Germano Tremiliosi-Filho

Electro-oxidation of ethanol on gold : analysis of the reaction products and mechanism

Although the electrocatalytic properties of gold are not as good as those of platinum, the oxidation of small alcohol molecules, such as ethanol, is possible at gold both in acid and alkaline media. By using programmed potential electrolysis coupled with chromatographic analysis of the products formed, the nature and the variation of their concentrations as a function of pH and time have been determined. In alkaline medium, the only product detected was acetic acid. On the other hand, in perchloric acid medium, the reaction products are highly dependent on the value of the oxidation potential, with the following successive steps: ethanol→acetaldehyde→acetic acid. A general tentative mechanism is proposed and discussed.

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.

Limit to extent of formation of the quasi-two-dimensional oxide state on Au electrodes

Potentiostatic polarization of Au electrodes in 0.5 M aqueous H2SO4 and KOH solutions at polarization potentials Ep between 2.00 and 2.43 V (RHE) for polarization times tp up to 104 s leads to the formation of thick oxide films which comprise up to four oxide states, designated OC1, OC2, OC3 and OC4, as resolved from linear sweep voltammetry oxide reduction profiles. The oxide growth proceeds by inverse logarithmic kinetics with two distinguishable kinetic regions leading to linear relations between 1/qox and log tp. The first 1/qox vs. log tp linear region corresponds to development of the quasi-2D state (OC1) and the very initial growth of the OC2 state, and the oxide growth in this tp domain is slow. Further growth of OC2 and formation of the OC3 and OC4 states is significantly faster. The oxide growth kinetics change when 1/qox reaches a value of ca. 0.10 mC−1 cm2. In situ ellipsometry measurements indicate the presence of two distinct layers within the oxide films, the inner layer designated α and the outer one designated β. The α film corresponds to the OC1 state whereas the β film corresponds to the OC2, OC3 and OC4 states. The OC1 state reaches a limiting thickness of 3 equiv. monolayers of AuO or Au(OH)2 in aqueous H2SO4 and 1 monolayer of AuO or Au(OH)2 in aqueous KOH. The β state which resides on top of the α one grows without reaching any limiting thickness and up to 100 equiv. monolayers of Au2O3 or Au(OH)3, depending on the electrolyte composition, can be formed under the conditions described in the present paper.

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.

Straightforward Synthesis of Carbon-Supported Ag Nanoparticles and Their Application for the Oxygen Reduction Reaction

In this paper we report a simple and environmentally friendly synthesis of silver nanoparticles (AgNps) and their activities towards the oxygen reduction reaction (ORR). Ultraviolet spectroscopy (UV–vis) and transmission electron microscopy confirmed the formation of poly(vinyl pyrrolidone)-protected colloidal AgNps through direct reduction of Ag+ by glycerol in alkaline medium at room temperature. For the ORR tests, the AgNps were directly produced onto carbon to yield the Ag/C catalyst. Levich plots revealed the process to occur via 2.7 electrons, suggesting that the carbon support contributes to the ORR. We discuss here possibilities of improving the catalytic properties of the Ag/C for ORR by optimizing the parameters of the synthesis.

A Study of the Hydrogen Evolution Reaction on a Ni/NiFeS Electrodeposited Coating

Composite cathode materials based on nickel, such as NiFe, NiMo, NiMoCd, NiS, and NiMoS 2 , are of considerable practical interest as possible electrocatalysts for hydrogen production in alkaline water electrolyzers. This work describes the development of a new composite material of Ni and FeS (referred to as Ni/NiFeS) that shows a high electrocatalytic activity toward the hydrogen evolution reaction. Ni/NiFeS coatings were obtained by codeposition of Ni and FeS particles on Ni previously electrodeposited on a mild steel substrate. The evaluation of this material as H 2 -evolving cathodes was done in alkaline solution through steady-state techniques. The polarization behavior of the material shows two distinct regions exhibiting different Tafel slopes over the temperature range 273 to 353 K. A detailed mechanistic study of the hydrogen evolution reaction on Ni/NiFeS was carried out by ac impedance techniques. The rate constants for the elementary steps were obtained and suggest a mechanism that involves an electron transfer followed by an electrochemical desorption step. The consistency of the analysis was confirmed by establishing that the apparent activation energy calculated from the dependence of the rate constant of the rate-determining step with T -1 has the same value as that found for the overall reaction.

