【作者】 姚淑娟；

【导师】 程寒松；

【作者基本信息】 中国地质大学 ， 岩石矿物材料学， 2009， 博士

【摘要】 过渡金属催化剂具有良好的催化活性、稳定性和选择性,其中铂、镍等过渡金属催化剂尤为重要,被广泛应用于有机催化加氢和脱氢反应、消除汽车尾气污染、电催化氧化和还原过程等,具有重要的工业价值。氢催化过程中,氢在金属表面的吸附活化和氢原子的脱附是催化反应的重要步骤。对原子或分子氢在过渡金属团簇或其单晶模型上吸附行为的微观理论研究,可以获得氢的表面吸附和扩散动力学信息,有助于我们理解吸附物与催化剂之间相互作用的本质,加深对相应催化过程的理解,并为解决实际工业催化问题提供有益的理论支撑。本论文采用密度泛函理论方法,应用更加贴近实际情况的催化剂模型和处理方法,系统研究了氢在过渡金属Pt单晶表面连续解离吸附行为;研究了饱和氢气氛下的Pt和Pt-Ru合金团簇模型上吸附的CO通过氧化或还原反应而被除去的反应热力学和动力学;研究了氧化铝负载的Pt团簇模型连续解离吸附氢的反应性能:进一步研究了氢在非贵金属Ni团簇催化剂上的解离吸附行为。确定了过渡金属Pt、Ni催化剂模型的活性与表面吸附物覆盖度、团簇构型和尺度大小的依赖关系以及载体效应等参数,并对吸附物与催化剂之间相互作用的本质进行了探讨。论文先研究了氢在贵金属Pt的不同活性单晶表面连续解离吸附行为,以Pt团簇模型解离吸附氢的研究方式为基础,确定了H₂在单晶模型表面的连续解离吸附能和H原子的脱附能以及饱和吸附态时形成的金属氢化物的几何结构和电子结构。研究结果表明,Pt（111）,Pt（100）和Pt（531）三种表面不同吸附位对氢的反应活性不同,Pt（111）表面顶点位、桥位和fcc空洞位对H的吸附能相差无几,而Pt（100）表面桥位对H的吸附活性明显高于顶点位和空洞位,Pt（531）表面缺陷明显而尤以最外层桥位和顶点位吸附活性最大。各表面最稳定吸附位比较,Pt（531）表面桥位比Pt（100）面桥位吸附能高0.06 eV,比Pt（111）面的顶点位或桥位高了约0.15 eV。随着表面H原子覆盖率增加,对H₂的连续解离吸附能和对H原子的脱附能逐渐降低。在饱和吸附态时,三种不同活性的Pt单晶表面对H₂分子的解离吸附能分别降到0.60～0.88 eV的微小范围内,比Pt小团簇模型对氢气的解离吸附能稍低;而对H原子的脱附能为2.08～2.84 eV,与团簇模型对H原子脱附能接近。电荷布局分析表明,H₂在Pt单晶表面能够发生稳定地解离吸附,是由于金属Pt和H原子间发生了明显的电荷转移,并且随着H原子覆盖度增大,由Pt原子向H原子平均转移电子逐渐减小。随着金属氢化物的形成,体系由金属键向共价键转变。另外,在饱和吸附态时单晶模型较团簇模型相比,表面容纳H原子的数量少得多。在已确定的Pt团簇模型吸附大量氢的结构的基础上,论文进一步研究了催化剂模型上吸附的少量CO选择性氧化或还原反应的热力学和动力学。研究表明,当催化剂模型表面覆盖少量氢时,纯Pt₆团簇催化剂模型表面吸附的CO被临近的活性氧原子（O₂的解离吸附）氧化形成类CO₂的反应是吸热的,反应热和活化能较高。CO₂进一步从催化剂模型表面脱除的反应也是强吸热的。从化学反应动力学角度而言,该反应过程也需要克服较高的活化能垒。故催化剂表面少量氢覆盖时,CO的氧化和脱除反应不容易发生。当催化剂模型上有大量氢覆盖时,CO被临近的活性O氧化形成类CO₂的反应是放热性的,且活化能也大大降低。研究还表明,饱和吸附氢状态下的纯Pt₆团簇催化剂模型上吸附的CO被临近的活性O氧化的反应在低温下仍较难发生,因为该反应仍需要克服较大的活化能,若适当升高反应温度,将容易克服反应活化能垒,反应变得容易发生。这与实际工艺中升高燃料电池的操作温度可提高Pt电极的抗CO中毒的事实相吻合。另外,许多实验研究表明如果Pt催化剂中掺入Ru后其抗CO能力会显著提高,为了揭示CO被氧化成CO₂及除去的反应过程中Ru所起的作用,计算了CO在饱和吸附氢状态的Pt/Ru合金团簇催化剂模型上氧化的反应热化学能和动力学能。研究表明,氧源（氧原子）在Pt-Ru键之间的吸附强于在Pt-Pt键之间的吸附:Pt催化剂中掺杂了Ru使CO的氧化反应放热性增大。依据进攻CO的活性O吸附位置不同,该氧化反应活化能有可能被适当减弱或适当增加。CO在合金团簇的Ru位吸附比Pt位吸附能量上更大,此时通过氧化反应去除CO则变得更加困难。同时,进一步研究了饱和吸附氢状态的纯Pt团簇催化剂模型上吸附的CO被临近的H原子还原形成类甲醛基团的反应和类甲醛基团进一步脱附的反应可行性。研究过程中只允许与CO紧邻的两个H原子相继进攻CO。研究发现,每一步反应过程都是较吸热的,而反应的活化能也相对氧化反应高,即使在500K温度下的MD也未发现有甲醛分子生成。这些结果表明,没有氧化剂存在下大量氢覆盖的纯Pt₆团簇上吸附的CO被临近的H原子还原的反应在热力学和动力学上都是难发生的。虽然所使用的催化剂模型过于简单,但是从催化剂表面吸附物覆盖度不同的角度出发,对认识实际催化反应中CO在Pt电极催化剂上吸附和除去的反应机理以及设计更高效的抗CO中毒催化剂是非常有益的。氧化物负载的过渡金属纳米颗粒在许多异相催化反应中起着非常重要的作用。将贵金属Pt团簇负载于γ-Al₂O₃载体上,建立了负载Pt团簇催化剂模型,研究了不同尺度和构型的Pt小团簇与载体之间相互作用强度以及氢在负载Pt团簇催化剂模型上连续解离吸附行为。负载Pt团簇催化剂的活性与团簇的大小、结构以及分散度密切相关。研究结果表明:（1）Pt小团簇能够在γ-Al₂O₃（001）面上稳定吸附。Pt与表面O原子的作用强于与表面Al的作用,缘于Pt团簇向基底O原子转移了电子而使Pt显正电性。Pt和Al同是正电性的,二者之间存在排斥作用,只能形成较弱的键。吸附过程伴随着团簇结构和基底变形和松弛。随着金属团簇粒径增大,与载体之间的相互作用减弱。由于Pt-Pt之间相互作用强于Pt-O和Pt-Al的作用,γ-Al₂O₃（001）面上Pt团簇担载量较高的情况下,团簇均匀平铺相对较难,而团聚则很容易发生:（2）γ-Al₂O₃负载Pt金属粒子增加了氢在Pt团簇的桥位活化而降低了在顶端位的活化。负载型Pt团簇上随着氢覆盖率增大,对H₂的解离吸附能逐渐降低。氢高覆盖情况下,随着负载金属团簇粒径大小和结构的变化,金属团簇和基底发生变形和松弛程度大。对于较大尺度的Pt₄～Pt₆团簇负载于γ-Al₂O₃（001）面时,对H₂的解离吸附能和对H原子的脱附能基本在1.00～1.10 eV,2.46～2.80 eV微小范围内变化。与无载体时相比,团簇的Pt原子数与吸附的H原子数之比（Pt:H）降低到将近1:3,这是由于与H相互作用的Pt原子需要调整部分取向与基底成键作用。电荷布局分析表明,由于Pt团簇向吸附的H原子转移了电子而削弱了与载体的相互作用。本文研究了氢在非贵金属Ni团簇催化剂模型上的吸附行为,并与贵金属Pt团簇对H₂解离吸附性质对比分析。研究结果表明,H在Ni团簇上最稳定吸附位是棱位和空洞位,而与Pt团簇的最稳定吸附位不同。氢在Ni团簇上解离吸附反应在热力学上是放热反应,反应活化能小,但总体上比H₂在Pt团簇上的解离吸附能略低。随着Ni团簇上H原子覆盖度增大,H₂分子在团簇上连续解离吸附能和H原子的脱附能呈下降的趋势。饱和吸附态时不同尺度和构型的Ni团簇对H₂分子的解离吸附能和对H原子的脱附能分别降到0.71～1.0eV和2.08～2.73 eV微小范围内。