G. Kokkinidis

Electroless deposition of Pt on Ti : catalytic activity for the hydrogen evolution reaction

Abstract 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.

Electrooxidation of ethylene glycol on Pt-based catalysts dispersed in polyaniline

Platinum dispersed in a polyaniline film is a better catalyst than smooth Pt for ethylene glycol electrooxidation in perchloric acid aqueous solutions. The catalytic activity of the platinum microparticles is further enhanced when Ru, Sn or both are codeposited. The PAni/Pt–Sn assembly shows the highest electrocatalytic activity of the electrodes examined. Underpotential deposition of Tl and Bi on dispersed Pt inhibits EG electrooxidation while Pb causes significant catalysis only with a specific preparation method electrocatalyst. The morphology and the identity of the metallic dispersion is examined by transmission electron microscopy.

Electrocatalytic oxidation of methanol and formic acid on dispersed electrodes: Pt, Pt–Sn and Pt/M(upd) in poly(2-hydroxy-3-aminophenazine)

Platinum particles dispersed in a poly(2-hydroxy-3-aminophenazine) film (pHAPh/Pt) provide a better catalyst than smooth Pt for the electrooxidation of methanol and formic acid in perchloric acid aqueous solutions. The catalytic activity of the Pt particles is further enhanced when Sn is codeposited in the polymer film. In the case of formic acid oxidation, the activity of Pt nanoparticles is influenced by adatoms of Tl, Pb and Bi deposited undepotential conditions. The upd-modified Pt particles are much more active than bare Pt particles. The morphology and identity of the metallic dispersion were examined by transmission electron microscopy.

Electroless deposition of Pt on Ti: Part II. Catalytic activity for oxygen reduction

Abstract The reduction of oxygen on electroless deposited platinum on a freshly polished titanium electrode was studied in aqueous 0.1 M HClO 4 and 0.2 M NaOH solutions, utilising the rotating-disc electrode technique. TEM studies show that at short deposition times Pt is highly dispersed on titanium. At longer deposition times the degree of dispersion decreases due to coalescence and agglomeration processes and larger clusters are formed. The kinetics of oxygen reduction were affected substantially by the time of Pt deposition on Ti at the open-circuit potential. A kinetic analysis of the current–potential curves was made in the kinetically controlled region in the form of Tafel plots and in the mixed kinetic-diffusion control region in the form of Koutecky–Levich plots. The catalytic activity of the Pt crystals decreases with increasing deposition time. The results were interpreted in terms of the effect of the Pt crystal size. The larger the Pt particles the lower the catalytic activity. The role of titanium oxide formation and the electronic effects were also discussed.

Nucleation and growth of metal catalysts. Part I. Electrodeposition of platinum on tungsten

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.

Electrochemical behaviour of nitroindoles: oxidative electropolymerization and reduction of the nitro group of polymerized and non-polymerized 4-nitro and 5-nitroindole

Nitroindoles give polymer films after oxidative electropolymerization on a Au rotating-disc electrode in acetonitrile. Films of poly(4-nitroindole) and poly(5-nitroindole) were grown potentiostatically at potentials where electrodissolution of Au substrate occurs simultaneously with the electropolymerization. The films are electroactive. Their cyclic voltammograms in acetonitrile present an immense cathodic peak corresponding to the reduction of the nitro group. On transfer to aqueous electrolyte, films of poly(5-nitroindole) show well-defined reduction peaks, while those of poly(4-nitroindole) show ill-defined peaks. The reduction of the nitro group of the monomers has also been studied. Nitroindoles show the usual behaviour of aromatic nitro compounds both in aqueous solutions and in acetonitrile. Finally, it was found that underpotentially deposited layers of Pb and Tl on Au and Pt have a marked influence on the catalytic activity and selectivity of these electrodes for the nitro group electroreduction.

Electrochemical studies of ferrocene derivatives and their complexation by β-cyclodextrin

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 β-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 β-cyclodextrin causes a decrease in the currents and shifts the potentials positively. The formation constants of the inclusion complexes of compounds with β-cyclodextrin were determined by quantitative evaluation of E1/2 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 β-cyclodextrin decreases the rate of its hydrolysis substantially.

Pt-Ni carbon-supported catalysts for methanol oxidation prepared by Ni electroless deposition and its galvanic replacement by Pt

Pt–Ni particles supported on Vulcan XC72R carbon powder have been prepared by a combination of crystalline Ni electroless deposition and its subsequent partial galvanic replacement by Pt upon treatment of the Ni/C precursor by a solution of chloroplatinate ions. The Pt-to-Ni atomic ratio of the prepared catalyst has been confirmed by EDS analysis to be ca. 1.5:1. No shift of Pt XPS peaks has been observed, indicating no significant modification of its electronic properties, whereas the small shift of the corresponding X-ray diffraction (XRD) peaks indicates the formation of a Pt-rich alloy. No Ni XRD peaks have been observed in the XRD pattern, suggesting the existence of very small pockets of Ni in the core of the particles. The surface electrochemistry of electrodes prepared from the catalyst material suggests the existence of a Pt shell. A moderate increase in intrinsic catalytic activity towards methanol oxidation in acid has been observed with respect to a commercial Pt catalyst, but significant mass specific activity has been recorded as a result of Pt preferential confinement to the outer layers of the catalyst nanoparticles.

Catalytic influence of underpotential-deposited submonolayers of heavy metals on d-glucose oxidation on various noble metal electrodes in alkaline media

Abstract The catalytic influence of underpotential-deposited (upd) submonolayers of heavy metals (i.e. Pb, Tl, Bi) on the electrooxidation of D-glucose on various noble metal electrodes (i.e. Pd, Rh, Ir) is studied in alkaline media, and the results are compared with those observed for these systems when Pt was used as the electrode. In the case of Rh and Ir electrodes the catalytic activity is expressed mainly through the considerable increase of the respective current peaks as well as through the negative shift of the oxidation peak in the double-layer region, while in the case of the Pd electrode no significant catalytic action of upd ad-layers on the oxidation of D-glucose is observed. An explanation of the enhancement of the catalytic action of these electrodes (except Pd) is given on the basis that the upd ad-atoms decrease the electrode poisoning, due to an intermediate gluconolactone-type adsorbate, according to the third body mechanism in electrocatalysis. Finally, from a volcano-type diagram it is found that for the catalysis of D-glucose oxidation by upd Pb, Tl and Bi ad-atoms the catalytic activity of the electrode metal used decreases according to the order: Pt > Pd > Rh > Ir.

Electrooxidation of β-D(+)glucose on bare and u.p.d. modified platinum particles dispersed in polyaniline

Platinum particles dispersed in a polyaniline film (PAni/Pt) provide a better catalyst than smooth Pt for the electrooxidation of β-d(+)glucose in perchloric acid aqueous solutions. The Pt particles are less sensitive to poison formation than smooth platinum. Their catalytic activity is influenced by modification with underpotential deposition (u.p.d.) of thallium, lead or bismuth. Up to 0.4V vs RHE the Pt particles show a better activity than u.p.d.- modified platinum particles. However, above 0.4V the u.p.d.-modified particles are more active than bare Pt particles. Underpotential deposition of cadmium inhibits the electrooxidation of β- d(+)glucose. The morphology and the nature of the metallic dispersion were examined by transmission electron microscopy.