Presence Of Carbon Monoxide Research Articles

Heterobimetallic Cobalt/Rhodium Nanoparticle-Catalyzed Carbonylative Cycloaddition of 2-Alkynylanilines to Oxindoles

The cobalt-rhodium heterobimetallic nanoparticle-catalyzed synthesis of oxindoles from 2-alkynylanilines in the presence of carbon monoxide is described.

Folding Mechanism of Reduced Cytochrome c: Equilibrium and Kinetic Properties in the Presence of Carbon Monoxide

Despite close structural similarity, the ferric and ferrous forms of cytochrome c differ greatly in terms of their ligand binding properties, stability, folding, and dynamics. The reduced heme iron binds diatomic ligands such as CO only under destabilizing conditions that promote weakening or disruption of native methionine–iron linkage. This makes CO a useful conformational probe for detecting partially structured states that cannot be observed in the absence of endogenous ligands. Heme absorbance, circular dichroism, and NMR were used to characterize the denaturant-induced unfolding equilibrium of ferrocytochrome c in the presence and in the absence of CO. In addition to the native state (N), which does not bind CO, and the unfolded CO complex (U-CO), a structurally distinct CO-bound form (M-CO) accumulates to high levels (∼ 75% of the population) at intermediate guanidine HCl concentrations. Comparison of the unfolding transitions for different conformational probes reveals that M-CO is a compact state containing a native-like helical core and regions of local disorder in the segment containing the native Met80 ligand and adjacent loops. Kinetic measurements of CO binding and dissociation under native, partially denaturing, and fully unfolded conditions indicate that a state M that is structurally analogous to M-CO is populated even in the absence of CO. The binding energy of the CO ligand lowers the free energy of this high-energy state to such an extent that it accumulates even under mildly denaturing equilibrium conditions. The thermodynamic and kinetic parameters obtained in this study provide a fully self-consistent description of the linked unfolding/CO binding equilibria of reduced cytochrome c.

Stereoselective Synthesis of 4‐Dehydroxydiversonol Employing Enantioselective Palladium‐Catalysed Domino Reactions

The stereoselective synthesis of 4-dehydroxydiversonol (4) employing enantioselective palladium-catalysed domino processes such as the domino Wacker-Heck and the domino Wacker-carbonylation reaction for the formation of the central chroman moiety is described. Thus, reaction of 8 with palladium(II) trifluoroacetate [Pd(OTFA)2] in the presence of carbon monoxide, methanol and the 2,2'- bis(oxazolin-2-yl)-1,1'-binaphthyl (BOXAX) ligand 17 led to 19 in 80% yield and 96% ee. Similarly, the chroman 7 was prepared using 8 and methyl acrylate (9) as starting material. Hydrogenation of the double bond, oxidation of the benzylic methylene group and intramolecular acylation of chromanone 6 provided the tetrahydroxanthenone core 5, from which the synthesis of 4 was completed. The relative configuration of 4 could be established by crystal structure analysis.

Transient analysis of carbon monoxide poisoning and oxygen bleeding in a PEM fuel cell anode catalyst layer

The presence of carbon monoxide in the fuel stream hinders the performance of a polymer electrolyte membrane (PEM) fuel cell, known as carbon monoxide (CO) poisoning. Introducing oxygen in the fuel stream lessens CO poisoning. Since CO poisoning is a phenomenon that occurs over a substantial period of time, a transient model has been developed in this study, taking into account the effect of CO concentration, operating pressure and temperature, as well as oxygen bleeding on the performance of the cell. It is found that at a lower CO concentration the poisoning effect takes a much longer time to reach the steady state, even though for a better steady state anode performance. A higher operating temperature results in a better steady state performance, but the performance drops faster toward the steady state value at higher temperature. A higher operating pressure leads to an enhanced performance over the entire transient history, although the benefit diminishes as pressure is increased. Even with a small amount of oxygen (0.5%) introduced into the fuel stream, the anode performance can be improved significantly. Finally, it is observed that the use of pure hydrogen interspersed in carbon monoxide containing fuel improves the anode performance. However, performance recovery when operating on pure hydrogen is much slower than the performance degradation due to the CO poisoning.

