The kinetics of reduction of synthetic ulvospinel in a hydrogen atmosphere

Ulvospinel, having the composition Fe₂TiO₄, was prepared by heating an intimate mixture of Fe₂O₃ and TiO₂ in a controlled atmosphere of CO₂ and H₂. The kinetics of its reduction by H₂ was studied. The parameters controlling the rate of reduction, such as temperature, granulometry and hydrogen partial pressure were investigated. The optimum temperature range for reduction of the ulvospinel to TiO₂ and Fe was established to be between 600° and 800°; above 800° reduction of the TiO₂ commenced.     

A study of TiO system between Ti3O5 and TiO2 at high temperature by means of electrical resistivity

The TiO system between the compositions TiO₂ and Ti₃O₅ has been studied in following the electrical conductivity against the oxygen partial pressure. Several features are discussed : the dependence of electrical conductivity versus oxygen pressure, the kinetics of approach to equilibrium, and reversibility during oxidation and reduction paths. The results suggest an important contribution of point defects for small departures from the composition TiO₂ at high temperature.     

Catalytic Disproportionation of the Superoxide Intermediate from Electrochemical O2 Reduction in Nonaqueous Electrolytes

Tris(pentafluorophenyl)borane (TPFPB) was found to be an efficient catalyst for rapid superoxide (O₂^?) disproportionation. The kinetics for the catalytic disproportionation reaction is much faster than the reaction between O₂^? and propylene carbonate. Therefore, the negative impact of the reaction between the electrolyte and O₂^? produced by the O₂ reduction is minimized. The cathodic current for O₂ reduction can be doubled in the presence of TPFPB. The high reduction current resulted from the pseudo two‐electron O₂‐reduction reaction due to the replenishment of O₂ at the electrode surface. This discovery could lead to a new avenue for the development of high‐capacity, high‐rate, rechargeable Li–air batteries.     

Kinetics and Mechanism of the H2O2 Electroreduction on the Peroxidase-Promoted Electrodes

The regularities of the bioelectrocatalytic reduction of H₂O₂ in the presence of peroxidase (POD) or a POD–Nafion composite adsorbed on the surface of carbon black or isotropic pyrocarbon are considered. The effect of the surface coverage with the enzyme, the H₂O₂ concentration, and the H₂O₂ supply rate to the electrode on the system activity is studied. Specific activities of three electrode types (POD adsorbed on carbon black or pyrocarbon, and the composite deposited on pyrocarbon) are close to one another. A mechanism for the bioelectrocatalytic reduction of H₂O₂ in a wide potential range is proposed. At potentials near the steady-state value, the reaction rate is limited by the electrochemical kinetics. At high polarizations, the formation of a POD compound with H₂O₂ is the limiting stage.     

Thermodynamic modeling of magnesiothermic reduction of niobium and tantalum from pentoxides

Results obtained in thermodynamic modeling of the magnesiothermic reduction of niobium and tantalum from pentoxides under adiabatic conditions are given. The dependences of equilibrium temperature of the reactions on the composition and initial temperature of the raw material, along with the equilibrium composition, were determined in the systems Nb₂O₅-Mg-NaCl and Ta₂O₅-Mg-NaCl in the temperature range 1200–3000 K.     

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.     

First principles calculations of oxygen reduction reaction at fuel cell cathodes

The efficiency of solid oxide fuel cells (SOFC) depends critically on materials, in particular for the cathode where the oxygen reduction reaction (ORR) occurs. Typically, mixed conducting perovskite ABO₃-type materials are used for this purpose. The dominating surface terminations are (001) AO and BO₂, with the relative fractions depending on materials composition and ambient conditions.Here, results of recent large-scale first principles (ab initio) calculations for the two alternative polar (La,Sr)O and MnO₂ (001) terminations of (La,Sr)MnO₃ cathode materials are discussed. The surface oxygen vacancy concentration for the (La,Sr)O termination is more than 5 orders of magnitude smaller compared to MnO₂, which leads to drastically decreased estimated ORR rates. Thus, it is predicted for prototypical SOFC cathode materials that the BO₂ termination largely determines the ORR kinetics, although with Sr surface segregation (long-term degradation) its fraction of the total surface area decreases, which slows down cathode kinetics.     

