Modification of platinum surfaces by spontaneous deposition: Methanol oxidation electrocatalysis

The presence of a second metal on platinum surfaces affects the performance of methanol oxidation. However, most of the electrocatalytic reactions are studied by using electrochemically deposited platinum alloys, but in the case of spontaneous deposition the situation is not so clear since the surface distribution, stability and morphology are usually not well documented. The formation of surface decorated samples on mono- and poly-crystalline platinum is followed by electrochemical and spectroscopic techniques and analysis of their performance towards methanol adsorption and oxidation compared with that on pure platinum. Pt/Sn and Pt/Ru are of special interest because of their well-known performance in methanol fuel cells. Methanol oxidation on Pt(111)/Ru, Pt(111)/Sn and Pt(111) shows that ruthenium is the only one able to promote the reaction since the simultaneous dissolution of tin occurs and competes with the process of interest. The in situ infrared spectroscopy is used to compare methanol oxidation on Pt(111)/Ru and Pt(111) in acid media using p-polarized light. The formation of bridge bound carbon monoxide is inhibited in the presence of ruthenium ad-species, whereas on Pt(111) the three adsorption configurations are observed. Linear sweep polarization curves and Tafel slopes (calculated from steady state potentiostatic plots) for methanol oxidation are compared on polycrystalline surfaces modified by tin or ruthenium at different coverages. There is almost no change in the Tafel slopes due to the presence of the foreign metal except for Pt/Ru, where a 0.09 V decade⁻¹ slope was calculated below 0.55 V due to hydroxyl adsorbates on ruthenium islands. The anodic stripping of methanol residues on the three surfaces indicates a lower amount of carbon monoxide-type adsorbates on Pt/Ru, and the simultaneous tin dissolution process leading to residues oxidation on Pt/Sn electrodes.     

Oxidation of some aliphatic polyols on anodically deposited MnO2

Galvanostatic steady state current potential measurements were carried out for oxidation of a series of aliphatic alcohols having varying number of hydroxyl groups. The anodically deposited layer of MnO₂ on platinum was used as the electrode material. The deposit was characterised by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and electrode potential measurements. The catalytic role of MnO₂ in the electro-oxidation of alcohols was indicated by the chronopotentiograms and the cyclic voltammograms. An analysis of the electrochemical data indicated a catalytic EC mechanism in which Mn (V) is generated electrochemically and consumed chemically in succession. Based on this and the hydrogen bonding interaction between alcoholic hydroxyl groups and MnO₂ layer, a mechanism was proposed which accounts for the variation in the observed electrochemical reaction orders. Tafel behaviour was found to be followed only approximately. Current efficiency of the electrochemical oxidation of polyols was studied. Replacement of platinum by carbon as current collector was found to leave the electrocatalytic activity of the MnO₂ deposit practically unaltered.     

Electrochemical behaviour of phenylethanolamine at gold in aqueous solution

The electrooxidation of phenylethanolamine (2-amino-1-phenylethanol) at a gold electrode in alkaline electrolyte has been studied. Measurement of the differential capacitance of the electric double layer versus the electrode potential has shown that the adsorption of phenylethanolamine at the gold-solution interface plays a significant role in the oxidation mechanism. The effect of amine concentration, electrolyte pH and potential scan rate on the electrooxidation is analysed.     

Structural and surface properties of thermally prepared Ti/RhxTi(1−x)Oy electrodes: I—influence of preparation parameters

Electrodes of Rh_xTi_(1−x)O_y nominal composition were prepared by thermal decomposition of the chloride or nitrate precursor salts dissolved in strongly acidic medium and applied by brush to both sides of a Ti° support. A systematic study of the influence of calcination temperature and time as well as oxygen flux was conducted. The coatings were characterised by SEM, EDAX, XRD, open circuit potential measurements and cyclic voltammetry (CV). Visible-ultraviolet spectrophotometry was employed to identify the chemical form of the precursor in solution while thermogravimetric analysis (TGA) was used to assess the decomposition temperature ranges. Optimisation of the coating preparation parameters showed coatings obtained from [Rh(H₂O)₆](NO₃)₃ precursor dissolved in HNO₃ 1:2 (v/v) and fired at 430 °C for 2 h in a 5 l min⁻¹ oxygen stream-furnished stable electrodes having the highest electrochemically active surface area.     

Nile blue adsorbed onto silica gel modified with niobium oxide for electrocatalytic oxidation of NADH

A study of modified carbon paste electrode employing Nile blue (NB) adsorbed on silica gel modified with niobium oxide (SN) for electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) is described. The adsorbed organic dye on SN was used to prepare a modified carbon paste electrode to investigate its electrochemical properties. The formal potential (E°′) of the adsorbed NB (−230 mV vs. saturated calomel electrode, SCE) showed a shift of 70 mV towards a more positive potential value, compared to NB dissolved in aqueous solution. In solutions with pH between 6.0 and 8.0 did stability and E°′ remained almost constant. However, for a solution pH lower than 6.0 the E°′ was affected by the acidity of the contacting solution, shifting the E°′ towards more positive values. For the solution pHs between 6.0 and 8.0 the electrocatalytic activity remained almost constant. A linear response range for NADH between 1.0×10⁻⁵ and 5.2×10⁻⁴ mol l⁻¹, at pH 7.0, was observed for the electrode, with an applied potential of −200 mV versus SCE. The formation of an intermediate charge transfer (CT) complex was proposed to the CT reaction between NADH and adsorbed NB. The heterogeneous electron transfer rate, k_obs, was 1400 M⁻¹ s⁻¹ and the apparent Michaelis-Menten constant, was 0.21 mM at pH 7.0 evaluated from rotating disk electrode (RDE) experiments with an electrode coverage of about 5.2×10⁻⁹ mol cm⁻². The increase in the reaction rate between NADH and the immobilized NB compared to those obtained with dissolved NB was assigned to the shift of the E°′ towards more positive values.     

