Isotope Separation Factor and Kinetic Isotopic Effect of the Hydrogen Evolution Reaction
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- Krishtalik, L.I. Russian Journal of Electrochemistry (2001) 37: 102. doi:10.1023/A:1009092015225
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The hydrogen isotope separation factor decreases with increasing overvoltage on mercury-like electrodes in acid solutions. This dependence qualitatively differs from that for the ratio between hydrogen evolution rates in light and heavy water, which increases with polarization. The reason for the difference is that the isotope separation factor depends on the kinetic isotopic effect (KIE) in two reaction stages: discharge and electrochemical desorption. Experimental data show that a decrease in KIE with potential in the latter stage outweighs the increase in KIE in the former. The KIE decreases in the desorption reaction because this process is activationless; therefore, there is no potential dependence of the relative contribution made by vibrationally excited states of the product in this case, i.e. no dependence that is responsible for the increase in KIE at the discharge stage. The same factor has no effect on KIE for a barrierless discharge, which also decreases with increasing overvoltage. Thus, all experimentally observed dependences of KIE on potential are explained in the framework of a unified approach.