Fast Oxygen Reduction Catalyzed by a Copper(II) Tris(2‐pyridylmethyl)amine Complex through a Stepwise Mechanism

Catalytic pathways for the reduction of dioxygen can either lead to the formation of water or peroxide as the reaction product. We demonstrate that the electrocatalytic reduction of O₂ by the pyridylalkylamine copper complex [Cu(tmpa)(L)]²⁺ in a neutral aqueous solution follows a stepwise 4 e^?/4 H⁺ pathway, in which H₂O₂ is formed as a detectable intermediate and subsequently reduced to H₂O in two separate catalytic reactions. These homogeneous catalytic reactions are shown to be first order in catalyst. Coordination of O₂ to Cu^I was found to be the rate‐determining step in the formation of the peroxide intermediate. Furthermore, electrochemical studies of the reaction kinetics revealed a high turnover frequency of 1.5×10⁵ s^?1, the highest reported for any molecular copper catalyst.     

Untersuchungen an Nickeloxid-Mischkatalysatoren. XVI. Reduktionsverhalten amorpher NiO?Al2O3/SiO2-Katalysatoren

Studies on Nickel Oxide Mixed Catalysts. XVI. Reduction Behaviour of Amorphous NiO? Al₂O₃/SiO₂ Catalysts The reduction behaviour of NiO? Al₂O₃/SiO₂ catalysts prepared by precipitationdeposition is influenced by the phase composition (amorphous nickel layersilicates and nickel alumino layersilicates, nickel spinels, nickel oxide) and the differences of the composition between surface and bulk. TPR measurements, determinations of the reduction degree, and the nickel particle sizes by static magnetic measurements showed that the reducibility of the NiO? Al₂O₃/SiO₂ catalysts is enhanced and the nickel dispersity is decreased at low Al₂O₃ contents. The decrease of the reducibility at Al₂O₃ contents >5 mole% is caused by the formation of nickel spinels and the decrease of the Ni^II ion surface concentration.     

Physico-chemical properties of CuO−Bi2O3 mixed oxides and modification of their hydrogen reduction reactivity by ionizing radiation

This paper describes a study on some physico-chemical properties of CuO?Bi₂O₃ mixed oxides of various composition and their reactivity during hydrogen reduction in the range 290–460°C. Depending on the composition, the changes in the morphology of the samples, their specific surface areas, phase composition of Bi₂O₃ as well as the change in the amount of chemisorbed surface oxygen (specific surface oxidation ability) were found. The last mentioned parameter is strongly affected positively or negatively (depending on the dose absorbed) by the pre-irradiation with gamma-rays and accelerated electrons. Either reduction of CuO or consecutive reduction of both components with a maximum rate on the surface or sub-surface layers of grain, proceed in different temperature ranges. The retarding effect of liquid bismuth is partially compensated for by the sponging and trapping effects, so that the overall reduction rate changes non-monotonously with the composition of the samples. Pre-irradiation leads in all cases to the lowering of reduction rate, which can be correlated with the increase in the concentration of strongly bound oxygen forms, creating centres of donor hydrogen chemisorption. The effect of gamma-radiation appears to be function of the threshold dose absorbed.     

