【作者】 陈博昊；

【导师】 黄伟新；

【作者基本信息】 中国科学技术大学 ， 物理化学， 2014， 博士

【摘要】 氧化铈作为金属催化剂载体和助剂在水煤气变换、CO氧化、汽车尾气净化等催化反应过程中有着广泛地应用。其中,金属/氧化铈催化剂的催化活性与氧化铈表面结构(尤其是表面上氧缺陷的浓度和分布)、金属-氧化物相互作用以及金属-氧化铈界面结构等因素密切相关。在超高真空体系中的可控条件下制备的表面结构规整的模型催化剂为微观角度上了解实际多相催化体系提供了较为重要的研究手段。在本论文中,我们通过制备具有不同结构和组成的氧化铈/金属单晶倒载型模型催化剂,在表征其表面结构和电子结构的基础上,考察该模型催化剂上无机探针分子(D2O, CO等)的化学吸附和反应性能,取得了如下主要研究成果：(1)Cu(111)表面及CuOx/Cu(111)表面的吸附反应性质考察了Cu(111)上NO低温下在Cu(111)表面的解离过程以及表面氧在此过程中的影响,通过调节NO的暴露量和衬底温度,可以在Cu(111)表面获得不同类型的氧物种：亚稳态的吸附氧物种O528(Ols结合能为528.7eV)、表面吸附氧0529(Ols结合能为529.5eV)和化学吸附氧O531(Ols结合能为531.0eV)。实验发现,这些不同的氧物种对NO的吸附和解离行为有着显著的影响。低温下,表面吸附氧O531的存在主要影响NO不同吸附状态的相对分布,同时改变了NO的解离脱附的选择性；而同样条件下,化学吸附氧O529对于表面NO的吸附有着更为显著的抑制作用,同时该吸附氧的存在明显抑制了NO的解离脱附。在超高真空中原位制备了CuOx/Cu(111)模型体系,同时考察了原子氘与该模型体系的相互作用及其形成的表面物种的反应行为。研究结果表明,在低温下(115K),原子氘可以与CuOx薄膜中的氧物种发生还原反应,生成表面羟基(D(g)+OL→OD(a))和表面吸附水(OD(a)+D(g)→D2O(a))。同时,实验中观察到与Cu+离子相关的表面吸附氢物种。与Cu(111)相比,氧化后形成的CuOx薄膜对原子氘在体相的吸附有显著的抑制作用。同时,CuOx表面羟基的稳定性与表面的组成和氧化还原状态密切相关。(2)CeO2(111)模型表面的制备和表面化学性质：以原位制备的CeO2(111)有序薄膜为基础,系统地研究了该模型表面与原子D(g)和D2O的相互作用,D(g)在CeO2表面吸附生成表面羟基(OD(a))和吸附水(D2O(a)),同时将表面Ce4+离子还原成Ce3+离子,而在还原CeO2-x表面上,原子氢在表面的吸附主要生成表面羟基,生成水的反应通道被明显抑制。在化学计量比CeO2(111)表面上,D2O以解离吸附和分子吸附形式存在；而在还原CeO2-x(111)表面上,D2O主要为不可逆解离吸附。在加热过程中,表面羟基经历两个反应途径：(1)羟基饼合生成水,同时生成表面氧缺陷,对应于表面的还原(OD(a)+OD(a)→D2O(g)+Olattice+Ovacancy);(2)羟基发生脱氢反应,对应于表面恢复原状(OD(a)+OD(a)→D2(g)+2Olattice)。在氧化表面上,脱水反应为主要反应通道,而在还原表面上,羟基倾向于发生脱氢反应。表面氧缺陷的存在有利于增加表面羟基的稳定性。采用X-射线光电子能谱(XPS)考察了CeO2模型表面与O2的相互作用。结果表明,O2在氧化CeO2(111)表面粘附几率很低,基本观察不到表面吸附。在还原CeO2-x表面上,O2在低温下吸附生成表面超氧物种(O2-)和过氧物种(O22-),其O1s结合能分别位于533.6-534eV和531.7～532eV。进一步地,我们利用原子氢还原和低温氩刻两种不同方法分别得到具有相同还原程度但具有不同氧缺陷结构的CeO2-x薄膜,结果表明吸附氧物种与CeO2-x表面氧缺陷的结构紧密相关,低温氩刻表面上更有利于过氧物种的形成。我们同样利用Au(110)表面蒸镀得到不规整的CeO2多晶薄膜,并对O2的吸附行为做了初步研究。TPD结果表明,在该完全氧化的多晶薄膜表面低温下吸附O2,可观察到位于230K O2的脱附峰。对于原子氘高温还原后的表面,进一步观察到了脱附温度位于250-400K的O2脱附峰。虽然目前还无法对上述O2的脱附峰进行归属,但与CeO2-x(111)表面的结果进行比较,也充分表明了O2的吸附性能与CeO2表面结构之间的密切关系。(3)倒载型CeO2/Cu(111)表面的结构和吸附反应性能。原位制备了覆盖度为0.5ML的Ce02(111)/CuOx/Cu(111)倒载型模型催化剂,通过控制原子氢的暴露量和衬底温度,可以对表面结构、组成和表面物种进行有效控制,进而考察了CO, D2O和原子氢等吸附分子与模型表面的相互作用。一方面,CeO2的存在对Cu表面的小分子吸附行为没有明显的影响,主要表现为位阻效应；另一方面,CeO2薄膜表面的吸附反应行为与薄膜厚度密切相关,相对于较厚CeO2(111)薄膜(10个单层),厚度为0.5ML的CeO2(111)薄膜的还原性能显著降低；进一步地,对于还原CeO2-x(111)表面,减少薄膜厚度明显抑制了H2O的解离吸附,上述性能的差异可能与Cu-CeO2界面结构以及衬底Cu的电子修饰作用相关。另外,通过制备条件的控制,我们成功制备出Cu上吸附CO,而CeO2表面吸附OD的结构,以考察CO+OD界面反应的可能性,但实验结果并没观察到明显的CO2生成,说明在超高真空实验条件下,该界面反应可能在Cu-CeO2催化体系催化水汽变换反应中不是主要的反应通道。上述研究系统考察了CeO2/Cu(111)倒载型模型催化剂的几何电子结构和吸附反应性能之间的相互关联,有利于在微观水平上进一步揭示实际金属/氧化铈催化体系中的结构一性能关系以及相应的微观作用机制。更多还原

【Abstract】 Ceria-based materials has been widely applied as supporters/promoters in the field of catalysis such as water gas shift (WGS), CO oxidation and three-way catalysis. Generally, the excellent catalytic performances of ceria-based catalysts are closely related to the surface strcuture of ceria (especially the concentration and distribution of oxygen vacancy), the ceria/metal interaction and the composition and structure of ceria/metal interface etc. The well-defined model catalyst, which has been prepared in-situ under Ultra-high Vacuum (UHV) condition, can be used as a model system for a detailed understanding of the reaciton mechanism of heterogeneous catalysis. In this thesis, the inverse ceria/metal model surfaces with various surface composition and structure have been prepared