| Abstract
| - The oxygen reduction reaction (ORR) in acid and alkaline solutions is investigated on thin Pd films (0 < θPd< 1.5 ML) supported on a Pt(111) electrode. In 0.1 M KOH, the general form of the current vs ϑPd relationshipfor the ORR has a volcano shape, with the maximum catalytic activity exhibited by a surface modified with1 ML of Pd. However, if the amount of Pd deposited exceeds 1 ML, the reaction is inhibited and 1.5 ML ofPd are less active than unmodified Pt(111). In 0.1 M HClO4, none of the Pd films are more active thanPt(111). In solution containing Cl- (ca. 106−10-5 M), the inhibition on the Pt(111)−Pd electrode is muchmore severe than on bare Pt(111) due to stronger anion adsorption on Pd. We present an interpretation of thekinetics of the ORR on Pt modified by thin Pd films in alkaline and acid solution based on the electronicproperties of the Pd films and how the electronic properties effect specific anion adsorption, the formation ofOHad and the adsorption of the reaction intermediates.
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