Adsorption surface coverage versus potential

Fig. 2 Comparison of SO4, Cl , Br , and 1 adsorption at the Au (111) electrode surface from 0.1 M HCIO4 + 10 M K2SO4 and 0.1 M KCIO4 + 10 M KCl, KBr, and KI solutions, (a) Cyclic voltammograms at u = 10 mV s (b) Gibbs surface excess versus potential, (c) Gibbs energy of adsorption versus potential. The standard state corresponds to the surface coverage / = 1 ion per cm and bulk concentration c = 1 mol dm [36].

The reason for these discrepancies is quite probably to be ascribed to a strong dependence of the activity coefficient of /, f(rs)irs, upon E and rs at the high surface coverages employed for the estimate of l. In particular, I values obtained from Ms versus E plots at constant Es are affected by the potential dependence of the activity coefficient at constant rs (see Eq. 21) conversely, the l values obtained from the dependence of crM upon rs at constant E are affected by the 7s-dependence of the activity coefficient at constant E. These different dependences may have opposite effects on the l values obtained on the basis of the two alternative thermodynamic definitions. This may also explain the anomalously high l values for bromide and chloride adsorption on polycrystalline Ag obtained by Schmidt and Stucki16 from Ms versus E plots at constant Es. For this reason, at high surface... 

At = const, when the adsorption isotherm is congruent with respect to the electrode potential and the surface coverage is low, the effective standard rate constant may be determined with the posterior correction for the double-layer effects. Besides, in some cases (for instance, in the case of M j M"" electrode), faradaic elements do not depend on the bulk concentration of M"" [18]. This effect may be useful in studies of Cjj in the presence of faradaic process. At last, no surface area of the electrode is required to obtain the parameter k (see Eq. (5.56)). Dependence of directly measured capacitance (unrelated to the surface unit) versus direct current (instead of i) may be used for this purpose. 

As a conclusion, the promoting effects of adsorption site blockers on H entry can be understood not by considering the effective diminution of 0h the overall coverage of OPD H (determined by integration of adsorption pseudocapacitance versus potential curves [109,110,129]), but by considering the variation of Xh the local OPD H coverage in the sites not blocked (obtained by normalization of 0jj or from the Tafel and permeation slopes see Table 2.1). If the surface-bulk transfer step is in equilibrium or quasiequilibrium, a significant increase of the bulk H concentration beneath the surface can be induced by ASB surface effects only for the HER mechanism where the steps of electroadsorption and chemical combination are coupled. This analysis provides a quantitative explanation of the effects of promotion of H absorption into iron and ferrous alloys. 

Peng et al. combined electrochemical surface-enhanced infrared spectroscopy (EC-SEIRAS) and DFT calculations to probe the Sb adatom enhancement mechanism on polycrystalline Pt surfaces [27]. The forward cyclic voltammogram in 0.1 M formic acid and 0.5 M H2SO4 showed a 2.7 x decrease in COads at potentials below 0.2 V versus RHE for a 0.6 mraiolayer (ML) Sb, with a tenfold current increase at 0.5 V in the forward scan. They concluded at coverages >0.25 ML that the [Sb] [Pt] dipole interacticHi enhances CH-down adsorption. This is consistent with Leiva et al. s work presented above. They additionally attributed this coverage dependence to a decrease in the Pt-COads bond strength with increased Sb coverage. 

The OH adsorption isotherm on various Ag (hkl) surfaces as a function of the electrode potential was reported by Blizanac et al. [47], as shown in Fig. 15.10. It is seen that the OH coverage (0qh) was quite low at a cell voltage of about 0.1 V versus RHF and increased with increasing electrode potentials (0.2-0.4 V vs. RHF). Then, a Oon plateau was observed at potentials of 0.4—0.7 V versus RHF, and a new steep increase in 0aa was observed when the electrode potential was higher than 0.7 V... 

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