J.L. Rodríguez

Electrochemical surface reactions of intermediates formed in the oxidative ethanol adsorption on porous Pt and PtRu

The oxidative ethanol adsorption and electrochemical surface reactions of adsorbed intermediates were studied on electrodeposited Pt, Ru, Pt0.92Ru0.08 and Pt0.85Ru0.15 in 1 M HClO4. A flow cell procedure was performed in order to separate surface reactions of intermediates formed in the oxidative ethanol adsorption from those reactions for ethanol in the bulk electrolyte solution. Oxidation and reduction reactions of adsorbed species were studied by potential-controlled electrodesorption spectrometry. No adsorbate formation was observed on a pure Ru electrode. In all cases, CO2 was the sole product formed in the oxidative electrodesorption of the adsorbates. Using 12CH312CH2OH and 12CH313CH2OH, the oxidation reaction pathways of each C-atom to CO2 were followed. On the other hand, methane and ethane were detected during the reductive electrodesorption of adlayer species. The onset potential for these reduction products shifts to more negative potentials as the Ru content of the alloy increases. The methane to ethane yield ratio decreases in the sequence Pt>Pt0.92Ru0.08>Pt0.85Ru0.15. A critical interpretation of the experimental data leads to a tentative yield of different adsorbed species as function of the Ru surface concentration.

Spectroscopic evidence for intermediate species formed during aniline polymerization and polyaniline degradation

Spectroscopic methods are used to investigate the formation of low molecular mass intermediates during aniline (ANI) oxidation and polyaniline (PANI) degradation. Studying ANI anodic oxidation by in situ Fourier transform infrared spectroscopy (FTIRS) it is possible to obtain, for the first time, spectroscopic evidence for ANI dimers produced by head-to-tail (4-aminodiphenylamine, 4ADA) and tail-to-tail (benzidine, BZ) coupling of ANI cation radicals. The 4ADA dimer is adsorbed on the electrode surface during polymerization, as proved by cyclic voltammetry of thin PANI films and its infrared spectrum. This method also allows, with the help of computational simulations, to assign characteristic vibration frequencies for the different oxidation states of PANI. The presence of 4ADA retained inside thin polymer layers is established too. On the other hand, FTIRS demonstrates that the electrochemically promoted degradation of PANI renders p-benzoquinone as its main product. This compound, retained inside the film, is apparent in the cyclic voltammogram in the same potential region previously observed for 4ADA dimer. Therefore, applying in situ FTIRS is possible to distinguish between different chemical species (4ADA or p-benzoquinone) which give rise to voltammetric peaks in the same potential region. Indophenol and CO(2) are also detected by FTIRS during ANI oxidation and polymer degradation. The formation of CO(2) during degradation is confirmed by differential electrochemical mass spectroscopy. To the best of our knowledge, this is the first evidence of the oxidation of a conducting polymer to CO(2) by electrochemical means. The relevance of the production of different intermediate species towards PANI fabrication and applications is discussed.

Reactivity of acetaldehyde at platinum and rhodium in acidic media. A DEMS study

The electrochemical behaviour of acetaldehyde at platinum and rhodium electrodes in acid solutions was studied applying differential electrochemical mass spectrometry (DEMS). In an acetaldehyde-containing solution, CO2 was the sole electrooxidation product detected at both electrodes, whereas only methane was recorded in the hydrogen region. The production of acetic acid was indirectly established at platinum from DEMS. Residues were studied through the design of flow cell experiments. As for the acetaldehyde-containing solution, the adsorbates produce CO2 and methane during the oxidation and reduction processes, respectively. However, ethane was also observed at platinum during the cathodic stripping of the adspecies. The yield of these substances depends on the adsorption potential (Ead). These results suggest that the fragmentation of acetaldehyde occurs during adsorption and reduction reactions, and the extent of the CC bond scission is influenced by the Ead and the nature of the electrode, being favoured at rhodium.

A comparative study on the adsorption of benzyl alcohol, toluene and benzene on platinum

Abstract The adsorption of benzyl alcohol, toluene and benzene on platinum was studied using cyclic voltammetry combined with on-line mass spectrometry (DEMS). Flow cell procedures allow the detection of volatile products formed during adsorption of these molecules and their displacement with CO, as well as during the oxidative and reductive stripping of the adsorbed layer. Three adsorption potentials were chosen: 0.20, 0.35 and 0.50 V versus reversible hydrogen electrode (rhe). Toluene and benzene adsorb without dissociation. Total hydrogenation of the ring with formation of methyl-cyclohexane and cyclohexane, respectively, was observed suggesting that the aromatic ring of the adsorbed species lies on the surface. For benzyl alcohol, the presence of the OH group favours the dissociative adsorption: the rupture of the CC bond between the ring and the CH 2 OH group produces CO and benzene. Hydrogenolysis of benzyl alcohol also occurs in the Pt(H) region with formation of toluene. Both adsorbed toluene and benzene from benzyl alcohol react with H 2 producing methyl-cyclohexane and cyclohexane, respectively. Thus, benzene and toluene formed from benzyl alcohol also seem to adsorb with the aromatic ring parallel-oriented on the surface.

