What can we learn about electrode–chemisorbate bonding energetics from vibrational spectroscopy? An assessment from density functional theory

Abstract

An analysis is presented of the manner and extent to which the metal surface–chemisorbate bond energetics and geometries as functions of the metal and the applied field can be correlated with vibrational frequencies, with specific reference to electrochemical systems. Emphasis is placed on metal–adsorbate stretching frequencies, ν_M–A, using oxygen and carbon monoxide chemisorption as illustrative examples; the intramolecular stretch (ν_CO) of the latter adsorbate is also examined in view of the extensive experimental utilization of this vibrational mode. Results based on Density Functional Theory (DFT) are presented for finite-cluster models of Pt-group and coinage-metal (111) surfaces. The DFT calculations enable a separation between steric repulsion and orbital contributions to the potential-energy surface (PES), and additionally, in the case of CO chemisorption, between the 5σ and 2π* orbital components. While rough metal-dependent correlations between ν_M–A and the surface binding energy, −E_b, are observed, such a relationship is not expected in general. Thus for CO chemisorption, the variations in −E_b are affected more by changes in the 5σ rather than 2π* orbital energies, whereas these components influence the M–CO stretching frequency, ν_M–CO, to a comparable extent. Moreover, the metal-dependent ν_CO frequencies do not correlate even qualitatively with −E_b; this is because the former are dominated by 2π*, rather than 5σ, interactions. The factors influencing the field (F) (and hence electrode potential) dependence of E_bversusν_M–CO and ν_CO mirror somewhat this pattern. While the field-dependent influence of the 5σ and 2π* interactions are offsetting, the latter affects the ν_M–CO–F, and especially the ν_CO–F, behavior to a greater extent than the −E_b–F dependence. Generally, then, the lack of broad-based correlations between chemisorbate vibrational frequencies and binding energetics can be understood in terms of the differing influence of the individual interaction components on the PES well shape and depth. The description of such bonding contributions in terms of dipole-moment parameters is illustrated. Also considered are relations between vibrational frequencies and bond lengths.