K.B. Kokoh

Fourier transform infrared reflectance spectroscopic investigation of the electrocatalytic oxidation of d-glucose: Identification of reactive intermediates and reaction products

In situ reflectance infrared spectroscopic techniques (under the SPAIRS and SNIFTIRS variants) have been applied to the investigation of the electrooxidation process of d-glucose at platinum electrodes in alkaline medium. Various reactive or poisoning intermediates were identified and their potential dependence monitored. The following mechanism was postulated. At low potentials, the first step leads to an adsorbed intermediate (A) formed by dehydrogenation of the anomeric carbon of glucose. Oxidation of (A) at higher potentials follows two possible routes. At E < 0.6 V vs. rhe, species (A) is oxidised as weakly adsorbed gluconate, either linked by two oxygens (species I, at E < 0.3 V), or by only one oxygen at 0.3 < E < 0.6V (species II). The second route is observed at 0.6 < E < 1.6V. In this potential range, ie, all over the range where the surface is covered by oxygenated species, species (A) is oxidised as weakly adsorbed δ-gluconolactone (B). Desorption of (I), (II) and (B) gives the corresponding solution species, ie gluconate (C) and δ-gluconolactone (D). In solution, slow hydrolysis of (D) leads to (C), so that gluconate is the only final product accumulating in solution.

Electrosynthesis in aqueous medium: a kinetic study of the electrocatalytic oxidation of oxygenated organic molecules

Abstract The electrocatalytic oxidation of small organic molecules (methanol, ethylene-glycol, glyoxal, propane-diols and glucose) at electrodes modified or unmodified by lead adatoms was investigated in order to follow the formation and to determine the concentration of the different products involved. Prolonged electrolyses of the organic compounds were carried out at controlled-programmed potentials with different potential programmes including cleaning sequences of the working electrode. Analyses of products were performed with “on-line” gas and liquid chromatographies. The kinetics of the overall reaction can be determined from these results and the different factors which modify the electroactivity of the electrode and the selectivity of the reaction are discussed.

On the electrochemical reactivity of anomers: electrocatalytic oxidation of α- and β-d-glucose on platinum electrodes in acid and basic media

Abstract Polarimetric studies of d -glucose solutions were carried out at low temperature (2°C). This procedure allowed us to stabilize the solutions of d -glucose and to differentiate the behaviour of the two main dissolved anomeric forms (α or β). In particular, the dependence of their stability on the pH of the electrolytic solutions was followed and their electrochemical reactivity was tested. Using cyclic voltammetry it was possible to demonstrate that, of the two possible pyranic forms of d -glucose, only β- d -glucose exhibited an appreciable reactivity at platinum electrodes, i.e. α- d -glucose was almost unreactive at low temperature. This difference in reactivity between anomers has never been shown clearly in any of the previous electrochemical studies of glucose. Long-term electrolysis was also performed. Ionic chromatography as well as in situ IR spectroscopic experiments allowed us to conclude that the oxidation of d -glucose in basic media at low temperature leads selectively to formation of the δ-gluconolactone, which later undergoes hydrolysis forming d -gluconate as a final product. Mechanisms are proposed and discussed.

Electrochemically induced surface modifications of mesoporous spinels (Co3O4−δ, MnCo2O4−δ, NiCo2O4−δ) as the origin of the OER activity and stability in alkaline medium

Co3O4−δ, MnCo2O4−δ, NiCo2O4−δ materials were synthesized using a nanocasting process consisting in replicating a SBA-15 hard template. Catalysts powders obtained were characterized using different physico-chemical techniques (X-ray scattering, transmission electron microscopy, N2 physisorption and X-ray photoelectron spectroscopy) in order to deeply characterize their morphostructural properties. Electrochemical measurements performed with cyclic voltammetry and electrochemical impedance spectroscopy techniques have shown that these catalysts were liable to surface modifications induced by the applied electrode potential. These surface structural modifications as well as their effect on the electroactivity of the catalyst towards the OER in alkaline medium are discussed. The activated NiCo2O4−δ material showed particularly excellent catalytic ability towards the OER in 0.1 M KOH electrolyte. In this material Co(IV) is found to be the active species in the catalyst composition for the OER. It exhibits an overpotential as low as 390 mV at a current density of 10 mA cm−2. This catalytic activity is especially high since the oxide loading is only of 0.074 mg cm−2. Furthermore, this anode catalyst showed high stability during an accelerated durability test of 1500 voltammetric cycles.

On some mechanistic aspects of the electrochemical oxidation of lactose at platinum and gold electrodes in alkaline medium

Abstract The electrocatalytic oxidation of lactose was performed in alkaline medium at both platinum and gold electrodes. Electrolysis carried out on Au electrodes showed that the conversion yield decreases when the initial concentration of lactose increases. This does not affect the selectivity in the production of lactobionic acid which is evaluated to be nearly 100%. The kinetic study on lead modified platinum electrodes showed that the oxidation of lactose into lactobionic acid follows an overall first order law with k0 = 8.2 × 10−4 min−1 cm−2. An infrared reflectance spectroscopic investigation reveals the presence of a lactone intermediate at low potentials. On a gold electrode, the chromatographic analyses confirm the IR spectroscopic studies, leading us to assume that the reaction mechanism depends on the applied electrode potential.

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.

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.

Selective oxidation of lactose to lactobionic acid on lead-adatoms modified platinum electrodes in Na2CO3 + NaHCO3 buffered medium

Abstract The electrocatalytic oxidation of lactose was performed in carbonate buffered solutions. The electrolysed solutions, analysed at regular periods of time by chromatography, showed that lactobionic acid was the only important reaction product when the oxidation was carried out on lead-adatom-modified platinum electrodes. Confirmation was obtained by means of additional spectroscopic techniques like 1 H nuclear magnetic resonance and gas chromatography-mass spectroscopy. A mechanism for the first steps of the reaction is tentatively suggested.

Size‐Dependent Electrocatalytic Activity of Free Gold Nanoparticles for the Glucose Oxidation Reaction

Understanding the fundamental relationship between the size and the structure of electrode materials is essential to design catalysts and enhance their activity. Therefore, spherical gold nanoparticles (GNSs) with a mean diameter from 4 to 15 nm were synthesized. UV/Vis spectroscopy, transmission electron microscopy, and under-potential deposition of lead (UPDPb ) were used to determine the morphology, size, and surface crystallographic structure of the GNSs. The UPDPb revealed that their crystallographic facets are affected by their size and the growth process. The catalytic properties of these GNSs toward glucose electrooxidation were studied by cyclic voltammetry, taking into account the scan rate and temperature effects. The results clearly show the size-dependent electrocatalytic activity for glucose oxidation reactions that are controlled by diffusion. Small GNSs with an average size of 4.2 nm exhibited high catalytic activity. This drastic increase in activity results from the high specific area and reactivity of the surface electrons induced by their small size. The reaction mechanism was investigated by in situ Fourier transform infrared reflectance spectroscopy. Gluconolactone and gluconate were identified as the intermediate and the final reaction product, respectively, of the glucose electrooxidation.

Electrocatalytic oxidation of sucrose: analysis of the reaction products

The analysis of sucronic acids produced by the electrocatalytic oxidation of sucrose is described. The main reaction products, l′-monocarboxylic and 6-monocarboxylic acids of sucrose, were identified by means of gas and liquid chromatography, mass spectrometry, NMR and i.r. spectroscopy, and acid hydrolysis. The quantitative analysis of reaction products other than the sucronic acids was realized by ionic chromatography (Dionex), whereas that of the sucronic acids was carried out using their acid hydrolysis products.