CO在CeO2(111)表面的吸附与氧化

采用密度泛函理论计算了CO在CeO2(111)表面的吸附与氧化反应行为. 结果表明, O2在洁净的CeO2(111)表面为弱物理吸附, 而在氧空位表面是强化学吸附, 且O2分子活化程度较大, O—O键长为0.143 nm. CO在CeO2(111)表面吸附行为的研究表明, CO在洁净表面及氧空位表面上为物理吸附, 吸附能均小于0.42 eV; 当表面氧空位吸附O2后, CO可吸附生成二齿碳酸盐中间体或直接生成CO2, 与原位红外光谱结果相一致. 表面碳酸盐物种脱附生成CO2的能垒仅为0.28 eV. 计算结果表明, 当CeO2表面存在氧空位时, Hubbard参数U对CO吸附能有一定的影响. CeO2载体在氧化反应中可能的催化作用为, 在氧气氛下, CeO2表面氧空位吸附O2分子, 形成活性氧物种, 参与CO催化氧化反应.     

Effects of deposited Pt particles on the reducibility of CeO2(111)

The interaction of Pt particles with the regular CeO(2)(111) surface has been studied using Pt(8) clusters as representative examples. The atomic and electronic structure of the resulting model systems have been obtained through periodic spin-polarized density functional calculations using the PW91 exchange-correlation potential corrected with the inclusion of a Hubbard U parameter. The focus is on the effect of the metal-support interaction on the surface reducibility of ceria. Several initial geometries and orientations of Pt(8) with respect to the ceria substrate have been explored. It has been found that deposition of Pt(8) over the ceria surface results in spontaneous oxidation of the supported particle with a concomitant reduction of up to two Ce(4+) cations to Ce(3+). Oxygen vacancy formation on the CeO(2)(111) surface and oxygen spillover to the adsorbed particle have also been considered. The presence of the supported Pt(8) particles has a rather small effect (～0.2 eV) on the O vacancy formation energy. However, it is predicted that the spillover of atomic oxygen from the substrate to the metal particle greatly facilitates the formation of oxygen vacancies: the calculated energy required to transfer an oxygen atom from the CeO(2)(111) surface to the supported Pt(8) particle is only 1.00 eV, i.e. considerably smaller than 2.25 eV necessary to form an oxygen vacancy on the bare regular ceria surface. This strongly suggests that the propensity of ceria systems to store and release oxygen is directly affected by the presence of supported Pt particles.     

A theoretical study of surface reduction mechanisms of CeO(2)(111) and (110) by H(2).

Reaction mechanisms for the interactions between CeO(2)(111) and (110) surfaces are investigated using periodic density functional theory (DFT) calculations. Both standard DFT and DFT+U calculations to examine the effect of the localization of Ce 4f states on the redox chemistry of H(2)-CeO(2) interactions are described. For mechanistic studies, molecular and dissociative local minima are initially located by placing an H(2) molecule at various active sites of the CeO(2) surfaces. The binding energies of physisorbed species optimized using the DFT and DFT+U methods are very weak. The dissociative adsorption reactions producing hydroxylated surfaces are all exothermic; exothermicities at the DFT level range from 4.1 kcal mol(-1) for the (111) to 26.5 kcal mol(-1) for the (110) surface, while those at the DFT+U level are between 65.0 kcal mol(-1) for the (111) and 81.8 kcal mol(-1) for the (110) surface. Predicted vibrational frequencies of adsorbed OH and H(2)O species on the surfaces are in line with available experimental and theoretical results. Potential energy profiles are constructed by connecting molecularly adsorbed and dissociatively adsorbed intermediates on each CeO(2) surface with tight transition states using the nudged elastic band (NEB) method. It is found that the U correction method plays a significant role in energetics, especially for the intermediates of the exit channels and products that are partially reduced. The surface reduction reaction on CeO(2)(110) is energetically much more favorable. Accordingly, oxygen vacancies are more easily formed on the (110) surface than on the (111) surface.     