and characterized. Furthermore, the adsorption and reaction behavior of small probe molecules (including D2O CO and atomic D) has been investigated on the above inverse model catalysts. The main results are summarized as follows:(1) Adsorption/reaction behavior on Cu(111) and CuOx/Cu(111)The adsorption behavior of NO on clean Cu(111) and the effect of pre-covered oxygen species have been investigated. NO adsorbs dissociatively on Cu(111) surface even at low temperature and left NO(a) and O(a) on the surface. By controlling NO exposure and the substrate temperature, several different types of O(a) can be produced: the metastable species O528(O1s BE-528.9eV), chemisorbed species0529(O1s BE-529.5eV), chemisorbed species O531(O1s BE～531.0eV). In the presence of O531species, the relative occupation of different NO adsorption states was largely affected and most of the adsorbed NO(a) underwent dissociative desorption, releasing N2O and N2upon heating. In contrast, in the presence of O529species, the dissociative desorption of NO(a) was largely suppressed. Furthermore, the O529species show a more significant blocking effect on NO adsorption than the chemisorbed O531species.The CuOx/Cu(111) surface was prepared by the oxidation of Cu(111) under UHV condition and the adsorption behavior of atomic deuterium (D(g)) on the CuOx/Cu(111) was studied. Exposure of CuOx/Cu(111) to atomic D(g) even at low temperature (115K) can produce surface hydroxyl groups (D(g)+OL→OD(a))and leads to the formation of the adsorbed water(OD(a)+D(g)→D2O(a))at higher D(g) exposure. The adsorption of D(a) on the Cu+sites of CuOx was also observed. Compared to Cu(111), the diffusion of D(a) into the bulk phase was significantly suppressed for D(a) adsorption on CuOx/Cu(111). The stability of surface hydroxyl species was found to depend largely on the surface composition and oxidation/reduction state of CuOx/Cu(111).(2) The preparation and chemical properties of CeO2(111) model surfaceThe adsorption behavior of D2O and atomic D was investigated on the well-defined CeO2(111) thin film, which was prepared on Cu(111) substrate by physical vapor deposition. On the stoichiometric CeO2(111) surface, the adsorption of atomic D leads to the formation of surface hydroxyl and D2O as well as the reduction of Ce4+into Ce3+. For D(g) adsorption on the reduced CeO2-x(111), surface hydroxyls are the main adsorption products while the formation of D2O(a) is largely suppressed. Moreover, D2O adsorbs both molecularly and dissociatively on the stoichiometric CeO2(111) surface while dissociative adsorption is much preferred on the reduced CeO2-x(111). Furthermore, on reduced CeO2-x(111) surfaces, the stability of OD(a) was enhanced by the presence of oxygen vacancies. Upon heating, surface hydroxyls undergo two competing reaction pathways:one is the associative desorption of OD(a) releasing D2O and creating oxygen vacancies (OD(a)+OD(a)→D20(g)+Olattice+Ovacancy), and the other one is to produce D2via OD(a)+OD(a)→D2(g)+2Oiattice. The presence of oxygen vacancies in CeO2favors the reaction pathway of D2formation.The interaction between oxygen and CeO2(111) thin film was also investigated by X-ray Photoelectron Spectroscopy(XPS). O2hardly adsorbs on the