The possible importance of hydrolysis effects in the early stages of metal surface electrooxidation reactions-with particular reference to platinum

Abstract

It is demonstrated, using cyclic voltammetry techniques, that the potential required to intiate oxidation of a platinum surface in aqueous media shows a variation of about 90, rather than 59 mV per unit change in solution pH. It is also claimed that this result is in agreement with previously published charging curve data for platinum, and cyclic voltammetry results for nickel. The effect is attributed to the formation of anionic oxyplatinum species at the electrode surface in a manner similar to that described recently in connection with the behaviour of hydrous gold oxide films. With increasing oxide coverage the process is complicated by a place-exchange reaction, so that a return to conventional (59 mV/pH unit) behaviour is observed. It appears also that the latter effectively masked the 90 mV/pH unit behaviour for this electrode system at low sweep-rates, i.e. place-exchange in this case is significant even at low sweep-rates. A shift of about 50 mV/pH unit is recorded for the main voltammetric peaks for hydrogen on platinum. This is tentatively ascribed to the presence of a slightly negative charge on the chemisorbed hydrogen species—a suggestion supported by literature data.