Activity of gold towards methylamine electrooxidation

The kinetics of methylamine oxidation at a gold electrode in contact with an alkaline electrolyte solution was studied. The adsorptive behaviour of substrate molecules was determined by changes in the differential capacitance of the double layer at the electrode/solution interface. The electrooxidation of methylamine occurs only in the potential range of gold oxide layer formation. The general reaction pathways of methylamine oxidation on the gold electrode is proposed.     

Electrochemical preparation of epinephrine/Nafion chemically modified electrodes and their electrocatalytic oxidation of ascorbic acid and dopamine

Two types of epinephrine and cyclized epinephrine quinone films have been prepared using cyclic voltammetry from the epinephrine in the strong acidic solutions and neutral aqueous solutions over different scanning potential ranges. The cyclic voltammogram of the epinephrine film is characterized by one redox couple at about +0.5 V (versus Ag|AgCl) and cyclized epinephrine quinone film exhibits one redox couples at about −0.15 V (versus Ag|AgCl) .In addition to cyclic voltammetry and an electrochemical quartz crystal microbalance (EQCM) were used to study the growth mechanism of the epinephrine and cyclized epinephrine quinone molecules. The electrocatalytic oxidation of catecholamines (dopamine and norepinephrine) and also ascorbic acid were investigated in acidic aqueous solutions using epinephrine films. The rotating ring-disk electrode technique was used to investigate the mechanism of electrochemical oxidation of dopamine and ascorbic acid.     

Electrochemical reduction of nitrate on Pt(S)[n(1 1 1) × (1 1 1)] electrodes in perchloric acid solution

The electrochemical reduction of nitrate ion was studied by cyclic voltammetry on Pt(1 1 1) and [n(1 1 1) × (1 1 1)] stepped Pt surfaces, where n (=14, 10, 7, 6, 5, 4, 3, 2) is the number of terrace atoms, in 0.1 M HClO₄ + 10 mM KNO₃. The electrocatalytic nitrate reduction was found to hardly proceed on Pt(1 1 1) in the hydrogen adsorption region, while the electrocatalytic activity was improved with the increase in the step density. Inactivation was observed in the presence of adsorbed hydrogen or nitrate-derived reduced adsorbate, i.e. adsorbed NO, on (1 1 1) step sites. It was, therefore, concluded that the electrocatalytically active NO₃⁻ species does not adsorb on the (1 1 1) terraces but on the (1 1 1) monoatomic steps. The nitrate reduction current increased with the step density in a non-linear relationship. The overall current density at 0.21 V (RHE) corresponding to the peak potential of the main electrocatalytic nitrate reduction wave which was maximum at n = 2, abruptly increased with short terraces, i.e. n < 5, where the current wave of adsorbed hydrogen on the Pt stepped surface with comparatively narrow (1 1 1) terraces, denoted as H_nt, also appeared unmodified for n < 5 on voltammograms recorded in 0.1 M HClO₄ in the absence of nitrate.     

Hydrogen adsorption and hydrogen evolution reaction on a polycrystalline Pt electrode studied by surface-enhanced infrared absorption spectroscopy

Hydrogen evolution reaction (HER) on a polycrystalline Pt electrode has been investigated in Ar-purged acids by surface-enhanced infrared absorption spectroscopy and electrochemical kinetic analysis (Tafel plot). A vibrational mode characteristic to H atom adsorbed at atop sites (terminal H) was observed at 2080-2095 cm⁻¹. This band appears at 0.1 V (RHE) and grows at more negative potentials in parallel to the increase in hydrogen evolution current. Good signal-to-noise ratio of the spectra enabled us to establish the quantitative relation between the band intensity (equivalently, coverage) of terminal H and the kinetics of HER, from which we conclude that terminal H atom is the reaction intermediate in HER and the recombination of two terminal H atoms is the rate-determining step. The quantitative analysis of the infrared data also revealed that the adsorption of terminal H follows the Frumkin isotherm with repulsive interaction.     

Electrocatalytic properties of NDGA and NDGA/FAD hybrid film modified electrodes for NADH/NAD redox reaction

The fabrication of monolayers composed of nordihydroguaiaretic acid (NDGA), and hybrid films composed of NDGA-flavin adenine dinucleotide (FAD) adsorbed films was performed in neutral aqueous solutions to produce electrochemically active thin films exhibiting one and two redox couples, respectively. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ growth of the NDGA and hybrid NDGA/FAD film monolayers. The NDGA modified film electrocatalytically oxidized NADH, ascorbic acid, dopamine, and N₂H₄ in neutral aqueous solutions. Well-separated voltammetric peaks were observed for dopamine and uric acid mixtures, and also for ascorbic acid and uric acid mixtures using the NDGA/GC modified electrode. When transferred to various aqueous buffered solutions, the two redox couples of the NDGA/FAD hybrid film and their formal potentials were observed to be pH-dependent. The electrocatalytic oxidation and reduction of NADH and NAD⁺ by a NDGA/FAD hybrid film in neutral aqueous solutions was carried out, and the electrocatalytic oxidation of NADH was performed using a NDGA/FAD hybrid film.