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Carbon paste electrodes modified with a family of ionic liquids (as binders) derived from N-octyl-pyridinium hexafluorophosphate, (composite-ionic liquid electrodes (CILEs)) were studied toward the oxidation of sulfite as an inner-sphere probe reaction and compared to a conventional carbon paste electrode (CPE) in which the binder was mineral oil. The ionic liquids were modified at para -position with substituents that are electron withdrawing: -CN (CILE/CN) and –CF3 (CILE/CF3) and substituents that are electron donating: -CH3 (CILE/CH3) and –OCH3 (CILE/OCH3), and compared with the ionic liquid without substituents (CILE/OPy). The results showed that CILEs are capable of catalyzing the sulfite oxidation, shifting the oxidation potential to more negative values compared to CPE. Also, they showed linear correlations between increasing sulfite concentration and increasing current density. The best system in terms of sensitivity was the electrode modified with the 4-methyl-N-octylpyridiniumhexafluorophosphate CILE/CH3 and, then, that modified with the non-substituted ionic liquid CILE/OPy measured by amperometry. In terms of potential, the best systems are the CILEs modified with ILs with inductive-substituents, -CH3 and –CF3 and OPy, indicating that the delocalization of the charge of the cation produced by the mesomeric-substituent (-OCH3, -CN) diminishes the electrocatalytic behavior of these binders for that oxidation. On the other hand, the analytical parameters of all the amperometric sensors studied here (all CILEs excepts CILE/OCH3) are good enough to be applied in food industry for samples without polyphenols at concentrations higher than ca. 1mM of sulfite. Their stability is very high (at least 100 cycles after obtaining a stable response without changing its current, exposed to air and humidity) and they can be used to remove sulfite from wastewaters. Finally, to our knowledge, there are not comparative studies about the effect of changing the substituent of the cation in electrocatalysis of modified IL-electrodes.

A mixed electrocatalyst that consists of multi walled carbon nanotubes (MWCNTs) and the ionic liquid (IL) N-octylpyridinum hexafluorophosphate (OPyPF6) as binder (MWCNT/IL system) was used in the study of hydrogen evolution reaction. This electrocatalyst was compared to graphite/mineral oil (Gr), graphite/OPyPF6 (Gr/IL), MWCNT/mineral oil (MWCNT) systems. All systems were characterized by cyclic voltammetry determining that MWCNT/IL system is highly electrocatalytic (E onset = -0.3 V vs Ag/AgCl) in relation to the other systems studied containing MWCNT or IL separately. The presence of each of the components that form the electrodic surface is corroborated by XRD, Raman, and FESEM-EDX, and it is demonstrated by chronoamperometric analysis that MWCNT/IL system is highly stable. The efficiency of the electrode, determined by its electrocatalytic activity, is confirmed by the quantification by gas chromatography of the hydrogen produced. Moreover, an elevated TON value (16274) and faradaic efficiency close to 60% are calculated, in comparison to other electrocatalysts found in literature. All these attractive properties are complemented by the fact that the MWCNT/IL system is simple, novel and easy to prepare.

This paper describes the electrochemical reduction of nitrite ion in neutral aqueous solution mediated by tetraruthenated metalloporphyrins (Co(II), Ni(II) and Zn(II)) electrostatically assembled onto a Nafion film previously adsorbed on glassy carbon or ITO electrodes. Scanning electron microscope (SEM-EDX) and transmission electron microscopy (TEM) results have shown that on ITO electrodes the macrocycles forms multiple layers with a disordered stacking orientation over the Nafion film occupying hydrophobic and hydrophilic sites in the polyelectrolyte. Atomic force microscopy (AFM) results demonstrated that the Nafion film is 35 nm thick and tetraruthenated metalloporphyrins layers 190 nm thick presenting a thin but compacted morphology. Scanning electrochemical microscopy (SECM) images shows that the Co(II) tetraruthenated porphyrins/Nf/GC modified electrode is more electrochemically active than their Ni and Zn analogues. These modified electrodes are able to reduce nitrite at −660 mV showing enhanced reduction current and a decrease in the required overpotential compared to bare glassy carbon electrode. Controlled potential electrolysis experiments verify the production of ammonia, hydrazine and hydroxylamine at potentials where reduction of solvent is plausible demonstrating some selectivity toward the nitrite ion. Rotating disc electrode voltammetry shows that the factor that governs the kinetics of nitrite reduction is the charge propagation in the film.

