Adsorbate coverage

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

We now consider how one extracts quantitative infonnation about die surface or interface adsorbate coverage from such SHG data. In many circumstances, it is possible to adopt a purely phenomenological approach one calibrates the nonlinear response as a fiinction of surface coverage in a preliminary set of experiments and then makes use of this calibration in subsequent investigations. Such an approach may, for example, be appropriate for studies of adsorption kinetics where the interest lies in die temporal evolution of the surface adsorbate density N. ... 

For other purposes, obtaining a measure of the adsorbate surface density directly from the experiment is desirable. From this perspective, we introduce a simple model for the variation of the surface nonlinear susceptibility with adsorbate coverage. An approximation that has been found suitable for many systems is... 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

The siipercell plane wave DFT approach is periodic in tliree dimensions, which has some disadvantages (i) thick vacuum layers are required so the slab does not interact with its images, (ii) for a tractably sized unit cell, only high adsorbate coverages are modelled readily and (iii) one is limited in accuracy by the fonn of the... 

Figure 8 shows an example of the most common behavior of AEam/0 as a function of adsorbate coverage. Linear behavior, if ever observed, is seen at the air/solution interface.93 At metal/solution interfaces, if chemical interactions with the metal can be ruled out, electrostatic interactions cannot be avoided, and these are responsible for the downward curvature.91 Upward curvatures are often observed at air/solution interfaces as a consequence of lateral interactions.95... 

If the pumping speed is infinitely high, readsorption may be ignored, and the relative rate of desorption, defined as the change in adsorbate coverage per unit of time, is given by... 

The data in Figs. 2 and 3 suggest a reaction which requires a delicate balance between adsorbate coverages, consistent with a Langmuir-Hlnshelwood mechanism. More extensive data of this type (24-27) indicate that molecularly adsorbed ethylene and O2 are the critical species, consistent with the mechanism proposed below. 

The effects of adsorbate coverage (film thickness) on the Pd 3d5/2 XPS peak positions of the Pd/W(l 1 0), Pd/ Re(0001), and Pd/Mo(l 10) systems were systematically investigated [63]. The peak positions reported for Pd coverage in excess of 1 ML represent a product of electrons emitted from surface and subsurface atoms. For the case of Pd(lOO), theoretical calculation suggest that the Pd 3d5/2 XPS BE of the surface atoms is 0.4 eV lower than that of bulk Pd. A similar difference has been observed experimentally for Ni and Pt surfaces. These shifts in BE are a consequence of variations in the coordination number of the surface atoms compared to bulk atom. If we reference the combined peak of bulk and surface atoms in 40 ML of Pd on W(1 1 0) to that of Pd(l 00) a difference of —0.8 eV is obtained between the Pd 3ds/2 BE of a pseudomorphic monolayer of Pd on W(110) and that of the surface atoms of Pd(l 00). The corresponding shifts... 

In this section, we will present and discuss cyclic voltammetry and potential-step DBMS data on the electro-oxidation ( stripping ) of pre-adsorbed residues formed upon adsorption of formic acid, formaldehyde, and methanol, and compare these data with the oxidative stripping of a CO adlayer formed upon exposure of a Pt/ Vulcan catalyst to a CO-containing (either CO- or CO/Ar-saturated) electrolyte as reference. We will identify adsorbed species from the ratio of the mass spectrometric and faradaic stripping charge, determine the adsorbate coverage relative to a saturated CO adlayer, and discuss mass spectrometric and faradaic current transients after adsorption at 0.16 V and a subsequent potential step to 0.6 V. 

Oxidation of the adsorbed species resulting from interaction with formaldehyde, formic acid, and methanol, respectively, leads to stripping peaks that are downshifted to more negative potentials. Furthermore, the adsorbate coverage is significantly lower... 

TABLE 13.1 Number of Electrons per Resulting CO2 Molecule Required for Oxidizing the Stable Adsorbed Decomposition Product from Adsorption of Ci Molecules to CO2, and Ci Adsorbate Coverage Relative to that of a Saturated CO Adlayer after Adsorption of Ci Reactants"... 

At higher potentials, positive of the Hupd OHad exchange, the C Vs of the Pt island-modified Ru(OOOl) surface closely resemble those of the ft-fiee Ru(OOOl) electrode, except for the lower currents/charges in the characteristic features. This simply reflects the fact that at these potentials, the surface reactivity is dominated by the electrochemical properties of the remaining exposed Ru surface. As already mentioned, the Pt monolayer islands themselves contribute only little to the voltammetric behavior, which is due to the weak bonding and hence low adsorbate coverages on these islands. 

Figure 14.11 Dlustration of CO electro-oxidation at Pt-modified Ru(OOOl) (a) mixed, non-leactive adlayer (b) Pt-assisted formation of OH d at high local adsorbate coverages on the Ru areas (c) CO oxidation at the Pt islands. For simplicity, H is used instead of H30. ...

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

Figure 3.3. Schematic representation of the adsorption, surface diffusion, and surface reaction steps identified by surface-science experiments on model supported-palladium catalysts [28]. Important conclusions from this work include the preferential dissociation of NO at the edges and defects of the Pd particles, the limited mobility of the resulting Nads and Oads species at low temperatures, and the enhancement in NO dissociation promoted by strongly-bonded nitrogen atoms in the vicinity of edge and defect sites at high adsorbate coverages. (Figure provided by Professor Libuda and reproduced with permission from the American Chemical Society, Copyright 2004).

In conclusion, TDS of adsorbates on single crystal surfaces measured in ultrahigh vacuum systems with sufficiently high pumping speeds provides information on adsorbate coverage, the adsorption energy, the existence of lateral interactions between the adsorbates, and the preexponential factor of desorption, which in turn depends on the desorption mechanism. Analysis of spectra should be done with care, as simplified analysis procedures may easily give erroneous results. 

Figure 3 shows plots of the relative IgER values against the relative adsorbate coverage, 0., for two such adsorbates at silver, Ru(NH3)3+ and Os(NHg) jpy3 (py - pyridine), extracted from the corresponding potential-dependent data in references 6a and 17, respectively. Both these species are adsorbed by electrostatic attraction to the chloride monolayer present under the experimental conditions employed (18.). The... 

Plots of relative SERS band Intensity, ISEH, against relative adsorbate coverage, 0rel, for Ru(NH3)j +, Os(NH3)5py3+ (py -pyridine), and Br adsorbed on silver, extracted from corresponding ISER-potential and -potential data. Experimental data taken from references 6a, 17, and 19, respectively (see text and the references for further details). 

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