Abstract
Adsorbate vibrational excitations are an important fingerprint of molecule/surface interactions, affecting temperature contributions to the free energy and impacting reaction rate and equilibrium constants. Furthermore, vibrational spectra aid in identifying species and adsorption sites present in experimental studies. Despite their importance, knowledge of how adsorbate frequencies scale across materials is lacking. Here, by combining previously reported experimental data and our own density-functional theory calculations, we reveal linear correlations between vibrational frequencies of adsorbates on transition metal surfaces. Through effective-medium theory, linear muffin-tin orbital theory, and the d-band model, we rationalize the squares of the frequencies to be fundamentally linear in their scaling across transition metal surfaces. We identify the adsorbate-binding energy as a descriptor for certain molecular vibrations and rigorously relate errors in frequencies to errors in adsorption energies. We also discuss the impact of scaling on surface thermochemistry and adsorbate coverage.
Highlights
Adsorbate vibrational excitations are an important fingerprint of molecule/surface interactions, affecting temperature contributions to the free energy and impacting reaction rate and equilibrium constants
A breakthrough in computational catalysis was the introduction of linear scaling relations (LSRs), by Nørskov and co-workers[1], which link the binding energy of a partially hydrogenated adsorbate AHX ðΔEAHX Þ to the binding energy of its respective atomic adsorbate A (ΔEA) across transition metal surfaces, ΔEAHX 1⁄4 mEΔEA þ bE; ð1Þ
While LSRs can predict the scaling of binding energies, there is not yet a way to scale zero-point energies (ZPE) and temperature contributions to Gibbs free energy across surfaces
Summary
Adsorbate vibrational excitations are an important fingerprint of molecule/surface interactions, affecting temperature contributions to the free energy and impacting reaction rate and equilibrium constants. Vibrational spectra aid in identifying species and adsorption sites present in experimental studies Despite their importance, knowledge of how adsorbate frequencies scale across materials is lacking. Through effective-medium theory, linear muffin-tin orbital theory, and the d-band model, we rationalize the squares of the frequencies to be fundamentally linear in their scaling across transition metal surfaces. While LSRs can predict the scaling of binding energies, there is not yet a way to scale zero-point energies (ZPE) and temperature contributions to Gibbs free energy across surfaces These temperature effects, which are composed of a molecule’s heat capacity and entropy, result entirely from vibrations for immobilized chemisorbed surface species[4]. The frequency (v) of a normal mode is given according to sffiffiffi ν
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