Molecule/surface site bonding energy

Owing to the spatial dependencies of electrostatic potentials, derivation of equations describing electrostatic influences on the adsorption process is difficult. The primitive interfacial is a model that solves this problem by regarding all adsorbed species as siuface sites in a monolayer (the Stem layer). Further, the surface sites (bonded or not bonded to adsorbed solute molecules or ions) are required to be spherical in geometry with the same diameter. For polymeric adsorbates, these restrictions would be a very poor electrostatic model. For polymer adsorption the same electrostatic and free energy equations can be applied, but the restriction of the entire polymer molecule residing in the Stem layer must be eliminated. 

Fundamental is that the atoms in the surface pha.se are not fully co-ordinated. These sites are often called Co-ordinatively Unsaturated Sites (CUS) . These sites chemisorb molecules because upon formation of bonds with the adsorbing molecules the Gibbs free energy is lowered. 

We will now compare the N2 system to the much more studied isoelectronic CO molecule adsorbed on Ni(100). Like N2, CO adsorbs in a c(2 x 2) overlayer structure on Ni(100), occupying on-top sites with the carbon end down with a C—Ni distance of 1.73 A, see Chapter 1 for details. However, the adsorption energy of 1.2 eV [63] is much higher in comparison to that of N2. It is therefore very interesting to see how the difference in electronegativity of the carbon and oxygen atoms influences the surface-chemical bond in comparison to the isoelectronic N2. 

If the pressure exceeds about 10 mm., a third layer is adsorbed. At least part of this is chemisorbed as molecules with a bond strength of about 2.0 ev. Even at room temperature these molecules may change their orientation with respect to the substrate. O2 and O4 molecules are observed frequently, O3 and Oe less frequently. Some of these molecules have vibrational and rotational degrees of freedom. These molecules do not migrate over many atom diameters or lattice sites before they disappear in the substrate or evaporate. There are probably physisorbed molecules in the third layer which are held with smaller energies and which migrate over the surface even at 300°K. 

The goal of theoretical surface chemistry is to understand the surface chemical bond, and from this to be able to describe and predict the properties of atoms and molecules adsorbed on surfaces. The primary properties of interest include adsorption sites and geometries, bond lengths and angles, the electronic structure of adsorbed species, adsorption energies, diffusion energies, and the... 

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