Cytochrome b5 Inhibits Electron Transfer from NADPH-Cytochrome P450 Reductase to Ferric Cytochrome P450 2B4

Experiments demonstrating that cytochrome (cyt) b5 inhibits the activity of cytochrome P450 2B4 (cyt P450 2B4) at higher concentrations suggested that cyt b5 was occupying the cyt P450 reductase-binding site on cyt P450 2B4 and preventing the reduction of ferric cyt P450 (Zhang, H., Im, S.-C., and Waskell, L. (2007) J. Biol. Chem. 282, 29766-29776). In this work experiments were undertaken with manganese-containing cyt b5 (Mn-cyt b5) to test this hypothesis. Because Mn-cyt b5 does not undergo oxidation state changes under our experimental conditions, interpretation of the experimental results was unambiguous. The rate of electron transfer from cyt P450 reductase to ferric cyt P450 2B4 was decreased by Mn-cyt b5 in a concentration-dependent manner. Moreover, reduction of cyt P450 2B4 by cyt P450 reductase was incomplete in the presence of Mn-cyt b5. At a Mn-cyt b(5):cyt P450 2B4:cyt P450 reductase molar ratio of 5:1:1, the rate of reduction of ferric cyt P450 was decreased by 10-fold, and only 30% of the cyt P450 was reduced, whereas 70% remained oxidized. It could be demonstrated that Mn-cyt b5 had its effect by acting on cyt P450, not the reductase, because the reduction of cyt c by cyt P450 reductase in the presence of Mn-cyt b5 was not effected. Furthermore, under steady-state conditions in the cyt P450 reconstituted system, Mn-cyt b5, which lacks the ability to reduce oxyferrous cyt P450 2B4, was unable to stimulate the activity of cyt P450. Mn-cyt b5 only inhibited the cyt P450 2B4 activity. In conjunction with site-directed mutagenesis studies and experiments that strongly suggested that cyt b5 competed with cyt P450 reductase for binding to cyt P450, the current investigation demonstrates unequivocally that cyt b5 inhibits the activity of cyt P450 2B4 by preventing cyt P450 reductase from binding to cyt P450, a prerequisite for electron transfer from cyt P450 reductase to cyt P450 and catalysis.

Solvent-Free Synthesis of Urea Derivatives from Primary Amines and Sulfur under Carbon Monoxide and Oxygen at Atmospheric Pressure

A solvent-free carbonylation and oxidation system aimed at green and sustainable chemistry was developed. With these reactions, an environmentally benign synthesis of urea derivatives could be carried out in good to excellent yields from primary amines and sulfur at ambient pressure of carbon monoxide and oxy-gen. For example, N, N′-dioctylurea was prepared in 99% yield from two equivalents octylamine and one equivalent sulfur in the presence of carbon monoxide (1 atm) at 80 °C and oxygen (1 atm) at room temperature, in the complete absence of solvent.

One-pot syntheses of alcohols from olefins through Co/Ru tandem catalysis

The one-pot synthesis of cyclohexylmethanol from cyclohexene has been realized using a tandem catalytic system formed by the Co 2(CO) 8 complex for hydroformylation and different [Ru(CO) 2(PPh 3)X 2] 2 (X: Cl, Br, I) complexes for hydrogenation. The sole ruthenium(II) complexes [Ru(CO) 2(PPh 3)X 2] 2 (X: Cl, Br, I) are also able to synthesize cyclohexylmethanol from cyclohexene but a large part of the olefin is hydrogenated to alkane. Furthermore the sole ruthenium complexes are able to hydrogenate aldehydes to the corresponding alcohols even in the presence of carbon monoxide. The tandem catalytic system (Co-Ru) allows for the syntheses of several primary alcohols from the corresponding olefins with a total conversion and a very high selectivity (up to 97%).

