Electrochemical rate-determining step

Williamson M. A. and Rimstidt J. D. (1994) The kinetics and electrochemical rate-determining step of aqueous pyrite oxidation. Geochim. Cosmochim. Acta 58, 5443—5454. Witzke T. (1999) Hydrowoodwardite, a new mineral of the hydrotalcite group from Konigswalde near Annaberg, Saxony/Germany and other localities. Neues Jahrb. Mineral. Mh., 75-86. 

Case I. Electrochemical Rate-determining Step For situations in which the rate determining step in Scheme 3.7 is the second electrochemical process (B), that is ... 

Figure 12.20 The rate of oxidation of pyrite (r = rf(FeS2l/t/< in mo /m s) near 25"C and 1 bar pressure. Whole model and leverage plots for multiple linear regression analysis of published and measured rate data for the aqueous oxidation of pyrite (a) Oxidation of pyrite by dissolved oxygen (b) Oxidation of pyrite by ferric iron under an N2 atmosphere and (c) Oxidation of pyrite by ferric iron in the presence of dissolved oxygen. Reprinted from Geochim. et Cosmochim. Acta, 58, M. A. Williamson and J. D. Rimstidt, The kinetics and electrochemical rate-determining step of aqueous pyrite oxidation, 5443-54, 1994, with permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, U.K.

Minor variations in hypophosphite oxidation rate on the electrode potential (Figures 19.2A(b) and 19.3A(a)) could be interpreted as the evidence of a non-electrochemical rate-determining step in the anodic oxidation of hypophosphite, for example, catalytic dehydrogenation [19], possibly, via hypophosphite radical intermediate formation, as was evidenced from the electron spin-trap resonance spectroscopy measurements [73], although this was not confirmed by in situ IR spectroscopy, apparently, due to a short lifetime of the radical intermediate [74]. 

Williamson, M.A. and J.D. Rimstidt, 1994, The kinetics and electrochemical rate-determining step of aqueous pyrite oxidation. Geochimica et Cosmochimica Acta 58, 5443-5454. 

Process (3.8) is a total 2e per cadmium atom and indicates that CdS formation occurs via a sulfur atom abstraction from 8203 . This reaction was called for in order to suggest that the reduction of Cd " is the only electrochemical step, whereby charge is consumed, followed by a subsequent chemical step comprising sulfur association to reduced cadmium. Sulfur is generated by the decomposition of thiosulfate. On the other hand, reaction (3.9) corresponds to an overall 4e /Cd process where reduction of S2O3 itself must occur as well as that of Cd ", the former comprising actually the rate-determining step. This route becomes more favorable as pH decreases for it requires additional protons. 

Polarization is much higher for the electrochemical reduction of hydrogen peroxide. The slope has the unusually high value of about 0.3 V. At a given current density the electrode potential in this reaction is again independent of solution pH. These and certain other data indicate that addition of the first electron to the peroxide molecule and simultaneous peroxide decomposition is the rate-determining step ... 

A few electrochemical reductions of formazans to hydrazidine have been reported.370,372 Ho wever, as discussed in Section 7.4.2.6, electrochemical techniques have been widely used to study the redox chemistry of tetrazolium salts and formazans. Opinions about the reversibility of the electron transfer step, the number of electrons involved, and the identity of the rate-determining step differ widely.369- 371,656 The electrochemical oxidation of some novel formazans, e.g., 215 produces the dicationic species 216 (Eq. 28). The mechanism is not clearly understood.372,373... 

In the past two chapters we have already encountered examples of reactions involving several steps, and introduced the notion of rate-determining step. Here we will elaborate on the subject of complex reactions, introduce another concept the electrochemical reaction order, and consider a few other examples. 

There are many possible reaction pathways between acrylonitrile and adiponitrile and, in each, there are several possible rate-determining steps. None of the reaction intermediates has yet been detected electrochemically or spectroscopically thus indicating very fast chemical processes with intermediates of half-lives of < 10-5 s. Bard and Feiming Zhou [104a] have recently detected the CH2 = CHCNT radical by Scanning Electrochemical Microscopy (SCEM) using a 2.5 fim radius Au electrode (1.5 mol CH2 = CHCN in MeCN/TBAPF6). The dimerization rate has been determined to 6.107 M-1 S l. 

