Hemin-mediated oxidation of dithiothreitol reduces oxygen to H2O

Hemin catalyses the oxidation of dithiothreitol. One mole of oxygen is consumed for every 2 moles of dithiothreitol oxidized and the product is shown by spectral studies to be the intramolecular disulphide. The reaction shows a specificity for dithiol and for free heme moieties. Hemin molecules exhibit cooperativity in oxygen reduction. Oxygen radicals do not seem to be involved. H2O2 is not required for this oxidation of dithiothreitol and does not appear to be an intermediate in the reduction of O2 to H2O. However, an independent minor reaction involving a 2-electron transfer with the formation of H2O2 also occurs. These studies on the hemin-catalyzed oxidation of dithiothreitol provide a chemical model for a direct 4-electron reduction of O2 to H2O.

Pd Supported on Titanium Nitride for Efficient Ethanol Oxidation

The excellent metal support interaction between palladium (Pd) and titanium nitride (TiN) is exploited in designing an efficient anode material. Pd-TN, that could be useful for direct ethanol fuel cell in alkaline media. The physicochemical and electrochemical characterization of the Pd-TiN/electrolyte interface reveals an efficient oxidation of ethanol coupled with excellent stability of the catalyst under electrochemical conditions. Characterization of the interface using in situ Fourier transform infrared spectroscopy (in situ FITR) shows the production CO2 at low overvoltages revealing an efficient cleaving of the C-C bond. The performance comparison of Pd supported on TiN (Pd-TiN) with that supported on carbon (Pd-C) clearly demonstrates the advantages of TiN support over carbon. A positive chemical shift of Pd (3d) binding energy confirms the existence of metal support interaction between pd and TiN, which in turn helps weaken the Pd-CO synergetic bonding interaction. The remarkable ability of TiN to accumulate -OH species on its surface coupled with the strong adhesion of Pd makes TiN an active support material for electrocatalysts.

Relation between effective atomic charge and chemical shifts in X-ray absorption spectra of transition-metal compounds

Chemical shifts of K absorption discontinuities, Delta E, of several manganese, iron and cobalt oxides with the metal in the formal oxidation states between +2 and +4, have been measured. These data, together with data in the literature on other compounds of these metals, can be fitted into the expression Delta E=aq+bq2, where q is the effective atomic charge on the metal. Theoretical considerations also support this functional relationship between Delta E and q.

Chemical Characterization of Urban Background Aerosol Using Online And Filter Methods

In order to evaluate the influence of ambient aerosol particles on cloud formation, climate and human health, detailed information about the concentration and composition of ambient aerosol particles is needed. The dura-tion of aerosol formation, growth and removal processes in the atmosphere range from minutes to hours, which highlights the need for high-time-resolution data in order to understand the underlying processes. This thesis focuses on characterization of ambient levels, size distributions and sources of water-soluble organic carbon (WSOC) in ambient aerosols. The results show that in the location of this study typically 50-60 % of organic carbon in fine particles is water-soluble. The amount of WSOC was observed to increase as aerosols age, likely due to further oxidation of organic compounds. In the boreal region the main sources of WSOC were biomass burning during the winter and secondary aerosol formation during the summer. WSOC was mainly attributed to a fine particle mode between 0.1 - 1 μm, although different size distributions were measured for different sources. The WSOC concentrations and size distributions had a clear seasonal variation. Another main focus of this thesis was to test and further develop the high-time-resolution methods for chemical characterization of ambient aerosol particles. The concentrations of the main chemical components (ions, OC, EC) of ambient aerosol particles were measured online during a year-long intensive measurement campaign conducted on the SMEAR III station in Southern Finland. The results were compared to the results of traditional filter collections in order to study sampling artifacts and limitations related to each method. To achieve better a time resolution for the WSOC and ion measurements, a particle-into-liquid sampler (PILS) was coupled with a total organic carbon analyzer (TOC) and two ion chromatographs (IC). The PILS-TOC-IC provided important data about diurnal variations and short-time plumes, which cannot be resolved from the filter samples. In summary, the measurements made for this thesis provide new information on the concentrations, size distribu-tions and sources of WSOC in ambient aerosol particles in the boreal region. The analytical and collection me-thods needed for the online characterization of aerosol chemical composition were further developed in order to provide more reliable high-time-resolution measurements.

