Principles of Monolayer Adsorption

Consider a uniform surface with a number no of equivalent adsorption sites. The ratio of the number of adsorbed atoms or molecules, n, and no is defined as the coverage, d = n/no. The coverage in the monolayer is usually less than or equal to unity for a uniform surface. For a heterogeneous surface that exhibits multiple binding sites, i.e., more than one site per substrate unit cell, small adsorbate atoms may build up coverages somewhat greater than unity. We shall, however, ignore this possibility for the present. 

When adsorption occurs on the clean surface, heat is liberated during the formation of the surface bond. The heat of adsorption, AH ds, associated with the layer of adsorbates reveals the strength of interaction between atoms and molecules in the monolayer and the surface on which they are adsorbed. These two macroscopic, experimentally measurable parameters, d and AH ds, usually well characterize the adsorbed monolayer and the form of their interdependence often reveals the nature of bonding in the adsorbed layer. 

Atoms or molecules may impinge on a surface from the gas phase where they establish a surface concentration [n Js [molecules/cm ]. Let us assume that only one type of species of concentration [n Jg [molecules/cm ] exists in the gas phase so that the adsorption process can be written as 

If the incident molecules stay on the surface long enough to achieve thermal equilibrium with the surface atoms, t has a form of t = Tq where Tq is related to the 

The coverage, 6, may be varied by changing the pressure over the surface while maintaining a well-chosen constant temperature. The 6 vs P(T) curve so obtained for any given gas-surface system is called the adsorption isotherm. The simplest adsorption isotherm is obtained from Eq. (4) which we can rewrite as 

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