Highly Efficient FeIII-initiated Self-cycled Fenton System in Piezo-catalytic Process for Organic Pollutants Degradation

Piezo-catalytic self-Fenton (PSF) system has been emerging as a promising technique for wastewater treatment, while the competing O₂ reductive hydrogen peroxide (H₂O₂) production and Fe^III reduction seriously limited the reaction kinetics. Here, we develop a two-electron water oxidative H₂O₂ production (WOR−H₂O₂) coupled with Fe^III reduction over a Fe^III/BiOIO₃ piezo-catalyst for highly efficient PSF. It is found that the presence of Fe^III can simultaneously initiate the WOR−H₂O₂ and reduction of Fe^III to Fe^II, thereby enabling a rapid reaction kinetics towards subsequent Fenton reaction of H₂O₂/Fe^II. The Fe^III initiating PSF system offers exceptional self-recyclable degradation of pollutants with a degradation rate constant for sulfamethoxazole (SMZ) over 3.5 times as that of the classic Fe^II-PSF system. This study offers a new perspective for constructing efficient PSF systems and shatters the preconceived notion of Fe^III in the Fenton reaction.     

Fabrication of Nb2O5/Carbon Submicrostructures for Advanced Lithium-Ion Battery Anodes

Nb₂O₅ possesses superior fast Li⁺ storage capability for LIB anodes, benefiting from its fast pseudocapacitive behavior and low volumetric change within the cycling processes. However, the poor electric conductivity for Nb₂O₅ restricts its reaction kinetics and rate property. Herein, Nb₂O₅/carbon (C) submicrostructures are fabricated by solvothermal method followed by calcination process. The Nb₂O₅/C submicrostructures exhibit outstanding rate behavior and cyclic performance (332 (194) mAh g⁻¹ after 1000 cycles at 1 (5) A g⁻¹). The superior electrochemical property is attributed to the distinctive structure for Nb₂O₅/C submicrostructures, in which Nb₂O₅ nanoparticles uniformly distributed within Nb₂O₅/C composite can protect Nb₂O₅ nanoparticles from agglomeration, and the porous carbon matrix can enhance electron/ion conductivity. This work furnishes a novel strategy for fabricating Nb₂O₅/C submicrostructures with superior Li⁺ storage performance, which can be potentially used to design other metal oxide/C submicrostructures for second battery anode.     

Influence of composition and ionizing radiation on kinetics of reduction of mixed CdO-ZnO oxides with hydrogen

The reduction of cadmium oxide and zinc oxide mixtures of various compositions with hydrogen at 410–450° was studied with thermogravimetry. The reduction kinetics, which depends markedly on the composition in the whole composition range, can be described by the equation (α+0.3)/(1?α)=A·e ^kt. Preliminary irradiation of the samples with⁶⁰Coγ-rays (applied doseD _γ=10⁵ Gy) causes an increase in the rate of reduction of samples containing an excess of CdO and decrease in the rate of reduction in the region with excess of non-reducible ZnO. Fast neutron irradiation with a flux density of 1.18·10¹⁷ n/m² (total doseD _n=4.5·10² Gy) from a²⁵²Cf source retards the reduction in the whole range of composition. The probable mechanism of mutual interaction of both components and of the applied ionizing radiation is discussed.     

Studies of selectivity of oxygen reduction reaction in acidic electrolyte on electrodes modified by products of pyrolysis of polyacrylonitrile and metalloporphyrins

The rotating disk electrode technique was used to study in 0.5 M H₂SO₄ catalytic properties of products of pyrolysis of the metal-free polyacrylonitrile/carbon black composite, polyacrylonitrile/iron/carbon black composite, and also supported pyropolymers of Co(II) tetramethoxyphenyl porphyrine (CoTMPP) and Fe(III) tetramethoxyphenyl porphyrin chloride (FeTMPPCl). It is shown that the metal-free polyacrylonitrile/carbon black composite catalyzes the oxygen reduction reaction via the parallel path. Addition of up to 2% of Fe into the composite results in abrupt growth of the catalytic activity and share of the four-electron reaction, which provides the parallel–serial reaction path. The parallel reaction with no further catalytic conversion of H₂O₂ occurs on catalysts of the CoTMPP/Vulcan XC72 and FeTMPPCl/Vulcan XC72 series. The chemical composition is one of the key factors affecting activity and selectivity of CoTMPP/Vulcan XC72 catalysts. An increase in the precursor content from 5 to 30% is accompanied by an increase in selectivity k₁/k₂ from 0.14–0.30 to 0.5–1.7, where k₁ is the rate constant of the reaction of O₂ reduction to H₂O, k₂ is the rate constant of the reaction of O₂ reduction to H₂O₂.     

