Cathodic behaviour of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel electrodes in alkaline medium

Spinel type CoFe₂O₄ thin films have been prepared, on stainless steel supports, by thermal decomposition of aqueous solutions of mixed cobalt and iron nitrates in 1:2 molar ratio at 400 °C. The electrochemical behaviour of the CoFe₂O₄/1 M KOH interface was investigated by cyclic voltammetry, chronoamperometry and impedance techniques. The studies allowed finding out the redox reactions occurring at the oxide surface. The results were compared with colloidal electrodes prepared by alkaline precipitation of Fe(II) or Fe(III) hydrous oxi-hydroxides on platinum electrodes. In addition, it has been concluded that the processes are diffusion-controlled and the diffusion of the hydroxide ion, through the oxide, acts as the rate-determining step. The diffusion coefficient of OH⁻ through the oxide film was determined using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy techniques.     

Maximization of total surface area of Pt-SnO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst by the Taguchi method

The maximization of the total surface area of Pt-SnO₂/Al₂O₃ catalyst was studied by using the Taguchi method of experimental design. The catalysts were prepared by sol-gel method. The effects of HNO₃, H₂O and aluminum nitrate concentrations and the stirring rate on the total surface area were studied at three levels of each. L₉ orthogonal array leading nine experiments was used in the experimental design. The parameter levels that give maximum total surface area were determined and experimentally verified. In the range of conditions studied it was found that, medium levels of HNO₃ and H₂O concentration and lower levels of aluminum nitrate concentration and stirring rate maximize the total surface area.     

New oxygen evolution anodes for metal electrowinning: MnO<Subscript>2</Subscript> composite electrodes

Several modifications of manganese dioxide (MnO₂) were investigated for use in composite electrode materials for oxygen evolution, the target application being anodes for the industrial electrowinning of metals. It is demonstrated that the performance of this material depends strongly on the modifications of MnO₂. All modifications investigated were found to be more active than the usual anode of lead alloyed with silver (PbAg) used in zinc electrowinning. A composite sample containing chemical manganese dioxide (CMD) was found to give an oxygen evolution overpotential 0.25 V lower than the standard PbAg anode material. In the second part of the article, we investigate the effect of varying several parameters of the composite electrode assembly, including the size of the catalyst particles and percentage of the catalyst material used. A model is proposed where the performance of the material is proportional to the total length of the boundaries between the lead matrix material and the MnO₂ catalyst particles. Physicochemical processes contributing to the observed data are discussed.     

Polyaniline/g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> composites as novel media for anticorrosion coatings

In this paper, graphitic carbon nitride (g-C₃N₄) was first proposed for the pioneer application of anticorrosion coatings. Original g-C₃N₄ was facilely treated using HNO₃ and the exfoliated g-C₃N₄ sheets (E-g-C₃N₄) were fabricated, and then, polyaniline/E-g-C₃N₄ composites (PANI/E-g-C₃N₄) as novel anticorrosive media were synthesized by chemical oxidative polymerization and devoted to the corrosion protection of coatings. The E-g-C₃N₄ sheets and PANI/E-g-C₃N₄ composites were characterized by X-ray diffraction, Fourier transform infrared, thermogravimetric analysis, and transmission electron microscopy. The anticorrosion properties of the samples prepared were investigated by electrochemical measurements including Tafel plots, electrochemical impedance spectra, and open-circuit potential. Accelerated corrosion tests of iron panels coated by PANI/E-g-C₃N₄ were performed in 3.5 wt% NaCl solution. Anticorrosive mechanism of PANI/E-g-C₃N₄ was discussed in detail. PANI/E-g-C₃N₄-3 fillers possessed superior corrosion inhibition than individual components on iron coatings, which was due to the synergetic effect of anticorrosion between E-g-C₃N₄ and PANI.     

Synthetic spinels of the system MgO-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>

Synthetic spinels of the system MgO-Cr₂O₃-Al₂O₃-Fe₂O₃ are considered and the desirability of organizing their production for the refractory industry is demonstrated. Translated from Novye Ogneupory, No. 6, pp. 32–35, June 2008.     

