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 diversity of fields from catalysis to batteries. Preventing the desorption of alcohol at the gamma-Al2O3/water interface is key to increase the stability of this catalyst support to perform reactions in water. Taking ethanol as a typical example, we investigate here the mechanism of desorption of two adsorption modes, namely chemisorbed ethanol and ethoxy, from the interface to the bulk water using three DFT-based simulations. Our 3D well-tempered metadynamics simulations include a bias in solvation, which triggers possible proton transfers with water. They evidence that solvation needs to be increased prior to desorption in both cases. Comparison with static approaches and thermodynamic integration simulations unambiguously identifies ethoxy as the more stable adsorption mode. It is more stable by at least 40 kJ/mol when considering adsorption at the gas/solid interface. And the presence of liquid water yields to a desorption barriers ranging from 89 kJ/mol (thermodynamic integration) to 149 kJ/mol (well-tempered metadynamics). The observed difference between the two biased ab initio molecular dynamics methods can be ascribed to the intrinsic difficulty of sampling the desorbed state vs. the adsorbed state.