The rate of adsorption
The rate at which a surface is covered by adsorbate depends on the ability of the substrate to dissipate the energy of the incoming particle as thermal motion as it crashes on to the surface. If the energy is not dissipated quickly, the particle migrates over the surface until a vibration expels it into the overlying gas or it reaches an edge. The pro portion of collisions with the surface that successfully lead to adsorption is called the sticking probability, s:
s =
The denominator can be calculated from the kinetic model, and the numerator can be measured by observing the rate of change of pressure. Values of s vary widely. For example, at room temperature CO has s in the range 0.1–1.0 for several d-metal surfaces, but for N2 on rhenium s < 10−2, indicating that more than a hundred collisions are needed before one molecule sticks success fully. Beam studies on specific crystal planes show a pronounced specificity: for N2 on tungsten, s ranges from 0.74 on the (320) faces down to less than 0.01 on the (110) faces at room temperature. The sticking probability decreases as the surface coverage increases (Fig. 25.22). A simple assumption is that s is proportional to 1 − θ, the fraction uncovered, and it is common to write
s =(1−θ) s0
wheres0is the sticking probability on a perfectly clean surface. The results in the illustration do not fit this expression because they show that s remains close to s0 until the coverage has risen to about 6 × 1013 molecules cm−2, and then falls steeply. The explanation is probably that the colliding molecule does not enter the chemisorbed state at once, but moves over the surface until it encounters an empty site.
Fig. 25.22 The sticking probability of N2 on various faces of a tungsten crystal and its dependence on surface coverage. Note the very low sticking probability for the (110) and (111) faces. (Data provided by Professor D.A. King.)
