Platinum Metals Rev., 1964, 8, (3), 90
Alternating Current Polarisation of Noble Metal Surfaces
The Formation of Platinum and Palladium Blacks
It has been observed that the reproducibility of a platinum electrode can be improved by a pre-anodisation treatment. The explanation of this effect has been in some doubt, and a recent paper by Dr J. P. Hoare of the General Motors Research Laboratories (Electrochimica Acta, 1964, 9, (5), 599-605), throws new light on the matter.
The effect of AC currents on small beads of noble metals was studied and it was noted that whereas black films were formed on the surface of platinum and palladium when treated in this way, no such films were observed on gold, iridium or rhodium. Some oxidation did occur on the gold and iridium and, if a DC current was superimposed on the AC so that the AC swing was not centred about a point more oxidising than that originally used, then the ‘black’ formation did not occur on platinum or palladium and oxides were formed here also.
Dr Hoare deduces from his study that the mechanism of the formation of these blacks is via the dissolution of hydrogen, since it is well known that platinum and palladium will react readily with hydrogen, whereas gold, iridium and rhodium will not. The break-up of the surface is attributed by the author to the successive absorption and desorption of this hydrogen.
Further, measurements of double layer capacity, a technique that can detect changes of surface area, confirmed that the platinum and palladium surfaces increased in area after polarisation whereas the rhodium surface did not. The increases of area noted for the gold and iridium are explained by the formation and reduction of oxide films.