Marc T. M. Koper

Field-dependent chemisorption of carbon monoxide and nitric oxide on platinum-group (111) surfaces: Quantum chemical calculations compared with infrared spectroscopy at electrochemical and vacuum-based interfaces

Density Functional Theory (DFT) is utilized to compute field-dependent binding energies and intramolecular vibrational frequencies for carbon monoxide and nitric oxide chemisorbed on five hexagonal Pt-group metal surfaces, Pt, Ir, Pd, Rh, and Ru. The results are compared with corresponding binding geometries and vibrational frequencies obtained chiefly from infrared spectroscopy in electrochemical and ultrahigh vacuum environments in order to elucidate the broad-based quantum-chemical factors responsible for the observed metal- and potential-dependent surface bonding in these benchmark diatomic chemisorbate systems. The surfaces are modeled chiefly as 13-atom metal clusters in a variable external field, enabling examination of potential-dependent CO and NO bonding at low coverages in atop and threefold-hollow geometries. The calculated trends in the CO binding-site preferences are in accordance with spectral data: Pt and Rh switch from atop to multifold coordination at negative fields, whereas Ir and Ru e...

A THEORY FOR ADIABATIC BOND BREAKING ELECTRON TRANSFER REACTIONS AT METAL ELECTRODES

Abstract A theory is formulated for bond breaking and electron transfer at metal electrodes, based on an adaptation of the Anderson–Newns Hamiltonian. The model provides an extension of Saveants model for concerted bond breaking and electron transfer and yields Saveants predictions in the limit of vanishing electronic coupling. Solvent dynamical effects are investigated by assuming an overdamped solvent motion and a ballistic bond breaking motion. It is found that in a sufficiently slow solvent the transfer coefficient and the activation enthalpy may become temperature dependent, although the effect is probably small for most combinations of redox couples and solvents at room temperature.

Molecular dynamics simulations of solvent reorganization in electron-transfer reactions

We present molecular dynamics simulations of solvent reorganization in electron-transfer reactions in water. Studying a series of solutes with the same core radius (typical for chlorine) but with varying charge from −3 to +3, the simulations show that the single-solute solvent reorganization energy depends quite strongly on the solute’s charge, in contrast with the continuum Marcus theory. Due to the ion-dipole interactions, electrostriction plays an important role for charged species. The effective radius of a neutral species is comparatively larger, making the solvent reorganization energy small. Strong increases in the solvent reorganization energy occur when the solute is charged to either −1 to +1, due to the significantly smaller effective radius caused by the ion-dipole interactions. However, the effect is nonsymmetric because the center of the water dipole can approach closer to the negative species than to the positive species. Hence, the nonlinearity occurs mainly in the transition from 0 to –1....

Metal electrode–chemisorbate bonding: General influence of surface bond polarization on field-dependent binding energetics and vibrational frequencies

An analysis is outlined in which the dependence of the binding energy, Eb, stretching frequency, νM–A, and equilibrium bond length, req, of metal–adsorbate bonds on the external interfacial field (F), and hence surface potential of relevance to electrochemical systems, are described in terms of potential-energy surface and bond-polarization parameters. Density Functional Theory (DFT) calculations for finite metal clusters with monoatomic adsorbates forming polar surface bonds are utilized both to examine metal-, adsorbate-, and field-dependent trends in the required parameters and to examine the applicability of the analytic relations in comparison with numerical calculations. To a very good approximation, the Eb–F dependence is described by the field-dependent static dipole moment, μS, of the metal–adsorbate bond. As expected, the field-induced changes in the potential-energy surface (PES) are determined by the r-dependent μS values, i.e., the dynamic dipole moment, μD. The factors determining the νM–A–F...

Potential-dependent chemisorption of carbon monoxide on platinum electrodes : new insight from quantum-chemical calculations combined with vibrational spectroscopy

