【作者】 王莹莹；

【导师】 刘成卜；

【作者基本信息】 山东大学 ， 理论与计算化学， 2010， 博士

【摘要】 金催化近年来己成为研究的热点。上世纪八十年代末Haruta等发现负载在过渡金属氧化物上的金纳米粒子在CO氧化反应中具有极高的催化活性并表现出独特的湿度增强效应,改变了人们长期以来一直认为金没有催化活性的传统观念,从而在世界范围内迅速掀起了纳米金及其合金团簇的研究热潮。金团簇及其合金结构和性能的研究是理解金及其合金纳米材料物理化学性能的基础,开展相关的实验和理论研究具有重要的科学意义和应用价值。然而,当前实验手段还没有做到真正从原子尺度分析金基催化剂的性质,限制了人们进一步开发其在不同领域的应用。而理论研究可以克服实验研究在这方面的弱点,当前已经成为团簇及表面科学研究的一个重要手段。本论文运用密度泛函方法研究了CO,NO等气体分子在金团簇,金表面,合金表面的吸附和反应。通过量子化学计算系统地研究了它们的结构和催化性能,获得了有关的微观结构信息,探讨了其结构与催化性能的内在联系,揭示了其催化活性的本质,找出了控制其催化活性的关键因素,为相关的实验提供了一定的理论指导,为开发与设计基于纳米金的新型催化剂提供了新的思路。主要研究内容及创新性研究结果归纳如下：1.概括论述了金基催化剂的实验和理论研究现状,总结了目前已知的影响金催化剂的活性因素,金催化剂的活性中心及催化机制。从理论与实验两个方面,介绍了金催化剂团簇与表面研究的基本方法。简述了密度泛函的基本原理,并介绍了本文的研究目的、特色与创新之处。2.以金的二聚体(Au2,Au2-和Au2+)为例用密度泛函理论研究了金催化的水煤气转化反应。研究了金的二聚体与反应物CO和H2O的结合能力,计算结果显示,催化剂与CO和H2O的结合能与金的氧化态有关,不同二聚体与CO和H2O的结合能数值的顺序为：Au2+>Au2>Au2-。计算结果表明,反应遵循所谓的甲酸盐机理,即在金的催化作用下H2O分解产生的OH与CO反应产生AuH-COOH中间体,并继而分解产生H2和CO2；其中水的分解是反应的速控步骤：外来的正负电荷可明显降低水解离的势垒,有效地提高金的催化性能。我们也在相同基组下计算了氧化还原机理,结果显示,计算所得的中间体和过渡态并不合理。因此,金的二聚体催化的WGS反应并不遵循氧化还原机理。总之,通过DFT计算提供了详细的金的二聚体(Au2,Au2-和Au2+)催化WGS反应的机制,解释了实验现象。3.以Au4+和Au4为例用密度泛函理论研究了金催化的NO／H2还原反应,弄清了其微观反应机理,分析了金上过剩的正电荷对其催化性能的影响。结果表明,H2的分解是反应的速控步骤；与文献报道一致,外来的正电荷可明显降低H2的分解的势垒,有效地提高金的催化性能。这些结果对理解NO／H2反应机理有一定科学意义,对相关的实验研究有一定指导作用。4.用密度泛函理论研究了NO分子在Au(111)面还原生成N2O的过程。为了更好地理解NO分子在Au(111)面的还原机理,论文首先讨论了NO分子在Au(111)面所有可能的的吸附方式：NO以N端与金属原子作用,NO以O端与金属原子作用,NO分子同时以N端和O端与金属原子作用。计算结果显示,在所有作用方式中,NO以N端与金属原子作用比其它两种作用方式稳定,而且NO在金属表面顶位弯曲型结构比直线型结构稳定。此外,NO与金表面的作用比较弱。在已确定NO分子最稳定的吸附构型的基础上,继而研究了NO分子直接分解还原的可能性。研究结果显示,NO分解为O原子和N原子的势垒高达3.9 eV,而且末态的能量比初态高3.03 eV和2.91 eV。如此高的势垒和反应过程中的强吸热表明,从化学动力学和热力学两方面而言,NO分子在Au(111)表面的分解都是不利的。因此,论文排除了NO分子在Au(111)表面分解的可能性,进而排除了NO分子通过分解在Au(111)表面还原生成N2O的可行性。同时,论文研究表明,NO还原易通过形成二聚体而发生,即两个NO分子首先结合生成(NO)2二聚体,继而二聚体分解产生N2O分子。这一计算结果也证实了实验中在金表面可以观察到NO二聚体的现象。论文详细地提供了三条反应通道,包括梯形OadNNOad通道,倒置梯形ONadNadO通道,锯齿形ONadNOad通道。计算结果发现,梯形OadNNOad通道是最佳反应通道,速控步的能垒仅为0.34 eV。总之,通过DFT计算提供了详细的NO分子在Au(111)面还原生成N2O的机理,充分解释了实验现象。5.使用密度泛函理论(DFT)研究了NO分子在中性及带正、负电荷的Au(111),Au(100),Au(310)和Au／Au(111)表面的吸附行为。研究结果表明,NO倾斜地吸附在金表面。中性及带正、负电荷的Au(111),Au(100),Au(310)和Au／Au(111)表面不同吸附位对NO的反应活性不同,NO易吸附于各个金表面的顶位。计算结果显示,NO分子在Au(111)面几乎不吸附,而在Au／Au(111)的吸附能高达0.89 eV。对表面金原子d态电子偏态密度分析表明,金表面对NO分子的吸附活性随着金原子配位数的减少而增强。当金表面增加或减少一个电子时,金表面对NO的吸附能有明显变化。正电荷的金表面对NO吸附的活性比中性的表面活性高,而带负电荷的表面对NO分子吸附的活性比中性的表面活性低。为了揭示电荷对金表面吸附活性的影响,我们计算了不同电荷态金表面的d带中心能量数值。研究表明,随着表面正电荷的聚集,金表面的d带中心能量逐渐增加。此外,本文进一步研究了NO分子在不同电荷态的金表面吸附时N—O键长的变化。研究发现,由于不同表面电子转移方向不同,使得在不同表面上分子构型将发生不同变化。在正电荷的金表面N—O键缩短,而在中性和带负电荷的金表面N—O键拉长。虽然实际催化剂颗粒绝非少数单一的单晶模型表面可以描述,但本文总结出的结论,即金表面的活性来源于表面低配位数的金原子和正电荷的聚集,是非常重要的,为研究过渡金属吸附NO分子的真实反应过程及催化剂的活性提供了有价值的信息,对理解NO在金表面的行为有一定科学意义,对相关的实验研究有一定指导作用。6.通过研究人们发现,在金上添加修饰成分或者使金合金化都能够有效的改进其催化活性。将1-3个Pd原子掺杂在Au(111)表面上,建立了不同Pd-Au(111)合金模型,用密度泛函理论研究了不同含量的Pd与金表面之间相互作用强度。研究结果显示,Pd易掺杂在Au(111)表面。同时,进一步研究了CO分子在不同Pd-Au(111)表面的吸附。研究发现,由于金原子影响Pd原子的d态电子,Pd原子为合金的活性位点,而且随着Pd原子数目的增多,CO分子的吸附能增加。为了揭示金原子的配合基作用和协同作用,比较了Au(111),Pd(111),和各个合金表面的总的电子轨道的投影态密度和Pd原子及靠近Pd原子的Au原子的d态电子轨道的投影态密度。研究表明,由于Au原子的影响,Pd原子的d带变窄,而且随着Pd数目的增多,Pd原子的d态越靠近费米能级,表明其催化活性增加。本文的研究取得了许多具有理论价值的创新性成果,对实验研究及金催化剂的进一步研究有重要的理论指导意义。更多还原

【Abstract】 Catalysis by gold has attracted significant attention in the last decades, since Haruta’s discovery that these catalysts have ultrahigh catalytic activity in the low temperature CO oxidation and that the activity is even higher in humid atmosphere. This discovery changes the traditional concept that gold does not have the catalytic activity, so a research fever is raised all over the world on the nano-gold and its alloy clusters. The studies on their structures and properties are the foundation on which the studies on the chemical and physical properties of gold and its alloy