团簇对H₂的解离吸附能比Pt（0.91～1.10 eV）稍低,而对H原子的脱附能与Pt（2.02～2.70eV）的接近。金属的HOMO和氢的LUMO能够达到最大程度重叠,解离吸附过程中伴随着电子转移。Hirshfeld电荷布局分析表明,随着H原子覆盖度增加,金属原子向H原子转移电子数逐渐减少。随着金属氢化物的形成,体系由金属键向共价键化合物转变。但与Pt团簇不同的是,个别Ni团簇在饱和吸附态时形成的金属氢化物仍保持着磁性特征,如Ni₈和Ni₁₃。除了团簇内核心的金属原子不容易直接接触H原子外,在饱和吸附态时表面金属原子数和吸附的H原子数之比值几乎保持常数为1:2,但只有Pt团簇表面容纳氢的量的一半。虽然本论文研究的亚纳米级尺度的团簇较实际异相催化反应体系中应用的纳米催化剂颗粒尺寸小得多,而且实际催化剂颗粒也绝非少数单一的单晶模型表面可以描述,但论文总结出团簇模型的催化性质并不随一定范围内的团簇大小和构型的变化而发生明显的改变的结论是非常重要的,在原子/分子水平上为研究过渡金属吸附解离氢的真实反应过程及催化剂的活性提出了有价值的信息,为真正理解异相催化反应提供了非常有益的帮助。更多还原

【Abstract】 Transition metals have been shown to be the most effective catalysts in heterogeneous catalysis due to their highly catalytic reactivity, stability and selectivity. Among which, platinum （Pt） and nickel （Ni） have been received the most attention and already applied in many industrial chemical processes such as hydrogenation and dehydrogenation in organic and petrochemical productions, toxic gas reduction of automobile exhausts, oxidation and reduction in electrolysis reactions. For a hydrogen involved reaction, the H₂ molecular adsorption with the successive activation and the H atoms desorption were regarded as the most important steps in the catalytic processes. A detailed knowledge of the dissociative Chemisorption of hydrogen molecules on the transition metal clusters or crystalline surfaces at an atomistic/molecular level would be very important to gain useful insight into the dynamic behaviors of surface adsorption and migration, and the nature of the interaction between the adsorbates and substrates, and consequently to understand the underlying catalytic mechanisms. Such an understanding would be benefit on finding appropriate solutions towards specific industrial catalytic problems. In this thesis, we present a systematic density functional theory （DFT） study of hydrogen dissociative Chemisorption on Pt crystalline surfaces and the alumina supported Pt clusters using a more realistic model which is capable of describing the catalytic processes. To address the CO poisoning issue, we further adopted a hydrogen-saturated subnano Pt and Pt/Ru cluster model to explore the CO removal mechanisms via O₂ oxidation and/or hydrogenation. We further present the comparably study of hydrogen sequential dissociative Chemisorption on Ni and Pt clusters. The structural characteristics, energetics and electronic structures of the hydrogen+catalyst complex systems were thoroughly calculated and analyzed by considering the H coverage dependence. The nature of the interaction between the adsorbates and substrate had also been discussed.Hydrogen sequential dissociative Chemisorption on precious metal Pt crystalline surfaces was first studied using DFT-based methods according to the study of hydrogen adsorption behaviors on Pt clusters models. Successive H₂ decomposition and sequential H desorption and the geometric and electronic structures of metal hydrides at full H saturation were identified. The results indicate that the difference is the preferred sites for H atom loading on Pt（111）, Pt（100） and Pt（531） surfaces. The energies are very close for H adsorption on