Electrolytic oxygen evolution on Ni-Co-P alloys

Electrolytically obtained Ni–Co–P alloys exhibit amorphous structure. On heating these alloys to 773 K crystalline nickel and cobalt phosphide phases, and also nickel crystallites, are formed. After cathode-anode polarization of the amorphous and crystalline Ni–Co–P alloys in 5m KOH for 18 h, oxygen-cobalt compounds together with crystalline or amorphous nickel phosphides appear on the electrode surface. The evolution of oxygen from 5m KOH was studied on Ni–Co–P alloys prepared in this way. The Tafel parameters were determined and it was ascertained that the values of directional coefficients of the Tafel lines are comparable with those obtained for spinel NiCo₂O₄ oxides, while calculated values of the exchange currents for the oxygen evolution reaction are dependent on the chemical composition of the alloy. The rate of electrolytic oxygen evolution is virtually identical for amorphous and crystalline Ni–Co–P alloys of the same chemical composition. The highest rate of oxygen evolution was found for the alloy containing 15–20% Ni, 66–73% Co and 12–14% P.     

Reduction Characteristics of Iodate Ion on Copper: Application to Copper Chemical Mechanical Polishing

Potentiodynamic and potentiostatic polarization, and the rotating disk electrode technique were used to study the reduction characteristics of iodate (IO₃ ^–) ion on copper (Cu). Depending on the relative concentrations of IO₃ ^– and H⁺ two pH regimes were observed. The cathodic current in the first regime (pH > ,3) was controlled by H⁺ diffusion from the solution to the metal surface. In the second regime (pH > 3 and up to 10^–2 m IO₃ ^– concentration) the cathodic current was found to be under mixed control, involving reaction control via the electrochemical reduction of IO₃ ^– and transport control via the diffusion of I₂(aq). It was concluded that IO₃ ^– was an effective oxidant for Cu chemical mechanical polishing (CMP) with strongly acidic (pH < 3) slurries but it was not convenient reagent as an oxidant for Cu CMP with weakly acidic (pH > 3) slurries.     

The reaction occurring between gaseous CF4 and oxides dissolved in different molten fluorides. Some practical aspects

When CF₄ is bubbled through molten fluorides containing dissolved oxides within the range 900–1020° C, a chemical reaction takes place which decreases the oxide concentration and yields CO₂ and F^– ions. The possibility of the reaction between the CF₄ evolved at the anode and dissolved alumina, occurring during the anode effect in aluminium reduction cells, is discussed. This reaction provides a simple and convenient method for removing oxides and hydroxides from molten fluorides.     

Thin films of Co3O4 and NiCo2O4 obtained by the method of chemical spray pyrolysis for electrocatalysis III. The electrocatalysis of oxygen evolution

The electrocatalytic properties of thin Co₃O₄ and NiCo₂O₄ films prepared on CdO-nonconductive glass by the method of chemical spray pyrolysis were investigated for oxygen evolution in varying KOH concentrations. The Tafel slopes were close to (2.3RT/F) V per decade for both oxides. The reaction order with respect to [OH^–] was found to be approximately 1.3. It was observed that Co₃O₄ is more active than NiCo₂O₄ towards oxygen evolution. A mechanism for oxygen evolution involving the electrochemical adsorption of OH^– as a fast step and the electrochemical desorption of OH forming an intermediate H₂O₂ as the rate-determining step has been suggested.     

Partial oxidation of CH4 with air to produce pure hydrogen and syngas

The gas–solid reaction between methane and the lattice oxygen of Ni, Co, and Fe-oxides loaded on various support materials produced a synthesis gas (hydrogen and carbon monoxide) at 600–800 °C. Metal oxides were reduced to metals or lower valence oxides, and they were re-oxidized to oxides by introducing air after the reaction. Thus, production of hydrogen or synthesis gas free from nitrogen can be achieved alternatively without using pure oxygen. As a metal oxide, Fe₂O₃ and Rh₂O₃-loaded on Y₂O₃ exhibited the highest H₂ selectivity of 60.1% with a moderate CH₄ conversion of 54% and a high lattice oxygen utilization of 84% at 800 °C.     

An Investigation into Silane Evolution from Porous Silicon by Temperature Programmed Desorption Method

The gaseous species desorbed from porous silicon (PS) were investigated using the method of temperature programmed desorption (TPD) and fourier transform infrared spectroscopy (FTIR). Silicon wafers (25–50 cm, p^–, FZ) were anodised in 40% HF and HF/C₂H₅OH electrolytes. The PS samples were linearly heated at 1.5 K s^–1 using a custom built heating unit in a oil-free pump backed vacuum chamber at a base pressure of <10^–8 torr. A quadrupole mass spectrometer, which was used as the detector, was fitted in line of sight of the sample at a distance of about 6 mm. It was observed that silane was liberated during the heating of porous silicon samples produced from both electrolytes. The peak temperature at which this occurred was at 570 ± 10 K. This temperature coincides with the temperature of silicon-silicon bond breakage in Si–SiH₃ groups on the pore walls, as shown by the FTIR results. It is proposed that silane formation involves the reaction of the Si-silyl group with moisture: Si–SiH₃ + H₂O Si–OH + SiH₄.     

