Platinum electrodes oxygen adsorption

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

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. 

A bare platinum electrode was cleaned as previously described and then allowed to come to adsorption equilibrium with air in the gas chamber. When dry nitrogen was passed through the chamber, the potential difference between the platinum surface and the FEP resin-coated electrode increased rapidly, reaching an equilibrium value 0.5 volt higher in approximately 2 hours. Dry oxygen was then introduced into the chamber and the surface potential of the platinum decreased to a steady-state value of + 0.2 volt relative to the reference electrode (see Figure 6). This system was allowed to remain in that condition for 24 hours, during which time there was only a 20-mv. increase in the contact... 

This rather simple experiment demonstrates quite clearly the poisoning of the oxygen reduction process by copper. Incidentally, Cu(II) is a rather common impurity in distilled water and mineral acids, and this experiment demonstrates that underpotential deposition of a monolayer of copper from solutions containing as little as 1 ppm Cu can drastically affect the behavior of a platinum electrode. Adsorption of small amounts of other impurities (i.e., organic molecules) can also have an effect on solid-electrode behavior. Thus, electrochemical experiments often require making great efforts to establish and maintain solution purity. 

Complications with ruthenium are induced by early oxygen adsorption, as well as by the shift of research interest to its catalytically outstanding alloy with platinum (see recent review). Only one paper contains some data on free electrode charge of electrodeposited Ru and Pt-Ru in alkaline and acidic chloride medium. The most important result is the shift of pzfc for Pt-Ru (ca. 0.06 V towards less positive values), as compared to Pt imder the same conditions. As can be judged from some curves, pzfc of pure (non-oxidized) Ru finds itself in the region of negative RHE potentials. 

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