Tafel slope intermediates

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

Ito et a/.18 supported the above reaction pathways for various cathode materials, such as In, Sn, Cd, and Pb, from the similarity in Tafel slopes. Hori and Suzuki46 verified the above mechanism in various aqueous solutions on Hg. Russell et al.19 also agreed with the above mechanism. Adsorbed CO J anion radical was found as an intermediate at a Pb electrode using modulated specular reflectance spectroscopy.47 This intermediate underwent rapid chemical reaction in an aqueous solution the rate constant for protonation was found to be 5.5 M-1 s-1, and the coverage of the intermediate was estimated to be very low (0.02). 

The use of Tafel slopes and reaction orders to explore the mechanism of an electrochemical reaction involving adsorbed intermediates is clearly fraught... 

Tafel slope for the chlorine evolution reaction follows an electrochemical desorption-type mechanism, it can be expressed [36, 37] in terms of the electrode surface coverage by the adsorbed Cl intermediates, 0aa, as ... 

At 60 minutes only, dc potentiodynamic curves were determined from which the corrosion current was obtained by extrapolation of the anodic Tafel slope to the corrosion potential. The anodic Tafel slope b was generally between 70 to 80 mV whereas the cathodic curve continuously increased to a limiting diffusion current. The curves supported impedance data in indicating the presence of charge transfer and mass transfer control processes. The measurements at 60 minutes indicated a linear relationship between and 0 of slope 21mV. This confirmed that charge transfer impedance could be used to provide a measure of the corrosion rate at intermediate exposure times and these values are summarised in Table 1. 

The intermediate energy depends on the interaction with the electrode surface. Materials binding OHads weakly as a rule show high overpotentials for O2 evolution. Their mechanism is dominated by step (7.25) as the rate-determining step and the Tafel slope is close to 120 mV. 

Fig. 8.5. In electrochemical reactions involving one or more adsorbed reaction intermediates (sometimes involved in the rate-determining step), the steady-state concentration of the intermediate changes with the potential. However, each intermediate has a time constant to reach the surface coverage corresponding to a given overpotential. The downside of too low a pulse time, or too fast a sweep rate, is that the intermediate concentration does not relax to its appropriate concentration in time. The Tafel slope (sometimes a significant mechanism indicator) may then differ from that calculated for the assumed path and rate-determining step.

Consider the following information on the electrochemical oxidation of methanol on platinum It is known that upon adsorption of CH3OH upon platinum, dehydrogenation occurs. The final product is C02. Several analyses made on the basis of potentiodynamic sweep data suggest the presence on the electrode surface of c -OH. However, further spectroscopic work, particularly that in potentiostatic work in steady state finds linear CO, i.e., S=0, the intermediate. The Tafel slope is 60 mV at low current densities and 120 mV at higher values of the current. Potentiodynamic profiles show that the overall number of electrons in the oxidation is 1.2—1.5 (i.e., two kinds of CO radicals of somewhat different character as to their adsorption are on the surface). 

A relatively constant Tafel slope for reactions not involving adsorption, and those involving adsorption with complete charge transfer across the double layer, distorted by second order effects, may also be explained in terms of a non-Franck-Condon process. Since adsorbed intermediates in charge transfer processes also show adsorption energies depending on potential in the same way as the potential energy barrier maxima, these should also follow the same phenomena. 

Table 11 Tafel Slopes and Reaction Orders Calculated for Two Different Mechanisms, Assuming Low, Intermediate, and High Coverage...

It should be noted that the Tafel slopes just given were calculated for the combined isotherm under Langmuir conditions, namely at very low coverage. The same type of calculation can be repeated to obtain the kinetic parameters for different mechanisms both at low and at intermediate values of the coverage. The effect on the Tafel slope of competition with water is rather small for small molecules. Thus, for n = 1 the Tafel slope changes only by about 2 mV for the two mechanisms just discussed. This is within experimental error in most cases, perhaps explaining why the need to use the combined adsorption isotherm is not... 

The involvement of chemisorbed intermediates in electrocatalytic reactions is manifested in various and complementary ways which may be summarized as follows (i) in the value of the Tafel slope dK/d In i related to the mechanism of the reaction and the rate-determining step (ii) in the value of reaction order of the process (iii) in the pseudocapacitance behavior of the electrode interface (see below), for a given reaction (iv) in the frequency-response behavior in ac impedance spectroscopy (see below) (v) in the response of the reaction to pulse and linear perturbations or in its spontaneous relaxation after polarization (see below) (vi) in certain suitable cases, also to the optical reflectivity behavior, for example, in reflection IR spectroscopy or ellipso-metry (applicable only for processes or conditions where bubble formation is avoided). It should be emphasized that, for any full mechanistic understanding of an electrode process, a number of the above factors should be evaluated complementarily, especially (i), (ii), and (iii) with determination, from (iii), whether the steady-state coverage by the kinetically involved intermediate is small or large. Unfortunately, in many mechanistic works in the literature, the required complementary information has not usually been evaluated, especially (iii) with 6(V) information, so conclusions remained ambiguous. 

VII. Tafel Slope Factor in Electrocatalysis and Its Relation to Chemisorption of Intermediates... 

The reason for this difference arises from the strength of chemisorption of the intermediate at the two materials. If conditions are such that the coverage by the intermediate at material I is appreciable and potential dependent, as discussed in Section III, then the Tafel slope b, is given by... 

Thus, it is seen that in practical evaluation of electrocatalysis at various materials, the relative Tafel slope b values, and associated conditions of coverage by intermediates, are as important as the material dependence of logi o values, as discussed in Ref. 131. 

IX. Tafel Slopes and Potential Dependence of Coverage by Intermediates... 

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