【作者】 刘冰；

【导师】 赵震；

【作者基本信息】 中国石油大学(北京) ， 化学工程与技术， 2016， 博士

【摘要】 工业废气及机动车尾气排出的CO和NO_x是城市大气污染的主要来源之一,严重影响环境质量和人体健康。催化净化技术是解决CO和NO_x污染的重要途径。因此,设计和研发高性能的CO和NO_x净化催化剂具有非常重要的意义。从原子尺度上深入认识催化反应机理和催化作用本质可以为理性设计新型高效的催化剂提供重要的理论指导与科学依据。本论文采用密度泛函理论计算的方法研究了四种不同的二氧化铈基催化剂体系,包括二氧化铈负载Pd纳米棒、二氧化铈负载Pd基双金属纳米棒、Pd掺杂二氧化铈以及W掺杂二氧化铈,从原子尺度上解释了金属-氧化物界面效应以及双金属协同效应的本质,并系统研究了CO氧化的反应机理以及NO选择性催化还原反应循环过程。主要研究内容和结论如下:（1）研究了CeO₂负载Pd纳米棒催化剂上三种不同的CO氧化反应机理,分别是Pd-Ce³⁺双活性位反应机理、Mars-van Krevelen机理和Pd-only反应机理。能量分析结果表明,Pd-Ce³⁺双活性位反应机理和Mars-van Krevelen机理是最可行以及最合理的反应机理。Pd-CeO₂界面效应在这两种反应机理中起着至关重要的作用。Pd-CeO₂界面效应的化学本质可以归因于界面处的结构变形和CeO₂载体与Pd纳米棒之间电子转移。从结构变形的角度来看,Ce O₂载体与Pd纳米棒之间的相互作用拉长了界面处的Ce-O键,促进了界面处CeO₂载体晶格氧的活性。从电子转移的角度来看,CeO₂载体在反应中扮演着电子储存器的角色,它能够接受来自Pd纳米棒的电子从而使Pd-CeO₂界面处形成Ce³⁺阳离子;而当O₂分子吸附到界面上以后,Ce³⁺的电子又会转移到吸附的O₂上形成活性超氧物种O₂-,该超氧物种对CO氧化具有非常高的活性。本论文还研究了CeO₂负载单原子Pd上CO氧化反应的Mars-van Krevelen机理,结果表明Pd/CeO₂催化剂上CO氧化活性与金属Pd的尺寸大小是密切相关的。（2）研究了六种不同的双金属Pd-X/CeO₂催化剂体系（X=Ag,Au,Cu,Pt,Rh,Ru）。通过计算CO和O₂在这六种双金属体系上的吸附,发现Pd-Ag/CeO₂和Pd-Cu/CeO₂这两个双金属体系可以很好地分离CO与O₂的吸附位;而其他四种双金属体系中没有适合于O₂吸附的吸附位,会造成CO中毒。对Pd-Ag/CeO₂和Pd-Cu/CeO₂这两个体系上的CO氧化反应机理进行了详细的研究。结果表明,在Pd-Ag/CeO₂体系中,吸附在Ag上的分子氧和CeO₂载体上的晶格氧原子都能够以较低的活化能参与到CO氧化中;而在Pd-Cu/CeO₂体系中,CeO₂载体的晶格氧原子很难参与到CO氧化的过程中,表明CeO₂载体的晶格氧原子在该体系中不能被很好地利用。因此,Pd-Ag/CeO₂双金属体系是适用于CO氧化反应的最佳体系。Pd-Ag界面和Pd-CeO₂界面在CO氧化反应中都起着至关重要的作用。Pd-Ag界面结合了这两种金属的不同性质,可以作为双功能催化活性位来调节CO氧化的催化活性。Pd-CeO₂界面处金属与氧化物载体的强相互作用促进了CO被载体晶格氧氧化的M-vK机理。（3）研究了Pd掺杂CeO₂的催化作用机制及CO氧化反应机理。与未掺杂的CeO₂相比,Pd的掺杂不仅极大地促进了CO的氧化,还改变了反应产物和反应途径,阻止催化剂表面形成稳定且难以脱附掉的碳酸盐物种,促进了气相二氧化碳分子的直接形成,防止催化剂中毒现象的发生。Pd掺杂CeO₂催化体系表面晶格氧的活性与吸附分子氧O₂的能力共同影响着CO氧化的活性。（4）系统研究了W-CeO₂催化剂上NO选择性催化还原反应循环与反应机理,提出了一个包括四个反应步骤的完整催化循环,即L酸位反应、B酸位反应、氧空位反应以及催化剂再生。在这四个反应步骤中计算出的关键中间体与前期的实验结果很好的相符合,这表明本论文提出的循环过程是非常合理的。催化剂表面的氧空位在反应过程中起着非常重要的作用,当两个NO分子吸附到氧空位上以后,两个Ce³⁺的4f电子转移到这两个NO分子上,形成了带负电的N2O₂2-物种。N2O₂2-物种可以作为选择性催化还原反应的前驱体。从Ce³⁺到N2O₂2-的电子转移增强了N-N键,大大促进了N2的形成。W-CeO₂催化剂在选择性催化还原反应中的催化本质可以归因于表面酸性和氧化还原性能之间的协同效应。更多还原

