Oxygen and silver clusters: transition from chemisorption to oxidation

We use the adsorption probabilities of molecular nitrogen and oxygen to study the physi- and chemisorption on small silver particles. The physisorption of nitrogen is governed by the structure of the particle surface. The sticking of oxygen additionally involves the electronic configuration of the metal cluster. At 77 K molecular oxygen sticks chemisorbed to the particles with a transfer of one electron. At temperatures above 105 K the chemisorption transforms into oxidation, invoking the dissociation of the oxygen molecule and the loss of a single oxygen atom.     

Reactivity of small supported particles

Transition metals are frequently used in heterogeneous catalytic reactions. In many cases catalysts have specific properties related to crystallographic structure and morphology. One point of actual interest concerns the reactivity of specific sites as steps and kinks. These low co-ordination sites are characteristic for surfaces of small supported particles ranging from 1 to 10 nm. The structure sensitivity for CO chemisorption on supported Pd particles is explained by formation of (100)×(111) surface steps.     

Momentum-resolved inverse photoemission

Inverse photoemission including isochromat spectroscopy is shown to be a versatile technique to probe empty electronic states in solids and at clean and adsorbate covered surfaces. The complete set of quantum numbers of an electronic state can be determined and examples will be discussed for bulk and surface electronic states. For sufficiently low kinetic energy of the primary electrons, inverse photoemission is shown to be applicable to adsorbates also. This allows one to assess directly the unoccupied electronic states of the adsorbate which play an important role in the formation of the surface chemical bond. Examples are discussed for atomic and molecular chemisorption as well as adsorption on alkali promoted surfaces.     

Chemisorption on a model insulator

The adsorption both of a single atom and a monolayer of atoms on the (001) surface of a model two-band crystal with the CsCI structure is investigated using the Green's function formalism and the phase shift technique. The electronic structure of the surface is described within the Linear Combination of Atomic Orbitals (LCAO) scheme and the Tight Binding (TB) approximation. Each adatom is represented by a single non-degenerate energy level. The adatoms are placed on the surface in both the on-site and the centered fourfold-site configuration. The change in the density of electronic states upon chemisorption is found, and comparison is made with similar results on a metal surface. It is shown that many, but not all, of the qualitative features in chemisorption on metallic surfaces can be transferred to the case of an insulating surface. In addition, it is shown that there are systematic variations in the density of states with adatom coverage which depend upon the absorption site.     

Alkali-metal-affected adsorption of molecules on metal surfaces

The study of coadsorption of alkali metals and simple molecules on transition metal surfaces has been a favored topic of research ever since the pioneering work by Langmuir in 1923. The main reasons for this continued interest are both of fundamental and applied nature. There are a number of interesting physical effects, such as work function changes, charge transfer, two-dimensional ordering, bond energy and molecule orientational changes, and altered surface reactive properties, which have been investigated by a large number of different surface analytical techniques. From an applied point of view, alkali metal covered surfaces are important in the areas of electron and ion emission and heterogenous catalysis, for example.In this paper we will give a short review of the adsorption of alkali metals on well-defined transition metal surfaces. The interaction of these adsorbed alkali metals with subsequently adsorbed atomic or molecular species will be treated more extensively. The emphasis will be on recent experiments dealing with well-characterized surfaces. In particular we will consider questions of adsorption energetics and kinetics, but also review in detail the vibrational, electronic, structural and reactive properties of the coadsorbed complex. Based on a wealth of experimental data, several models of the coadsorbed alkali metal-molecule complex will be introduced and discussed.     

Comparaison des méthodes magnétiques à fort et à faible champ pour l'étude de l'adsorption de H2 et de O2 par des catalyseurs NiSiO2

Magnetic properties of nickel catalysts vary by gas adsorption. In this paper, the variation of magnetic susceptibility and of saturation magnetization with chemisorption are derived taking into account the possibility of preferencial fixation on ferromagnetic particles of given size: high field techniques enable one to calculate a parameter α, the overal magnetic moment change in the fixation of one molecule; if the sample is superparamagnetic, low field techniques give (1 + A) α, where A depends upon the particle size distribution and upon a function which describes the preferencial adsorption. Experimental results concerning adsorption of O₂ and H₂ on NiSiO₂ catalysts are reported; they are interpreted by assuming that H₂ is uniformly adsorbed on the metal surface and that O₂ is preferencially fixed on small particles. Some comments are added about the adsorption of hydrocarbons on nickel.     

