Coverage limiting

Figure 10.21 Heats of adsorption for CO on (A) 0.5 ML Au/Ti02, (B) 0.25 ML Au/Ti02, and (C) 0.125 ML Au/Ti02. For comparison, the heat of adsorption of CO on bulk gold at the zero coverage limit is 10.9 kcal/mol. (Reprinted from Meier, D.C. and Goodman, D.W., J. Am. Chem. Soc., 126, 1892-1899, 2004. Copyright 2004. With permission from American Chemical Society.)...

Chemisorption requires direct contact between the chemisorbed molecule and the electrode surface as a result, the highest coverage achievable is usually a monomolecular layer. This may be contrasted with several of the methods to be discussed later that allow the electrode surface to be covered with thick films (i.e., multimolecular layers) of the desired molecule. In addition to this coverage limitation, chemisorption is rarely completely irreversible. In most cases, the chemisorbed molecules slowly leach into the contacting solution phase during electrochemical or other investigations of the chemisorbed layer. For these reasons, electrode modification via chemisorption was quickly supplanted by other methods, most notably polymer-coating methods. 

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It is important to note that the migration barrier Em of isolated adsorbates and the barrier encountered in collective diffusion Ed are a priori unequal, although they are related and become identical in the zero coverage limit [13,20],... 

The first qualitative observation of vacancy-induced motion of embedded atoms was published in 1997 by Flores et al. [20], Using STM, an unusual, low mobility of embedded Mn atoms in Cu(0 0 1) was observed. Flores et al. argued that this could only be consistent with a vacancy-mediated diffusion mechanism. Upper and lower limits for the jump rate were established in the low-coverage limit and reasonable agreement was obtained between the experimentally observed diffusion coefficient and a theoretical estimate based on vacancy-mediated diffusion. That same year it was proposed that the diffusion of vacancies is the dominant mechanism in the decay of adatom islands on Cu(00 1) [36], which was also backed up by ab initio calculations [37]. After that, studies were performed on the vacancy-mediated diffusion of embedded In atoms [21-23] and Pd atoms [24] in the same surface. The deployment of a high-speed variable temperature STM in the case of embedded In and an atom-tracker STM in the case of Pd, allowed for a detailed quantitative investigation of the vacancy-mediated diffusion process by examining in detail both the jump frequency as well as the displacement statistics. Experimental details of both setups have been published elsewhere [34,35]. A review of the quantitative results from these studies is presented in the next subsections. 

From the calculated adsorbate-substrate bond length a substantial covalent character of alkali bonding to transition metal surfaces is deduced in the low coverage limit. A spin-polarized calculation shows that the unpaired spin of the alkali atom is almost completely quenched upon chemisorption. 

In the case of Cu 100 /In, much work has been performed in the low coverage limit. Breeman and Boerma have used time-of-flight LEISS with 6 keV Ne" ions as probes to investigate the In adsorption site as a function of temperature of a stepped Cu sample (Cu 17,l,l =8.5 100 x 100 ) at an In coverage of 0.013 ML. At such low coverages, all In atoms deposited are alloyed with 92% on terrace sites and the remaining 8% embedded at step edges... 

Figure 14 The DFT calculated potential energy profile for ethylene hydrogenation over Pd in the zero coverage limit. The sequence involves the dissociative adsorption of H2, the adsorption of ethylene, the addition of hydrogen to form ethyl, and the addition of hydrogen to ethyl to form ethane. (Adapted from Ref. [84].)...

The deposition of Os on Pt(l 11) occurred much faster than that of Ru. Comparing the surfaces obtained under the deposition conditions of 1 min from a 0.1 mM Os solution and 3 min from a 1 mM Ru solution (see Fig. 16a), a similar Os coverage vs. the Ru coverage limit was attained even though the concentration of Os in the depositing solution was ten times lower than that of Ru and the depositiontime was shorter. This was observed irrespective of the electrolyte composition or upon the change from perchloric to sulfuric acid medium. 

In the special but not uncommon situation where the intermediates of a reaction are adsorbed onto the surface of the electrode substratef the activities of the intermediates in the surface region, as they are used in the present derivation, would have to be replaced with surface coverage fractions (0jS) since in this case the available electrode substrate area limits the extent of the reaction. If the electrode were to become completely covered by one or more of the intermediates (as with the application of a larger driving force q ), the potential dependence of the rate (i.e the Tafel slope, b) would no longer include the contribution that would otherwise arise for the non-rds coverage-limited reaction steps. 

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