(214b) DFT Studies of Coverage Dependent Dipole-Dipole Interactions and Effect of Uniform Electric Field On Chemisorption of CO and NO On Pt(111)

In the present study periodic DFT calculations are used to study adsorption of CO and NO molecules on Pt(111) surface at different coverage. The effect of uniform electric field applied normal to Pt surface on adsorption energy and vibrational frequency of the adsorbates has also been studied. The dipole moment of adsorbates is calculated by taking the first moment of electronic charge distribution. The infrared absorption intensities are assumed to be proportional to the dynamic dipole moments for the vibrational modes. CO and NO are molecules with small dipole moments in the gas phase which undergo a change when the molecules are adsorbed on Pt surface. At low coverage the dipole moment of these molecules changes sign with change in adsorption site. For example, the value of dipole moment is positive for NO adsorbed on atop site (0.065 eÅ) but negative for fcc site (- 0.056 eÅ) at ¼ ML coverage. At higher coverages there is increased lateral electrostatic interaction between neighboring dipoles which leads to reduction or enhancement of dipole moments depending on whether the neighboring molecules occupy identical or mixed adsorption sites. The coverage dependent changes in dipole moments have important implications for adsorption site preferences, vibrational frequencies, infrared absorption intensity and electric field effects. The zero field results are consistent with experimentally observed ordered structures of adsorbates on Pt(111) studied using LEED and STM, and the experimentally observed changes in RAIRS spectra with coverage.