The role of electron affinity in electrochemical anion adsorption on metals

Anion adsorption is an important phenomenon in surface science, electrochemistry, and catalysis, where it can dictate (electro)catalyst performance and corrosion resistance. In recent work, we found that the adsorption energies of carboxylate anions to a metal electrode were linearly correlated with their acid dissociation constant (pKa) and electron affinity. To determine if these linear trends are general across anions, we have now used density functional theory to compare the thermodynamics of adsorption across a series of thiolates, carboxylates, alkoxides, and small molecular anions (OH*, SH*, SCF3*, and SCH3*) to a Pt(111) surface. We find that electron affinity is a general predictive descriptor of anion adsorption energy and, in fact, dominates over differences in the radical adsorption energy, representing the covalent contribution to adsorption, even across structurally different anions. In examining descriptors for radical adsorption, we find that the X–H bond dissociation energy (where X is the atom the radical uses to bind to the surface), used as a descriptor in prior work, only strongly correlates with radical adsorption energy for some species. This work highlights the importance of electron affinity in controlling adsorption and that the correlation between radical adsorption energy and (X–H) bond dissociation energy may not be as transferrable of a relationship as previously thought. Both these factors impact how we understand the physics and chemistry of adsorption, as well as the development of predictive descriptors of adsorption strength. A simple model using radical electron affinity and X–H bond dissociation energy to predict anion adsorption energetics is demonstrated.