Francisco Huerta

Voltammetric and in situ FTIRS study of the electrochemical oxidation of aniline from aqueous solutions buffered at pH 5

The electrochemical oxidation of aniline at low concentration was carried out in phosphate buffer solutions of pH 5. Both electrochemical and spectroscopic results indicate that during the initial stages of aniline oxidation, a dimerisation process takes place. The dimer has been identified as p-aminodiphenylamine (ADPA). No evidence supporting the formation of other dimerisation products, such as benzidine or hydrazobenzene has been found. The hydrolysis of ADPA seems to be a very slow process at pH 5 and, consequently, the formation of hydroquinone/p-benzoquinone has not been observed. ADPA is poorly soluble in aqueous medium and a part of these species precipitates on the electrode surface when formed. This precipitated dimer could act as an active centre for the further growth of the polymeric chains.

Voltammetric and spectroscopic characterization of cyanide adlayers on Pt(h,k,l) in an acidic medium

Irreversibly adsorbed cyanide adlayers formed on single-crystal platinum electrodes were studied by cyclic voltammetry and in-situ FTIR spectroscopy in a perchloric medium. A vibrational band around 2100 cm−1 was assigned to the C-N stretching vibration of adsorbed cyanide on the three electrode surfaces. Cyanide adlayers seem to be stable on Pt(111) in a wide range of potentials. Tuning rates of 100 and 30 cm−1 V−1 were measured for the band at 2100 cm−1 below and above 0.5 V, respectively. Adsorbed cyanide has a complex behavior on Pt(100). The polarization of the cyanide-covered Pt(100) electrode in the potential region below 0.4 V (RHE) leads to the formation of a CO adlayer (band at 1820 cm−1). The oxidation of adsorbed cyanide on the Pt(100) electrode produces CO2 (band at 2344 cm−1) and adsorbed NO (band at 1624 cm−1), which shows a characteristic voltammetric behavior. Adsorbed cyanate (band at 2175 cm−1) was also identified as an intermediate product during adsorbed cyanide oxidation on Pt(100). Finally, for Pt(110), adsorbed cyanide is slowly desorbed below 0.1 V (RHE).

Electrochemical behaviour of amino acids on Pt(h,k,l): a voltammetric and in situ FTIR study. Part 1. Glycine on Pt(111)

Abstract A spectroelectrochemical study of the adsorption and oxidation of glycine on a Pt(111) electrode has been carried out in 0.1 M HClO4 solution. Two types of adsorbate were identified. On the one hand, strongly adsorbed cyanide which comes from the oxidative adsorption of the molecule. This adsorbate has been isolated and characterized. On the other hand reversibly adsorbed glycinate anions, which adsorb at potentials above 0.3 V, have been detected. As in the case of acetate anions, glycinate is two-fold coordinated to the Pt(111) surface through the carboxylate group. The analysis of the potential dependent band centre frequencies and integrated band intensities suggests that lateral interactions between adsorbed glycinate anions are at the origin of the small frequency shift observed with increasing potential.

Electrochemical behaviour of benzene on platinum electrodes

Abstract The adsorption and oxidation of benzene in acidic media on platinum electrodes (polycrystalline and single-crystal electrodes) have been studied by cyclic voltammetry and in-situ Fourier transform infrared spectroscopy. The oxidation characteristics of benzene depend on the surface structure of the platinum electrode used. In all platinum electrodes studied, the main reduction product of benzene is cyclohexane, and the oxidation products detected by infrared spectroscopy have been CO 2 and benzoquinone. Pt(100) is the surface more active for the oxidation of benzene to CO 2 and Pt(110) is the surface more active for the reduction of benzene to cyclohexane.

Spectroelectrochemical study on CN− adsorbed at Pt(111) in sulphuric and perchloric media

Abstract Cyclic voltammetry and in situ FTIR spectroscopy were combined to study irreversibly adsorbed cyanide adlayers in both perchloric and sulphuric media. Cyanide adlayers were formed on Pt(111) single-crystal electrodes and are stable in a wide range of potentials. A vibrational band at around 2100 cm −1 was assigned to the C–N stretching vibration of adsorbed cyanide, for which tuning rates close to 100 cm −1 V −1 below 0.5 V and 30 cm −1 V −1 above this potential were measured. These values are independent on the supporting electrolyte employed. A reorientation of the adsorbed cyanide was proposed to explain the changes observed in both tuning rate and integrated intensity at around 0.5 V.

