Electrocatalytic reduction of chlorophenoxy acids at palladium-modified glassy carbon electrodes

Palladium species can be immobilized on a glassy carbon electrode by voltage cycling between 0.0 and −0.4 V versus SCE in solutions containing 0.5 mM Na₂PdCl₆ in order to facilitate the electrocatalytic reduction of chlorophenoxycarboxylic acids in solutions buffered at pH 7. Cyclic voltammetry, measurements at the rotating disc electrode (RDE) and chronoamperometric techniques were used in order to investigate the electrochemical behaviour of the modified electrodes (GC/Pd) towards the catalytic reduction of chlorophenoxycarboxylic acids. A reaction mechanism is proposed and discussed. A probable scheme for the electroreduction of chlorophenoxycarboxylic species in neutral medium involves a simultaneous and competitive adsorption of the organic molecules and hydrogen atoms on the catalytic sites, followed by an irreversible hydrodechlorination reaction. 2,4-Chlorophenoxyacetic acid can be dehalogenated to a chlorine-free product in neutral aqueous solutions at relatively low cathodic polarizations and at ambient temperature using a GC/Pd electrode.     

Catalytic wet-air oxidation of carboxylic acids on carbon-supported platinum catalysts

Catalytic wet-air oxidation (CWAO) of aqueous solutions (5 g · l⁻¹) of car☐ylic acids (formic, oxalic, and maleic) was carried out with air at 293–463 K on carbon-supported platinum catalysts. Platinum was loaded on active charcoal by cationic exchange, then reduced under H₂. Homogeneous dispersions of 1–2 nm metal particles were obtained. CWAO reactions were performed at 1 or 15 bar air pressure in stirred, batch reactors. Total conversion of formic and oxalic acids into carbon dioxide was obtained under very mild conditions (air at atmospheric pressure, 326 K). The Pt/C catalyst was almost inactive for the oxidation of acetic acid but maleic acid was oxidized under moderate conditions (15 bar of air pressure, 405 K) which indicates that the degradation of this acid does not occur via acetic acid.     

Oxygen reduction on platinum in mixtures of phosphoric and trifluoromethane sulphonic acids

A rotating disk electrode study of O₂ reduction on platinum has been carried out in mixtures of concentrated H₃PO₄ and CF₃SO₃H at room temperature. The diffusion limiting current was found to increase with increasing amounts of CF₃SO₃H added to the H₃PO₄. The apparent first-order rate constant defined in terms of O₂ concentration (M) is lower in the mixed acids than in H₃PO₄ (86% by weight), but the concentration of O₂ increases by a much larger factor, with the result that the kinetics are faster in the mixed acids.     

Polyoxyethylene esters of fatty acids: an alternative synthetic route for high selectivity of monoesters

Esterification reaction of fatty acids with polyoxyethylenes has been studied in the presence of solid acid catalysts and the results are compared with those obtained with a classical homogeneous catalyst, p-toluene sulphonic acid. Solid acid catalysts showed very high selectivities for monoesters even under less favorable reaction conditions of 1:1 molar ratios of oleic acid to PEG. Excess amount of PEG is normally required to ensure high selectivity for monoesters in the presence of homogeneous catalysts. The yields for monoesters given by heterogeneous catalysts such as zeolites, HPA and nafion were comparable to that with p-toluene sulphonic acid after 24 h of reaction. Results suggest that solid acid catalysts are potential catalysts for selective synthesis of monoesters from oleic acids and PEG even under severe reaction conditions. A screening study of different solid acid catalysts showed ZSM-5, zeolite-β, and nafion to be better catalysts than zeolite USY and mordenite, the former giving higher selectivity and yield to monoesters. The results suggest that the most important parameters in optimization catalyst performance for monoesters production is the acid strength and less so on other properties such as shape selectivity and concentration of active sites.     

Effect of rhodium on the properties of bifunctional MxOy/ZrO2 catalysts in the reduction of nitrogen oxides by hydrocarbons

The catalytic properties of transition metal oxides (Cr, Ce, and Co) supported on ZrO₂ synthesized by various methods, as well as the effect of rhodium on the performance of the M_xO_y/ZrO₂ oxide systems in NO reduction with hydrocarbons (methane, propane–butane mixture, and propene) were studied. Scanning electron microscopy, ammonia thermoprogrammed desorption (NH₃-TPD), XPS, and IR spectroscopy were used to study the physicochemical indices of rhodium-promoted M_xO_y/ZrO₂ oxide catalysts. The enhancement of the redox properties of the oxide catalysts upon the introduction of rhodium does not alter their bifunctional nature in SCR activity: these catalysts have both redox and strong acid Brønsted-sites.     

