Electrooxidation of Methanol and Ethylene Glycol Mixture on Platinum and Palladium in Alkaline Medium

The performance of mixture of methanol and ethylene glycol (EG) oxidation has been studied on both Pt and Pd electrodes in alkaline medium. The activity of EG oxidation is better than that of methanol oxidation and the stability of EG oxidation is better than that of methanol and ethanol oxidation on the Pd electrode. The onset potential for ethanol oxidation is more negative 200 mV than that of EG, however the stability of EG oxidation on the Pd electrode is better than that of ethanol oxidation. The performance of methanol oxidation improves pronouncedly by adding a small amount of EG on both Pt and Pd electrodes. The onset potential and peak potential of mixture of methanol and EG oxidation are close to or more negative than that of sole methanol and EG oxidation on the Pd electrode. The mixture of methanol and EG is more easily to be electrochemically oxidized and gives a better performance than sole methanol and EG on the Pd electrode. The results show that the mixture of methanol and EG is a promising candidate as fuel in direct alcohol fuel cells.     

Efficient Anchorage of Palladium Nanoparticles on the Multi‐Walled Carbon Nanotubes as Electrocatalyst for the Hydrazine Electrooxidation in Strong Acidic Solutions

A facile homogeneous precipitation–reduction reaction method, which involves PdCl₂ → PdO · H₂O → Pd⁰ reaction path, is used to synthesize the multi‐walled carbon nanotubes (MWCNTs) supported Pd nanoparticles (Pd/MWCNTs) catalysts. The particle size of Pd/MWCNTs catalysts can be easily tuned by controlling the hydrolysis temperature of PdCl₂. X‐ray diffraction (XRD) and transmission electron microscopy (TEM) measurements show the particle size of Pd/MWCNTs catalysts increases with hydrolysis temperature of PdCl₂, which is ascribed to the fact that the particle size of PdO · H₂O nanoparticles increases with hydrolysis temperature of PdCl₂. At the lower hydrolysis temperature, the as‐prepared Pd/MWCNTs catalyst possesses the higher dispersion and the smaller particle size. Consequently, the resultant Pd/MWCNTs catalyst exhibits the big electrochemical active surface area and the excellent electrocatalytic performance for hydrazine electrooxidation in strong acidic solutions. In addition, the electrochemical measurement indicate that particle size effect of Pd‐NPs occurs during the N₂H₄ electrooxidation. In brief, the mass activity and specific activity of the Pd/MWCNTs catalyst increases and decreases with decreasing the particle size of Pd‐NPs for the N₂H₄ electrooxidation, respectively.     

Application of electrooxidation process for treating concentrated wastewater from distillery industry with a voluminous electrode

Experiments in a laboratory scale were carried out to reduce color and chemical oxygen demand (COD) in distillery wastewater by using electrooxidation processes. A cylindrical electrochemical reactor constructed in an axial configuration with 0.2m diameter and 0.35 m height was employed in this study. Two materials including graphite particles and titanium sponge were used as the voluminous anodes. A cathode made from Ti/RuO(2) was placed 0.04-0.05 m above the upper level of anode particles. Effect of parameters including initial pH of wastewater (1-5), time of dilution, current intensity (1-10A), type of additive (H(2)O(2) and NaCl), and additive concentration were investigated. The results indicated that the anode made from titanium sponge showed a higher potential to treat wastewater than the another one. The treatment in acidic condition (pH=1) provided the maximum oxidation of organic pollutants in wastewater. The presence of additives can promote the reduction of COD and color in wastewater approximately 89.62% and 92.24%, respectively. The maximum current efficiency was reached at the first 30 min and decreased slightly as electrolysis time proceeded due to the formation of passivation on the electrode surface. The energy consumption was obtained in the range of 2.82-4.83 kWh/kgCOD or 24.08-28.07 kWh/m(3) wastewater depending upon the concentration of additive. The kinetics of COD reduction was the pseudo first-order reaction with a fast rate constant of 6.78 min(-1).     

