【作者】 王芳；

【导师】 丁轶；

【作者基本信息】 山东大学 ， 物理化学， 2010， 博士

【摘要】 20世纪80年代,Hutchings在研究乙炔氢氯化反应的机理时预测负载的金将对这一反应有较高的催化活性。随后,Haruta发现尺寸缩小到纳米量级、并且高度分散在Fe2O3、Co3O4、NiO等载体氧化物上的金,在低至-70℃的温度下,对CO的氧化仍具有很高的催化活性。这两大重要发现,彻底改变了人们一直以来对Au化学惰性的认识,引起了人们的高度关注,在世界范围内掀起了研究金纳米粒子的热潮。随着研究的深入,人们发现Au纳米粒子对很多反应都具有较高的催化活性,如CO的氧化、氮氧化物的还原、不饱和烃的氢化及烃类的燃烧等。同时,人们还研究了制备方法、载体氧化物种类、Au纳米粒子的尺寸、形貌及氧化态等因素对其催化性能的影响。通过这些探索,人们对金纳米材料的催化活性有了一定的认识,但尚不完善。比如,Au纳米粒子高催化活性的起因一直是备受争议的问题,目前人们还没有统一认识。另外,Au纳米粒子催化的CO氧化、H2O2的直接合成等一系列重要反应的机理也没有完全弄清楚。Pt及Pt基催化剂是所有元素中催化性能最好、选择性最高、应用最为广泛的催化剂。特别值得一提的是,它们在质子交换膜燃料电池中有极高的潜在应用价值,如直接甲醇燃料电池等。然而,Pt电极易被反应生成的副产物CO毒化,导致其失活,加之Pt在全球的储量少、价格昂贵,人们一直在寻求提高Pt催化活性及利用率的方法,并不断开发高性能、低成本的催化剂。许多研究发现Pt-Au二元金属纳米粒子具有很好的抗CO中毒能力,而且对甲醇氧化、甲酸氧化等反应的催化活性较单一纯金属的高。另外,Au的加入使催化剂的成本大为降低,符合人们对“高性能、低成本”催化剂的要求。然而Pt-Au二元金属纳米粒子的微观结构还存在很多争议,有人认为该双金属可以形成均匀混合的合金结构,而有的研究小组则发现Au易析出到催化剂的表面。另外,Au的加入使Pt催化剂的催化活性提高的原因也一直困扰着科学工作者。基于以上两方面,本文利用密度泛函理论方法,从理论上研究了Au团簇催化CO氧化与H2O2直接合成的反应机理,探讨了量子尺寸效应、电荷态效应等对Au催化活性的影响；研究了Pt-Au二元金属团簇的几何结构及电子结构,并探讨了Pt-Au双金属团簇及表面催化CO氧化的微观反应机制,阐明了Au的加入对Pt催化性能的影响。本论文从原子、分子水平上提供了金及金铂贵金属纳米材料微观结构及其催化性能的认识,加深了人们对这些纳米材料微观结构及催化性能的理解,可以为相关的实验研究提供一定的理论指导。本文主要内容和创新性研究结果如下：一、概括论述了Au及Pt-Au双金属纳米粒子的研究进展和现状。与体相的Au不同,纳米尺寸的Au对很多反应都有良好的催化活性,其中低温催化CO氧化是人们研究最多最深入的反应。另外,以负载Au为催化剂,由H2和O2直接合成H2O2的反应也是当前研究较多的反应之一。因此,首先以这两个反应为主,从催化剂制备、载体的选择、尺寸和形貌对Au粒子催化活性的影响、Au纳米粒子的氧化态、理论研究概况等方面简要介绍了Au纳米粒子的研究现状。然后从实验和理论两个方面简述了Pt-Au二元金属纳米粒子催化剂的研究进展,并介绍了本文的研究思路和主要内容。二、简要介绍了论文的理论计算研究基础。简述了定态薛定谔方程和从头算方法所采用的五个近似。密度泛函理论是本论文最重要的理论基础,对其做了较为详细的介绍。简要介绍了基组、赝势的种类、ADMP从头算分子动力学模拟方法以及论文所采用的计算软件程序包。三、基于密度泛函理论,在BPW91／LANL2DZ／6-311G(d,p)水平上系统研究了Au负离子团簇Aun-(n=1-4)催化H2和O2直接制备H2O2的反应机理。研究发现,所有Au负离子团簇Aun-(N=1-4)催化的反应均经过两个基元步骤：首先H2解离,与O2作用形成含OOH的中间体,紧接着这一中间体异构化形成类似于产物的中间体。从能量上来讲,Au2-和Au4-催化的反应比Au-和Au3-催化的反应更容易进行,表明即使是在最小的尺寸,Au负离子团簇的催化活性依然呈现出明显的奇偶震荡效应,即含偶数个Au原子的负离子团簇表现出较高的催化活性。Au-对该反应的活性相对较低,因为其决速步的势垒高达40.60 kcal mol-1。计算结果还表明量子尺寸效应可能对Au负离子团簇的反应活性影响不大,而附加的电荷却是影响偶数Au团簇高活性的重要因素。四、基于密度泛函理论,在PW91／LANL2DZ／6-311+G(d)水平上详细地研究了不同电荷态的Au三聚体(Au3+、Au3及Au3-)催化CO氧化的反应机理,阐明了电荷态效应对Au团簇催化性能的影响。对每一种电荷态的Au三聚体催化的反应都探讨了三种可能的反应通道：第一种是O2首先吸附到Au三聚体上形成初始复合物,接着CO与该复合物反应；第二种正相反,CO首先与Au三聚体相互作用形成复合物,接着O2与该复合物反应；第三种为自催化机理,即预先吸附的CO分子释放出较多的热量,能够促进第二个CO分子的氧化。计算结果显示,Au3+、Au3及Au3-均可催化CO的氧化,只是反应的基元步骤不同,其中阳离子团簇催化的反应沿着自催化反应通道最为有利。这一结果说明Au团簇的电荷态效应对CO氧化的基元反应步骤有影响,但不是影响其催化活性的关键因素。所得理论结果很好地解释了实验上的发现,即不同电荷态的Au团簇均对CO氧化有活性。另外,稳定的碳酸盐中间体不是CO氧化最小能量路径上必须的中间体。ADMP分子动力学模拟的结果表明它是通过Au氧化物中间体与新生成的CO2有效碰撞的结果,即CO2中的C直接进攻金属氧化物中的O原子。五、基于密度泛函理论,在PW91／LANL2DZ水平上详细研究了AumPtn(m+n=4-6,13)二元金属团簇的几何和电子结构。计算结果表明Pt-Au二元金属团簇易形成Pt为核Au为壳的几何结构,Au、Pt均匀混合的结构在能量上不稳定。这归因于Pt-Pt,Pt-Au和Au-Au键不同的成键本质,研究发现这三种键的强度存在如下次序：Pt-Pt>Pt-Au>Au-Au。本章的研究对Pt-Au二元金属团簇的几何结构和电子结构提供了原子水平上的理解,有利于帮助人们了解此类二元金属团簇的微观结构。六、通过DFT计算,系统研究了PtmAun(m+n=4)团簇催化CO氧化反应的微观机制,讨论了Pt-Au双金属团簇催化剂比单一金属团簇催化活性高的原因。计算结果表明,除了以Au4为催化剂的反应,其他反应均依照单中心机理进行,且Pt是双金属团簇中的活性位,Au在形式上只是一个旁观者,它的存在避免了过多的CO吸附到活性中心Pt的周围。另外,研究还发现Pt的催化活性与它周围的原子是Au或Pt无关,因为比较双金属团簇与纯金属团簇催化的反应,发现决速步的势垒无明显变化。根据上述计算结果,我们提出了一种对CO氧化“低廉且高效”的理想Pt-Au催化剂结构模型。在这种催化剂中,Au原子将Pt原子(活性中心)有效的隔开,这样由于Au与CO相互作用较弱,使得Pt原子周围不会被过多的CO占据,能为O2的加入预留足够的空间,而O2是CO氧化所必需的氧化剂。七、运用密度泛函理论,对Pt(111)和PtAu3(111)面催化的H2O的分解及COads与OHads发生氧化反应的机理进行了系统研究。通过研究弄清了Pt-Au双金属纳米材料对CO氧化性能提高的可能原因。计算结果表明,反应的活性位仍然是Pt位。与Pt(111)相比,PtAu3(111)对H2O、CO和OH的吸附没有明显的改变。不过,PtAu3(111)对H2O的分解反应具有较高的催化活性,我们将其归因于PtAu3(111)中两种金属电子结构的改变,即两种原子的d带中心向费米能级方向移动使它们的反应活性升高。对COads和OHads的反应,Pt(111)和PtAu3(111)的催化活性相当。本章的研究加深了我们对Pt-Au双金属纳米材料催化活性的认识。更多还原

