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采用从头算程序对MgO表面3种不同配位位置吸附Cl2的构型进行优化,并用扩展休克尔紧束缚(EHT)晶体轨道方法对MgO的缺陷和不规则表面只附C12的可能带计算,讨论了吸附前后能带组成和成键性质的变化。研究表明:MgO表面吸附Cl2将更超向于吸附在O原子上而非Mg原子上,而且在3种配位中MgO表面三配位氧最有利于吸附Cl2;吸附时,电子从O原子转移到Cl2分子的反键轨道,但是各种吸附的MgO表面对C 相似文献
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基于密度泛函理论的第一性原理从头计算方法,建立了Cl2和O2在TiC(100)表面的共吸附模型.通过分析吸附能、电荷密度和偏态密度(PDOS)等参数,研究了Cl2和O2在TiC(100)表面的反应机理,发现解离后的Cl原子和O原子与TiC(100)表面的原子均成键,从而破坏了Ti—C键.Cl2分子在吸附过程中充当电子的受体,得到与之成键的Ti原子贡献的电子,O2分子在吸附过程中也充当电子的受体,得到C原子贡献的电子.TiC(100)表面在吸附分子后,Ti—C成键轨道上电子占据数变少,反键轨道上电子占据数增多,Ti原子与C原子之间的成键作用减弱.同时,Ti3d与Cl3s,Cl3p发生轨道重叠杂化作用,O2p轨道和C2p轨道存在较强的共振峰,Cl原子和O原子与TiC表面相互作用强烈. 相似文献
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采用基于第一性原理的分子动力学和量子力学相结合的方法,对W3O9团簇在经Li和Al原子掺杂的MgO(001)表面的负载构型、稳定性以及体系的电子结构进行了系统研究.结果表明,当掺杂发生在表层时,杂质原子的类型对W3O9团簇的负载构型有显著影响.对于缺电子的Li掺杂,负载后W3O9团簇环状构型并不稳定,转化为链状结构;而Al原子的掺杂则使得MgO(001)表面电子富余,此时W3O9团簇存在平躺和垂直两种吸附方式,二者能量稳定性相近,其中前者存在同时与三个W原子成键的帽氧结构.当掺杂发生在次表层时,两种掺杂体系W3O9的负载构型相似,团簇仍保持环状结构并倾向于采用垂直方式沉积在表面上.与Li掺杂体系相比,富电子的Al掺杂可显著增强W3O9与MgO(001)表面之间的结合能力,负载后有较多电子从表面转移到团簇中特定的W原子上,这将对W3O9团簇的催化性能产生显著影响. 相似文献
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基于密度泛函理论的第一性原理方法,通过计算表面能确定La Fe O_3(010)表面为最稳定的吸附表面,研究了H_2分子在La Fe O_3(010)表面的吸附性质。La Fe O_3(010)表面存在La O和Fe O_2两种终止表面,但吸附主要发生在Fe O_2终止表面,由于La Fe O_3(010)表面弛豫的影响,使得凹凸不平的表面层增加了表面原子与H原子的接触面积,表面晶胞的纵向体积增加约2.5%,有利于H原子向晶体内扩散。研究发现,H_2分子在La Fe O_3(010)表面主要存在3种化学吸附方式:第一种吸附发生在O-O桥位,2个H原子分别吸附在2个O原子上,形成2个-OH基,这是最佳吸附位置,此时H原子与表面O原子的作用主要是H1s与O_2p轨道杂化作用的结果,H-O之间为典型的共价键。H_2分子的解离能垒为1.542 e V,说明表面需要一定的热条件,H_2分子才会发生解离吸附;第二种吸附发生在Fe-O桥位,1个H原子吸附在O原子上形成1个-OH基,另一个H原子吸附在Fe原子上形成金属键;第三种吸附发生在O顶位,2个H原子吸附在同一个O原子上,形成H_2O分子,此时H_2O分子与表面形成物理吸附,H_2O分子逃离表面后容易形成氧空位。此外,H_2分子在La Fe O_3(010)表面还可以发生物理吸附。 相似文献
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采用密度泛函理论(DFT)中广义梯度近似(GGA)方法,对Pt原子与y-Al2O3(001)面的相互作用及迁移性能进行了研究.分析了各种可能吸附位及吸附构型的松弛和变形现象,吸附能和迁移能垒的计算结果表明:Pt团簇能够稳定吸附在该表面.Pt原子在表面O位的吸附能明显较高,这主要是由Pt向基底O原子转移了电子所致.电荷布居分析表明,Pt原子显电正性,Pt和Al原子之间存在排斥作用,导致与Al原子产生较弱相互作用.计算的平均吸附能大小依赖于Pt团簇的大小和形状,总体趋势是随着Pt原子数增多,吸附能降低.Pt原子在y-Al2O3(001)表面迁移过程所需克服的迁移能垒最高值为0.51 eV.随着吸附的Pt原子数增多,更倾向于形成Pt团簇.因此,Pt原子在y-Al2O,(001)表面的吸附演变不可能形成光滑、均匀平铺的吸附构型,而在一定条件下容易出现团聚. 相似文献
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采用密度泛函理论中的广义梯度近似,计算了CO在α-U(001)表面的吸附、解离和扩散.结果表明:CO分子以CU3OU2构型化学吸附在α-U(001)表面,吸附能为1.78-1.99eV;吸附后表层U原子向上迁移,伴随着褶皱的产生;CO分子与表面U原子的相互作用主要是U原子的电子向CO分子最低空轨道2π*转移,以及CO2π*/5σ/1π-U6d轨道间杂化而生成新的化学键;CO解离吸附较分子吸附在能量上更为有利,h1(C)+h2(O)和h1(C)+h1(O)(h:空位)解离态吸附能分别为2.71和3.08eV;近邻三重穴位之间C、O原子的扩散能垒分别为0.57和0.14eV,预示O原子较C原子更易在U(001)表面扩散迁移. 相似文献
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基于密度泛函理论的第一性原理方法,通过计算表面能确定LaFeO3(010)表面为最稳定的吸附表面,研究了H2分子在LaFeO3(010)表面的吸附性质。LaFeO3(010)表面存在LaO和FeO2两种终止表面,但吸附主要发生在FeO2终止表面,由于LaFeO3(010)表面弛豫的影响,使得凹凸不平的表面层增加了表面原子与H原子的接触面积,表面晶胞的纵向体积增加约2.5%,有利于H原子向晶体内扩散。研究发现,H2分子在LaFeO3(010)表面主要存在3种化学吸附方式:第一种吸附发生在O-O桥位,2个H原子分别吸附在2个O原子上,形成2个-OH基,这是最佳吸附位置,此时H原子与表面O原子的作用主要是H1s与O2p轨道杂化作用的结果,H-O之间为典型的共价键。H2分子的解离能垒为1.542 eV,说明表面需要一定的热条件,H2分子才会发生解离吸附;第二种吸附发生在Fe-O桥位,1个H原子吸附在O原子上形成1个-OH基,另一个H原子吸附在Fe原子上形成金属键;第三种吸附发生在O顶位,2个H原子吸附在同一个O原子上,形成H2O分子,此时H2O分子与表面形成物理吸附,H2O分子逃离表面后容易形成氧空位。此外,H2分子在LaFeO3(010)表面还可以发生物理吸附。 相似文献
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在密度泛函理论的框架下,采用嵌入点电荷簇模型研究了O2在具有氧缺陷和镁缺陷MgO(001)表面上的吸附.用电荷自洽的方法确定了点电荷的值.计算结果表明,O2倾向吸附在具有氧缺陷的MgO(001)表面上.通过和我们近期研究过的O2在低配位的边、角上吸附结果相比较,发现具有氧缺陷的MgO(001)表面更加有利于O2的吸附和解离. Mülliken电荷分析表明,电荷由底物向吸附的O2反键轨道上转移是导致O2键强削弱的主要原因.势能曲线表明,O2在具有氧缺陷的MgO(001)表面上发生解离所需要克服的能垒比在角阳离子端发生解离所需克服的能垒有大幅度降低. 相似文献
