Thermodynamics and Kinetics in Weakly Chemisorbed Phases

Abstract

In a heterogeneous surface reaction, the reactants, intermediates, and products can be weakly chemisorbed along their respective reaction paths. For example, considering the fragmentation of a molecule into strongly adsorbed atoms, dissociation will occur out of a less strongly bonded molecular entity. But not only the importance for heterogeneous catalytic reactions stimulates the investigation of weakly chemisorbed phases, they also exhibit interesting sequences of phase transitions which are reflected in their thermodynamic functions and in the adsorption and desorption kinetics of the ad- sorbate. For the present purpose, we define weak chemisorption as a chemical interaction between adsorbate and substrate with a low adsorption energy (E_B ≲70 kJ/mol), resulting in a facile balance between the interaction of the individual species with the substrate and lateral forces between the chemisorbed molecules. The distinction between “weak chemisorption” and “chemisorption” is rather arbitrary, but a difference is obvious with respect to pure physisorption involving only Van der Waals forces between adsorbate and substrate.