Adsorption energy surface

To model the substrate we have used the Dual Site-Bond Model (DSBM) [S], which has proved to be useful to analyze topography effects on several molecular processes on heterogeneous surfaces. DSBM provide a statistical description of the disordered media based on two elements sites, and the corresponding saddle points (bonds) the adsorptive energy surface is described by site and bond probability density functions Fs( s) Fb(Eb)- The distribution functions S and B associated to F and Fb are defined by... 

Figure 10.1 Adsorption Energy Surface (AES) for a crystal of atoms A with 20% impurity of atoms B. Adapted from Ref. 25.

Structural and dynamic properties of pure water in contact with uncharged realistic metal surfaces are obtained by molecular dynamics simulations. The influences of adsorption energy, surface corrugation, electronic polarizability and surface inhomogeneity are investigated. The adsorption energy of water on the metal surface is found to be the most important parameter. 

Alternatively, an integral distribution function F may be defined as giving the fraction of surface for which the adsorption energy is greater than or equal to a given Q,... 

With the aid of (B1.25.4), it is possible to detennine the activation energy of desorption (usually equal to the adsorption energy) and the preexponential factor of desorption [21, 24]. Attractive or repulsive interactions between the adsorbate molecules make the desorption parameters and v dependent on coverage [22]- hr the case of TPRS one obtains infonnation on surface reactions if the latter is rate detennming for the desorption. 

Olsen R A, Philipsen P H T, Baerends E J, Kroes G J and Louvik O M 1997 Direct subsurface adsorption of hydrogen on Pd(111) quantum mechanical calculations on a new two-dimensional potential energy surfaced. Chem. Phys. 106 9286... 

The potential energy 0(z) depends not only on the distance z hut also on the position of the gas molecule in the xy plane parallel to the surface of the solid and distant z from it. For any given position, the adsorption energy will be equal to the value of 0 = 0o minimum of the potential curve (cf. Fig. 1.2), which of course represents the equilibrium position. 

It should be loted that with low-energy surfaces the sudden fall in the heat of adsorption is absent. This is illustrated in Fig. 2.15, where the contrast between the behaviour of nitrogen on the carbons (high-energy surfaces) and on the molecular solids (low-energy surfaces) is very clear. 

FIG. 4 Normalized oxygen density profile perpendicular to the surface from simulations of pure water with adsorption energies of 12, 24, 36, and 48 kJ/mol (from bottom to top). The lower curves are shifted downwards by 0.5, 1.0, and 1.5 units. The inset shows the height of the first (diamonds) and second peak (crosses) as a function of adsorption energy. Water interacts with the surface through a Morse potential. (From Ref. 98.)... 

Fig. 5(a) contains the oxygen and hydrogen density profiles it demonstrates clearly the major differences between the water structure next to a metal surface and near a free or nonpolar surface (compare to Fig. 3). Due to the significant adsorption energy of water on transition metal surfaces (typically of the order of 20-50kJmoP see, e.g., [136]), strong density oscillations are observed next to the metal. Between three and four water layers have also been identified in most simulations near uncharged metal surfaces, depending on the model and on statistical accuracy. Beyond about...

---