M.C. Arévalo

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.

Electrochemical and AFM characterization on gold and carbon electrodes of a high redox potential laccase from Fusarium proliferatum

The redox potential of the T1 copper site of laccase from Fusarium proliferatum was determined by titration to be about 510 mV vs. SCE (750 mV vs. NHE), which makes it a high redox potential enzyme. Anaerobic electron transfer reactions between laccase and carbon and gold electrodes were detected, both in solution and when the enzyme was adsorbed on these surfaces. In solution, a single high-potential signal (660 mV vs. SCE) was recorded at the carbon surfaces, attributable to the T1 copper site of the enzyme. However, a well-defined oxidative process at about 660 mV and an anodic wave at 350 mV vs. SCE were recorded at the gold electrode, respectively associated with the T1 and T2 copper sites. Laccase-modified carbon electrodes behaved analogously when the enzyme was in solution, unlike laccase adsorbed on gold, which showed only a low-potential signal. Laccase molecules were successfully imaged by AFM; obtaining a thick compact stable film on Au(111), and large aggregates forming a complex network of small branches leaving voids on the HOPG surface. Laccase-modified carbon electrodes retained significant enzymatic activity, efficiently oxidising violuric acid and reducing molecular oxygen. Explanations are proposed for how protein-film organisation affects the electrode function.

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.

Adsorption, oxidation and reduction reactions of propargyl alcohol on palladium as studied by electrochemical mass spectrometry

Abstract The electrochemical behavior of propargyl alcohol (PA) on palladium electrodes in 0.1 M HClO 4 was studied by differential electrochemical mass spectrometry (DEMS). Experiments with the alcohol present in the bulk of the solution have shown that the sole oxidation product is CO 2 . During potential cycling in the hydrogen adsorption/absorption region, C 3 - (propylene and propane) and C 2 - (ethane and traces of ethene/ethyne) hydrocarbons, as well as allyl alcohol, were produced. On the other hand, PA forms strongly bonded species on palladium which can be studied using a flow cell procedure. The maximum adsorption of PA was observed in the potential range 0.25–0.65 V. As for bulk studies, only CO 2 was detected during the oxidation of the residues. However, some differences should be mentioned in respect of the reduction reactions: only propane, propylene and allyl alcohol were observed from the adlayer. According to these results, and taking into account the values of the charges involved in the adsorption ( Q t ) and oxidation ( Q ox ) processes, different structures were proposed for the adsorbed species. From these structures, the formation of the reduction products was justified. Results were compared with previously reported data for platinum and gold in acid media.

A DEMS study of the electroreduction and oxidation of 3-buten-2-one and 2-butanone adsorbates on platinum in sulphuric solutions

Abstract The electrochemical adsorption characteristics of 3-buten-2-one and 2-butanone on porous platinum electrodes were investigated by differential electrochemical mass spectrometry (DEMS) in aqueous 0.5 M H2SO4. The study was conducted through the oxidation and reduction of residues formed at different adsorption potentials (Ead). The maximum adsorption for both organic substances was observed at the potential of zero charge of the interface, that is, ca 0.20 V. At this potential an adsorbate species with the C–O bond lying parallel to the surface is proposed. The sole oxidation product for both molecules was carbon dioxide. Anodic current transients were detected for 3-buten-2-one at adsorption potentials higher than 0.20 V. Between 0.40 and 0.60 V, it is possible to admit a 1–3e− loss during adsorption with a progressive fragmentation of the original molecule at increasing Ead. After three cycles in the H-adatom region, about 75% of the 3-buten-2-one adsorbate is desorbed; the main products being butane, 1-butene and propane. The ratio of these products depends on Ead, as is expected for the fragmentation of the molecule upon increasing potentials. On the other hand, 2-butanone was adsorbed in the 0.10–0.60 V potential range without current transients associated with the dissociative reactions of the molecule. The surface coverage diminished drastically at both sides of the maximum adsorption (0.20 V). Butane was the main reduction product of 2-butanone.