【Abstract】 In the 1980’s, supported Au has been prognosticated by Hutchings having higher catalytic activity toward the hydrochlorination of acetylene. Subsequently, it was found by Haruta et al that Au can exhibit surprisingly high catalytic activity for CO oxidation as low as-70℃when it is highly dispersed on Fe2O3、Co3O4、NiO. These vital findings make Au nanoparticles attract sustained experimental and theoretical interest and change the traditional impression about its inert activity. Using supported gold nanoparticles as catalysts, many important reactions have been achieved so far, including the oxidation of CO, the direct synthesis of H2O2 from H2 and O2, the reduction of nitrogen oxides, selective hydrogenation of unsaturated hydrocarbon, the combustion of hydrocarbons and so on. In the past decade, many experimental and theoretical studies have been devoted to unveiling the origin of the catalytic activity of Au nanoparticles. The effects of preparation method, support oxides, oxidation state, size and morphology of Au particles have been investigated. However, there is no consensus on the crucial factors that govern the catalytic activity of Au nanoparticles. And our understanding for the mechanisms of CO oxidation and H2O2 formation from H2 and O2 are still far from complete.Pt and Pt-based particles, which have extensive application, are regarded as the most active catalysts with the highest selectivity among the chemical elements. It is worthy to note that they have potential application in proton exchange membrane fuel cell, such as direct methanol fuel cell. However, the Pt electrode can be easily poisoned by the byproduct CO and loss the catalytic activity. Moreover, the scarce world reserves of Pt make it much more expensive than the other noble metals, such as Au. Thus, the enhanced activity and most effective utilization of Pt catalyst are desired. Up to date, much effort has been devoted to develop the "less expensive and more efficient" Pt-based catalysts. Several groups have consistently observed the higher catalytic activity of Pt-Au bimetallic nanoparticles toward low temperature CO oxidation compared to the corresponding monometallic catalysts. Moreover, the cost of Au modified Pt catalysts is much lower than pure Pt catalysts. Therefore, Pt-Au bimetallic catalysts have attracted special interest in recent years. However, for the microstructures of Pt-Au bimetallic catalysts, some groups reported that Pt-Au NPs exhibit alloy properties while the others found that Au tends to migrate to the surface and cover the active Pt sites. The existing