Large vibrational free energy of tightly bonded small chemicals on metal surfaces

We demonstrate that the vibrational free energy of strongly adsorbed chemicals on metal surfaces can be considerably large based on density functional theory (DFT) calculations. In the contrary to the common belief that vibrational free energy of chemisorbed molecule is negligible, we find that even diatomic radicals (e.g. OH) could have larger than 100 meV of vibrational free energy at room temperature. Calculations were conducted for H, O, CO, OH, OCH3, and OC2H5 on Pt and Cu surfaces. The vibrational free energy is mostly contributed by only a few low-lying external modes regardless of the chemicals, and their frequencies strongly depend on the adsorbed geometry and mass. A low frequency appeared in the vibration of constant bond length due to its smooth energy curve. Top site adsorption has always the largest vibrational free energy as it has many sideward oscillation modes, followed by the bridge site. The mass effect is equally significant to the bond characters; the larger chemical, the lower the frequency. The vibrational free energy was higher than the zero-point energy (ZPE) when the chemicals is diatomic or larger, which implies that ZPE correction alone worsens the adsorption prediction.