Adsorption geometries

Figure Bl.22.2. RAIRS data from molecular ethyl bromide adsorbed on a Pt(l 11) surface at 100 K. The two traces shown, which correspond to coverages of 20% and 100% saturation, illustrate the use of the RAIRS surface selection nde for the detemiination of adsorption geometries. Only one peak, but a different one, is observed in each case while the signal detected at low coverages is due to the asymmetric defomiation of the...

Figure Bl.22.3. RAIRS data in the C-H stretching region from two different self-assembled monolayers, namely, from a monolayer of dioctadecyldisulfide (ODS) on gold (bottom), and from a monolayer of octadecyltrichlorosilane (OTS) on silicon (top). Although the RAIRS surface selection rules for non-metallic substrates are more complex than those which apply to metals, they can still be used to detemiine adsorption geometries. The spectra shown here were, in fact, analysed to yield the tilt (a) and twist (p) angles of the molecular chains in each case with respect to the surface plane (the resulting values are also given in the figure) [40].

Figure Bl.22.6. Raman spectra in the C-H stretching region from 2-butanol (left frame) and 2-butanethiol (right), each either as bulk liquid (top traces) or adsorbed on a rough silver electrode surface (bottom). An analysis of the relative intensities of the different vibrational modes led to tire proposed adsorption structures depicted in the corresponding panels [53], This example illustrates the usefiilness of Raman spectroscopy for the detennination of adsorption geometries, but also points to its main limitation, namely the need to use rough silver surfaces to achieve adequate signal-to-noise levels.

Figure Bl.22.10. Carbon K-edge near-edge x-ray absorption (NEXAFS) speetra as a fiinotion of photon ineidenee angle from a submonolayer of vinyl moieties adsorbed on Ni(lOO) (prepared by dosing 0.2 1 of ethylene on that surfaee at 180 K). Several eleetronie transitions are identified in these speetra, to both the pi (284 and 286 eV) and the sigma (>292 eV) imoeeupied levels of the moleeule. The relative variations in the intensities of those peaks with ineidenee angle ean be easily eonverted into adsorption geometry data the vinyl plane was found in this ease to be at a tilt angle of about 65° from the surfaee [71], Similar geometrieal detenninations using NEXAFS have been earried out for a number of simple adsorbate systems over the past few deeades.

As an adsorption geometry one considers a semi-infinite system with an impenetrable wall at z = 0, such that monomer positions are restricted to the positive half-space z > 0. At the wall acts a short-range attractive potential, either as a square well... 

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

Schematic diagrams for adsorption geometries of (c) and (e) are shown in (d) and (f), respectively a linear atop and a tilted off-site CO are implicated. The black (red) circles represent carbon (oxygen) atoms and the large gray circles are silver atoms. The sizes of the circles are scaled to the atomic covalent radii. (Reprinted with permission from Ref. [25]. Copyright 2001, The American Physical Society.)...

The physical properties of probe molecules adsorbed in the confined space of porous materials are known to vary in dependence of structural constraints on molecular motion. Detailed investigations of adsorption geometries are possible, when well-defined sites and loadings exist. This was the case for the adsorption of strongly interacting probe molecules, such as pyridine, on SiOH groups in the... 

Figure 2.9 Thermal desorption of carbon monoxide from two rhodium surfaces in ultrahigh vacuum, as measured with the experimental set-up of Fig. 2,10. Each curve corresponds to a different surface coverage of CO. At low coverages CO desorbs in a single peak indicating that all CO molecules bind in a similar configuration to the surface. At higher coverages, an additional desorption peak appears, indicative of a different adsorption geometry (courtesy of M.J.P. Hopstaken and W.E. van Gennip [141).

The most recent advances in structure determination by LEED make use of holographic effects. In short, adsorbed atoms in an ordered superstructure on the surface act as beam splitters, reflecting a reference wave and transmitting a wave that reflects from the surface as the object wave. Both waves together constitute the holographic image, from which the adsorption geometry can in principle be reconstructed [25]. 

A part of Figure 3 in Ref. 207, reproduced on the right, reports radial EXAFS data around the S Is absorption edge for sulfur adsorbed on the (100) plane of a g nickel single-crystal surface. The top trace corresponds to the deposition of atomic S sulfur by dehydrogenation of H2S, while g, the bottom data were obtained by adsorb- M ing thiophene on the clean surface at 100 K. Based on these data, what can be learned about the adsorption geometry of thiophene Propose a local structure for the sulfur atoms in reference to the neighboring nickel surface. 

Adsorption geometry Adsorption energy (eV) 0—0 bond length (nm) Fe —0 bond length (nm) Fe —0 bond length (nm)... 

From Fig. 4.25, the Q and B transitions appear for coverages of 0.3 nm and higher. This threshold value corresponds, in the hypothesis of a fiat-lying adsorption geometry, to the saturation of the surface with one ML. In this framework molecules... 

The adsorption and reduction of N03 ions at Au and Pt electrodes was studied by in situ fourier transform infrared (FTIR) spectroscopy [55]. Possible adsorption geometries were suggested for adsorbed nitrate ions and for nitrite ions formed by reduction. 

Figure 17 The adsorption geometry of (R,R)-bitartrate on Cu(llO). The dashes protruding from the tartrate oval represent the tartrate hydroxyl groups sticking out...

Fig. 6.2. Top and side views (in top and bottom sketches of each panel) of adsorption geometries on various metal surfaces. Adsorbates are drawn shaded. Dotted lines represent clean-surface (relaxed) atomic positions arrows show atomic displacements due to adsorption...

Turning to metal substrates, in most cases of atomic adsorption on metal surfaces where the adsorption geometry has been determined (cf. Table 6.1), only one adsorption site is involved, i.e., all adatoms have identical surroundings [the exceptions are Ni(l 11)... 

Fig. 7.1 a-c. The co-adsorption geometry of S (small shaded circles) and Na (large shaded circles) on Ni(lOO) (open circles), in top and side views (a) half-monolayer of S and half-monolayer of Na (b) half-monolayer of S and quarter-monolayer of Na (c) quarter-monolayer of S and quarter-monolayer of Na... 

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