Effect of palladium particle size on the appearance of superstructure of graphite in palladium/graphite model catalyst

Scanning tunneling microscopy images of palladium particles supported on highly oriented pyrolytic graphite as a model catalyst in ultra-high vacuum have been observed. We found superstructures on graphite lattice due to electronic interaction between palladium particles and graphite in the vicinity of small two-dimensional palladium particles (lateral size <2 nm, height <0.5 nm). However, such superstructures could not be observed near larger three-dimensional palladium particles (lateral size ∼4 nm, height ∼2 nm). The results indicate the importance of not only the size but also the dimension of metal particles in interaction between palladium and graphite, the nature of which can be interpreted by the difference in electronic properties of atomic and bulk palladium. This has important implications to the understanding of metal-support electronic interaction and its effect on the surface catalytic reactivity of supported metal catalysts.     

Adsorption and valence electronic states of nitric oxide on metal surfaces

Among fundamental diatomic molecules, the adsorption of carbon monoxide (CO) and nitric oxide (NO) on metal surfaces has been a subject of intensive research in the surface science community, partly owing to its relevance to heterogeneous catalysis used for environmental control. Compared to the rather well-defined adsorption mechanism of CO, that of NO is less understood because the adsorption results in much more complex reactions. The complexity is ascribed to the open-shell structure of valence electrons, making the molecule readily interact with the metal surface itself as well as with co-adsorbed molecules. Furthermore, the interaction crucially depends on the local structure of the surface. Therefore, to elucidate the interaction at the molecular scale, it is essential to study the valence state as well as the bonding geometry for individual NO molecules placed in a well-defined environment on the surface. Scanning tunneling microscopy (STM) is suitable for this purpose. In this review, we summarize the knowledge about the interaction of NO with metal surfaces, mainly focused on the valence electronic states, followed by recent studies using STM and atomic force microscopy (AFM) at the level of individual molecules.     

Optical second-harmonic generation by supported metal clusters: Size and shape effects

Optical Second Harmonic Generation (SHG) by metal clusters has been investigated. For this purpose clusters were generated by the deposition and nucleation of metal atoms on a LiF(100) single crystal surface under ultrahigh vacuum conditions. The size and shape of the metal particles was characterized by optical transmission spectroscopy. The SHG intensity was detected in situ as a function of cluster size during the nucleation. Fundamental wavelengths of =1064 and 532 nm were used and the SHG signal was measured for different polarization combinations of the incident and registered light. SH radiation is detectable for particles as small as approximately 1 nm. The signal grows monotonically as a function of particle size, passes a maximum and finally drops off. This behavior is discussed in terms of resonant enhancement of the signal by surface plasmon excitation and changes of ⁽²⁾ as a function of particle size and shape. In further experiments the chemisorption of oxygen on the surface of the metal particles was studied. The SH signal decreases as a function of oxygen coverage and amounts to only about 15% of the initial value upon chemisorption of one monolayer. This indicates that the SH signal originates almost exclusively from the surface of the clean clusters and that higher order bulk contributions are negligible.     

Challenges in hydrogen adsorptions: from physisorption to chemisorption

In this short review, we will briefly discuss the story of hydrogen storage, its impact on clean energy application, especially the challenges of using hydrogen adsorption for onboard application. After a short comparison of the main methods of hydrogen storage (high pressure tank, metal hydride and adsorption), we will focus our discussion on adsorption of hydrogen in graphitic carbon based large surface area adsorbents including carbon nanotubes, graphene and metal organic frameworks. The mechanisms, advantages, disadvantages and recent progresses will be discussed and reviewed for physisorption, metal-assisted storage and chemisorption. In the last section, we will discuss hydrogen spillover chemisorption in detail for the mechanism, status, challenges and perspectives. We hope to present a clear picture of the present technologies, challenges and the perspectives of hydrogen storage for the future studies.     

“Small and beautiful” – The novel structures and phases of nano-oxides

Low-dimensional oxide nanostructures supported on well-defined metal surfaces raise scientific interest both on a fundamental level and for potential technological applications. These systems may be regarded as artificially created hybrid materials with novel emergent properties, supporting new concepts of geometrical structure, electronics and magnetism, complex phase diagrams and particular chemical reactivity. The coupling of the oxide nano-phase to the metal support surface by electronic and elastic interactions together with the low dimensionality aspects are at the origin of the particular behaviour of these hybrid structures. By way of prototypical examples, this Prospective article highlights some of the novel properties of nano-oxides and, as a side aspect, comments on the aesthetics of their structural motifs.     