Electrochemical reactions of benzoic acid on platinum and palladium studied by DEMS. Comparison with benzyl alcohol

Abstract The electrochemical reactivity of benzoic acid on porous platinum and palladium electrodes has been studied by cyclic voltammetry and differential electrochemical mass spectrometry (DEMS) in aqueous 0.1 M HClO 4 . The objective of this work is to investigate the adsorption processes and the reactivity of this compound on different noble metals, in order to compare these results with those obtained for related aromatic compounds. On-line mass spectroscopy analysis of volatile products reveals that the adsorption of benzoic acid is irreversible at platinum while it is mainly reversible on palladium. Accordingly, different catalytic activity of platinum and palladium is demonstrated during the electrooxidation process. The anodic stripping of the adsorbates formed from benzoic acid yields CO 2 as the sole oxidation product of platinum. Cathodic stripping of the residues results in the partial desorption of the organic adlayer from the electrode. At palladium, only small amounts of irreversibly adsorbed species, which also oxidise to CO 2 are detected. The effect of the nature of the electrode on the electrochemical behaviour of these substances is also analysed.

DEMS study on the adsorption and reactivity of benzyl alcohol on palladium and platinum

Abstract The catalytic oxidation of benzyl alcohol is studied by means of on-line differential electrochemical mass spectrometry (DEMS) on both palladium and platinum electrodes. The objective of this work is to investigate the reactivity of the adsorbed species remaining on the electrode surface after a flow-cell experiment and the influence of electrode composition. It is found that the adsorbates formed during the oxidative adsorption of benzyl alcohol are desorbed in the positive-going potential scan as CO 2 for platinum and palladium, although benzene is also detected on the latter. Adsorbate yields depend on both potential and electrode composition.

Investigation of the electrochemical reactivity of benzyl alcohol at platinum and palladium electrodes

Abstract The electrochemical reactions of 2 mM benzyl alcohol (BA) on platinum and palladium electrodes in 0.1 M HClO 4 were studied by means of differential electrochemical mass spectrometry (DEMS) and cyclic voltammetry. The objective of this work is to investigate the influence of electrode nature on the reactivity of BA. On-line mass spectroscopy analysis of volatile products reveals that the interaction of BA with these metals partially occurs dissociatively producing benzene. CO 2 is the sole oxidation product detected. The corresponding mass signal exhibits three different contributions. The extent of each contribution is found to depend strongly on the electrode nature, though in the case of palladium, it also varies with changes in the electrode potential range covered during the voltammetric scan. The different catalytic activity of platinum and palladium is demonstrated during the electroreduction process. Both hydrogenolysis of the molecule and hydrogenation of the aromatic ring occur on platinum, whereas the latest process is of little significance on palladium.

Consecutive adsorption as studied by electrochemical mass spectrometry : Coadsorption, desorption and displacement reactions on platinum

Abstract The consecutive adsorption of formic acid, benzene or benzyl alcohol with CO on Pt in acid media was studied by electrochemical mass spectrometry (DEMS). In all experiments, CO 2 is the sole oxidation product detected from the chemisorbates. Using isotopic labelled H 13 COOH the contribution of species from formic acid and CO during the electrooxidation of the coadsorbate can be distinguished: the oxidation of the chemisorbates from formic acid is followed by the mass signal m / z =45 ( 13 CO 2 ) whereas for CO the corresponding mass signal is m / z =44 ( 12 CO 2 ). It is observed that 35.5% of the residues formed from formic acid are replaced by CO at E ad =0.20 V. The total amount of CO 2 is increased in relation to the individual adsorbates, suggesting that the coadsorbed layer becomes more compact. On the other hand, the adsorption of intact benzene molecules occurs at E ad =0.20 and 0.50 V, and these molecules can be displaced by CO: at E ad =0.20 V benzene is almost completely desorbed whereas at E ad =0.50 V only a small part is displaced and a coadsorbate is formed. For benzyl alcohol, the interaction with CO produces the desorption of benzene and toluene molecules, the extent of this reaction depending on E ad .

The Influence of H 2 O 2 on the Adsorption and Oxidation of CO on Pt Electrodes in Sulfuric Acid Solution

Electrochemical and nonelectrochemical reactions of the H 2 O 2 /CO system on a Pt electrode were studied in 2 M H 2 SO 4 with cyclic voltammetry and on-line mass spectrometry. It was found that a complete CO monolayer is chemically stable in the presence of dissolved H 2 O 2 . However, H 2 O 2 oxidizes adsorbed CO to CO 2 at lower potentials than those required for electro-chemical CO adsorbate oxidation when the Pt electrode is partially covered by CO. Furthermore, it was observed that a complete CO monolayer on Pt inhibits the electrochemical reduction of H 2 O 2 to H 2 O in CO-containing electrolyte.

Elucidation of the reaction pathways of allyl alcohol at polycrystalline palladium electrodes

Abstract The electrochemical behaviour of allyl alcohol at palladium electrodes was studied by cyclic voltammetry, chronoamperometry and on-line mass spectrometry (DEMS). The latter allows the detection of the volatile products generated during the electroreduction and electrooxidation processes. C 3 -hydrocarbons (propene and propane) and acrolein were detected as the bulk products, whereas C 2 -hydrocarbons and CO 2 are related to the adsorbed species. The dissociation of the alcohol produces ethine in the 0.20–0.35 V potential range, which reduces to ethane. Adsorbed acrolein and C 2 -hydrocarbonated residues seem to be formed in addition to CO-like species. The results were compared with those previously obtained at platinum and gold, as well as with other unsaturated alcohols, namely benzyl and propargyl alcohol.