R. Amadelli

Oxygen and ozone evolution at fluoride modified lead dioxide electrodes

This work examines the behaviour of fluorine modified β-PbO2 electrodes in the processes of O2 and O3 evolution in sulphuric acid. The electrochemical kinetic analyses of these processes are based on quasi-steady-state polarisation and impedance data. The good agreement between the two sets of measurements allows some basic conclusions to be drawn. In particular, the O2 evolution process is always inhibited at F-doped PbO2 electrodes, and impedance results suggest possible changes in the mechanism, with electrodesorption of intermediates becoming more important as the concentration of the doping element increases. The interpretation of the data for the less positive potentials region invokes the specific adsorption of SO42− as a factor influencing the kinetics of O2 evolution. The current efficiency for O3 formation as a function of the amount of NaF added to the PbO2 growth solution reaches a maximum for a concentration of 0.01 mol dm−3. A plausible cause for the decrease on the higher concentration side is the discharge of adsorbed SO42− (or HSO4−) eventually yielding persulphate. This reaction is known to be favoured in the presence of a relatively high amount of fluoride in the electrolyte. An analysis of the results of modified neglect of diatomic differential overlap (MNDO) calculations on Pb cluster models and of X-ray photoelectron spectroscopy (XPS) data suggests that the coverage by weakly adsorbed oxygen species (OH and H2O) is an important parameter that is influenced by F-doping.

Electrosynthesis and Physicochemical Properties of PbO2 Films

An electrochemical and X-ray diffraction study has been conducted on the formation of lead dioxide deposits on platinum, from nitric acid solutions, as a function of potential and temperature. It has been shown that these parameters strongly influence the morphology and electrocatalytic activity of the PhO 2 films. The electrodeposition process is satisfactorily described by an electrochemical, chemical, electrochemical mechanism: (i) H 2 O → OH ads + H + + e; (ii) Ph 2+ + OH ads → Pb(OH) 2+ ; (iii) Pb(OH) 2+ + H 2 O → PbO 2 + 3H + + e; the second electron transfer stage and Pb 2+ diffusion control the dioxide formation in the lower and higher overpotential range, respectively. Temperature and potential (or current) are important parameters in the electrodeposition process. Depending on the potential region, the process can he kinetically or diffusion controlled. In an acid electrolyte, where mainly the β-PbO 2 modification is electrodeposited, the amount of α-phase impurity increases with increasing potential in the kinetically controlled region and decreases in the diffusion controlled domain. In addition, relatively low electrodeposition potentials and high temperatures favor an increase of the crystallite size with preferred crystallographic orientation for both α- and β-PbO 2 modifications. The temperature of the growth solution affects the crystallinity of the resulting oxide deposits and has a marked effect on their performance as anodes in processes at high positive potentials such as ozone generation.

Electrochemical oxidation of trans-3,4-dihydroxycinnamic acid at PbO2 electrodes: direct electrolysis and ozone mediated reactions compared

A comparative study of the oxidative degradation of trans-3,4-dihydroxycinnamic acid was carried out using three different methods, two of which have in common the use of electrogenerated ozone as a potential oxidant: (i) direct electrolysis; (ii) external cell chemical reactions with O3; and (iii) a ‘cathodic oxidation’ in which an O2/O3 mixture is fed into the cathodic compartment containing the organic substrate; H2O2 produced reacts with ozone to yield radicals that bring about the demolition of the organic compound. The nature products formed by the direct electrochemical oxidation depends on potential. The degradation of the dihydroxy-acid is observed only at relatively high potentials, where radical oxygen species are formed in a high amount by the oxidation of water. At relatively lower potentials the process shows stirring dependence and leads essentially to one oxidation product which is adsorbed on the electrode. A comparative examination of the results, considering also the requested amount of ozone as a parameter, shows that the external cell and cathodic oxidation are the most efficient methods. This confirms our previous data on the oxidation of phenolic compounds, and the cathodic oxidation with ozone emerges as a promising approach in the abatement of pollutants.

Influence of the electrode history and effects of the electrolyte composition and temperature on O2 evolution at β-PbO2 anodes in acid media

An investigation is reported on O2 electrogeneration at β-PbO2 electrodes in HClO4, H2SO4 and CF3SO3H (TFMSA) using steady-state and impedance measurements. It is established that desorption of reaction intermediates is rate limiting in all the cases examined, and it is proposed that elementary reactions leading to oxygen evolution take place at a surface consisting of crystal line (PbO2) and hydrous [PbO(OH)2] zones. Contribution of the latter becomes particularly pronounced at high potentials, probably on account of PbO2 dissolution and re-deposition in the form of Pb(OH)4 that subsequently evolves to give PbO(OH)2. This process is indicated as a possible cause of an inflexion point in the Tafel plots between 1.95 and 2.0 V SCE. Different important factors are found to affect the electrochemical process: (i) pre-treatment of the electrode, which makes electrodes more active by reason of an increase of the hydrous zones; (ii) the temperature at which electrodeposition of PbO2 is carried out has an effect on the hydration state of the surface and hence the electrocatalytic activity; (iii) electrolyte anions, particularly SO42− and CF3SO3−, are adsorbed and inhibit both water discharge and desorption of reaction intermediates; besides, they undergo oxidation at the more positive potentials; (iv) fluoride added to the electrolyte is strongly adsorbed and suppresses and/or modifies the structure of the hydrous layer with the consequence that water discharge is inhibited in the lower potential range and, prevalently at the more positive potentials, desorption of oxygen intermediates is strongly retarded; and (v) a low electrolyte temperature also inhibits O2 evolution mainly by inhibition of the desorption of reaction intermediates in the whole potential range examined, in contrast to the selective effect of fluoride.

