Surface Chemical Reactivity of Ultrathin Pd(111) Films on Ru(0001): Importance of Orbital Symmetry in the Application of the d-Band Model

TL;DR

This study investigates how ultrathin Pd(111) films on Ru(0001) exhibit variable chemical reactivity based on film thickness, highlighting the importance of orbital symmetry and specific d-orbital contributions in surface catalysis.

Contribution

It demonstrates the orbital-specific effects on the d-band center and reactivity in ultrathin Pd films, advancing understanding of surface chemistry beyond bulk properties.

Findings

Ultrathin Pd(111) films below five monolayers are inert to oxygen.

Reactivity correlates with shifts in 4dxz and 4dyz orbital densities.

d-band center shifts explain the reactivity variation with film thickness.

Abstract

The chemical bonding of adsorbate molecules on transition-metal surfaces is strongly influenced by the hybridization between the molecular orbitals and the metal d-band. The strength of this interaction is often correlated with the location of the metal d-band center relative to the Fermi level. Here, we exploit finite size effects in the electronic structure of ultrathin Pd(111) films grown on Ru(0001) to tune their reactivity by changing the film thickness one atom layer at a time, while keeping all other variables unchanged. Interestingly, while bulk Pd(111) is reactive towards oxygen, Pd(111) films below five monolayers are surprisingly inert. This observation is fully in line with the d-band model prediction when applied to the orbitals involved in the bonding. The shift of the d-band center with film thickness is primarily attributed to shifts in the partial density of states associated with the 4dxz and 4dyz orbitals. This study gives an in-depth look into the orbital specific contributions to the surface chemical reactivity, providing new insights that could be useful in surface catalysis.