Raman spectra and surface structure of CaO-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass leached in potassium hydroxide solutions

It is difficult to research on the surface structure of amorphous phase in fly ash during leaching reaction due to crystalline phase and complex structure. In the present work, in order to reveal the effects of leaching reaction on the surface structure of amorphous phase in fly ash, the modelling CaO-Fe₂O₃-Al₂O₃-SiO₂ glass was prepared by the traditional melting methods. The leaching reaction of CaO-Fe₂O₃-Al₂O₃-SiO₂ glass with 7.5 M KOH was investigated by spectroscopy, spectrophotometer and wet chemical method. The results show that the content of Q ¹, Q ², Q ³ and Q ⁴ of glass without corrosion was 4.21, 9.51, 23.03 and 52.55%, respectively, which shows that the network polymerization of glass is compact. The leaching reaction of glass can be described by the following equation: dS/dt = k/(r + S ₀). Leaching in KOH for various times induces the content of Q ⁴ and Q ¹ to be decreased, and Q ² and Q ³ increased, resulted in the depolymerization of network and the surface glass dissolved in alkaline solution to form a gel phase. In stage one of leaching reaction, the rate of iron ion leached from glass surface was slow, which resulted in the small slope of straight-line relationship of leaching curve. In the following stage, the leaching rate of iron ion increased with the prolongation of time.     

Electrical conductivity of CuI-AsI<Subscript>3</Subscript>-As<Subscript>2</Subscript>Se<Subscript>3</Subscript> and CuI-SbI<Subscript>3</Subscript>-As<Subscript>2</Subscript>Se<Subscript>3</Subscript> chalcogenide films prepared by chemical deposition

The electrical conductivity of chalcogenide semiconductor films in the CuI-AsI₃-As₂Se₃ and CuI-SbI₃-As₂Se₃ systems, which have been prepared by chemical deposition from mono-n-butylamine, has been studied as a function of the temperature and film composition. It has been established that the electrical conductivity of the CuI-AsI₃-As₂Se₃ and CuI-SbI₃-As₂Se₃ films is predominantly determined by the copper iodide content. It has been demonstrated that the electrical properties of the chalcogenide glasses and the related films are characterized by the same values to within the experimental error, which is explained by the same model of dissolution of vitreous semiconductors in amines with the retention of the electrical properties of chalcogenide glasses after the deposition of films from their solutions.     

Optimization of High Conducting Na<Subscript>3</Subscript>Zr<Subscript>2</Subscript>Si<Subscript>2</Subscript>PO<Subscript>12</Subscript> Phase by new Phosphate Salt for Solid Electrolyte

A composition of NASICON (Na₃Zr₂Si₂PO₁₂) was synthesized by the solid-state reaction method using a new compound Na₂HPO₄2H₂O. The X-ray diffraction patterns of all samples exhibit monoclinic Na₃Zr₂Si₂PO₁₂ as a major phase with a very small amount of monoclinic-ZrO₂. The maximum relative density (97 %) and maximum conductivity is obtained in the samples sintered at 1200 °C (N3) which is slightly higher than β-Al₂O₃. The activation energy is ～ 0.20 eV for the N3 sample which is lower than for β-Al₂O₃. The dilatometeric study and Arrhenius plots confirmed a phase transition of NASICON from monoclinic to rhombohedral. The micro-structural study of the samples done by scanning electron microscopy (SEM) indicated a significant influence of the processing conditions on the microstructures. Raman spectroscopy demonstrated that the sample N3 exhibits minor structural changes compared to other samples.     

EPR study of compounds in the LaAlO<Subscript>3</Subscript>-La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> system

The EPR spectra of compounds in the LaAlO₃-La_0.67Sr_0.33Mn _y O₃ system at a frequency of 9.4 GHz have been investigated at the temperatures T = 77 and 300 K as a function of the manganese concentration y (y = 0.015, 0.030, 0.080). It has been revealed that, in the paramagnetic state at y = 0.015, there exist isolated Mn²⁺ and Mn⁴⁺ ions, which has been confirmed by simulating the EPR spectra. The parameters of the EPR spectra have been determined. The effective magnetic moments μ_eff of the Mn²⁺ and Mn⁴⁺ ions have been calculated from the EPR spectra. It has been demonstrated that an increase in the Mn concentration leads to a decrease in the number of isolated ions and to the formation of new spin clusters. This manifests itself in the predominance of a broad line with weak traces of the hyperfine structure due to the isolated manganese ions.     

