We have analyzed, by means of density functional
theory calculations and the embedded cluster model,
the adsorption and spin-state properties of Cr, Ni, Mo, and
Pt deposited on a MgO crystal. We considered deposition at
the Mg2+ site of a defect-free surface and at Li+ and Na+
sites of impurity-containing surfaces. To avoid artificial
polarization effects, clusters of moderate sizes with no
border anions were embedded in simulated Coulomb fields
that closely approximate the Madelung fields of the host
surfaces. The interaction between a transition metal atom
and a surface results from a competition between Hund's
rule for the adsorbed atom and the formation of a chemical
bond at the interface. We found that the adsorption energies
of the metal atoms are significantly enhanced by the cation
impurities, and the adsorption energies of the low-spin
states of spin-quenched complexes are always more
favorable than those of the high-spin states. Spin polarization
effects tend to preserve the spin states of the adsorbed
atoms relative to those of the isolated atoms. The metal–
support interactions stabilize the low-spin states of the
adsorbed metals with respect to the isolated metals, but the
effect is not always enough to quench the spin. Spin
quenching occurs for Cr and Mo complexes at the Mg2+
site of the pure surface and at Li+ and Na+ sites of the
impurity-containing surfaces. Variations of the spin-state
properties of free metals and of the adsorption and spinstate
properties of metal complexes are correlated with the
energies of the frontier orbitals. The electrostatic potential
energy curves provide further understanding of the nature
of the examined properties. |
