Modeling of Electrode Interactions with Metal Clusters

10 Modeling of Electrode Interactions with Metal Clusters 

Anna Ignaczak and Jose Alberto Nunes Ferreira Gomes 

This area has received special attention from theoreticians. The most commonly used methodology for the calculation of the particle(ion)-metal interaction is to approximate the metal surface with a cluster of several atoms with the crystallographic organization typical of the metal studied. Although such an approach has many limitations and introduces certain difficulties, cluster-model calculations are becoming more popular in studies of interfacial interactions. Section 3.10.2 gives a brief review of quantum studies related to adsorption on metal surfaces in the cluster model approximation. In Section 3.10.3 a more detailed analysis of some aspects of this methodology is described, and is related to some recently published work on the problem of specific adsorption phenomenon. 

The metal cluster as a model of the electrode has been used in the theoretical investigations of specific adsorption phenomena for approximately twenty years. 

In 1976 Leban et used this approach to study the interaction of a platinum electrode with several particles, among them the water molecule and halide ions. The cluster used was taken to be a model of the Pt( 111) surface and contained only five platinum atoms. In this work the iterative extended Huckel molecular orbital (lEHT) method was used. The stability of the adsorption of the water molecule and of the ions was tested by computing the charge transfer to the cluster and the total energy of the system for various positions of the adsorbate on the surface. 

---

An important frontier in cluster chemistry is the effect of electron count on reactivity. Are clusters merely passive electron reservoirs or does the number of electrons have a critical influence on reactions other than electron transfer Conversely, how does the number of metal atoms, the relative ratio of heterometals, and the specific ligand set relate to the ability of clusters to enter into redox reactions These questions are of fundamental interest, but the answers also may have practical consequences, for example, in the development of metal nanoparticles or colloids to act as electrocatalysts. In a final group of articles Longoni and coworkers review the ability of homoleptic carbonyl clusters to act as electron-sinks , Zanello and Fab-rizi de Biani examine the effect of heterometallic interactions on cluster redox aptitude, and Ignaczak and Gomes report on modelling of electrode interactions with metal clusters. 

Theoretical quantum-chemical study of pyridine adsorption at Hg electrode (including its charged surface) has been described by Man ko et al. [137,138]. An ab initio Hartree-Fock-Roothaan method has been employed. The electrode was modeled as a planar seven-atomic Hg-7 cluster. The deepest minimum of the total energy of the adsorption system was found for positive charge density and Py interacting with the metal through the lone electron... 

After the brief introduction to the modem methods of ab initio quantum chemistry, we will discuss specific applications. First of all, we will discuss some general aspects of the adsorption of atoms and molecules on electrochemical surfaces, including a discussion of the two different types of geometrical models that may be used to study surfaces, i. e. clusters and slabs, and how to model the effect of the electrode potential in an ab initio calculation. As a first application, the adsorption of halogens and halides on metal surfaces, a problem very central to interfacial electrochemistry, will be dealt with, followed by a section on the ab initio quantum chemical description of the adsorption of a paradigmatic probe molecule in both interfacial electrochemistry and surface science, namely carbon monoxide. Next we will discuss in detail an issue uniquely specific to electrochemistry, namely the effect of the electric field, i. e. the variable electrode potential, on the adsorption energy and vibrational properties of chemisorbed atoms and molecules. The potential-dependent adsorption of carbon monoxide will be discussed in a separate section, as this is a much studied system both in experimental electrochemistry and ab initio quantum electrochemistry. The interaction of water and water dissociation products with metal surfaces will be the next topic of interest. Finally, as a last... 

Brief consideration is now given to the solvent structure at metal/aqueous electrolyte interfaces.Several molecular models have been proposed which treat a single layer of water molecules at the metal surface. Within the layer, the individual water molecules (or clusters of molecules) are allowed to have certain orientations. In the earliest and simplest molecular model, an inner-layer water molecule is oriented as a result of its dipole interaction with the charge on the metal electrode. Orientation is limited to either of the two positions in which the molecular dipole is perpendicular to the electrode surface. More realistic treatments have since been described which variously... 

---