contrary results indicate that the atomic ordering in nano-scaled Pt-Au bimetallic particles, which importantly influences the catalytic activity of nanoparticles, is a complex issue and still not understood well. Therefore, it is worthy to theoretically ascertain the stable atomic ordering of Pt-Au NPs. Moreover, the origin for the improved catalytic activity of Pt-Au bimetallic particles remains unclear.Considering the issues concerned above, employing density functional theory we explored the mechanism of CO oxidation and H2O2 formation from H2 and O2 which are promoted by Au clusters, and investigated the quantum size effect and charge state effect on gold catalytic activity in this thesis. At the same time, we also studied the geometrical and electronic structures of Pt-Au bimetallic clusters, researched the mechanism of CO oxidation mediated by Pt-Au bimetallic clusters or slabs and elucidated the influence of Au addition for Pt catalytic activity.The major innovative conclusions in this thesis are listed as follows:1. A theoretical investigation of the formation of H2O2 from H2 and O2 over anionic gold clusters Aun-(n=1-4) were performed at the BPW91/LANL2DZ/6-311G(d,p) level. In all cases, the reactions proceed via two elementary steps:the initial H2 dissociation to form an OOH-containing intermediate and the subsequent isomerization of this intermediate into the product-like intermediate. Energetically, the reactions over Au2- and Au4- are significantly less demanding than the ones over Au-and Au3-. In particular, Au-is relatively less active to hydrogenate O2 because the barrier of the rate-determining step is as high as 40.60 kcal mol-1. The barriers for both the odd-and even-membered sequences slightly decrease with the cluster size. The present results shows that quantum size effects appear to play a less important role for the reactivity of anionic Au clusters, in contrast, the additional charge on even-numbered gold clusters seems to be a dominant factor for the high reactivity.2. By carrying out density functional theory calculations, we studied the CO oxidation promoted by cationic, neutral, and anionic Au trimers, which represent the prototypes of Au-cluster-based catalysts with different charge states. The reaction is explored along three possible pathways:one involves the reaction of the initial complexes between Au trimers and O2 with CO; another is related to O2 interacting with the complexes between Au trimers and CO; the third refers to a self-promoting mechanism, i.e., the second CO oxidation is promoted by a pre-adsorbed CO molecule. The theoretical results show that all three species may promote the reaction, as indicated by calculated low energy barriers and high exothermicities, supporting the fact that cationic, neutral, and anionic Au species were all observed to present catalytic activity toward CO oxidation. Along the reaction coordinates for all the reactions, Au-carbonate species are not found to be