Atomistic models for CeO(2)(111), (110), and (100) nanoparticles,supported on yttrium-stabilized zirconia

Ceria is an important component in three-way catalysts for the treatment of automobile exhaust gases owing to its ability to store and release oxygen, a property known as the oxygen storage capacity. Much effort has been focused on increasing the OSC of ceria, and one avenue of exploration is the ability to fabricate CeO(2)-based catalysts, which expose reactive surfaces. Here we show how models for a polycrystalline CeO(2) thin film, which expose the (111), (110), and dipolar (100) surfaces, can be synthesized. This is achieved by supporting the CeO(2) thin film on an yttrium-stabilized zirconia substrate using a simulated amorphization and recrystallization strategy. In particular, the methodology generates models which reveal the atomistic structures present on the surface of the reactive faces and provides details of the grain-boundary structures, defects (vacancies, substitutionals, and clustering), and epitaxial relationships. Such models are an important first step in understanding the active sites at the surface of a catalytic material.     

Chain structures of surface hydroxyl groups formed via line oxygen vacancies on TiO2(110) surfaces studied using noncontact atomic force microscopy

Structures of surface hydroxyl groups arranged on a reduced TiO2(110) surface that had line oxygen vacancies were studied using noncontact atomic force microscopy (NC-AFM). NC-AFM results revealed that by increasing the density of oxygen vacancies on the TiO2(110) surface, line oxygen vacancies were formed by removal of oxygen atoms in a bridge oxygen row on the TiO2(110) surface. After the TiO2(110) surface with the line oxygen vacancies was exposed to water, the surface showed hydroxyl chain structures that were composed of hydroxyl groups linearly arranged in a form of two rows on the line oxygen vacancies and on a neighboring bridge oxygen row. In-situ NC-AFM measurements of these surfaces exposed to water at room temperature revealed that hydroxyl chain structures were formed at the line oxygen vacancy. Annealing above 500 K was sufficient to remove the hydroxyl chain structures on the TiO2(110) surface and allowed line oxygen vacancies to reappear on the surface. The line oxygen vacancies are active sites for water dissociation. In conclusion, the formation of the hydroxyl chain structure suggests that the surface hydroxyl groups on a TiO2(110) surface can be controlled by preparing oxygen vacancy structures on the surface.     

Atomic simulation of the point defects in three low‐index surfaces of BCC transition metals with the MAEAM

The favorable position of an adatom and the formation energies of a single vacancy and an adatom‐vacancy pair in three low‐index surfaces of body‐centered cubic (BCC) transition metals have been calculated by using the modified analytical embedded atom method (MAEAM). The favorable position of an adatom is at the fourfold and twofold positions above the (100) and (110) surfaces respectively, but it is deviated from the threefold position of the (111) surface. Either the heights of the adatom from the top atomic layer, or the formation energies of a single vacancy, or an adatom‐vacancy pair decrease in sequence of the (110), (100) and (111) surfaces for each metal. Furthermore, the formation energy of an adatom‐vacancy pair is always lower than that of a single vacancy for each low‐index surface of each metal, which shown the formation of adatom‐vacancy pair is more energetically favorable than the vacancy for the BCC transition metals. Copyright © 2007 John Wiley & Sons, Ltd.     

CeO2(110)表面上CO氧化反应的密度泛函理论研究

利用密度泛函理论系统研究了O2与CO在CeO2(110)表面的吸附反应行为. 研究表明, O2在洁净的CeO2(110)表面吸附热力学不利, 而在氧空位表面为强化学吸附, O2分子被活化, 可能是重要的氧化反应物种. CO在洁净的CeO2(110)表面有化学吸附与物理吸附两种构型, 前者形成二齿碳酸盐物种, 后者与表面仅存在弱的相互作用. 在氧空位表面, CO可分子吸附或形成碳酸盐物种, 相应吸附能均较低. 当表面氧空位吸附O2后(O2/Ov), CO可吸附生成碳酸盐或直接生成CO2, 与原位红外光谱结果相一致. 过渡态计算发现,O2/Ov/CeO2(110)表面的三齿碳酸盐物种经两齿、单齿过渡态脱附生成CO2. 利用扩展休克尔分子轨道理论分析了典型吸附构型的电子结构, 说明表面碳酸盐物种三个氧原子电子存在离域作用, 物理吸附的CO及生成的CO2电子结构与相应自由分子相似.     

