Platinum oxygen adsorption

Oxygen adsorption that occurs at platinum at potentials more positive than 0.9 to 1.0 V is irreversible, in contrast to hydrogen adsorption. Oxygen can be removed from the surface by cathodic current, but the curves obtained in the anodic and cathodic scan do not coincide cathodic oxygen desorption occurs within a narrower region of potentials, and these potentials are more negative than the region where the... 

The first step of oxide-layer formation is oxygen adsorption (chemisorption). In the case of platinum, the process stops at this stage, and depending on the conditions, an incomplete or complete monolayer of adsorbed oxygen is present on the platinum surface. In the case of other metals, layer formation continues. When its thickness 5 has attained two to three atomic diameters, the layer is converted to an individual surface phase that is crystalline (more seldom, amorphous) and has properties analogous to those of the corresponding bulk oxides. 

Platinum electrodes are widely used as an inert electrode in redox reactions because the metal is most stable in aqueous and nonaqueous solutions in the absence of complexing agents, as well as because of its electrocatalytic activity. The inertness of the metal does not mean that no surface layers are formed. The true doublelayer (ideal polarized electrode) behavior is limited to ca. 200-300 mV potential interval depending on the crystal structure and the actual state of the metal surface, while at low and high potentials, hydrogen and oxygen adsorption (oxide formation) respectively, occur. 

The heat and entropy associated with the above reaction implied by the temperature dependence of the critical oxygen activity were —46 kcal mol 02 1 (-192 kJ mol 02 ) and -45 kcal K-1 mol O 1 (-188 kJ K-1 mol 02- ) in close agreement with the results of Berry.81 Oxygen adsorption and oxide formation on platinum have also been studied using exchange currents in a solid electrolyte cell.82... 

In summary, a variety of LEED patterns have been observed for oxygen adsorption on the platinum metals. However, their interpretation is complicated by the uncertainty in the oxygen coverage. A second complication is the uncertainty introduced by oxygen-induced surface reconstruction. The stability of 22 high index Miller planes upon exposure to oxygen has recently been investigated by Blakely and Somorjai (160). 

The potential cycling illustrated by Figure 5.20 is a commonly used pre-treatment procedure for attainment of a reproducible active surface. Less widely known is the fact that in aqueous solution this cycling procedure causes the dissolution of appreciable quantities of metal. The discrepancy between the integrated anodic and cathodic oxygen adsorption-desorption peaks has been shown to be due to dissolution of the metal. Typical values are given in Ref. 68 and indicate that platinum and gold dissolve to a much lesser extent than do palladium and rhodium. 

Courbon H, Herrmann JM, Pichat P. Effect of platinum deposits on oxygen adsorption and oxygen isotope exchange over variously pretreated, UV-illuminated powder Ti02. J Phys Chem 1984 88 5210-5214. 

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