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 mechanisms for two distinct adsorption modes of ethanol at the water/alumina interface using various density functional theory-based approaches. Thermodynamic integration (TI) simulations unambiguously identify ethoxy as the more stable adsorption mode. The presence of liquid water yields adsorption barriers of at least 20 kJ·mol−1. To better assess the effect of water, we perform three-dimensional well-tempered metadynamics simulations that include a bias accounting for solvation effects and proton transfers at the interface. Activating the proton shuffling allows us to explore a variety of protonation and hydration configurations and yields higher barriers (up to 40 kJ·mol–1) than the ones predicted by TI where the solvent reorganization was assumed to be decoupled from the desorption. This study illustrates the importance of explicitly treating solvation effects when modeling reactions at the solid/liquid interface.