Lead dioxide electrodeposition and its application: influence of fluoride and iron ions

Abstract The electrodeposition of PbO2 on Pt in the presence of F− and Fe(III), separately or together in the growth solution, is examined. The electrodeposition follows the same reaction scheme previously reported in the literature for PbO2. The electrocatalytic activity of the resulting oxides is assessed for the reactions of O2 and O3 evolution. Fluoride and iron into PbO2 have different effects on the electrocatalytic activity of PbO2 for O2 evolution and O3 formation. With F–PbO2 the first process is favoured and the second one inhibited, whereas with Fe–PbO2 electrodes we observed the opposite trend. Lead dioxide grown from F− and Fe(III) containing solutions showed the highest activity for O3 formation. The larger amount of iron incorporated into PbO2 in the presence of F− is proposed as a possible cause of the enhanced activity for ozone formation of (F–Fe)–PbO2 electrodes.

Electrosynthesis and physicochemical properties of Fe-doped lead dioxide electrocatalysts

Abstract Doping of electrodeposited PbO 2 with Fe 3+ and Fe 3+ +F − is investigated with the aim to produce electrodes of improved catalytic activity and stability for use in processes at high positive potentials. The amount of Fe incorporated into PbO 2 depends on two main factors; (i) the charge of the electrode surface and (ii) the charge of Fe(III) species. Surface analysis data show substantial differences in the physicochemical properties of Fe-doped and (Fe, F)-doped PbO 2 in comparison with undoped oxide. In particular, X-ray photoelectron spectroscopy (XPS) measurements show a marked decrease in the signal due to adsorbed oxygen species, water and hydroxyl groups. On the basis of these results and of tritium radioactive tracer measurements, it is proposed that incorporation of the foreign species in PbO 2 occurs at defect sites: F − replaces OH groups while Fe 3+ can replace Pb(II) or Pb(IV) at low and high temperature, respectively.

Electrochemical treatment of bisphenol-A containing wastewaters

The effectiveness of electrochemical methods in purification of synthetic wastewaters containing bisphenol-A has been tested. The role of electrode material and electrolysis parameters have been considered. The kinetics of bisphenol oxidation have been followed using different analytical techniques and a degradation mechanism has been proposed.

Electro-oxidation of Some Phenolic Compounds by Electrogenerated O3 and by Direct Electrolysis at PbO2 Anodes

The oxidative degradation of phenolic compounds (4-chlorophenol and 4-nitrophenol) was studied using different electrochemical systems involving ozone formation at PbO 2 anodes: (i) direct electrolysis at constant current; (ii) ex-situ use of O 3 and (iii) combined use of anodically generated stream of O 3 /O 2 fed into the cathode where H 2 O 2 is electrogenerated by O 2 reduction. We show that the latter advanced oxidation method gives the best results: it is a Fenton-type degradation of the target pollutants taking place in the cathodic compartment by reason of the highly oxidizing environment brought about by radicals that are formed mainly in the reactions of 0 3 with OH ― and HO 2 ― .

Phororedox and photocatalytic processes on Fe(III)–porphyrin surface modified nanocrystalline TiO2

Abstract Surface derivatization of titanium dioxide nanoparticles with a Fe(III)–porphyrin has been carried out following a new procedure whereby the complex, rather than the surface, contains the aminopropylsilane functional group. This avoids the problems of surface deactivation by silane groups, reported in earlier investigations, on analogous systems. Characterization of the light-transparent dispersions by laser flash photolysis, UV-vis spectroscopy and photo-electrochemical methods has shown that the nature of the solvent is an important parameter in determining the redox processes involving the grafted porphyrin. In particular, one observes marked effects on the stability of the Fe(II)–porphyrin formed upon capture of the photogenerated electrons. The photocatalytic activity of the composite systems was assessed in the process of monooxygenation of cyclohexane and cyclohexene by molecular oxygen. The bonded porphyrin enhances the yield and the formation of the monooxygenation products with respect to total degradation to CO2 for both the examined substrates. On this basis, we can claim an increase in the efficiency and selectivity with the composite photocatalytic system. In the case of cyclohexane, we observed, in addition, that the iron–porphyrin complex also changes the selectivity of the process, increasing the alcohol to ketone ratio.

Influence of anions on oxygen/ozone evolution on PbO2/spe and PbO2/Ti electrodes in neutral pH media

The formation of O2/O3 on PbO2 from the electrolysis of water in neutral solutions is shown here to present some analogies and some differences with respect to the same process in acid media. The electrolyte composition affects the current efficiency for O3 formation (η) and the cell potential (E) of a Membrel type electrochemical assembly. An improvement in η and a decrease in E is observed upon addition of relatively low amounts of Na2SO4 or NaClO4 in pure water. We observe no effect of NaNO3 on either parameters. In agreement with literature data in acid solutions, F− causes an increase in both η and E. The results of electrochemical kinetic investigations with electrodes of PbO2 electrodeposited on Ti confirm the above data. Current—potential curves constructed from measurements in NaNO3 show a region of Tafel linearity with slopes of 2RT/F and RT/F in the low and high currents range, respectively. Addition of Na2SO4 and NaClO4 to NaNO3 has an effect on the process at more positive potentials only: the RT/F slope decreases toward a value of RT/2F as the concentration of the “foreign” salt is increased. As an explanation of the observed behaviour, the possibility is advanced that a step following the discharge of water is rate determining at high positive potentials with the adsorption of intermediates described by Temkin conditions. The composition of the electrolyte is expected to influence the yield of ozone formation by affecting the coverage and free energy of adsorbed oxygen intermediates.