Summary: This paper describes the electrochemical reduction of nitrite ion in 0.1MNaClO 4 , on glassy carbon or ITO electrodes modified with mixtures PVC- tetraruthe-nated metalloporphyrins. This electrode is able to keep the extraordinary electro-catalytic properties of the macrocycle allowing multielectronic transfers and a greatstability as a consequence of the inclusion of the macrocycles into a polymericsupport such as PVC. On the other hand, the electrochemical reduction of nitrite ionon these modified electrodes studied by cyclic voltammetry shows an enhancementin the current values and lower overpotential compared with the activity of the bareglassy carbon electrode. Controlled potential electrolysis experiments verify theproduction of ammonia, hydrazine and hydroxylamine, showing the electrocatalyticcharacter and the stability of this modified electrode. Keywords: electrocatalysis; modified electrodes; nitrite; PVC; tetraruthenated porphyrins Introduction Nitrite ions are produced by photolysis ofnitrate ions and by photodegradation ofaquatic humic substances present in theaquatic environment. Nitrite is a keyspecies in global nitrogen cycle. Nitritesshow environmental and biological impor-tance. Moreover, they have an importantrole as additive in the alimentary industryas preservative, antimicrobial and colorfixative agent.

A new conducting film derived from the complex [Fe (diaphen)3]2+, (diaphen=5,6-diamino-1,10-phenanthroline) was electropolymerized by cyclic voltammetry onto a glassy carbon electrode. Poly-[FeII (diaphen)3] was studied by cyclic voltammetry, SEM, UV-vis and micro-Raman spectroscopy. Poly-[FeII (diaphen)3] shows electrocatalytic activity in HSO3− reduction in an ethanol/water solution. Electrocatalysis is centered at the π ring of phenanthroline. Rotating disk electrode studies showed a 0.117 V/dec Tafel slope, suggesting an EC process where the electrochemical process is the determining step. The chemical step was studied by UV-vis spectroelectrochemistry. Amperometric behavior showed a linear range between 47.5 µM to 417 µM and the LOD was 19.5 µM.

This work describes the preparation of a new tetraruthenated porphyrin and the corresponding polymeric film. This macromolecule consists of a Ni(II) tetrapyridylporphyrin coordinated to four [Ru(5-NO 2 -phen) 2 Cl] + moieties. The 5-NO 2 -phen ligands are reduced at −0.90 V, generating a radical anion that is able to generate polymeric coatings onto glassy carbon (GC) electrodes. These polymeric films are very stable in aqueous solutions. The polymeric films are characterized by electrochemistry, infrared spectroelectrochemistry (IR-SEC) and atomic force microscopy (AFM).

This work describes electrocatalytic detection of S(IV) compounds in water-ethanol solutions under acidic conditions, on a glassy carbon electrode modified with Co(II) tetraruthenated porphyrin electrostatically assembled onto a Nafion film. The Ipc current shows a linear relationship with the concentration of S(IV) oxo-anions. Controlled potential electrolysis shows that thiosulfate was detected as reaction product. Rotating disk electrode and UV-visible spectroelectrochemistry experiments showed a kinetic limitation to the electron transfer controlled by charge propagation in the film and the formation of an intermediary between Co center and S(IV) species. The reproducibility of the modification methodology presents a RSD of 4.1%.

