First-principles study of K and Cs adsorbed on Pd(111)

Area of Science:

• Surface Science
• Computational Materials Science
• Physical Chemistry

Background:

• Understanding alkali metal adsorption on transition metal surfaces is crucial for catalysis and materials science.
• Palladium (Pd(111)) is a key catalyst, and its surface properties are significantly influenced by adsorbates.
• Alkali metals like Potassium (K) and Cesium (Cs) act as promoters or poisons in catalytic processes.

Purpose of the Study:

• To investigate the adsorption behavior of Potassium (K) and Cesium (Cs) on the Pd(111) surface.
• To determine preferred adsorption sites, bonding characteristics, and the impact on work function and electronic structure.
• To elucidate the nature of the chemical bond between alkali metals and the Pd(111) surface.

Main Methods:

• Utilizing density functional calculations (DFT) within the generalized gradient approximation (GGA).
• Analyzing adsorption geometries, site preferences, and work function changes.
• Examining the electron structure and charge distribution at the adsorbate-surface interface.

Main Results:

• The hexagonal close-packed (hcp) hollow site is the energetically preferred adsorption site for both K and Cs across all studied coverages.
• Calculated adsorption geometries for (2x2) and (√3x√3)R30° phases show good agreement with experimental observations.
• Adsorption of K and Cs leads to a decrease in work function, attributed to adsorbate-induced dipole moments and charge redistribution.

Conclusions:

• The bonding between alkali metals (K, Cs) and the Pd(111) surface exhibits a transition from mixed ionic/metallic character at low coverage to predominantly covalent metallic character at high coverage.
• The observed work function reduction is explained by the polarized nature of the adsorbate layer and charge accumulation at the interface.
• The findings provide fundamental insights into alkali metal-surface interactions on transition metals, relevant for catalytic applications.