Water Formation under Silica Thin Films: Real‐Time Observation of a Chemical Reaction in a Physically Confined Space

Using low energy electron microscopy and local photoelectron spectroscopy, we investigated the water formation from adsorbed O and H₂ on a Ru(0001) surface covered with a vitreous SiO₂ bilayer (BL) and compared it to the same reaction on bare Ru(0001). In both cases the reaction is characterized by moving reaction fronts. The reason for this might be related with the requirement of site release by O adatoms for further H₂ dissociative adsorption. We find apparent activation energies (E_a^app) for the front motion of 0.59 eV without cover and 0.27 eV under cover. We suggest that the smaller activation energy but higher reaction temperature for SiO₂ BL covered Ru(0001) surface is due to a change of the rate‐determining step. Other possible effects of the cover are discussed. Our results give the first values for E_a^app in confined space, thus leading to potentially new approaches of physical confinement effects on reaction rates, including theoretical modelling