Kinetics and mechanisms of iron sulfide reductions in hydrogen and in carbon monoxide

The reduction of iron sulfides by hydrogen and by carbon monoxide has been studied using plug flow and thermogravimetric methods. The reactions were studied in the 523–723°K temperature range and were found to be first-order processes. Plug flow studies were used to correlate reaction rates between pyrite and the gases as a function of the surface area of the pyrite. The rate of H₂S formation increases with the surface area of the pyrite sample. The results of thermogravimetric experiments indicate that the reactions consist of several steps. Rate constants for the pyrite reduction by H₂ and by CO were obtained. The activation energies increased with degree of reduction. Values of E_a were 113.2 (step I) and 122.5 kJ/mole (step II) for pyrite reduction with CO and 99.4 (step I), 122.4 (step II), 125.2 (step III), and 142.6 kJ/mole (step IV) for pyrite reduction with hydrogen.     

Mechanistic aspects of reductions of allylic ethers and acetates with organocuprates

The rate of substitution to reduction has been investigated for reactions of three phenyl-substituted allylic ethers and the corresponding acetates with EtMgBr plus 10 or 25% copper(I) bromide in THF. It is found that the relative amount of reduction increases with increased electron delocalization in the postulated copper(III)-bound allyl ligand, and is also dependent on the nature of the leaving group; methoxy giving much more reduction product than acetoxy. Furthermore, for one acetate investigated there was more reduction at −65° than at −25°C. The results are interpreted in terms of relative binding strength of allyl ligands to a copper(III) intermediate.     

镁铁和镁铁铝催化剂氢还原过程的研究

以水滑石为前体 ,制备了镁铁和镁铁铝复合氧化物催化剂 ,运用原位穆斯堡尔谱研究了催化剂在H2 气氛中的还原行为。结果表明 :由于Mg、Al的加入和固溶体的形成 ,相对地稳定了FeO物相 ,阻碍了H2 对铁离子的还原 ,使得Fe2 进一步还原为金属Fe0 的能力减弱 ;在还原过程中催化剂首先生成含Fe2 的固溶体FeO MgO或FeO MgO Al2O3,然后再完全还原成金属Fe0。     

Uranyl-Bound Tetra-Dentate Non-Innocent Ligands: Prediction of Structure and Redox Behaviour Using Density Functional Theory

Computational methods have been applied to understand the reduction potentials of [UO₂-salmnt-L] complexes (L=pyridine, DMSO, DMF and TPPO), and their redox behavior is compared with previous experiments in dichloromethane solution. Since the experimental results were inconclusive regarding the influence of the uranyl-bound tetra-dentate ‘salmnt’ ligand, here we will show that salmnt acts as a redox-active ligand and exhibits non-innocent behavior to interfere with the otherwise expected one-electron metal (U) reduction. We have employed two approaches to determine the uranyl (VI/V) reduction potentials, using a direct study of one-electron reduction processes and an estimation of the overall reduction using isodesmic reactions. Hybrid density functional theory (DFT) methods were combined with the Conductor-like Polarizable Continuum Model (CPCM) to account for solvation effects. The computationally predicted one-electron reduction potentials for the range of [UO₂-salmnt-L] complexes are in excellent agreement with shoulder peaks (∼1.4 eV) observed in the cyclic voltammetry experiments and clearly correlate with ligand reduction. Highly conjugated pi-bonds stabilize the ligand based delocalized orbital relative to the localized U f-orbitals, and as a consequence, the ligand traps the incoming electron. A second reduction step results in metal U(VI) to U(V) reduction, in good agreement with the experimentally assigned uranyl (VI/V) reduction potentials.     

Automated high performance liquid chromatography with on-line reduction of disulfides and chemiluminescence detection for determination of thiols and disulfides in biological fluids

In general, the reduction of disulfide bonds with tris(2-carboxyethyl)phosphine (TCEP) is performed using off-line operation, which is not only time-consuming but also vulnerable to the spontaneous re-oxidation of thiols during sample preparation and subsequent analysis procedures. To the best of our knowledge, there has been not any case on the on-line reduction for biological disulfides coupled with high performance liquid chromatography (HPLC). In this study, these obstacles are overcome by packing Zn(II)-TCEP complexes into a home-made column. The as-synthesized Zn(II)-TCEP complexes enable efficient reduction of disulfide bonds at pH 3.0. This acidic pH value was compatible with that of the mobile phase for HPLC separation of thiols and disulfides. Therefore, using fluorosurfactant-prepared triangular gold nanoparticles as HPLC postcolumn specific chemiluminescence (CL) reagents for thiols, the feasibility of the established on-line reduction column has been confirmed for the direct identification of both thiols and disulfides by incorporating this reduction column into a single chromatographic separation. Detection limits for these analytes range from 8.3 to 25.4 nM and the linear range in a log–log plot can comprise three orders of magnitude. Finally, the utility of this automated on-line reduction of disulfides-HPLC-CL system has been demonstrated for the reliable determination of thiols and disulfides in human urine and plasma samples.     