Theoretical Study on the Structural and Electronic Properties of the Reduced SnO2 （110） Surface

The reduced SnO2(110) surface has been investigated by using first-principles method with a slab model. By examining the vacancy formation energy of three kinds of reduced SnO2(110) surfaces, the most energetically favorable defect surface is confirmed to be the surface with the coexistence of bridging and in-plane oxygen vacancies, which is different with the traditional model by only removing bridging oxygen. The results of band structure calculations indicate that the electronic structure of this defect surface is similar to the SnO surface.     

A computational modelling study of oxygen vacancies at LaCoO3 perovskite surfaces

Atomistic computational modelling of the surface structure of the catalytically-active perovskite LaCoO(3) has been undertaken in order to develop better models of the processes involved during catalytic oxidation processes. In particular, the energetics of creating oxygen ion vacancies at the surface have been investigated for the three low index faces (100), (110) and (111). Two mechanisms for vacancy creation have been considered involving dopant Sr(2+) cations at the La(3+) site and reduction of Co(3+) to Co(2+). For both mechanisms, there is a general tendency that the smaller the cation defect separation, the lower the energy of the cluster, as would be expected from simple electrostatic considerations. In addition, there are clear indications that oxygen vacancies are more easily created at the surface than in the bulk. The results also confirm that the presence of defects strongly influences crystal morphology and surface chemistry. The importance of individual crystal surfaces in catalysis is discussed in terms of the energetics for the creation of oxygen vacancies.     

A theoretic insight into the catalytic activity promotion of CeO2 surfaces by Mn doping

In this paper, we investigated the primary reduction and oxygen replenishing processes over Mn substitutionally doped CeO(2)(111) surfaces by density functional theory with the on-site Coulomb correction (DFT + U). The results indicated that Mn doping could make the surface much more reducible and the adsorbed O(2) could be effectively activated to form superoxo (O(2)(-)) and/or peroxo species (O(2)(2-)). The Mn doping induced the Mn 3d-O 2p gap state instead of Ce 4f acting as an electrons acceptor and donor during the first oxygen vacancy formation and O(2) replenishing, which helped to lower the formation energy of the first and second oxygen vacancies to -0.46 eV and 1.40 eV, respectively. In contrast, the formation energy of a single oxygen vacancy in the pure ceria surface was 2.08 eV and only peroxo species were identified as the O(2) molecule adsorbed. Our work provides a theoretical and electronic insight into the catalytic redox processes of Mn doped ceria surfaces, which may help to understand the enhanced catalytic performances of MnO(x)-CeO(2) oxides, as reported in previous experimental works.     

过渡金属掺杂铈中晶格氧的活化

采用密度泛函理论计算研究了在铈表面掺杂的过渡金属(TM)离子对表面晶格氧原子活化的影响.为此,测定了经TM离子修饰的CeO2最稳定(111)表面终端的结构和稳定性.除了保持八面体氧配位的锆和铂掺杂剂外, TM掺杂剂在取代表面Ce离子时更倾向于正方形平面配位.除了Pt(1.14 eV)和Zr(正方形平面配位不稳定)外,所有TM掺杂剂的表面结构从八面体到正方形平面都很容易.通常,四价TM阳离子的离子半径比Ce^4+的小得多,从而导致了显著的拉伸应变晶格,并解释了氧空位形成能量的降低.除Zr外,当产生一个氧空位时,优先形成正方形平面结构.热力学分析表明, TM掺杂CeO2表面在典型环境催化条件下存在氧缺陷.一个具有实际意义的例子是锆掺杂CeO2(111)中的晶格氧容易活化,从而有利于CO氧化.研究结果强调了晶格氧活化的本质和TM掺杂剂在TM-铈固溶催化剂中的优选位置.     