A topographic view of the Pt(1 1 1) surface at the electrochemical interface in the presence of carbon monoxide

In this communication we present topographic images of the Pt(1 1 1) surface in CO saturated 0.1 M HClO 4, obtained by scanning tunneling microscopy. The topography presents two different structures, depending on the CO adsorption potential ( E ad = 0.15 V or E ad = 0.5 V vs RHE). For adsorption at 0.15 V the system presents a heterogeneous appearance, which totally covers the surface and impedes the observation of steps on the substrate surface. When CO is adsorbed at 0.5 V large clusters forming chains along the steps are observed. These aggregates can be, tentatively, correlated with the H-bonded water structure suggested earlier on the basis of FTIR spectroscopy. The clusters have inhibitory effects on CO oxidation.

Ronald Estabrook's Early Guidance of a Postdoctoral Fellow Concerning the Intricacies of Steroid Metabolism by Cytochromes P450

Ronald Estabrook made his initial impact studying cytochrome P450 by demonstrating the oxidative metabolism function of this unique class of enzymes, which had an unusual spectral peak at 450 nm when reduced and in the presence of carbon monoxide. Utilizing a photochemical action spectrum, he demonstrated that a cytochrome P450 was responsible for steroid 21 hydroxylation catalyzed by microsomes prepared from adrenal cortex tissue. As a young postdoctoral student, I was given the unique opportunity to learn from a true pioneer in this field. Ron had a surprisingly small laboratory at that time that allowed me to closely interact with a great scientist to learn about the important role cytochrome P450 proteins play in a wide variety of different organisms catalyzing oxidative metabolism reactions essential to life and to provide organisms, with the means to defend against xenobiotics.

Evaluation of the dioxin and furan formation thermodynamics in combustion processes of urban solid wastes

Specific combustion programs (Gaseq, Chemical equilibria in perfect gases, Chris Morley) are used to model dioxin and formation in the incineration processes of urban solid wastes. Thanks to these programs, it is possible to establish correlations with the formation mechanisms postulated in li- terature on the subject. It was found that minimum oxygen quantities are required to obtain a significant formation of these compounds and that more furans than dioxins are formed. Likewise, dioxin and furan formation is related to the presence of carbon monoxide, and dioxin and furan distribution among its different compounds depends on the chlorine and hydrogen relative composition. This is due to the fact that an increased chlorine availability leads to the formation of compounds bearing a higher chlorine concentration (penta-, hexa-, hepta-, and octachlorides), whereas an increased hydrogen availability leads to the formation of compounds bearing a lower chlorine number (mono, di-, tri-, and tetrachlorides)

Preferential catalytic oxidation of carbon monoxide in presence of hydrogen over bimetallic AuPt supported on zeolite catalysts

A series of AuPt/A zeolite and Pt/A zeolite catalysts prepared by incipient wetness impregnation are investigated for the preferential oxidation (PROX) of carbon monoxide in the presence of hydrogen over the temperature range of 50–310 °C under atmospheric pressure. The results indicate that when a small amount of gold is added to the Pt/A zeolite catalyst, the CO selectivity is improved at low temperatures, and 1% AuPt/A zeolite (at a weight ratio of Au:Pt = 1:2) gives the best performance, which provides a high CO conversion in combination with a high CO selectivity. In more realistic simulated reformate gases containing 10% CO 2 and 10% H 2O, there is not much difference between those in the presence and the absence of CO 2 and H 2O. Transmission electron microscopic and X-ray diffraction studies show that the two metals, Au and Pt, appear to be severely phase separated, which is confirmed by energy dispersive investigations.

Simplified models for SnO 2 sensors during chemical and thermal transients in mixtures of inert, oxidizing and reducing gases

In this work, a set of models able to predict the behavior of tin oxide (SnO 2) sensors during chemical and thermal transients is presented. These models are obtained through simplification of more complex physicochemical ones, taking into account both the intrinsic behavior of the sensors, observable during the exposure to an inert gas, and the extrinsic one, responsible for the sensor response in the presence of oxidizing and reducing gases. In particular, three simple models were developed, one for the intrinsic behavior, one for operation in oxygen (O 2), and one for operation in the presence of carbon monoxide (CO) and an inert gas. Finally, a complete model for operations in mixtures of CO and O 2 was derived from these latter.

