Calculation of Adsorbate Free Energy Using the Damping Function Method

Adsorbate free energies are important parameters in surface chemistry and catalysis. Because of its simplicity, the harmonic oscillator (HO) model remains the most widely used method for calculating adsorbate free energy in many fields, including microkinetic modeling. However, it is well-known that the HO method is ineffective for weak adsorption. In this study, we propose a translational model with a diffusion barrier to calculate the state functions of near free translation. Furthermore, an effective mass is introduced in this model. To address hindered translation uniformly, a diffusion barrier-based damping function (DF) is proposed that effectively links the harmonic vibration and translation limits. Adsorbates are divided into three categories according to their adsorption strength and diffusion barrier height. Adsorbed hydrogen atoms have a strong binding energy and relatively high vibrational frequency but a low diffusion barrier. The HT and our proposed DF methods predict that the adsorbed hydrogen atoms behave as translation above room temperature, while the previous DF method predicts that they behave as vibration at any temperature. At last, the dehydrogenation reaction of propane on the Pt(111) surface was taken as an example to illustrate the influence of different methods on the thermodynamic functions.