Influence of Dipole−Dipole Interactions on Coverage-Dependent Adsorption: CO and NO on Pt(111)

摘要

Density functional theory (DFT) calculations of energetic, geometric, vibrational, and electrostatic properties ofndifferent arrangements of CO and NO at quarter and half monolayer coverage on Pt(111) are presented. Differences in thenextents of electron back-donation from the Pt surface to these molecules cause the low-coverage adsorbate dipoles to havenopposite signs at atop and more highly coordinated bridge or fcc sites. These dipoles of opposite sign occupy adjacent positionsnin the experimentally observed atop−bridge or atop−fcc high -coverage arrangements, leading to attractive electrostaticninteractions and concomitant changes in dipole moments, bond lengths, and vibrational frequencies. The interaction energies arenestimated by charge partitioning to extract individual dipoles from the mixed arrangement and by calculations of field−dipoleninteractions. These estimated dipole interactions contribute significantly (20−60%) to the DFT-calculated relative stability ofnmixed arrangements over atop-, bridge-, or fcc-only arrangements and thus play an important role in coverage-dependentnadsorption. We further extend these analyses to a range of molecules with varying dipole moments and show that the generalnnature of these interactions is not limited to CO and NO.