Mechanism of the Accelerated Water Formation Reaction under Interfacial Confinement

Subnanoscale
spaces at the interface between weakly coupled thin
films and their metal supports offer exciting opportunities for studying
chemical reactions under confinement. Here, we investigated the kinetics
of water formation (from hydrogen and chemisorbed oxygen) in the confined
space at the interface between bilayer (BL) silica and a Ru(0001)
support, compared to the reaction on the bare Ru(0001) surface. Ambient
pressure X-ray photoelectron spectroscopy (AP-XPS) experiments were
carried out at different temperatures at elevated pressures of H2 to follow the reaction kinetics. The apparent activation
energy at the BL-silica/Ru(0001) interface was found to be 0.38 eV
lower than that on bare Ru(0001), consistent with a recent report
by Prieto et al. (Angew. Chem., Int. Ed. 2018, 57(28), 8749–8753) carried
out at lower H2 pressures using low-energy electron microscopy.
Density functional theory calculations revealed that the rate-limiting
step in the direct hydrogenation pathway on the Ru(0001) surface is
the first hydrogen addition step (*H + *O ↔ *OH). The confinement
at the BL-silica/Ru(0001) interface only marginally affects the energy
barrier of the first hydrogen addition. Instead, it activates an alternative
disproportionation reaction pathway (*H2O + *O ↔
2*OH). On the bare Ru(0001) surface, the disproportionation pathway
can only occur at cryogenic temperatures or under high water vapor
pressures. However, the presence of the BL-silica increases the desorption
barrier for water molecules at the interface. The increased residence
time allows trapped water molecules to react with chemisorbed oxygen
to produce two *OH per H2O with an activation energy 0.25
eV lower than that of the first hydrogen addition step. This work
reveals the origin of the observed accelerated water formation reaction
at the BL-silica/Ru(0001) interface in the low-temperature regime
(T < 350 K) and points to a route to engineer
chemical reaction pathways by leveraging subnanoscale confined spaces
at metal–oxide interfaces.