When the surface is completely covered by an oxide film, dissolution becomes independent of the geometric factors such as surface curvature and orientation, which are responsible for the formation and directional growth of pores. Fundamentally, unlike silicon, which does not have an atomic structure identical in different directions, anodic silicon oxides are amorphous in nature and thus have intrinsically identical structure in all orientations. Also, on the oxide covered surface the rate determining step is no longer electrochemical but the chemical dissolution of the oxide.1... 

The electrochemical reaction occurs at the surface of graphite anode [37 39]. At potentials lower than 1.25 V, chlorine is formed by a Volmer/Heyrovsky mechanism with the latter being the rate determining step. Chloride ions are initially discharged on surface sites that are not covered by chlorine atoms (Volmer reaction (14.4a)), followed by the discharge of chloride ions on adsorbed chlorine ions (Heyrovsky reaction (14.4b)) [39] ... 

Fig. 14 Reduction of PhjCSPh by electrochemically generated aromatic anion radicals (in DMF at 25°C). Variation of the rate-determining step rate constant, A , with the standard potential of the aromatic anion radical, p,g (from left to right azobenzene, benzo[c]cinnoline, 4-dimethylaminoazobenzene, terephthalonitrile, naphthacene, phlhalonitrile, perylene, fluoranthene, 9,10-diphenylanthracene). The dotted lines are the theoretical limiting behaviours corresponding to the concerted (right) and stepwise (left) pathways. (Adapted from Severin et al 1988.)...

Of hundreds of theoretically possible pathways, the list can be trimmed to four using linear sweep voltammetry (LSV) and chemical arguments [22]. The LSV method is an exceptionally powerful one for analyzing electrochemical processes [24-27]. From LSV studies, it was concluded that a single heterogeneous electron transfer precedes the rate-determining step, cyclization is first order in substrate, and that proton transfer occurs before or in the rate-determining step. The candidates include (a) e-c-P-d-p (radical anion closure). 

Thus, there are two kinetic paths for the hydrogen evolution. The first path consists of charge transfer (CT) followed by chemical desorption (CD) path CT-CD. The second path consists of charge transfer (CT) followed by electrochemical desorption (ED) path CT-ED. Within each path, either of the consecutive steps can be slow and thus can be the rate-determining step (RDS). Each of these paths has two pKJSsible mechanisms. 

In this equation, aua represents the product of the coefficient of electron transfer (a) by the number of electrons (na) involved in the rate-determining step, n the total number of electrons involved in the electrochemical reaction, k the heterogeneous electrochemical rate constant at the zero potential, D the coefficient of diffusion of the electroactive species, and c the concentration of the same in the bulk of the solution. The initial potential is E/ and G represents a numerical constant. This equation predicts a linear variation of the logarithm of the current. In/, on the applied potential, E, which can easily be compared with experimental current-potential curves in linear potential scan and cyclic voltammetries. This type of dependence between current and potential does not apply to electron transfer processes with coupled chemical reactions [186]. In several cases, however, linear In/ vs. E plots can be approached in the rising portion of voltammetric curves for the solid-state electron transfer processes involving species immobilized on the electrode surface [131, 187-191], reductive/oxidative dissolution of metallic deposits [79], and reductive/oxidative dissolution of insulating compounds [147,148]. Thus, linear potential scan voltammograms for surface-confined electroactive species verify [79]... 

There are several places in electrochemical reactions where rate-determining steps can occur. First, if a cathode potential is sufficiently negative, transport of reactants to the electrode will not be able to keep pace with the events that transfer charge as the electrode demands. Then, transport in solution and the electrode events have to be satisfied with what the transport rate can bring to the interface. Transport is the rds. 

The current density for an overall electrochemical reaction (A + 2e - C) was measured as a function of the overpotential listed in Table P.l. Develop the mechanism of the reaction and find the rate-determining step, assuming a symmetry factor of 0.5. 

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.

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