Oxidising CO to CO2 using ceria nanoparticles

We calculate, using simulated amorphisation and recrystallisation (A&R), that ceria (CeO2) nanoparticles, about 8 nm in diameter, comprise a high concentration of labile surface oxygen species, which we suggest will help promote the oxidation of CO to CO2. In particular, the ceria nanoparticle contains a high proportion of reactive {100} surfaces, surface steps and corner sites. When reduced to CeO1.95, the associated Ce3+ species and oxygen vacancies decorate step, corner and {100} sites in addition to plateau positions on {111}. The energetics of CO oxidation to CO2, catalysed by a ceria nanoparticle, is calculated to be lower compared with CO oxidation associated with the lowest energy surface (i.e. CeO2(111)) of the corresponding 'bulk' material. Our calculated morphologies for the ceria nanoparticles are in accord with experiment.     

CeO2(100)极性面结构与活性的密度泛函理论研究

CeO2是一类使用非常广泛的稀土氧化物催化材料,在许多重要的催化反应过程,如机动车尾气净化、水汽转换、石油裂解等,表现出很高的活性.大量研究表明,CeO2的高活性来源于其表面晶格氧,正是由于这些晶格氧能够直接参与氧化反应,同时反应留下的氧空位又能够被气相氧分子吸附填补,因而体现出很好的储放氧催化性能.目前多数研究采用CO氧化为模型反应,研究了CeO2常见的(111)和(110)晶面的晶格氧活性,但对于其另外一种重要低指数晶面(100)的结构和活性研究却非常有限.需要指出的是,CeO2(100)是一种极性表面,这给该表面的模型构建和理论研究带来了困难.为了深入了解这种极性表面的结构稳定性和催化活性,本文运用在位库仑力校正的密度泛函理论(DFT+U)方法系统研究了CeO2(100)极性面的可能结构及相关稳定性,并且深入分析了CO在该表面上的吸附和反应.本文首先利用板层模型尝试构建稳定的CeO2(100)极性面结构,方法是在保证整个板层化学计量配比完整的前提下,在表层或体相去除氧原子,同时使得整个板层上下对称不存在极性以利于计算.通过计算发现,在CeO2(100)表层分布氧空位的结构比体相中分布氧空位的结构要稳定,同时,氧空位的分布越接近表面,CeO2(100)面的结构稳定性就会越高,其最稳定的结构是将表层满覆盖氧离子移除一半.对CeO2(100)面不同结构的稳定性及相关电子结构分析表明,CeO2(100)表层满覆盖的氧离子间存在很强的相互排斥作用,因此倾向于降低表面氧浓度来提高表面的稳定性.另外,这种相互作用会降低相邻氧离子的价态,并能引起体相铈离子在整体表面维持完整的化学计量比的情况下,仍能出现局域4?电子而被还原为三价铈.随后我们研究了CO在CeO2(100)最稳定和次稳定表面上的氧化反应.发现CO在不同CeO2(100)表面的氧空位处吸附较强,另外,CO在CeO2(100)最稳定结构上可与表面晶格氧反应形成吸附的CO2中间物种,中间物种可直接解离成气相CO2,也可以继续与表面晶格氧反应形成碳酸盐.而在CeO2(100)次稳定表面上,CO很难与表面晶格氧形成吸附的CO2中间态,而直接产生气态CO2.     

