Abstract
A particular choice of experimental conditions permits study of the electrochemical behavior of hydrogen on the surface of palladium. As in the case of platinum, two adsorbed states are distinguished. The anodic desorption of these superficial species as a function of pH shows the same behavior only for the more strongly bound form on these two metals. The oxidation reactions of the two adsorbed forms present the same “barrier-less” mode of activation in neutral media. The difference in free energy of adsorption between the two superficial forms decreases from 4.8 kcal per at. g for palladium to 3.2 kcal per at. g for platinum. These results can be explained by means of two different features of the electron screening of the proton by the conduction electrons of the metal. At low concentrations the screening for the more strongly bound form occurs with an electron contribution which is primarily of d character; whereas at higher concentrations, after the d vacancies are saturated, the electron contribution becomes of s character.
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