Theory of surface chemical reactivity

Fundamental reactivity concepts of relevance to the reactivity of transition metal surfaces are reviewed using elementary quantum-chemical concepts. The Newns-Anderson tight binding model of chemisorption is presented and subsequently used to outline the electronic structure characteristics of weak versus strong chemisorption. Fundamental concepts such as electron localization and surface complex embedding energies are defined and used to help explain surface reactivity. The emphasis here is on establishing an understanding of the surface chemical bond as a function of adatom coordination number, degree of coordinative unsaturation of the surface atoms and electron occupation of the d-type valence electron band. We derive from formal chemisorption theory the important relationships that exist between measured chemisorption properties and the average position of the d-valence electron band. The Newns-Anderson model is also used to show the relationship that exists between the d-band center and the coordinative unsaturation of the metal surface atoms. The general conclusion is that for Group VIII metals the shift of the average energy of the surface local density of states correlates with the strength of the interaction of the surface atoms with the metal atoms next to the surface layer. The same model is then used to analyze the Shustorovich bond order conservation model. The BOC or its modern version UBI-QEP is found to be consistent with a surface interaction potential comprised of a two-body repulsive term along with a constant attractive interaction independent of the number of coordinating atoms. The concepts of weak and strong chemisorption provide a very good basis for the subsequent analysis of the Brønsted-Eyring-Polanyi (BEP) relation. The extreme values of the BEP proportionality constant are related to the concept of loose and tight transition states. This proportionality constant between transition state energy and reaction energy can be expressed in parameters from the Newns-Anderson model by identifying loose transition states with intermediates in which the bond to be activated has not yet been broken, whereas in tight transition states this bond can be considered to be broken. We conclude the paper with an analysis of surface reconstruction. The power of the surface-molecule complex view of chemisorption will be quite apparent. The paper has an extensive introductory section to relate the topics of the four sections that follow with important classical catalytic notions.     

Spectroscopic and chemical reactivity analysis of D-Myo-Inositol using quantum chemical approach and its experimental verification

Glow discharge conditioning (GDC) has long been accepted as one of the basic wall conditioning techniques for achieving ultrahigh vacuum in an unbaked chamber. As a part of this fundamental experimental study, a test chamber has been fabricated from stainless steel 304 L with its inner surface electropolished on which a detailed investigation has been carried out. Both helium and hydrogen gases have been employed as discharge cleaning medium. The discharge cleaning was carried out at 0.1 A /m ² current density with working pressure maintained at 1.0 × 10 ^-2 mbar. It was experimentally observed that the pump-down time to attain the base pressure ~10 ^-8 mbar was reduced by 62% compared to the unbaked chamber being pumped to this ultimate vacuum. The results were similar irrespective of whether the discharge cleaning medium is either hydrogen or helium. It was also experimentally established that a better ultimate vacuum could be achieved as compared to theoretically calculated ultimate vacuum with the help of discharge cleaning.     

Application of exact occupation statistics to surface coverage effect on heat of adsorption

Using an exact expression for the number of nearest-neighbor pairs, the exact relations for the change in heat of adsorption of single particles with surface coverage have been calculated within the framework of random distribution taking into account the first, second and third nearestneighbor interactions. It is shown that (1) when the total number of sites is less than about 30, the influence of the corners and edges of a lattice on the heat of adsorption cannot be neglected, and that (2) when the third nearest-neighbor interactions are taken into account for a rectangular or a hexagonal lattice, the heat of adsorption varies with coverage obeying a quadratic equation.     

Following local adsorption sites through a surface chemical reaction: CH3SH on Cu(111)

Studying the interaction of CH3SH, methanethiol, with Cu(111) as a model system, we demonstrate the ability of chemical-shift normal incidence x-ray standing wave field measurements to identify the local adsorption geometries of coadsorbed reaction products at different temperatures, a technical problem of broad chemical significance. In the present case the local geometries of four distinct S-containing adsorbate species (intact CH3SH, two thiolate (CH3S-) reaction intermediates and atomic S) are determined.     

Interplay between gas adsorption and dislocation structure on a metal surface

The influence of oxygen and sulfur on the dislocation patterns of the strained two monolayer Cu film on Ru was observed by scanning tunneling microscopy. Both oxygen and sulfur adsorption lead to the formation of vacancies that aggregate over existing dislocations in the film, thereby modifying the dislocation structure. With increasing adsorbate coverage the overall dislocation structure and pattern are transformed. The atomic mechanisms and general nature of this transformation can be explained in terms of generic dislocation reactions. This interpretation is also supported by atomistic simulations.     

The adsorption site of the surface formate species chemisorbed on Ni{110}

The selection rules pertinent to the dipole scattering of electrons involved in the vibrational excitation of surfaces are exploited to provide information on the nature of the adsorption site occupied by the surface formate species chemisorbed on a Ni{110} single crystal surface. In particular, attention is concentrated on the observation, in specular scattering geometry, of a surface phonon mode which has its origin at the S̄ point of the clean surface Brillouin zone, and clearly indicates that short bridge sites are occupied.     