clusters are based and have very significant scientific meanings and application values for conducting related experimental and theoretical studies.In this dissertation, we studied the interaction and reaction of molecules with Au-based catalysts with density functional theory (DFT) calculations. Our purposes are to (a) shed light on the mechanistic details of the Au-based catalysts and hence obtain a better interplay between theory and experiment, (b) understand the intrinsic catalytic activity of gold-based catalysts, (c) provide a general profile of the catalytic reaction by gold-based catalysts. Our results provide detailed information on the transition states of gold catalyzed reactions and on the behavior of small molecules adsorption. This should be helpful for the designing the new efficient gold-based catalysts.The valuable results in this dissertation can be summarized as follows:1. The research history and current state on gold-based catalysts have been briefly reviewd. A number of important chemical reactions at low temperature catalyzed by gold-based catalysts are reported. Different explanations have been proposed to account for the apparently high catalytic activity of gold-based catalysts. Moreover, the theory of quantum chemistry and the calculation methods of this paper are summarized. The contents of these reports were the basis and background of our studies and offer us with useful and reliable quantum methods.2. While nanoscale gold particles show exceptional catalytic activity towards the water-gas-shift (WGS) reaction, not much is known about the detailed reaction mechanism and the influence of charge state of Au nanoparticles on the reactivity. We here report a systematic theoretical study by carrying out density functional theory calculations for the WGS reaction promoted by cationic, neutral, and anionic Au dimers, which represent three simplest prototypes of Au nanoparticles with different charge states. To better understand the catalytic activities of the Au dimers towards the WGS reaction, we first study their complexes with CO and H2O molecules. We find that the calculated values of the binding energies of H2O and CO molecules on Au dimers are closely related to the oxidation state of gold. From the positively charged to neutral and to the negatively charged dimers, the value substantially decreases. The reaction mechanism is explored along two possible entrances:one involves the complexes of the dimers with CO and the other is related to the complexes of the dimers with H2O. In all cases, it is found that the catalytic cycle proceeds via the formate mechanism and involves two sequential elementary steps:the rupture of the O-H bond in H2O and the formation of H2 molecule. Great efforts have also been made to locate the intermediates and first-order saddle points proposed in the redox mechanism, however, our requests were always unsuccessful. It is found that the atomic O intermediates and transition states supposed in the redox mechanism are led to either the reactant-like intermediates or product-like intermediates in our calculations. This fact proposes that the WGS reaction mediated by small Au clusters may not adopt this mechanism. The calculated results show that the reaction mediated by Au2+ is energetically most favorable compared to those promoted by Au2 and Au2-, indicating that the charge state of Au dimers plays an essential role for the catalysted WGS. The notable catalytic activity of Au2+ may originate from the action of the cation, which stabilizes the intermediates and transition states by trsnsferring its charge