Spectroelectrochemical studies of tetrasulfonated metallophthalocyanines adsorbed on the basal plane of graphite in the presence of cysteine

Cobalt and iron tetrasulfonated phthalocyanines (M-TSPc) have been studied by electroreflectance spectroscopy in the adsorbed state on the basal plane of HOPG in a borate buffer (pH 9). Since both complexes exhibit electrocatalytic activity for the electro-oxidation of cysteine, we looked at the changes of the visible spectra of the adsorbed complexes upon adding millimolar amounts of cysteine to the electrolyte. Under zero-current conditions (open circuit) the addition of cysteine causes large changes in the reflectance spectra of both Co-TSPc and Fe-TSPc. In the case of Co-TSPc the spectra resemble that of Co(I)TSPc whereas for Fe-TSPc bands at 424 and 626 nm are observed. The open circuit potential shifts to values close to the standard potential of the M(II)/M(I) couple. We attribute these changes, particularly in the case of Fe-TSPc, to the formation of a radical adduct between the adsorbed phthalocyanine and the cysteine molecule, with partial electron-transfer from the cysteine to the metal center. Evidences for these adducts is not found when polarizing the electrode, which shows that they behave as very unstable intermediates in the catalytic process.     

Electrodeposition of copper into functionalized polypyrrole films

Copper microparticles have been dispersed in anion-exchange polymer films coated on carbon electrodes by oxidative electropolymerization of pyrrole–alkylammonium monomers. Incorporation of copper in polymeric films was effected by impregnation of copper-oxalate anionic complexes followed by an electroreductive precipitation to copper metal, or by electroreduction of polymer coated electrodes immersed in copper–oxalate aqueous solutions. The deposition process was examined by voltammetry. Scanning electron microscopy showed a dispersion of metal particles from 50–100 up to 300 nm, according to the lipophilic character of the polymer, located in a layer near the carbon ∣ polymer interface. A study of the electrocatalytic activity in the reduction of nitrobenzene to aniline at these materials was performed. The cathodes appeared much more active than bulk copper electrodes.     

Electrochemical behaviors of dimethyl ether on platinum single crystal electrodes. Part II: Pt(1 0 0)

The electrochemical behaviors of dimethyl ether (CH₃–O–CH₃, DME), which is a promising fuel for the fuel cell, on Pt(1 0 0) electrode in 0.5 M H₂SO₄ solution have been investigated in detail by electrochemical and in situ infrared (IR) measurements. As the potential is swept from 0.05 V (vs. RHE) to positive direction at 50 mV s⁻¹, the dehydrogenation peak of DME is observed around 0.33 V to generate a reaction intermediate and is further converted to carbon monoxide (CO) in more positive potential region. The main peak for DME bulk oxidation locates around 0.80 and 0.72 V in the positive- and negative-going potential sweep, respectively. The positions of these peaks strongly depend on the scan rate. The in situ IR observations show that (CH₃OCH₂–)_ad is an intermediate for the first dehydrogenation step of DME on Pt(1 0 0) surface and can serve as a precursor of the subsequent intermediate of adsorbed CO (CO_ad). Cyclic voltammograms of Pt high index single crystal planes Pt(hkl) show that the direct oxidation of DME is suppressed by decreasing the (1 0 0) terrace width. Based on these results, a possible reaction mechanism for DME electro-oxidation on the platinum single crystal electrode surface is proposed.     

Small signal (local) analysis of electrocatalytic reaction. Pole-zero approach

The small signal response of a model electrocatalytic reaction is analysed following the author’s earlier analysis of non-linear, large signal responses of this reaction under steady-state conditions. The kinetic parameters assumed generated peculiarities in large signal responses such as multiple steady states with hystereses and non-monotonic relations of surface coverage versus potential. The corresponding potential dependences of the electrode immittance parameters were calculated: high frequency limit (modulus), zero and pole of the immittance. Their advantage for the characterisation of small signal responses consists in a direct definition of the system’s stability and the distinction between the ‘capacitive’ and the ‘inductive’ character of the electrode immittance. Equality of the kinetic parameters of constituent reactions results in equal values of zero and pole and hence in a purely resistive character of the electrocatalytic reaction immittance.     

