Adsorbate structures

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.)... 

In many cases the number of occupied adsorption sites is not equivalent to the number of unit cells. Often, repulsive interactions bet veen adsorbed species prevents filling of all sites and adsorption may only be possible if all neighboring sites are unoccupied. Adsorbate structures are described according to hoiv the neiv unit cell of adsorbate and substrate relate to the original unit cell of the substrate. Figure 5.7 shows a few examples along with the nomenclature used. 

The unit cell of the p(2x2) adsorbate structure in Fig. 5.7 is twice as large in both directions as the unit cell of the substrate and hence the structure is called p(2x2), where the p stands for primitive. The coverage corresponds to 0.25 monolayers (commonly abbreviated to ML). 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

The application of infrared photoacoustic spectroscopy to characterize silica and alumina samples is reported. High quality infrared photoacoustic spectra illuminate structural changes between different forms of silica and alumina, as well as permit adsorbate structure to be probed. Adsorption studies on aerosil suggest adsorbed species shield the electric fields due to particle-particle interactions and induce changes in the vibrational spectra of the adsorbates as well as in the bulk phonon band. It is shown that different forms of aluminum oxides and hydroxides could be distinguished by the infrared spectra. 

Photoemission has been proved to be a tool for measurement of the electronic structure of metal nanoparticles. The information is gained for DOS in the valence-band region, ionization threshold, core-level positions, and adsorbate structure. In a very simplified picture photoemission transforms the energy distribution of the bounded electrons into the kinetic energy distribution of free electrons leaving the sample, which can easily be measured ... 

Figure 8.2 In situ SXS electrochemical cell WE, working electrode CE, counter-electrode RE, reference electrode. On the left is shown the transition from (1 x 1) to (hex) for a Au(lOO) surface and on the right the characteristic adsorbate structures of CO on Pt(lll) commonly observed by SXS.

Lines represent [1 1 0] directions of underlying platinum. A unit cell ofthe adsorbate structure is drawn, (b) Diagram ofthe proposed (y/7 x yj7) R19.T model for the Jt-allyl structure in Figure 7.18a. Inset shows bonding structure. 

Cyclohexadiene and benzene form identical structures on Pt(l 1 1) at low pressures (Figures 7.23 and 7.24). 1,3-Cyclohexadiene dehydrogenates to form benzene on the surface, while benzene adsorbs molecularly. Figure 7.24b schematically shows the adsorbed benzene structure at low pressure. The STM images of the C6 cyclic hydrocarbons show three different adsorbed structures on Pt(l 1 1). Cyclohexene and cyclohexane partially dehydrogenate to form rc-allyl, 1,4-cyclohexadiene adsorbs in a boat configuration, and both 1,3-cylohexadiene and benzene adsorb as molecular benzene on the surface. 

These systematic studies suggest that an intrinsic connection between the adsorbate structure, mobility, and the formation of product can be established with the aid of structural information obtained from high-pressure STM. It further demonstrated the importance of STM in studies of heterogeneous catalysis at high pressure. 

Polarization-dependent surface EXAFS measurements have provided some of the best-defined characterizations of adsorbate structures. 

The relative rate of isobutane isomerization has been shown by Anderson and Avery 24) to be markedly increased by using a (111) platinum film surface. On the other hand, this did not occur with n-butane, nor did it occur with either iso- or n-butane over a (100) platinum surface (cf. Table II). A triangular array of adjacent sites on a (111) platinum surface can be readily fitted by an adsorbed isohydrocarbon, and this structure also fits to allow the carbon orbitals to be directed normally to the surface. On simple geometric grounds, this adsorbed structure is specific to the (111)/... 

Projections of molecular axes onto the surface plane form chain-like structures in which the chains with identically oriented molecules alternate (with the exception of oxygen molecules). The Davydov splitting of spectral lines represents the main spectroscopic manifestation of adsorbed structures with several orientationally inequivalent molecules in the unit cell of a two-dimensional adsorbate lattice. Many... 

Adsorbates may form ordered overlayers, which can have their own periodicity. The adsorbate structure is given with respect to that of the substrate metal. For simple arrangements the Wood notation is used some examples are given in Fig. A.3. The notation Pt (110) - c(2x2) O means that oxygen atoms form an ordered overlayer with a unit cell that has twice the dimensions of the Pt (110) unit cell, and an additional O in the middle. Note that this abbreviation does not specify where the O is with respect to the Pt atoms. It may be on top of the Pt atoms but also in bridged or fourfold sites, or in principle anywhere as long as the periodic... 

Increasing the size of PAHs makes their deposition on surfaces difficult because they can neither be sublimed nor made sufficiently soluble for solution processing. A precursor route has thus been designed according to which molecules are deposited on a surface and transformed into the final disc-type adsorbate structures in a thermal solid-state reaction with the substrate surface acting as a template.1261 An exciting example is the hexaether 41 (scheme 11) which is sublimed onto a Cu-(1U) sur-... 

The Cu adsorbate structure was studied using STM and EXAFS (extended x-ray absorption fine structure) techniques, but it is not yet well understood. UPD-OPD transition is in the range —82 to —71 mV. Bulk fee Cu spacing is reached after deposition of about lOCu monolayers. Holzle et al. (72) have shown that UPD Cu deposition on Au(l 11) is a combined adsorption-nucleation and growth process. 

The adsorbent structure was dependent on the structure of the conjugated polymer precursor and carbon formation conditions. 

Substrate Adsorbed Structure Nearest Heat of Deposition Substrate Technique of Surface structures observed References... 

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