Volmer step

TABLE 3.8 Calculated energies of activation for hydrogen adsorption (Volmer step) on various... 

We shall, however, not dwell on this matter since this would not affect very much the general analysis to be dealt with below, except that the kinetics might be different from those anticipated on the basis of an ordinary Volmer step. Although the formation of Na(a) on the electrode is probably well established, particularly on soft metals, there might still remain a discussion as to Na(a) being actually a reaction intermediate of the HER or simply a by-product. Other reaction routes, involving reaction intermediates such as H2(a), H"(a), and solvated electrons,etc. have not gained sufficient experimental support to demonstrate that these species should be... 

Similarly, i t is expressed, assuming quasiequilibrium Volmer step, hence taking tip = 2 and n, = 0,... 

Similarly, for the rate-determining Volmer step, we obtain from Eqs. (44) and (45),... 

Results of the experiments have been summarized elsewhere, and only brief comments will be given here (a) results which support the Tafel-Volmer reaction route were obtained on Rh (in acid) and Rh and Ni (in alkaline solution), and partly on Pt, Pd, Ir, Au, and Ag (in acid solutions) (b) elementary step rates are frequently of comparable magnitudes, particularly on Rh and Ni (c) correspondingly, the stoichiometric numbers often deviate from integer values and (d) kinetics of the cathodic component rate of the Tafel step was second order in au, while that of the Volmer step indicated its symmetry factor (1 - /3) to be close to unity. Some results are summarized in Table 1/ ... 

This means that the overall affinity is more or less evenly distributed between both steps. Hence, the affinity values of the steps become appreciable, and the rapid step should eventually become irreversible as seen from Eq. (13). In Figure 8 is shown the growth of rjy for the case mo = 10 as a function of 17. For example, it is seen that the approximation for the Volmer step to be in quasiequilibrium (say, i7v < 5 mV) is permissible only in the overpotential range, -60 < rj/mV < 150, for the case 0 = 0.01. A much larger value of mo would be needed in order for rfv to be sufficiently small at cathodic overpotentials, of magnitudes encountered in ordinary experiments. 

Heyrovsky step is rate determining. However, the latter view may not be justified, because the difference of Eq. (86) from Eq. (84) originates from Z, or the (1 - ) term. It seems more appropriate to say that the rate is still determined by the Volmer step but, since the free electrode surface now plays a decisive role, and its magnitude depends on the rate of the Heyrovsky step, the rate expression involves kinetic parameters of that step. ... 

In conclusion, the break of the cathodic Tafel line in the Heyrovsky-Volmer route is not due to onset of surface saturation with H(a) with the Heyrovsky step rate determining, but to the Volmer step becoming rate determining. ... 

It can readily be understood that Eq. (108) would be valid only when mechanism A-1 (or B-1), Section 3 is operative, because in this case we have, from the quasiequilibrium condition of the Volmer step,... 

The HER on platinum proceeds through two, successive, elementary reaction steps H2 oxidation takes place by the reverse sequence. The first common step in the HER is the Volmer step, in which a proton is discharged to form adsorbed hydrogen atoms ... 

AG /RT for an HER mechanism with the Volmer step in quasi-equilibrium and for die coupled Volmer-Tafel mechanism. From Table 2, the maximum of the HER current occurs at 0jj = Vi if the S5mmetiy factors are equal to Vi or for the two coupled mechanisms if P = 8 or P = y. For the coupled Volmer-Tafel mechanism. Figure 10b shows a case where X is minimum for 0jj V2 and maximum for extreme values of coverage (0jj 1 or 0jj 1), i.e., of the adsorption fiiee energy. 

This situation is the aqueous solution analog of gas-phase charging from H2. For example, let us consider the case where the Volmer step is in quasi-equilibrium, i.e.,Vg Vg (this mechanism is operating on noble metds at low overpotentials). By combining Eqs. (25) and (26), the following expression is obtained ... 

Figure 2.10a and b show the theoretical variations of log 4 (here 4 = x her), 0H/ and Xjj versus -AG JRT for a HER mechanism with the Volmer step in quasiequilibrium and for the coupled Volmer-Tafel mechanism. The symmetry factors are taken equal to 2, and a Langmuir adsorption model is taken, where AG ds is not coverage dependent. 

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