Voltammetric and in situ FTIRS study on CN- and Au(CN)-x complexes at the polycrystalline gold surface in citrate medium

Abstract The adsorption and reactivity of citrate anions, cyanide anions, and the Au(I) and Au(III) cyanocomplexes (KAu(CN) 2 and KAu(CN) 4 , respectively) on gold polycrystalline electrodes have been investigated by means of cyclic voltammetry and in situ FTIR spectroscopy in pH 7 sodium citrate medium. It was found that citrate anion adsorbs reversibly through the carboxylate groups from 0.4 V (RHE) on bare gold surfaces. However, the onset of this adsorption process is shifted by 600 mV in the positive direction when adsorbed cyanide species are present at the electrode surface. The Au(I) cyanocomplex produces a film on the gold electrode that inhibits the oxidation of both citrate anions and the gold surface. The reduction of the Au(III) cyanocomplex gives rise to the formation of the Au(I) cyanocomplex at potentials less negative than those required for gold deposition. Upon reduction, both cyanocomplexes release adsorbed cyanide on the gold electrode, which can be subsequently oxidized to cyanate at higher potentials.

Synthesis and Characterization of Electroactive Films Deposited from Aniline Dimers

This work has been cofinanced by the European Commission (ERDF funds) and the Spanish Ministerio de Educacion y Cultura (MAT2004-01479). University of Alicante assisted in meeting the publication costs of this article.

Electrochemical behaviour of amino acids on Pt(h, k, l). A voltammetric and in situ FTIR study. Part II. Serine and alanine on Pt(111)

Abstract A spectroelectrochemical study of the adsorption and oxidation of serine and alanine on Pt(111) electrodes has been carried out in 0.1 M HClO4 solutions. From the voltammetric and FTIR spectroscopic results, we concluded that adsorbed cyanide is formed at the Pt(111) surface as a product of serine and alanine oxidation. In the case of serine, adsorbed cyanide is detected at potentials above 0.4 V, whereas a higher potential is needed in the case of alanine. Linear and multibonded CO have also been detected as poisoning species during serine oxidation. These adsorbed species have been isolated by irreversible adsorption experiments of amino acids. Dissolved CO2 has been found by FTIR spectroscopy as an oxidation product of both amino acids. The role played by the R group in the electrochemical behaviour of these molecules is significant. Reversibly adsorbed serinate and alaninate anions have been also detected. As in the case of the anions present in glycine solutions, the serinate and alaninate anions are two-fold coordinated to the Pt(111) surface through the car☐ylate group.

Potential modulated reflectance spectroscopy of Pt(111) in acidic and alkaline media: cyanide adsorption

The electroreflectance (ER) technique has been applied to the study of a Pt(111) electrode in different acidic and alkaline electrolytes. The variation in the normalised potential derivative of the reflectance signal, 1/R(dR/dE) is observed in sulphuric and perchloric acid media as the potential is scanned 1.0 V positive from the hydrogen evolution region. In both these media, the usual hydrogen adsorption-desorption region corresponds to a zone in which the ER signal is practically zero and does not change with the potential. However, in the anomalous region, an increase in the ER signal was observed indicating that here, the adsorbed species increased the electron density of the platinum surface. The ER spectrum recorded for the cyanide-covered Pt(111) surface showed a large decrease in the electron density of the platinum surface and this is attributed to electron back-donation from the platinum to the adsorbed cyanide. In alkaline solutions, the integrated ER spectrum showed relatively small changes which corresponded to the features in the cyclic voltammogram and were attributed to the reorientation of the hydrogen-bonded surface species and water dipoles.

Conducting films obtained by electro-oxidation of p-aminodiphenylamine (ADPA) in the presence of aniline in buffer aqueous solution at pH 5

Abstract The electrochemical oxidation of aniline and p-aminodiphenylamine (ADPA) was carried out in a phosphate buffer solution at pH 5. Cyclic voltammetry and “in situ” FTIR spectroscopy were combined to study the redox behavior of the oxidation products. When ADPA is oxidized in the presence of aniline, two redox processes are observed. One of these redox processes is related to a soluble species, while the other pertains to a species attached to the electrode surface. Only those films synthesized from ADPA oxidation in the presence of aniline are electroactive in this buffered medium. No evidence pointing to the occurrence of autocatalytic polymerization exists under the experimental conditions employed.