High-Surface Step Density on Dendritic Pd Leads to Exceptional Catalytic Activity for Formic Acid Oxidation

Dendrite Pd with corrugated surfaces, obtained by a novel AC technique, exhibits an exceptionally high catalytic activity for the oxidation of formic acid because of the presence of a high density of surface steps. The formation of twinned dendrites leads to a predominance of exposed 111 facets with a high density of surface steps as evident from high resolution electron microscopy investigations. These surface sites provide active sites for the absorption of the formic acid molecules, thereby enhancing the reaction rate. Control experiments by varying the time of deposition reveal the formation of partially grown dendrites at shorter times indicating that the dendrites were formed by growth rather than particle attachment. Our deposition method opens up interesting possibilities to produce artisotropic nanostructures with corrugated surfaces by exploiting the perturbations involved in the growth process.

Chemical Shift of the X-Ray K-Absorption Edge of Cu in Some of Its Compounds, Complexes and Superconductors

Chemical shifts, ΔE, of the X-ray K-absorption edge in several compounds, complexes of copper including its superconducting oxides possessing formal oxidation states +1 and +2 have been measured. It has been shown that the chemical shift is primarily governed by the effective ionic charge on the absorbing ion and the nature of the atoms in the first coordination shell around the absorbing ion. The relation between the chemical shift, ΔE , and the effective charge q on the absorbing ion is found to be ΔE=Aq+Bq2+Cq3+Dq4 (A, B, C and D are constants). The effects of electronegativity, atomic number, oxidation state, crystal structure, the valence d-orbital electrons, etc. on the X-ray absorption chemical shift have been discussed. ©1990 The Physical Society of Japan

Modelling of Fe2 + oxidation by Thiobacillus ferrooxidans

The kinetics of oxidation of aqueous acidic ferrous sulphate by Thiobacillus ferrooxidans has been studied in a batch reactor. The contribution of cell wall envelopes to the oxidation rate has been shown to be negligible. A model which accounts for the oxidation of Fe2 +, death of bacteria due to Fe3 + poisoning, existence of an optimal pH and precipitation of Fe3 + has been proposed. The model is able to predict the concentration of Fe2 + and pH quite satisfactorily. The predictions of Fe3 + are not so accurate because of simplifying assumptions made about its precipitation.

Design and fabrication of an automated thermal desorption gas-solid reaction system: Oxidation of ammonia and methanol over YBa2Cu3O7−δ(123) oxide systems

A fully automated, versatile Temperature Programmed Desorption (TDP), Temperature Programmed Reaction (TPR) and Evolved Gas Analysis (EGA) system has been designed and fabricated. The system consists of a micro-reactor which can be evacuated to 10−6 torr and can be heated from 30 to 750°C at a rate of 5 to 30°C per minute. The gas evolved from the reactor is analysed by a quadrupole mass spectrometer (1–300 amu). Data on each of the mass scans and the temperature at a given time are acquired by a PC/AT system to generate thermograms. The functioning of the system is exemplified by the temperature programmed desorption (TPD) of oxygen from YBa2Cu3−xCoxO7 ± δ, catalytic ammonia oxidation to NO over YBa2Cu3O7−δ and anaerobic oxidation of methanol to CO2, CO and H2O over YBa2Cu3O7−δ (Y123) and PrBa2Cu3O7−δ (Pr123) systems.

Heterogeneous permanganate oxidation of 1,5-dienes: a novel synthesis of 5-substituted butanolides

In the presence of a catalytic amount of water, 1,5-dienes undergo novel and unusual oxidation with potassium permanganate�copper sulfate in dichloromethane to give substituted butanolides in good yields under very mild conditions.

