Elena Pastor

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.

Synthesis of atomic gold clusters with strong electrocatalytic activities.

Small atomic gold clusters in solution, Au n , stabilized by tetrabutyl ammonium bromide (TBABr), have been synthesized by a simple electrochemical technique, based on the anodic dissolution of a gold electrode in the presence of TBABr salt, and using acetronitrile as solvent. The presence of clusters in the range Au3-Au11 were detected by MALDI-TOF spectroscopy, and further characterized by UV-vis absorption spectroscopy, TEM, AFM, X-ray diffraction, and cyclic voltammetry. Clusters display a semiconductor behavior with a band edge of approximately 2.5 eV. We report here their extraordinarily high electrocatalytic activity toward the O2 reduction reaction in acid solutions, which can explain Zhangs results, showing that a four-electron mechanism seems to occur because of the facile reduction of H2O2 on gold clusters compared to bulk gold or larger gold nanoparticles.

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.

Electroreactivity of isopropanol on platinum in acids studied by DEMS and FTIRS

Abstract The reactivity of isopropanol on polycrystalline Pt in acid solutions was investigated using in-situ Fourier transform IR spectroscopy (FTIRS) and on-line differential electrochemical mass spectrometry (DEMS). The electro-oxidation products are acetone and CO2; the former is observed when isopropanol is present in the bulk of the solution, and the latter is produced from strongly adsorbed species. Both bulk and adsorbed isopropanol electro-reduction yield propane. HD exchange is observed during propane formation when the reaction takes place in a D2O + DClO4 solution. This observation suggests the formation of adsorbates bonded to the surface through the Cα atom of the isopropanol molecule. FTIR spectra of adsorbed species show the presence of CH3 and COH groups. The most probable adsorbate structures are (CH3CCH3)Pt, (CH3CHCH3)Pt and (CH3COHCH3)Pt, presumably accompanied by (CH3COCH3)Pt.

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.

Tuning CO2 electroreduction efficiency at Pd shells on Au nanocores

The faradaic efficiency of CO2 electroreduction is significantly affected by the thickness of Pd nanoshells on Au cores. The ratio of hydrogen evolution to CO2 reduction was determined by differential electrochemical mass spectrometry. Decreasing the Pd shell thickness from 10 to 1 nm leads to a twofold increase in faradaic efficiency.

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.

Electrochemical Behavior of TiOxCy as Catalyst Support for Direct Ethanol Fuel Cells at Intermediate Temperature: From Planar Systems to Powders

To achieve complete oxidation of ethanol (EOR) to CO2, higher operating temperatures (often called intermediate-T, 150-200 °C) and appropriate catalysts are required. We examine here titanium oxycarbide (hereafter TiOxCy) as a possible alternative to standard carbon-based supports to enhance the stability of the catalyst/support assembly at intermediate-T. To test this material as electrocatalyst support, a systematic study of its behavior under electrochemical conditions was carried out. To have a clear description of the chemical changes of TiOxCy induced by electrochemical polarization of the material, a special setup that allows the combination of X-ray photoelectron spectroscopy and electrochemical measurements was used. Subsequently, an electrochemical study was carried out on TiOxCy powders, both at room temperature and at 150 °C. The present study has revealed that TiOxCy is a sufficiently conductive material whose surface is passivated by a TiO2 film under working conditions, which prevents the full oxidation of the TiOxCy and can thus be considered a stable electrode material for EOR working conditions. This result has also been confirmed through density functional theory (DFT) calculations on a simplified model system. Furthermore, it has been experimentally observed that ethanol molecules adsorb on the TiOxCy surface, inhibiting its oxidation. This result has been confirmed by using in situ Fourier transform infrared spectroscopy (FTIRS). The adsorption of ethanol is expected to favor the EOR in the presence of suitable catalyst nanoparticles supported on TiOxCy.

The electroformation and the anodic stripping characteristics of adsorbed residues formed on platinum electrodes from acid solutions containing different alcohols

Abstract The electroadsorption at constant potential of propargyl alcohol, allyl alcohol, methanol and propanol, and the anodic stripping voltammetry of the corresponding adsorbed residues were investigated on platinum (polycrystal) electrodes in 0.5 M H 2 SO 4 by using the flow cell technique. The results allowed us to established the stoichiometries and a qualitative relation of the adsorption processes and the structures of the different alcohols, and to conjecture the possible formation of coadsorbates when the adsorption processes yield simultaneously various adsorbed residues.