DIFFERENT ROUTES IN THE FORWARD AND BACKWARD OCCURRENCES OF THE HYDROGEN ELECTRODE-REACTION ON PT SINGLE-CRYSTAL ELECTRODES IN ACID-SOLUTION

The hydrogen evolution and ionization reactions were studied on Pt single crystal electrodes in N2 or H-2 saturated, acidic solutions. The hydrogen evolution reaction was structure insensitive and the ionization reaction was structure dependent. The Tafel constants of the respective reactions, alpha over arrow pointing right and alpha over arrow pointing left, and the reaction orders. z(H+) over arrow pointing right and z(H+) over arrow pointing left, with respect to proton, did not satisfy the relationships, alpha over arrow pointing right + alpha over arrow pointing left = 2 and z(H+) over arrow pointing left-z(H) over arrow pointing right = -2, deduced by the rate-determining step which is the same for the hydrogen evolution and ionization reactions; The results were, alpha over arrow pointing right + alpha over arrow pointing left almost-equal-to 4 and z(H+) over arrow pointing left - z(H+) over arrow pointing right almost-equal-to -4. These suggest that the rate-determining steps is different for the evolution and ionization reactions in acidic solutions. A tentative model for the reaction routes was proposed.