L. A. Khanova

Adsorption characteristics of tetraphenylporphyrins on an amalgamated platinum electrode

Abstract An irreversible adsorption of tetraphenylporphyrin (H2TPP) and its zinc derivative (ZnTPP) from aqueous and aprotic solutions has been found. The amount adsorbed depends on whether the solution concentration consists of one (H2TPP) or several (ZnTPP) plateaux corresponding to different adsorption states which are characterized by specific potentials and kinetics of redox reaction. The irreversible adsorption fixes the metastable states formed in the course of the adsorption process. A transformation into the most stable form during potential cycling has been observed. The positive shift of redox potential upon adsorption, protonation of one of the adsorbed forms of H2TPP, and retarding effect of adsorbed species on oxygen reduction have been discussed.

Electrochemical Behavior of Complex Based on Ruthenium(II) Phthalocyaninate

The electrochemical reactions of ruthenium(II) bis(triethylenediamine)tetra-tret-butyl-phthalocyaninate in dimethylformamide are studied. Two reversible redox reactions on the platinum amalgam electrode are revealed at the potentials of −0.73 and −1.16 V (Ag/AgCl). Similarly to several other phthalocyanines, these redox reactions correspond to the successive transfer of two electrons to phthalocyanine ring. A new phenomenon, which has not been reported in the literature for phthalocyanines, namely, the cathodic polymerization, is discovered. Thus formed polymer is redox-active, and only one cathodic reaction at the potentials from −0.78 to −0.84 V (a shift in the cathodic direction takes place as the film thickness increases) is observed in the polymer. In addition, the polymer exhibits also considerable electron conductivity that enables one to perform various electrochemical reactions in a wide potential range on the electrode modified with the polymer.

Reduction of oxygen and hydrogen peroxide on electrodes with adsorbed monolayer of aliphatic compounds

Cathodic reduction of oxygen and hydrogen peroxide on amalgamated platinum electrodes, which are coated with monolayers of long-chain aliphatic compounds cetyl alcohol (CA) and stearic acid (SA), is retarded as compared with the same reactions on clean mercury (or amalgam) surface. The oxygen reduction kinetics differ from that on mercury. The difference is explained by that oxygen diffuses into the monolayer and is reduced in it at a certain distance from the metal surface and only at the limiting current the reaction is forced onto the monolayer surface. In contrast to the oxygen reduction, the hydrogen peroxide reduction kinetics on electrodes with SA and CA monolayers is much closer to that on mercury, but with some quantitative distinctions. All results favor the H2O2 reduction at the monolayer/solution interface. The difference in the behavior of O2 and H2O2 is explained by different polarity of these molecules: it is significantly more difficult to penetrate the hydrocarbon monolayer for polar H2O2 molecule than for nonpolar O2 molecule.

Redox reactions of dissolved substances on amalgamated platinum electrode modified with adsorbed porphyrin

In order to develop the electrodes convenient for investigating the high-rate redox reactions not distorted by a reagent adsorption, the modification of amalgamated Pt electrodes with a monolayer of irreversibly adsorbed porphyrin is studied. A specific feature of applied procedure is that the adsorption was performed at a controlled electrode potential from the aqueous solution, in which porphyrin in a low concentration was solubilized. The adsorbed porphyrin monolayer prevents immediate contact of dissolved reagent with the metal. At the same time, due to a strong electronic interaction of the system of conjugate π-electrons with the metal, the monolayer does not hamper the electronic transfer between the reagent and the electrode.

Hydrogen peroxide reduction on amalgamated platinum electrodes covered with a monolayer of stearic acid

The reduction of hydrogen peroxide and, for comparison, oxygen on an amalgamated platinum electrode covered with a monolayer of stearic acid is studied by methods of polarization curves and impedance spectroscopy. In contrast with the oxygen reduction, the reduction of dissolved hydrogen peroxide occurs predominantly on the monolayer surface, rather than inside it. This is explained by the difficulty of penetration of the polar molecule of hydrogen peroxide into hydrocarbon environment.