A. Papoutsis

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.

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.

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.

Electrocatalytic reduction of nitrocompounds on gold UPD modified electrodes: Part 1. Reduction of nitrobenzene and 3-nitro-1,2,4-triazole on Au and Au/M(UPD) (M = Pb, Tl)

Abstract The underpotential deposition (UPD) of Pb and Tl has been found to exhibit a marked influence on the performance of the Au electrode for the electroreduction of nitro groups. In acid solutions, the UPD of Pb shifts the four-electron reduction wave of nitrobenzene to phenylhydroxylamine towards more negative potentials and also causes a complete inhibition of the reduction step of phenylhydroxylamine to aniline. However, Pb(UPD) and Tl(UPD) adatom layers on Au catalyse the four-electron reduction of 3-nitro-1,2,4-triazole in acid solutions. The catalytic effect is characterized by a positive shift of the reduction wave and the attainment of diffusion-controlled limiting currents at medium degrees of UPD adatom coverages. At complete UPD coverages, the catalytic activity decreases and the limiting current becomes kinetically controlled as on bare Au. The kinetic character of the limiting current was attributed to the dehydration reaction of a dihydroxylamine intermediate in an ECCE reaction sequence. The catalytic effect observed at medium degrees of UPD adatom coverages was interpreted in terms of strong adsorption of the nitro group with a bridged complex involving a substrate (Au) and an adsorbate atom (Pb,Tl) which favour NO bond cleavage. Finally, in alkaline solutions it has been found that Pb(UPD) and Tl(UPD) monolayers catalyse the four-electron reduction of nitrobenzene and 3-nitro-1,2,4-triazole.

Electrocrystallisation of metal catalysts: nucleation and growth of platinum on a titanium electrode

Abstract 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.

Study of the kinetics of adsorption and electro-oxidation of MeOH on Pt(100) in an acid medium by programmed potential voltammetry

The electrosorption and electro-oxidation of methanol on Pt(100) single-crystal surfaces have been studied using programmed potential voltammetry (PPV). Deconvolution of the main oxidation peak of methanol adsorption residues leads to three secondary peaks at short adsorption times (tads < 1 s) and two secondary peaks at medium and long adsorption times. The correlation of the quantities of electricity of the secondary peaks with the decrease in the quantity of electricity related to hydrogen desorption was obtained by means of an approximation method. Thus the number Neps of electrons per site was calculated for each peak. The Neps values for the three peaks resulting from the deconvolution leads to the conclusion that at short tads multibonded or bridged CO species coexist with linearly bonded CO and formyl species, while at medium and long tads only linear CO and formyl species exist.

Electrocatalytic reduction of nitro compounds on gold UPD modified electrodes: Part 2. Reduction of 2-nitro- and 4-nitroimidazole and 4-nitropyridine-N-oxide on Au and Au/M(UPD) (M = Pb, Tl)

Abstract The reduction of N -heterocyclic nitro compounds on Au, an sp metal eletrocatalyst, in acid solutions can follow an electrocatalytic and/or electron exchange mechanism. The electrocatalytic mechanism proceeds through chemisorption of the nitro group and reductive cleavage of one of the two NO bonds and gives diffusion-controlled limiting currents. The electron exchange mechanism proceeds through a dihydroxylamine which is reduced further after an irreversible loss of water to produce the nitroso intermediate and leads to kinetically controlled limiting currents. The strong electron-attracting protonated heterocyclic nuclei, as well as rendering the nitro group more easily reducible, help to stabilize the hydrated form of the nitroso intermediate. On Au/M(UPD) electrocatalysts the reduction mechanism depends on the UPD adatom coverage. On Au surfaces, with almost complete adatom layers, the reduction proceeds through the dihydroxylamine. However, on Au partially covered by adatoms the electrocatalytic mechanism appears to be the only reaction path for the reduction. A bridge adsorption complex of the nitro group was assumed to explain its stronger chemisorption on Au partially covered by adatoms than on Au without adatoms.

Electropolymerization of 2-hydroxy-3-aminophenazine (HAPh): properties of pHAPh films and kinetics of oxidation and incorporation of dihydroxybenzenes at Pt/pHAPh electrodes

Abstract 2-Hydroxy-3-aminophenazine (HAPh) gives stable redox active polymer (pHAPh) films after oxidative electropolymerization on Pt, Au, glassy carbon and ITO electrodes. pHAPh is a new ladder polymer with structures and redox properties closely related to those of poly( o -phenylenediamine) and poly( o -aminophenol). The electropolymerization of HAPh on Au is accompanied by electrodissolution of the Au substrate. Dissolved Au species are redeposited producing dispersed gold crystallites in the organic polymer film. The pHAPh film can respond to dissolved organic species, such as dihydroxybenzenes and p -benzoquinone, in the electroinactive potential range. For the oxidation of dihydroxybenzenes the rate controlling processes are all three contributory steps, i.e. charge transport and diffusion of substrate within the film and cross-exchange electron transfer reaction between the substrate and the redox sites of the film. For p -benzoquinone, the reduction process is solely governed by substrate diffusion within the film. RRDE measurements have shown that the dihydroxybenzenes are incorporated in the pHAPh film in the reduced state. There is also evidence that p -benzoquinone is also incorporated in the film when it is in the oxidized state.

Dispersion of crystallites of the metal substrate during the electropolymerization of 2-hydroxy-3-aminophenazine (HAPh) on Au : effect of Au crystallites on the kinetics and mechanism of charge-transfer reactions between the polymer and redox species in electrolyte

Abstract The electrochemical polymerization of 2-hydroxy-3-aminophenazine (HAPh) has been studied in aqueous perchloric acid solutions. Under potential cycling the polymerization reaction is accompanied by electrodissolution/electrodeposition of the Au substrate onto which the polymer is deposited. TEM observations have revealed two kinds of crystalline grains in the pHAPh polymer film. The first kind consists of spherical grains of pure gold which are randomly oriented in the polymer matrix. The second kind consists of oriented well-shaped crystals of unknown composition, but most probably crystals of an intermediate Au(III) complex. The dispersed Au crystallites affect the kinetics and mechanism of charge-transfer reactions between the polymer and redox species in electrolyte, such as dihydroxybenzenes and hydrazine, as well as their permeability and incorporation into the polymer film.