Kinetic studies of the electroreduction of dimedone on mercury

The reduction of 5,5-dimethyl-1,3-cyclohexandione (dimedone) in anhydrous alcohols (n-butanol, 2-propanol and ethanol) and in anhydrous dimethylformamide (DMF) on mercury electrodes has been studied. Tetra-ethylammonium-p-toluene sulphonate (Et₄Np-TS) was used as the electrolyte. The main product found was 1,1′-diol-3,3′-dione-5,5,5′5′-tetramethyl bicyclohexyl, formed by transfer of one electron per molecule of reactant. This compound easily loses two molecules of water to yield 3,3′-dione-5,5,5′5′-tetramethyl bicyclohexenyl. Addition of up to 10% water to the solvent had no effect on the product and caused very little change in the current/potential curves. The rate-determining step is the combination of two radicals on the surface. Other rate-determining steps and reaction pathways were rejected because they would not account for the kinetic behaviour found experimentally. The results were essentially the same in all solvents tested, although the energies of activation were found to be 2–3 kcal/mole lower in DMF than in n-butanol. The dimer was found to be electrochemically active at less negative potentials than those required to reduce the reactant.     

碳化钨/碳纳米管纳米复合材料的制备及其对硝基苯的电催化活性

采用表面修饰技术和原位还原碳化技术制备了纳米碳化钨（WC）/碳纳米管（CNT）纳米复合材料,通过X射线衍射（XRD）、扫描电子显微镜（SEM）和热重-差热分析（TG-DTA）等手段对WC/CNT纳米复合材料的晶相组成、形态结构和热稳定性进行了表征,结果显示样品是由WC和CNT两相构成,纳米WC颗粒均匀地分散在碳纳米管上,粒径细小;在空气气氛中,WC/CNT纳米复合材料在470℃以下保持稳定。采用循环伏安法研究了WC/CNT纳米复合材料对硝基苯的电催化活性和电化学稳定性。结果表明,WC/CNT纳米复合材料对硝基苯的电催化活性优于纳米WC和CNT,且WC/CNT纳米复合材料在硝基苯电还原过程中保持良好的化学稳定性。     

Comparative evaluation of electroreduction of nitrobenzene and m-dinitrobenzene on Ti/TiO2 electrodes in H2SO4

Cyclic voltammetric studies of nitrobenzene and m-dinitrobenzene in 1M H₂SO₄ on thermally coated Ti/TiO₂ electrodes indicate heterogeneous redox catalysis for both these compounds. Significant adsorption and blocking effect of these compounds are noticed at higher concentrations. m-Dinitrobenzene (MDNB) exhibits greater inhibitive adsorption when compared to nitrobenzene. These effects, however, do not significantly influence the yield and current efficiency of the reduction of these two nitro compounds to their corresponding amines. Under optimum experimental conditions yields of over 87% nitrobenzene reduction and over 83% m-dinitrobenzene reduction could be achieved. The electrolyte could also be reused by recovering the amines as their sulphates at lower temperatures.     

A study of the electrochemical reduction of nitrobenzene to p-aminophenol

The electrochemical reduction of nitrobenzene has been studied in aqueous organic solvents containing sulphuric acid. It is shown that the yield ofp-aminophenol and the maximum current density for the reaction is dependent upon the solvent, the cathode metal, the electrode potential, the concentration of acid and the rate of stirring of the catholyte. The changes of products would not be obvious from theI-E curves but it is possible to find conditions where the yield ofp-aminophenol is > 75% at a current density of 150 mA cm^–2.     

A kinetic study of the electroreduction of anodically formed cobalt oxide layers

The kinetics of the electroreduction of anodically formed cobalt oxide layers in different carbonate-bicarbonate aqueous solutions were investigated using combined voltammetric and potential step techniques. The composition of the electrolyte was varied to cover wide pH and ionic strength ranges. The electroreduction reaction can be explained through a nucleation and three-dimensional growth process, involving cobalt phase formation from either soluble Co(II) or Co(III) species.     

对硝基苯乙酮生产废水处理工程实例

介绍了处理对硝基苯乙酮生产废水的化学/生物/深度处理组合工艺及相关反应条件。其中化学法采用混合酸析和Fenton催化氧化法,通过小试发现,对硝基苯乙酮生产废水混合酸析最佳pH为2.0,Fenton催化氧化的最佳反应条件为pH 3.0、H_2O_2质量分数0.9%、FeSO_4·7H_2O 2.5 g/L、反应时间2 h。经过混合酸析-Fenton催化氧化-水解酸化-A/O-臭氧氧化-接触氧化工艺处理后,出水的COD、硝基苯、色度、总氮等指标达到国家污水综合排放三级标准(GB 8978—1996)。     