Impedance Analysis of Working PEMFCs in the Presence of Carbon Monoxide

A three-electrode membrane electrode assembly including anode, cathode, and pseudoreference electrode was used in a working proton exchange membrane fuel cell (PEMFC). The pseudoreference electrode was directly prepared onto a Nafion membrane by an impregnation-reduction method. The electrochemical behaviors of the anode and cathode were separately studied when this three-electrode cell performed. The CO tolerance of the fuel cell with various anode compositions of 0.12–3.33 was investigated with both polarization curves and ac impedance analysis. An equivalent circuit model was proposed to explain the impedance spectra for both the anode and the cathode. With the presence of CO, the overpotential due to CO poisoning on the anode was due to the increase of both charge-transfer resistance and ohmic resistance in the catalytic layer.

Properties of Selective Gas-Sensitive SnO2 ∕ RuO2 ∕ Pt Composition and Detection Mechanism

We present new gas-sensitive material that has been obtained by mixing the powders of tin dioxide (semiconductor) and ruthenium dioxide (metallic conduction). The powder mixture was doped with platinum by the impregnation method. The X-ray and scanning electron microscopy investigation has shown that this material is a three-phase composition. The resistance of material is much higher than pure . Applied as a gas-sensitive layer in thick-film sensors, the material presents high selectivity to methane in the presence of carbon monoxide. The three-phase composition reacts in a way similar to when applied as a catalyst in a test reaction of -butyl alcohol condensation. Considering the microstructure, numerous research studies, and literature, the authors suggest the following explanation of physicochemical effects appearing in this gas-sensitive material. The authors think that two potential barriers appear in this composition: the first barrier on the boundary of tin dioxide grains, the second one is Schottky’s barrier on the boundary of grain and grain. The appearance of Schottky’s barrier on boundary causes an increase in resistance of studied composition. The definite improvement in sensor selectivity is caused by specific catalytic properties of . The catalyst reveals high activity in the heterogeneous reaction of carbon monoxide conversion by water vapor. Analyzing the results of conducted research, the authors think the selectivity of resistive gas sensors may be significantly improved by applying multiphase gas-sensitive material.

Single-walled carbon nanotube synthesis using ferrocene and iron pentacarbonyl in a laminar flow reactor

A gas-phase process of single-walled carbon nanotube (SWCNT) formation, based on thermal decomposition of iron pentacarbonyl or ferrocene in the presence of carbon monoxide (CO), was investigated in ambient pressure laminar flow reactors in the temperature range of 600–1300 °C. Ferrocene was found to be a better catalyst precursor in the studied conditions since iron pentacarbonyl decomposes at lower temperatures resulting in the excessive growth of catalyst particles. In situ sampling carried out at 1000 °C showed that the SWCNT growth occurred from individual metal particles in the heating section of the furnace in the temperature range of 891–928 °C, in which the growth rate was estimated to exceed 2 μ m / s . FT-IR measurements of gaseous products revealed that the majority of the CO disproportionation took place on the reactor walls. Raman measurements confirmed the results of TEM observations, namely, the formation of very high purity SWCNT product. On-line aerosol number size distributions were measured to detect the conditions of SWCNT formation and the product morphology changes. Mechanism of SWCNT formation in the gas phase from ferrocene and CO is discussed.

Influence of Aprotic Solvent on Selectivity of an Amperometric Sensor with Nafion Membrane

AbstractThis paper presents the results of investigation on selectivity of the sulfur dioxide amperometric sensor with Nafion membrane in the presence of carbon monoxide and nitrogen dioxide as the interferents. There have been compared selectivity coefficients, for the sensors containing the following internal electrolytes: solution of sulfuric acid (concentration 5 mol dm−3) in pure water (A) and solution of sulfuric acid (concentration 5 mol dm−3) in mixed solvent dimethylsulfoxide‐water with an DMSO: H2O mole ratio of 1 : 2 (B). Values of the selectivity coefficients have been calculated based on the calibration curves. Analysis of both calibration curves and selectivity coefficients plays a significant role in optimization of a working point of a particular sensor. The investigated sensor operates in a three‐electrode system, where the working and counter electrodes are vacuum sublimation deposited on the membrane surface.

