Voltammetric profile

Similar studies have been carried out with Pt(l 11) and stepped surfaces with (111) terraces [Angelucci et al., 2007a, b]. The voltammetric profiles of these surfaces agree qualitatively with those depicted in Fig. 6.9. For the stepped surfaces, the potentials Ex and 2 depend linearly on the step density for terraces wider than 5 atoms. This hnear dependence is a consequence of the dependence of the oxidation rate on the step density, as was observed in the chronoamperometric CO stripping experiments. In H2SO4... 

Figure 6.13 Voltammetric profile for (a) Pt(lll) and (b) Pt(lOO) electrodes in 0.1 M HCOOH + 0.5 M H2SO4 on electrode. The full lines show the first cycle and the dashed line the second cycle. The scan rate was 50 mV/s.

This general scheme for the oxidation reaction is very sensitive to the surface stmcture. The first studies with single-crystal electrodes revealed that the voltammetric profiles for the three basal planes, Pt(lll), Pt(lOO), and Pt(llO), were completely different (Fig. 6.13) [Clavilier et ah, 1981 Lamy et al., 1983 Adzic et ah, 1982]. The lowest currents are obtained for the Pt(l 11) electrode, which in mm has a very low poisoning rate, as suggested by the small hysteresis. In fact, the reaction on this... 

The Ge adatoms do not remain adsorbed after oxidation, and the characteristic voltammetric profile of the blank is recovered after about 2-3 cycles up to 1.2 V. The Ge reduction process does not take place in a well-defined peak. 

In the negative-going scan, the current remains at the same constant value for the medium and high Pt content samples (x = 0.25 and 0.47), until it decays steeply at about 0.75 V, and vanishes completely at 0.51 V. For the low Pt content sample, the simation is somewhat different, since the current increases first, essentially following the current trace of the positive-going scan, down to about 0.8 V, where it bends off and decays steeply at 0.69 V. For all surface alloys, we observe a negative current below 0.55 V, which for the low Pt content electrode (x = 0.07) has a voltammetric profile very similar to that of the Pto.23ML/Ru(0001) surface. This points to a similar... 

Adsorption of acetic acid on Pt(lll) surface was studied the surface concentration data were correlated with voltammetric profiles of the Pt(lll) electrode in perchloric acid electrolyte containing 0.5 mM of CHoCOOH. It is concluded that acetic acid adsorption is associative and occurs without a significant charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(lll) than on polycrystalline platinum. A classification of adsorption processes on catalytic electrodes and atmospheric methods of preparation of single crystal electrodes are discussed. 

Figure 2 Typical cyclic voltammetric profile exhibited by a species which undergoes a reversible reduction at E° = 0.00 V...

The potential/current signs in the IUPAC convention follows the criterium positive currents for processes occurring at positive potentials , and vice versa. The American convention still adopts the opposite criterium positive currents for processes occurring at negative potentials , and vice versa. Even if the present theoretical treatment follows the American convention (positive sign to reduction processes), in presenting cyclic voltammetric profiles the IUPAC recommendation will be adopted. 

Figure 7 Typical cyclic voltammetric profile of an irreversible reduction process...

In order to understand better the distinction between reversibility, quasireversibility and irreversibility in cyclic voltammetry, the typical cyclic voltammetric profiles for the three cases are represented in Figure 8. 

As an example, Figure 43 shows the cyclic voltammetric profiles of a species which undergoes sequential electron transfers. 

To obviate possible erroneous interpretation of the cyclic voltammetric profiles (particularly in those cases in which an oxidation process occurs at negative potential values as well as a reduction process occuring at positive potential values), it is always wise to perform preliminarily hydrodynamic tests. 

It should be kept in mind that controlled potential electrolysis is indispensable in ascertaining the stability of every electrogenerated species (i.e. to determine the chemical reversibility of a redox process). Such a determination simply requires the recording of a cyclic voltammogram on the exhaustively electrolysed solution. The chemical reversibility implies that such a response must be quite complementary to that initially recorded. For instance, if the voltammetric profile of Figure 46 had been obtained before electrolysis, one must obtain a complementary response of the type illustrated in Figure 47 after electrolysis. 

As seen, it affords a single oxidation process the cyclic voltammetric profile of which is very close to that expected for the mechanism ... 

Figure 63 shows the cyclic voltammetric profiles exhibited in dmso solution by [Fem(salen)]+ and [Fen(saloph)], respectively.94,95... 

UV-photoirradiation causes trans-to-cis isomerization and the resulting solution exhibits a new voltammetric profile, Figure 2b, in which traces of the original waves are preceded by an anodic process (Eof = +0.03 V), which has been attributed to the single-stepped two-electron oxidation of cw-[Fc—N=N—Fc].3b This suggests that the apparent simple isomerization really involves important electronic effects in that (based on the discussions in Section 1.3, Chapter 4) one must conclude that in trans-[Fc—N=N—Fc] the two ferrocenyl units are interacting with each other, while in cz>[Fc—N=N—Fc] they do not interact. 