Bioimprinted protein exhibits glutathione peroxidase activity

A strategy for design of bioimprinted proteins with glutathione peroxidase (GPX) activity has been proposed. The proteins imprinted with a glutathione derivative were converted into selenium-containing proteins by chemical modifying the reactive hydroxyl groups of serines followed by sodium hydrogen selenide displacement. These selenium-containing proteins exhibited remarkable GPX activities and the GPX activities of reduction of H₂O₂ by glutathione (GSH) were found to be 101-817 U μmol⁻¹, which approaches the activity of a selenium-containing catalytic antibody elicited by a hapten similar to our template. The steady state kinetic study for imprinted protein catalysis revealed Michaelis-Menten kinetics for both H₂O₂ and GSH, e.g. the pesudo-first-order rate constant k_cat (H₂O₂) and the apparent Michaelis constant K_m (H₂O₂) at 1 mM GSH were calculated to be 784 min⁻¹ and 1.24×10⁻³ M, respectively, and the apparent second-order rate constant k_cat (H₂O₂)/K_m (H₂O₂) was determined to be 6.33×10⁵ (M min)⁻¹. The kinetics and the template inhibition showed that the strategy might be a remarkably efficient one for generating artificial enzyme with GPX activity.     

The electrochemical reduction of oxygen to hydrogen peroxide at the dropping mercury electrode: Part I. Its kinetics at 605 pH<12.5

Via the combination of data obtained from dc and ac experiments it is shown that, contrary to earlier beliefs, the kinetics of the reaction O₂ + 2e + 2 H₂O ? H₂O₂ + 2 OH^? do not depend on pH between pH = 6.5 and pH = 12.5. The rate of reduction is obtained in a large potential range that allows proof of the existence of an electrochemical step followed by a chemical step and the determination of the kinetic parameters of these steps.The analyses of the ac data have some novel aspects.     

Light production in alkaline mixtures of reducing agents and dimethylbiacridylium nitrate.

Abstract— Kinetic studies were made of light production and 0₂ absorption elicited by treatment of dimethylbiacridylium hydroxide [D(OH)₂] with reducing agents in alkaline aqueous solutions. D(OH)2 addition stimulated O₂ uptake which proceeded with zero-order kinetics until nearly all of the O₂ or of the D(OH)₂ was converted to end products. At the termination of the reactions with fructose as reductant the D(OH)₂ was converted to methylacridone and to dimethylbiacridene each compound accounting for approximately one-half the D(OH)₂ consumed. O₂ was reduced to H₂O₂. With hydroxylamine as the reducing agent the emitted light intensity was related to the first power of the D(OH)₂ concentration and the rate of O₂ uptake to the square root of the D(OH)₂. The disappearance of D(OH)₂ in these circumstances was by a first order or pseudo-first order rate. Fructose as a reducing agent by contrast resulted in an O₂ uptake nearly independent of D(OH)₂ concentration over a range from 1 × 10^-5M-1 × 10^-4M, while the light intensity and disappearance of D(OH)₂ followed a one-half order rate. O₂ uptake was by zero order kinetics and the oxidation of fructose proceeded at the same rate as was found with ferricyanide as oxidant. The kinetics, quantum yields and temperature dependence of the fructose reactions were compared with similar reactions employing H₂O₂ as the light eliciting reagent. The results are interpreted as indicating that D(OH)₂ acts as a chain initiator in a manner analogous to better known, radical producing compounds found to accelerate hydrocarbon autooxidations.     