Nitroxyl Radical‐Mediated Oxidation of Alcohols in Continuous Microreactors

Primary and secondary alcohols are oxidized to corresponding aldehydes and ketones, respectively, with nitroxyl radicals. The stable radical 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) is used as a mediator for selective oxidation of anisyl alcohol to anisaldehyde. This reaction is operated in different continuous microreactors either in single‐phase or in multiphase applications like double emulsions. The latter are used for a simple separation of the hydrophilic coproduct 1‐hydroxy‐2,2,6,6‐tetramethylpiperidine (TEMPO‐H) and the lipophilic product anisaldehyde. In addition, cyclic voltammetry is applied to determine conditions and parameters for electrochemical recycling of TEMPO‐H. TEMPO‐H is reactivated in a continuous electrolysis cell by anodic oxidation for reuse as mediator.     

Insights on the electrooxidation of ethanol with Pd-based catalysts in alkaline electrolyte

In this work, we report a facile method of synthesis of carbon supported Pd, PdRu, and PdNi nanoparticles, and a comparative study of their catalytic behavior for the electrooxidation of ethanol in alkaline media. The addition of metals such as Ru or Ni increases the oxophilicity of the Pd surface, as observed from the shifting of the Pd oxide reduction peaks. As a consequence, the onset potential for the electrooxidation of ethanol shifts to less positive values on the bimetallic catalysts. The nature and evolution of the species formed during the electrooxidation of ethanol over the catalysts under study has been monitored using in situ infrared spectroscopy. In order to assess properly the evolution of the species formed during the electrooxidation of ethanol, infrared spectra have been recorded in both H₂O and D₂O electrolytes. The results presented in this work demonstrate that the scission of the C–C bond of ethanol takes place at the surface of Pd/C and PdM/C (M = Ni and Ru) at potentials as low as 30 mV. However, at potentials above E ≥ 400 mV, acetates are the main species formed during the electrooxidation of ethanol.     

Oxidation chemistry of indole-2-carboxylic acid: Mechanism and products formed in neutral aqueous solution

The oxidation chemistry of indole-2-carboxylic acid has been investigated in phosphate containing supporting electrolytes in the pH range 1.4-9.8 at a pyrolytic graphite electrode by voltammetric studies, spectral studies, controlled potential electrolysis and related techniques. The kinetics of decay of the UV-absorbing intermediate generated during electrooxidation was followed spectrophotometrically and the decay occurred in a pseudo-first-order reaction. The products of the electrode reaction were characterized as 2,4-, 2,6- and 2,7-dioxindoles, COC- and CC-linked dimers by using GC-MS, IR and ¹H NMR. A detailed interpretation of the redox mechanism of indole-2-carboxylic acid in neutral aqueous medium has been presented to account for the formation of various products.     

Novel electrochemical surface modification method of carbon fiber and its utilization to the preparation of functional electrode

The electrooxidative and -reductive methods for the surface modification of carbon fiber were developed. The carbon fiber surface was first oxidized under anodic conditions to introduce phenolic hydroxyl groups on the carbon fiber, and then the resulting oxidized carbon fiber was treated under cathodic conditions in the presence of various kinds of electrophiles such as alkyl halides and alkyl tosylates introducing the alkyl groups on the carbon fiber. The changes of the functional groups on the carbon fibers were confirmed by the X-ray photoelectron spectroscopy (XPS) study and the observation of the hydrophilicity. The functional carbon fiber electrode introduced β-cyclodextrin (β-CD) was then prepared by this electrochemical method. The electroreduction of acetophenone (1) was carried out using the resulting carbon fibers as cathodes. The use of the carbon fiber modified with β-CD gave relatively high dl selectivity of the reductive coupling products 2 (the dl/meso ratio: 5.2), while the dl/meso ratio using the untreated carbon fiber was 3.0, and the formation of 1-phenyl ethanol (3) was observed only by using the β-CD-modified carbon fiber. The observed products selectivity is discussed from the viewpoints of the interaction between the electrogenerated species and the functional groups on the carbon fibers.     

