Specific adsorption structure

A freshly prepared flame-annealed Au(100) surface has been found to be reconstmcted188,487,534,538 and the surface atoms exhibit a hexagonal close-packed structure to yield the (hex)-stmcture. One-directional long-range corrugation of 1.45 nm periodicity and 0.05 nm height has been found on the Au( 100) surface.188,488 When the reconstruction is lifted due to specific adsorption of SO - anions at more positive , the surface changes to a (1 x 1) structure.538... 

The specific adsorption of OH" ions depends on the electrode surface structure increasing in the order Au(l 11) < Au(100) < Au(311).391 The similarity of the results obtained in alkaline solutions and those observed in acid and neutral media have led the authors of many papers to conclude that surface reconstruction occurs at a < 0 and is removed at 0. 

The potentials of zero charge considered in this chapter are those in the absence of specific adsorption of ionic as well as nonionic species. There has been no attempt to review the enormous amount of data on the effect of specific adsorption on Ea+j, except for the few cases where extrapolation back to zero specific adsorption has been used as a more accurate way to determine <7-o- However, specific adsorption is difficult to relate quantitatively to the structure of interfacial water as well as to the effect of the metal. 

Reactant concentrations Cyj in the bulk solution, as well as the Galvani potential between the electrode and the bulk solution (which is a constituent term in electrode potential E), appear in kinetic equations such as (6.8). However, the reacting particles are not those in the bulk solution but those close to the electrode surface, near the outer Helmholtz plane when there is no specific adsorption, and near the inner Helmholtz plane when there is specific adsorption. Both the particle concentrations and the potential differ between these regions and the bulk solution. It was first pointed out by Afexander N. Frumkin in 1933 that for this reason, the kinetics of electrochemical reactions should strongly depend on EDL structure at the electrode surface. 

Finally, it should be remarked that, as long as the interfacial region is extended sufficiently to include all structural and electronic deviations from the reservoirs, (5.18) and (5.19) are valid for any type of connection between a metallic electrode and an electrolyte. They also include the cases of nonspecific and specific adsorption on the electrode. 

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

At present it is impossible to formulate an exact theory of the structure of the electrical double layer, even in the simple case where no specific adsorption occurs. This is partly because of the lack of experimental data (e.g. on the permittivity in electric fields of up to 109 V m"1) and partly because even the largest computers are incapable of carrying out such a task. The analysis of a system where an electrically charged metal in which the positions of the ions in the lattice are known (the situation is more complicated with liquid metals) is in contact with an electrolyte solution should include the effect of the electrical field on the permittivity of the solvent, its structure and electrolyte ion concentrations in the vicinity of the interface, and, at the same time, the effect of varying ion concentrations on the structure and the permittivity of the solvent. Because of the unsolved difficulties in the solution of this problem, simplifying models must be employed the electrical double layer is divided into three regions that interact only electrostatically, i.e. the electrode itself, the compact layer and the diffuse layer. 

The structure of the compact layer depends on whether specific adsorption occurs (ions are present in the compact layer) or not (ions are absent from the compact layer). In the absence of specific adsorption, the surface of the electrode is covered by a monomolecular solvent layer. The solvent molecules are oriented and their dipoles are distorted at higher field strengths. The permittivity of the solvent in this region is only an operational quantity, with a value of about 12 at the Epzc in water,... 

Activated carbon is an amorphous solid with a large internal surface area/pore structure that adsorbs molecules from both the liquid and gas phase [11]. It has been manufactured from a number of raw materials including wood, coconut shell, and coal [11,12], Specific processes have been developed to produce activated carbon in powdered, granular, and specially shaped (pellet) forms. The key to development of activated carbon products has been the selection of the manufacturing process, raw material, and an understanding of the basic adsorption process to tailor the product to a specific adsorption application. 

The good agreement between electrochemical and UHV data, documented in Figure 4, is a very important result, because it proves for the first time that the microscopic information which one obtains with surface science techniques in the simulation studies is indeed very relevant to interfacial electrochemistry. As an example of such microscopic information, Figure 5 shows a structural model of the inner layer for bromide specific adsorption at a halide coverage of 0.25 on Ag 110 which has been deduced from thermal desorption and low energy electron diffraction measurements /12/. Qualitatively similar models have been obtained for H2O / Br / Cu( 110) /18/and also for H2O/CI /Ag 110. ... 

While modeling the structure and properties of porous materials one usually is interested in structural properties of a desirable hierarchical level. For example, for chemical properties the molecular structure is major, and the specific adsorption and catalytic properties are guided by the structure and composition of particle surface. Diffusion permeability is determined by the supramolecular... 

For surface area determinations the ideal adsorbate should exhibit BET C values sufficiently low to preclude localized adsorption. When the adsorbate is so strongly tied to the surface as to be constrained to specific adsorption sites, the adsorbate cross-sectional area will be determined more by the adsorbent lattice structure than by the adsorbate dimensions. This type of epitaxial adsorption will lead to decreasing measured surface areas relative to the true BET value as the surface sites become more widely spaced. 

There are a number of factors that determine crystal size. Probably the two most important are the deposition mechanism (ion-by-ion growth, in general, will result in larger crystal size than the hydroxide mechanism, discussed in detail in Chap. 3) and specific adsorption of anions onto the growing crystal (this can affect both crystal structure and size). 

At the time that this survey of chemisorption bondlengths was undertaken, the database of molecular adsorption structures [139] was even more sparse, inadequate for any clear pattern to be established. More recently, we have returned to this problem through a detailed study of one specific chemisorption system, namely CO on Ni surfaces [142,143]. The objective was two-fold. First, by conducting PhD experimental structure determinations for different phases of CO on Ni(100) and Ni(lll) it was possible to obtain Ni—CO chemisorption bondlengths for 1-,... 

Alkynes have also been shown to form the [2 + 2] cycloaddition product. Acetylene (H—C=C—H), the simplest alkyne, forms an interesting adsorption case, because the specific adsorption geometries of acetylene on Si(100)-2 x 1 have been debated [11,201,207,210,224-236]. Acetylene was first found experimentally to form a [2 + 2] C=C cycloaddition product that exhibits a cyclobutene-like surface structure on Si(100)-2 x 1 [210,227]. Later STM measurements revealed that at least two different surface products were present [228,231,233], and identified a product that is oriented perpendicularly to the dimer row. From these images, it was argued that in addition to an intradimer [2 + 2] C=C cycloaddition geometry, acetylene also forms a surface adduct that bridges two dimers along a row. Several theoretical... 

Specific adsorption may involve short-range, strong interactions due to the overlapping of the electronic orbitals of the adsorbate and the electrode and ionic species or dipoles in the electrolyte. These will be considered in Sect. 6.1 together with the effect of changes of the structure of the interfacial region on electrode kinetics (double layer effects [3,5]). 

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