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
SiO2 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 (Eaapp) 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, H2O accumulation cannot explain the change
in Eaapp for the confined cases studied because its concentration
remains low. Instead, numerical simulations of the proposed kinetic
model suggest that the H2 adsorption process plays a decisive
role in reproducing the Arrhenius plots.