Abstract
The chemisorption of various organic functional groups from aqueous solutions onto smooth single-crystal and polycrystalline electrodes, and the resistance of the resulting monolayer intermediates towards electrochemical oxidation have been studied; these investigations were motivated by the need to understand, at the atomic level, metal passivation by monolayer organic coatings. The electrodes employed were Rh, Pd, Ir, Pt, and Au whose anodic dissolution is preceded by surface-oxide formation even in highly acidic media. Resistance towards anodic oxidation can thus be associated with the retardation of the formation of surface metal oxides, and the degree of retardation can be taken as one measure of the passivation properties of the organic monolayer. Thirty-eight organic compounds containing at least one surface-active functional group were investigated: alkenes, alkynes, aromatics, heterocyclic aromatics, alcohols, phenols, sulfides, amines, amides, nitriles, CO, aldehydes, carboxylates, esters, and their selected derivatives. Experimental measurements were based primarily upon thin-layer electrochemical methods; selected interfacial systems were investigated by X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy, and Auger electron spectroscopy. The preliminary data suggest the following trends: (i) The stronger the organic monolayer is chemisorbed, the more efficient its corrosion inhibition, (ii) The strength of chemisorption of a given functional group varies with electrode material in the order Pt > Pd ≥ Rh ≥ Ir > Au. (iii) For a given electrode material, the strength of organic chemisorption decreases as -SH > aromatic N > aromatic ring ≥ alkyne > alkene >R2S ≥ aryl -CN > aliphatic N > -OH > alkyl -CN > -COOH. (iv) For a specific surface-active substituent, the higher the molecular weight of the parent organic compound, the better the corrosion inhibition, (v) The more reactive the metal, the greater the extent of organic chemisorption-induced passivation.