the top, bridge and fee hollow sites of Pt（111） surface, while the bridge site is more preferable than on-top and hollow sites of Pt（100） surface. The first bridge and top sites are energetically the most favorable for H dissociation on Pt（531） surface. In contrast to the most stable adsorption energy, the first bridge site of Pt（531） is about 0.06 eV higher than that of Pt（lOO） surface, and about 0.15 eV higher than the on-top and bridge sites of Pt（111） surface. We further found that dissociative Chemisorption energy of H₂ and desorption energy of H atom in general decline with H coverage. For the three Pt crystalline surfaces at the threshold of saturation, the H₂ Chemisorption energies fall within a narrow range of 0.60-0.88 eV, which are slightly lower than the values 0.90-1.1 eV of Pt cluster models at high coverages. The calculated thresholds of H desorption energy vary in a range of 2.08-2.84 eV, which are comparable to the values of 2.02-2.70 eV on Pt cluster models. H₂ dissociative Chemisorption is largely controlled by charge transfer from metal atoms to H atoms. Bader charge population analysis indicates that charge transfer increases with H loading, resulting in sequential change of metallic bonds to covalent bonds in the metal hydrides. Moreover, our calculations suggest that the capacity of Pt crystalline surfaces to adsorb H atoms is essentially much lower than what was found for Pt clusters at full saturation.Based on the studies of sequential H₂ dissociative Chemisorption on small Pt clusters, we attempt to investigate different CO mitigation techniques by employing a small Pt subnano cluster as model to understand the hydrogenation and oxidation of CO poison in the presence of H atoms. At low H coverage, CO oxidation by oxygen to form CO₂ on the selected Pt₆ cluster was found to be endothermic with moderate overall thermochemical energy. However, the subsequent CO₂ desorption from the cluster is highly endothermic. Kinetically, the oxidation process is also unfavorable and needs to overcome a significant activation barrier. The unfavorable energetics makes the CO oxidation at low H-coverage unlikely to occur. Upon saturation of the Pt₆ cluster by H atoms, the activation energy required to form a transition state that leads to the formation of surface CO₂ is reduced substantially and, thermochemically, the oxidation reaction becomes exothermic. Our results suggest that CO oxidation by oxygen on the H-saturated Pt₆ cluster would be difficult at low temperatures due to the moderate activation energy in spite of the favorable thermochemical energy. However, at an elevated temperature, the relatively moderate barrier can be readily overcome and thus the reaction can become facile. This conclusion is consistent with experimental observations. Many experimental results suggest that CO tolerance can be signifieantly improved by doping the Pt catalysts with Ru. To reveal the role of Ru in the CO₂ removal process, we calculated the thermochemical energies and activation barriers for the CO oxidation on a Pt₅Ru cluster fully covered by H atoms. Our results indicate