还原方式及还原温度对甲烷部分氧化镍催化剂结构和反应性能的影响

采用H2 TPR、TEM及活性评价等手段,考察了还原方式（等温和程序升温还原）及还原温度对不同温度（550℃和950℃）焙烧制备的镍基催化剂结构和甲烷部分氧化反应性能的影响。结果表明,与程序升温还原方式相比,等温还原的催化剂中镍物种的还原度较低、Ni晶粒度较小。还原方式对550℃焙烧制备的催化剂（POM-1）的甲烷部分氧化反应性能影响不明显,但等温还原的催化剂反应过程中床层温度较低。随着等温还原温度的提高,POM-1催化剂的镍还原度有所降低,而950℃焙烧制备的催化剂（POM-5）还原度略有增加,且具有较小的镍晶粒。随着等温还原温度的提高,POM-1催化剂反应性能无明显差异,但床层热点温度提高;POM-5催化剂反应性能随还原温度的提高而提高,且床层温度呈现降低趋势。通过分析发现,催化剂床层温度与催化剂镍晶粒大小密切相关,较小的镍晶粒利于床层热点温度的降低,这与较大镍晶粒利于甲烷完全氧化反应有关。     

SAR Models for Futile Metabolism: One-Electron Reduction of Quinones,Phenols and Nitrobenzenes

Abstract

Benzoquinones, naphthoquinones and aziridinylbenzoquinones, can be reduced by flavoproteins to semiquinones that react with molecular oxygen to form superoxide anion with the subsequent regeneration of the parent compounds. This redox cycling, a form of futile metabolism, produces reactive oxygen species and depletes the reducing equivalents of cells without concomitant energy production. The ability of a toxicant to redox cycle is related to its one-electron reduction potential, and this study attempted to estimate reduction potential from structure using semi-empirical quantum chemical models for a diverse set of chemicals. The results of this study suggest that one-electron reduction potentials, within structural classes of benzoquinones, naphthoquinones, phenols and nitrobenzenes, can be estimated from local and global electronic indices that are related to delocalization. Smaller absolute charge on the carbonyl carbon in the quinone moiety correlated with more positive one-electron reduction potentials of 1,4-benzoquinones, naphthoquinones and two-electron reduction potentials of aziridinylbenzoquinones. The energy of frontier orbitals of the quinones, phenols and nitrobenzenes also co-varied with reduction potential. More positive reduction potentials of 1,4-benzoquinones, 1,4-naphthoquinones and phenols were correlated with more negative values of E_HOMO, while more negative values of E_LUMO were correlated with more positive potentials of nitrobenzenes and aziridinylbenzoquinones. Delocalization of electron density also correlated with reduction potentials within individual classes.     

Polarographic study of acridine and 9-chloroacridine and its derivatives in dimethylformamide

Polarographic data on the reduction of acridine (I) and its 9-chloro (II), 2-methoxy-9-chloro (III), 4-methoxy-9-chloro (IV), 2-methyl-9-chloro (V), 4-methyl-9-chloro (VI), 2,9-dichloro (VII), and 4,9-dichloro (VIII) derivatives and a mechanism for the reduction are presented. In dimethylformamide (DMF) the polarographic reduction of acridine I takes place in two steps with participation of one electron in each step and the intermediate formation of an anion radical. Acridine II is reduced in four steps: The second and fourth waves correspond to the reduction of the heteroring, while the first and third waves are associated with detachment of a chlorine atom. The addition of phenol as a proton donor facilitates the reduction but does not affect the number of electrons consumed by the depolarizer during its reduction. A linear correlation between E_1/2 and the corresponding constants of quinoline is observed for acridines II, III, V, and VII; this made it possible to calculate the previously unknown constants for acridines that contain a substituent in the 4 position. The E_1/2 ¹ values for acridine II and its derivatives correlate with the rate constants for chemical dehalogenation.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1660–1665, December, 1978.     