Studies on the mechanism of the hydrazine reduction reaction: Applications to selected monoethylenic,diethylenic and triethylenic fatty acids ofcis configurations

The hydrazine reduction ofcis ethylenic bonds has been studied using a number of C₁₆, C₁₈, and C₂₀ monoethylenic, C₁₈ diethylenic, and C₁₈ plus C₂₀ triethylenic fatty acids. Relative reduction rates were determined and correlated with the positions of the ethylenic bonds in the fatty acids. The terminal ethylenic bond was the most reactive and among the others, reactivity was greater nearer the ends of the chain than in the center. Also, the ethylenic bonds closer to the carboxyl group showed greater reactivity than those closer to the methyl end of the chain. Differences in reactivity among the centrally located double bonds were not significant. These reactivity trends are consistent with a mechanism involving protonated diimide as an intermediate in the reduction process. In diethylenic fatty acids, the non-methylene-interrupted dienes showed slightly lower reactivity than the methylene-interrupted counterparts, which indicated that both the position of ethylenic bonds and the relative distance between two ethylenic bonds in the diene influence the rate of reaction. The information obtained from the rate studies should be useful for predicting the product distribution from partial hydrazine reductions of long-chain polyethylenic compounds.     

Cathodic reduction of benzil in acetone and in dichloromethane

The cathodic reduction of benzil has been carried out at a controlled potential on a mercury cathode in two different SSE (solvent-supporting-electrolyte) conditions: (a) acetone/lithium perchlorate in absence of electrophile where 2,3-diphenyl-5-methyl-furan and 1,2-diphenyl-2-hydroxy-1,4-pentanedione were obtained as main products and (b) dichloromethane/tetrabuthylammonium chloride with the addition of oxalyl chloride as electrophile, where a fast electron transfer took place.     

Electrolytic reduction improves treatability of humic acids containing water streams

BACKGROUND: An understanding of the structure of humic acids is essential for their degradation or physical removal from wastewaters. This work aims at targeting the reactivity of these molecules by modifying their properties. Structural alterations were carried out by electrolytically reducing the solution containing humic acid in an electrolytic cell to convert them into less polar structures. RESULTS: Overall it was observed that electrolytic reduction of humic acids strongly facilitated their further treatability. First, the reduced forms of humic acids exhibited improved adsorption on activated carbon. For 1 kW h of electrical energy consumed during electrolytic reduction, the additional chemical oxygen demand (COD) adsorbed was 60 g for a synthetic humic acid solution. Similarly, the additional COD adsorbed (kW h)^?1 was found to be 35 g and 112 g for humic acid‐rich effluent and landfill leachate, respectively. In comparison with non‐reduced control samples, a 200‐fold decrease in the chloroform formation was observed when electrolytically reduced drinking water samples were supplemented with a chlorine dosage of 150 mg L^?1. Moreover, an enhanced membrane flux was obtained with electrically reduced samples, indicating their improved membrane filterability. CONCLUSION: The electrolyzed humus species were characterized by analyzing their surface tension and particle size. This work addresses an alternative technology for the treatment of water streams containing humic acids. Copyright © 2007 Society of Chemical Industry     

一段炉转化催化剂的还原性能研究

用定温还原迎头色谱法和程序升温还原法对加与不加稀土氧化物的两种转化催化剂的还原性能进行了研究、测定了两种催化剂的还原动力学参数。结果表明，温度对催化剂还原性能有明显影响；讨论了稀土促进镍催化剂还原的原因和还原进行的方式．认为低温时遵从速率控制成核作用机理．高温时按瞬间成核收缩球模型机理。     

Acidity of oryzanol and its contribution to free fatty acids value in vegetable oils

Model oil systems containing physically refined rice bran oil to which oryzanol was added were examined to determine the effects of oryzanol concentration on FFA values. When oryzanol was added to the model oils at a 0.5% level and FFA was determined, increases in FFA value were 0.28% as determined with phenolphthalein, 0.58% with thymolphthalein, and 0.07% with alkali blue 6B. Oils containing added oryzanol at 0.5–1.5% showed a proportionate increase in FFA values with an average increase of 0.413% per gram of oryzanol. A direct titration of purified oryzanol showed an acidity of 42.5% expressed as FFA. In spectroscopic studies, the phenolic group in the ferulic acid moiety of oryzanol was titrated by sodium hydroxide. Based on these data, indicator correction factors for oryzanol's acidity and a formula for calculating real FFA content of vegetable oils containing oryzanol were developed.     

