Adsorbed substrate

Strong adsorbate-substrate forces lead to chemisorption, in which a chemical bond is fomied. By contrast, weak forces result inphysisorption, as one calls non-chemical physical adsorption. 

The balance between these different types of bonds has a strong bearing on the resulting ordering or disordering of the surface. For adsorbates, the relative strength of adsorbate-substrate and adsorbate-adsorbate interactions is particularly important. Wlien adsorbate-substrate interactions dominate, well ordered overlayer structures are induced that are arranged in a superlattice, i.e. a periodicity which is closely related to that of the substrate lattice one then speaks of commensurate overlayers. This results from the tendency for each adsorbate to seek out the same type of adsorption site on the surface, which means that all adsorbates attempt to bond in the same maimer to substrate atoms. 

Maiik i J and Trenary M 1989 infrared refieotion-absorption study of the adsorbate-substrate stretoh of CO on Pt (111) Surf. Sol. 214 L237-45... 

At potentials positive to the bulk metal deposition, a metal monolayer-or in some cases a bilayer-of one metal can be electrodeposited on another metal surface this phenomenon is referred to as underiDotential deposition (upd) in the literature. Many investigations of several different metal adsorbate/substrate systems have been published to date. In general, two different classes of surface stmetures can be classified (a) simple superstmetures with small packing densities and (b) close-packed (bulklike) or even compressed stmetures, which are observed for deposition of the heavy metal ions Tl, Hg and Pb on Ag, Au, Cu or Pt (see, e.g., [63, 64, 65, 66, 62, 68, 69 and 70]). In case (a), the metal adsorbate is very often stabilized by coadsorbed anions typical representatives of this type are Cu/Au (111) (e.g. [44, 45, 21, 22 and 25]) or Cu/Pt(l 11) (e.g. [46, 74, 75, and 26 ]) It has to be mentioned that the two dimensional ordering of the Cu adatoms is significantly affected by the presence of coadsorbed anions, for example, for the upd of Cu on Au(l 11), the onset of underiDotential deposition shifts to more positive potentials from 80"to Br and CE [72]. 

Vacuum-compatible solids Adsorbate-substrate bond lengths... 

The rates of the elementary steps can be formulated in a conventional manner, and the quasi-steady state hypothesis is applied to the adsorbed substrate (A ). The... 

Watanabe, K., Takagi, N. and Matsumoto, Y. (2004) Direct time-domain observation of ultrafast dephasing in adsorbate-substrate vibration rmder the influence of a hot electron bafh Cs adatoms on Pt(lll). Phys. Rev. Lett., 92, 057401. 

G. A. Somorjai and Y. Borodko, Adsorbate (substrate)-induced restructuring of active transition metal sites of heterogeneous and enzyme catalysts, Catal. Lett., 1999, 59, 89. 

Conventional bulk measurements of adsorption are performed by determining the amount of gas adsorbed at equilibrium as a function of pressure, at a constant temperature [23-25], These bulk adsorption isotherms are commonly analyzed using a kinetic theory for multilayer adsorption developed in 1938 by Brunauer, Emmett and Teller (the BET Theory) [23]. BET adsorption isotherms are a common material science technique for surface area analysis of porous solids, and also permit calculation of adsorption energy and fractional surface coverage. While more advanced analysis methods, such as Density Functional Theory, have been developed in recent years, BET remains a mainstay of material science, and is the recommended method for the experimental measurement of pore surface area. This is largely due to the clear physical meaning of its principal assumptions, and its ability to handle the primary effects of adsorbate-adsorbate and adsorbate-substrate interactions. 

The electrocatalytic hydrogenation (ECH) of an unsaturated organic substrate (Y=Z) in aqueous or mixed aqueous-organic media (eqs [2] to [4] where M represents an adsorption site of the catalyst, and M(H) and M(Y=Z), chemisorbed hydrogen and the adsorbed substrate respectively) involves the same hydrogenation steps as those of classical catalytic hydrogenation (CH) (steps [2] to [4] stoichiometry for adsorbed species only, not for surface M) (1) ... 

