Mechanistic investigation and free energies of the reactive adsorption of ethanol at the alumina/water interface

Controlling the adsorption/desorption of molecules at the solid/water interface is central to a wide range of fields from catalysis to batteries. For instance, adsorbing alcohols at the surface of γ-Al2O3 can prevent its chemical weathering. To make sure that γ-Al2O3 remains a stable catalyst support under operating conditions in liquid water, it is crucial to design alcohols that cannot desorb easily. Taking ethanol as a typical example, we here compare the adsorption/desorption mechanism for two distinct adsorption modes of ethanol at the water/alumina interface using various DFT-based approaches. Thermodynamic integration simulations unambiguously identify ethoxy as the more stable adsorption mode. The presence of liquid water yields to adsorption barriers fo adsorption barriers of at least 20 kJ·mol-1. To better assess the effect of water, we perform 3D well-tempered metadynamics simulations that include a bias accounting for solvation effects and proton transfers at the interface. Activating the proton shuffling allows to explore a variety of protonation and hydration configurations and yields to higher barriers (up to 40 kJ·mol−1) than the ones predicted by thermodynamic integration where the solvent reorganisation was assumed to be decoupled from the desorption. This study illustrates the importance of treating explicitly solvation effects when modelling reactions at the solid/liquid interface.