Insights into Reaction Kinetics in Confined Space: Real Time Observation of Water Formation under a Silica Cover

We offer a comprehensive approach to determine how physical confinement can affect the water formation reaction. By using free-standing crystalline SiO 2 bilayer supported on Ru(0001) as a model system, we studied the water formation reaction under confinement in situ and in real time. Low-energy electron microscopy reveals that the reaction proceeds via the formation of reaction fronts propagating across the Ru(0001) surface. The Arrhenius analyses of the front velocity yield apparent activation energies ( E a app ) of 0.32 eV for the confined and 0.59 eV for the nonconfined reaction. DFT simulations indicate that the rate-determining step remains unchanged upon confinement, therefore ruling out the widely accepted transition state effect. Additionally, H 2 O accumulation cannot explain the change in E a app for the confined cases studied because its concentration remains low. Instead, numerical simulations of the proposed kinetic model suggest that the H 2 adsorption process plays a decisive role in reproducing the Arrhenius plots.