Electrical conductivity and chemical diffusivity of NiAl2O4 spinel under internal reforming fuel cell conditions

NiAl₂O₄ spinel was formed by solid state reaction. Its electrical conductivity was measured in the temperature range of 680–940 °C and under various oxygen-rich environments, as well as under reducing conditions. From the temperature dependence of the conductivity, the activation energies for conduction increase for decreasing oxygen partial pressures. From the partial oxygen pressure dependence, the defect structure of the material was analysed. The conductivity change with respect to P _O2 can be attributed to singly and doubly ionized nickel vacancies. The chemical diffusivity of the oxide was determined by conductivity relaxation upon abrupt change in P _O2 in the surrounding atmosphere. The oxygen chemical diffusion coefficient is of the order of magnitude of 10^–4 cm² s^–1.     

Feasibility of Na-based thermochemical cycles for the capture of CO2 from air—Thermodynamic and thermogravimetric analyses

Three Na-based thermochemical cycles for capturing CO₂ from air are considered: (1) a NaOH/NaHCO₃/Na₂CO₃/Na₂O cycle with 4 reaction steps, (2) a NaOH/NaHCO₃/Na₂CO₃ cycle with 3 reactions steps, and (3) a Na₂CO₃/NaHCO₃ cycle with 2 reaction steps. Depending on the choice of CO₂ sorbent – NaOH or Na₂CO₃ – the cycles are closed by either NaHCO₃ or Na₂CO₃ decomposition, followed by hydrolysis of Na₂CO₃ or Na₂O, respectively. The temperature requirements, energy inputs, and expected products of the reaction steps were determined by thermodynamic equilibrium and energy balance computations. The total thermal energy requirement for Cycles 1, 2, and 3 are 481, 213, and 390 kJ/mol of CO₂ captured, respectively, when heat exchangers are employed to recover the sensible heat of hot streams. Isothermal and dynamic thermogravimetric runs were carried out on the pertinent carbonation, decomposition, and hydrolysis reactions. The extent of the NaOH carbonation with 500 ppm CO₂ in air at 25 °C – applied in Cycles 1 and 2 – reached 9% after 4 h, while that for the Na₂CO₃ carbonation with water-saturated air – applied in Cycle 3 – was 3.5% after 2 h. Thermal decomposition of NaHCO₃ – applied in all three cycles – reached completion after 3 min in the 90–200 °C range, while that of Na₂CO₃ – applied in Cycle 1 – reached completion after 15 min in the 1000–1400 °C range. The significantly slow reaction rates for the carbonation steps and, consequently, the relatively large mass flow rates required, introduce process complications in the scale-up of the reactor technology and impede the application of Na-based sorbents for capturing CO₂ from air.     

Selective oxidation of C3–C4 olefins over Mo-containing catalysts with tetragonal tungsten bronze structure

Mo–V–Nb–P–O-based catalysts with a tetragonal tungsten bronze-type (TTB) structure have been prepared hydrothemally from a H₃PMo₁₂O₄₀ Keggin-type heteropolyacid. These catalysts have been tested in the oxidation of C₃–C₄ olefins (propene, isobutene and 1-butene). Although the catalytic performance depends on the nature of the olefin fed the TTB-type catalysts prepared in the presence of elements of the V and VI groups such as Te, Sb and Bi have shown a high selectivity to partial oxidation products, especially that with Te. However, in the absence of these elements the TTB-catalysts present a high catalytic activity to deep oxidation. The selectivity to partial oxidation products decreases in the order: MoVNbPTe- > MoVNbPSb- > MoVNbPBi- > MoVNbP-TTB catalysts. The reaction products obtained in the oxidation of each olefin will be discussed according to their corresponding reaction mechanism and the characteristics of catalysts.     

Structural and Chemical Features of Silicon Nanocrystallites in Nanocrystalline Hydrogenated Silicon Thin Films

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited on Si wafers at room temperature by plasma-enhanced chemical vapor deposition (PECVD): a mixture of SiH₄ and H₂ was introduced into the evacuated reaction chamber. The films were postdeposition annealed at temperatures of 400–1100°C. The silicon nanocrystallites (nc-Si) in the films range from ∼4.0 to ∼8.0 nm in size, depending on the hydrogen flow rates as well as the annealing conditions. The relative fractions of the Si-H₃, Si-H₂, and Si-H bonds in the nc-Si:H films varied sensitively with the heat-treatment conditions. Local radial distribution function (RDF) analysis of the films was performed by using selected area electron diffraction methods. A model for the nanostructure of the nc-Si:H films was suggested to take into account the variation in the size and chemical bonds of the nanocrystallites as well as the amorphous matrix.Original English Text Copyright © 2005 by Fizika i Khimiya Stekla, Shim, Cho.This article was submitted by the authors in English.     

