Papers by Georgios Kokkinidis
Canadian Journal of Chemistry, 1996
The anodic oxidation of 6-hydroxydopamine and 5-hydroxydopamine and their chloride and bromide sa... more The anodic oxidation of 6-hydroxydopamine and 5-hydroxydopamine and their chloride and bromide salts was studied in aqueous acidic solutions. The electrodes used were Pt and Pt/M (UPD) (M = Bi, Tl, Pb) modified surfaces. The electrochemical studies revealed that the first two-electron oxidation leads to formation of reactive quinoid intermediates. Reactions of these quinoid intermediates with nucleophiles yield substituted quinones that are not stable in solution. The reactions are initiated by the oxidation of the third hydroxyl group at more positive potentials. Cyclic voltammetry and rotating ring-disc electrode voltammetry were used to elucidate the reaction mechanisms. These techniques provide powerful tools to generate and detect transient intermediates and products and give valuable information regarding the pathways and mechanisms of the redox reactions. Key words: 6-hydroxydopamine, 5-hydroxydopamine, voltammetry, electrolysis, oxidation mechanism, nucleophilic attack.
Materials Chemistry and Physics, 2001
The possibility for platinum electrodeposition on metallic and nonmetallic electrodes has been ex... more The possibility for platinum electrodeposition on metallic and nonmetallic electrodes has been examined. Several materials were used to prepare the working electrodes for platinum electrodeposition: tungsten, titanium, rhenium, zirconium, stainless steel, glassy carbon ...
The Journal of Physical Chemistry C, 2010
Platinized Cu, Ni, and Co deposits have been formed on glassy carbon (GC) electrode substrates by... more Platinized Cu, Ni, and Co deposits have been formed on glassy carbon (GC) electrode substrates by a twostep process, whereby a controlled amount of the transition metal was electrodeposited onto GC and subsequently partially exchanged for Pt upon immersion of the Cu/GC, Ni/GC, and Co/GC electrodes into a chloroplatinic acid solution. The spontaneous partial replacement of the transition metal by Pt resulted in Pt(Cu)/GC, Pt(Ni)/GC, and Pt(Co)/GC electrodes whose composition and depth profile were obtained by energy-dispersive spectrometry (EDS) and sputter-etch Auger electron spectroscopy (AES). Following electrochemical conditioning (involving electrode exposure to the oxygen and hydrogen evolution potential regimes), all deposits displayed typical Pt surface electrochemistry in acid solutions. Pt(Cu) catalysts exhibited enhanced electrocatalytic activity for methanol oxidation both during voltammetric and constant potential experiments. The behavior of Pt(Ni) and Pt(Co) depended on the method of assessment; in short-term voltammetric experiments, they were inferior to pure Pt, whereas in long-term constant potential experiments they outperformed it. The superior methanol oxidation activity of Pt(Cu) among all catalysts tested is interpreted in terms of the effect of Cu, Ni, and Co on methanol dissociative chemisorption and CO poison removal at/from the Pt surface.
A rotating disc electrode study of oxygen reduction at platinised nickel and cobalt coatings
Journal of Solid State Electrochemistry, 2010
Abstract Platinized nickel and cobalt coatings, Pt(Ni) and Pt(Co), have been prepared on glassy c... more Abstract Platinized nickel and cobalt coatings, Pt(Ni) and Pt(Co), have been prepared on glassy carbon, GC, rotating disc electrode substrates by a two-step room temperature procedure that involved the electrodeposition of nickel and cobalt layers and their spontaneous partial ...
Effect of the extent and structure of upd adlayers on the reduction of 2-nitroimidazole on Au(111) in acidic solutions
Journal of Electroanalytical Chemistry, 2002
The influence of ad-atom layers of Pb, Tl, Cu, Bi and Ag, deposited in the underpotential region,... more The influence of ad-atom layers of Pb, Tl, Cu, Bi and Ag, deposited in the underpotential region, on the reduction of 2-nitroimidazole on the Au(111) single crystal electrode was studied in aqueous acid solutions. It was found that the kinetics and mechanism of the nitro group reduction depend on the degree of coverage and the superlattice structure of the ad-layers. On Au(111) covered by a low coverage of disordered ad-atoms, the reduction follows an electrocatalytic mechanism associated with a four-electron reduction to 2-hydrohylaminoimidazole. At higher coverage where the ad-atoms form ordered monolayers, the reaction follows an electronation–protonation mechanism associated with a two-electron reduction to a N,N′-dihydroxyamine. This product, rapidly stabilized by two intramolecular hydrogen bonds, cannot be reduced further. Of particular interest is the effect of Cu upd for which a stepwise decrease of the reduction current was observed at potentials closely related to those of the peaks associated with the formation of Cu adlattice structures on Au(111) in the absence of 2-nitroimidazole.
