M.I.S. Lopes

Integrated electromodulated IR reflectance spectroscopy bands: Part 2: Methanol adsorbates at polycrystalline platinum and Pt(111) single-crystal electrodes in acid medium

Using electromodulated IR reflectance spectroscopy (EMIRS), absorption spectra of adsorbed species resulting from the chemisorption of HCHO, HCOOH and methyl formate, HCOOCH3, on both polycrystalline and (111) platinum surfaces, were first obtained in the range 1400–1900 cm−1, to cover completely the carbonyl stretching mode region. The same experimental conditions were used for all systems. EMIRS bands were integrated and analysed, using the procedure described in Part 1 (B. Beden, J. Electroanal. Chem., 345 (1993) 1). Characteristic frequencies were located precisely, allowing the spectral discrimination of several types of adsorbates. The same procedure was then used to follow the chemisorption of methanol on platinum in perchloric medium with time. As expected, formate species were found to dominate on the two types of platinum surface. However, during the initial stages of chemisorption, and particularly on the Pt(111) electrode, the formation of adsorbed methyl formate is likely. A mechanism is suggested, taking into consideration the possibility of surface reactions between adsorbates.

Electrocatalytic oxidation of d-sorbitol on platinum in acid medium: analysis of the reaction products

Abstract The electrocatalytic oxidation of d -sorbitol has been studied on polycrystalline platinum in 0.1 M HClO 4 . Prolonged electrolyses were carried out using a three potential plateau program with different values of the oxidation potentials. Chromatographic analyses showed that d -sorbitol was transformed into gluconic acid via the formation of glucose. Glucuronic acid and some degradation side products were also detected. When increasing the applied potential, the degradation of the initial molecule skeleton became more and more important. Moreover, a reaction mechanism for the electrooxidation of d -sorbitol was proposed and it was demonstrated that the consumption of d -sorbitol followed a first-order reaction law.

Electrosorption of sorbitol at platinum electrodes: Effect of the superficial structure

Abstract The electrosorption of sorbitol was investigated at polycrystalline and single-crystal platinum electrodes in an acidic medium. The coverage degree and the number of electrons per site involved in the electro-oxidation of the adsorbed species coming from the sorbitol were estimated from voltammetric measurements carried out at high sweep rates and for adsorption times varying from 0.1 to 500 s. The adsorption process depends on the superficial structure. The electrode surface was never saturated by the organic species coming from sorbitol adsorption and the number of electrons per site was found to be around 3, though this decreased to about 2 after long adsorption times. One of the main adsorbed species present at the electrode surface for long adsorption times is linearly adsorbed CO, as confirmed by in situ IR reflectance spectroscopy. The presence of other kinds of adsorbed species is also discussed.

On the oxidation of d-sorbitol on platinum single crystal electrodes: a voltammetric and in situ FTIRS study

The electrooxidation of d-sorbitol on Pt(100), Pt(110), Pt(111) and stepped surfaces vicinal to the (111) pole was studied in perchloric acid medium. The reaction is markedly influenced by the crystalline surface structure. Long range order effects of (111) terraces were also observed. Adsorbed CO (linearly and bridge bonded to the surface, on a level which depends on the electrode surface orientation) was identified by in situ Fourier transform IR (FTIR) spectroscopy as the main poisoning species. Moreover, for Pt(111), glucono-δ-lactone and CO2 were identified as the main soluble reaction products.

Electrosorption of d-sorbitol on Pt(110) and Pt(111) electrodes in acid medium: a study by programmed potential voltammetry

Abstract The adsorption and electro-oxidation of d -sorbitol on Pt(110) and Pt(111) single-crystal surfaces in acid medium was studied using programmed potential voltammetry (PPV). The coverage ( θ org ) of the surface and the number of electrons per site ( N eps ) involved in the electro-oxidation of the adsorbed species coming from the d -sorbitol electrosorption was estimated for both electrodes. Limiting values for θ org of 0.90 and 0.85, were obtained for Pt(111) and Pt(110), respectively, although the limit is reached at shorter adsorption times (100s) in the case of Pt(111). For Pt(111), N eps was found to increase the adsorption time reaching a value around 2 for long adsorption times, related to the formation of linearly-bonded CO, while for Pt(110) its value is near 1 in the range 0.5–300 s, corresponding to a mixture of different species, including multibonded and bridge-bonded CO, as confirmmed by EMIRS. A very fast oxidation process occurs at Pt(111), in contrast to Pt(110), where the strongly adsorbed species formed upon adsorption remain attached to the surface, blocking it almost completely. The reoxidation process taking place at Pt(111) prevails over the reduction of the oxygenated species formed on the platinum surface when a sufficiently high converage of the electrode surface ( θ org ≥ 0.8) is attained.