The aim of this work is to study four types of modification of a glassy carbon electrode by Fe(III)-tetrakis(p-tetraaminophenyl)porphyrin and determine the influence of the method of immobilization of the complex on glassy carbon in electrocatalytic properties for the sulfite and hydrogensulfite oxidation in ethanol–water. The first modification was deposition of a drop of solution containing the porphyrin on a glassy carbon electrode and evaporation of the solvent (dry-drop method). The second method was immersion of the electrode at 54°C in a solution of dimethylformamide containing the porphyrin for 2 h. The third method consisted of the same heating treatment but after formation of a chemical bond of 4-aminopyridine on the glassy carbon surface, which acts as an axial ligand for the first layer of porphyrin. The fourth method involves electropolymerization of the porphyrin on the electrode surface. Important differences in stability, the potential where the oxidation wave begins and selectivity of the...

The complex [FeII(Imz-phen)3](PF6)2, (Imz-phen = imidazolidine-[4,5-f]-1,10-phenanthroline) has been prepared and characterized using X-ray diffraction, UV–vis and IR spectroscopy, elemental analysis, fast atomic bombardment (FAB) mass spectrometry, and cyclic voltammetry. Its crystal lattice includes acetonitrile (π–π bound to phenanthroline), methanol, and water molecules. Scanning continuously between 1000 mV and 1650 mV in CH3CN, a modified electrode that includes the iron (II) complex is obtained; after the 25th continuous cycle a stable film is formed that is electrocatalytically active in the reduction of sulfur oxoanions. When the electrocatalytic properties are evaluated in ethanol/water solution, the current achieved from the electroreduction of these sulfur species is linearly dependent on the respective concentrations, suggesting potential application in sulfite determination.

The electro-oxidation of sulfite was studied by using a glassy carbon electrode coated with a polymeric film of Fe-tetra-4-aminophenylporphyrin, in a wide pH range. The polymeric complex-modified electrode catalyses the electro-oxidation of sulfite to sulfate in acid and basic media, but it is more active at pH higher than 8.5. The polymer film coating is obtained by cycling the electrode in a solution containing the complex (monomer). The polymerization process takes place after the oxidation of the amino groups via a radical mechanism. When the modified electrode is obtained, the polymeric system needs to be over-oxidized in order to avoid the further oxidation of the remaining amino groups in the potential range where the oxidation of the sulfite takes place. The over-oxidation process destroys the monomer-modified electrode. Then, the electropolymerization of the iron complex is the only way to obtain a catalyst for this reaction with this porphyrin. The specie postulated for the oxidation is Fe(IV). The polymer-modified electrode is very stable and its activity remains almost constant for more than 500 consecutive cycles. Also, its activity remains unchanged during more than 30 days of exposure to air and light. The oxidation process occurs during the anodic and the cathodic potentiodynamic scans, showing that it is only necessary the recovery of the active sites; the formation of Fe(IV) is probably needed to promote the oxidation. However, this regeneration is low and becomes the slow step of the reaction.

In this study, six polymer-modified electrodes were used to electrocatalyze the reduction of nitrate in two aqueous media: 0.5 M HClO 4 and 0.1 M NaClO 4 . These six polymers were prepared from the electropolymerization of tetrakis( x -aminophenyl)porphyrin (where x = o , p ), and their Cu(II) and Ni(II) complexes in HCl solution on the surface of a conducting glass electrode. The results indicate that the catalytic response depends on the position of the amino group, the metal and the pH of the solution. In HClO 4 , the modified electrodes show two reduction processes. The voltammetric responses of the modified electrodes show a reduction process that takes place during the negative scan and another reduction process occurring during the return potential sweep. Indeed, there is a change in the number of reduction processes depending on the direction of the scan at low scan rate (5 mV s −1 ). In HClO 4 , the products of electrolysis are nitrite and nitrous oxide for Cu and Ni- ortho -polymers and nitrite for the others polymers. In NaClO 4 , only one reduction process takes place, independently of the direction of the scan, giving nitrite and nitrous oxide for the Ni and Cu- ortho systems and only nitrite for the other systems. In both cases, HClO 4 and NaClO 4 , the best polymers are poly Cu(II) and Ni(II)tetrakis( o -aminophenyl)porphyrin as electrocatalysts.

