Speaker
Description
The physics at an electrified interface formed by a planar metal electrode and a liquid electrolyte is commonly understood within a plate-capacitor picture. Varying the applied potential away from the point of zero charge (PZC) leads to a charging of the electrode. This surface net charge is balanced by the built-up of a corresponding counter charge of equal magnitude and opposite sign in the electrolyte. The potential drop then occurs over the thus formed narrow electric double layer (DL), leading generally to quite high electric fields at such electrochemical interfaces. In the widely employed dipole-field interpretation, a dipole moment of an adsorbate at the electrode will interact with this electric field to yield potential-dependent adsorption energies.
Even a macroscopically planar electrode is never really completely flat on smaller scales though. The idealized plate-capacitor picture of a laterally evenly distributed net charge and a concomitant homogeneous DL field must therefore be refined by recognizing that under an applied potential, charge will particularly accumulate in any protrusion of the surface. This will, in turn, lead to a local electric field enhancement at this local curvature. While this is well established for the mesoscale, we here scrutinize whether it also holds for atomic-level protrusions like ubiquitous step edge or kink sites between flat terraces. Stronger potential dependencies of adsorption energies due to a local field enhancement would then add to the special catalytic properties of such undercoordinated sites.
For our analysis we exploit that novel fully grand-canonical (FGC) density-functional theory calculations allow to capture the capacitive charging of the electric DL and thus give efficient access to potential-dependent adsorption energies [1]. The computed dependencies for prototypical adsorbates at vicinal Pt(111) surfaces can indeed largely be interpreted in terms of an effective dipole-field interaction [2]. However, importantly, this concerns the surface dipole that forms upon adsorption, which thus generally precludes simple estimates for the strength of the adsorption energy variation on the basis of accessible molecular dipoles. The charge rearrangement upon specific adsorption also affects the local field experienced by the adsorbate, which precludes estimates merely based on the local atomic-scale electrode geometry. As an important corollary, no particularly strong adsorption energy variation can thus generally be concluded for protruding defect sites like steps and kinks. Notwithstanding, as illustrated for several prototypical adsorbates, the dependencies can well reach 300 meV/V, even for adsorbates for which no variation may intuitively have been expected. Changes of this magnitude may well critically affect reaction mechanisms, in particular when it concerns more subtle selectivities. Our findings thus question mechanistic analyses of surface catalytic reactions made on the basis of the prevalent computational hydrogen electrode approach that is agnostic to these potential-induced adsorption energy variations.
[1] N.G. Hörmann, N. Marzari, and K. Reuter, npj Comput. Mater. 6, 136 (2020).
[2] S.D. Beinlich, N.G. Hörmann, and K. Reuter, ACS Catal. 12, 6143 (2022).
| Abstract Number (department-wise) | TH 08 |
|---|---|
| Department | TH (Reuter) |
