L. N. Kuznetsova

Peculiarities of direct bioelectrocatalysis by laccase in aqueous–nonaqueous mixtures

The effect of concentration of ethanol and dimethyl sulfoxide on the catalytic activity of laccase is studied for the enzymatic reaction of catechol oxidation and bioelectrocatalytic reaction of oxygen reduction under the conditions of direct electron transfer. Laccase-Nafion composite is elaborated ensuring the enzyme stability in a wide potential range and a content of organic solvents. Based on the STM measurements, the structure of composite layer is proposed. It is shown that the mechanism of oxygen reduction reaction by laccase in organo-aqueous mixtures is similar to that earlier proposed for aqueous solutions. A decrease in the electrocatalytic activity of laccase in the oxygen reduction correlates with a decrease in the laccase enzymatic activity in the substrate oxidation. However, a decrease in the laccase activity in the composite is observed at a higher content of organic solvent in the mixture. The mechanism of laccase inactivation by organic solvents is proposed.

Bioelectrocatalytic reduction of oxygen in the presence of laccase adsorbed on carbon electrodes

The kinetics of electroreduction of molecular oxygen on isotropic pyrocarbon with adsorbed laccase or a laccase–Nafion composite is studied. Kinetic parameters thus obtained are compared with those determined previously for electrodes of carbon black with adsorbed laccase. The closeness of kinetic parameters of the reaction of bioelectrocatalytic reduction of oxygen by laccase adsorbed on smooth (pyrocarbon) and disperse (carbon black) carbon materials led to a refined reaction mechanism. The slow stage of the reaction of bioelectrocatalytic reduction of oxygen is a synchronous transfer of two first electrons onto the oxygen molecule, similar to the mechanism of enzymatic catalysis by laccase.

Composition, surface segregation, and electrochemical properties of binary PtM/C (M = Co, Ni, Cr) catalysts

The effect of the nature of transient metal and chemical treatment of binary cathodic PtM/C (M = Co, Ni, Cr) catalysts, which were prepared by high-temperature synthesis, on their structure, surface segregation, and characteristic properties (activity and stability) is studied. It is shown that, in the course of treatment in 0.5 M H2SO4 at the elevated temperature (60°C), the surface of nanoparticles becomes enriched in platinum with the formation of core-shell structures. The PtCo/C catalyst is the most efficient one. In this case, a compromise between the corrosion resistance and electrocatalytic activity is reached due to a higher, as compared with PtNi/C and PtCr/C, degree of alloy formation and enriching of surface in platinum in the course of corrosive attack. Thereby, the properties of platinum on the core surface change as a result of a pronounced ligand effect of the core. Thus, depending on the nature of transient metal, the binary cathodic PtM/C catalysts differ in their activity and stability, which depend on the degree of alloy formation and a possibility of formation of core-shell structure as a result of surface segregation in the course of synthesis and chemical treatment.

Oxygen electroreduction at catalysts PtM (M = Co, Ni, or Cr)

Bimetallic catalysts PtM (M = Co, Ni, or Cr) are synthesized. They exceed purely platinum commercial catalyst E-TEK (20 wt % Pt) in its mass activity (mA/mgPt) and specific activity (mA/cPt2) in the oxygen reduction reaction. According to XRD data, the high-temperature synthesis involving metal N4-complexes, chloroplatinic acid, and XC72 carbon black as precursors, yields alloys (or solid solutions) of the metals. The higher activity of the bimetallic catalyst PtCo/C is likely to be caused by the practically entire formation of solid solutions (Pt3Co and PtCo), unlike PtNi and PtCr where nickel and chromium exist also as oxides that decorate the electrode surface and partly block active centers. It is shown that the mechanism of the oxygen reduction reaction at the synthesized catalysts is similar to that of oxygen reduction at the purely platinum catalyst. The slow stage in the process is transfer of the 1st electron; at potentials more positive than 0.6 V the reaction mainly yields water. The higher electrocatalytic activity of the bimetallic systems is caused by the alloy formation, which leads to changes in the bond length between platinum atoms. The achieving of the optimal bond length, as a result of the alloy formation, provides appropriate conditions for dissociative adsorption of oxygen molecules; the surface coverage with oxygen-containing particles adsorbed from water (which block active centers for O2 adsorption) decreased. The increase in the activity may also be caused by the formation of the “core-shell” structures whose surface is enriched with platinum whose surface properties are changed under the ligand action of the core formed by the metal alloy