Cathodic processes in the leaching and electrochemistry of covellite in mixed sulfate–chloride media

The cathodic processes that occur on a covellite (CuS) surface in mixed sulfate–chloride solutions in the absence and presence of copper(II) ions have been studied using potentiostatic transients and cyclic voltammetry at rotating disk electrodes in the potential range 0.3–0.7 V (versus SHE). This range is relevant to the oxidative leaching of this copper mineral in sulfate and chloride lixiviants. Variations in the concentrations of sulfate and chloride ions had a small effect on the cathodic reduction of covellite in the potential range of 0.5–0.3 V, although the presence of chloride ion resulted in a significant increase in the anodic current on the reverse sweep. On the other hand, addition of copper(II) ions resulted in enhanced cathodic currents and subsequent anodic currents in both sulfate and chloride solutions due to reduction of covellite to an undefined reduced copper sulfide species. Reduction of copper(II) to copper(I) ions becomes the preferred cathodic reaction as the concentration of chloride ions increases, becoming mass transport controlled at a rotating disc electrode at potentials below about 0.4 V. Potentiostatic measurements at potentials negative to the mixed potential in acidic chloride solutions have shown that reduction of copper(II) ions is reversible and have been used to estimate the rate of oxidative dissolution of the mineral which value agrees reasonably well with previously reported leaching rates under similar conditions. Reduction of dissolved oxygen has been found to be very much slower that that of copper(II) ions under ambient conditions.     

羧酸类配位聚合物的研究进展

羧酸类金属配位聚合物是一种新型功能性分子材料,近年来得到科学家的普遍关注。本文对羧酸类金属配位聚合物的分类进行了综述,综述了羧酸类金属配位聚合物研究的重要性和国内外羧酸类金属配位聚合物的研究工作,对苯多羧酸类、吡啶羧酸类、草酸类、脂肪族二羧酸类配合物在手性拆分和催化、分子磁体、非线性光学方面的研究进行了详细介绍,列举了近年来这类配位聚合物的研究成果和开发进展,并对其发展趋势进行了展望。     

氧化还原酶催化还原硝基化合物的研究进展

用纯化的氧化还原酶催化还原污染环境的硝基化合物,反应条件温和、容易控制。归纳了几种可以催化还原硝基化合物的氧化还原酶的结构、生理作用和还原硝基化合物的过程机理。对氧化还原酶催化还原硝基化合物的研究前景进行了展望。     

A simple mathematical analysis on the effect of sand in Cr(VI) reduction using zero valent iron

A simple mathematical model was proposed to analyze the enhancement of Cr(VI) reduction when sand materials are added to the zero valent iron (ZVI). Natural decay of Cr(VI) in a control experiment was analyzed by using a zero-order decay reaction. Adsorption kinetics of Cr(VI) to sand was modeled as a first-order reversible process, and the reduction rate by ZVI was treated as a first-order reaction. Natural decay of Cr(VI) was also included in other experiments, i.e., the adsorption to sand, the reduction by ZVI, and both adsorption and reduction when sand and ZVI are present together. The model parameters were estimated by fitting the solution of each model to the corresponding experimental data. To observe the effect of sand addition to ZVI, both adsorption and reduction rate models were considered simultaneously including the natural decay. The solution of the combined model was fitted to the experimental data to determine the first-order adsorption and reduction rate constants when sand as well as ZVI is present. The first-order reduction rate constant in the presence of sand was about 35 times higher than that with ZVI only.     

Mechanistic study of the reduction of copper oxides in alkaline solutions by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy was used to study the mechanism by which copper oxides are reduced in alkaline solutions. For the reductions of CuO and Cu₂O, a capacitive loop and also an inductive loop under certain conditions were observed in the complex plane. The electrochemical impedance for CuO reduction was not greatly dependent on the solution alkalinity and the kind of alkali hydroxide. However, the electrochemical impedance for Cu₂O reduction was considerably affected by the kind and concentration of alkali hydroxide. The diameter of the capacitive loop, i.e., the charge-transfer resistance (R_ct), was increased with increase in solution alkalinity. It should also be noted that R_ct was increased in the order of KOH < NaOH < LiOH. These dependences were consistent with the good separation between the reduction potentials of CuO and Cu₂O in chronopotentiometric and voltammetric measurements with strongly alkaline electrolytes containing Li⁺. The inductive loop observed for the Cu₂O reduction at higher concentrations of KOH (>6 M) and LiOH (>0.2 M) suggested the existence of an intermediate species (probably CuOH). The specific inhibitory effect of Li⁺ ions on the reduction of Cu₂O might be explained by a possible stabilization of CuOH by Li⁺ ions.     