Chemisorption and surface barrier structure

The effects of chemisorption on the potential barrier at a metal surface are discussed. Simple physical considerations predict an outward shift in the barrier origin, an increased saturation of the barrier, and changes in the inner potential. However, an analysis of low-energy electron diffraction (LEED) data for p(2 × 1) O/W(110) and p(2 × 1) H/W(110) indicates that these changes are small, and that the observed modifications are due primarily to changes in the surface scattering properties accompanying chemisorption. LEED fine structure analysis should then be a useful technique for identifying adsorption sites on metal surfaces.     

Catalyst design using an inverse strategy: From mechanistic studies on inverted model catalysts to applications of oxide-coated metal nanoparticles

Metal-oxide interfaces are of great importance in catalytic applications since each material can provide a distinct functionality that is necessary for efficient catalysis in complex reaction pathways. Moreover, the synergy between two materials can yield properties that exceed the superposition of single sites. While interfaces between metals and metal oxides can play a key role in the reactivity of traditional supported catalysts, significant attention has recently been focused on using “inverted” oxide/metal catalysts to prepare catalytic interfaces with unique properties. In the inverted systems, metal surfaces or nanoparticles are covered by oxide layers ranging from submonolayer patches to continuous films with thickness at the nanometer scale. Inverse catalysts provide an alternative approach for catalyst design that emphasizes control over interfacial sites, including inverted model catalysts that provide an important tool for elucidation of mechanisms of interfacial catalytic reactions and oxide-coated metal nanoparticles that can yield improved stability, activity and selectivity for practical catalysts.This review begins by providing a summary of recent progress in the use of inverted model catalysts in surface science studies, where oxides are usually deposited onto the surface of metal single crystals under ultra-high vacuum conditions. Surface-level studies of inverse systems have yielded key insights into interfacial catalysis and facilitated active site identification for important reactions such as CO oxidation, the water-gas shift reaction, and CO₂ reduction using well-defined model systems, informing strategies for designing improved technical catalysts. We then expand the scope of inverted catalysts, using the “inverse” strategy for preparation of higher-surface area practical catalysts, chiefly through the deposition of metal oxide films or particles onto metal nanoparticles. The synthesis techniques include encapsulation of metal nanoparticles within porous oxide shells to generate core-shell type catalysts using wet chemical techniques, the application of oxide overcoat layers through atomic layer deposition or similar techniques, and spontaneous formation of metal oxide coatings from more conventional catalyst geometries under reaction or pretreatment conditions. Oxide-coated metal nanoparticles have been applied for improvement of catalyst stability, control over transport or binding to active sites, direct modification of the active site structure, and formation of bifunctional sites. Following a survey of recent studies in each of these areas, future directions of inverted catalytic systems are discussed.     

Characterization of a Pd-Fe bimetallic model catalyst

Small bimetallic Pd-Fe particles supported on a well ordered alumina film grown on NiAl (1 1 0) were studied focusing on the geometric, electronic, adsorption, as well as magnetic properties. The morphology, growth mode and surface composition were investigated by combining scanning tunneling microscopy (STM), temperature-programmed desorption (TPD) and infrared spectroscopy (IRAS) using CO as a probe molecule. Information on the electronic properties of the bimetallic systems was obtained by means of X-ray photoelectron spectroscopy (XPS). These measurements were amended by in situ ferromagnetic resonance spectroscopy to address the magnetic properties of the bimetallic particles. The subsequent deposition of the metals at 300 K varying the order of metal deposition resulted in two distinct bimetallic systems. Pd deposited on existing Fe particles forms a shell, however, FMR and XPS suggest that intermixing of Pd and Fe occurs to some extent. For the reverse order, a larger amount of Fe is required to coat Pd particles, due to the strong tendency of Pd to segregate to the surface of the particles.     

A method for measuring the proportion of different plane orientations in metal supported catalysts by gas chemisorption

A method is presented, which allows us to calculate the percentage of metal surface atoms with different coordination numbers, on metal supported catalysts. This method is based on gas-metal stoichiometry values reported for the chemisorption of H₂, O₂ and CO on well defined monocrystals, and on gas chemisorption measurements on metal supported catalysts. It has here been applied to a series of Pd/SiO₂ catalysts.     

