THE MECHANISM OF OCCLUSION OF HYDROGEN BY PALLADIUM IN CONTACT WITH SULFURIC ACID SOLUTION¹

The Kinetics of Absorption and Evolution of Hydrogen by Palladium and Palladium/Platinum Electrodes

“…Dependence of rates on hydrogen ion concentration and temperature.--Potential decay during the evolution of hydrogen was determined for the 2.73 % Pt alloy in 1.0N, 0.2N, and 0.02N HC1; no significant dependence of k~ on hydrogen ion concentration was observed in this range. This result is in accord with the relative independence of the rate of absorption on hydrogen ion concentration when desorption can be neglected (1,2).…”

“…It was concluded from these results that diffusion of hydrogen within the metal cannot be the rate-determining step in absorption from solution. In addition, Fallon and Castellan (1) have calculated coefficients of diffusion for hydrogen dissolved in solution from their absorption rates; these coefficients proved to be in good agreement with values previously established, indicating that diffusion of hydrogen molecules up to the surface is most probably the slow step under the experimental conditions employed. [From similar calculations based on absorption curves of palladium presented earlier (3), Von Stackelberg has also suggested this to be the slow step (4)].…”

“…The accompanying electrode potentials indicate that the rate of desorption should be significant at a relatively early stage of this absorption experiment (see Eq. [1] and [2]).…”

“…1). Advantage has been taken of this (1,2,3) to measure the rate of absorption of hydrogen over the = ~ fl transition regions when the constant electrode potential corresponds to pressures, p, which are sufficiently low for desorption to be unimportant in Eq. [2].…”

“…The kinetics of the absorption of molecular hydrogen from acidic solutions by palladium have recently been examined (1,2). Under conditions where the rate of desorption could be neglected, these studies showed that the rate of absorption depends on the hydrogen pressure to the first power and that the rate-determining step had an activation energy of < 2 kcal/mole H~.…”