Methanol oxidation on carbon-supported platinum-tin electrodes in sulfuric acid

A study is made of the electrooxidation of methanol in sulfuric acid on carbon-supported electrodes containing platinum-tin bimetal catalysts that are prepared by an in situ potentiometric-characterization route. The catalysts are investigated by employing chemical analyses, X-ray diffraction (XRD), X-ray absorption-near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) data in conjunction with electrochemical measurements. From the electrochemical data, it is inferred that while an electrode with (3:1) Pt-Sn/C catalyst involves a two-electron rate-limiting step akin to platinum-on-carbon electrodes, it is shifted to a one-electron mechanism on electrodes with (3:2)Pt-Sn/C, (3:3)Pt-Sn/C, and (3:4)Pt-Sn/C catalysts. The study suggests that the tin content in the platinum-tin bimetal catalyst produces: (i) a charge transfer from tin to platinum; (ii) an increase in the coverage of adsorbed methanolic residues with increase in the tin content, as indicated by the shift in rest potential of the electrodes towards the reversible value for oxidation of methanol (0.043 V versus SHE), and (iii) a decrease in the overall content of higher valent platinum sites in the catalyst.

Methanol oxidation on carbon-supported Pt---Sn electrodes in silicotungstic acid

Electro-oxidation of methanol was studied on carbon-supported Pt---Sn/C electrodes in silcotungstic acid (SiWA) at various concentrations. The porous-carbon electrodes employing Pt---Sn/C catalyst have been characterized using chemical analyses, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) in conjunction with electrochemistry. The presence of Pt---Sn and Pt3Sn alloys along with Pt and SnO2 phases in the catalyst were identified by XRD. XPS analysis showed a lower amount of PtO species in the Pt---Sn/C catalyst with respect to the corresponding Pt/C sample. From the steady-state galvanostatic polarization data on Pt---Sn/C electrodes in SiWA, it is inferred that a one-electron process is the rate determining step. The performance of the electrodes in 0.084 M SiWA was better than in 2.5 M H2SO4 under similar conditions up to load currents of about 100 mA cm−2 indicating the promoting behaviour of the electrolyte. At currents larger than 100 mA cm−2, the performance of the electrodes in 0.084 SiWA was poorer than that in 2.5 M H2SO4 mainly due to the dominance of mass polarization in the former owing to the large size of keggin units associated with the structure of SiWA. This aspect was supported by cyclic voltammetry and ac impedance studies on Pt---Sn/C electrodes. Simulation of the electrochemical impedance response for the oxidation of methanol in SiWA was carried out using the equivalent electrical circuit model.

Methanol oxidation on carbon-supported platinum-tin electrodes in sulfuric acid

A study is made of the electrooxidation of methanol in sulfuric acid on carbon-supported electrodes containing platinum-tin bimetal catalysts that are prepared by an in situ potentiometric-characterization route. The catalysts are investigated by employing chemical analyses, X-ray diffraction (XRD), X-ray absorption-near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) data in conjunction with electrochemical measurements. From the electrochemical data, it is inferred that while an electrode with (3:1) Pt-Sn/C catalyst involves a two-electron rate-limiting step akin to platinum-on-carbon electrodes, it is shifted to a one-electron mechanism on electrodes with (3:2)Pt-Sn/C, (3:3)Pt-Sn/C, and (3:4)Pt-Sn/C catalysts. The study suggests that the tin content in the platinum-tin bimetal catalyst produces: (i) a charge transfer from tin to platinum; (ii) an increase in the coverage of adsorbed methanolic residues with increase in the tin content, as indicated by the shift in rest potential of the electrodes towards the reversible value for oxidation of methanol (0.043 V versus SHE), and (iii) a decrease in the overall content of higher valent platinum sites in the catalyst.

