The Adsorbate-Adsorbent Bond

The immediate site of the adsorbent-adsorbate interaction is presumably that between adjacent atoms of the respective species. This is certainly true in chemisorption, where actual chemical bond formation is the rule, and is largely true in the case of physical adsorption, with the possible exception of multilayer formation, which can be viewed as a consequence of weak, long-range force helds. Another possible exception would be the case of molecules where some electron delocalization is present, as with aromatic ring systems. 

The physisorption bond, being relatively weak, is even more difficult to characterize than the chemisorption one. Some aspects of this are covered in Section XVII-10. 

A phenomenon that certainly involves the adsorbent-adsorbate interaction is that of surface-enhanced resonance Raman spectroscopy, or SERS. The basic observation is that for pyridine adsorbed on surface-roughened silver, there is an amazing enhancement of the resonance Raman intensity (see Refs. 124—128). More recent work has involved other adsorbates and colloidal 

The following data have been obtained for a sample of coal mine dust (see Ref. 2) (mean particle diameter, /tm surface area, m /g) (2.40 8.61), (2.82 6.93), (5.95 5.28), (8.00 4.51). Make the appropriate plot and obtain D. Comment on the result. 

The contact angle for water on single-crystal naphthalene is 87.7° at 35°C, and ddjdT is -0.13 deg/K. Using data from Table III-l as necessary, calculate the heat of immersion of naphthalene in water in cal/g if a sample of powdered naphthalene of 10 m /g is used for the immersion study. (Note Ref. 135.) 

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The activity of a catalyst depends on the nature, the number, the strength and the spatial arrangement of the chemical bonds that are transiently created between the reactants and the surface. The objective of the chemical characterization of the surface is a detailed description of the adsorbate-adsorbent bonds that a given catalyst will develop when contacted with a given reaction mixture. Therefore, chemical characterization should be done in situ in the course of the reaction itself. However, because of experimental limitations, this is seldom possible and catalyst surfaces are usually characterized by means of separate experiments. It is important to characterize the catalyst surface both before and after its use in a reaction. 

Another point in interpreting infrared spectra of adsorbed layers is the coupling interaction between adsorbates. This primarily depends on the nature of the adsorbate-adsorbent bond. For CO this bond is mainly covalent-dative, while for NO it is mostly... 

The preexponential factors for each of the reaction steps can, in principle, be estimated using either gas kinetics or transition state theory. Desorption occurs when the adsorbate-adsorbent bond acquires the required activation energy for desorption in the form of vibrational energy. To a first approximation the vibrational frequency can be assumed to be approximately 10 s for the temperature at which desorption proceeds at a significant rate. The frequency of bond rupture is given by... 

The consequence of the localization of the charge is that the strength of the adsorbate-adsorbent bond will only be significantly modified if adsorption occurs... 

Another problem eonneeted with the frustrated modes at the surfaee is their influenee on the intramoleeular vibrations of the adsorbed moleeule. Nitzan and Persson [30] suppose that the high-frequeney intramoleeular vibrations of adsorbed moleeules interaet with the phonons of the solid via low-frequeney oseillations of the adsorbent—adsorbate bond, the latter being regarded as frustrated translational and rotational degrees of freedom of the adsorbed moleeule. In the quantum ease the Hamiltonian of the system ean be expressed as... 

Information on the binding energy, deduced from calorimetric data, is needed to achieve a theoretical description of the adsorbate-adsorbent bond. It has been shown, for instance, that, in the case of the adsorption of hydrogen on nickel-copper alloys, a correlation between heats of adsorption and surface magnetic properties can be found. The correlation indicates that the energy of the bond between adsorbed hydrogen and nickel atoms is regulated by the electron density of states, near the Fermi level, for the metal surface [6-8]. 

The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

The theory developed permits spectral line shift and width to be calculated from Taylor power series for interatomic potential energies in a concrete system. Various methods of tackling this problem can be found in the literature140,169,171,176 180 (see also survey 181 and references cited therein). Here we invoke a realistic model for the coupling of two mutually perpendicular vibrations which was reported by Burke, Langreth, Persson, and Zhang.1 As in Ref. 1, write the Hamiltonian for the interaction between the modes uh and w, in polar coordinates r and 6, where 6 is the angle between the adsorbate bond and the perpendicular to the surface plane ... 

In the gas phase the dipole moment determined through Eq. (4.10) refers to an individual adsorbed particle. This is not so in the electrochemical situation. The dipole moment of an adsorbed species will tend to align neighboring solvent molecules in the opposite direction, thereby reducing the total dipole potential drop (see Fig. 4.3). Only the total change in dipole potential can be measured, and there is no way of dividing this into separate contributions from the adsorbate bond and the reorientation of the solvent. The apparent dipole potential of an ion adsorbed from a solution on a particular metal is often substantially smaller than that of the same ion adsorbed in the vacuum (see Table 4.1), since it contains a contribution from the solvent. For comparison we note that the dipole moments of alkali ions adsorbed from the vacuum are usually of the order of the order of 10 29 C m. 