Mixed platinum–gold electrocatalysts for borohydride oxidation prepared by the galvanic replacement of nickel deposits
Journal of Electroanalytical Chemistry, 2009
Mixed platinum/gold coatings were prepared by partial galvanic replacement of nickel layers that ... more Mixed platinum/gold coatings were prepared by partial galvanic replacement of nickel layers that had been electrodeposited onto glassy carbon electrode substrates. The process (termed “transmetalation”) involves the spontaneous replacement of surface Ni atoms by Pt and Au upon immersion of the former metal into equimolar mixed chloroplatinic and chlorolaurate acid solutions. The multimetallic deposits on glassy carbon substrates, PtAu(Ni)/GC, were
Journal of Electroanalytical Chemistry, 2000
The deposition of platinum on titanium at open-circuit potential from an aqueous 0.1 M HClO 4 sol... more The deposition of platinum on titanium at open-circuit potential from an aqueous 0.1 M HClO 4 solution containing K 2 PtCl 6 was studied. On freshly polished titanium, spontaneous deposition of platinum takes place through a displacement reaction between Ti(0) and dissolved Pt(IV). At very short times of deposition, fine dispersions of platinum crystals were formed on titanium. At longer deposition times, the size of the platinum crystals increases whereas their number decreases due to coalescence and agglomeration processes. On titanium covered by a thin oxide film, the rate of deposition is very low and photocatalytic deposition of Pt on the TiO 2 surface occurs simultaneously with the displacement reaction. The hydrogen evolution reaction (her) was used as a probe reaction for testing both the catalytic activity and deposition conditions. There is evidence that the catalytic activity of the Pt crystals for the her increases with decreasing crystal size.
Journal of Electroanalytical Chemistry, 1999
The voltammetric behaviour of (ferrocenylmethyl)trimethylammonium hexafluorophosphate, 2-ferrocen... more The voltammetric behaviour of (ferrocenylmethyl)trimethylammonium hexafluorophosphate, 2-ferrocenylacrylic acid, N-ferrocenylmethyl-N%-methyl-4,4%-bipyridinium hexafluorophosphate and 9-ferrocenylmethyladenine was studied in aqueous and/or acetonitrile solutions by means of cyclic voltammetry and rotating-disc electrode voltammetry. The influence of b-cyclodextrin on the voltammetric behaviour of the compounds was also studied in aqueous solutions. (Ferrocenylmethyl)trimethylammonium hexafluorophosphate and 2-ferrocenylacrylic acid, studied in aqueous solutions at pH 9.2, exhibit typical reversible one electron transfer behaviour, whereas the redox reaction of 9-ferrocenylmethyladenine, studied in acetonitrile, is strongly affected by adsorption. N-ferrocenylmethyl-N%-methyl-4,4%-bipyridinium hexafluorophosphate gives voltammetric responses for both ferrocene and viologen redox centres in acetonitrile. In aqueous solutions it is hydrolyzed to ferrocenylmethylalcohol and N-methylbipyridyl, a process facilitated by the neighbouring group participation of the iron atom. In all cases b-cyclodextrin causes a decrease in the currents and shifts the potentials positively. The formation constants of the inclusion complexes of compounds with b-cyclodextrin were determined by quantitative evaluation of E 1/2 values and possible explanations about their differentiation are discussed. In the case of N-ferrocenylmethyl-N%-methyl-4,4%-bipyridinium hexafluorophosphate it was found that b-cyclodextrin decreases the rate of its hydrolysis substantially.
Journal of Electroanalytical Chemistry, 2007
Oxygen reduction has been studied on glassy carbon rotating disc substrates modified by platinum-... more Oxygen reduction has been studied on glassy carbon rotating disc substrates modified by platinum-and gold-coated copper particles. The deposits have been formed by a two-step process, whereby controlled amounts of Cu were electrodeposited onto the substrate and ...
Oxygen reduction at platinum- and gold-coated iron, cobalt, nickel and lead deposits on glassy carbon substrates
Journal of Electroanalytical Chemistry, 2008
Platinum- and gold-coated iron, cobalt, nickel and lead particles deposited on glassy carbon elec... more Platinum- and gold-coated iron, cobalt, nickel and lead particles deposited on glassy carbon electrode substrates have been tested for oxygen reduction in acid solutions. Fe, Co, Ni or Pb layers were electrodeposited onto glassy carbon and subsequently immersed into chloroplatinic or chlorolaurate acid solutions where the spontaneous surface exchange of M (M=Fe, Co, Ni or Pb) for Pt or Au
Journal of Electroanalytical Chemistry, 1998
ABSTRACT The paper is an experimental study of the initial stage of platinum electrodeposition on... more ABSTRACT The paper is an experimental study of the initial stage of platinum electrodeposition on a tungsten electrode. Current transients are recorded at different electrode potentials and data are obtained for the stationary nucleation rate and for the diffusion coefficient of platinum ions in an aqueous 0.1 M HClO4 solution containing K2PtCl6.