Photoelectron spectroscopy in the energy region 30 to 800 eV using synchrotron radiation

With the advent of synchrotron radiation, the photoemission techniques were extended to a continuous range of excitation energies in the far ultraviolet and soft x-ray regions, adding tremendously to the usefulness of photoemission as a probe of the electronic structure of materials. In this paper, we discuss the application of photoelectron spectroscopy using synchrotron radiation to the studies of oxygen chemisorption/oxidation of Si surfaces, metal overlayers on III-V semiconductor surfaces, chemisorption on transition metal surfaces, and the surface electronic structure of CuNi alloys.     

采用(meta)-GGA泛函对Ag(111)、Rh(111)和Ir(111)上乙烯吸附的理论研究

本文针对在多种催化反应的重要中间体乙烯,使用(meta)-GGA等级的包含PBE,BEEF-vdW,SCAN以及SCAN+rVV10在内的多种交换关联泛函,系统研究了在过渡金属表面(Ag,Rh和Ir)上乙烯吸附势能面对泛函的依赖关系. 研究发现,对于乙烯在贵金属Ag(111)上的吸附,除了PBE外,BEEF-vdW,SCAN以及SCAN+rVV10均能预测出物理吸附态的存在. 对于乙烯在Rh(111)面的吸附,SCAN和SCAN+rVV10预测在化学吸附位之前存在有物理吸附前驱态,而基于PBE和BEEF-vdW的势能面并没有发现前驱态的存在. 而对于乙烯在Ir(111)上的吸附,BEEF-vdW也能微弱地预测出化学吸附前驱态的存在. 研究结果表明,无论在哪一种金属表面上,四种泛函中SCAN+rVV10给出的吸附能最强,其次是SCAN,最后是PBE或者BEEF-vdW.     

Liénard-type chemical oscillator

Using first-principles calculation, we have studied the properties of a series of M _x Co_1?x /Co(0001) (M = Pd, Pt) bimetallic surface alloys with atom M ratios from 0.25 to 1.0, then the effect of alloyed M metal on the properties of S adsorbed on these surfaces are discussed. Our calculations show that the alloying of metal Pd, Pt on Co(0001) weakens the S-M (M = Pd, Pt, Co) bond strength compared to monometallic surfaces and the site preference of sulfur atom is dependent on the alloyed metal M and its surface concentration. Moreover, bimetallic surface electronic structure modifications with and without sulfur are analyzed in comparison with clean Co(0001), and the correlation between the sulfur adsorption energy and the bimetallic surface d-band center is presented.     

Atomic and electronic properties of tert-butanol on the Si(001)-(2×1) surface

The atomic and electronic properties of the adsorption of tert-butanol [(CH₃)₃OH] molecule on the Si(001)-(2×1) surface have been studied by using the ab-initio density functional theory (DFT) based on pseudopotential approach. We have found that tert-butanol bonded the Si(001) surface by oxygen atom, cleaving a O–H bond and producing a Si-H bond and tert-butoxy surface species. We have also investigated the influence of chemisorption of tert-butanol on the electronic structure of the clean Si(001)-(2×1) surface. Two occupied surface states situated entirely below the bulk valence band maximum have been identified, which means that the clean Si(001)-(2×1)surface was passivated by the chemisorption of tert-butanol. In order to explain the nature of the surface components we have also plotted the total and partial charge densities at the [`(K)]\bar{K} point of the surface Brillouin zone (SBZ).     

XPS and TPD investigation of CO adsorption on mixed Rh-V layers supported by gamma-alumina

Alumina-supported mixed bimetallic Rh-V thin films, with the overall thickness of 0.8 ML, were prepared under the ultrahigh vacuum (UHV) conditions and characterized with respect to their electronic and CO adsorption properties. X-ray photoelectron spectroscopy (XPS) was utilized to characterize electronic changes accompanying bimetallic Rh-V interaction and interaction between metal and polycrystalline γ-Al₂O₃ substrate. The chemisorption properties were probed by temperature-programmed desorption spectroscopy (TPD) of CO molecules. The electronic and chemisorption properties of the mixed layers were compared with pure Rh and V layers grown on the same γ-Al₂O₃ substrate and with a model bimetallic Rh-V system prepared by V deposition on a polycrystalline Rh foil. By varying the preparation conditions, we observed a strong dependence of the studied properties on the position of the V atoms. The presence of V atoms on the surface led to a fast deactivation, while vanadium presented under the surface resulted in a weakening of CO-metal surface bond, a change in the proportion of the adsorption side species, and an increase of CO dissociation.