Dipole–dipole coupling effects upon infrared spectroscopy of compressed electrochemical adlayers: Application to the Pt(111)/CO system

Abstract

Experimental infrared spectra for CO adlayers on Pt(111) electrodes having known real-space structures as deduced by scanning tunneling microscopy are compared with predictions extracted from conventional dipole–dipole coupling models in order to test the validity of such treatments for compressed electrochemical adlayers, especially with regard to band-intensity transfer effects. The specific structures considered are (2×2)–3CO and (√19×√19)R23.4°–13CO hexagonal adlayers; the former is especially close packed (θCO=0.75) with a pair of threefold hollow and one atop CO per unit cell, while the latter has a lower coverage (θCO=13/19) and involves largely asymmetric binding sites. The comparisons between dipole-coupling theory and experiment include infrared spectra for various 13CO/12CO mixtures, thereby exploiting the well-known systematic alterations which are induced in the degree of coupling for a given adlayer. Consistent with an earlier assessment (Ref. ) the conventional dipole–dipole treatment can account semiquantitatively for the marked higher intensity of the atop relative to the threefold hollow C–O stretching band in the observed infrared spectra even though the occupancy on the latter site is twofold greater and the singleton frequencies are substantially (∼280 cm−1) different. This coupling-induced intensity transfer toward the higher-frequency band component is likely to be a widespread phenomenon for densely packed adlayers. For the (2×2) adlayer, however, the isotope composition-dependent spectral band frequencies and relative intensities deviate markedly from the experiment. While the inclusion of stochastic broadening effects associated with adlayer disorder improves the situation, a satisfactory fit between theory and experiment requires the incorporation of vibrational coupling associated with short-range intermolecular interactions. For the (√19×√19) adlayer, on the other hand, dipole–dipole coupling with stochastic broadening accounts well for the observed spectral behavior. The more pronounced limitation of the conventional theory for the (2×2) structure may well be due to the abnormally high adsorbate packing density enhancing the importance of short-range interactions.

CO oxidation on stepped-Pt(111) under electrochemical conditions: insights from theory and experiment

“…atop, bridge, multi-bonded positions, and even combinations of these [54]), providing a clear indication that the adsorption energies are indeed very similar at the different surface sites. In order to cover the effects induced by the electrode potential on the systems, different groups performed DFT studies on the CO adsorption on Pt(111) under the influence of a constant electric field [55][56][57], concluding that the preferred CO adsorption position changes from atop to fcc under decreasing external electric field [55,56].…”

Surface electrochemistry of CO on Pt(): anion effects

“…The stronger a-top signal observed with IR spectroscopy can be explained by "intensity stealing" by which the higher-frequency a-top mode gains intensity over the lower frequency hollow-site mode 27 . Quantitation of the intensity changes to fractional coverages can only be done with modeling of the dipole coupling, and even then would entail assumptions about the CO ad -CO ad coordination as the coverage changes 27 .…”

DME Dissociation Reaction on Platinum Electrode Surface: A Quantitative Kinetic Analysis by In Situ IR Spectroscopy

“…First, it was assumed in the present work that the IR band intensity of adsorbed CO species is linearly proportional to the CO coverage on the Pt electrode surface. This assumption is not exactly correct, especially for the surface covered by a high coverage where the strong dipole-dipole interaction between adsorbed CO molecules can greatly affect the IR absorption of CO. 26,27 Second, the coverage for CO adsorption on the Pt electrode surface is estimated only from CO L for simplicity in the study. This is also not exactly correct because CO B has been clearly observed from the IR measurements.…”