A novel electrochemical method for efficient reduction of disulfide bonds in peptides and proteins prior to MS detection

A novel electrochemical (EC) method for fast and efficient reduction of the disulfide bonds in proteins and peptides is presented. The method does not use any chemical agents and is purely instrumental. To demonstrate the performance of the EC reactor cell online with electrospray mass spectrometry, insulin and somatostatin were used as model compounds. Efficient reduction is achieved in continuous infusion mode using an EC reactor cell with a titanium-based working electrode. Under optimized conditions, the presented method shows almost complete reduction of insulin and somatostatin. The method does not require any special sample preparation, and the EC reactor cell makes it suitable for automation. Online EC reduction followed by collision-induced dissociation fragmentation of somatostatin showed more backbone cleavages and improved sequence coverage. By adjusting the settings, the EC reaction efficiency was gradually changed from partial to full disulfide bonds reduction in α-lactalbumin, and the expected shift in charge state distribution has been demonstrated. The reduction can be controlled by adjusting the square-wave pulse, flow rate or mobile phase composition. We have shown the successful use of an EC reactor cell for fast and efficient reduction of disulfide bonds for online mass spectrometry of proteins and peptides. The possibility of online and gradual disulfide bond reduction adds a unique dimension to characterization of disulfide bonds in mid- and top-down proteomics applications.

A Rapid Method for the Analysis of Diquat and Paraquat in Potatoes

Abstract

A rapid method was developed for the analysis of paraquat dication (I) and diquat dication (II) as their diene reduction products (ID and IID respectively) in potatoes. Macerated potato tissue was spiked with various levels of the dications, 0.05, 0.5, and 5.0 ppm. The dications were reduced using a sodium borohydride/ethanol reduction procedure with reduction and simultaneous extraction in a single 50 ml glass-stoppered centrifuge tube. The diene reduction products were extracted into hexane and analyzed by gas-liquid chromatography (GLC) and a N/P-thermionic detector. Quantification by external standard method or internal standard method resulted in recoveries of 35, 38, 48 and 39,42, 64% for diquat and paraquat respectively. When a spiked macerate was well-mixed, recoveries were low due to adsorption; precision was within 5 5.4% (standard deviation). The importance of spiking technique was also illustrated. Upon thorough mixing and equilibration, of diquat spiked potatoes, recoveries were reduced dramatically. Adsorption of the dications and diene reduction products to glassware reduced recoveries and accounted for poorer precision; this latter effect was overcome by prior silanization of the glassware. Modification of existing NaBH₄ methods resulted in a procedure for I and II analysis requiring only 30 minutes per sample for complete analysis. Use of NaBH₄ reduction and analysis of I in potatoes has not been previously reported.     

The Combination of Phthalocyanines and Polymers for Electrochemically or Photoelectrochemically Induced Processes

Phthalocyanines 1-3 were used in combinations with polymers in order to investigate reduction/reoxidation of thin film electrodes, photoinduced reduction of dioxygen at the junction of thin film electrodes to an electrolyte, and dioxygen reduction in the dark (fuel cell reaction) at dispersions of phthalocyanines on carbon supports. Electrochromic reduction/reoxidation of phthalocyanines also occurs readily inside a polymer matrix if intermolecular particle contact exists. A significant photoreduction of dioxygen is observed if O₂ can form trapped states inside of the bulk of the film, with the result of enhanced photoconductivity. The kind of polymer is of striking influence. Phthalocyanines with central metals forming localized ionized states with dioxygen are active electrocatalysts for dioxygen reduction in the dark if they are finely dispersed on a carrier. The influence of both phthalocyanine and polymer component in each of the three-mentioned processes is discussed.     

Electrochemical Measurement of Gold Oxide Reduction and Methods for Acid Neutralization and Minimization of Water in Wet Ionic Liquid

The formation and reduction of gold oxide in wet ionic liquid (IL), N‐trimethyl‐N‐butylammonium bis(trifluoromethanesulfonyl)imide, ([Me₃NⁿBu][TFSI]) is examined. The water concentration is determined using both the charge and peak current from the reduction of gold oxide and compared directly with Karl Fischer data. The quantitative determination of water in the IL is demonstrated for concentrations between 0.09 and 0.74 by weight (w%). The treatment of wet IL with dry nitrogen or molecular sieves reduces the water below background levels. Finally, methods for acid neutralization and the reduction of water with molecular sieves are conducted to minimize their impact on subsequent electrochemical measurements.     

