M.C. Giordano

Multiple current components for methanol electrosorption and electro-oxidation at platinum in acidic solutions

Abstract Methanol electrosorption and residue electro-oxidation was studied on activated platinum electrodes by means of potentiodynamic profiles. Results obtained point to CO adsorbed on two sites as the main stable intermediate after methanol adsorption. Current peaks for both electrosorption and oxidation processes show the splitting on two components that strongly depend on electrode pretreatment and ionic composition of the base electrolyte. An explanation of the different behaviour observed is attempted in terms of two different surface states for the same intermediate.

Electrooxidation of adsorbed CO on polycrystalline platinum in alkaline solutions

Abstract The electrooxidation of adsorbed CO on polycristalline platinum in 1 M KOH was investigated under potentiodynamic conditions. A micro flux cell was used to avoid CO readsorption during the oxidation process. Three oxidation current peaks are observed at low sweep rates for the electrooxidation of (CO)ad obtained from CO-saturated electrolyte. The corresponding peak potentials shift towards more positive values, but the three peaks gradually overlap to give a single broad asymmetric current peak as v is increased. The multiplicity in the oxidation current peaks for (CO)ad under adsorption saturation is interpreted in terms of reactions involving different OH—substrate and CO—substrate interactions, instead of the existence of the “linear” or “bridge” adsorbed structures for (CO)ad. For (CO)ad below saturation values, a complex model involving different (CO)ad structures interacting with energetically different (OH)ad should be considered. From the dependence of peak potential on log v, a chemical reaction between adsorbed CO and platinum oxides can in principle be postulated as the rds. The splitting of the initial stages of oxide electroformation in the presence of adsorbed CO is evident from the triangular modulated linear potential sweep (TMLPS).

Complex kinetic behaviour of “reduced” CO2 electro-oxidation at Pt electrodes

The electro-oxidation of “reduced” CO2 intermediate adsorbed at different adsorption times and potentials was studied by means of potentiodynamic oxidation profiles and current transients at constant potential. At potentials >0.75 V a Langmuir isotherm is obeyed, while a ·er isotherm is found at potentials <0.75 V. The kinetic behaviour of (CO2)r electro-oxidation can be explained through the existence of two intermediate species of similar chemical composition but different energy content. These species are related through an exchange equilibrium, assisted by the catalyst surface.

Zinc dissolution and passivation in buffered phosphate solutions: Part I. A comparative study with sodium hydroxide solutions

Abstract Zn dissolution and passivation in buffered phosphate electrolyte in the pH range between 7 and 13.5 has been studied under a wide variety of experimental conditions. The electrode rest potential shows three regions depending on pH. Between pH 7 and 10 a constant value was found while a slope of 60 mV is obtained from a plot of E r vs. pH. In the pH range 10–11.5 a mixed potential could be established due to the minimum buffer capacity of the system. The necessary charge for passivation was found to be 1.5 mC cm −2 and this was attributed to a monolayer film. From l.s.v. and RDE results a change on Zn dissolution mechanism from a dissolution precipitation to a solid phase process with pH can be postulated. Time and potential effects on product reduction would show that there are chemical transformations on the electrode surface as a second step on the electrode poassivation. From experimental kinetic parameters an empirical rate equation for Zn dissolution is found as i a =2 k a Fc OH − c (PO 4 3− ) t 0.5 exp[(3/2) EF/RT )] Accordingly a reaction mechanism is postulated where PO 4 3− ions act as dissolution promoters, while HPO 4 2− , through the formation of NaZnPO 4 ·H 2 O glass, would act as a dissolution inhibitor.

Para-phenylenediamine oxidation at a platinum electrode in acetonitrile solutions*

Abstract p -Phenylenediamine oxidation at platinum electrodes in acetonitrile solutions has been studied under a very wide range of experimental conditions. Chronopotentiometry, rotating disc electrode and cyclic voltammetry were used as electrochemical techniques. Coulometry at constant potential and product analysis were also performed. The electrochemical reaction appears as a fast and reversible one electron exchange per molecule of PPD. The electrode reaction is further complicated by follow-up chemical reactions giving unknown products in the bulk of the solution. The whole polarization curve under steady state conditions shows two waves, while under non-steady state conditions a small wave at intermediate potentials is also apparent. The reaction pathway for the first wave was interpreted as a non-conventional e.c.e. mechanism where the parent molecule acts as a base in the chemical step. These assumptions were confirmed through experiments performed with pyridine or water addition.