stoichiometric CeO2(111) surface even at a low temperature of115K. In contrast, on reduced CeO2-x(111) film, two kinds of adsorbed oxygen species are formed:peroxide species(O22-) and superoxide species (O2-), which exhibit the O1s feature with the binding energy of531.7～532eV and533.6～534eV, respectively. By using atomic Dreduction and low-temperature Ar-sputtering, two types of reduced ceria films with different vacancies structure were prepared and the related oxygen adsorption behavior were compared. After O2adsorption on the D(g) reduced CeO2-x(111) film, the amount of superoxide(O2-) and peroxide(O22-) species are comparable while peroxide(O22-) species are predominant on the low-temperature Ar-sputtered ceria films. Furthermore, We also prepared a disorder polycrystalline CeO2thick layer(about28ML) on Au(110)-(1×2) substrate and some preliminary results of O2 adsorption behavior were obtained. Surprisingly, O2desorption was observed in TPD results with the desorption temperature of～230K upon O2exposure on CeO2/Au(110) at100K. In the case of D(g)-reduced CeO2-x film, additional O2desorption peaks were detected in the temperature range of250-400K. Although a clear assignment of these O2desorption peaks are not possible at moment, the TPD results may provide some evidences for the structure-dependent O2adsorption behavior on ceria film.(3)The preparation and surface chemistry of inverse CeO2/Cu(111) model surfaceThe inverse CeO2/Cu(111) model surface was firstly prepared with the ceria coverage of0.5ML. It was found that, by choosing the D(g) exposure and substrate temperature, the surface structure, composition and surface species of the inverse model surface can be controlled to some extent, on which the adsorption and reaction of CO, D2O and atomic D was studied. The partly-covered ceria layer mostly shows a "site-blocking" effect on the adsorption properties of Cu substrate. The adsorption behavior on the ceria layer is largely dependent on the thickness. Compared to the thick CeO2(111) film(10ML), the removal of the lattice oxygen (the reduction ability) is much more difficult on the thin CeO2(111) layer (0.5ML) upon D(g) exposure at higher temperature. Furthermore, the dissociative adsorption of D2O was largely suppressed on the reduced thin CeO2-x(111) layer(θ=0.5ML). The above thickness-dependent properties of ceria layer was ascribed to the structure of Cu-CeO2interface and the electronic modification of Cu substrate. In addition, by tuning the D(g) exposure and substrate temperature, a model system was successfully prepared, in which CO adsorbs on Cu substrate and OD on ceria layer, and the possibility of the interfacial CO+OD reaction was further examined. No CO2formation was found via the interfacial CO+OD reaction, which suggests that such reaction mechanism is not likely on the inverse CeO2/Cu(111) model catalyst under the studied experimental condition.In summary, the geometric/electronic structure and the adsorption/reaction properties of various CeO2/Cu(111) model surface were investigated in the thesis. The obtained results will deepen our understanding on the structure-activity relationship as well as the related reaction mechanism in the ceria-based catalytic systems.更多还原