How molecular is the chemisorptive bond?

Area of Science:

• Surface Science
• Materials Chemistry
• Computational Chemistry

Background:

• Adsorption energies on transition metals depend on adsorbate binding sites (atop vs. high coordination).
• Comparing early and late transition metals reveals significant variations in adsorbate bond energies.
• Understanding these variations requires detailed analysis of electronic structure and chemical bonding.

Purpose of the Study:

• To provide a theoretical understanding of adsorption energy trends on transition metals.
• To investigate the influence of electronic structure on adsorbate-surface interactions.
• To compare adsorption on surfaces with adsorption in transition metal molecules.

Main Methods:

• Density Functional Theory (DFT) electronic structure computations.
• Analysis of partial density of states (PDOS) and Crystal Orbital Hamiltonian Population (COHP).
• Calculations of Bader charge densities and electron density topology.

Main Results:

• Adsorption energy trends differ for atop vs. high coordination sites, with larger variations for early/late transition metals.
• Electronic structure parameters like Fermi level position and bond polarity dictate bond energies.
• Adsorbed species (adatoms, molecular fragments) show trends similar to analogous transition metal molecules when adsorbed atop.

Conclusions:

• Embedding energy comprises spin state quenching, weakened adsorbate-surface interaction, and weakened metal-metal bonds.
• Scaling rules for CHx fragments are generally followed only at high coordination sites.
• Deviations from scaling rules occur for early transition metals, particularly for C, CH, N, and NH at atop sites or in molecules.