恩德炉煤灰处理硝基苯污染的研究

本文利用黑化集团有限公司恩德造气炉产生的恩德炉煤灰作吸附剂进行处理突发硝基苯污染的研究,分析了吸附剂用量和搅拌时间对吸附效果的影响,实验采用还原-偶氮分光光度法进行硝基苯浓度的测定,实验结果表明恩德炉煤灰对硝基苯有较好的吸附效果。     

Mechanism of boron tribromide electroreduction

This study was carried out to investigate the electrochemical behavior of boron tribromide in dimethlyformamide. The reduction of the compound was found to follow a CE mechanism. The kinetic parameters and the diffusion coefficient were calculated by the use of ultramicrodisc electrodes and chronoamperometry. The number of electrons transferred was found to be 2 by rotating disc and ultramicro disc electrodes and 3 by coulometry. These results are in good accordance with those obtained from molten boron salts. This study is important in regard to electrochemical boronizing at low temperatures.     

Mechanism of the electroreduction of 1,1-dicyanoethylene derivatives

The electroreduction behaviour of some substituted 1,1-dicyanoethylene derivatives in acetonitrile was investigated. The mechanism for the electroreduction of different substituted compounds was proposed using the results of cyclic voltammetry, rotating disc voltammetry, dc and differential pulse polarography and controlled potential electrolysis experiments. The effect of the presence of protons on the mechanism was also elucidated. The products of large scale electrolysis experiments were identified by the aid of electroanalytical and chromatographic methods.     

Kinetic study of the depolymerization of Bruceton coal

The depolymerization of coals using phenol and p-toluenesulphonic acid, was studied.In this reaction the acid ‘catalyst’ is destroyed, its rate of destruction being different for different coals. A kinetic study of the depolymerization of Bruceton coal under conditions of gradually declining acid concentration has been carried out. Parameters which were monitored include the weight increase of the products, the solubility of the products in benzene-ethanol, the extractability of the products into pyridine, the amount of colloidal material present in the pyridine-extractables, and the molecular weight distribution of the pyridine-soluble products. Under the conditions used, the reaction produces primarily polymeric products from coal. The molecular weight distribution of the pyridine-soluble material seems to undergo no major changes with reaction time.     

磷石膏脱水反应动力学研究

利用Netzsch STA 409PC型TG-DSC同步热分析仪,在升温速率分别为2.5,5.0,10,20℃/min的条件下对磷石膏脱水进行测试,研究其动力学和反应机理。结果表明,磷石膏脱水分为2步进行,产物分别是CaSO4.0.5H2O和CaSO4。2步脱水都遵循Avrami-Erofeev方程,脱水速率受晶核的形成及生长控制,活化能分别为97.67 kJ/mol和92.63 kJ/mol。利用得到的动力学方程对磷石膏在不同温度下的脱水过程进行模拟计算,结合工业上采用磷石膏制备建筑石膏的工艺和控制问题进行了讨论和分析。     

Kinetic investigations of the deactivation by coking of a noble metal catalyst in the catalytic hydrogenation of nitrobenzene using a catalytic wall reactor

The vapour phase hydrogenation of nitrobenzene to aniline is a highly exothermic reaction deactivated by coking of the palladium catalyst supported on -alumina carrier. For studying the deactivation of the catalyst a catalytic wall reactor was used in order to ensure isothermal reaction conditions for the kinetic measurements. Furthermore, the catalytic wall reactor allowed the determination of axial coke profiles by total carbon analysis of different wall segments. On the assumption that the main reaction and the deactivation of the catalyst can be assumed separable both the steady state and the unsteady state kinetics were studied. Nitrobenzene was identified as the relevant coke precursor whereas aniline has neither an influence on the main reaction nor on the deactivation. It could be shown that the hydrogenation of nitrobenzene to aniline follows a Langmuir–Hinshelwood mechanism considering the surface reaction of the adsorbed nitrobenzene molecule and one adsorbed hydrogen atom as the rate determining step. The differentiation of coke on the active sites and coke on the support must be taken into account to model the kinetics of coke formation with sufficient accuracy.     

厌氧折流板反应器处理硝基苯废水的研究

采用厌氧折流板反应器(ABR)中温处理硝基苯废水,研究了工艺条件和硝基苯的降解特点。试验结果表明：在进水COD．质量浓度为2088mg／L,硝基苯质量浓度为16．8mg／L,水力停留时间(HRT)为24h条件下,ABR能有效地处理硝基苯废水,COD去除率为86．4％,硝基苯去除率为91．1％;在厌氧条件下,硝基苯降解为苯胺,但苯胺很难再进一步分解;硝基苯的去除历程推断为先吸附后分解。     

Dynamic aspects of the electroreduction of chromic acid solutions

Potentiodynamic and impedance spectroscopy measurements were performed on ARMCO iron, steels (C30, C40, C50), cast iron and graphite electrodes in a bath of industrial composition: CrO₃ 250 g dm^–3, H₂SO₄ 2.5 g dm^–3. Attention focused on the electrochemical reactions occurring before the start of the chromium deposition. The electrochemical evidence is markedly affected by the chemical nature of the cathode. This is rationalized by proposing a reaction mechanism involving the formation, depending on the chemical nature of the substrate, of a passive adsorbed film consisting of mixed chromium and iron oxides. Moreover, the proposed overall scheme accounts for the presence of periodic current oscillations observed both in potentiodynamic and potentiostatic curves. In close agreement, impedance spectra show negative loops when measured at the corresponding potentials.     

