Nature of the Oxygen Layers on Platinum Metals

The following reactions have to be considered besides reaction 1 in acid electrolytes  

Here reactions 3 and 4 stand for the formation of the hydroxide M(OH) and the oxide MO respectively. Similar reactions apply in alkaline electrolytes. It is conceivable that the chemisorbed layers and metal hydroxide or oxide are formed simultaneously on different parts of the surface in a certain potential range. The conditions [42] under which the voltammetric i — U curves give evidence for the occurrence of reaction 1 and 2 are discussed subsequently. The electrolyte concentrations are assumed such that concentration gradients of OH or H are negligible in the diffusion layer during the formation or reduction of the oxygen layer. 

In contrast to phase hydroxides or oxides the reversible potential of layer formation becomes more positive with increasing coverage for chemisorbed layers  

Oxygen Layers on Different Materials and Inhibition of Fuel Oxidations 

The following conclusions may be drawn for the case of simultaneous formation of chemisorbed layers and hydroxide or oxide. The sweep reversal will lead to a current reversal if the formation and reduction of one of the chemisorbed layers occur with sufficiently small overvoltage and if the rate of hydroxide or oxide formation is smaller than that of the reduction of the chemisorbed layers. 

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