that O atoms are chemisorbed on the Pt-Ru bond more strongly than on the Pt-Pt bond. As a consequence, the oxidation process becomes much more exothermic. Depending on where the attacking O atom resides, the associated activation barrier can be moderately reduced or slightly increased. CO occupation on Ru was found to be energetically more favorable than on Pt, and thus makes the CO removal via oxidation at this site more difficult. We further investigated the feasibility of removing CO from the H-saturated Pt₆ cluster by considering CO reduction by H atoms to form formaldehyde. This was done by allowing two H atoms adsorbed nearby the active site to sequentially attack the CO molecule. It was found that each reaction step is moderately endothermic. However, the calculated activation barriers are relatively high. Even at 500K, no formaldehyde formation was observed in our ab initio MD simulations. The results suggest that CO reduction by H atoms on the Pt₆ cluster is energetically difficult. The present study utilizes an exceedingly small Pt cluster to represent the catalyst particles for exploration of CO removal mechanisms. However the catalyst model is undoubtedly oversimplified, we catch an eye on the dependency of CO poison and removal mechanisms on the size of Pt clusters.Oxide supported precious metals play an important role in many heterogeneous catalytic reactions. We present a systematic study using the DFT method to understand the adhesion of small Pt_n clusters for n up to 6 on theγ-Al₂O₃ （001） surface and the catalytic behaviors of Pt_n/γ-Al₂O₃ system with respect to H coverage. Our calculations indicate that the catalytic performance of supported Pt subnano-catalyst is dependent upon the size and shape of metal particles. Results show that （1）. The Pt clusters can be stably anchored on the surface. Energetically the most favorable adsorption sites were identified and substantial structural relaxation upon adsorption was observed. The significantly higher adsorption energy at the O site is largely attributed to the face that the charge transfer from the Pt atoms to the O atoms makes the Pt atom positively charged. The Al atom underneath Pt atom is also positively charged. The repulsion between the two positively charged atoms, Pt and Al, leads to much weaker bonds. The calculated average adhesion energies were found to be size and shape dependent. The adhesion energy of Pt atoms in general decrease with the size of cluster increases. Since the Pt-Pt interaction would become stronger than Pt-O and Pt-Al at large cluster size, the formation of metal cluster would be strongly preferred upon high Pt loading, consequently, the growth of metal films on theγ-Al₂O₃（001） surface is unlikely to be smooth and agglomeration could occur under certain conditions; （2）. The support changes the site preference of H₂ adsorption, increasing H₂ activation for bridge sites but decreases it for on-top sites. Our results also indicate that H₂ dissociative Chemisorption on supported small Pt clusters in general decrease with the coverage of H atoms. At H high coverage, the