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在密度泛函理论的框架下, 采用嵌入点电荷簇模型研究了O2在MgO(001)完整和缺陷表面上的吸附.用电荷自洽的方法确定了点电荷的值.计算结果表明, O2倾向吸附在低配位的角Mg2+端.并且发现, 当O2为平躺吸附时,键长有较大的拉伸,将有利于O2的解离.同时,分别计算了使用裸簇和嵌入表观±2.0 e点电荷簇模型时的吸附能,并与采用电荷自洽方法的计算值进行了比较.结果表明,电荷自洽方法更能有效反映簇周围的环境,得到的计算结果能够较好地与实验值吻合.最后,分别计算了不同吸附情况下O2的振动频率. 相似文献
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1 INTRODUCTION The interfaces between metals and oxide play a vital role in many industrial applications: hetero- geneous catalysis, microelectronics, thermal barriers, corrosion protection, metal processing and so on[1]. In catalysis, the choice of metal and oxide support is critical in order to obtain a desired reactivity and selectivity[2]. This is due in part to the inherent reac- tivity of the two components. Also the size and shape of the metal particle, which depend on the choice… 相似文献
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The adsorption and decomposition of NzO at regular and defect sites of MgO (001) surface have been studied using cluster models embedded in a large array of point charges (PCs) by DFT/B3LYP method. The results indicate that the MgO (001)surface with oxygen vacancies exhibits high catalytic reactivity toward N2O adsorptive-decomposition. It is different from the regular MgO surface or the surface with magnesium vacancies.Much elongation of O—N bond of N2O after adsorption at oxy-gen vacancy site with O end down shows that O—N bond has been broken with concurrent production of N2, leaving a regu-lar site instead of the original oxygen vacancy site (F center ).The MgO (001) surface with magnesium vacancies hardly ex-hibits catalytic reactivity. It can be concluded that N2O dissoci-ation likely occurs at oxygen vacancy sites of MgO (001) sur-face, which is consistent with the generally accepted viewpoint in the experiments. The potential energy surface (PES) reflects that the dissociation process of N2O does not virtually need to surmount a given energy barrier. 相似文献
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Combined molecular dynamics (MD) and quantum mechanics (QM) calculations have been performed for CO adsorbed on MgO(001) at 50 K. The changes in the adsorption energy caused by the surface dynamics have been analyzed, and a clear correlation was found between the dynamic variation of the adsorption energy and the electrostatic field above the adsorption site. By separating the electrostatic contributions arising from the local structure at the adsorption site from those originating from the rest of the slab, a linear expression of these contributions could be fitted which closely reproduces the dynamic changes in the adsorption energy. Using this simple linear expression, the distribution of adsorption energies for CO above the Mg(2+) sites on the MgO(001) surface at 50, 80, and 150 K have been predicted. 相似文献
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Neyman KM Inntam C Moskaleva LV Rösch N 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(1):277-286
Cu(4), Ag(4), and Au(4) species adsorbed on an MgO(001) surface that exhibits neutral (F(s)) and charged (F(s) (+)) oxygen vacancies have been studied using a density functional approach and advanced embedding models. The gas-phase rhombic-planar structure of the coinage metal tetramers is only moderately affected by adsorption. In the most stable surface configuration, the plane of the tetramers is oriented perpendicular to the MgO(001) surface; one metal atom is attached to an oxygen vacancy and another one is bound to a nearby surface oxygen anion. A very similar structural motif was recently found on defect-free MgO(001), where two O(2-) ions serve as adsorption sites. Following the trend of the interactions with the regular