Flavio Colmati

Surface structure effects on the electrochemical oxidation of ethanol on platinum single crystal electrodes

Ethanol oxidation has been studied on Pt(111), Pt(100) and Pt(110) electrodes in order to investigate the effect of the surface structure and adsorbing anions using electrochemical and FTIR techniques. The results indicate that the surface structure and anion adsorption affect significantly the reactivity of the electrode. Thus, the main product of the oxidation of ethanol on the Pt(111) electrode is acetic acid, and acetaldehyde is formed as secondary product. Moreover, the amount of CO formed is very small, and probably associated with the defects present on the electrode surface. For that reason, the amount of CO2 is also small. This electrode has the highest catalytic activity for the formation of acetic acid in perchloric acid. However, the formation of acetic acid is inhibited by the presence of specifically adsorbed anions, such as (bi)sulfate or acetate, which is the result of the formation of acetic acid. On the other hand, CO is readily formed at low potentials on the Pt(100) electrode, blocking completely the surface. Between 0.65 and 0.80 V, the CO layer is oxidized and the production of acetaldehyde and acetic acid is detected. The Pt(110) electrode displays the highest catalytic activity for the splitting of the C-C bond. Reactions giving rise to CO formation, from either ethanol or acetaldehyde, occur at high rate at any potential. On the other hand, the oxidation of acetaldehyde to acetic acid has probably the lower reaction rate of the three basal planes.

Ethanol Oxidation on Carbon Supported Pt-Sn Electrocatalysts Prepared by Reduction with Formic Acid

Carbon supported Pt-Sn alloy catalysts were prepared by reduction of Pt and Sn precursors with formic acid and characterized by X-ray diffraction, high-resolution transmission electron microscopy, and X-ray absorption spectroscopy techniques. Their electrocatalytic activity for ethanol oxidation was compared with commercial Pt/C and Pt3Sn/C electrocatalysts and the differences discussed in terms of intrinsic properties of the materials. The activity for ethanol oxidation in linear sweep voltammetry experiments at room temperature is greatly enhanced, mainly at low potentials, on Pt-Sn 3:1 and Pt-Sn 2:1 catalysts prepared by the formic acid method, while only a slight improvement is observed on Pt-Sn 9:1 prepared with formic acid and on Pt3Sn from E-TEK. The commercial Pt3Sn catalyst showed a higher current performance as anode material in a direct ethanol fuel cell operating at 90 and 110°C. At the low temperatures of the electrochemical experiments, the rate determining step rds of ethanol oxidation is the removal of CO-species, which is enhanced by the presence of Sn oxides. At the higher temperatures of the fuel cell, instead, the rds is the dissociative adsorption of ethanol and/or the oxidation of acetaldehyde to acetic acid: both these reactions are favored by an increase of the Sn content in the alloyed state.

The role of the steps in the cleavage of the C–C bond during ethanol oxidation on platinum electrodes

Ethanol oxidation has been studied on stepped platinum single crystal electrodes in acid media using electrochemical and Fourier transform infrared (FTIR) techniques. The electrodes used belong to two different series of stepped surfaces: those having (111) terraces with (100) monoatomic steps and those with (111) terraces with (110) monoatomic steps. The behaviors of the two series of stepped surfaces for the oxidation of ethanol are very different. On the one hand, the presence of (100) steps on the (111) terraces provides no significant enhancement of the activity of the surfaces. On the other hand, (110) steps have a double effect on the ethanol oxidation reaction. At potentials below 0.7 V, the step catalyzes the C-C bond cleavage and also the oxidation of the adsorbed CO species formed. At higher potentials, the step is not only able to break the C-C bond, but also to catalyze the oxidation of ethanol to acetic acid and acetaldehyde. The highest catalytic activity from voltammetry for ethanol oxidation was obtained with the Pt(554) electrode.

Carbon monoxide oxidation on Pt-Ru electrocatalysts supported on high surface area carbon

This work describes the preparation and characterization of Pt-Ru alloys dispersed on high surface area carbon, which were evaluated for CO oxidation on thin porous coating rotating disk electrodes and for hydrogen oxidation on polymer electrolyte fuel cells fed with hydrogen containing 100 ppm CO. A thermal treatment (H2, 300 oC) applied to the catalysts improves the tolerance to small quantities of CO and, in some cases, reduces the potential necessary to promote the CO oxidation during a linear potential scan. Under operational conditions in a fuel cell in the presence of CO it was observed that the best results were obtained when the Pt-Ru/C alloy was prepared by simultaneous reduction of the ions Pt (IV) and Ru (III), as opposed to a sequential reduction.

Ethanol Oxidation on Pt-Sn Electrocatalysts Supported on Carbon Prepared by Reduction with Formic Acid

Carbon supported Pt-Sn alloy catalysts were prepared by reduction of Pt and Sn precursors with formic acid and characterised by X-ray diffraction, high resolution transmission electron microscopy and X-ray absorption near- edge structure analyses. Their electrocatalytic activity for ethanol oxidation was compared with commercial Pt/C and Pt3Sn/C electrocatalysts. The synthetized PtSn catalysts presented a low degree of alloy formation and high contents of Sn oxides. The electrocatalytic activity for ethanol oxidation in linear sweep voltammetry experiments at room temperature is greatly enhanced, mainly at low potentials, on Pt3Sn and Pt2Sn catalysts prepared by reduction with formic acid, while only a slight improvement is observed on Pt9Sn prepared with formic acid. On the other hand, a commercial Pt3Sn catalyst with a high degree of alloying showed the highest current performance as anode material in a direct ethanol fuel cell (DEFC) operating at 90 and 110 oC.