NO x -Catalyzed Gas-Phase Activation of Methane: In Situ IR and Mechanistic Studies

The products of reactions occurring in a CH₄–O₂–NO _x mixture have been investigated by in situ FTIR spectroscopy. The results show that low temperatures favor the formation of HCHO and CH₃OH, while high temperatures favor that of C₂H₄. Possible reaction mechanisms based on the in situ observations are briefly discussed.     

Conditions for Preparation of Oxide Components and Their Effect on Properties of Al2O3 – ZrO2 – SiC Composite

Results of a study of phase formation in the Al₂O₃ – ZrO₂ – SiC system sintered under vacuum and in a reducing medium at 1350 – 1550°C are reported. Conditions for preparation of Al₂O₃ and ZrO₂ powders from hydroxides are specified and the effect of specific surface, temperature, and holding time on the reaction between oxide and carbide components is considered. Results of microstructural, x-ray phase, thermogravimetric, and chemical analyses of precursor materials and end products are discussed. Optimum composition (20% SiC, 15% ZrO₂, 65% Al₂O₃ ) and dispersity of the mixture for obtaining a composite with a strength of about 200 MPa by a method other than high-temperature toughening are determined.     

Propane Oxidative Dehydrogenation over Metal Pyrophosphates Catalysts

Metal pyrophosphates (M–P₂O₇, where M is V, Zr, Cr, Mg, Mn, Ni or Ce) have been found to catalyze the oxidative dehydrogenation of propane to propene. The reaction was conducted at 1 atm, 450–550°C and feed flowrate of 75 cm³/min (20 cm³/min propane, 5 cm³/min oxygen and the balance is helium). All catalysts showed increase in degrees of conversion and decrease in olefins selectivity with increase in reaction temperature. At 550°C, MnP₂O₇ exhibited the highest activity (40.7% conversion) and total olefins (C₃H₆ and C₂H₄) yield (29.3%). The other catalysts, indicated by their respective metals, may be ranked (based on olefins yield) as V (16.9%) < Cr (17.5%) < Ce (25.1%) < Zr (26.2%) < Ni (26.8%) < Mg (27.9%). The reactivity of the lattice oxygen was estimated from energy of formation of the corresponding metal oxides. Correlation between the selectivity to propene and the standard energy of formation was attempted. Although there was no clear correlation, the result suggested that the lattice oxygen play a key role in the selectivity-determining step.     

Reprint of: Oxidative dehydrogenation of cyclohexane and cyclohexene over supported gold, –palladium catalysts

Supported gold, palladium and gold–palladium catalysts have been used to oxidatively dehydrogenate cyclohexane and cyclohexenes to their aromatic counterpart. The supported metal nanoparticles decreased the activation temperature of the dehydrogenation reaction. We found that the order of reactivity was Pd ≥ Au–Pd > Au supported on TiO₂. Attempts were made to lower the reaction temperature whilst retaining high selectivity. The space-time yield of benzene from cyclohexane at 473 K was determined to be 53.7 mol/kg_cat/h rising to 87.3 mol/kg_cat/h at 673 K for the Pd catalyst. Increasing the temperature in this case improved conversion at a detriment to the benzene selectivity. Oxidative dehydrogenation of cyclohexene over AuPd/TiO₂ or Pd/TiO₂ catalysts was found to be very effective (conversion >99% at 423 K). These results indicate that the first step in the reaction sequence of cyclohexane to cyclohexene is the slowest step. These initial results suggest that in a fixed-bed reactor the oxidative dehydrogenation in the presence of oxygen, palladium and gold–palladium catalysts are readily able to surpass current literature examples and with further modification should yield even higher performance.     

Thermal stability of palladium on ceria-doped alumina

Several palladium on alumina and ceria/alumina catalysts were prepared and oxidized in air between 400 and 1000°C. The metal dispersion was determined by hydrogen titration of adsorbed oxygen. Dispersions above 50% were maintained on 0.2% Pd/Al₂O₃ up to 900°C. Adding 5.0% ceria, or increasing the metal loading to 2.5%, greatly reduces the thermal stability of the palladium, such that the dispersion falls rapidly at 600°C. The rates of methane oxidation (moles of CO₂/g Pd h) at 250°C and 5% excess oxygen are nearly equal on 0.22–2.50% Pd/3.5–5.2% CeO₂/Al₂O₃, dispersion 14–42%, and 0.20–0.46% Pd/Al₂O₃, dispersion 59–86%, but are 10 to 20 times lower than the rate on 2.3% Pd/Al₂O₃, dispersion 11%. The lower rate of methane oxidation on ceria-promoted and highly dispersed palladium on alumina might be due to the conversion of the palladium into less active palladium oxide during reaction.     

