Atomically and molecularly adsorbed

Ohta, T. (2002) Surface XAFS applied to atomic and molecular adsorbates, in Chemical Application of Synchrotron Radiation, Part IT. X-ray Application, Advance Series in Physical Chemistry, Vol. 12B (ed. T.-K. Sham), World Scientific, London, pp. 664—706. 

In this section we review studies of atomic and molecular adsorbates on alloy surfaces. The section is organized from simplest adsorbates to most complex. Within each subsection we have organized the papers reviewed from most general to most specific. We begin with hydrogen in section 3.1 and oxgyen in 3.2. Carbon monoxide, carbon dioxide and other small molecules such as NO are presented in 3.3. Hydrocarbons such as ethylene and acetylene are discussed in 3.4. Finally, carbon and sulfur are presented in section 3.5. 

Several such minima have been computed in the interaction of atomic and molecular adsorbates with Ni clusters P. E. M. Siegbahn, private communication. 

The quantitative analysis of adsorption on metal clusters, as discussed earlier, can be sensitive to the cluster chosen to model the chemisorption site. In a systematic series of studies, we examined the adsorption of various atomic and molecular adsorbates on different metal surfaces. We found that it was important to optimize 1) size, 2) structural configuration, 3) spin state, and... 

Baetzold, R. Surface Diffusion of Atomic and Molecular Adsorbates, in "Metal-Surfaee Reaetion Energetics," Chap. 3, Shustorovich, E. (ed.). New York VCH, 1991. 

The emphasis is on the atomic-scale structure as defined by atomic positions, relaxations and reconstructions, structural models and bonding configurations. Included are both atomic and molecular adsorbates. The data for atomic adsorbates far outnumber those for molecular adsorbates, but the effects are rather similar, which is interesting in its own right the tables in this chapter therefore show them side by side for direct comparison. 

It is clear that the effect of varying the t/-band center for a given adsorbate and a fixed coupling matrix element must be similar for any adsorbate with low-lying adsorbate states. This includes H, S, C, N, and many other atomic and molecular adsorbates. 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

H. Ohtani, C.-T. Kao, M.A.V. Hove, and G. Somorjai, A tabulation and classification of the stmctures of clean solid surfaces and of adsorbed atomic and molecular monolayes as determined from low energy electron diffraction patterns, Progress in Surface Science 23(2,3), 155-316 (1986) and reference therein. 

The latter conclusion is reliably confirmed by experimental results [40] in which the studies of effect of the structure on the character of adsorption change in electric conductivity of monocrystal or partially reduced polycrystalline ZnO adsorbents were conducted. The comparative studies of the character and the value of response of electric conductivity in both types of adsorbents on adsorption of various atoms and molecular particles led the authors to conclusion on identical origin of both the mechanisms of electric conductivity and mechanisms of its adsorption induced change. 

The above results put forward the question, whether or not one can directly compare in experiment the influence produced by adsorbed atomic and molecular silver particles on electric conductivity of a zinc oxide film. 

Cant et al. [125] suggested a three-route mechanism in which the reactive forms were adsorbed CO and atomic and molecular (physically adsorbed) oxygen. They suggested the existence of step... 

The present state of the theories of atomic and molecular processes in condensed phases is characterized by great non-uniformity of its development. Matters are much problematic in the theory of the kinetics of processes at a molecular level. The kinetics of surface processes mainly employs models taking no account of the interaction of the adsorbed particles (the law of masses or surface action) [14-16]. This does not reflect the real properties of a gas solid interface. There is also a diversity of models when considering the interaction of the particles because various approximations are used (equilibrium is described with a view to the correlation effects, while kinetics ignores them). The problem of approximations is of a fundamental significance in the theory of condensed systems. Interaction between the particles causes all the particles to be bound to... 

LEED has been used to determine the structure of a wide variety of surfaces, including clean and reconstructed surfaces of metals and semiconductors, and atomic and molecular physisorption and chemisorption on many different substrates (see part 5). As the theoretical and experimental tools of LEED have improved, the structure of systems with larger and more complex unit cells have been determined. Successful LEED structure determinations have been carried out for systems with several molecules adsorbed in unit cells up to 16 times larger than the substrate unit cell,/1 / and for reconstructed surfaces where the structural rearrangement involves several surface layers./2/... 

