The role of hydrophobic hydration in the free energy of chemical reactions at the gold/water interface: size and position effects

Metal/water interfaces catalyze a large variety of chemical reactions, which often involve small hydrophobic molecules. In the present theoretical study we show that hydrophobic hydration at the Au(100)/water interface actively contributes to the reaction free energy by up to several hundreds of meV. This occurs either in adsorption/desorption reaction steps, where the vertical distance from the surface changes in going from reactants to products, or in addition and elimination reaction steps, where two small reactants merge into a larger product and viceversa. We find that size and position effects cannot be captured by treating them as independent variables. Instead, their simultaneous evaluation allows to map the important contributions, and we provide examples of their combinations for which interfacial reactions can be either favoured or disfavoured. By taking a N2 and a CO2 reduction pathway as test cases, we show that explicitly considering hydrophobic effects is important for the selectivity and rate of these relevant interfacial processes.