Electrochemical desorption step

Step 2b Electrochemical desorption step (ED step) or ion-atom combination step ... 

In previous material—when discussing the mechanisms of hydrogen evolution— the value 0H has been an important factor in the argument. For example, if Gjj is laige (approaching unity), it is likely that the mechanism of desorption of H from the surface to form H2 will be via the electrochemical desorption step rather than that of recombination. 

Thfel slopes different from the usual ones may also result if the r. d. s. proceeds barrierless (i.e., with a = 1) (or quasi-barrierless [201]). Thus, hydrogen evolution with barrierless discharge - step (3) - or barrierless electrochemical desorption - step (4) - as the r. d. s., is expected to occur with a Tafel slope of 60 and 30 mV, respectively [202], This behavior has been reportedly observed with Hg [203], Bi [204], Ag [205], and Au [206]. However, such an experimental observation takes place only under very special conditions and cannot have any relevance to practical electrolysis. 

In certain cases encountered experimentally, for example, for the HER at Ni or Ni-Mo alloys (75), the electrochemical barrier symmetry factor for the initial proton-discharge step [Eq. (4)] may be close to that for the electrochemical desorption step [Eq. (5)] then a limiting coverage ([Pg.42]

When appreciably strong lateral interactions between electrosorbed intermediates arise, for instance, when they are partially charged species, the R values and the corresponding Tafel slopes can be worked out for various mechanisms they are usually closely related, functionally. We show the results of calculations for the heterogeneous recombination and the electrochemical desorption steps, for example, for the case of H in the HER or Cl in CI2 evolution. The equations for recombination desorption are... 

Tafel slope, b. In the case of electrochemical desorption (step S), R can only change from 2 to a lower limit of 1 as b increases from RT/(1 + / )F (0 1) to RT/pp (0 - 1). It is trivial to extend these relations for the following reaction schemes ... 

Integration of the Q versus r profiles (Fig. 18) gave the A0 versus rj relations reproduced in Fig. 19. Interestingly, these approach limits of 0 below 1 this, however, is the required behavior according to the mechanism of the HER in which the rate is controlled by the electrochemical desorption step [reaction (5)] then 0 attains limiting values determined by the rate constant ration kj[k kj) for the coupled reaction [Eqs. (4) and (5)] if / ... 

Harrison and co-workers (405, 406), based on a Tafel slope value of 30-38 mV and an Rq value of 1, first proposed a fast discharge-slow recombination mechanism (note that this mechanism predicts R j = 2) and later attributed (405) a b value of 40 mV to the slow electrochemical desorption step. As these results do not fit with a fast discharge-slow electrochemical desorption, for which Rd should be 2, they proposed (397) an alternate mechanism involving HCIO, which was inferred from investigations on cathodic reduction of CI2 ... 

From these studies, it may be concluded that the mechanisms of the various metals fall into three groups in acid solution on the high overpotential metals (e.g., Hg, Pb, Sn), the discharge step is ratedetermining on the medium overpotential metals (transition metals), the electrochemical desorption step is rate-determining and on the low overpotential metals (noble metals), the recombination step is rate-determining. Table II illustrates the mechanisms predicted for some metals from a systematic investigation. 

The overall reaction (A) may involve the following three combinations of elementary steps (a) double occurrence of the discharge step (B) in the cathodic direction followed by the recombination step (C) (b) a single occurrence of the discharge step (B) followed by the electrochemical desorption step (D) and (c) a single occurrence of the dissociative adsorption of the H2 molecule (C), directed from right to left, with the electrochemical desorption (D) subsequently occurring twice. These three possibilities determine three different paths of the reaction as a whole. 

In other barrierless reactions, particularly chlorine evolution on graphite, no limiting current in the backward process was observed, the reason being that, in these cases, the slow step of the forward reaction was the transfer of the first electron, followed by that of the second, e.g., in an electrochemical desorption step. In the backward process, the slow activationless step is, in this case, preceded by the transfer of a single electron. The relationship between the rate of this process and the potential masks the limiting current phenomenon. 

The nature of intermediate has been rarely disputed. The suggestion [14] that reaction (2) might involve (H2 )ad, has not received any experimental confirmation. The possibility that the various steps proceed at comparable rates [15], rather than with a single rds, has also been suggested and theoretical calculated values of Tafel slopes for these cases are also presented in Table 1. Hydrogen evolutiOTi with barrierless discharge - step (1) or barrierless electrochemical desorption step - (3) as the rds [16] is expected to occur with a Tafel slope of —60 and —30 mV, respectively. This behavior has been observed with Au [17] and Ag. It has been also observed that hydrogen evolution could occur at Ni... 

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