Classical and quantum-mechanical effects in electrochemical proton discharge, and the kinetics at low temperatures

Abstract

In previous studies of the kinetics of electrocheniical proton discharge, it has been implicitly assumed that an " adiabatic " electron transfer mechanism is involved, i.e., the reaction involves an activated complex which is partially (or totally) neutralized. In a previous treatment of the activated complex in rate-determining proton discharge, it was shown that the H/D separation factor SD could be expected to be related to the ratio of exchange current densities R only on the assumption that the activated complex retains its full charge in the activation process. In the present paper, SD and R are shown to be related by reference to experimental and theoretical considerations for an assumed fully charged transition state, and on the basis that proton tunnelling contributions to the rate are not appreciable. Experimental evidence in support of the latter contention is given by low-temperature kinetic studies down to -125 and -150°C in supercooled alcoholic solutions. A model of the activated complex in proton discharge is considered and involves a " non-adiabatic " electron transfer process. In these terms, the activated complex (particularly that for H+ transfer into Pd) is analogous to that in acid-base proton transfer processes ; the electrode plays the role of a base with variable base strength ; the theoretical treatment of Bader based on Platt's calculations of force constants can be applied, giving an account of the potential dependence of SD.

Interpretation and Significance of Heats of Activation for Electrochemical Reactions Exhibiting Anomalous Tafel Slopes

“…Usually the apparent heat of activation is evaluated at zero overpotential from the derivative (3 In io/0 1/T),=o or from the temperature coefficient of overpotential at a given current density (5). At finite overpotential o, the derivative (~ In i/8 1/T)~ also gives the effective heat of activation at potential which differs from that deduced for the reversible potential by the energy fl~F.…”

Primary and Solvent Isotope Effects in the Anodic Evolution of Oxygen

“…A value of 1200 cm -1 for v*mb was chosen empirically because it seems to be quite reasonable by comparison to observed frequencies for stable molecules (29,32,35,42).…”

“…~ qoo--" )2 q*H exp [25r176[27] q-~H-/ q4 D and R2b ~-( qOD--)2 q=~H [28] qOH--~' q4D For the intermediate Temkin type behavior R2b ~-( ~qoD-. )2 q:Dq=~H exp [A~OF/RT][29] …”

Observations and Theories of Quantum Effects in Proton Transfer Electrode Processes

“…In 1934, Bawn and Ogden suggested that a magnitude of proton tunneling in the hydrogen evolution reaction (HER) at metal electrodes can be investigated by measuring the H/D (hydrogen/deuteron) separation factors, S H/D , and they reported that EQPT should have a major contribution in the HER [3]. Following this work and other reports on measuring S H/D , Conway and his co-workers performed careful experiments in order to obtain S H/D by using a mercury electrode (Hg) [5][6][7][8][9]. Their result on EQPT was negative: There were no clear evidence of quantum effects and they concluded that EQPT in the HER at Hg is not significant.…”