Flow reactor studies and kinetic modeling of the H2/O2 reaction

Profile measurements of the H₂/O₂ reaction have been obtained using a variable pressure flow reactor over pressure and temperature ranges of 0.3–15.7 atm and 850–1040 K, respectively. These data span the explosion limit behavior of the system and place significant emphasis on HO₂ and H₂O₂ kinetics. The explosion limits of dilute H₂/O₂/N₂ mixtures extend to higher pressures and temperatures than those previously observed for undiluted H₂/O₂ mixtures. In addition, the explosion limit data exhibit a marked transition to an extended second limit which runs parallel to the second limit criteria calculated by assuming HO₂ formation to be terminating. The experimental data and modeling results show that the extended second limit remains an important boundary in H₂/O₂ kinetics. Near this limit, small increases in pressure can result in more than a two order of magnitude reduction in reaction rate. At conditions above the extended second limit, the reaction is characterized by an overall activation energy much higher than in the chain explosive regime. The overall data set, consisting primarily of experimentally measured profiles of H₂, O₂, H₂O, and temperature, further expand the data base used for comprehensive mechanism development for the H₂/O₂ and CO/H₂O/O₂ systems. Several rate constants recommended in an earlier reaction mechanism have been modified using recently published rate constant data for H + O₂ (+ N₂) = HO₂ (+ N₂), HO₂ + OH = H₂O + O₂, and HO₂ + HO₂ = H₂O₂ + O₂. When these new rate constants are incorporated into the reaction mechanism, model predictions are in very good agreement with the experimental data. ©1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 113–125, 1999     

Preparation and Electrochemical Performance of V2O3-C Dual-Layer Coated LiFePO4 by Carbothermic Reduction of V2O5

The V₂O₃-C dual-layer coated LiFePO₄ cathode materials with excellent rate capability and cycling stability were prepared by carbothermic reduction of V₂O₅. X-ray powder diffraction, elemental analyzer, high resolution transmission electron microscopy and Raman spectra revealed that the V₂O₃ phase co-existed with carbon in the coating layer of LiFePO₄ particles and the carbon content reduced without graphitization degree changing after the carbothermic reduction of V₂O₅. The electrochemical measurement results indicated that small amounts of V₂O₃ improved rate capability and cycling stability at elevated temperature of LiFePO₄/C cathode materials. The V₂O₃-C dual-layer coated LiFePO₄ composite with 1wt% vanadium oxide delivered an initial specific capacity of 167 mAh/g at 0.2 C and 129 mAh/g at 5 C as well as excellent cycling stability. Even at elevated temperature of 55 oC, the specific capacity of 151 mAh/g was achieved at 1 C without capacity fading after 100 cycles.     

Reduction reactivity of CuO-Cr2O3 mixed oxides with hydrogen and the effect of gamma pre-irradiation

The physico-chemical properties and parameters of CuO-Cr₂O₃ mixed oxides as well as their reactivity toward reduction with hydrogen have been investigated by means of various methods (X-ray analysis, electron spectroscopy for chemical analysis (ESCA), DTA, IR spectroscopy and scanning electron microscopy). A series of mixed oxides of various compositions in the range 0–100% of each component were prepared by calcinating the mass of coprecipitated basic carbonates in air. The reduction kinetics were studied by isothermal thermogravimetry in the temperature range 180–470 ° C. The effect of the origin of the oxides on the reactivity of the oxide system during its hydrogen reduction was proved by comparing its properties with those of a previously studied series of analogous mixed oxides, which were prepared from different precursors.The pre-irradiation of the system by ⁶⁰Co γ radiation results in a positive kinetic effect (an increase in the reduction rate) if the lowest dose applied is 2.57 × 10⁶ Gy. The magnitude of the radiation effect is greatly dependent on the composition of the mixed oxides.