Influence of surface phase transitions on desorption kinetics: The compensation effect.

In heterogeneous chemical kinetics, the Arrhenius parameters often used to describe the chemical rate process, the activation energy E(a) and pre-exponential factor A, vary with coverage or adsorbate density. It is sometimes observed that these parameters continuously change so as to maintain a constant rate, i.e., the changes in A and E(a) offset or compensate. This compensation effect is a poorly understood but not uncommon phenomenon in surface chemistry. In this paper we demonstrate that phase transitions in the adsorbate-substrate system will give rise to such compensation effects in thermal desorption kinetics. The argument is based on the continuity of the chemical potential through the adsorbate-substrate phase transition.