Electrochemical behavior of acidic acetonitrile solutions at platinum and gold electrodes

Hydrogen evolution reactions from acidic MeCN solutions were studied at Pt and Au electrodes. The acids studied were HCl and HPic in a wide range of concentrations. LiClO4 and Et4NClO4 (0.4 M) were used as supporting electrolytes. Quasi steady state polarization curves show that the reaction is irreversible with a Tafel slope of 2 RT/F after diffusion polarization and pseudo-ohmic drop have been corrected. Non-stationary techniques reveal that the electrochemical reaction is preceded by a chemical reaction attributed to slow dissociation of ACN·2 HCl and ACN·3 HPic species. Equilibria and chemical rate constants for both species were evaluated by chronopotentiometry. The values obtained explain satisfactorily the results accomplished under potentiodynamic conditions. Data at the RDE confirm the existence of a previous chemical reaction. MeCN electroadsorption in the double layer potential region at Pt accounts for the solvent levelling effect towards H adsorption between Pt and Au. Furthermore, an inhibitory effect over hydrogen evolution and hydrogen oxidation is reported at more negative potentials at both Pt and Au electrodes.

Electrochemical behavior of iodide-iodine and bromide-bromine redox systems in nitromethane solutions

Summary Iodide-iodine and bromide-bromine electrode processes at platinum electrodes in nitromethane solutions are studied. Results were obtained by means of the rotating disk electrode technique under a wide range of experimental conditions. Diffusional and kinetic parameters were evaluated for the different reactions involved. This study supports the behavior found for the same halogen species in other non-aqueous solvents. The differences encountered are mainly related to specific properties of the solvent on ion solvation and on the stability of the X 3 − complex.

Zinc dissolution and passivation in buffered phosphate solutions: Part II. About the nature of the passivating film

Abstract The nature of the passivating film formed on Zn electrodes in Na 2 HPO 4 solutions at pH 10.0 was investigated. The effect of different aggressive ions (I − , Cl − , CH 3 COO − ) showed that the breakdown potential depends on anion nature, but a positive shift of about 0.2 V is observed when the film has been formed in phosphate solutions. Low energy electron diffraction patterns as well as i.r. spectra were obtained with deposits potentiodynamically and potentiostatically formed. Good agreement between OZn pattern and the experimental lines was obtained by l.e.e.d., while by i.r. spectra an amorphous compound of the P-O-Zn type can be inferred. Cathodic galvanostatic charging curves after applying different anodization programs would indicate species transformation at the interphase. Chemical stability of potentiodynamically formed species was determined by cathodic charging curves after different open circuit times. The most passivating nature of the film formed in presence of phosphate species is demonstrated.

Electrochemical oxidation of mercury under sodium iodide in acetonitrile solutions

Summary Mercury electrode oxidation in the presence of iodide ions in MeCN solutions has been studied by direct and reverse chronopotentiometry. The reaction steps proposed correspond to mercuric iodide complex formation, with a lower coordination number as the potential goes towards more positive values. Both anodic and cathodic processes are found to be reversible and mass-transfer controlled. The distortion of the chronopotentiograms at high iodide concentrations and with changes in the cation of the supporting electrolyte and temperature is interpreted as iodide adsorption at the mercury surface.

Electro-oxidation of o-phenylenediamine at platinum electrodes in acetonitrile solutions

Abstract The study of o-phenylenediamine electro-oxidation at platinum electrodes in acetonitrile solutions under different experimental conditions is presented. Cyclic voltammograms show 4 oxidation peaks, which are assigned to o-phenylenediamine, o,o′-diamineazobenzene and protonated o-phenylenediamine anodic oxidation. An additional prepeak system is apparent at potentials less anodic than the first peak in successive scans. This prepeak system evidences the presence of two redox systems diphenylamine and H+, derived from the initial oxidation product o-phenylenediamine cation radical. The effect of base and acid addition is also studied. o,o′-Diamineazobenzene was identified as one of the principal soluble products in preparative scale electrolysis and a general mechanism for o-phenylenediamine oxidation is proposed.