A composite material based on yttrium oxide

Conclusions It was established that a high-density yttrium ceramic (relative density 95–98%) can be produced by adding 20–30% TiB₂ and sintering in vacuo or hot molding at 1500°C. It is assumed that the high density of the material is the result of the formation of reaction products in the interaction of the components of the TiB₂-Y₂O₃ system.Translated from Ogneupory, No. 3, pp. 53–55, March, 1977.     

XPS analysis of wet chemical etching of GaAs(110) by Br2–H2O: comparison of emersion and model experiments

We have applied photoelectron spectroscopy to investigate the surface composition after different surface treatments involving Br₂–H₂O mixtures in order to study wet chemical etching. Emersion experiments from Br₂–H₂O solution are compared with model experiments, in which Br₂–H₂O adsorbate and coadsorbate mixtures react with clean GaAs(110) surfaces. Our results indicate that Ga- and As-bromides formed initially are hydrolyzed to form the respective oxides. Without addition of Br₂, only slight oxidation of the surface takes place. There is an enrichment of Ga due to loss of As both in adsorption as well as in emersion experiments. Since in emersion experiments only a final situation is analyzed, the relative influence of surface reactivity and subsequent solvation effects cannot be distinguished easily, while model experiments give clear information on reaction products formed intermediately. However, model experiments differ in environment and temperature from the real solid–liquid interface. The presented results demonstrate that a combination of emersion and model experiments provide valuable insight into the mechanism of wet chemical etching on a microscopic level.     

Mechanism and Kinetics of Formation of Solid Solutions in the Nd2SrAl2O7–Ho2SrAl2O7 System

The processes of phase formation in the Nd₂O₃–Ho₂O₃–SrO–Al₂O₃ system are investigated in the temperature range 1100–1530°C. It is revealed that the structural–chemical mechanism of formation of (Nd _x Ho_{1 – x} )₂SrAl₂O₇ solid solutions depends on the temperature and composition. An increase in the holmium content and in the temperature leads to a crossover from the mechanism including the formation of LnAlO₃ and LnSrAlO₄ as intermediate products to the mechanism in which the interaction of Ln ₂O₃ and SrAl₂O₄ is a limiting stage.     

Oxygen reduction on stainless steel

Oxygen reduction was studied on AISI 304 stainless steel in 0.51 m NaCl solution at pH values ranging from 4 to 10. A rotating disc electrode was employed. It was found that oxygen reduction is under mixed activation-diffusion control. The reaction order with respect to oxygen was found to be one. The values of the Tafel slope depend on the potential scan direction and pH of the solution, and range from – 115 to – 180 mV dec^–1. The apparent number of electrons exchanged was calculated to be four, indicating the absence of H₂O₂ formation.Nomenclature B =0.62 nFcD ^2/3 –^1/6 - c bulk concentration of dissolved oxygen (mol dm^–3) - D molecular diffusion coefficient of oxygen (cm² s^–1) - E electrode potential (V) - EH standard electrode potential (V) - E _H ⁰ Faraday constant (96 500 As mol^–1) - I current (A) - j current density (A cm^–2) - j _k kinetic current density (A cm^–2) - j _L limiting current density (A cm^–2) - m reaction order with respect to dissolved oxygen molecule - M molar mass (g mol^–1) - n number of transferred electrons per molecule oxygen - density (g cm^–3) - kinematic viscosity (cm² s^–1) - angular velocity (s^–1)     

Oscillation of electrical current during direct methane oxidation over Ni-added LSCF–GDC anode of solid oxide fuel cells

Solid oxide fuel cells are studied under direct methane feeding with 10–70% CH₄. When either La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)–Ce_0.9Gd_0.1O_1.95 (GDC) or Ni-added LSCF–GDC composite is used as the anode, the oscillations of the electrical current and the formation rates of CO and CO₂ occur. The oscillation of the electrical current can be explained by a mechanism of periodic oxidation–reduction of the bulk lattice of the anode, with the determining factor being the build-up of the concentration of the oxygen vacancies to a certain extent. As the methane concentration increases, the current density increases and becomes larger with Ni addition. Higher methane concentration leads to higher possibility to induce the oscillation, to start it earlier, and to result in a larger amplitude. Ni addition inhibites the occurrence of the oscillation of the electrical current but promotes that of the CO₂ formation rate.