MgO(001) surface, Au(4) and Cu(4) bind substantially stronger to F(s) and F(s) (+) sites than Ag(4). This stronger adsorption interaction at oxygen vacancies, in particular at F(s), is partly due to a notable accumulation of electron density on the adsorbates. We also examined the propensity of small supported metal species to aggregate to adsorbed di-, tri- and tetramers. Furthermore, we demonstrated that core-level ionization potentials offer the possibility for detecting experimentally supported metal tetramers and characterizing them structurally with the help of calculated data. 相似文献
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Porphyrin Metalation at MgO Surfaces: A Spectroscopic and Quantum Mechanical Study on Complementary Model Systems
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Johannes Schneider Matthias Franke Martin Gurrath Dr. Michael Röckert Dr. Thomas Berger Prof. Dr. Johannes Bernardi Prof. Dr. Bernd Meyer Prof. Dr. Hans‐Peter Steinrück Dr. Ole Lytken Prof. Dr. Oliver Diwald 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(5):1744-1749
We show that both single‐crystalline and nanostructured MgO surfaces convert free‐base tetraphenyl porphyrin (2HTPP) into magnesium tetraphenyl porphyrin (MgTPP) at room temperature. The reaction can be viewed as an ion exchange between the two aminic protons of the 2HTPP molecule with a Mg2+ ion from the surface. The driving force for the reaction is the strong stability of the formed hydroxyl groups along the steps and at defects on the MgO surface. We have used an integrated characterization approach that includes UV/Vis diffuse reflectance measurements on nanostructured powders, X‐ray photoelectron spectroscopic investigation of atomically clean MgO(100) single‐crystalline thin films, and density functional theory (DFT) calculations on model systems. The DFT calculations demonstrate that MgTPP formation is strongly exothermic at the corners, edges and steps, but slightly endothermic on terrace sites. This agrees well with the UV/Vis diffuse reflectance, which upon adsorption of 2HTPP shows a decrease in the absorption band associated with corner and edge sites on MgO nanocube powders. 相似文献
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A thorough investigation of the adsorption and diffusion of Mg, O, and O(2) on MgO(001) terraces is performed by first-principles calculations. The single Mg adatom weakly binds to surface oxygens, diffuses, and evaporates easily at room temperatures. Atomic O strongly binds to surface oxygens, forming peroxide groups. The diffusion of the O adatom is strongly influenced by the spin polarization, since energy barriers are significantly different for the singlet and triplet states. The crossing of the two Born-Oppenheimer surfaces corresponding to the distinct spin states is also analyzed. Although the O(2) molecule does not stick to the perfect surface, it chemisorbs on surface nonstoichiometric point defects such as O vacancies or Mg adatoms, forming in the latter case new chemical species on the surface. We show that the oxidation rate limiting factor in an O(2) atmosphere is the concentration of point defects (O vacancies and Mg adatoms) in the growing surface. The simulated O core-level shifts for the various adsorption configurations enable a meaningful comparison with the measured values, suggesting the presence of peroxide ions on growing surfaces. Finally, the computed energy barriers are used to estimate the Mg and O surface lifetimes and diffusion lengths, and some implications for the homoepitaxial growth of MgO are discussed. 相似文献