Hydrogen evolution reaction description

These descriptions about the rate of the mechanism of the hydrogen evolution reaction and how the mechanism changes with increase of M-H are rather simple, but they are consistent with the observations and the proposed change of r.d.s. can be regarded as probable. In fact, over the 78 years that have passed, it is still current thinking. 

The hydrogen evolution reaction is an example where its electrocatalytic character shows that it is necessary for both the description of the adsorption process and the knowledge of the kinetic parameters. Most analysis of the electrocatalytic properties involve correlations from the estimated exchange current densities with characteristic electric potentials, free energies of adsorption, enthalpy of sublimations for the metal electrode, etc. [60]. 

The quantity a is defined, as is well known, as the ratio of the slopes of the potential curves at the point of their intersection. Since these curves are not straight, the ratio of slopes generally varies with AU and can be assumed to be constant only over a comparatively narrow range of heats of reaction. Indeed, in the analysis of some chemical reactions (in particular, the proton transfer reactions), a smooth variation of a was observed over a wide range of AUq (of the order of 1 eV/molecule)[16], but as a rule, a smooth variation of the true value of a with potential has not been reliably established for electrode reactions. Moreover, for the hydrogen evolution reaction the value of a is found to be fairly constant for an overpotential interval of about 1.5 V[l,17j. This difference in the behavior of chemical and electrode reactions has not received a satisfactory explanation in any of the existing molecular theories of an elementary act. We shall return to this problem later here, it should suffice to note that although a detailed description of the dependence of a on AU (or n) cannot be obtained at the moment. 

In 2003, Shima and Ue from Mitsubishi Chanical Corporation and Yamaki from Kyushu University presented the mechanism behind the overcharge prevention. According to the description in the relevant reference, the aromatic compounds without hydrogen at the benzylic position (e.g., t rt-butylbenzene) evolve mainly carbon dioxide (CO2) gas, which is generated by the indirect decomposition of carbonate solvents. The CO2 gas evolution reaction using a redox mediator is expected as a new overcharge protection method [3]. 

The building of heavier elements from hydrogen is the source of energy for stars. Such fusion reactions are exoergic only through iron, however. Yttrium and the lanthanides are products of nuclear reactions incidental to stellar evolution, e.g., explosions of supernovae. They are the products of the stepwise capture of many neutrons by nuclei of iron or by heavier nuclei already synthesized from iron. Some lanthanide isotopes are produced when successive neutrons are captured on a slow time scale. Under those conditions, each nucleus produced by the capture of a neutron, if radioactive, had time to convert the extra neutron to a proton by beta decay before the next neutron was absorbed. Other isotopes resulted from neutron capture on an incredibly rapid time scale. Parent nuclei were exposed briefly to a flux of neutrons so intense that they absorbed all the neutrons that could be contained in energetically bound states. Afterwards, a series of beta decays ensued until a stable ratio of neutrons to protons was reached. The most proton-rich (and relatively rare) lanthanide isotopes were produced by nuclear reactions that absorbed protons. Detailed descriptions of nucleosynthetic processes are given by Clayton (1968) and in the more recent literature of astrophysics. 

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