Developments on carbon dioxide reduction: Their promise,achievements, and challenges

CO₂ reduction processes continue to be developed for electrosynthesis, energy storage applications, and environmental remediation. A number of promising materials have shown high activity and selectivity to target reduction products. However, the progress has been mainly at a small laboratory scale, and the technical challenges of large scale CO₂ reduction have not been considered adequately. This review covers recent advancements in catalyst materials and cell designs. The leading materials for CO₂ reduction to a number of useful products are presented with their corresponding cell and reactor designs. The latest efforts to progress to industrially relevant scales are discussed, along with the challenges that must be met for carbon dioxide reduction to be a viable route for mass scale production.     

Effect of polycarbonate structure and reduction time on graphene oxide dispersion

Polycarbonate (PC)/graphene oxide (GO) composites with different GO reduction time and PC types were prepared by using a twin screw extruder at 260 °C after solution mixing with chloroform. The chemical reaction degree of PC/GO composites with GO reduction time was confirmed by C–H stretching peak at 3000 cm ^?1, and the chemical reaction degree decreased with GO reduction time. The slope for storage (G′) versus loss (G″) modulus plot decreases with an increase in heterogeneous property of the polymer melts. So we can check the GO dispersion of the PC/GO composites using by the slop for G′–G″ plot. According to the G′–G″ slopes for PC/GO composite with GO reduction time, GO was well dispersed within PC matrix when the reduction time decreased. It was re‐confirmed by atomic force microscope (AFM) results. Based on the degradation temperature by Thermogravimetric analysis, G′–G″ slopes, and surface roughness by AFM, the branched PC was better than linear PC for the GO dispersion within PC matrix. The fact was also confirmed by tensile test results that the Young's modulus increased with the improvement of GO dispersion. Copyright © 2015 John Wiley & Sons, Ltd.     

Electrochemical reduction of 5,5′-dichlorohydurilic acid. mechanism and analytical applications

The electrochemical reduction of 5,5′-dichlorohydurilic acid has been studied at the dropping mercury electrode (DME) and the pyrolytic graphite electrode (PGE). At the DME the single polarographic reduction wave observed at pH 6–11 involves a direct 4e—2H⁺ reduction of the carbon-halogen bond to give hydurilic acid and chloride. The state of hydration or ionization of the 5,5′-dichlorohydurilic acid has no effect on the electrochemical reaction. At the PGE, 5,5′-dichlorohydurilic acid shows two voltammetric peaks. Peak I_c, observed between pH 5 and 7, arises from an overall 4e—2H⁺ reduction of 5,5′-dichlorohydurilic acid via a mechanism that involves initial electron attack at a carbonyl group alpha to a carbon-halogen bond with simultaneous elimination of chloride ion. The peak II_c process involves an initial 2e—1H⁺ reduction of a partially hydrated form of 5,5′-dichlorohydurilic acid with only one unhydrated halocarbonyl moiety available for reaction. Attack is again via the carbonyl group with simultaneous elimination of chloride and formation of 5-chlorohydurilic acid. A chemical dehydration step then occurs with a rate constant of ca. 0.24 s^?1 at pH 8.2, with formation of a further reducible halocarbonyl group. This is again reduced in an overall 2e—2H⁺ reaction to give hydurilic acid and chloride ion. The peak II_c process hence proceeds via an ECE mechanism. The different mechanisms observed for reduction of 5,5′-dichlorohydurilic acid at mercury and pyrolytic graphite electrodes are unusual. Analytical methods have been developed for the polarographic determination of 5,5′-dichlorohydurilic acid via its reduction wave at the DME, and for the voltammetric determination of hydurilic acid via its first oxidation peak at the PGE.     