to the ligand molecules. The present theoretical study rationalizes the early experimental findings well and enriches our understanding of the catalytic WGS by Au-based catalysts.3. Density functional theory (DFT) is used to study the NO reduction by H2 on Au4+ and Au4 clusters. The reaction mechanism is explored along two possible entrances:one involves the complexes of the clusters with H2 and the other is related to the complexes of the clusters with NO. In all cases, it is found that the catalytic cycle involves two sequential elementary steps:the rupture of the H-H bond in H2 and the formation of H2O and N2O molecule. The calculated results show that the reaction mediated by Au4+ is energetically most favorable compared to that promoted by Au4, indicating that the charge state of Au clusters plays an essential role for the catalysted NO reduction. The present theoretical study rationalizes the early experimental findings well and enriches our understanding of the catalytic NO reduction by Au-based catalysts.4. Density functional theory calculations have been performed to elucidate the mechanism of N2O formation over Au(111) surface during NO reduction. Initial adsorption manner of a molecule on a metal surface is expected to affect the following surface reaction. To better understand the reactivity of NO on Au(111) surface, we examine the NO adsorption behavior on Au(111) surface by considering three possible adsorption manners:the N atom close to the surface, the O atom close to the surface, and both the N and O atoms close the surface. It is found that NO adsorption occurs with the N atom close to the surface. It is noted that in all situations the NO molecular axis is tilted with respect to the surface normal and the most stable adsorptions occur at the top site. In addition, NO binds weakly to the Au(111) surface. During the catalytic NO reduction on metal surfaces, the formation of N2O via the direct dissociation mechanism is considered as the most straightforward pathway. At such, we first investigate the possibility of the direct dissociation mechanism. Our calculations show that the dissociation of NO into an N atom and an O atom involves a barrier as high as 3.9 eV, and the final states are more unstable than the initial states by 3.03 and 2.91 eV, respectively. These extremely high barriers and the strong endothermicity of the reaction indicate that the NO dissociation over the Au(111) surface is both kinetically and thermodynamically very unfavorable. We thus rule out the possibility of direct NO dissociation on Au(111) and hence the direct dissociation mechanism for N2O formation during NO reduction on Au(111) surface. Alternatively, we find that the reaction may occur via a dimer mechanism, i.e. two NO molecules initially associate into a dimeric (NO)2, which then dissociate into a N2O molecule and a N atom. We find that the formation of dimer (NO)2 over the Au(111) surface is a thermodynamically favorable process. We have scanned the potential energy surface forming N2O along different pathways, which involve trapezoid OadNNOad dimer, inverted trapezoid ONadNadO dimer, zigzag ONadNOad dimer, or rhombus ONadOadN dimer. The trapezoid dimer, OadNNOad is found to be a necessary intermediate for the formation of N2O, and the calculated barrier for the rate-determining step along the energetically most favorable pathway is only 0.34 eV. The present results rationalize the early experimental findings well, and enriches our understanding of the reduction of NO on Au surface.5. The adsorption of gas molecules on transition metal surfaces