Potentials of Zero Charge General Trends

The number of platinum group metals in Fig. 5a is hmited to three because other metals are less studied. For Pd, complications appear because of hydrogen absorption at potentials below ca. 0.1-0.15 V (RHE). This interval is so wide because highly defective dispersed palladium absorbs a lot of hydrogen in the a-phase, to 

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. 

For osmium, early oxidation problems are even more pronounced, and no systematic thermodynamic study was completed. A number of important experimental facts about electrodeposited osmium (or better to say, osmium oxohydroxide ) electrochemistry are collected in Ref. 79. 

The data on zero charge potentials in alkaline solutions are mostly available for platinum and rhodium, being closely related to the studies of oxygen adsorption. Oxygen UPD phenomena is beyond the frames of this review, but some intersections are inescapable, as just for these systems an interesting phenomenon of inverted pzfc is the most pronounced. 

Direct experimental observations of both pzfc in solutions of certain composition were reported for the following systems for Ir in alkaline iodide-containing solutions for Pt in sulphate solution, pH 6 for Rh in chloride solutions, pH 5-9 for Ru in chloride solution, pH 2.  

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