铜-氧化铈界面:几何及电子结构

纳米催化材料的性能主要由粒子尺寸、形貌和界面决定,即活性位点的电子及几何结构.尺寸、形貌可控的纳米催化材料的合成及其反应性能的研究,即催化剂的构效关系,一直是催化领域的研究热点.氧化物负载的金属催化剂广泛应用于多相催化反应过程.基于氧化铈优异的氧化还原性能, Cu/CeO2催化剂在CO氧化、N2O消除、水气变换、甲醇合成等反应中表现出优异性能.其中,通过铜物种与氧化铈表面化学键合形成的金属-载体界面通常被认为是催化活性中心.铜物种和氧化铈的相互作用主要体现在氧化铈固定铜物种,而铜物种促进氧化铈的氧化还原能力,涉及Cu^2+/Cu^+/Cu^0和Ce^3+/Ce^4+之间电子的传输和转移.Cu/CeO2催化剂活性位的原子结构与金属-载体相互作用程度密切相关.氧化铈形貌和铜负载量是决定界面电子和几何结构的重要因素.常见的纳米氧化铈形貌包括纳米粒子(多面体)、纳米棒和纳米立方体,可分别选择性暴露(111)、(110)和(100)晶面;这些晶面上原子配位环境和化学性能决定了铜-氧化铈的键合方式和界面结构.与暴露{100}晶面的纳米立方体相比,主要暴露{100}/{110}镜面的氧化铈纳米棒、暴露{111}/{100}晶面的纳米粒子与铜物种具有更强的金属-载体相互作用程度,也更有利于铜物种的分散.铜的负载量也显著影响铜物种在特定氧化铈表面的分散度和化学状态;随着铜负载量的增加,可在氧化铈表面形成层状铜、铜团簇和铜纳米粒子.通常情况下,低负载量有利于单层、双层铜物种的形成,高负载量时则出现多层铜和铜纳米粒子.催化活性位通常是由铜原子与氧化铈上的氧空穴相互作用产生,与氧化铈表面氧空穴的数量和密度密切相关,即氧化铈形貌.本文总结了Cu/CeO2催化剂的研究进展,讨论了氧化铈形貌和铜负载量对铜物种分散度和化学状态的影响规律,总结了铜氧化铈界面结构的多维度表征结果,比较了Cu/CeO2催化剂在CO氧化、水气变换及甲醇合成中的活性位结构和反应机制.     

Surface studies of gas sensing metal oxides

The relation of surface science studies of single crystal metal oxides to gas sensing applications is reviewed. Most metal oxide gas sensors are used to detect oxidizing or reducing gases and therefore this article focuses on surface reduction processes and the interaction of oxygen with these surfaces. The systems that are discussed are: (i) the oxygen vacancy formation on the surface of the ion conductor CeO(2)(111); (ii) interaction of oxygen with TiO(2) (both adsorption processes and the incorporation of oxygen into the TiO(2)(110) lattice are discussed); (iii) the varying surface composition of SnO(2)(101) and its consequence for the adsorption of water; and (iv) Cu modified ZnO(0001)-Zn surfaces and its interaction with oxygen. These examples are chosen to give a comprehensive overview of surface science studies of different kinds of gas sensing materials and to illustrate the potential that surface science studies have to give fundamental insight into gas sensing phenomena.     

Oxygen vacancies on TiO2 (110) from first principles calculations

We have carried out a systematic study of oxygen vacancy formation on the TiO2 (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have used models with the mean number of vacancies per surface unit cell being theta=0.25 and theta=0.5. The study comprises several kind of vacancies within the outermost layers of the surface. The use of a suitable set of technical parameter is often essential in order to get accurate results. We find that the presence of bridging vacancies is energetically favored in accordance to experimental data, although the formation of sub-bridging vacancies might be possible at moderate temperatures. Surprisingly, the spin state of the vacancy has little influence on the results. Atomic displacements are also analyzed and found to be strongly dependent on the particular arrangement of vacancies.     

Density-functional calculation of CeO2 surfaces and prediction of effects of oxygen partial pressure and temperature on stabilities

We have used density-functional theory to investigate (111), (110), (210), (211), (100), and (310) surfaces of ceria (CeO2). Compared with previous interatomic-potential-based studies, our calculations reported a slightly different relative stability ordering and significantly lower surface energies for the stoichiometric surfaces. Using a defect model, the surface stabilities were evaluated as functions of oxygen partial pressure and temperature. Our investigations were restricted to ideal surface terminations, without considering defect formation on those surfaces. We found that at 300 K, the stoichiometric (111) has the lowest free energy for a wide range of oxygen partial pressures up to 1 atm, and only at ultrahigh vacuum does the Ce-terminated (111) becomes the most stable one. The transition point for the Ce-terminated (111) surfaces moves to higher oxygen partial pressures when temperature increases. To improve the prediction of electron density of states, we used the local-density approximation plus U(J) correction method to correct the on-site Coulomb correlation and exchange interaction due to the strongly localized Ce-4f electrons. The optimal parameter combination of U = 7 eV and J = 0.7 eV was found to improve the O 2p-Ce 4f gap without much degradation of ground-state bulk properties or the O 2p-Ce 5d gap. The bulk and surface electronic structures were then analyzed based on the improved density of states.     