Methanol oxidation on carbon-supported Pt---Sn electrodes in silicotungstic acid

Electro-oxidation of methanol was studied on carbon-supported Pt-Sn/C electrodes in silcotungstic acid (SiWA) at various concentrations. The porous-carbon electrodes employing Pt-Sn/C catalyst have been characterized using chemical analyses, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) in conjunction with electrochemistry. The presence of Pt-Sn and Pt3Sn alloys along with Pt and SnO2 phases in the catalyst were identified by XRD. XPS analysis showed a lower amount of PtO species in the Pt-Sn/C catalyst with respect to the corresponding Pt/C sample. From the steady-state galvanostatic polarization data on Pt-Sn/C electrodes in SiWA, it is inferred that a one-electron process is the rate determining step. The performance of the electrodes in 0.084 M SiWA was better than in 2.5 M H2SO4 under similar conditions up to load currents of about 100 mA cm-2 indicating the promoting behaviour of the electrolyte. At currents larger than 100 mA cm-2, the performance of the electrodes in 0.084 SiWA was poorer than that in 2.5M H2SO4 mainly due to the dominance of mass polarization in the former owing to the large size of Keggin units associated with the structure of SiWA. This aspect was supported by cyclic voltammetry and ac impedance studies on Pt-Sn/C electrodes. Simulation of the electrochemical impedance response for the oxidation of methanol in SiWA was carried out using the equivalent electrical circuit model.

Oxygen-Exchange Reactions Accompanying Oxidation of Vanadyl Sulfate by Diperoxovanadate

The absorption spectrum in the visible range and the, ESR spectrum of vanadyl sulfate were lost on addition of diperoxovanadate. The V-51-NMR spectra revealed that diperoxovanadate was reduced to vanadate and its oligomers. With excess vanadyl, tetrameric vanadate was found to be the major product, During this reaction oxygen was released into the medium. The oxygen-release reaction was inhibited by a variety of organic ligands-imidazole, benzoate, formate, mannitol, ethanol, Tris, DMPO, malate, and asparagine. An oxygen-consuming reaction emerged at high concentrations of some of these compounds, e.g. benzoate and ethanol. Using DMPO as the spin-trap, an oxygen-radical species with a 1:2:2:1 type of ESR spectrum was detected in the reaction mixtures resulting from vanadyl oxidation by diperoxovanadate which was unaffected by addition of catalase or ethanol. The results showed that secondary oxygen-exchange reactions occur which depend on and utilize the intermediates in the primary reaction during diperoxovanadate-dependent oxidation of vanadyl sulfate.

New uniform solid catalyst for the low-temperature oxidation of carbon monoxide: A triple-layered rare earth perovskite containing Co and Cu ions

Oxygen reactivity and catalytic activity of the cobalt-containing layered defect perovskites, YBa2Cu2CoO7+delta and LaBa2Cu2CoO7+delta, in comparison with LaBa2Cu3O7-delta have been investigated employing temperature-programmed desorption (TPD) and temperature-programmed surface reactions (TPSR) in the stoichiometric and catalytic mode using carbon monoxide as a probe molecule. TPD studies showed evidence for the presence of two distinct labile oxygen species, one at (0 0 1/2) sites and the other at (0 1/2 0) sites in LaBa2Cu2CoO7+delta against a single labile species at (0 1/2 0) in the case of two other oxides. The activation energies for the catalytic oxidation of carbon monoxide by oxygen over LaBa2Cu3O7-delta, YBa2Cu2CoO7+delta, and LaBa2Cu2CoO7+delta have been estimated to be 24.2, 15.9, and 13.6 kcal/mol, respectively. The reactivity and catalytic activity of the oxide systems have been interpreted in terms of the structural changes brought about by substituents, guided by a directing effect of the larger rare earth cation. TPSR profiles, structural analysis, and infrared spectroscopic investigations suggest that the oxygen present at (0 0 1/2) sites in the case of LaBa2Cu2CoO7+delta is accessible to catalytic oxidation of CO through a Mars-Van Krevelen pathway. Catalytic conversion of CO to CO2 over LaBa2Cu2CoO7+delta occurs at 200 degrees C. The enhanced reactivity is explained in terms of changes brought about in the coordination polyhedra around transition metals, enhanced basal plane oxygen diffusivity, and redox potentials of the different transition metal cations.