Mediated bioelectrocatalytic O2 reduction to water at highly positive electrode potentials near neutral pH

Combinations of bilirubin oxidase and metal complexes: [W(CN)₈]^3−/4−, [Os(CN)₆]^3−/4− and [Mo(CN)₈]^3−/4− (the formal potentials, E^0′(M), being 0.320, 0.448, and 0.584 V vs. Ag|AgCl, respectively, at pH 7.0), allowed bioelectrocatalytic reduction of O₂ to water at their formal potentials near neutral pH. The O₂ reduction current appeared even at the standard potential of the O₂/H₂O redox couple, E^0′(O₂/H₂O), when [Mo(CN)₈]^3−/4− was used at pH 7.4, though the magnitude was small. The magnitude of the bioelectrocatalytic current systematically decreased with the decrease in the potential difference between E^0′(O₂/H₂O) and E^0′(M). A limiting current as large as 17 mA/cm² of a projected electrode surface area was obtained at 0.25 V (−0.37 V vs. E^0′(O₂/H₂O)) for the O₂ reduction at pH 7.0 with a carbon felt electrode modified with electrostatically entrapped bilirubin oxidase and [W(CN)₈]^3−/4− at the electrode rotation rate of 4000 rpm.     

Spectroelectrochemical investigation of Azotobacter vinelandii bacterial ferritin

Bacterial ferritin of Azotobacter vinelandii (AvBF) contains a function in accepting electrons from platinum electrode directly for complete iron release in the absence of a mediator. The reduction potentials of electron tunnels of −125, −310, and −370 mV for iron release are determined by direct spectroelectrochemical technique, which suggests which should be defined as midpoint potentials of electron–tunnel–heme on the surface of protein shell. A kinetic study for complete iron release by the electrode reduction at −600 mV fits the zero-order reaction law.     

Visible absorption spectral measurements of the formation of the cobalt porphyrin dioxygen complex using a high-pressure cell ag

The 1:1 complex formation between dioxygen and the meso-tetraphenylporphinatocobalt(II)pyridine complex in toluene was followed spectrophotometrically using a high-pressure cell. The values of the equilibrium constant K_p obtained are 0.0278, 0.0513, 0.0840, 0.157 and 0.278 atm⁻¹ at −30, −36, −42, −48 and −54dgC, respectively. The thermodynamic data are ΔH^θ = −10.1±0.6 kcal mol⁻¹ and gDS^θ = −49±3 eu at a standard pressure of 1 atm.     

Das tautomere Gleichgewicht des Bilirubins Kurze Mitteilung

The equilibrium between lactam and lactim forms of bilirubin was studied by means of optical pK-measurements of model compounds of the arylmethylidenepyrrolinone and pyrromethenone series as well as bilirubin, bilirubin dimethyl ester and dimethoxybilirubin dimethyl ester. The lactam form of bilirubin was found to be more stable by a factor of at least 10⁴ compared with the lactim form. Therefore the latter is not responsible for the spectroscopic anomalies of bilirubin.     

Comparison of the chemical stability of the high energy density cathodes of lithium-ion batteries

With an objective to assess the chemical stabilities and their consequences in cell performance, the variations of oxygen content with lithium content (1−x) in chemically delithiated Li_1−xCoO₂, Li_1−xNi_0.85Co_0.15O₂, and Li_1−xMn₂O₄ cathodes have been monitored with redox titrations. The Li_1−xCoO₂ system tends to lose oxygen from the lattice at deep lithium extraction, while the Li_1−xNi_0.85Co_0.15O₂ system does not lose oxygen at least for (1−x)>0.3. The chemical instability with a tendency to lose oxygen at deep lithium extraction could be the reason for the limited practical capacity of the Li_1−xCoO₂ system (140 mA h/g) compared to that realized with the Li_1−xNi_0.85Co_0.15O₂ system (180 mA h/g). The Li_1−xMn₂O₄ spinel maintains an oxygen content of 4.0 without losing any oxygen for 0.15⩽(1−x)⩽1.     