The electrochemical reduction of carbon dioxide using hexa-aza-macrocycles derived from the condensation of 1,10-phenanthroline and its Co(II) complex as an electrocatalyst dissolved in dimethylformamide has been studied by cyclic voltammetry and UV-visible spectroscopy. The ligand does not show catalytic activity and only generates hydrogen when it is reduced under carbon dioxide. The cobalt complex shows electrocatalytic activity toward the reduction of carbon dioxide, generating carbon monoxide and formic acid. Cyclic voltammetry and UV-visible spectroscopy show that the active site for the reduction is the metal center in oxidation state (I), although the reduced cobalt center alone is not enough to promote reduction of the carbon dioxide. Electrolysis at controlled potential shows that only at potentials corresponding to reduction of the ligand (second reduction) does carbon dioxide reduction occur. Cobalt(I) probably reacts with CO2 forming a non-isolated intermediate which, when reduced, gives CO a...

Recent literature results on inorganic models related to metalloenzymes which transport electrons or activate small molecules are discussed in this review. The molecular systems are essentially restricted to macrocyclic or polydentate ligands preferentially containing nitrogen atoms and pendant arms. The discussion is mainly concerned with aspects such as spectroscopic, electrochemical and chemical characterization of the systems and electro- or photo-activation of small molecules.

Cobalt phthalocyanine (CoPc) and cobalt naphthalocyanine (CoNPc) exhibit different redox and electronic properties. Redox processes on CoPc are more reversible than on CoNPc. The pH dependence of those processes is also different for these complexes, which could indicate that in the case of CoNPc, the redox couples observed involve the macrocyclic ligand and not the metal as with CoPc. This seems to explain the different redox properties and kinetics for O2 reduction and N2H4 oxidation of these two macrocyclics. In this study we compare the electrocatalytic activity of these two phthalocyanines, adsorbed on glassy carbon, for the electrochemical reduction of O2 and for the oxidation of hydrazine.

The aim of this work is to study the sulfite oxidation mediated by glassy carbon electrodes modified with copolymers formed by ortho-phenylenediamine (OPD) and Co(II)-tetrakis(para-aminophenyl)porphyrin (CoTAPP). The copolymer was obtained through continuous cycling of potential of a glassy carbon electrode in acid medium containing each respective homopolymer in 1:1 feeding volume, in which the ortho-phenylenediamine concentration was 1·10-1 M and the CoTAPP concentration was 1·10-4 M. The film obtained is thin, stable and contains a low amount of porphyrin in comparison to the amount in orthophenylenediamine. Its electrocatalytic activity was tested toward the sulfite oxidation. In this case, this film is more electrocatalytic than the modified electrode modified with the CoTAPP electropolymer, even though ortho-phenylenediamine is inactive in this reaction. In this work the electrocatalysis process, the morphology of the film, and the difference of electrocatalytic behavior between the ortho-phenylenediamine and cobalt porphyrin homopolymers and the orthophenylenediamine-porphyrin copolymer were studied. The morphologic studies were done using Au electrodes since the electrocatalytic activity on Au and on glassy carbon are quite similar.

Glassy carbon electrodes modified with conducting polymers of Ni(II), Zn(II) and metal free tetraruthenated porphyrin were evaluated for reduction and oxidation processes of S(IV) oxoanions in Na2SO3/water-ethanol at pH 1.0 and 3.5, showing electrocatalytic activity. A Ni(II) film was able to reduce the S(IV) oxoanions selectively in presence of high concentration of gallic acid. The Ni(II) film was also used as an amperometric sensor toward S(IV) oxoanions reduction in white wine samples showing a detection and quantification limit of 1.40 mg L−1 and 4.68 mg L−1, respectively. These results are promising for the electrochemical determination of S(IV) using conducting polymers from these macrocycles.