Electromagnetic interaction between local surface plasmon polaritons and an atmospheric surface wave plasma jet

We analyze the electromagnetic interaction between local surface plasmon polaritons （SPPs） and an atmospheric surface wave plasma jet （ASWPJ） in combination with our designed discharge device. Before discharge, the excitation of the SPPs and the spatial distribution of the enhanced electric field are analyzed. During discharge, the critical breakdown electric field of the gases at atmospheric gas pressure and the surface wave of the SPPs converted into electron plasma waves at resonant points are studied. After discharge, the ionization development process of the ASWPJ is simulated using a two- dimensional fluid model. Our results suggest that the local enhanced electric field of SPPs is merely the precondition of gas breakdown, and the key mechanism in maintaining the discharge development of a low-power ASWPJ is the wave-mode conversion of the local enhanced electric field at the resonant point.     

The nature of the adsorption bond between graphite islands and iridium surface

To reveal the nature of adsorption bonds between two-dimensional graphite islands and iridium (111) and (100) faces, a study has been made of the adsorption of potassium and cesium atoms on the surface of these systems, using thermal desorption and Auger electron spectroscopy, as well as surface ionization and thermionic emission techniques. The graphite islands are shown to be weakly bound to the iridium substrate by Van der Waals forces. The unsaturated valence bonds at the periphery of the graphite islands are “lowered down” on to the metal. The recess between the graphite layer and the metal is filled by adsorbing particles through defects in the graphite layer. The atoms can penetrate into the recess in two ways: at T > 1000 K directly from the flux incident on the surface, and at T < 1000 K also by migration from the graphite island surface. The adsorption capacity of this state is ～ (2?3) × 10¹⁴cm^-2. Thermal destruction of the islands at T > 1900 K liberates the potassium and cesium atoms from under the graphite islands. Our study suggests that the reason for the “raised” position of the islands lies in the valence bonds of the graphite layer being saturated, the valence bonds of the metal and its crystallographic orientation being less significant. Therefore one may expect the graphite layer to be raised also above other metals as well. The filling by cesium of the recess between the graphite layer and iridium and of the adsorption phase on the graphite surface, does not change the general “graphitic” shape of the carbon Auger peak. This cesium results, however, in a pronounced splitting of the negative spike on the carbon peak (which provides information on its location relative to the graphite layer) indicating the appearance in the valence band of graphite near the Fermi level of two narrow (～ 2?3 eV) regions with an enhanced density of states originating from the presence of the alkali metal.     

Physical adsorption and surface plasmons

The quantum mechanical formulation of the screening of a point charge by surface plasmons is extended to describe the coupling of a fluctuating atomic dipole with a metallic surface of planar, spherical or cylindrical shape. This allows for the calculation of the nonretarded Van der Waals attraction of an atom by a solid surface in the three different geometries. Applications of the theoretical formulae are made to obtain numerical values of the dispersion energy by a spherical particle, a spherical pore, a thin film, a slot-like prore, a cylindrical fiber or a cylindrical pore.     

The local adsorption site for sulphur on Ni {111} in the low coverage lattice gas

Angle-resolved sulphur L_2.3VV Auger electron spectra have been taken from sulphur adsorbed on Ni {111} at a range of coverages both below and above those corresponding to the ordered (2 × 2) structure. These data indicate that the local adsorption site in the low coverage lattice gas is the same single three-fold hollow site adopted in the ordered overlayer. This contrasts with the low coverage occupation of both three-fold hollow sites for the system I/Ag<{111}.     

Electronic modifications and surface reactivity of an ion bombarded graphite surface

The first steps of structural and electronic modifications of a graphite surface bombarded with argon, hydrogen and deuterium ions were investigated using high resolution electron energy loss spectroscopy (HREELS). The energy and the damping of the low energy plasmon mode of graphite (E//C mode) were studied with respect to the bombardment settings. We show that argon bombardment affects the energy of the plasmon mode, while no similar change is observed after hydrogen (deuterium) bombardments. This can be related to the variation of inter-planar distance between two graphene layers. Moreover, the damping of the plasmon mode can be correlated with the interstitial defect concentration. Concerning the reactivity of the bombarded surfaces, we demonstrate that deuterium bombardment produce a non-deuterated surface. This last is very reactive to a further atomic deuterium exposure, as it is shown by the formation of C-D bondings. The deuterated sites can be removed after thermal annealings between 473 and 783 K. The occurrence of a chemical erosion mechanism accompanying this deuteration is discussed.     

Hydrogen adsorption and surface structures of silicon

The adsorption of atomic hydrogen on silicon (111)2 × 1 cleaved, (111) 7 × 7, and (100) 2 × 1 surfaces has been studied by using electron energy loss spectrscopy (ELS) and Photoemission spectroscopy (UPS). On all surfaces the hydrogen removes the “dangling bond” surface state and a new peak in the density of states at lower energies corresponding to the SiH bond is found. The LEED pattern of the equilibrium surfaces (111) 7 × 7 and (100) 2 × 1 is not altered by hydrogen adsorption, while on the cleaved (111) 2 × 1 surface the fractional order spots are extinguished. The Haneman surface-buckling model therefore provides an explanation for the surface reconstruction of the cleaved (111) 2 × 1 surfaces. For the equilibrium surfaces, (111) 7 × 7 and the (100) 2 × 1, the data are consistent with the Lander-Phillips model.     

