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Evaluating Thermal Corrections for Adsorption Processes at the Metal/Gas Interface

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posted on 13.11.2019 by Romain Réocreux, Carine Michel, Paul Fleurat-Lessard, Philippe Sautet, Stephan N. Steinmann
Adsorption and desorption steps are key for active catalysts and rely on a subtle balance between enthalpic and entropic terms. While the enthalpic term is becoming ever more accurate through density functional development, the entropic term remains underrated and its precise determination a great challenge. In this work, we have performed extensive first-principles thermodynamic integration (TI) simulations for the adsorption of small (e.g., CO) to larger (e.g., phenol) molecules at metallic surfaces and compared their adsorption free energies to the values obtained by vertical, static statistical mechanics approximations to thermal corrections invoking three different approximations for the low-frequency modes. We have found an excellent agreement between the vertical corrections and the TI for minima, for both weakly bound systems (e.g., CO2 and formic acid) and strongly chemisorbed molecules such as phenol or CO. While the treatment of the low-frequency modes has a minor impact on the agreement with TI, all vertical corrections systematically overestimate activation energies by 0.1–0.2 eV compared to TI, demonstrating a noticeable lowering of activation barriers. As a result of this study, we suggest that the vertical corrections and in particular the standard harmonic approximation can be safely applied to chemisorption minima, while the activation energies are likely to be overestimated. Hence, if a greater accuracy than ∼0.2 eV is required for activation free energies, we recommend to use thermodynamic integration, which for small- to medium-sized molecules in the gas phase is accessible with a reasonable computational effort but requires a dense sampling in the transition state region.