Electrocatalytic oxidation of p-nitrophenol from aqueous solutions at Pb/PbO2 anodes

In this work, the elimination of p-nitrophenol (p-NPh) from aqueous solutions by electrochemical oxidation at Pb/PbO₂ anodes was investigated. The process was studied under galvanostatic polarization mode in acidic and alkaline media, as a function of the temperature (20, 40 and 60 °C) and of the anodic current density (J = 10, 20 and 30 mA cm⁻²). In acidic media (0.5 M H₂SO₄), the oxidation process allowed a 94% p-NPh conversion in 7 h, at 20 °C and with J = 20 mA cm⁻², with a wide distribution of degradation products (in particular: 39% p-benzoquinone and 26% hydroquinone, as given by a mass balance at the above electrolysis time). Under these conditions, the current efficiency for the substrate oxidation was 15.4% ([Ah L⁻¹]_exp = 7 versus [Ah L⁻¹]_theo = 1.08 Ah L⁻¹). In alkaline media (0.1 M NaOH, pH 8.5), the most effective p-NPh elimination (97%) was obtained at 60 °C, 20 mA cm⁻² and 420 min of electrolysis time, again with the production of p-benzoquinone and hydroquinone (52.7 and 15.1%, respectively). Under the latter conditions, an almost complete chemical oxygen demand (COD) abatement was attained, with a high level of p-NPh mineralization (>80%), a yield of p-NPh conversion greater than 95% and a scarce formation of aliphatic acids (most probably maleic acid). From the degradation curves ([p-NPh] versus t), in both acidic and alkaline media, the UV analyses and/or COD measurements, a complete oxidation of aliphatic acids to form CO₂ could be predicted for electrolysis time >420 min, according to a suggested oxidation pathway.     

Homolytic cleavage C-C bond in the electrooxidation of ethanol and bioethanol

Nowadays, the studies are focused on the search of better electrocatalysts that promote the complete oxidation of ethanol/bioethanol to CO₂. To that end, amorphous bi-catalytic catalysts of composition Ni₅₉Nb₄₀Pt_1−xY_x (Y = Cu, Ru, x = 0.4% at.) have been developed, obtained by mechanical alloying, resulting in higher current densities and an improvement in tolerance to adsorbed CO vs. Ni₅₉Nb₄₀Pt₁ catalyst. By using voltammetric techniques, the appearance of three oxidation peaks can be observed. The first peak could be associated with the electrooxidative process of ethanol/bioethanol to acetaldehyde, the second peak could be the oxidation of acetaldehyde to acetic acid, and the last peak might be the final oxidation to CO₂. Chrono-amperometric experiments show qualitative poisoning of catalytic surfaces. However, the in situ Fourier Transformed Infrared Spectroscopy, FTIR, is used for the quasi-quantitative determination with which can be observed the appearance and evolution of different vibrational bands of carbonyl and carboxylic groups of different species, as it moves towards anodic potential in the electrooxidative process.     

Electrochemical degradation of amaranth aqueous solution on ACF

The degradation of Amaranth, a kind of azo dye, has been studied under galvanostatic model with activated carbon fiber (ACF) electrode in aqueous solution with electrochemical method. The ACF was used as anode and cathode, respectively for the decolorization process. The onset oxidation potential and reduction potential for Amaranth on ACF were respectively ascertained at 0.6 and -0.4 V. During the range of -1.1 to 0.50 mA cm(-2), the decolorization was clarified into three processes as electroreduction, adsorption and electrooxidation. There were little contributions to the color and COD removals for the process of adsorption. The color removal can be up to 99% when the current density was 0.50 mA cm(-2). The maximum COD removal was 52% for the process of electrooxidation. Hundred percent color removal was obtained when the current density of -1.0 mA cm(-2) was applied. The maximum COD removal was 62% for the electroreduction. The COD removal results from the adsorption of products for the decolorization process of electrooxidation or electroreduction.