Homogeneous redox catalysis of the electroreduction of benzyl chloride

The reactions between benzyl chloride and radical anions derived from anthracene and 9,10-diphenylanthracene were studied by cyclic voltammetry and controlled-potential electrolyses in N,N-dimethylformamide solutions, for several concentrations of the aromatic hydrocarbons. The controlled-potential electrolyses for direct and catalytic homogeneous reduction of benzyl chloride on mercury showed toluene as being the main product in both cases. These results were used in the formulation of the overall catalytic mechanisms. The application of a new theoretical treatment for the cyclic voltammetric results, developed recently by Savéant and coworkers, allowed the establishment of the rate determining step and the calculation of the rate constant for the homogeneous electron transfer reaction between the anthracene radical anion and benzyl chloride. This resulted in a value of 1.8 × 10⁴ M^?1 s^?1. For 9,10-diphenylanthracene it was established the existence of a mixed kinetic control under normal experimental conditions. Using very low initial concentrations of the depolarizer it was possible to obtain for the rate constant the value of 5.0 × 10³ M^?1 s^?1.     

Kinetic study of the acetate-catalysed hydrolysis of phenyl acetate

The results of an investigation aimed at obtaining the kinetic characteristics of the hydrolysis reaction of phenyl acetate catalysed by sodium acetate are presented. The hydrolytic reaction giving phenol and acetic acid starting from phenyl acetate is particularly suitable to run in a microwave-irradiated environment. In fact, since the considered reaction is characterised by a very low rate, small variations occurring in an irradiated system can be detected. Furthermore, since this reaction runs in an aqueous environment, it can use the energy transmitted by microwave radiation efficiently, thanks to the high capacity of water absorbance at microwave frequencies. The reaction was run in a buffered aqueous solution of sodium acetate and acetic acid at equal concentrations (pH=5). Kinetic constants were obtained in the 293–318 K temperature interval and at a concentration of both sodium acetate and acetic acid in the 0.4–0.06 mol dm⁻³ range. The derived activation energies decreased as the concentration of the sodium acetate/acetic acid increased. The kinetic study of the hydrolysis was carried out to obtain experimental data suitable for a fast comparison with the data that will be obtained from the development of this reaction in a microwave-irradiated environment. This paper also presents the analytical and experimental techniques suitable to run the experimental procedure in a microwave-irradiated reactor.     

Kinetic study of urease-catalysed urea hydrolysis

The kinetics of the urease-catalysed hydrolysis of urea in phosphate and citrate buffers have been studied. The effects of urease concentration, substrate concentration, pH and temperature of solutions, and particularly the concentrations of buffers and product on the reaction rate of urea hydrolysis have been examined. The activation energies in two buffer systems indicate that the reaction mechanism catalysed by urease may be the same in each case. A substrate inhibition model has been developed that describes the effect of substrate concentration ranging from 0.001 to 4 mol dm^?3. The experimental data indicated that the inhibition of urease by phosphate is of partially mixed type, whilst that by citrate is uncompetitive. In contrast ammonium ions exhibited no significant influence on the rate of urea hydrolysis by urease.     

Kinetic study of the polycondensation of diisocyanates with polyols

On the basis of kinetic studies of noncatalyzed synthesis of carbamates in chlorobenzene solution, a kinetic model of polyurethane synthesis has been proposed. The model was built on a set of elementary, sequential—parallel reactions between functional groups of the main reagents involved in polyurethane synthesis. The assumption that reactivity of reagents did not depend of their mol wt allowed simulation of the kinetics of formation of allophanate branches for various process parameters. It has been found that the process temperature and initial ratio of reagents are of primary importance. A set of equations, describing the synthesis of linear polyurethanes, has been derived. The equations afford calculation of the degree of polymer chain branching and their application yields results similar to those obtained by the more general method based on numerical solution of the differential equations involved. Advantages resulting from the mathematical modeling of the process kinetics have been pointed out. © 1992 John Wiley & Sons, Inc.     

Kinetic study of photoinitiated frontal polymerization

The frontal polymerization of a monomer exposed continuously to UV radiation in the presence of a photobleachable initiator has been studied on the basis of the rate equations. The time dependence of photoinitiator concentration profile and that of monomer conversion within the irradiated sample have been calculated, assuming steady‐state conditions. It is thus possible to visualize how fast the polymerization profile is moving within the sample and how effectively the polymerization of a monomer layer located at a given distance from the surface proceeds upon exposure to UV radiation. © 2001 Society of Chemical Industry