(6a) Modeling the Effect of Electrolyte Composition on the Aqueous-Phase Adsorption of Phenol, Catechol, and Benzene on Ag(111)

Electrocatalytic reactions of organic molecules can be influenced by electrolyte composition through competitive adsorption of ions and changes in the electrochemical double layer.¹ Here we model the effect of electrolyte composition on the adsorption strength of phenol, catechol, and benzene on Ag(111) using a classical molecular dynamics approach that includes the polarization of the metal surface. We predict that 0.2–1 M of LiCl, NaCl, CsCl, LiClO₄, and NaClO₄ changes the free energy of adsorption for these organics by 0.5–9 kJ/mol compared to pure water. We investigate two factors that contribute to the change in the organic adsorption strength based on a bond-additivity (BA) model: solvation of organics and interfacial water-displacement during organic adsorption. We find that electrolytes decrease the solvation energy by 0.5–4.2 kJ/mol. The predicted solvation free energies of organics in pure water are compared to estimates derived from measured Henry’s constants, which agree within 2.7% for catechol and phenol. Electrolyte composition changes the free energy penalty for water-displacement by up to ~12 kJ/mol. Both these factors partly explain the effect of electrolytes on typically weaking organic adsorption on silver. Because these organics adsorb flat to the Ag surface and do not have a strong dipole normal to the surface, electrostatic effects of the ions on the adsorption energy are small. We compare our results to the BA equation to identify the parameters that can help modify the equation to better understand the effect of electrolytes on adsorption of organics. These changes in the organic adsorption free energy may change the rate of catalytic conversion by 1-2 orders of magnitude.

References:

(1) Mathanker, A.; Yu, W.; Singh, N.; Goldsmith, B. R. Effects of Ions on Electrocatalytic Hydrogenation and Oxidation of Organics in Aqueous Phase. Current Opinion in Electrochemistry 2023, 40, 101347. https://doi.org/10.1016/j.coelec.2023.101347.