In this study, we compared four glassy carbon electrodes modified with adsorbed porphyrins of cobalt containing one, two, three, or four bis(methoxyphenyl) groups as substituents of the ligand. The aim of this work was to determine the effect of these donating groups on the electrocatalyst behavior of the modified electrodes for the reduction or oxidation of sulfite in water or in mixtures of 12% ethanol in water (v/v) at different pHs. The results showed that the modified electrodes are not catalytic toward the electro-reduction of sulfite in the whole range of pH studied (2-12). However, all the electrodes showed a catalytic toward the oxidation of sulfite at basic pHs. We found that the best catalytic system was the four-substituted porphyrin modified electrode at pH 9.8. This system performed well as an amperometric sensor for sulfite in ethanol-water mixtures and in aqueous solutions at the same pH. When the calibration curve was carried out by cyclic voltammetry, a detection limit of 0.10 mg L-1 for the four-substituted cobalt porphyrin modified electrode in water and in ethanol-water mixtures was found.

A comparison is made of the behavior of three Cu(II)-porphyrins (with the amino substituents in different positions on the periphery of the ligand) electropolymerized on glassy carbon electrodes in HCl as solvent. The electropolymerized films show a certain degree of demetallation but the Cu amount of porphyrin in each case is sufficient to catalyze the oxidation of sulfite and hydrogensulfite in ethanol–water solutions. The ligands of the porphyrins are almost inactive in the electrocatalysis of the sulfite. The oxidation processes are favored at low scan rates. The foot of the oxidation waves depends on the positions. The most active porphyrin bears the substituents with the amino groups in para position, the oxidation occurring at low positive potentials. All the modified electrodes can be used as amperometric sensors for sulfite; they show linear relationships between the current and sulfite concentration in the range in which sulfites are present in red wines. The modified electrodes are stable and all of them promote the oxidation of sulfite to sulfate.

The electrochemical reduction of carbon dioxide was studied on a glassy carbon electrode modified with either polymeric M-tetrakis aminophthalocyanines (M = Co, Ni, Fe) or with the polymeric free ligand, in aqueous electrolyte. The reaction products are dependent on the central ion: for Co-polymer the only reaction product found was formic acid; for Ni polymer, formic acid and formaldehyde were found, whereas formaldehyde and H2 were detected for Fe polymer. For the free ligand polymer only H2 was detected. Spectroelectrochemical experiments show that in the case of Co-polymer, Co(I) is the active site of the electrocatalysis but the reduced cobalt center and the reduced ligand are not enough to promote the reduction of the carbon dioxide and an extra overpotential is necessary. In the case of the Ni polymer, the reaction takes place at the same potential where the complex is double reduced and it is not necessary to apply more potential. On the other hand, there are important differences between the morphologies of both polymers as demonstrated by electrochemical impedance spectroscopy. The experiments show that the metallic center affects the kinetics of polymerization and the polymer morphology. On the other hand, the chemical nature of the metal center of the catalyst is the most important factor in the electrochemical reduction of CO2 and the products involved.

Porphyrins and phthalocyanines of transition metals are good catalysts for many redox reactions [1-7]. In order to increase their catalytic activity it is often necessary to modify the macrocycle with donor or acceptor substituents making the species more reductive or oxidant depending on the analyte. However, this process is a one-way modification, because the molecule enhances or diminishes the electronic density of the metal by the substituent effect

We have investigated the electrocatalytic activity of five iron phthalocyanines for the electrooxidation of 2-mercaptoethanol, with the complexes preadsorbed on a ordinary pyrolytic graphite electrode. The potential of Fe[II]/Fe[I] and Fe[III]/Fe[II] redox processes on the adsorbed phthalocyanines are shifted to more positive potentials by the effect of electron withdrawing groups −CO2−, −SO3− and −Cl and to more negative potentials by the electron-donating methoxy group compared to −H in unsubstituted FePc. The electrocatalytic activity, measured as current at constant potential, decreases as: Fe-octamethoxyphthalocyanine>Fe-phthalocyanine>Fe-tetracarboxyphthalocyanine>Fe-tetrasulfonatedphthalocyanine>Fe-perchlorinatedphthalocyanine. The most oxidant complex (Fe-perchlorinatedphthalocyanine) is the least active, which is contrary to what would be expected on thermodynamic grounds. This can be attributed to the fact that the reaction proceeds via an inner-sphere mechanism when the metal center is in state of oxidation II.