The effects of potential and p H on the rate of the hydrogen evolution reaction on zone‐refined titanium have been determined in acidic sulfate and chloride media over the p H range of 0 to 4.0. In all cases, the cathodic Tafel slope is equal to −120 m.V, i.e. , ( ∂ E / ∂ log | i H , ∝ | ) pH = − 2 ( 2.303 RT / F ) . The reaction order with respect to proton activity, ( − ∂ log | i H , ∝ | / ∂ p H ) E , exhibits a minimum value of 0.49 at p H = 1.83 , and increases to a limiting value of 1.0 with increasing or decreasing p H. A mechanism in agreement with the aforementioned observations has been proposed. The mechanism consists of two parallel paths for rate‐determining proton discharge, one path involving the usual solvated proton and the other involving the adsorbed surface species ( TiOH ) ads + , followed by the fast recombination reaction. Langmuir adsorption kinetics apply to the adsorbed hydrogen atoms. The proposed mechanism is consistent with what is known concerning the effect of potential upon the rate of absorption of hydrogen by the metal. The mechanism is also consistent with the mechanism of active‐state dissolution and passivation of titanium in acidic media, i.e. , the hypothetical existence of a 3‐d phase oxide (as opposed to adsorbed oxy and hydroxy titanium species on the submonolayer level) on titanium in the active state is rejected.
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Kelly et al. (1984) studied this question.