Thermodynamics is not enough - The case of the Volmer reaction on silver

- The adsorption energies of hydrogen on the hollow sites of Ag(1 0 0) and Ag(1 1 5) are similar.
- There is a lower activation barrier for the hollow sites of Ag(1 0 0) than for Ag(1 1 5).
- The surfaces have different electronic environments at the saddle.
- Thermodynamic properties as single descriptors are not enough to explain reactivity.

The energy of activation for the Volmer step of the hydrogen evolution reaction on silver electrodes is evaluated using our own theory of electrocatalysis. The reactivity at the hollow sites of the flat Ag(1 0 0) surface and the vicinal Ag(1 1 5) surface is compared. Although the adsorption energy calculated by Density Functional Theory is similar for both systems, we found a substantially higher value for the activation energy in the case of the (1 1 5) crystal orientation. Our analysis shows that other important factors, such as the electronic interactions between reactant and substrate at the transition state, play a decisive role. Thermodynamics is not enough to explain reactivity.