Continuous adlayer

Fig. 23 Cross sections of models used to account for the heterogeneity of the zone analyzed by XPS (a) Continuous adlayer of constant thickness r, (b) distribution of an element at the atomic scale (c) adlayer of thickness t and fractional coverage

The influence of the adventitious contamination present on high surface energy sohds, such as oxides or metals, has been discussed in the sections Influence of Adsorbed Contaminants and Continuous Adlayer. Note that these organic compounds also contain oxygen. " ... 

In the following, after a brief description of the experimental setup and procedures (Section 13.2), we will first focus on the adsorption and on the coverage and composition of the adlayer resulting from adsorption of the respective Cj molecules at a potential in the Hup range as determined by adsorbate stripping experiments (Section 13.3.1). Section 13.3.2 deals with bulk oxidation of the respective reactants and the contribution of the different reaction products to the total reaction current under continuous electrolyte flow, first in potentiodynamic experiments and then in potentiostatic reaction transients, after stepping the potential from 0.16 to 0.6 V, which was chosen as a typical reaction potential. The results are discussed in terms of a mechanism in which, for methanol and formaldehyde oxidation, the commonly used dual-pathway mechanism is extended by the possibility that reaction intermediates can desorb as incomplete oxidation products and also re-adsorb for further oxidation (for the formic acid oxidation mechanism, see [Samjeske and Osawa, 2005 Chen et al., 2006a, b Miki et al., 2004]). 

Combining the results of the spectroscopic and kinetic experiments allows us to partially confirm one aspect of the mechanism for CO oxidation. The IRRAS results showed that the nitrile molecules are able to disrupt the continuity of the CO adlayer at the molecular scale by blocking a fraction of the surface Pt atoms. This is in contrast to the procedure of preparing a partial CO adlayer by controlling the exposure to bulk solution, which produces a uniformly distributed CO layer without blocking Pt atoms by a second adsorbate (12). The fact that there is no evidence for enhanced CO oxidation kinetics in the solutions with added nitriles indicates that the presence of an exposed Pt atom next to the... 

Although in-situ STM provided overall information of the structural changes in both phases and helped to obtain accurate lattice parameters in the LEED analysis [13, 14], the STM technique alone could not capture such small variations in the adlattice. Our work clearly demonstrates that complementary use of LEED and STM is a powerful technique more easily available in ordinary laboratories compared with surface X-ray scattering to determine accurate structural parameters of adlayers on electrode surfaces. We have recently found that the iodine adlayers on Ag(lll) were also continuously compressed with changing electrode potential [15], which is in contrast to the result reported previously [25]. 

Phase-formation phenomena at electrode-electrolyte interfaces can be conveniently treated with lattice gas concepts [38, 60, 67]. Such models consider that the entities, atoms, ions, or molecules, are fixed to particular cells /, j. (M. Fisher explicitly pointed out ... that instead of imagining the particles confined to lattice sites, one may suppose that they move continuously in space divided into cells, but that their interactions are determined solely by which particular cells are occupied [52].) The configurational energy of the adlayer on a (L x L) square lattice is given, as an example, by the following Grand Canonical Hamiltonian [38, 56, 57, 63]... 

The continuous change of the bromide coverage with potential upon the transition c(.y2 X 2. 2) R45° —> c(. 2 x p)R45 can be approximated by the power law 6 = 0.122 (E - Ecf + 0.50 with p = 0.40 and Ec = 0.375 V. The measured exponent is smaller than the theoretical value p = 0.50 predicted by Pokrovsky and Talapov for the C UIC transition within a model of noninteracting domain walls [70]. The theoretical prediction is only supposed to apply close to the transition, where the domain walls are narrow relative to their separation. The experimental precision does not permit to quantitatively extract the exponent close to the transition, in which the theoretical prediction could be unambiguously tested. At higher incommensurabilities the bromide coverage is not only determined by the proximity to the C/UIC phase transition, but rather by the compressibility of the bromide adlayer, which may explain the deviations in the exponent. The width of the X-ray diffraction (XRD) peaks in the incommensurate direction scale quadratically with the wave vector component in this direction, and increase continuously by an order of magnitude as the C/UIC transition is approached from the UIC side. 

Fig. 21 Proposed mechanism of the UPD of Pb-mediated deposition of Ag. The Ag and Pb atoms are light and dark shaded spheres, respectively. Arrows indicate depositing metal cations, (a) Pb mediation at potentials close to reversible Pb deposition, a Pb monolayer covers the surface. The deposited Ag adatoms undergo interlayer place-exchange with Pb adatoms (light green sphere) forming 2D islands below the Pb adlayer. (b) On the reverse cycle, the Pb adlayer is stripped from the surface as Ag continues to deposit resulting in island growth. (Reproduced with permission from Ref [198].)...

After 30 years of continuing investigation, the adsorption properties of the noble ses on metal and semiconductor surfaces have recently attracted renewed interest. On the one hand, some fundamental aspects have come within the reach of modem experimental and theoretical techniques, sueh as the very nature of physisorption and the noble gas - substrate interaction, the possibility to study growth and surface kinetics at the atomic scale, and the recent interest in nanoscale surface friction and related tribological issues, where noble gas adlayers serve as model systems [99P]. On the other hand, noble gas adsorption is being used as a non-destmctive and quantitative surface analytical tool as, for instance, in photoemission of adsorbed xenon (PAX) [97W] and for titration analysis of heterogeneous surfaees based on the site specificity of the interaction strength [96S, 98W]. 

In Tables 6-8 we have listed the submonolayer and monolayer stractures of metals on the principal low index surfaces of metal substrates from 2- to 4-fold rotational symmetry. This compilation comprises heteroepitaxial systems only since the structure of homoepitaxial systerrrs is in most cases trivial. For umeconstructed surfaces the bulk stacking is pseudomorphically continued and for reconstructed ones the reconstruction is lifted below the adlayer and at the same time taken on by the adsorbate layer. Only few homoepitaxial cases are worth mentioning since their reconstructions can metastably be lifted, as seen for Au/Au(110)-(lx2) [97Giin], or a reconstruction can be induced at a lower tenqreratrrre by homoepitaxial adsorption, as seen for Pt/Pt( 111) [93Bot]. 

---