Browsing by Author "Kalcheva S."

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    A study on the effect of potentiostatically grown gold oxide films on the oxidation of phenol in base
    (2001-01-01) Iotov P.; Kalcheva S.
    The general behaviour of potentiostatically grown gold oxide films during the oxidation of phenol in basic solutions has been studied using cyclic voltammetry. No catalytic activity is observed and phenol oxidation is prevented when the oxide film consists of both α- and β-oxides. The depletion of β-oxide is accompanied by rapid electrode passivation, showing that the presence of Au(OH)3 is essential for the oxidation process to take place. A tentative reaction mechanism based on the IHOAM model of electrocatalysis is proposed to account for the participation of the redox mediator couple Au(I)/Au(III). In solutions having high concentrations of KClO4 or KCl and short reaction times chlorination occurs at gold oxide electrode films, without polymerization at temperatures lower than 308 K and 321 K respectively. Oxidation and chlorination reactions occur with apparent activation energies of ca 17 and 70 kJ mol-1 respectively : this indicates that oxidation is favoured at lower concentrations of the additives.
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    Kinetics of borohydride electrooxidation: Revisited
    (2014-01-01) Kalcheva S.; Kanazirski I.
    The initial steps of sodium borohydride oxidation are studied at PtAu alloy electrodes of Au bulk composition of 20, 40, 60 and 80 at. %. Linear sweep voltammetry is applied at low scan rates in 0.01 M NaBH4 in 1.00 M NaOH. The experiments are performed at four temperature values in the range from 293.2 K to 323.2 K. The data obtained are compared to those referring to Pt and Au as well. The values of the exchange current density and the total number of electrons exchanged are determined following the effect of the temperature applied. The activation energy for the exchange current density is found dependent on the bulk composition of the alloys studied. The lowest value is expected for an alloy of Au bulk content of ca 50 at. %. The dependence on the surface composition is only a tentative one in view of the sensitivity of the reaction studied. Its proceeding requires the presence of four adjacent active centers on the electrode surface whose heats of adsorption are strongly affected by the alloying.
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    On the electrocatalytic activity of PtAu bimetallic systems in highly concentrated methanol solutions
    (2013-05-21) Iotov P.; Kalcheva S.
    The electrocatalytic behavior of Pt and PtAu bimetallic electrodes of bulk composition of 20, 40, 60 and 80 at.% is studied in 0.5 M solution of H2SO4 in dried CH3OH and 0.5 M solution of H2SO4 in CH3OH:H2O in a ratio of 1:1. Cyclic voltammetry is applied varying the potential ranges studied and the scan rate applied. The experiments are performed at four temperature values in the interval from 293.2 K to 323.2 K. The data obtained show that O-H bond scission occurs preferably in the first medium leading to formation of formaldehyde and formic acid, while sequential C-H rupture takes place in the second one to yield COads . The apparent activation energy of the process is found to decrease in both media with Au bulk increase till about 60 at. %. It is concluded that the amount of water present controls the pathway of methanol transformation while alloying favors predominantly the kinetics of the process through its effect on the surface susceptibility to poisonous intermediates.