Hydrogen evolution reaction on NiS electrodes in alkaline solutions

Abstract The hydrogen evolution reaction mechanism on electrodeposited NiS coatings of low sulphur content was initially studied in alkaline solutions through steady-state polarization measurements. Tafel plots were analysed at several temperatures and varying pH. The results suggested a mechanism involving fast electron transfer followed by a slow electrochemical desorption step. This Volmer-Heyrovsky mechanism was fully confirmed by impedance spectroscopy measurements through the fit of the results with those derived theoretically for the different mechanisms. The values of the corresponding rate constants were obtained for temperatures ranging from 0 to 60°C.

The hydrogen evolution reaction on amorphous nickel and cobalt alloys

Abstract The mechanism of the hydrogen evolution reaction (HER) on Ni70P30, Co72P28, Ni40Co23P37 and Ni72Fe1P27 was studied by impedance spectroscopy and steady-state polarization measurements. Only the Ni-Fe-P presents a good electrocatalytic activity with an overpotential ≈200 mV lower than that for mild steel or Ni (the traditional materials used as cathodes in water electrolysers). Impedance studies of the HER were carried out on these amorphous alloys and it was concluded that Ni-Fe-P is the only material to present a Volmer-Heyrovsky mechanism with the Heyrovsky step being the r.d.s. In contrast, on Co-P, Ni-P and Ni-Co-P the mechanism is Volmer-Tafel with the Volmer step being the r.d.s. From the rate constants the following catalytic sequence was determined: Ni-Fe-P > Co-P > Ni-Co-P > Ni-P.

Ethanol electrooxidation on Pt-Sn and Pt-Sn-W bulk alloys

Ethanol oxidation has been studied on Pt-Sn and Pt-Sn-W electrodes prepared in an arc-melting furnace. Different electrochemical techniques like cyclic voltammetry and chronoamperometry were used to evaluate the catalytic activity of these materials. The electro-oxidation process was also investigated by in situ infrared reflectance spectroscopy in order to determine adsorbed intermediates and reaction products. Experimental results indicated that Pt-Sn and Pt-Sn-W alloys are able to oxidize ethanol mainly to acetaldehyde and acetic acid. Adsorbed CO was also detected, demonstrating the viability of splitting the C-C bond in the ethanol molecule during the oxidation process. The adsorbed CO was further oxidized to CO2.This reaction product was clearly detected by SNIFTIRS. Pt-Sn-W catalyst showed a better electrochemical performance than Pt-Sn that, in it turn, is better than Pt-alone.

The oxidation of formaldehyde on high overvoltage DSA type electrodes

Neste trabalho a oxidacao eletroquimica do formaldeido e estudada sobre eletrodos dimensionalmente estaveis preparados por decomposicao termica de percursores (correspondentes cloretos). Foram utilizados como eletrodos de trabalho: Ti/Ir0.3Ti0.7O2, Ti/Ru0.3Ti0.7O2 e Ti/Ir0.2Ru0.2Ti0.6O2. As eletrolises foram realizadas galvanostaticamente em uma celula do tipo filtro prensa em solucoes 0,5 mol L-1 H2SO4 com concentracao inicial de formaldeido de 100 mmol L-1. A concentracao de formaldeido decresce rapidamente com o tempo de eletrolise sendo que o eletrodo ternario (Ir + Ru + Ti) e o que apresenta maior atividade catalitica. O ânodo contendo somente Ir, apesar de maior carga superficial e o de menor atividade eletrocatalitica. Para a oxidacao de acido formico, formado pela oxidacao de formaldeido, a presenca de Ir na composicao do ânodo nao favorece o processo, sendo o ânodo contendo somente Ru o mais efetivo para este processo.