Diffusionless electron transfer of microperoxidase-11 on gold electrodes

Microperoxidase-11, MP-11, is made by proteolytic digestion of cytochrome c, cyt. c. It consists of a polypeptide of 11 amino residues attached covalently to the heme. Given that MP-11 has a more exposed heme than the complete protein, it would seem that electron transfer, ET, between immobilized MP-11 and electrodes would be at least as fast as for intact cyt. c. However, while the maximal heterogeneous ET rate for immobilized cyt. c is around 1000 s^?1, that reported previously for immobilized MP-11 does not exceed 20 s^?1. This work attempts to understand this difference in measured ET rates. The MP-11 was immobilized on gold electrodes using several protocols: (electrode A) the immobilization was done following a previously published carbodiimide based recipe yielding ET rates of the order of 20 s^?1; (B) MP-11 was bound to gold electrodes by Lomant’s reagent and gave an ET rate close to 4000 s^?1; (C) physisorbed MP-11 on gold electrodes with a self assembled monolayer, SAM, of alkane thiols gave an ET rate approaching 2000 s^?1 for the shortest length alkane thiol. Inspection of the immobilization chemistries suggests that the procedure employed in producing electrodes B and C are likely to lead to a monolayer or less of immobilized MP-11 while the procedure employed for electrode A may lead to a film comprised of a multilayer of MP-11. The presence of such a film on electrode A complicates the ET process since the MP-11 in the layer adjacent to the electrode could have fast ET rates while the MP-11 in the outer layers may have significantly slower ET rates. The net result would be an apparent ET rate constant which is much smaller than the value for the first layer. The measurements and calculations are presented in support of such an interpretation.     

Kinetics of oxygen reduction on gold nanoparticle/multi-walled carbon nanotube hybrid electrodes in acid media

In this paper, the electrochemical reduction of oxygen has been studied on gold nanoparticle/multi-walled carbon nanotube (AuNP/MWCNT) modified glassy carbon (GC) electrodes in 0.5 M H₂SO₄ using the rotating disk electrode (RDE) method. The AuNP/MWCNT catalysts were prepared by chemical deposition of AuNPs onto MWCNTs spontaneously grafted with 4-nitrophenyl groups. The composite electrode was characterised by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The oxygen reduction behaviour of these electrodes was compared with that of a bulk gold electrode. The AuNP/MWCNT catalyst showed a pronounced electrocatalytic activity towards O₂ reduction in acid media. The half-wave potential of O₂ reduction on the AuNP/MWCNT catalyst shifted ca 80 mV to more positive potentials as compared to that of a polished Au electrode. The kinetic parameters of oxygen reduction were determined and the specific activity of the hybrid electrode was slightly higher than that of the bulk Au electrode.     

Catalytic electro-oxidation of 2-mercaptoethanol using cobalt phthalocyanine + poly(2-chloroaniline) modified electrodes

The electrochemical oxidation of 2-mercaptoethanol has been examined on cobalt phthalocyanine (CoPc) + poly(2-chloroaniline) (PANCl) modified electrodes. A simple and reproducible procedure is described to obtain the modified-polymer electrodes. PANCl electrodes are modified by dispersing three different percentages of CoPc (0.1, 1 and 20%). These modified electrodes show electrocatalytic activity for 2-mercaptoethanol electro-oxidation in an acid medium. The evaluation of the electrocatalytical response of PANCl + CoPc films suggests that the diffusion rate of electroactive species into the film is improved when the polymer reaches the oxidized state.     