Performance of a poly(2,5-benzimidazole) membrane based high temperature PEM fuel cell in the presence of carbon monoxide

The performance of a poly(2,5-benzimidazole) (ABPBI) membrane based high temperature PEM fuel cell in presence of carbon monoxide, at various temperatures is reported here. The ABPBI was synthesized by polymerization of 3,4-diaminobenzoic acid in a polymerization medium containing methanesulfonic acid (CH 3SO 3H) and phosphorous pentoxide (P 2O 5). The ABPBI membranes were characterized by fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). A maximum conductivity of 0.026 S cm −1 at 180 °C was obtained for the membrane doped with 1.2 molecules of phosphoric acid (H 3PO 4) per polymer repeat unit. Fuel cell performance was evaluated using dry hydrogen/oxygen gases and was comparable with that reported in the literature. Performance of a single cell at different temperatures was studied with 0.48 and 1.0 vol.% of CO in the hydrogen fuel. The studies lead to the conclusion that CO poisoning is not a serious problem above 170 °C. Performance of the fuel cell operating at 210 °C is not at all affected by 1.0 vol.% of CO in the hydrogen feed.

Production, properties and application to biocatalysis of a novel extracellular alkaline phenol oxidase from the thermophilic fungus Scytalidium thermophilum

Scytalidium thermophilum produces an extracellular phenol oxidase on glucose-containing medium. Certain phenolic acids, specifically gallic acid and tannic acid, induce the expression of the enzyme. Production at 45 degrees C in batch cultures is growth-associated and is enhanced in the presence of 160 microM CuSO4 x 5 H2O and 3 mM gallic acid. The highest enzyme activity is observed at pH 7.5 and 65 degrees C, on catechol. When incubated for 1 h at pH 7 and pH 8, 95% and 86% of the activity is retained. Thermostability decreases gradually from 40 degrees C to 80 degrees C. Estimated molecular mass is c. 83 kDa, and pI is acidic at c. 5.4. Substrate specificity and inhibition analysis in culture supernatants suggest that the enzyme has unique properties showing activity towards catechol; 3,4-dihydroxy-L-phenylalanine (L-DOPA); 4-amino-N, N-diethylaniline (ADA); p-hydroquinone; gallic acid; tannic acid and caffeic acid, and no activity towards L-tyrosine, guaiacol, 2,2'-azino-bis(3-ethyl-benzthiazoline-6-sulphonic acid) (ABTS) and syringaldazine. Inhibition is observed in the presence of salicyl hydroxamic acid (SHAM) and p-coumaric acid. Enzyme activity is enhanced by cetyltrimethylammonium bromide (CTAB) and polyvinylpyrrolidone (PVP), and the organic solvents dimethyl sulfoxide (DMSO) and ethanol. No inhibition is observed in the presence of carbon monoxide. Benzoin, benzoyl benzoin and hydrobenzoin are converted into benzil, and stereoselective oxidation is observed on hydrobenzoin. The reported enzyme is novel due to its catalytic properties resembling mainly catechol oxidases, but displaying some features of laccases at the same time.

Hydrogenation of Acetone in Supercritical Water Using Formic Acid: Rapid Hydrogenation Observed at a Long Retention Time

To determine the capability of formic acid as a hydrogenating agent in supercritical water, we conducted hydrogenation of acetone using formic acid. It was found that the formic acid enabled hydrogenation of acetone to produce 2-propanol. We varied the concentration of formic acid available for hydrogenation by changing the retention time in the formic-acid preheater, prior to mixing with acetone. The highest yield of 2-propanol occurred when the concentration of formic acid was a maximum. The yield decreased with thermal decomposition of the formic acid, as the retention time in the preheater was increased. However, an unexpected and rapid increase in the 2-propanol yield was observed when the retention time in the preheater was further increased. We theorize that the hydrogenating agent responsible for this phenomenon is closely related with the presence of carbon monoxide although further work is required to elucidate the mechanism.

Selectivity in Cobalt Carbonyl Mediated Cycloaddition of Dienynes

Dicobalt octacarbonyl mediated cycloaddition of dienynes in the presence of carbon monoxide has been studied. Three main competing reaction routes, two carbonylative cycloaddition reactions and a Diels-Alder reaction, have been recognized depending upon the substrate and reaction conditions. Judicious design of the substrate and selection of reaction conditions allow control of the reaction pathway.