Thermodynamic and Kinetic Insights into Hydrogen Adsorption and Dissociation on the Ru (0001) Surface under STM and Catalytic Conditions

The
study of the interaction of hydrogen with transition metal
surfaces holds great importance in numerous technological applications
such as hydrogen storage and catalysis. Among the promising catalysts,
ruthenium (Ru) stands out for its role in hydrogenation and hydrogenolysis
reactions. The atomic hydrogen coverage can play a critical role in
determining the outcome of these reactions, but the exact atomic hydrogen
coverage of the Ru surface under catalytic conditions remains unknown.
To address this knowledge gap, we conducted a comprehensive investigation
of hydrogen adsorption and dissociation on the Ru (0001) surface using
dispersion-corrected density functional theory calculations. Our study
reveals that ab initio thermodynamic surface phase diagrams, based
on the direct dissociation of gas-phase H2, are unable
to predict the coverage observed in vacuum scanning tunneling microscopy
(STM) experiments, which typically show submonolayer coverages. This
discrepancy is explained by the fact that above 0.75 monolayer (ML)
coverage, H2 adsorption becomes unfavorable due to the
electrostatic repulsion between the adsorbed hydrogen atoms and the
incoming gas-phase H2 molecules. Additionally, the activation
energy for adsorbed H2 dissociation increases at higher
coverages. Thus, to accurately predict the atomic hydrogen coverage
under experimental conditions, we computed a kinetic phase diagram
based on the comparison of the rates for adsorbed H2 dissociation
and desorption. This approach successfully predicts a maximum achievable
atomic hydrogen coverage of 0.875 ML under vacuum STM conditions.
Furthermore, our findings show that the surface is not fully saturated
by hydrogen under typical catalytic conditions, thus allowing for
the adsorption of molecules required for catalytic processes. Overall,
this study shows that H2 adsorption and dissociation kinetics
must be considered for the accurate prediction of the atomic hydrogen
coverage under experimental conditions.