Simultaneous determination of epinephrine and uric acid at a gold electrode modified by a 2-(2,3-dihydroxy phenyl)-1, 3-dithiane self-assembled monolayer

In the present paper, the use of a gold electrode modified by 2-(2,3-dihydroxy phenyl)-1,3-dithiane self-assembled monolayer (DPDSAM) for the determination of epinephrine (EP) and uric acid (UA) was described. Initially, cyclic voltammetry was used to investigate the redox properties of this modified electrode at various scan rates. The apparent charge transfer rate constant, k_s, and transfer coefficient, α, were calculated. Next, the mediated oxidation of EP at the modified electrode was described. At the optimum pH of 8.0, the oxidation of EP occurs at a potential about 155 mV less positive than that of an unmodified gold electrode. The values of electron transfer coefficients (α = 0.356), catalytic rate constant (k = 1.624 × 10⁴ M⁻¹ s⁻¹) and diffusion coefficient (D = 1.04 × 10⁻⁶ cm² s⁻¹) were calculated for EP, using electrochemical approaches. Based on differential pulse voltammetry, the oxidation of EP exhibited a dynamic range between 0.7 and 500.0 μM and a detection limit (3σ) of 0.51 μM. Furthermore, simultaneous determination of EP and UA at the modified electrode was described. Finally, this method was used for the determination of EP in EP ampoule.     

Search for multi-functional catalysts: The electrooxidation of acetaldehyde on Platinum–Ruthenium–Rhodium electrodeposits

Preliminary studies on the influence of Platinum–Rhodium electrodeposits upon the rate of acetaldehyde electrooxidation showed that Rhodium plays a dual role during electrooxidation, because it favors the break of C–C bonds whilst prevents the deliverance of oxygen-containing species, thus inhibiting the oxidation steps. Based on these results, in this work Platinum–Ruthenium and Platinum–Ruthenium–Rhodium electrodeposits were prepared and characterized by electrochemical and spectroscopic methods (FTIR in situ). The addition of Ruthenium is justified by its ability to promote a facile oxidation of adsorbed species by activation of water molecules. The results indicate a compromise between the existence of superficial sites able to break carbon chains (provoked by Rhodium) and responsible by the oxidation of adsorbed species (provoked by Ruthenium). Among the atomic compositions investigated, PtRuRh (73:23:04) shows the maximum activity towards acetaldehyde electrooxidation, but the production of acetic acid seems to be the main responsible for this activity.     

Heat-treated iron(III) tetramethoxyphenyl porphyrin chloride supported on high-area carbon as an electrocatalyst for oxygen reduction: Part II. Kinetics of oxygen reduction

Oxygen reduction was investigated at FeTMPP–Cl adsorbed on the Black Pearls carbon and heat-treated at temperatures from 200 to 1000°C. Kinetic parameters of the reaction and the dependence of the reaction rate on the heat-treatment temperature were established in acid and alkaline media. It was found that the O₂ reduction rate increases with increase in the heat-treatment temperature of the electrocatalyst, reaching a plateau at 700≤T≤1000°C. The reaction rate was higher in alkaline solution on all electrocatalysts examined, but its dependence on the heat-treatment temperature was the same as in acid media. Like similar other macrocycle catalysts, FeTMPP–Cl/BP was found to be methanol-tolerant. The effect of sulfate, perchlorate, and phosphate anions was also investigated and no influence on the reaction rate was found. Activities of FeTMPP–Cl/BP and Pt/BP electrocatalysts were compared. FeTMPP–Cl/BP heat-treated at 700≤T≤1000°C showed similar activity to Pt/BP if they are compared in alkaline solution, but in acid solution, the reaction rate on the Pt electrocatalyst was higher. The controversy in the literature on the role of metallic Fe particles in O₂ reduction catalysis is discussed. Comparison of present results with previous cyclic voltammetry and TEM investigations of the same electrocatalyst gave further evidence that some other products of the heat-treatment are responsible for the high activity of the catalyst.