is the first elementary step in a heterogeneous catalysis and it is fundamental to the understanding of catalytic mechanism. Nowadays it has being one of key subjects in the field of surface science and numerous studies have been carried out in experiments as well as theories. The first principles density function theory (DFT) calculation plays more and more important role in understanding of adsorption mechanics and explaining of experiment phenomenon in atomic scale. Here, DFT calculations are performed to study NO adsorption on neutral, anionic, and cationic Au(111), Au(100), Au(310), and Au/Au(111). We carefully study the NO adsorption at different adsorption sites on each surface, and find that NO prefers to bond at the top site with the NO molecular axis tilted to the surface normal. The adsorption energy of NO on the surfaces increases as the coordination number of Au atoms decreases:NO binds weakly to the Au(111) surface, while the adsorption energy of NO on Au/Au(111) is as high as 0.89 eV. In addition, the charge state of Au surfaces has a very strong effect on the Au activity:the cationic surfaces generally present stronger reactivity towards the NO molecule than the neutral and anionic surfaces. We have also provided detailed evidence for the origin of these trends:the low coordinated gold atoms and the surface with the concentration of positive charges have d states closing to the Fermi level, resulting in the high activity toward NO adsorption. The N—O bond length are also taken into account. For the cationic surfaces, the NPA charge on the NO molecule in all situations is positive. This indicates that electrons transfer from NO to the cationic surfaces, which will reduce the occupation of theπ* orbital of NO. Thus, the N-O bonding will be enhanced and the N-O bond length will shorten. In contrast, for the anionic and neutral surfaces, electron transfer occurs from the surfaces to molecule and the transferred electrons enter the 2π* orbital of NO. As a result, the N-O bonding becomes weaker, as indicated by the calculated longer N-O bond lengths. The present results enrich our understanding of the adsorption of NO on Au surfaces.6. Here configurations of different Pd-containing Au(111) bimetallic surfaces with Pd substituents varying from one to three atoms have been studied using density functional theory within the generalized gradient approximation. The stability of the so-formed Pd atoms in the surface of a Au(111)-(2×2) unit cell and their influence on the adsorption of CO molecule have been investigated. The influences of surface-ligand effect and lattice strain effect on activity were demonstrated. We have furthermore analyzed local trends by considering different adsorption sites on the different surfaces. The catalytic efficiency of Pd-Au bimetallic systems depends largely on the surface composition of Pd and Au. The addition of Pd significantly improves the activity of a Pd-Au bimetallic slab on CO adsorption. The surface Pd atoms are active and serve as independent attractive centers towards CO. The results can be rationalized within the d-band model. The Pd-d band becomes narrow and well below the Fermi level, very different from those in a bulk Pd. The work provides an effective method which can be used to link experiment and theory result.In this dissertation, the study rationalizes the early experimental findings well and enriches our understanding of the catalytic activity of gold-based catalysts. The valuable results have provided reliable verification and theoretical guide for the development of gold-based catalysts.更多还原