Tight-binding Ising-model study of chemisorption effects on surface segregation in transition-metal alloys

The tight-binding Ising model is generalized for the system consisting of a transition-metal alloy with chemisorbed adatoms. The effect of chemisorption on the surface segregation is calculated within the scope of a semi-infinite linear chain model for hydrogen chemisorbed on a Cu-Ni alloy. The charge-neutrality condition is incorporated into the model and its effect on the calculated surface segregation is demonstrated.     

CO和O在无序二元合金NiCu表面上的化学吸附

本文用一维紧束缚模型和单电子化学吸附理论,在平均T矩阵近似下,研究了CO和O在无序二元合金Ni_xCu_1-x表面上的化学吸附特性,结果表明,CO和O在Ni_xCu_1-x表面上的化学吸附具有类似的性质,随合金中Ni浓度的增加,化学吸附能降低,化学吸附加强,Cu在NiCu合金表面的偏析在一定程度上减弱了CO和O在NiCu表面上的吸附。 关键词：     

无序二元合金（NixCu1-x）表面CO吸附及对表面偏析的影响

根据计算机编程构造出了存在和不存在表面偏析的无序二元合金Ni_xCu_1-x（x=0.4）的原子集团模型，然后按覆盖度θ=0.5，构造出了CO表面吸附的模型 ，应用Recursion方法计算了CO在（Ni_xCu_1-x）（存在偏析和不存在偏析时）合金表面不同位置（顶位和芯位）吸附的电子结构 .由此得出：1)CO在顶位吸附时较稳定；2)CO吸附使合金表面态密度峰降低，带宽加宽，使d轨道的局域性变弱；3)CO的吸附抑制了Cu 关键词： 化学吸附 表面偏析 Recursion方法 态密度     

甘氨酸吸附对Ni-Cu合金(110)表面偏析的影响

通过构造无序二元合金NixCu1-x(x=0.4)(110)表面的原子集团模型用来研究表面偏析. 根据这个模型，按覆盖度μ=0.33，应用Recursion方法计算了甘氨酸在NixCu1-x无序二元合金(110)表面吸附的电子结构. 结果表明，Ni0:4Cu0:6合金表面存在着Cu的偏析，甘氨酸的吸附抑制了Cu在表面的偏析，并使得合金表面的态密度在费米能级附近发生了很大变化，吸附时合金表面与甘氨酸之间发生了电荷转移.     

Influence of the electronic structure on chemisorbed CO

The chemisorption of carbon monoxide on amorphous and crystalline alloys such as Pd-Si, Ni---Zr, Ir---Si, and Pd---U---Si has been studied by ultraviolet photoelectron spectroscopy (UPS). Systematic trends in the CO chemisorption behavior have been found for the various alloy systems. A correlation between the position of the valence band d-electron states of the LTM (late transition metals) and the molecular orbitals of the chemisorbed CO molecules is reported. The influence of the substrate electron states on the C-O bond strength is explained in a qualitative model involving the symmetry and the binding energy of the substrate valence electrons and those of the molecular orbitals.     

The calculation of the chemisorption energy using the new diagram technique for the Anderson model

Using the perturbation expansion in the rebonding interaction near the surface molecule limit, the new diagram technique for the calculation of the chemisorption energy in the Anderson model is proposed. The new expression for the chemisorption energy in the ring diagram approximation is presented. The approximate expression for the contribution of the non- ring diagrams is suggested. The chemisorption energies are calculated and compared with the available exact results and others in the literature. A simple explanation of observable trends in hydrogen chemisorption energies along the transition metal series is given based on the rebonding surface molecule picture. Received 8 July 1999 and Received in final form 24 January 2000     

无序二元合金（NixCu1－x）不同解理面上O吸附对Cu偏析的影响

应用计算机编程构造出了存在和不存在表面偏析的无序二元合金Ni_xCu_1-x (x=0.4)(１００)表面及(１１０)表面的原子集团模型，然后按覆盖度θ=0.5，构造 出了O吸附后的原子集团模型，应用Recursion方法计算了O在Ni_xCu_{1-x(存在偏析和不存在偏析时)无序二元合金(１００)和(１１０)表面吸附的电子结构.由此 得出：1)O吸附使合金表面态密度峰降低，带宽加宽，并且表面Ni原子的d电子与吸附质O原 子的s，p电子的共价作用比Cu更强烈；2)O吸附在NixCu1-x(x=0.4) (１１０)表面比(１００)表面更稳定；3)O的吸附抑制了Cu在表面富集，且这种作用主要表 现在表面一层. 关键词： 化学吸附 表面偏析 Recursion方法 态密度}     