Figure 19 shows that the irreversible (one-electron) oxidation of [Co(H2)(PP3)j+ generates [CoH(PP3)]+. In fact, the voltammetric profile which appears in the reverse scan of Figure 19a is perfectly complementary to that of the oxidation of the neutral complex [CoH(PP3)], Figure 19b. 

Figure 23 shows that the perpendicular monochloro cation undergoes an irreversible (two-electron) oxidation that generates a voltammetric profile coincident with that of the parallel dichloro dication. 

However, in confirmation of the appearance of the peak-system E/F in the backscan, exhaustive three-electron oxidation in correspondence to the overall anodic process affords the cyclic voltammetric profile shown... 

The slight variations of the voltammetric profile with time are due to slight absorption of the protein on the carbon electrode. 

Unfortunately, electronic tongue variables are very often considerably intercorrelated in voltammetric profiles, for instance, currents evaluated at two consecutive potential values frequently carry almost the same information, so that their correlation coefficient is nearly 1. In such cases, standard OLS is absolutely not recommendable. Furthermore, the number of objects required for OLS regression must be at least equal to the number of predictors plus 1, and it is difficult to satisfy such a condition in many practical cases. 

Fig. 1 Cyclic voltammogram of Au(l 11) electrode in 0.10 M sulfuric acid containing 1.0 mM Hg + scan rate 2 mV s. Inset voltammetric profile over the potential range +1.05 to +0.70 V versus Ag/AgCl (3 M KCl) (from Ref 23).

Herrero and Abruna [25] have also studied the kinetics and mechanism of Hg UPD on Au(lll) electrodes in the presence and absence of bisulfate, chloride, and acetate ions. In the absence of the interacting anions (in perchloric acid), the Hg UPD was significantly controlled by gold-mercury surface interactions. In sulfuric acid solutions, the kinetics of the initial and final stages of mercury deposi-tion/dissolution was altered. The presence of two well-ordered structures at potentials below and above mercury deposition led to the formation of two pairs of sharp spikes in cyclic voltammograms. In the chloride medium, the voltammetric profile exhibited two sharp peaks and thus it was very similar to that obtained in sulfuric acid solution. Neither nucleation, nor growth kinetics mechanism was found to be linked to the process of formation/disruption of the mercury chloride adlayer. The transients obviously deviated from the ideal Langmuir behavior. 

In the reverse (cathodic) scan, ferricyanide ions remaining in the vicinity of the electrode surface are reduced to ferrocyanide ones. This process can be represented by the inverse of Eq. 2.1, with the voltammetric profile being interpreted from considerations similar to those made for the anodic peak. 

Figure 6.26. CO stripping voltammetry of UHV sputter-cleaned electrodes in 0.5 M H2S04 on (a) Pt and (b) Ru. Solid curves represent the stripping of CO in the first positive-going sweep dotted lines represent the voltammetric profiles in the absence of CO (adapted from Ref. [153]).

Figure 8.5 illustrates the precolumn oxidation of a phenacetin solution with LC-EC-Array detection (discussed in more detail in the following section). A peak, which eluted at 3.8 min, shows a characteristic voltammetric profile (i.e., reduction followed by oxidation) of a quinone species. Based on EC-Array and MS data (not shown), this peak has been identified as NAPQI, the expected reactive intermediate. This peak was not evident in a microsomal incubate of phenacetin analyzed using the same conditions (not shown). A possible explanation for this is that... 

The electrochemical behavior of the ferrocenediyl complex has also been studied and from its cyclic voltammetric profile it can be deduced that it exhibits a ferrocene-centered one-electron oxidation, followed by the decomposition of the electrogenerated cation [Fc,(AuPPh3)2]+. 

In the case of an irreversible reaction of the type 0 + we - R, linear sweep and cyclic voltammetry lead to the same voltammetric profile, since no inverse peak appears on inversing the scan direction. 

The details of the calculation of the voltammetric profiles can be consulted in the specialized literature8. Here we give the expression for a reversible reaction in which only the adsorbed species O and R contribute to the total current. The reason for this is to enable a comparison between the expressions for this situation and for thin-layer... 

If the adsorption isotherm is of Langmuir type and (bQ/bR) = 1, then the voltammetric profile is described by the function 0(1 + 0) 2. From this it can be calculated that the peak width at half height is 90.6In mV. This is all shown schematically in Fig. 9.7. 

The extent of chemical reversibility of the ECE electron transfers depends either on the type of enol or on the solvent. In general, non-coordinating dichlomethane favours the chemical reversibility as opposed to the coordinating acetonitrile. Furthermore, the ECE mechanism can be in some cases enriched or complicated by further intermediates, thus making in some cases the voltammetric profiles more intriguing. Such a complication can be observed in the redox mechanism involved in the oxidation pathway of the dimesityl... 

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