Evidence for electrosorption of halides on polarized iron

Abstract

The effect of temperature on the cathodic polarization of Fe is reported in H 2SO 4 solutions containing Br − and I −. Both ions decrease the rate of the hydrogen-evolution reaction (her). The results are discussed in connexion with (i) changes in the structure of the double layer which influence both the apparent heat of activation Δ H* for the her, and the pre-exponential term of the exchange cd, and (ii) blocking of the surface. While their effect on Δ H* is relatively small, halides produce a considerable decrease in the pre-exponential term. Because double-layer contributions to the latter cannot be calculated separately, the whole change is related to surface blocking. This allows the calculation of apparent surface coverage as a function of temperature, and based on the Langmuir isotherm estimates are obtained for the enthalpy and entropy of adsorption. Positive values are found in both cases. The results provide evidence for electrosorption of halides on Fe.