Theoretical study of the chemisorption of CO on bimetallic RhCu surfaces and nanoparticles

The CO interaction with bimetallic RhCu surface models representing several compositions has been studied by first principles density functional theory calculations. The analysis of the bare bimetallic clusters Rh(4s) and Cu(3s) core-level binding energies indicates that is not possible to extract information about the oxidation state of the alloy components. The present calculations predict that CO does always sit on top sites, the influence of the alloy composition on the equilibrium geometry and vibrational frequency of CO chemisorbed at a given Rh or Cu site being very small. However, there is a large difference in the structural properties corresponding to CO chemisorption above either Rh or Cu. Therefore, the absolute value of the vibrational frequency of chemisorbed CO does not permit to extract any information about the alloy composition but afford to assign the chemisorption site. Finally, the CO adsorption energy does not follow a monotonic trend with composition. The use of the Constrained Space Orbital Variation analysis permits one to firmly establish that the difference in adsorption energy for different compositions cannot be explained through differences in the σ-donation and π-backdonation mechanisms.     

On the selective catalytic behaviour of the Cu-NI alloy surface

The selective catalytic behaviour of the Cu_xNi_1?x alloy surface is studied. The concentration dependence of the catalytic activity with respect to chemical reactions involving cracking and non-cracking of C-C bonds is calculated for a simple model for C-C bonds. Surface segregation and local environment effects on chemisorption both calculated from a microscopic electronic theory using tight-binding approximation are taken into account.     

Hydrogen Chemisorption on Disordered Binary Alloys

The one-dimensional tight-binding model and the Green's function method are used to study the chemisorption energy of an atomic H on disordered binary alloys within the average T-matrix approximation. The one-electron chemisorption theory is used to describe the chemisorption process. The chemisorption energy is calculated as a function of the concentration percentages of the two components of the disordered binary alloys. The effect of the surface segregation of the CuNi system is also investigated.     

Field adsorption of helium and neon on metals: an integrated theory

Density functional theory has been used to carry out self-consistent calculations relating to the field adsorption of helium and neon on metal surfaces. The transition from physisorption in low electric fields to field-induced chemisorption in high electric fields is explicitly demonstrated with electron density maps. The higher binding energies of rare gases above protruding metal surface atoms are explained as resulting from local field enhancement, and an approximate formula for the helium-on-tungsten system is derived that gives the total short-range field-adsorption binding energy as a function of the external field and a local field-enhancement factor. A survey of earlier theoretical approaches to rare-gas adsorption in electric fields is included. The present treatment can be regarded as an integrated theory in which both covalent and polarisation effects have their place; it brings out the role of polarisation contributions to the total binding energy, and allows us to clarify the relationship between the earlier classical models and the present quantum theory of field-induced chemisorption.     

On the theoretical description of the chemisorption of the hydrogen atom on the metallic surface. Calculation of energy interaction of H with hcp Co and Ni surface clusters

A model, based on the generalized valence bond theory, capable of describing the interaction of hydrogen atom (H) with large molecular systems is proposed. The accuracy of the model has been checked by calculating the and H₂ systems. Atomic spin densities for clusters having 1243 atoms have been obtained. The surface magnetic and chemisorption property differences between the Ni and Co metals have been analyzed. An extrapolation scheme suitable to obtain H chemisorption energies on infinite systems from the finite ones has been proposed. Size-converged results have been obtained for the Co. Potential energy surfaces of hydrogen, on and below, the Co surface have been provided.     

The surface composition of the nickel-copper alloy system as determined by Auger electron spectroscopy

The Ni-Cu alloys were prepared by evaporation of the specpure metals in UHV onto a quartz substrate. Spectra were obtained from clean as well as from gas covered surfaces. The Auger signal intensity of a monolayer of both metals was determined for the low energy electrons (102–105 eV) and for the high energy electrons (716–920 eV). The overlapping peaks of Cu and Ni in the low energy region (102–105 eV) were evaluated by comparing them with computer simulated alloy spectra. The results of the sintered alloys are interpreted by means of a model by Gallon and Jackson, using the experimentally determined signal intensity of a monolayer. Several surface enrichment data were used to predict the experimentally observed Auger signal intensities. A clear indication of surface enrichment of Cu was obtained; this is in good agreement with previous conclusions based upon hydrogen adsorption and work function measurements. An explanation is suggested why previous work with AES and CO chemisorption did not reveal any surface enrichment.     

