Vibrational frequencies of CO on Pt(111) in electric field: A periodic DFT study

Toward a quantitative theoretical method for infrared and Raman spectroscopic studies on single-crystal electrode/liquid interfaces

In situ electrochemical infrared spectroscopy and Raman spectroscopy are powerful tools for probing potential-dependent adstructures at solid/liquid electrochemical interfaces. However, it is very difficult to quantitatively interpret the observed spectral features including potential-dependent vibrational frequency and spectral intensity, even from model systems such as single-crystal electrode/liquid interfaces. The clear understanding of electrochemical vibrational spectra has remained as a fundamental issue for four decades. Here, we have developed a method to combine computational vibrational spectroscopy tools with interfacial electrochemical models to accurately calculate the infrared and Raman spectra. We found that the solvation model and high precision level in the self-consistent-field convergence are critical elements to realize quantitative spectral predictions. This method's predictive power is verified by analysis of a classic spectroelectrochemical system, saturated CO molecules electro-adsorbed on a Pt(111) electrode. We expect that this method will pave the way to precisely reveal the physicochemical mechanism in some electrochemical processes such as electrocatalytic reactions.

Influence of dipole-dipole interactions on coverage-dependent adsorption: CO and NO on Pt(111)

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

An ab initio study of electrochemical vs. electromechanical properties: the case of CO adsorbed on a Pt(111) surface

We have studied electrochemical vibrational and energy properties of CO/Pt(111) in the framework of periodic density functional theory (DFT) calculations. We have used a modified version of the previously developed Filhol-Neurock method to correct the unphysical contributions arising from homogeneous background countercharge in the case of thick metallic slabs. The stability of different CO adsorption sites on Pt(111) (Top, Bridge, Hcp, Fcc) has been studied at constant electric field. The energies are dominated by the surface dipole interaction with the external electric field: a strong positive electric field favors the surfaces with the lower dipole moment (that correspond to the ones with the lower coordination). The Stark tuning slope of the CO stretching frequency for a Top site was calculated for different surface coverages in very good agreement with both experimental and other theoretical results. Finally, we have performed an analysis of the origin of Stark shifts showing that the total Stark effect can be split into two competing components. The first one corresponds to the direct effect of charging on the C-O chemical bond: it is referred as an electrochemical effect. The second is the consequence of the surface dipole interaction with the applied electric field that modifies the C-O distance, inducing a change of the C-O force constant because of C-O bond anharmonicity: it is referred as the electromechanical effect. In the CO/Pt(111) case, the dominant contribution is electromechanical. The electrochemical contribution is very small because the electronic system involved in the surface charging is mostly non-bonding as analyzed by looking at the surface Fukui function.

A periodic density functional theory analysis of CO chemisorption on Pt(111) in the presence of uniform electric fields

Periodic DFT calculations are used to study the effect of a homogeneous electric field applied perpendicular to a Pt(111) surface on the bond distances, binding energies, and vibrational frequencies of atop- and fcc-adsorbed CO at various coverages. The observed structural and energetic modifications can be understood in terms of modest field-induced charge transfer between charged metal surface and adsorbate and are well-described by classical first and second-order Stark models. Electronic differences between atop and fcc adsorption cause CO in these sites to respond differently to applied fields. After correcting for the GGA site preference error, CO adsorption is predicted to shift from atop to fcc at potentials <-0.19 V A(-1). The results are in qualitative agreement with previously reported cluster-based DFT models but differ quantitatively due to difference in modeled coverage, surface relaxation, and finite size effects. The calculated 44.4 cm(-1) V(-1) A shift in C-O stretch frequency with electric field (Stark tuning rate) compares favorably with UHV experiments but is significantly lower than the value obtained in electrochemical measurements, highlighting the importance of adsorbate environment on the magnitude of the tuning rate. The calculated coverage dependence of the tuning rate is in good agreement with previous UHV experiments.