的Ce02纳米棒能够稳定带部分正电荷的Agn+团簇,而相同条件下具有较低氧缺陷浓度和大尺寸氧缺陷结构的Ce02纳米立方体只能稳定Ag纳米粒子；Ag纳米粒子比Agn+团簇具有更强的活化Ce02晶格氧的能力和促进Ce02还原的能力。Ag纳米粒子-具有合适氧缺陷浓度和结构的Ce02界面表现出最高的催化CO氧化反应活性。Ce02立方体是制备低Ag担载量但高CO氧化催化活性的Ag/CeO2催化剂的合适载体。3.合成出暴露特定晶面的Ag纳米晶,包括暴露{100}面的Ag纳米立方体,暴露{111}面的Ag纳米片,暴露{100}面和{110}面的Ag纳米棒及暴露多种晶面的Ag纳米多面体,并理解了PVP和HC1存在下乙二醇还原AgNO3方法中Ag纳米立方体和Ag纳米棒的生长机理,进一步探索并发展了氧气气氛下水热合成方法成功地在Ag纳米晶表面生长CeO2纳米粒子,制备基于Ag纳米晶的CeO2/Ag倒载催化剂。观察到Ce02纳米粒子的存在能够增强Ag纳米粒子的表面增强拉曼效应；同时CeO2/Ag倒载催化剂也表现出较好的催化CO氧化反应性能,表明Ag-CeO2界面是该催化反应的活性结构。上述研究结果对于基于纳米晶的金属／氧化物模型催化剂的构筑及其金属-氧化物载体相互作用和结构-性能关系研究具有积极的指导意义。更多还原

【Abstract】 The fundamental understanding of structure-catalytic performance relation of catalysts is of great importance in the design and exploring of novel efficient catalysts. The employed main approach is to study the catalytic surface chemistry of model catalysts with well-defined surface structures. However, there exist the so-called "materials gap" and "pressure gap" between the catalytic surface chemistry study of traditional model catalysts based on single crystals and the real catalytic systems. The rapid development of the synthesis of nanomaterials realizes the controllable preparation of metal and metal oxides nanocrystals with uniform and well-defined structures. Nanocrystals with uniform and well-defined structures consist of novel model catalysts for the catalytic surface chemistry study without the "material gap" and the "pressure gap", meanwhile, they also are nice candidates for active and selective catalysts.In this dissertation, Ag/CeO2catalyst was chosen as the target system, Ag/CeO2model catalysts based on CeO2and Ag nanocrystals have been fabricated and systematically investigated. The Ag-CeO2interaction and the structure-activity relation of Ag/CeO2catalyst were derived, including::1. Ag/CeO2catalysts with various Ag loadings were synthesized via traditional deposition-precipitation method using CeO2powder as support. It was found that the calcination temperature significantly affects the Ag-CeO2interaction structure and catalytic performance of Ag/CeO2catalysts. Ag/CeO2catalysts calcined at200℃contain Ag+dissolved in the CeO2lattice; calcinations at500℃result in the segregation of these dissolved Ag+from the CeO2lattice. Interestingly the segregation of dissolved Ag+from the CeO2lattice leads to the restructuring of Ag nanoparticles supported on CeO2and such a restructuring process depends on the size (Ag loading) of these Ag nanoparticles. The segregation of dissolved Ag+from the CeO2lattice results in the redispersion of supported Ag nanoparticles for Ag/CeO2with low Ag loadings and fine supported Ag nanoparticles but the aggregation of supported Ag nanoparticles for Ag/CeO2with high Ag loadings and large supported Ag nanoparticles. The catalytic activity of Ag/CeO2catalysts correlates positively with the dispersion of supported Ag nanoparticles and the Ag NPs-CeO2interface of Ag/CeO2catalysts is the active structure to catalyze CO oxidation.2. Uniform CeO2nanocrystals with different morphologies including cubic nanocrystals exposing{100} crystal planes, rod-like CeO2nanocrystals exposing{100} and{110} crystal planes were synthesized and employed as the supports to prepare Ag/CeO2catalysts by impregnation method. The shape-dependent interplay between oxygen vacancies concentration/type and Ag-CeO2interaction in Ag/CeO2catalysts and their influence on the catalytic activity of CO oxidation have been successfully elucidated. CeO2nanorods with a high oxygen vacancy concentration and small-sized/large-sized oxygen vacancies can stabilize the partially positively-charged Agn+clusters whereas under the same condition CeO2nanocubes with a low oxygen vacancy concentration and only largely-sized oxygen vacancies can only stabilize Ag nanoparticles; Ag nanoparticles exhibit stronger abilities to activate the lattice oxygen of CeO2and to promote the reducibility of CeO2than Agn+clusters. Ag nanoparticles--CeO2interface with suitable concentration and structure of oxygen vacancies are most active to catalyze CO oxidation. CeO2nanocubes are the suitable support for the preparation of Ag/CeO2with a low Ag loading but active in CO oxidation.3. Uniform Ag nanocrystals with different morphologies including cubic Ag nanocrystals exposing{100} crystal planes, plate-like Ag nanocrystals exposing{111} crystal planes, rod-like Ag nanocrystals exposing{100} and{110} crystal planes and Ag polyhedron exposing a variety of crystal faces were successfully synthesized. The growth mechanisms of Ag nanocubes and nanorods via the EG reduction of AgNO3in the presence of PVP and HCl have been elucidated. A novel hydrothermal method (oxygen-assisted hydrothermal method) was successfully developed for the growth of CeO2nanoparticles on Ag nanocrystals, i.e., the inverse CeO2/Ag model catalysts based on Ag nanocrystals. It was observed that the surface enhanced Raman effects of Ag nanoparticles can be enhanced by the presence of CeO2adparticles. CeO2/Ag inverse catalysts also exhibit nice activity in catalyzing CO oxidation, supporting that the Ag NPs-CeO2interface is the active structure.Above experimental results provide deep insights into the fabrication of metal/oxide model catalysts based on nanocrystals and the metal-oxide support interactions and structure-activity relation of supported catalysts.更多还原