Simultaneous monitoring of toxic metals on white poplar (populus) by SWASV

A tecnica electroanalitica, Voltametria de Redissolucao Anodica de Onda Quadrada (VRAOQ), foi utilizada na determinacao simultânea de quantidades traco dos metais toxicos Pb(II), Cd(II) e Cu(II) em folhas de choupo (populus), utilizadas como bio-indicador, recolhidas numa area de trafego automovel intenso da cidade de Lisboa. As folhas, apos secagem, foram submetidas a um processo de digestao acida por microondas. Para a aplicacao desta tecnica a analise das folhas foi realizado um estudo de optimizacao dos varios parâmetros voltametricos. O eletrodo de trabalho consistiu num filme de mercurio depositado numa superficie de carbono vitreo. O sistema Ag/AgCl foi utilizado como referencia e um fio de Platina como eletrodo auxiliar. Concentracoes medias (em mg de metal/kg de materia seca - folhas) de 2,6, 0,18 e 5,0 foram obtidas para o Pb(II), o Cd(II) e o Cu(II), respectivamente. O valor obtido para o chumbo coincide com o obtido pelo metodo de referencia baseado na Espectrofotometria de Absorcao Atomica em Forno de Grafite (EAAFG).

An in situ IR reflectance spectroscopic study of the electro-oxidation of d-sorbitol on platinum

Abstract An in situ infrared (IR) reflectance spectroscopic study of the electrooxidation of d -sorbitol on polycrystalline platinum in perchloric acid medium was carried out by Fourier transform IR spectroscopy (FTIRS), under the SNIFTIRS and SPAIRS variants. Additional information was obtained by electrochemically modulated IR reflectance spectroscopy (EMIRS). In situ IR spectra show that the electrooxidation of d -sorbitol on platinum proceeds via a dual-path mechanism involving both reactive and poisoning intermediates. Linearly and bridge-bonded CO, resulting from the dissociative adsorption of d -sorbitol, which were identified as the main poisoning species present at the surface, are fully oxidised to CO 2 at higher potentials. The superficial coverage in adsorbed CO was found to be function of the electrode potential. Moreover, the electrooxidation process can occur without breaking the initial molecule skeleton, indicating that molecular adsorption of d -sorbitol is also possible. d -glucono-(δ and γ)-lactones, D -glucose and CO 2 were identified as the main reaction products.

Electro-oxidation of d-mannitol on platinum single crystal surfaces

Abstract The adsorption and oxidation reactions of d -mannitol at platinum single crystal electrodes with basal orientations have been studied by combining cyclic voltammetry and in situ FTIR spectroscopy. Cyclic voltammetric results have shown that, as far as current densities are concerned, the Pt(110) surface is the least active and more poisonable surface among the three platinum basal planes. The deactivation of Pt(100) and Pt(111) surfaces was found to be similar, although, higher current densities are obtained for Pt(100). FTIR spectroscopic experiments have indicated that CO is formed upon d -mannitol dissociative adsorption on the three basal platinum surfaces. This adsorbed intermediate, which was identified as the main poisoning species, is bonded to the platinum surface both in a linearly and bridge bonding mode and on a ratio which depends on the electrode surface orientation. The electro-oxidation process was found to be strongly dependent on the platinum crystalline surface structure. Moreover, additional voltammetric experiments performed on the three sets of platinum stepped surfaces have shown that the adsorption and oxidation reactions of d -mannitol are sensitive to the symmetry of the bidimensional domains as well as to the distribution of orientated defects (monoatomic steps) on the platinum surfaces.