Electrooxidation cyclic voltammetry

The second type of porphyrin electrosynthesis discussed in this paper is controlled potential electrooxidation of a-bonded bis-alkyl or bis-aryl porphyrins of Ge(lV) and Si(IV). This electrooxidation results in formation of a-bonded mono-alkyl or mono-aryl complexes which can be isolated and characterized in situ. Again, cyclic voltammetry can be coupled with this method and will lead to an understanding of the various reaction pathways involved in the electrosynthesis. 

Wang et al240 reported the electrooxidation of MeOH in H2S04 solution using Pd well-dispersed on Ti nanotubes. A similar reaction was studied by Schmuki et al.232 (see above), but using Pt/Ru supported on titania nanotube which appear a preferable catalyst. Only indirect tests (cyclic voltammetry) have been reported and therefore it is difficult to understand the real applicability to direct methanol fuel cell, because several other aspects (three phase boundary to methanol diffusivity, etc.) determines the performance. 

In this chapter, two carbon-supported PtSn catalysts with core-shell nanostructure were designed and prepared to explore the effect of the nanostructure of PtSn nanoparticles on the performance of ethanol electro-oxidation. The physical (XRD, TEM, EDX, XPS) characterization was carried out to clarify the microstructure, the composition, and the chemical environment of nanoparticles. The electrochemical characterization, including cyclic voltammetry, chronoamperometry, of the two PtSn/C catalysts was conducted to characterize the electrochemical activities to ethanol oxidation. Finally, the performances of DEFCs with PtSn/C anode catalysts were tested. The microstmc-ture and composition of PtSn catalysts were correlated with their performance for ethanol electrooxidation. 

Recently, the electrochemical behavior saturated alcohols, that is, propargyl alcohol (HCSCCH2OH, PA) [145], benzyl alcohol (C6H5CH2OH, BA) [146] andallylalcohol [147], has been studied at Pd electrodes in an acid medium by cyclic voltammetry, chronoamperometry, and on-line mass spectrometry. For BA, it was observed that the fragmentation of the molecules occurs at potentials in the hydrogen ad-sorption/absorption region of palladium, whereas for PA the adsorbates maintain the C3-chain. On the other hand, the yields of the electroreduction and electrooxidation products for both PA and BA differ from those obtained at Pt [146,148,149]. 

Fleischmann et al s 34 report cyclic voltammetry data for the oxidation of a series of aromatic hydrocarbons in a molten salt electrolyte, AlCl3-NaCl-KCl at 150°. Electrooxidation in this medium occurs at unusually low oxidation potentials. Tris-(p-substituted phenyl)amines, with the exception of tri (p-nitrophenyl) amine, yield very stable radical cations by all electrochemical criteria 380>S42 Mono- and bis-p-substituted triphenylamines, however, dimerize with rate constants ranging from 101 to 10s M 1 sec 1 to benzidines 176 (Eq. (237)), which subsequently are oxidized to the radical cations 177, whose ESR-spectra are observed. Dimerization is fastest with the p-N02 andp-CN-derivative, in accordance with HMO calculations, which predict the highest spin sensity in the p-position of these compounds 542 ... 

Fritsch and Weingarten 386 have studied the electrooxidation of 22 substituted enamines by cyclic voltammetry and ESR spectroscopy. Oxidation potentials of these strong electron donors may be as low as -0,901 V (vs.S.C.E.). The life times of the initially formed radical cations range from 0,005 sec to days depending on the efficiency of reactive site blocking or stabilization by substituents. Coupling constants from the ESR spectra indicate that the unpaired electron is polarized away from the dimethylamino substituents. The opposite is true for the... 

The electrooxidation of several dibenzothiophenes substituted with electron-donating groups such as methoxy (LXXI) [217], methylenedioxy (LXXII) [217], methylsulfanyl (LXXIII) [206], amino (LXXIV) [218], and dimethylamino (LXXV) [218] has been studied. All of them exhibited a reversible first oxidation peak in cyclic voltammetry, and, when electrolyzed at Pt anode in MeCN/LiC104 stable cation radical salts were isolated in 70-81% yield, except for LXXIII, whose salt was unstable in the air. 

Fig. 14.23 Microtiter plate anodic electrooxidation from lb to form 3ba in the presence of CH3OH, c(lb) = 4mM, c(lu) = 50mM, c(CH3OH) = 2M, electrolysis potential E = +0.4 V vs. id fc+ (a) steady-state microdisk electrode (d = 25 pm) cyclic voltammetry during electrolysis, v = 0.02 V s 1, times after start of electrolysis indicated, (b) current development during electrolysis, (c) steady-state voltammograms before (1) and after 900 s of electrolysis with (3) and without (2) mixing by convection. (Figure reprinted from Markle et al.72). Copyright Elsevier Ltd. (2005)...