For atomic H adsorption on surfaces the electronic structure as obtained by UPS studies and DFT calculations on Ni, Pd, and Pt shows a similar picture. There is a strong bonding H-induced feature around 7-9 eV below the Fermi level observed both in UPS and band structure calculations [43]. This has been related to that the H Is level interacts with both the metal -and 7-bands. Since the H Is level is much lower in energy in comparison with the previously discussed adsorbates, for which the outer level was of p character, it is anticipated that the metal s-electrons will be more strongly mixed into the adsorbate bonding resonance. Since no X-ray spectroscopy measurements can be conducted on H it is difficult to derive how much H Is character there is in the 7-band region, respectively, above the Fermi... 

Together with Eq. (12) this shows AEd to be an appropriately weighted average of the contributions from the different metal atoms to which the adsorbate bonds (where the matrix element is non-negligible). 

Influence of the support acid/base properties on the adsorbate bonding... 

Determination of the adsorbate bonding geometries for CO chemisorbed on the Cu (100) surface provides an interesting example of the A a-SW method. 

Similar results have been obtained for Ni(100)c(2 x 2)-CO and Ni(100)c(2 X 2)-0. For CO best agreement between theory and experiment is found for the adsorbate bonded in the atop or terminal position, in agreement with LEED observations. It has been emphasized by Plummer and Gustafsson that at present the interpretation of experimental data depends heavily on comparison with theoretical calculations for each individual adsorbate-surface combination. General symmetry arguments (selection rules), however, are now being formulated which should allow identification of the symmetry of the initial-state orbital or adsorption site, without complicated calculations. 

For dissociative adsorption of H2 we pointed out the importance of reducing repulsive interactions between doubly occupied orbitals and the need for easily accessible occupied orbitals, asymmetric with respect to the reaction co-ordinate, that backdonate electrons into antibonding adsorbate orbitals. Occupation of the latter weakens the adsorbate bonds and reduces the activation energy for dissociation. 

The activation barriers of surface dissociation reactions are low compared to the energies of the adsorbate bonds that are to be broken. Often one finds a value -10% of the bond energy. This arises from the stabilization of the stretched bond by the stabilizing interaction with the metal surface. 

From Table 2 it appears that on passing from carbon black and aerosil to carbosil the thickness of the solvation shell of benzene increases and the hydration film decreases. The studies of changes of chemical potential of water molecules at the adsorbent/bonded wa-ter/ice interface depending on water layer thickness are presented in another paper [57]. For the initial silica the surface effect is confined to the adsorbent water monolayer. Poor carbonization of aerosil surface causes the increase of water layer thickness to 40-50 molecular diameters. With the increase of carbon constituent part on the complex adsorbent surface, the thickness of interfacial water layer decreases to 15 molecular diameters. 

The adsorbate bond is surface-structure-sensitive, and adsorbate-induced surface restructuring frequently occurs. Rough surfaces (with lower atomic coordination) restructure more readily. 

ESDIAD) [42]. This method was considerably improved by incorporation of a digital acquisition system and background subtraction [43] and provides some information about the s)rm-metry of the adsorption site and the direction of the adsorbate bond [35]. 

This attractive backdonative contribution to the adsorbate bond energy increases with increasing electron occupation of the bonding surface fragment orbitals of the surface complex. This happens when d-valence electron occupancy of the... 

The adsorbate bond energy increases with increase in the degree of coordinatively unsaturated metal atoms. This is due to the decrease in the localization energy of electrons on the Au surface atoms for structures with fewer neighboring atoms. 

As we will discuss in more detail in Chapter 3, the delocalization of electrons is proportional to the square root of the number of coordinating atomsl l. One would therefore expect adsorbate binding energies to increase with decreasing particle size, owing to the increased number of coordinatively unsaturated surface atoms. The reactivity of these particles with respect to cluster size will then depend the position of the adsorbate bond energy with resp>ect to the Sabatier curve maximum. 

Mavrikakis et al.I have nicely shown that the strain induced on the metal-metal bonds by the misalignment of the metal lattice to the registry of the oxide support leads to a shift in the center of the d-band. This change in the electronic structure alters the adsorbate bond strength at these sites, which ultimately dictates the reactivity of the metal. While these effects may die off for large particles on the support, they can clearly play a role for smaller nanoparticles that are in direct contact with the support. 

Calculations for NH3 chemisorbed to Cu clusters which simulate the Cu (100) surface illustrate the change in bonding character of the adsorbate bond with the metal surface nicely. Figure 3.4 shows the orbital interaction of the NH3, lone pair orbital with Cu(4s), Cu(4p) and Cu(3d ) orbitals. Both the OPDOS (the Overlap Population Densities of States) and Local Density of States (LDOS) are shown. 

If the d-valence electron band and the 5cr-orbital are both completely filled with electrons, the interaction energy will be repulsive since Pauli repulsion is proportional to the overlap of S and /3. When the d-electron valence bond is partially empty, this repulsive interaction is decreased because the antibonding orbital fragments become less occupied. The decrease in repulsive energy with a decrease in number of metal-atom neighbors of the surface atom involved in the adsorbate bond relates to an increase in the number of empty antibonding orbitals, determined by the electron density between ( ax (see Fig. 3.10). 

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