Reduction of oxygen and hydrogen peroxide on electrodes with adsorbed monolayer of aliphatic compounds

Cathodic reduction of oxygen and hydrogen peroxide on amalgamated platinum electrodes, which are coated with monolayers of long-chain aliphatic compounds cetyl alcohol (CA) and stearic acid (SA), is retarded as compared with the same reactions on clean mercury (or amalgam) surface. The oxygen reduction kinetics differ from that on mercury. The difference is explained by that oxygen diffuses into the monolayer and is reduced in it at a certain distance from the metal surface and only at the limiting current the reaction is forced onto the monolayer surface. In contrast to the oxygen reduction, the hydrogen peroxide reduction kinetics on electrodes with SA and CA monolayers is much closer to that on mercury, but with some quantitative distinctions. All results favor the H₂O₂ reduction at the monolayer/solution interface. The difference in the behavior of O₂ and H₂O₂ is explained by different polarity of these molecules: it is significantly more difficult to penetrate the hydrocarbon monolayer for polar H₂O₂ molecule than for nonpolar O₂ molecule.     

Stereoselective synthesis of (+)-boronolide and (−)-5-epi-boronolide

Stereoselective synthesis of boronolide and 5-epi-boronolide was achieved from d-(−)-tartaric acid. The key step involves the reduction of a keto Weinreb amide for the synthesis of boronolide, and a single pot construction of a diketone from the bis-Weinreb amide of tartaric acid and subsequent reduction with L-Selectride for 5-epi-boronolide.     

Reduction of Methylene Green by EDTA:Kinetic and Thermodynamic Aspects

The reduction of methylene green (MG) into protonated leuco dye with ethylenediamine tetraacetic acid (EDTA) in aqueous alkaline medium was studied spectrophotometrically at λ_max 660 nm. EDTA behaved as an effective electron donor during the reduction of MG in an aerobic condition. Consumption of EDTA in the reduction of MG means that it is oxidized. This is an unexpected result since EDTA does not normally function as a reducing agent. The nitrogen-containing chelating agents with secondary or tertiary nitrogen behaved as an electron donor in photochemical reaction of dye. The rate of reduction depends upon pH in the same way as the base titration of EDTA. Effects of salt and temperature have been investigated for the reduction process. The salting agent KNO₃ has been found to uniquely enhance the rate of reduction of MG by EDTA in the aerobic condition. Detailed kinetic and thermodynamic aspects have been discussed to realize the interaction between MG and EDTA. Kinetic studies revealed that reaction was sensitive and regeneration of oxidized form of the dye was observed. Reversible first order reaction kinetics with respect to EDTA, MG and NaOH was found.     

Reduction of MnFe2O4 without and with carbon

We succeeded in studying the mechanism of hydrogen added carbothermic reduction process of iron-manganese oxide by means of the new technique, simultaneous measurement of evolved gas analysis (EGA) and humidity sensor (HS). Water vapor evolved by the reduction with hydrogen can be detected by HS. Other gas was detected by TCD. Without carbon, the hydrogen reduction process was followed to the formation of the intermediate product between MnO and FeO and finally reduction to the mixture of MnO and Fe. With carbon, the intermediate products between MnO and FeO was formed at about 780 K. The methane was formed in higher temperature than 1073 K and the reduction with carbon proceeded mainly. At higher temperatures, methane decomposed to yield nascent carbon that tended to result in the acceleration of the reduction rate with carbon. The study is concerned with the mechanism of the hydrogen reduction of MnFe₂O₄ and the effect of without and with carbon on this reduction by means of combining EGA and HS. This revised version was published online in August 2006 with corrections to the Cover Date.     

Synthesis and electrochemistry of cis-dioxomolybdenum(VI) complexes with tridentate Schiff base ligands containing O,N and S donor atoms

The synthesis and characterization of a number of cis-dioxomolybdenum(VI) coordination complexes involving tridentate (ONS) ligands is described. The Schiff base ligands were obtained by condensation of 5-substituted salicylaldehydes with o-aminobenzenethiol or 2-aminoethanethiol. The chemical properties of these molybdenum complexes are compared with those having tridentate ligands with the ONO donor atom set. Cyclic voltammetry was used to obtain cathodic reduction potentials (E_pc) for the irreversible reduction of the Mo(VI) complexes. Although the reductions are irreversible, trends are observed in E_pc both within each series and when different series are compared. Cathodic reduction potentials for the four series examined span the range from ?1.53 to ?1.05 V versus NHE. There are three ligand features whose effect systematically alters the Mo(VI) cathodic reduction potentials. These include (1) the X-substituent on the salicylaldehyde portion of each ligand; (2) the degree of ligand delocalization; and (3) the substitution of a sulphur donor atom for an oxygen donor atom. Each of these effects is considered separately with regard to the Mo(VI) cathodic reduction potentials and then their cumulative effect is described.