Bioelectrocatalytic and Enzymic Activity of Laccase in Water–Ethanol Solutions

The effect the composition of a water–ethanol mixture has on the enzymic (in the pyrocatechol oxidation reaction) and bioelectrocatalytic in a broad potential range (in the oxygen reduction reaction) activity of laccase (L) is studied. On the basis of obtained results conclusions are made about the influence exerted by ethanol in the composition of the water–ethanol mixture on the activity of laccase solubilized and immobilized in the composition of a composite (laccase–Nafion). The decrease in the activity in both the enzymic and the bioelectrocatalytic reactions is probably caused by the denaturation of laccase, which is due to the replacement of the hydration shell of a protein globule by a solvation shell. Besides, there take place a retardation of the kinetic stage of the formation of a laccase–substrate complex (LHO2OH) because of the slowness of the diffusion of water into an active center and an inhibiting effect of ethanol, which is capable of binding itself to an enzyme globule in the vicinity of the active center of laccase.

Electroconductance and Penetrability of Polybenzimidazole Membranes in Alkaline Solutions

Effect of various factors on the conductance and penetrability of polybenzimidazole membranes is studied. The conductivity of alkali-doped membranes is no lower than 0.01 S cm–1. The membrane penetrability to methanol is close to that of Nafion.

The structure and characteristics of a PtCoCr nanosized polymetallic cathode catalyst on a carbon carrier

The paper presents X-ray and transmission electron microscopy data characterizing the structure of trimetallic PtCoCr catalysts synthesized on a disperse carbon carrier (carbon black KhS 72) and the influence of the structure on electrocatalytic activity in the reduction of oxygen in 0.5 M H2SO4. The mechanisms of oxygen reduction on platinum and trimetallic catalysts were shown to be similar. A higher activity of platinum contained in the trimetallic catalyst was caused by smaller PtCoCr/C catalyst surface coverage by oxygen-containing particles formed from water and interfering with the adsorption of molecular oxygen, which was, in turn, determined by the electronic structure of trimetallic system nanoparticles.

Electroreduction of oxygen on carbon black with adsorbed peroxidase or a peroxidase-nafion composite

The oxygen electroreduction on electrodes with adsorbed peroxidase (POD) or a POD-Nafion composite is studied. In the conditions of direct bioelectrocatalysis, the process occurs on both POD and POD in the composite. The higher the electrode coverage by POD, the higher the measured current. The composite electrodes have a higher activity and stability. A mechanism of the cathodic reduction of oxygen by immobilized POD is discussed

Kinetics and Mechanism of the H2O2 Electroreduction on the Peroxidase-Promoted Electrodes

The regularities of the bioelectrocatalytic reduction of H2O2 in the presence of peroxidase (POD) or a POD–Nafion composite adsorbed on the surface of carbon black or isotropic pyrocarbon are considered. The effect of the surface coverage with the enzyme, the H2O2 concentration, and the H2O2 supply rate to the electrode on the system activity is studied. Specific activities of three electrode types (POD adsorbed on carbon black or pyrocarbon, and the composite deposited on pyrocarbon) are close to one another. A mechanism for the bioelectrocatalytic reduction of H2O2 in a wide potential range is proposed. At potentials near the steady-state value, the reaction rate is limited by the electrochemical kinetics. At high polarizations, the formation of a POD compound with H2O2 is the limiting stage.

Macrokinetics of processes in glucose oxidase—peroxidase bienzyme systems when assaying glucose by the H2O2 reduction current: Effect of the enzyme ratio on the steady-state potential and H2O2 reduction current in glucose solutions of different concentrations

The effect of the enzyme ratio on the steady-state potential and H2O2 reduction current at a bienzyme glucose oxidase-peroxidase electrode in glucose solutions of different concentrations is studied. The derived equations adequately describe the experimental data and allow one to estimate characteristics of bienzyme electrodes in the linear domain of the current variation with the glucose concentration and the system’s sensitivity