Coadsorbate

Many of the adsorbents used have rough surfaces they may consist of clusters of very small particles, for example. It appears that the concept of self-similarity or fractal geometry (see Section VII-4C) may be applicable [210,211]. In the case of quenching of emission by a coadsorbed species, Q, some fraction of Q may be hidden from the emitter if Q is a small molecule that can fit into surface regions not accessible to the emitter [211]. 

Wimmer E, Fu C L and Freeman A J 1985 Catalytic promotion and poisoning all-electron local-density-functional theory of CO on Ni(001) surfaces coadsorbed with K or S Phys. Rev. Lett. 55 2618-21... 

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]. 

STM measurements suggested for the bilayer fonned by Cu and coadsorbed Cl either a (5 x 5) long range order similar to that of the (111) plane of CuCl or a (4 x 4)-based stmcture [46, 77, 78], while the bilayer fonned with Cu... 

The same effect of mutual enhancement of adsoi ption was observed in the spectra of amoi phous ice for ethylene and CHF coadsorbed at 77K. 

Fig. 4.1.17 Graphic illustration of Forster-type resonance energy transfer from aequorin to Aequorea GFP. In the vessel at left, a solution contains the molecules of aequorin and GFP randomly distributed in a low ionic strength buffer. The vessel at right contains a solution identical with the left, except that it contains some particles of DEAE cellulose. In the solution at right, the molecules of aequorin and GFP are coadsorbed on the surface of DEAE particles. Upon an addition of Ca2+, the solution at left emits blue light from aequorin (Xmax 465 nm), and the solution at right emits green light from GFP (Xmax 509 nm).

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

It is clear that in case (a) the rate, r, of the catalytic reaction (e.g. CO oxidation) will not be affected while in case (b) the rate increase, Ar, will at most equal I/nF (e.g. direct reaction of O2 with CO). In case (c), however, the new species introduced electrochemically onto the catalyst surface will interact with coadsorbed reactants and will change the catalytic properties of the catalyst surface in an a priori unpredictable manner, which is nevertheless not subject to Faraday s law. Thus in cases (a) and (b) there will be no NEMCA but in case (c) it is entirely logical to anticipate it. Even in case (b) one may anticipate NEMCA, if the product remains on the surface and has some catalytic or promotional properties. 

A classical example of promotion is the use of alkalis (K) on Fe for the ammonia synthesis reaction. Coadsorbed potassium (in the form of K20) significantly enhances the dissociative adsorption of N2 on the Fe surface, which is the crucial and rate limiting step for the ammonia synthesis5 (Fig. 2.1). 

There is a very rich literature and a comprehensive book6 on the role of promoters in heterogeneous catalysis. The vast majority of studies refers to the adsorption of promoters and to the effect of promoters on the chemisorptive state of coadsorbed species on well characterized single crystal surfaces. A... 

There are several reasons for deviations from the LHHW kinetics Surface heterogeneity, surface reconstruction, adsorbate island formation and, most important, lateral coadsorbate interactions.18,19 All these factors lead to significant deviations from the fundamental assumption of the Langmuir isotherm, i.e. constancy of AHa (and AHB) with varying coverage. 

Calorimetric chemisorption studies have clearly shown that even on single crystal surfaces -AHa varies with 0A. Usually (-AHA) decreases with 0A and this implies repulsive lateral interactions between the coadsorbed A molecules. In the simplest case of a linear variation ... 

Such an expression is sometimes found to provide an adequate description of catalyst poisoning, i.e. the reverse of promotion. But for a promoter, i.e. a coadsorbate which enhances r, it is clear that at least one of the following two events must happen ... 

One exception to this general behavior is the system p(2x2) 0.25 0/Pt( 111), where, due to the high tendency of coadsorbed CO and O for C02 formation on Pt(lll), the value of S0 for CO adsorption is the same to the one for the clean surface, although on other substrates oxygen behaves as a typical electronegative modifier of CO adsorption. 

It is also clear that in the present case oxygen is the electron acceptor (A) while CO is the electron donor (D). It has been already discussed that CO is an amphoteric adsorbent, i.e., its chemisorptive bond involves both electron donation and backdonation and that, in most cases, its electron acceptor character dominates. However, in presence of the coadsorbed strong electron acceptor O (see section 2.5.2.1) it always behaves as an electron donor. 

E. Umbach, Electronic structure and interactions in well-defined coadsorbed layers, Appl. Phys. 47, 25-36 (1988). 

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