Effect of Ti substitution on hydrogen storage properties of Zr1−xTixCo (x = 0, 0.1, 0.2, 0.3) alloys

Zr_1−xTi_xCo (x = 0, 0.1, 0.2, 0.3) alloys were prepared by arc-melting method and the effect of Ti substitution on hydrogen storage properties was studied systematically. Hydrogen desorption pressure-composition-temperature (PCT) measurements were carried out using Sievert's type volumetric apparatus for ZrCo (at 473 K, 573 K and 673 K) and Zr_1−xTi_xCo alloys (at 673 K), respectively. Products after dehydrogenation were characterized by X-ray diffraction (XRD). In addition, the kinetics of Zr_1−xTi_xCo hydride was investigated at 473 K and 673 K, respectively, under hydrogen pressure of 5 MPa. Results showed that Ti substitution for Zr did not change the crystal structure of ZrCo phase. With the increase of temperature from 473 K to 673 K, the extent of disproportionation for ZrCo alloy increased. With Ti content increasing at 673 K, the desorption equilibrium pressure of Zr_1−xTi_xCo-H₂ systems elevated and the disproportionation reaction of Zr_1−xTi_xCo alloys was inhibited effectively. Ti substitution decreased the kinetics rate and the effective hydrogen storage capacity of Zr_1−xTi_xCo alloys slightly. Generally speaking, it was found that Zr_0.8Ti_0.2Co alloy had better anti-disproportionation property with less decrease of effective hydrogen storage capacity which was beneficial to tritium application in the International Thermonuclear Experimental Reactor (ITER).     

Lipase catalyzed enantioselective desymmetrization of a prochiral pentane-1,3,5-triol derivative

The enantioselective desymmetrization of the prochiral 3-O-silyl protected pentanetriol derivative 3 was carefully investigated. At −10 °C, the bacterial lipase from Burkholderia cepacia immobilized on ceramic particles led to monoacetate (S)-4 in 52% yield and >99.9% ee. At a reaction temperature of −40 °C the yield and enantioselectivity were even higher, but the reaction time was very long. Theoretical simulations of the reaction progress indicated an enantioselectivity of 25:1 at −10 °C and 35:1 at −40 °C. (S)-4 was converted into the enantiomerically pure building block 5-azidopentane-1,3-diol (S)-7 in two steps. The absolute configuration of (S)-7 was determined by exciton-coupled circular dichroism (ECCD) of diester (S)-8.     

An ab initio study of the formation and structure of NO+-(N2)n (n = 1 and 2) clusters

The geometries and clustering energies of the NO⁺ (N₂)_n (n = 1 and 2) clusters have been determined by ab initio calculations at different levels. At the MP4SDQ/6-31G^* level, plus zero-point vibrational corrections making use of the HF/4-31G-optimized geometries, the energy differences between clusters and their relative fragments have been calculated to be −5.3 and −4.1 kcal/mol for n = 1 and n = 2, respectively. These values can be compared to the experimental enthalpies of clustering of −4.4 and −3.9 kcal/mol. The interaction between NO⁺ and N₂ and the structures of two stable cluster ions have also been examined.     

Equilibrium hydrogen pressures in vanadium alloys

The dissolution of hydrogen gas in vanadium-based alloys containing niobium, chromium and titanium was studied by measuring the equilibrium pressure, at various compositions, from 763 to 1125 K. Hydrogen followed Sieverts' law in all the alloys studied up to a hydrogen-to-metal atom ratio of 0.25. The values of the enthalpies of solution of hydrogen for these vanadium-based alloys ranged from −18.5 to −46.8 ± 1.5 kJ (mol H)⁻¹ and the standard entropies of solution of hydrogen ranged from −57.4 to −62.6 ± 5.5 J K⁻¹ (mol H)⁻¹. The present results agree extremely well with a previous low temperature study of these alloys which employed an isopiestic technique to measure indirectly the equilibrium hydrogen pressures.     

Effect of Ho substitution on the ionic conductivity of La2Mo1.7W0.3O9 oxygen ion conductor

Partial substitutions of Ho at the La-site of La₂Mo_1.7W_0.3O_9−δ were carried out. Compound La_2−xHo_xMo_1.7W_0.3O_9−δ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5) has been synthesized by solid state reaction technique. The specimens were characterized by XRD, SEM, DSC for crystal structure, surface morphology, phase transition and ac impedance spectroscopy for conductivity and other electrical parameter determination. Partial substitution of Ho at the La-site of La₂Mo_1.7W_0.3O_9−δ, increases the conductivity within the substitutional range, x ≤ 0.2. The phase transition of La₂Mo₂O₉ is suppressed in doped compound and a transition from Arrhenius to VTF behavior of temperature dependence of conductivity is observed around 500 °C. The conductivity is found to be high in the intermediate temperature region and at high temperature the conductivity of La_2−xHo_xMo_1.7W_0.3O_9−δ (0.05 ≤ x ≤ 0.2) is almost similar with that of La₂Mo₂O₉. The decrease in energy barrier enhances the thermally assisted process to start at lower temperature.     

