Effects of Electric Fields on the Energies of Adsorbates on Metal Surfaces in Vacuum and in Liquids

When thermal catalytic or electrocatalytic reactions occur on metal surfaces in liquid solvents like water, an electrical double layer develops near the metal surface with a large electric field which changes with reaction conditions. This electric field affects the energies of adsorbed reaction intermediates and transition states, and therefore also reaction rates. We review here what is known from ultrahigh vacuum (UHV) surface science studies regarding the effects of electric fields on the energies of adsorbed catalytic reaction intermediates and show how that can guide predictions about how changes in electric fields in the nearby double layer affect adsorbate energies on metal surfaces in liquids. In UHV, the electric field felt by an adsorbate can be strongly tuned by the addition of a different adsorbed species nearby. Alkali adatoms exert a very strong change in electric field near the metal surface, as quantified by work function measurements. The effects of these changes in field due to alkali adatoms on the energies of coadsorbed catalytic reaction intermediates, their electronic character, and their reaction rates have been studied extensively for decades on clean, well-ordered single-crystal metal surfaces in UHV. For example, their field stabilizes adsorbed CO, N2, and H, weakens their C–O and N–N bonds, and increases the dissociation rates of adsorbed CO and N2. These studies provide insight into the effects of applied electric fields on neutral and relatively nonpolar adsorbates in liquid environments, if we assume that changes in the field have only small effects on the strength of the weak attractions between adsorbate and solvent, so that the change in adsorbate energy with a change in its local electric field is the same change as when adsorbed in UHV. For example, this approach explains the well-known observation that the binding energy of hydrogen adatoms (Had) to many late transition metal surfaces, as probed by cyclic voltammetry in aqueous solutions, increases (bind more strongly) with increasing pH. This change in Had energy in turn explains pH-induced changes in thermal catalytic and electrocatalytic hydrogenation reaction rates whereby Had must add to another species.