Abstract Density functional theory (DFT) using the finite cluster approach is utilized to compute binding energies, bond geometries, and vibrational properties of carbon monoxide adsorbed on Pt(111) as a function of the external interfacial field, focusing attention on the metal–CO bond itself. Comparison with electrode potential-dependent frequencies for the metal–CO ( ν M–CO ) as well as the much-studied intramolecular CO ( ν CO ) vibration, as measured by in-situ Raman and infrared spectroscopy, facilitate their interpretation in terms of metal-chemisorbate bonding for this archetypal electrochemical system. Decomposing the calculated metal–CO binding energy and vibrational frequencies into individual orbital and steric repulsion components enables the role of such quantum-chemical interactions to the field- (and hence potential-) dependent bonding to be assessed. No simple relationship between the field( F )-dependent binding energies and the ν M–CO frequencies is evident. While the DFT ν M–CO – F slopes are negative at positive and small–moderate negative fields, reflecting the prevailing influence of back-donation, a ν M–CO – F maximum is obtained at larger negative fields for atop CO, and a plateau for hollow-site CO. This Stark-tuning behavior reflects largely offsetting field-dependent contributions from π and σ surface bonding, and can also be rationalized on the basis of changes in the electrostatic component of ν M–CO from increasing M–CO charge polarization. A rough correlation between the field-dependent ν M–CO frequencies and the corresponding bond distances, r M–CO , is observed for hollow and atop CO in that r M–CO shortens towards less positive fields, but becomes near-constant at moderate–large negative fields. A more quantitative correlation between the field-dependent CO frequencies and bond lengths is also evident. In harmony with earlier findings (and unlike the ν M–CO – F behavior), the ν CO – F dependence is due chiefly to changes in the back-donation bonding component. The overall vibrational frequency-field behavior predicted by DFT is also in semi-quantitative concordance with experimental potential-dependent spectra.

A three-dimensional potential energy surface for dissociative adsorption and associative desorption at metal electrodes

A simple model is constructed to calculate the potential energy surface of dissociative adsorption and associative desorption reactions at the metal/solution interface. The model is based on an extension of the Anderson–Newns Hamiltonian and has three reaction coordinates; the bond length or the distance between the fragments, the distance from the surface, and the generalized solvent coordinate familiar from the classical theory of electron-transfer reactions. The properties of the three-dimensional potential energy surfaces are studied and the activation energy for dissociative adsorption is calculated as a function of the applied potential and the metal work function. In the observed trends, the absorption energy and hence the electrosorption valency of the fragments play an important role. For certain “extreme” values of the bonding or antibonding energy levels, molecular ions may become metastable and affect the reaction mechanism.

Large-scale computer simulation of an electrochemical bond-breaking reaction

A novel Hamiltonian is employed to explicitly simulate an electrochemical bond-breaking reaction in which an electron-transfer reaction is directly coupled to the dissociation of a molecular species. The free energy surface as a function of both the collective solvation coordinate of the electron transfer and the intramolecular bond length of a CH3Cl molecule is computed by virtue of a classical molecular dynamics (MD) simulation. The method is also easily generalized to treat a variety of electrochemically catalyzed phenomenon. The simulation data show very significant deviations from the predictions of standard analytical theory.

Monte Carlo simulations of ionic adsorption isotherms at single-crystal electrodes

Abstract A lattice–gas model is considered for ionic adsorption at single-crystal electrodes, the statistical mechanics of which is solved by means of Monte Carlo simulations. The model assumes a short-range nearest-neighbor repulsion and a long-range Coulomb repulsion between adsorbed adions. By fitting the model isotherm to the experimental isotherm for bromide adsorption on Ag(100), which is lattice–gas-like, some conclusions can be drawn regarding the relative importance of these two interaction channels. Various ordered adlayers appearing through second-order order–disorder transitions are found on (100) and (111) surfaces. The inclusion of a simple model for mutual depolarization of the adions leads to first-order phase transitions in the isotherms. Finally some simulations on a uniaxially corrugated (110) surface are considered and compared to experimental results of halide adsorption on Ag(110).

Ab Initio Quantum-Chemical Calculations in Electrochemistry

In spite of Dirac’s pessimistic viewpoint on the applicability of laws of quantum mechanics to chemistry, the quantum-mechanical description of chemical bonds and reactions has been one of the most prominent and active areas of theoretical chemistry since the early days of quantum mechanics. As anticipated by Dirac, applying the laws of quantum mechanics to systems of chemical interest was frustrated by great computational difficulties for many years, with the exception perhaps of the simplest molecules. However, with recent developments both in conceptual quantum chemistry, i.e. the application of density

Isotherms of ionic adsorption at metal electrodes with coverage dependent lateral interactions due to mutual depolarization

Abstract Isotherms are calculated for a simple model describing ions absorbing onto a metal electrode, taking into account the effect of mutual depolarization. By mutual depolarization is meant the progressive discharge of the ions with increasing coverage, leading to weaker dipole–dipole lateral interactions. The mean-field isotherm predicts sigmoidal parts in the isotherm in the region of ionic discharge, which may even develop into hysteresis and first-order phase transitions. The qualitative predictions of the mean-field theory, in particular the first-order phase transition, are reproduced by Monte-Carlo simulations, but only if the lateral repulsive interactions are long ranged, as is expected for electrostatic interactions.