预还原型氨合成催化剂还原度的分析

对预还原型氨合成催化剂还原度的分析,目前暂无相对准确的方法。介绍了用滴定法测定预还原型氨合成催化剂中的金属铁和总铁的质量分数,二者之比即为预还原型氨合成催化剂的还原度。     

Electrode kinetics of oxygen reduction on platinum in trifluoromethanesulphonic acid

The kinetics of oxygen reduction at Pt in trifluoromethanesulphonic acid (TFMSA) (0.05–6.0 M) and in 1.0 M TFMSA with addition of small concentrations of phosphoric acid (0.003–0.1 M) was investigated using therrde technique. In TFMSA, the oxygen reduction current on the oxide-covered Pt was found to be smaller than that on the oxide-free Pt surface. This result is consistent with the greater amonut of hydrogen peroxide produced on the oxide-covered Pt. A reaction order of one-half with respect to the oxygen concentration for the oxygen reduction reaction was obtained from the ring-disc data. Addition of increasing amounts of phosphoric acid to TFMSA resulted in a progressive decrease in the oxygen reduction current and in an increase of the reaction order with respect to oxygen. On the basis of these experimental results the reaction mechanism proposed for the oxygen reduction of Pt in TFMSA is the fast dissociative adsorption of oxygen, followed by the slow electron transfer step.     

Catalyst optimization and reduction condition of continuous growth of carbon nanotubes on carbon fiber surface

Carbon nanotubes (CNTs)/carbon fiber (CF) reinforcements were synthesized under different catalyst compositions and reduction conditions. The effects of the catalyst, reduction temperature and reduction time on the surface morphology, graphitization, and single filament tensile strength of the prepared CNTs/CF samples were investigated. When nickel was used as the catalyst and copper as the catalyst promoter, with the increase of copper concentration, the catalytic activity increased. Thus, the carbon source was consumed more completely, improving the abundance of CNTs with good graphitization. And the effect of repairing CF defects was more obvious, hence the single filament tensile strength accordingly increased. Besides, the increase of catalyst reduction temperature and reduction time intensified the etching of CF by catalyst, and decreased the single filament tensile strength of CF. With the deposition of CNTs, the tensile strength of CF was enhanced in varying degrees. When the concentration of cooper was 0.01 mol/L with the reduction time of 10 min and reduction temperature of 450 °C, CNTs/CF had the highest tensile strength, which can reach up to 4.51 GPa. We determined that bimetallic catalysts could adjust the catalytic activity of nickel. The change of reduction time and temperature would affect the quality of CNTs, which was helpful to obtain high quality CNTs on CF surface and improve the mechanical properties of CNTs/CF and its composites.     

Oxygen reduction at Au nanoparticles electrodeposited on different carbon substrates

The electrocatalytic reduction of molecular oxygen (O₂) has been performed in O₂-saturated 0.5 M KOH solution at Au nanoparticles electrodeposited onto two different carbon substrates, namely glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG). Cyclic voltammetry (CV) technique has been used in this investigation. The electrocatalytic activity of the Au nanoparticle-based electrodes is inherently related to its electrodeposition conditions (i.e., the absence or presence of some additives) as well as the nature of the substrate. For instance, Au nanoparticles electrodeposited onto GC (nano-Au/GC) from K[AuBr₄] in the presence of 25 μM cysteine showed a high electrocatalytic activity towards the oxygen reduction reaction (ORR) as demonstrated by the largest positive shift of the cathodic peak potential (at ca. −0.165 V versus Ag/AgCl/KCl (sat)). On the other hand, two well-separated successive reduction peaks corresponding to the 2-step 4-electron reduction of oxygen were observed at the different nano-Au/HOPG electrodes. The relative ratio of the two peak current heights changed significantly depending on the electrodeposition conditions of the Au nanoparticles. The morphology of the different Au nanoparticles electrodeposited onto the different substrates was depicted by scanning electron microscope (SEM) technique.     

Oxygen reduction on platinum electrodes in base: Theoretical study

Electrode-potential-dependent activation energies for electron transfer have been calculated using a local reaction center model and constrained variation theory for the oxygen reduction reaction on platinum in base. Results for four one-electron transfer steps are presented. For the first, O₂(ads) is predicted to be reduced to adsorbed superoxide, O₂⁻(ads), which dissociates with a low activation barrier to O(ads) + O⁻(ads). Then a proton transfer form H₂O(ads) to O⁻(ads) takes place, forming OH(ads) + OH⁻(aq). The second electron transfer reacts O(ads) with H₂O(aq) to form a second OH(ads) + OH⁻(aq). The third and fourth electron transfers react the two OH(ads) with two H₂O(aq) to form two H₂O(ads) + two OH⁻(aq). All three different surface reduction reactions are predicted to have reversible potentials in the −0.24 V(SHE) to −0.29 V(SHE) range for 0.1 M base and activation energies for the superoxide formation step are close to the experimentally observed range in 0.1 M base for the overall four-electron to water over the three low index (1 1 0) (1 0 0) and (1 1 1) surfaces: 0.38-0.49 eV at 0.35 eV respectively at 0.88 V(RHE). Predicted reversible potentials for forming O₂⁻(ads) are compared with estimates from the experimental literature. The difference between the acid mechanism, where the peroxyl radical, OOH(ads) is the first reduction intermediate, and the base mechanism, where superoxide, O₂⁻(ads) is the first reduction intermediate, is discussed.