【Abstract】 Carbon monoxide（CO）and Nitrogen oxides（NO_x）emitted from industry and automobile exhausts have been regarded as one of major causes for air pollution,which seriously affects the environmental quality and human health.Catalytic purification technology is an important method to solve the problem of CO and NO_x pollution.Thus,the design and development of highly active catalysts for CO and NO_x purification is of great significance.Atomic-scale understanding of the reaction mechanism and catalytic nature can provide significant theoretical guidance for the rational design of novel highly active catalysts.In this dissertation,density functional theory calculations were used to study four different ceria-based catalyst systems,including ceria-supported Pd nanorod,ceira-supported Pd-based bimetallic nanorod,Pd-doped ceria,and W-doped ceria.The nature of metal-oxide interfacial effect and bimetallic cooperation effect was elucidated from atomic-scale insights.The reaction mechanism of CO oxidation and the catalytic cycle of selective catalytic reduction of NO with NH₃ were investigated systematically.The main conclusions are summarized as follows:（1）Three different reaction mechanisms for CO oxidation on CeO₂（111）-supported Pd nanorod were systematically studied,namely Pd-Ce³⁺ dual sites mechanism,Mars-van Krevelen（M-vK）mechanism,and Pd-only mechanism.On the basis of energetic analysis,it can be concluded that Pd-Ce³⁺ dual sites mechanism and M-vK mechanism are dominant and favourable.The interfacial effects play a crucial role and strongly affect the catalytic activity in these two mechanisms.The origin of the interfacial effects can be understood by analyzing the geometric and electronic properties.From the geometric perspective,the interaction between Pd nanorod and ceria support elongates the Ce-O bonds at the interface,enhancing the mobility and activity of interfacial lattice O atoms.From the electronic perspective,there occurs electron transfer from Pd nanorod to the interfacial Ce4+ cation,leading to the formation of Ce³⁺,and subsequent electron transfer from Ce³⁺ to the adsorbed O₂ at the Pd-Ce³⁺ dual sites significantly promotes the formation of active oxygen species for CO oxidation.These results provides atomic-scale insights into the nature of active sites and the interfacial effects that determine CO oxidation on Pd/CeO₂ catalysts.（2）CO oxidation on CeO₂（111）-supported Pd-based bimetallic nanorods was investigated.A series of Ce O₂（111）-supported Pd-based bimetallic nanorods（Pd-X,where X = Ag,Au,Cu,Pt,Rh,Ru）were studied.The results show that Pd-Ag/CeO₂ and Pd-Cu/CeO₂ are the two systems where the binding sites of CO and O₂ are distinct.In other words,in these two systems,CO and O₂ do not compete for the same binding sites.An analysis of the CO oxidation mechanisms suggests that the Pd-Ag/CeO₂ system is more effective for catalyzing CO oxidation as compared to Pd-Cu/CeO₂ because both CeO₂ lattice oxygen atoms and adsorbed oxygen molecules at Ag sites can oxidize CO with low energy barriers.Both the Pd-Ag and Pd-CeO₂ interfaces in Pd-Ag/CeO₂ were found to play important roles in CO oxidation.The Pd-Ag interface,which combines the different chemical nature of the two metals,not only separates the binding sites of CO and O₂,but also opens up active reaction pathways for CO oxidation.The strong metal-support interaction at the Pd-CeO₂ interface facilitates CO oxidation by the Mars-van Krevelen mechanism.（3）The origin of the high activity of Pd-doped ceria and the reaction mechanism of CO O_xidation on Pd-doped CeO₂ were systematically studied.The presence of a Pd dopant induces the Pd,instead of Ce,acting as an electrons acceptor and donor during the oxygen vacancy formation and O₂ replenishing.Pd dopant alters the reaction product and pathway to prevent the formation of stable carbonate species on the surface of catalysts and facilitate the direct formation of gas-phase CO₂ molecule,which reduces the effect of CO poisoning and enhances the activity of CO oxidation.Both the oxygen vacancy formation ability and the replenishment of O₂ molecule adsorbed at the vacancy site have a pronounced effect on CO oxidation.An over-facilitated oxygen vacancy formation process may result in difficulty in the oxygen healing process,decreasing the efficiency of the catalyst.（4）The reaction mechanism of selective catalytic reduction（SCR）of NO with NH₃ on W-doped CeO₂ catalysts was systematically investigated.A complete catalytic cycle was proposed,which consists of four steps,namely（i）Lewis acid site reaction,（ii）Br?nsted acid site reaction,（iii）oxygen vacancy reaction,and（iv）catalyst regeneration.The calculated key intermediates in these four steps are in good agreement with previous experimental results,which indicates that our suggested catalytic cycle is rational.The catalytic nature of W-doped CeO₂ catalysts for NH₃-SCR reaction was discussed by analyzing the role of oxygen vacancy,the synergistic effect between surface acidity and reducibility,and the difference from NH₃-SCR reaction on V2O5-based catalysts.Our results show that the oxygen vacancy on the surface which creates two Ce³⁺ cations plays a critical catalytic role in the NH₃-SCR reaction,where adsorbed N2O₂2-species can be readily formed and then acts as a precursor for SCR reaction,opening a unique reaction pathway.The formation of adsorbed NO₂ species on W-doped CeO₂ facilitates the SCR reaction via Langmuir-Hinshelwood mechanism with a relative low energy barrier.These results are significant for the design of highly active ceria-based SCR catalysts.更多还原