Beneficial modification of the properties of multiferroic BiFeO3 by cation substitution

The effects of modifying the well-known multiferroic BiFeO₃ by substituting Fe by Mn and Bi by La have been investigated. It is shown that both the substitutions have a favourable effect on the multiferroic properties of BiFeO₃. Thus, both BiFe_1−xMn_xO₃ and Bi_1−xLa_xFeO₃ (x = 0.0–0.3) show increased magnetization accompanied by hysteresis loops as well as improved dielectric properties. The ferroelectric transition temperature is lower than that of BiFeO₃, but is in a more accessible range. In Bi_1−xLa_xFeO₃, there is a change in structure at x = 0.2.     

Influence of In on the activity of zinc in liquid (tin + zinc + indium)

The activity of Zn in low-melting {(1−x₁−x₂)Sn+x₁Zn+x₂In}alloys has been determined by the method of equilibrium saturation with metal vapour at T=750 K. The total mole fraction of Zn and In was less than 0.25. As the method of equilibrium saturation is a comparative one, it is necessary to have an appropriate reference mixture. The {(1−x)Sn−xZn}alloy was taken as the reference mixture. The interaction parameter ε_Zn^In was determined by a least-squares method. The experimental value of ε_Zn^In was compared with the calculated value on the basis of a model of relative difference of ionic volumes.     

Surface modification of La0.3Sr0.7CoO3−δ ceramic membranes

The dependence of oxygen permeability of dense La_0.3Sr_0.7CoO_3−δ ceramics on membrane thickness indicates significant surface exchange limitations to the permeation fluxes, which suggests a possibility to increase membrane performance by surface activation. The cobaltite membranes with various porous layers applied onto the permeate-side surface were tested at 850–1120 K. Silver-modified La_0.3Sr_0.7CoO_3−δ membranes showed enhanced permeation at temperatures above 950 K; deposition of porous layers of PrO_x and Pr_0.7Sr_0.3CoO_3−δ had no positive effect. The maximum oxygen permeability at 850–1120 K was observed in the case of porous La_0.3Sr_0.7CoO_3−δ layers with surface density about 10 mg cm⁻². These results suggest that the surface exchange of lanthanum–strontium cobaltite membranes under an oxygen chemical potential gradient is limited by both oxygen sorption at the surface and ion diffusion through the surface oxide layers. Oxygen permeability of La_0.3Sr_0.7CoO_3−δ ceramics was found to increase with increasing grain size due to decreasing grain-boundary resistance to ionic transport.     

Determination of cystine and cysteine in seawater using cathodic stripping voltammetry in the presence of Cu(II)

A method is described for the determination of cystine and cysteine in seawater and freshwater using cathodic stripping voltammetry in the presence of added copper(II). The optimized conditions include a copper concentration of 150 nM, a pH of 8.5, and a collection potential of −0.15 V; the cathodic reduction peak is located at −0.55 V. The detection limit is 0.1 nM after a collection period of 4 min. The sensitivity is diminished by surfactants similar to Triton X-100 in natural waters; the sensitivity therefore needs to be calibrated by internal standard additions of amino acids. It is possible to differentiate between cystine and cysteine by employing a solution pH of 6.2, where the peak due to cystine is absent. The response in seawater is different from that previously reported in buffer solutions. It is shown that the amino acid reduction peak is due to Cu(I) reduction of the adsorbed Cu(I)-cysteinate complex; this complex is formed with Cu(I) generated from dissolved copper(II) at the electrode surface at −0.15 V in the presence of cysteine; cystine is reduced to cysteine at the electrode surface during the collection process.     

