Adsorption Entropies and Enthalpies and Their Implications for Adsorbate Dynamics

Experimental adsorption data is typically analyzed within the context of a Langmuir adsorption model. Such an analysis can yield values for adsorption entropies which are far too large based on a comparison with predictions from statistical mechanics. This is due to the fact that the Langmuir model does not always adequately replicate the dynamics of the adsorbate, and thus other prototypical models incorporating different dynamical assumptions should be considered. We extend prior work on adsorption entropies by providing a framework for the interpretation of adsorbate dynamics based on a comparison of experimentally determined adsorption entropies, saturation coverages, and enthalpies to those that are predicted for prototypical models of surface dynamics. Models that have been considered in this work include various Langmuir-type adsorption and 2D gas models. We demonstrate that a two-dimensional gas can display a low apparent saturation coverage when adsorption is facilitated by a subset of surface sites. Additionally, we consider a Langmuir replacement reaction and show that a large apparent adsorption entropy can arise from such a situation. Finally, we discuss how the choice of a model affects the range of plausible values for the adsorption entropy and equilibrium constant. Adsorption of acetone on Degussa P25 TiO2 is used as an illustrative example.