Influence de la chimisorption de l'oxygéne et l'azote sur l'émission ionique secondaire d'échantillons monocristallins de nickel et d'alliage nickel-chrome

The influence of an active gas adsorption on the secondary ion emission yields of a mono-crystalline target bombarded with a primary argon beam has been investigated. On the one hand, the chemisorption changes the electronic structure on the surface metal atoms and increases the positive ion emission yields, on the other hand, channeling conditions of the primary ions through the target lattice are modified according to the relation of the chemisorbed layer with the metal lattice. Two samples were examinated: pure nickel and a nickel chromium alloy in either an oxygen or a nitrogen partial pressure. Important modifications of the coverage are due to the sputtering yield variations. The experiments described here suggest that secondary ion emission could be used for chemisorption studies.     

Chemisorption on supported-metal catalysts

A Gree-function formalism is developed to describe the electronic and chemisorption properties of a supported-metal composite substrate. Within the framework of the tight-binding approximation, the metal catalyst is represented by a finite chain of d-orbitals, while the semi-infinite semiconductor support is characterized by a linear chain of alternating s- and p-orbitals. The Anderson-Newns model is used to calculate the chemisorption energy and adatom charge transfer for hydrogen chemisorption on the Ni/ZnO composite system.     

杂质对担载式催化剂化学吸附的影响

本文采用化学吸附的格林函数理论和复能量积分方法研究H在担载式催化剂ZnO/Ni上的化学吸附,使用由s轨道和p轨道交迭排列的有限原子链和由d轨道组成的半无限原子链,以描述半导体ZnO和金属Ni构成的复合衬底,用Koster-Slater模型来表示杂质原子,分别研究了化学吸附能随ZnO层厚度的变化和化学吸附能与掺杂的关系,结果表明:1)H的化学吸附能随ZnO层数的增加而单调地减小;2)在Ni中掺入杂质Cu和Pt会削弱H的化学吸附,而掺入Co和W将加强H的吸附;3)Ni中的杂质位于界面附近时对化学吸附能的影响最 关键词：     

Co-B amorphous alloy nanochains with enhanced magnetization and electrochemical activity prepared in a biphasic system

Co-B amorphous alloy in the form of nanochains was prepared by chemical reduction of cobalt ions with borohydride in a dodecanethiol/water biphasic system. Such Co-B nanochains were comprised of uniform Co-B nanoparticles connected in one-dimension through electrostatic force between the induced dipoles. Dodecanethiol molecules played a key role in the formation of Co-B nanochains by covering Co-B nanoparticles via strong chemisorption bond which stabilized Co-B nanoparticles against gathering and also generated induced electric dipoles. The linear array of Co-B amorphous alloy nanoparticles resulted in the superposition of magnetic moments and also facilitated the electron transfer, leading to the enhanced magnetization and electrochemical activity.     

Formation of a carbonaceous adsorbate layer on the surface of Pt-Cu alloys

Selective chemisorption of ethene and the chemisorption induced surface segregation of Pt upon the chemisorption of ethene (and its subsequent decomposition) on Pt-Cu alloys have been studied by Auger electron spectroscopy. The experimental results have also been evaluated by computer simulations of the various possible processes upon the chemisorption. In these computer simulations a modified Gallon model has been used in order to describe the variations in the intensities of the Pt and the Cu surface sensitive Auger transitions. Our present results indicate a preferential chemisorption of the carbonaceous species onto Pt sites, and a chemisorption induced surface segregation of Pt. Alloying of Pt with Cu results in a decreased coking of the catalyst.     

Ab initio hartree-fock investigation of the interaction between hydrogen monolayers and beryllium slabs

A Hartree-Fock SCF ab initio investigation is presented in which the hydrogen chemisorption at high coverage on the beryllium (0001) surface is simulated as a symmetrical interaction of two H monolayers with both sides of the Be film or as a nonsymmetrical model with one H monolayer interacting with one surface of a Be film. Relatively strong chemisorption bonds are found at two (“open” and “eclipsed” positions) of the three high-symmetry chemisorption sites. The results of this investigation are compared with the results obtained with the approaches based on cluster models     

Calculation of chemisorption and absorption induced surface segregation

Using a pair bond type model for the interatomic interactions, we determine surface segregation on clean, H, O and CO covered surfaces of various alloys. Furthermore, we study surface segregation caused by bulk hydrogen absorption. Numerical results are presented for Pd_x Zr_1?x H_y We find, that strong surface segregation may result from chemisorption of O and CO and from absorption of H in the bulk of an alloy.