structural distortion and relaxation of metal clusters and substrate occur with the selected cluster size and shape, the H₂ dissociative Chemisorption energy and H desorption energy fluctuate in the range of 1.00-1.10 eV, 2.46-2.80 eV for larger clusters, respectively. The capacity of Pt clusters on support at full saturation decreases to be 1:3（Pt:H ratio） compared with that of bare Pt clusters to absorb H atoms due to the fact that some of the possible orientations of the Pt atoms toward H were occupied by the Pt-substrate bonding. Bader charge analysis indicates that charge transfer from Pt clusters to H atoms increases with H loading, resulting in the interaction between Pt clusters and the substrate decreases.For purpose of comparison, we also studied the hydrogen sequential dissociative Chemisorption on subnano Ni clusters using the same computational scheme, together with our previous results on small Pt clusters to discuss the difference of Chemisorption/desorption behaviors between Pt and Ni. Our results show that at H low coverage, the edge and hollow sites are energetically the most favorable for H₂ dissociation, which are different to the Pt clusters. Dissociative Chemisorption of H₂ on the Ni clusters is facile with exothermic reaction energies and small activation barriers. However, the Chemisorption energy is generally lower than that of on Pt clusters. H₂ dissociative Chemisorption energies and H desorption energies are strongly coverage dependent. These energies in general decline with H coverage and for various sizes and shapes of Ni clusters at the threshold of saturation, the H₂ Chemisorption energies each fall within a narrow range of 0.71-1.00 eV, which are slightly lower than the values of 0.91-1.10 eV of Pt clusters at a high coverage. The calculated threshold values of H desorption vary in a range of 2.08-2.73 eV, which are comparable to the values of 2.02-2.70 eV on Pt clusters. The favorable orbital overlaps between the HOMO of metal clusters and the LUMO of H₂. Hirshfeld population analysis indicates that charge transfer from Pt clusters to H atoms increases with H loading, resulting in sequential change of metallic bonds to covalent bonds in the metal hydrides. However, the difference is that some of Ni metal hydrides still remain the magnetic characters, such as Ni₈ and Ni₁₃ clusers. Our calculations also suggest that the capacity of Ni clusters to adsorb H atoms is nearly constant at full saturation, except when some of the metal atoms residing at the core of the clusters are not accessible to H atoms. However, the H capacity on Ni clusters is essentially half of what was found for Pt clusters. Despite the relatively smaller size of the Ni clusters chosen in the present study compared with the size of catalyst particle size used in practice, some of the properties may not change significantly with the particle size and shape of catalysts. Useful insight into the catalytic activity of transition metal catalyst toward H₂ can be gained at atom and molecule levels. It will be very helpful in understanding real heterogeneous catalytic processes.更多还原