Physical and chemical nature of the scaling relations between adsorption energies of atoms on metal surfaces

Despite their importance in physics and chemistry, the origin and extent of the scaling relations between the energetics of adsorbed species on surfaces remain elusive. We demonstrate here that scalability is not exclusive to adsorbed atoms and their hydrogenated species but rather a general phenomenon between any set of adsorbates bound similarly to the surface. On the example of the near-surface alloys of Pt, we show that scalability is a result of identical variations of adsorption energies with respect to the valence configuration of both the surface components and the adsorbates.     

氢原子吸附对金表面金属酞菁分子的吸附位置、自旋和手征性的调控

实现单个功能有机分子构型、电子结构和自旋态的可逆调控, 是未来分子电子学和分子自旋电子学应用的关键. 近年来, 我们利用极低温强磁场超高真空扫描隧道显微镜系统, 结合第一性原理计算, 系统研究了氢原子吸附对金表面吸附的金属酞菁分子的自旋、手性和吸附位置的调控. 通过将金表面吸附的酞菁锰分子暴露于氢气或氢原子环境, 使得分子中心的磁性离子吸附单个氢原子, 从而实现了体系近藤效应由“开”到“关”的转变. 基于密度泛函理论的第一性原理计算表明, 氢原子吸附使得锰离子3d轨道内的电荷重排导致了分子的自旋由3/2降为1; 同时分子与金基底的间距增大, 使得近藤效应消失. 通过施加局域电压脉冲或者给样品加热, 可以实现单个或所有分子脱氢, 从而恢复体系的自旋态和近藤效应. 氢原子吸附还导致分子的优先吸附位置从金表面的面心立方堆垛区域变成了六角密排堆垛区域. 三个氢原子吸附于同一酞菁锰分子上, 可导致分子对称性的降低及分子镜面对称轴与金基底镜面对称轴的偏离, 从而导致手征性的出现. 这种分子吸附结构的手征性, 导致分子轨道也呈现出手征性. 这项工作为金属酞菁未来在分子电子学、自旋电子学、气体传感器等方面的应用提供了新思路.     

Measurement of the adsorption energy difference between ortho- and para-D2 on an amorphous ice surface

Molecular hydrogen interaction on water ice surfaces is a major process taking place in interstellar dense clouds. By coupling laser detection and classical thermal desorption spectroscopy, it is possible to study the effect of rotation of D(2) on adsorption on amorphous solid water ice surfaces. The desorption profiles of ortho- and para-D(2) are different. This difference is due to a shift in the adsorption energy distribution of the two lowest rotational states. Molecules in J'=1 rotational state are on average more strongly bound to the ice surface than those in J'=0 rotational state. This energy difference is estimated to be 1.4+/-0.3 meV. This value is in agreement with previous calculation and interpretation. The nonspherical wave function J' =1 has an interaction with the asymmetric part of the adsorption potential and contributes positively in the binding energy.     

The impact of carbon surface chemical composition on the adsorption of phenol determined at the real oxic and anoxic conditions

The results of phenol adsorption-desorption isotherms (at 310 K) measured on the series of activated carbons (D43/1, NORIT RO 0.8, D55/2) are presented. The effect of carbon surface chemical composition on phenol adsorption determined at real oxic and anoxic conditions is discussed. To obtain the real anoxic conditions the two station controlled atmosphere chamber with two catalyst heater units (Plas Labs, Lansing, MI, USA) was applied. It is shown that the adsorption under oxic conditions is always larger than that determined for anoxic ones for all studied carbons. The analysis of those differences shows that in the range of micropore filling they decrease with the equilibrium phenol mole fraction in solution. Contrary they increase after micropores being filled. The average differences between the adsorption properties are the linear function of the concentration of surface acidic groups (assigned from the Boehm's method as “carboxylic”) calculated per the apparent BET surface area of studied carbons.     

Local reactivity of ultrathin platinum overlayers and surface alloys on a gold surface

The geometric and electronic structure, the stability, and CO adsorption properties of pseudomorphic Pt overlayers and PtAu surface alloys on a Au(1 1 1) substrate have been addressed on the basis of first-principles calculations. We have found that two-monolayer thick surface alloys are more stable than one-monolayer thick alloys. The CO binding energies at the top sites of two-monolayer thick surface alloys increase gradually with the Pt concentration, while the energies are almost independent of the concentration for one-monolayer thick surface alloys. This difference is caused by the lower symmetry of the two-monolayer thick surface alloys, which makes the effect of neighboring atoms in the first layer more important.     

Comparison of reactivity on step and terrace sites of Pd (3 3 2) surface for the dissociative adsorption of hydrogen: A quantum chemical molecular dynamics study

The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface.