(6a) Hydrogenation of Functionalized Molecules in Presence and Absence of External Electric Potential

The presence of water and other solvents strongly influences the binding of reacting substrates and hydrogen. The binding is particularly complex in aqueous environment, as the electrochemical process at the surface influence both the metal and the adsorbed species. In addition to a competition between solvent components and hydrogen for sorption sites, part of the impact on surface stabilization is caused by differences in the sorption entropy induced by ordering in the Helmholtz layer and by the electronic impact of the dominating H⁺+e^- ⇌ H redox reaction. Applying an external electric potential changes these interactions markedly and influence hydrogenation rates. Overall, it will be shown that the surface excess potential of adsorbed hydrogen is the most important parameter for hydrogenation. Introducing a pathway via proton coupled electron transfer opens an additional and typically fast pathway, of the hydronium ions can be concentrated close to the metal- electrolyte interface. It will be discussed how an external electric potential and the chemical composition of the aqueous solution (pH, ionic strength) influences the binding of the reacting partners and how this influences activity and selectivity.