Adsorbate vibrational structure

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

At the time of a recent review [9], there remained very few examples of vibrational studies of adsorbate, or localised substrate modes, at metal oxide surfaces. By far the majority of studies concerned the characterisation by HREELS of phonon modes (such as Fuchs-Kliewer modes) pertaining to the properties of the bulk structure, rather than the surface, or to electronic transitions. Such studies have been excluded from this review in order to concentrate on the vibrational spectroscopy of surface vibrations on well-characterised metal oxide surfaces such as single crystals or epitaxially grown oxide films, for which there is now a substantial literature. Nevertheless, it is important to briefly describe the electronic and phonon properties of oxides in order to understand the constraints and difficulties in carrying out RAIRS and HREELS with sufficient sensitivity to observe adsorbate vibrations, and more localised substrate vibrational modes. 

Gaseous pyridine adsorbed on to a sublimed KCl film in vacuo displayed a broad absorption maximum at 255 mp without the vibrational structure of the vapor spectrum (47, 47a). With respect to the onset of the latter, this band was located at higher frequencies and occupied the same range as that of a microcrystalline sublimed layer of pyridine at — 180°C, which, however, exhibited a distinct vibrational structure of the spectrum. A similar behavior has been observed for a-picoline. 

SERS provides a means to study adsorbed molecules through their vibrational structure. However, a simple comparison is, alas, not straightforward in most cases. Besides the chemical changes which can occur, we still do not know in what way the enhancement mechanism affects the various vibrational bands, shifts them, and changes their relative intensities. It is established that the selection rules on the surface are different than those which pertain to a bulk situation. Hexter and Albrecht have analyzed the selection rules by assuming that the metal surface is approximately a perfect mirror. 

Electron tunneling spectroscopy applied in a different experimental configuration can yield the vibrational structure of adsorbates. For example, by adsorbing a monolayer of molecules at an aluminum oxide-lead interface, the vibrational spectrum of benzoic acid was obtained by plotting d V/dP, the second derivative of the applied voltage with respect to the tunnel current, versus the applied voltage V. The result is shown in Figure 5.19. The experiment was performed at 4.2 K. 

The next section will deal briefly with experimental techniques many of these have been introduced already, but the use of vibrational spectroscopy and of sum-frequency generation call for some further description. Section 4.4.1 describes the principal types of adsorbed hydrocarbon structure that have been found with alkenes and alkynes (aromatic hydrocarbons and cyclic Ce species will be considered in Chapters 10 and 12 respectively) Section 4.4.2 discusses the conditions under which the several chemisorbed forms of alkenes make their appearance. In Section 4.5 we look at detailed structural studies of a few adsorbed molecules, and Section 4.6 deals somewhat briefly with interconversions and decompositions of adsorbed alkenes, and structures of species formed. Finally there are sections on theoretical approaches (4.7), on the chemisorption of alkanes (4.8), and carbonaceous deposits that are the ultimate product of the decomposition process (4.9). 

Localized chemical processes, such as desorption and ablation, stimulated by resonant laser pulse-surface layer interaction have been discovered recently. In this lecture the essential theoretical features of the desorption induced by resonant excitation of adsorbate vibrations with laser infrared and their influence on yield, rate, and quantum efficiency are presented. Results on selective damage to pigmented biological structures by short resonant optical and ultraviolett laser pulses are briefly reported. 

The p(2x2) 0/ Ni(lll) system is reconstructed with a twist deformation of three of the top layer nickel atoms and a vertical displacement of all of the atoms in the top layer of the unit cell (LEED [90Gri]). The oxygen coverage is 0.25 ML. A schematic view of the structure of the p(2x2) overlayer is reported in Fig. 44. The oxygen lifts three of the nickel atoms away from their original bulk positions, while the fourth relaxes towards the second layer Ni atoms. The surface phonon dispersion measured by HREELS is reported in Fig. 45. Five optical modes are observed. The modes at 67 and 71 meV are assigned to oxygen adsorbate vibrations, while the lower modes lie within the bulk bands. The open (filled) circles... 

Thus the entropy of localized adsorption can range widely, depending on whether the site is viewed as equivalent to a strong adsorption bond of negligible entropy or as a potential box plus a weak bond (see Ref. 12). In addition, estimates of AS ds should include possible surface vibrational contributions in the case of mobile adsorption, and all calculations are faced with possible contributions from a loss in rotational entropy on adsorption as well as from change in the adsorbent structure following adsorption (see Section XVI-4B). These uncertainties make it virtually impossible to affirm what the state of an adsorbed film is from entropy measurements alone for this, additional independent information about surface mobility and vibrational surface states is needed. (However, see Ref. 15 for a somewhat more optimistic conclusion.)... 

Diffraction is not limited to periodic structures [1]. Non-periodic imperfections such as defects or vibrations, as well as sample-size or domain effects, are inevitable in practice but do not cause much difSculty or can be taken into account when studying the ordered part of a structure. Some other forms of disorder can also be handled quite well in their own right, such as lattice-gas disorder in which a given site in the unit cell is randomly occupied with less than 100% probability. At surfaces, lattice-gas disorder is very connnon when atoms or molecules are adsorbed on a substrate. The local adsorption structure in the given site can be studied in detail. 

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.

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