DEPENDENCE UPON pH OF THE RATE OF DEUTERIUM EXCHANGE BETWEEN HYDROGEN AND WATER ON NICKEL CATALYST

Experimental studies were carried out on the dependence upon solution pH (7-13) of the rate of deuterium exchange between hydrogen and water on nickel catalyst, both in presence and absence of supporting electrolyte. The exchange rate was found to be independent of the solution pH throughout the systems chosen: KOH, KOH+KCI, NaOH+NaCI04 and KOH+ K2S04• The facts strongly support the theory that the hydrogen electrode reaction on nickel in alkaline solution obeys the catalytic mechanism. Introduction The mechanism of the hydrogen electrode reaction (HER) III alkaline solution, ( 1 ) on nickel has been studied repeatedly in the past, and two opposite views have been expressed, viz. assuming that the overall reaction consists of two consecutive steps, and 2 [H20+e~H(a)+OH-], 2H(a) ----> H2 , (2. a) (2. b) some authors have proposed that the first step is rate-determiningl ) (in the slow-discharge mechanism), others hold that the second step (in the catalytic mechanism) is rate-determining. There has been a certain amount of experimental evidence for the slowdischarge mechanism to be operative on nickel: (i) the overpotential at constant polarization current depends upon pH of the solution l ), (ii) a linear Tafel relation with 0.12 v slope ,2) has been frequently observed, and (iii) the value * The Research Institute for Catalysis, Hokkaido Univ., Sapporo, Japan. 248 Deuterium exchange reaction on nickel of the stoichiometric number, ].I, of the rate-determining step has been reported to be tw0). The first piece of evidence has, however, been criticized) with regard to possible inclusion in the reported results of the ohmic overpotential which could depend on the solution pH. The second observation has been interpreted as well on the basis of the catalytic mechanism by taking into account the mutual interaction among the hydrogen intermediates on the electrode surface). The value of ].I has later been found to be unity5) in disagreement with the third observation. It seems, therefore, that the view of the slow-discharge mechanism to be operative on nickel is ill-founded. Conversely, these facts clearly support the catalytic mechanism and, further, later experimental findings such as the observation of a cathodic saturation current density6) at ca. 10 Amp/cm and of an anodic one7) at ca. 10-5 Amp/cm (the limiting current due to diffusion of molecular hydrogen in solution should be far greater than this figure) and the results of a detailed analysis of the transient behavior of the electrodeS) are all in good harmony with this mechanism. Studies on the isotopic composition of the gaseous hydrogen obtained by the exchange reaction between pure deuterium and light water catalyzed by nickel catalyst, however, did not support this mechanism). Thus, the composition of the hydrogen obtained was not very far from that in equilibrium with respect to the reaction,