When a molecule is adsorbed on a surface, the symmetry of the combined adsorbate-substrate system is very likely to be reduced compared to that of the isolated gas-phase species or the bare adsorption site. This raises the possibility that, if mirror planes present in the isolated achiral molecule and those at the relevant... 

The first four facets are rotationally equivalent to each other as are the final four. The two sets are related by reflectional symmetry to each other. When a chiral adsorbate, for example, S-lysine, is used, the reflectional symmetry is no longer valid and only rotationally equivalent facets should be formed. This was demonstrated elegantly by Zhao with STM [53], The driving force for facet formation is proposed to be a three-point interaction involving the carboxylate group, the a-amino group, and the amino-terminated side chain. The simultaneous optimization of adsorbate-adsorbate and adsorbate-substrate interactions determines the stereochemistry of the facet. 

Preferred adsorption of the unsaturated bond of the substrate occurs at that face which presents the least steric interactions between the adsorbed substrate and the surface. Since some amazingly sterically hindered molecules can be hydrogenated, at least some active sites must look like corners or edges or some other protuberances. 

The adsorbed substrate adopts that conformation which best balances the least strain within the adsorbed substrate and the fewest interactions... 

The shape of an isotherm depends on the choice of adsorbate, substrate, temperature T and, in a solution-phase system, the solvent. 

In non-electrochemical heterogeneous catalysis, the interface between the catalyst and the gas phase can often be characterized using a wide variety of spectroscopic probes. Differences between reaction conditions and the UHV conditions used in many studies have been probed extensively 8 as have differences between polycrystalline and single-crystalline materials. Nevertheless, the adsorbate-substrate interactions can often be characterized in the absence of pressure effects. Therefore, UHY based surface science techniques are able to elucidate the surface structures and energetics of the heterogeneous catalysis of gas phase molecules. 

The electrochemical interface between an electrode and an electrolyte solution is much more difficult to characterize. In addition to adsorbate-substrate and adsorbate-adsorbate interactions, adsorbate-electrolyte interactions play a significant role in the behavior of reactions on electrode surfaces. The strength of the adsorbate-substrate interactions is controlled by the electrode potential, which also determines the configuration of the electrolyte. With solution molecules, ions, and potential variation involved, characterization of the electrochemical interface is extremely difficult. However, by examining solvation, ion adsorption, and potential effects as individual components of the interface, a better understanding is being developed. 

An enantioselective catalyst must fulfill two functions (1) activate the different reactants (activation) and (2) control the stereochemical outcome of the reaction (controlling function). As an accepted general model, it is postulated that this control is achieved by specific interactions between the active centers of the catalyst, the adsorbed substrates, and the adsorbed chiral auxiliary (Figure 14.4). Experience has shown that most substrates that can be transformed in useful enantiomers have an additional functional group that can interact with the chiral active center. 

Adsorbate Substrate Atom density mismatch/ML (7o) Pseudomorphic/epitaxial layers Change in CO desorption T(K)... 

In physisorbed systems, the electronic ground state of the adsorbate is only weakly perturbed upon adsorption. The physisorption potential is rather flat and shallow, i.e. the restoring force of the vertical motion of adatoms is weak, and thus, the corresponding adsorbate-substrate vibrations are low-frequency modes " Radiative phonon processes are expected to dominate the relaxation and coupling processes. 

Two models of oxygen adsorption are considered, vertical form and parallel form, which are illustrated in Fig. 9.1 and Fig. 9.2. For all the cases, the adsorbate/substrate system is optimized by GGA. In optimization, all atoms on the pyrite substrate are fixed only the O atoms are allowed to move. The initial 0—0 double bond length and the distance between Fe atom and O atom are 0.121 nm and 0.196 run, respectively. To simplify the calculation, the adsorption coverage will not be considered. 

The adsorption energies are defined positive for a stable adsorbate/substrate system ... 

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