Potentialities and Peculiarities of Polycrystalline Silver as an Electrocatalytic Material for Organic Halide Detection

The impressive catalytic properties of polycrystalline silver electrodes for the cleavage of the C-X bond in organic halides (X = Cl, Br, I) have been the object of extensive investigations by our joint groups in the last years. The process can be interpreted, in the frame of the Marcus-Savéant theory, in terms of a concerted dissociative electron transfer hinging on a three-center intermediate including the metal surface, and results in huge anticipations of the reduction potentials (even >1 V with respect to the non-catalytic GC electrode). The electrocatalytic properties of the metal surface are widely modulated by the molecular structure, the presence of functional groups on the substrate and the reaction medium, both in terms of the nature of the solvent and the size of the tetralkylammonium cation of the supporting electrolyte. Such properties are extremely appealing not only in view of synthetic applications (affording milder reaction conditions and alternative reaction products), but also in the analytical and environmental fields, for the detection of trace amounts of organic halides. In fact the catalytic activity of silver specifically applies to the above halide functional groups, making possible easy detection at Ag of voltammetric signals, which otherwise, at non-catalytic electrodes, are partially or totally covered by the background limiting current. Most organic halides, including alkyl chlorides that are hardly detectable at conventional electrodes, give rise to neat peaks at Ag at potentials well before cathodic limit. Another important advantage of this electrode material is that it promotes peak differentiation in mixed samples, thanks to the modulation of the electrocatalytic properties by the molecular structure of RX. Moreover, protic solvents, including water, appear to enhance such catalytic activity. A series of model cases will be presented and discussed (concerning both qualitative and quantitative analysis topics), together with the optimized protocol for preparation and management of the silver surfaces.