The influence of the tunnel probability on the anodic oxygen evolution and other redox reactions at oxide covered platinum electrodes

Abstract

Potentiostatic and galvanostatic pulse measurements were carried out to investigate the anodic oxygen evolution at platinum electrodes in 1N H 2SO 4 in dependence on the oxide layer thickness d and the electrode potential ε. The thickness d (1·5–10 Å) was obtained from cathodic charging curves. Further, the temperature dependence (0°–81°C) was evaluated from Bowden's measurements. Summarizing, the current i o2 follows the relation, log i = A - ( E 0 a - α Fη)/2·3 RT - d/d o. The experimental activation energy E o = E o a = α Fη decreases linearly with increasing overvoltage η. The linear decrease of log i with increasing d, which is given by the term d/d o, is correlated to the probability of the quantum mechanical tunnel transition of the electron from adsorbed ions, OH − ad or O 2− ad respectively, through the oxide layer to the metal. Similar effects of the oxide layer thickness on the current density were observed in the case of the oxygen evolution at iridium, the CO-oxidation on platinum, and the reduction of Cl − and Ce 4+ at platinum. In these cases a rate determining electron transfer through the oxide layer is also assumed.