Court (5) used cyclic voltammetry for comparing the activity of different catal3rtic metals towards the electrooxidation of sucrose. However, in spite of some attempts, the anal3rtical techniques available at that time did not allow the identification of the reaction products. Other studies (6, 7), although they were also carried out by cyclic voltammetry on noble metal electrodes (Pt and Au) and on nickel electrodes, aimed to improve the amperometric detection of carbohydrates and were thus not directly related to our purpose. 

Fig. 3.10 Cyclic voltammetry using a electrochemical quartz crystal microbalance of formic acid electrooxidation on a polycrystalline Pt surface in 0.2 M formic acid and 0.2 M HCIO4 at 50 mV s (a) current and (b) frequency (corresponding to negative mass changes) response. The upper potential limit is sequentially increased with each subsequent cycle [66]...

Several iron porphyrins bound to diatomic molecules, such as CO, NO, CS, CSe, and O2, have also been examined as to their electrochemistry in nonaqueous media. Fe(II) porphyrins can coordinate CO to give mono- and his-CO derivatives [237-239], and the electrooxidation of these species by cyclic voltammetry results in irreversible waves because of a rapid loss of CO upon formation of Fe(III)[7, 30, 240). Studies of (TPP)FeCl and (TPP)FeCl04 in Py and CH2Cl2/Py mixtures under a CO atmosphere indicated that the following five types of iron(II) porphyrins could be formed (TPP)Fe, [(TPP)FeCl]-, [(TPP)Fe(CO)Clj-, (TPP)Fe(py)2, and (TPP)Fe(CO)(py), and that these could be electrochemically converted into two types of iron(I) porphyrins, namely [(TPP)Fe] and [(TPP)Fe(CO) (py)]- [240]. 

The co-catalytic role of Sb is somewhat similar to that of Bi. The redox behavior of Sb in conjunction with Had on Sb determines the electrocatalytic activity in a Pt structure-sensitive fashion [161, 162]. Sb modification had a beneficial effect on the HCOOH electrooxidation activation energies on Pt(lll) and Pt(331), while exercising an inhibitory role on Pt(100), Pt(llO), and Pt(320) [161]. But the same group presented cyclic voltammetry data showing increased HCOOH oxidation currents on Sb-modified Pt(llO) and Pt(320) [162]. The effect of Sbad was very dependent on its surface coverage and there was an interaction... 

Weaver et al. published an authoritative study comparing the particle-size dependence of the rates of methanol, formic acid, and formaldehyde electrooxidations on Pt/C catalysts with diameters between approximately 2-8 nm [230]. Using cyclic voltammetry and two different concentrations of species (10 mM and 0.1 M) it was found that the HCOOH and CH3OH oxidation rates had opposite trends with respect to particle size. In the case of HCOOH the effective oxidation current densities increased with a decrease of Pt particle size, in the... 

Fang et al. have investigated the influence of pH on the mechanism of ethanol electrooxidation on a palladium electrode [80]. Cyclic voltammetry and in situ Fourier Transform Infrared (FTIR) spectroelectrochemistry were used for identification of the oxidation products at different NaOH concentrations. The activity for ethanol oxidation on Pd was largely affected by pH as well as by the resulting product. Sodium acetate was the main product for NaOH concentrations higher than 0.5 M. Nevertheless, CO2 was identified as the pH was lowered (below 13). 

Cyclic voltammetry is a practical and simple method to characterize the elec-trocatalytic activity of catalysts. Figure 10.18 shows the C-V curves for PtSn electrode in 0.3M HCIO4+IM EtOH solution. The initial potentials of the EOR (at 0.2 mA) on PtSn-1 and PtSn-2 are 0.033 and 0.124 V, respectively. In all of the sweeping ranges, the ethanol electrooxidation current on PtSn-1 electrode is higher than that on PtSn-2 electrode. This indicates that PtSn-1 is a better catalyst for ethanol than PtSn-2. 

Although benzyl alcohol is not an environmental pollutant, it will be included here because it is the paradigm of aromatic alcohols. The electrooxidation of the aliphatic alcohols methanol and ethylene glycol on Au and on polyNiTSPc/Au/Q electrodes in a pH 11 carbonate/hydrogen carbonate buffer electrolyte has been studied by cyclic voltammetry (CV) and with an electrochemical quartz crystal... 

The growth of the polymer film can be followed by cyclic voltammetry since the current peaks related to the polymer redox transformations increase as more and more polymer is deposited. The increase of the surface mass can be detected by using an -> electrochemical quartz microbalance. Such an example is shown in the Figure. In this experiment the positive potential limit of cycling was gradually decreased in order to avoid overoxidation of the polyaniline (PANl) formed. It does not affect the rate of polymerization, i.e., the film growth, since the electrooxidation of aniline is an autocatalytic process. 

---