Velichenko Yu. A., Kopteva S. D., Posudiievska O. R.
Oles Honchar Dnepropetrovsk National University
ELECTRODEPOSITION OF PbO2-TiO2 AND PbO2- ZrO2 COMPOSITE ANODES AND THEIR ELECTROCATALYTIC ACTIVITY
Anodes based on oxide composite materials are widely used in various reactions as photo- and electrocatalysts, in electrochemical synthesis of strong oxidizing agents, in electrochemical removal of organic and inorganic contaminants in water. Various ways to produce materials of this type are known, e. g. sol-gel techniques, plasmochemical method, and so on. The electrochemical method should be distinguished as one of the most promising because it gives an opportunity of wide control over the composition and properties of composites due to its simple implementation and possibility of varying smoothly the technological parameters of the process. Lead dioxide is a promising material, widely used in different applications. Electrodeposited pure lead dioxide was demonstrated to exhibit a moderate electrocatalytic activity in various anodic reactions in acidic media. However, this activity can often be enhanced to a great extent by incorporation of some ions, surfactants, polyelectrolytes and oxides. In this study, the fundamental aspects of electrodeposition of PbO2-TiO2 and PbO2-ZrO2 composites are examined, as well as the physicochemical and electrocatalytical properties of these materials.
The composites were deposited in the galvanostatic mode onto pretreated platinum-plated titanium electrodes of the surface area 4 cm2. The deposition electrolyte contained 0.1 MPb(NO3)2 and 0.1 M HNO3. Additionally, TiO2 (35 nm) or ZrO2 (26 nm) powder was added into the electrolyte. Titanium and zirconium dioxide powders are monodisperse particles according to the producer data (Crimean Titanium Inc. – Ukraine). In some cases we used an anionic surfactant: sodium dodecylsulfate C12H25SO4Na (SDS) as an additive into the electrolyte.
Particles of the dispersed phase are incorporated into the growing PbO2 deposit to give a composite material, and the content of foreign oxide in the composite will be determined by the stages in which particles are delivered from the electrolyte bulk to the electrode surface. Additives introduced into the deposition electrolyte are adsorbed on oxides of the dispersed phase and cause the surface of the particles of the dispersed phase to be recharged. The negatively charged oxide/surfactant particles are transported towards the positively charged electrode where, in this case, their incorporation into the growing coating is assisted by the electric field. From a general point of view, the TiO2 and ZrO2 can be considered as electrochemically inert because their deposition does not require their oxidation/reduction, but they are not completely «inert», as they influence the deposition kinetics by providing extra nucleation sites like [MO2]Pb2+ads.or [MO2]OHads.. It means that in our case the oxide of the dispersed phase is involved in the electrochemical deposition of lead dioxide. Taking into account (i) the formation of colloidal PbO2 particles during the electrodeposition process, (ii) the adsorption of negatively charged additives on PbO2, TiO2 and ZrO2 particles, and (iii) the influence of electrolysis conditions and electrolyte composition mentioned above, one can suggest the colloidal-electrochemical mechanism of the composite material formation.
Electrocatalytic activity of composite materials was studied for oxygen evolution reaction (OER). It was shown that overpotential of oxygen evolution in the composite materials is much higher than at PbO2 electrode. The process of degradation of methyl tert-butyl ether (MTBE) on PbO2-TiO2 anodes was examined. Gas chromatographic measurements showed that tert-butanol (TBA), acetone, acetic acid and CO2 were the main by-products of electrochemical degradation of MTBE at PbO2 electrodes. The concentration of TBA reached its maximum after 1 hour of electrolysis, while the concentrations of acetone and acetic acid increased during treatment time of up to 3 hours. After 6 hours of electrolysis, only impurity level of acetic acid was found. According to our calculations performed on kinetic data (evolution of MTBE concentration vs time), electrochemical oxidation of MTBE is a pseudo -first -order reaction with main kinetic parameters. It is important to note that increasing TiO2 content in composite electrodes leads to the double increase in the rate of MTBE electrooxidation, along with the decrease in half-life of the reaction from 125 to 69 min. A stronger electrocatalytic activity of composite PbO2-TiO2 electrodes in the electron transfer reactions can be connected with the presence of the large number of particles, capable of creating a strong bond with the surface of the electrode.
Another interesting effect is the increase in the rate of MTBE electrooxidation under UV irradiation. In case of the composite containing 6 wt. % of TiO2 the constant rate of MTBE electrooxidation increases in two times under UV irradiation, along with decreasing half-life of the reaction from 99 to 58 min. This effect is related to the additional generation of OH-radicals due to photocatalysis.
The service life of composite anodes is at least two times higher than that of pure lead dioxide. According to the presented data, PbO2-TiO2 and PbO2-ZrO2 composite materials are interesting for the industrial applications as electrocatalysts with good mechanical properties and long service life.