Mechanistic analysis of the hydrogen evolution and absorption reactions on iron

The work in this thesis investigates the effect of additives on the kinetics of the hydrogen evolution reaction (HER) and hydrogen absorption reaction (HAR) on iron. The electrochemical hydrogen permeation cell has been used to collect data on both reactions in the absence and presence of the additives. The effect of two additives on the kinetics of both the HER and HAR on iron in acidic solutions was quantified. These two compounds have different behaviors towards both reactions. While benzotriazole (BTA) inhibits both reactions, iodide enhances hydrogen absorption while inhibiting the HER. Analysis of the results using the IPZ (Iyer, Pickering, Zamanzadeh) model shows that both compounds inhibit the HER by decreasing its discharge rate constant and hence the exchange current density. On the other hand, while BTA decreases the rate of hydrogen absorption by decreasing both the hydrogen surface coverage and the kinetic-diffusion constant, k (see chapter 5), iodide ions decrease the rate of hydrogen absorption by increasing the kinetic-diffusion constant, k, while decreasing the hydrogen surface coverage (see chapter 6).; A separate study was devoted to investigate the effect of thiosulfate on the kinetics of the HER and HAR on iron (chapter 7). It was shown that thiosulfate enhances both reactions in acidic solutions. The promoting effect was mainly due to its decomposition product H₂SO₃ with a small contribution from the colloidal sulfur and/or the undecomposed thiosulfate.; In chapter 8 it was shown that the polarization data of the hydrogen evolution reaction (HER) can be analyzed to calculate the hydrogen surface coverage and the rate constants of the hydrogen discharge and recombination reactions for metals which have very low permeabilities of hydrogen, and on which the HER proceeds through a coupled Volmer discharge-Tafel recombination mechanism. The analysis is applied to the results of the HER on copper and iron and the rate constants obtained using the analysis were comparable to some values reported in the literature on both metals.; Chapter 9 deals with the mathematics of the hydrogen oxidation reaction on metals where the mechanism is a Tafel-Volmer. This new analysis can predict the values of the hydrogen surface coverage and the rate constants of both the Tafel and Volmer reactions using the potential-current data which are readily available from polarization experiments.