10.2.5 Desorption

In each of the preceding cases, the products of the surface reaction adsorbed on the surface are subsequently desorbed into the gas phase. For the desorption of a species (e.g., C),

the rate of desorption of C is

10-20

where K_DC is the desorption equilibrium constant. Now let’s look at the above adsorption from right to left. We note that the desorption step for C is just the reverse of the adsorption step. Consequently, the rate of desorption of C, r_DC, is just opposite in sign to the rate of adsorption of C, r_ADC:

r_DC = –r_ADC

In addition, we see that the desorption equilibrium constant K_DC is just the reciprocal of the adsorption equilibrium constant for C, K_C:

in which case the rate of desorption of C can be written

10-21

In the material that follows, the form of the equation for the desorption step that we will use to develop our rate laws will be similar to Equation (10-21).