The theories of surface structure and bonding have been reviewed. It should be clear to the reader that surface structural chemistry is indeed a frontier area for both theorists and experimental researchers. From an experimentalists viewpoint the data base of atomic and molecular surface structures is very small at present. Most investigations have been carried out on flat, low Miller index surfaces of monatomic solids, either clean or with atomic or small molecules as adsorbates. 

In early proposals the species responsible for epoxidation was identified as the adsorbed molecular oxygen, Ag 02(ads)> while combustion was attributed to monoatomic Ag O(ads) (Equations 14-16). The oxidation step envisages the transfer of one atom of molecularly adsorbed oxygen to the double bond, while the other remains adsorbed on silver. The consumption of the latter by the total oxidation of ethylene restores the site vacancies necessary for the continuation of catalysis. Up to a maximum of six oxygen atoms are required for the combustion of one ethylene molecule. Thus, the combination of the reactions (Equation 14) and (Equation 15) predicts that the maximum attainable selectivity in the epoxidation of ethylene is 6/7, i.e., 85.7% (Equation 16). A lower selectivity should normally be expected because some monoatomic oxygen independently formed by dissociative adsorption (Equation 13) raises the level of ethylene combustion above that predicted by Equation 16. 

Atomic and molecular adsorption at vanadium oxide surfaces have been studied theoretically using both periodic slab and cluster models where so far studies are restricted to the pentoxide, V2O5, as a substrate due to its possible importance in catalytic applications as mentioned before. Further, adsorbate species include in all cases atoms (H [122-123, 126, 136-142], O (see below)) or rather small molecules (O2 (see below), H2O [143-144], NH3 [145-147], NO [146, 148], C2H4 [149], propene (CsHg) [140], toluene (CeHsCHj) [140]) that are of catalytic interest but also small enough to make meaningful calculations feasible. 

Hammer and Norskov have tested their model quite extensively [3] and have shown a good semiquantitative agreement between their model for the d contribution of the adsorption energy and the DFT-computed total adsorption energy of atomic adsorbates such as oxygen and hydrogen and molecular adsorbates such as carbon monoxide. Also, the activation energy, and hence the reactivity, of... 

Several earlier review articles are relevant to our subject. Slichter reviews the work done in his laboratory [16], most of it concerned with atoms or molecules adsorbed on the metal clusters, and the experimental techniques used in such studies [17]. Duncan s review [9] pays special attention to the C NMR of adsorbed CO. Most recently, one of us has given a rather detailed review of the held, in particular on metal NMR of supported metal catalysts [18]. While the topics and examples discussed in this chapter will inevitably have some overlap with these previous reviews, particular emphasis is directed toward highlighting the ability of metal NMR to access the iff-LDOS at both metal surfaces and molecular adsorbates. The iff-LDOS is an attractive concept, in that it contains information on both a spatial (local) and energy (electronic excitations) scale. It can bridge the conceptual gap between localized chemical descriptors (e.g., the active site or the surface bond) and the delocalized descriptors of condensed matter physics (e.g., the band structure of the metal surfaces). 

The kinetics of electrochemical reactions are often modified by the nature of the electrode material, and by the presence of atomic and molecular species either adsorbed on the surface or in the bulk solution [14]. Electrocatalysis is primarily concerned with the study of this phenomenon and, particularly, with the factors that govern enhancements in the rates of redox processes. Implicit in this general statement is the ability of the species responsible for these effects, or electrocatalyst, or the electrode itself, to carry out the reaction numerous times before undergoing possible deactivation. Electrocatalytic processes in which the electrode simply serves as a source or sink of electrons to generate solution phase species that... 

Adsorption. Oxygen is adsorbed on clean tungsten surfaces in a variety of atomic and molecular states. At low temperature (<0 °C), oxygen is adsorbed molecularly, but at room temperature this adsorption is a precursor state to the atomic adsorption. A covered surface shows an ordered oxygen superstructure. If the temperature is increased, a more extensive coverage occurs and oxide-like structures are formed. The surface layer can be described as adsorbed oxide. 

Summary. We describe work from our group utilizing in-situ scanning tunneling microscopy and atomic force microscopy in conjimction with additional surface characterization techniques. Systems described include catalytically active monolayers on electrode surfaces, oxygen or hydroxide adlattices on Cu, enhanced and additive-modified deposition of Cu, and molecular adsorbates on Au and Ag electrodes. 

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