Molecular dynamics simulation of surface segregation in a (110) B2-NiAl thin film

Surface segregation in (110) B2-NiAl film approximately 3 nm thick is investigated by using molecular dynamics simulation with a reliable embedded-atom potential. The simulation is performed for the stoichiometric composition at a temperature of 1500 K, just below the melting temperature of the film model. It is found that the (110) surface is structurally stable but develops adatoms, vacancies and antisites. The coverage of an adatom layer is estimated to be ～0.07 ML (monatomic layers) and it contains on average ～95% of Al atoms. The top (surface) and second (subsurface) layers of the (110) surface is the most enriched in Ni relative to the bulk composition. These layers contain on average ～51% of Ni atoms. The Ni fraction in the third and forth layers of the film is estimated as ～50.5%. The deeper layers have essentially the bulk composition. Vacancies in the film model are found only on the Ni sublattice. The vacancy concentration on the Ni sublattice in the top layer is ～7.5%. The second layer almost does not contain vacancies. The next layers have essentially the constant bulk vacancy composition which can be estimated as ～1.3-1.4%.     

Characterization of O2-CeO2 interactions using in situ Raman spectroscopy and first-principle calculations.

Interactions between O(2) and CeO(2) are examined experimentally using in situ Raman spectroscopy and theoretically using density-functional slab-model calculations. Two distinct oxygen bands appear at 825 and 1131 cm(-1), corresponding to peroxo- and superoxo-like species, respectively, when partially reduced CeO(2) is exposed to 10 % O(2). Periodic density-functional theory (DFT) calculations aid the interpretation of spectroscopic observations and provide energetic and geometric information for the dioxygen species adsorbed on CeO(2). The O(2) adsorption energies on unreduced CeO(2) surfaces are endothermic (0.91相似文献   

氢原子在Pt及Pt系双金属催化表面吸附的密度泛函理论研究

采用密度泛函理论(dFT)考察了Pt(100)、(110)、(111)三种表面氢原子的吸附行为, 计算了覆盖度为0.25 ML时氢原子在Pt 三种表面和M-Pt(111)双金属(M=Al, Fe, Co, Ni, Cu, Pd)上的最稳定吸附位、表面能以及吸附前后金属表面原子层间弛豫情况. 分析了氢原子在不同双金属表面吸附前后的局域态密度变化以及双金属表面d 带中心偏离费米能级的程度并与氢吸附能进行了关联. 计算结果表明, 在Pt(100), Pt(110)和Pt(111)表面, 氢原子的稳定吸附位分别为桥位、短桥位和fcc 穴位. 三种表面中以Pt(111)的表面能最低, 结构最稳定. 氢原子在不同M-Pt(111)双金属表面上的最稳定吸附位均为fcc 穴位, 其中在Ni-Pt 双金属表面的吸附能最低, Co-Pt 次之. 表明氢原子在Ni-Pt 和Co-Pt 双金属表面的吸附最稳定. 通过对氢原子在M-Pt(111)双金属表面吸附前后的局域态密度变化的分析, 验证了氢原子吸附能计算结果的准确性. 掺杂金属Ni、Co、Fe 的3d-Pt(111)双金属表面在吸附氢原子后发生弛豫, 第一层和第二层金属原子均不同程度地向外膨胀. 此外, 3d金属的掺入使得其对应的M-Pt(111)双金属表面d带中心与Pt 相比更靠近费米能级, 吸附氢原子能力增强, 表明3d-Pt系双金属表面有可能比Pt具有更好的脱氢活性.