Chemisorption field dependence

In general, two types of adsorption are distinguished, physical adsorption and chemisorption, which depend on the type of interaction established between the adsorbent and the adsorptive. In a chemisorption process, specific chemical interactions between the adsorbent and the adsorptive occur, and the process is not reversible. On the other hand, physical adsorption includes attractive dispersion forces and, at very short distances, repulsive forces, as well as contribution from polarization and electrostatic forces between permanent electrical moments and the electrical field of the solid, if the adsorptive or the adsorbent has a polar nature. In this case, the process is fully reversible (or almost reversible). Thus, the overall interaction energy ( >(z) of a molecule of adsorptive at a distance z from the surface of the adsorbent is given by the general expression... 

Field Dependent Chemisorption and the Interfacial Stark Effect General Relationships... 

It is a natural tendency of electrochemists to be interested in the potential or electric field dependence of a chemisorption bond, hi particular, electrochemists would like to know how the nature of the chemical bond, its coordination to the surface, the bond strength, and vibrational frequencies, both substrate-adsorbate and intramolecular, depend on the electrode potential. In recent years, some general rules concerning field-dependent chemisorbate bonding have emerged from extensive DPT calculations, and it is the aim of this section to review these advances. 

Ever since the first in situ Infrared spectral measurements of CO adsorbed on metal electrodes, the experimentally observed potential dependence of the vibrations of CO in its chemisorbed state has attracted much attention. This concerns both the intramolecular C-0 stretch and the metal-adsorbate M-CO vibration. Because of the great significance of this system to electrochemical surface science, a separate section devoted to the field-dependent chemisorption of CO on (transition-) metal electrodes is warranted. This is also the electrochemical system that has served as a paradigm for the application of quantum-chemical techniques to field-dependent electrochemistry, starting with the semi-empirical work of Anderson and the ab initio Hariree-Fock based calculations of Bagus and coworkers. ... 

In collaboration with the Purdue group, we have carried out detailed analyses of the field-dependent chemisorption of CO on transition-metal (111) surfaces by DFT-GGA calculations on 13-atom clusters.One of the objectives of our studies was to provide relationships between binding energetics, geometries, and vibrational properties, both of the intramolecular C-0 and the metal-adsorbate bond. 

Wasileski SA, Koper MTM, Weaver MJ (2001) Field-dependent chemisorption of carbon monoxide on platinum-group (111) surfaces relationships between binding energetics, geometries, and vibrational properties as assessed by density functional theory. J Phys Chem B 105 3518-3530... 

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

As we have seen, the chemisorption properties of the substrate depend on its electronic structure, so that changes in the latter are reflected in the former. In the case of electrified substrates, the strength of the applied electric field governs the substrate modification and, thereby, regulates the chemisorption process in a controllable manner. 

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