Journal of Crystal Growth, 2001
Electrodeposition of noble metals that can be used as catalysts of chemical and electrochemical r... more Electrodeposition of noble metals that can be used as catalysts of chemical and electrochemical reactions is a process of major importance both from a practical and a scientific point of view. This is particularly true for the very initial stage of the phase transition comprising nucleation and growth phenomena leading to the formation of a highly dispersed metal phase with a strongly developed active surface area. The paper contains information on the nucleation and growth stage of platinum electrodeposition on an oxidised titanium working electrode. Cathodic current transients are recorded under potentiostatic conditions and are analysed in terms of the theory of progressive nucleation with overlap of diffusion zones and limited number of active sites. Data are obtained on the diffusion coefficient D of the platinum ions, on the nucleation rate constant A and on the number N 0 of active sites on the titanium surface. #
Electrochemical reduction of some hypervalent iodine compounds
Electrochimica Acta, 1991
ABSTRACT The electrochemical reduction of [(hydroxy-sulfonyloxy)iodo] benzenes and some cyclic I(... more ABSTRACT The electrochemical reduction of [(hydroxy-sulfonyloxy)iodo] benzenes and some cyclic I(III) and I(V) compounds has been studied in aqueous neutral and acid solutions by employing polarography and rotating disk voltammetry. All compounds undergo a reductive cleavage of their IO bonds which yields the corresponding I(I) derivatives. The oxidizing properties of the I(III) and I(V) compounds are discussed in relation to those of other hypervalent iodine compounds previously studied. It is shown that differences in oxidizing power between cyclic and non-cyclic compounds are not due to differences in their reduction potential.
Electrochimica Acta, 1998
AbstractÐThis article describes the synthesis and the electrochemical properties of a redox polym... more AbstractÐThis article describes the synthesis and the electrochemical properties of a redox polymer, having a polyindole backbone and a viologen subunit, derived from the oxidative electropolymerization of Nmethyl-N'-(3-indol-1-yl-propyl)-4,4'-bipyridinium (In-V-Me 2+ ). Fairly stable ®lms of poly(In-V-Me 2+ ) were obtained on Pt and Au electrodes from acetonitrile containing Et 4 NClO 4 as supporting electrolyte. The best ®lms were grown on Au at potentials where electrodissolution of Au substrate occurs simultaneously with the electropolymerization. Poly(In-V-Me 2+ ) ®lms show interesting electrochemical properties in acetonitrile and water which stem from the behaviour of the pendant viologen redox centres. The redox reactions of the monomer have also been studied. The indole viologen shows the usual behaviour of viologen compounds both in aqueous solutions and in acetonitrile. In aqueous solutions, In-V-Me 2+ forms on mercury monolayers of radical cations deposited at underpotentials with parallel and vertical orientation on the electrode surface. #
Electrochemistry Communications, 2002
High performance gold-supported Pt electrocatalyst for the reduction of oxygen was prepared by re... more High performance gold-supported Pt electrocatalyst for the reduction of oxygen was prepared by replacing Cu adlayers, deposited potentiostatically on Au, with Pt at open-circuit potential in a 0.1 M HCl solution containing K 2 PtCl 6 . Auger Electron Spectroscopy and Atomic Force Microscopy reveal the surface modification. The kinetics of oxygen reduction on this platinum modified electrode was studied by the rotating-disc electrode technique. The activity of the electrode is lower than the activity of a smooth Pt electrode in the negative potential scan, but it is significantly higher in the positive scan. Ó
Catalysis Today, 2011
The electrochemical oxidation of borohydride has been studied by means of voltammetry at Pt and P... more The electrochemical oxidation of borohydride has been studied by means of voltammetry at Pt and Pt-Ni, Pt-Co bimetallic rotating disc electrodes (RDEs). The bimetallic catalysts are prepared by means of a galvanic replacement method (whereby electrodeposited Ni and Co layers are partially replaced by Pt when immersed in a chloroplatinic solution) and are shown to have a Pt shell-bimetallic alloy core. The effects of electrode history, potential scan direction, rotation speed and electrode material on borohydride oxidation have been investigated. For all Pt-based catalysts tested a gradual decrease of the voltammetric current from its initial value to a steady state response is observed in the kinetic control potential region, irrespective of scan direction and rotation speed. The initial deactivation of the catalyst at low overpotentials as well as the shape of the initial and steady-state voltammograms in that region point to the heterogeneous hydrolysis of borohydride and the subsequent oxidation of its products. As the catalytic activity for the hydrolysis reaction decreases the voltammograms shift to more positive potentials where direct borohydride oxidation dominates. The bimetallic Pt-Ni and Pt-Co catalysts exhibit a more negative open circuit potential and higher oxidation currents at low overpotentials than Pt, but lower apparent number of electrons transferred in the mass transport control region. These findings can be interpreted by the lowering of the d-band energy level of Pt in the presence of Ni and Co and the associated decrease in its adsorption affinity.
Preparation and characterisation of platinum- and gold-coated copper, iron, cobalt and nickel deposits on glassy carbon substrates
Electrochimica Acta, 2008
Pt-and Au-coated Cu, Fe, Co and Ni deposits have been formed on glassy carbon (GC) substrates by ... more Pt-and Au-coated Cu, Fe, Co and Ni deposits have been formed on glassy carbon (GC) substrates by electrodeposition of controlled amounts of the core metal onto the substrate and its subsequent partial replacement by Pt or Au upon immersion into a chloroplatinic or ...
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Papers by Georgios Kokkinidis