First-principles calculations of the structural,elastic and electronic properties of MNxC1−x (M = Ti,Zr, Hf; 0 ≤ x ≤ 1) carbonitrides at ambient and elevated hydrostatic pressure

The structural, electronic, and elastic properties of three mixed transition metal carbonitrides TiN_xC_1−x, ZrN_xC_1−x, and HfN_xC_1−x (0 ≤ x ≤ 1) with the rock-salt structure were calculated at ambient and elevated up to 50 GPa hydrostatic pressures in the framework of the density functional theory methods. The lattice constants, densities, and bulk moduli of the considered compounds were shown to behave as linear functions of the nitrogen concentration x. The obtained linear dependencies of all these parameters allow for getting their estimates at any value of x in the range from 0 to 1. Gradual enhancement of the ionicity of the chemical bonds with gradual replacement of carbon by nitrogen was demonstrated by calculating the bond orders and electron density difference distributions.     

Characterization and study of photochromism of hydroxylated polyazomethines

Polymers having a photochromic and/or thermochromic backbone have been synthesized by the condensation of bissalicylaldehydes with diamines. Photochromic activity was studied for hydroxylated polazomethines (HPAM) in the solid state and as thin films at room temperature, −75 and −180°. HPAM showed thermochromic behaviour but not photochromism. The lack of photochromic activity is attributed to the polymeric chain and the moieties attached to the azomethine groups which may increase the potential energy barrier and thus prevent the formation of trans-keto photoproduct.     

Possible explanation of unstable superconducting phase in KxC60 with Tc=21 K

EPR and MMMA methods revealed two K₃C₆₀ phases, the unstable one with T_c=21.0 K containing C¹⁻₆₀ centers and a stable one with T_c=18.0 K constituting of C₆₀ molecules in the form of C³⁻₆₀ ions. The metastable phase appears only in the first stage of potassium intercalation of fullerene at the C₆₀/K₃C₆₀ phase boundary. This phase is being transformed to a stable one on further doping to K₃C₆₀. The appearance of a metastable phase is related to the instability of the fullerene C₆₀ fcc structure during the process of octahedral sites being filled by potassium ions. The critical temperature T_c shift upwards from 18.5 to 21.0 K is ascribed to the possible process of potassium transient back charge transfer valency, i.e. K₃¹⁺C₆₀³⁻↔K₂¹⁺K¹⁻C₆₀¹⁻ [3K¹⁺+C³⁻₆₀⇔2K¹⁺+K¹⁻+C₆₀¹⁻], which could enlarge the unit cell necessary to shift a superconducting critical temerature according to T_c on volume dependence.     

Structure and magnetic properties of Ho2Co17−xMx pseudobinary compounds

The homogeneity range of the pseudobinary system Ho₂Co_17−xM_x (M ≡; Ni, Fe, Mn, Cr, V, Ti, Cu, Al) and the dependence of the lattice constants and the Curie temperature on composition were studied. Although Ho₂Co₁₇, Ho₂Ni₁₇ and Ho₂Fe₁₇ have identical crystal structures and similar lattice constants, the homogeneity range for the Th₂Ni₁₇-type compound in the Ho₂Co_17−xNi_x and Ho₂Co_17−xFe_x systems is limited to x < 5. The curves of Curie temperature versus composition have maxima at x values of 1, 3 and 0.9 for the systems Ho₂Co_17−xNi_x, Ho₂Co_17−xFe_x and Ho₂Co_17−xCu_x respectively. This suggests that the effect of nickel, iron and copper on the Co-Co exchange interaction in the cobalt sublattice at low concentrations may be different from that at high concentrations.     

Ab initio cluster study of the bilayer manganite La2−2xSr1+2xMn2O7

We have performed complete active space self-consistent field (CASSCF) cluster calculations for (MnO₆)⁹⁻ and (MnO₆)⁸⁻ complexes in the background potential appropriate for the bilayer manganite La_2−2xSr_1+2xMn₂O₇ at x=0.40. While the ground state of the (MnO₆)⁹⁻ complex is well-described by a restricted Hartree–Fock wavefunction, contributions from configurations with holes in oxygen orbitals are significant in the (MnO₆)⁸⁻ ground state wavefunction. Surprisingly, the CASSCF calculations including sufficient oxygen orbitals in the active space yields a doubly degenerate E (of the C_4v point group) as the ground state.