the necessary intermediates although they are calculated to be energetically very stable. In contrast, by performing atom-centered density matrix propagation molecular dynamics simulations, the formation of such highly stable species is attributed to the effective collision between Au-oxides and CO2 with the carbon atom of CO2 directly attacking the O atom in the oxides.3. The geometrical and electronic structures of the smallest AumPtn (m+n=4-6, 13) bimetallic clusters have been investigated by performing DFT calculations. It is found that Au and Pt atoms prefer to form the core-shell-like structure with Pt atoms assembling together forming the core while the Au atoms like to surround the Pt atoms forming the shell, and the evenly mixed clusters are structurally unstable. This is attributed to the distinct bonding nature of Pt-Pt, Pt-Au, and Au-Au bonds. The present studies have provided atomic-level insight into the geometrical and electronic structure of Au/Pt bimetallic clusters, which can offer assistance to some extent for understanding the microstructure of Au/Pt bimetallic nanomaterials.4. A theoretical exploration for the reactivity of PtmAun (m+n=4) clusters toward CO oxidation have been presented, aiming at understanding the improved catalytic activity of Pt-Au bimetallic catalysts. In all situations, the reaction proceeds according to the single-center mechanism except the Au4-involved reaction. The Pt sites in the bimetallic clusters are the active centers, while Au sites are formally spectators and their role is to avoid the excess adsorption of CO around the active centers. The activity of Pt active centers in the bimetallic clusters seems not to be dependent on its surroundings. Based on the calculated results, we show a picture of the ideal "less expensive and more effective" Pt-Au catalyst for CO oxidation, where Pt atoms (active center) are suitably spaced (stabilized) by Au atoms, which more weakly adsorb CO than Pt atoms and thus leave room for the coming O2 necessary to CO oxidation.5. The detailed mechanisms for the dissociation of H2O and the oxidation of s by OHads on Pt(111) and PtAu3(111) have been investigated in order to explicit the origin for the improved catalytic activity of Pt-Au bimetallic catalysts. The calculated results indicate that the active sites are still on Pt atoms in PtAu3(111) surface. The adsorption of H2O, CO and OH on PtAu3(111) show unapparent difference in comparison with that on Pt(111). However, PtAu3(111) shows enhanced activity toward the dissociation of H2O. We attribute this to the change of electronic structures of Pt in PtAu3(111) surface. The analysis of density of states (DOS) indicate the upshift of d band center of Pt in PtAu3(111), which make the activity of Pt improve. In the case of the COadS oxidation by OHadS, PtAu3(111) shows comparable activity. The present studies deepen the understanding for the improved catalytic activity of Pt-Au bimetallic catalysts.更多还原