Electrochemical Contact Adsorption Site Changes Driven by Field and Charge: Fact and Theory
When solvated anions like chloride or cyanide adsorb on metal electrodes from aqueous media they lose part of their solvation shell yielding a direct metal-ion bond. For certain anions this ‘contact’ adsorption can occur even when the electrode carries net negative charge. On the basis of classical electrochemical measurements and deductive reasoning using simple models, electrochemists have evolved a detailed ‘standard model’ of the disposition of ions, water, and organics adsorbed on the electrode in the range of potentials where the electric double layer is thermodynamically stable. There is evidence that the anions are physisorbed. However, it is only in more recent years through a variety of in situ probes of vibrational properties (ftir and raman) and geometry (sexafs and gixs) that this model is being really tested.
This paper briefly surveys what is known about contact adsorption and the electric double layer of noble metal electrodes. Some of the in situ experimental probes in current use are described. Then we describe how theory in the form of self consistent field (scf) model cluster calculations has provided some detailed insight into the nature of the adsorption bond. The interplay between experiment and theory is illustrated through discussion of a series of progressively more complex ligands. Starting with halides like chloride (Cl-), diatomics like cyanide (CN - ), small polyatomics like azide (N 3 -) and thiocyanate (SCN -) and concluding with some larger more complex ligands like sulphate (SO 4 --) and bisulphate (HSO 4 -).
KeywordsSurface Enhance Raman Scattering Electric Double Layer Compact Layer Tuning Rate Noble Metal Electrode
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