Effects of surface area on electrochemical performance of Li[Ni<Subscript>0.2</Subscript>Li<Subscript>0.2</Subscript>Mn<Subscript>0.6</Subscript>]O<Subscript>2</Subscript> cathode material

The effect of surface area on the electrochemical properties and thermal stability of Li[Ni_0.2Li_0.2Mn_0.6]O₂ powders was characterized using a charge/discharge cycler and DSC (Differential Scanning Calorimeter). The surface area of the samples was successfully controlled from ~4.0 to ~11.7 m² g⁻¹ by changing the molar ratio of the nitrate/acetate sources and adding an organic solvent such as acetic acid or glucose. The discharge capacity and rate capability was almost linearly increased with increase in surface area of the sample powder. A sample with a large surface area of 9.6–11.7 m² g⁻¹ delivered a high discharge capacity of ~250 mAh g⁻¹ at a 0.2 C rate and maintained 62–63% of its capacity at a 6 C rate versus a 0.2 C rate. According to the DSC analysis, heat generation by thermal reaction between the charged electrode and electrolyte was not critically dependent on the surface area. Instead, it was closely related to the type of organic solvent employed in the fabrication process of the powder.     

CeO<Subscript>2</Subscript>-La<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZSM-5 sorbents for high-temperature H<Subscript>2</Subscript>S removal

Performance of CeO₂-La₂O₃/ZSM-5 sorbents for sulfur removal was examined at temperature ranging from 500 oC to 700 oC. The sulfur capacity of 5Ce5La/ZSM-5 was much bigger than that of CeO₂/ZSM-5. H₂ had a negative impact on the sulfidation; however, CO had little influence on sulfur removal. The characterization results showed that CeO₂ and La₂O₃ were well dispersed on ZSM-5 because of the intimate admixing of La₂O₃ and CeO₂, the major sulfidation products were Ce₂O₂S and La₂O₂S, the XRD and SEM results revealed that ZSM-5 structure could remain intact during preparation and sulfidation process, the H₂-TPR showed that the reducibility of CeO₂ can be remarkably enhanced by addition of La.     

Low-temperature anti-icing reagents in the Ca(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>-Mg(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>-CO(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>-H<Subscript>2</Subscript>O system and their properties

The polytherms of ice melting in sections of the Ca(NO₃)₂-Mg(NO₃)₂-CO(NH₂)₂-H₂O system with different component ratios were studied in the temperature interval from 0 to −40°C. A series of nitrate and nitrate-carbonate reagents that are promising for the creation of anti-acing reagents were found, which form eutectics with ice at temperatures from −25 to −39°C. Their properties, viz., melting properties with respect to ice and corrosiveness on metals and alloys, were determined. An effective corrosion inhibitor was selected.     

Rhodium stabilized Prussian Blue-modified graphite electrodes for H<Subscript>2</Subscript>O<Subscript>2</Subscript> amperometric detection

Graphite electrodes chemically modified with Prussian Blue (G/PB) were obtained by spreading, on the electrode surface, appropriate volumes of 100 mM K₃[Fe(CN)₆] and 100 mM FeCl₃ solutions, both containing 10 mM HCl. In order to improve the electrochemical response stability, the potential of G/PB electrodes was cycled (in the domain where PB exhibits electrochemical activity) in 0.1 M KCl solution (G/PB-K), as well as in 2 mM RhCl₃ solution, containing 0.05 M KCl (G/PB-Rh). Compared with G/PB-K, the G/PB-Rh modified electrodes showed: (i) higher relative stability of the PB electrochemical response; (ii) better analytical parameters for H₂O₂ amperometric detection; (iii) slightly lower rate constant corresponding to the second order electrocatalytic reaction for H₂O₂ amperometric detection; (iv) an electrocatalytic activity not affected by the H₂O₂ concentration.     

Temperature programmed desorption of oxygen from Pd films interfaced with Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-doped ZrO<Subscript>2</Subscript>

The origin of the effect of non-faradaic electrochemical modification of catalytic activity (NEMCA) or Electrochemical Promotion was investigated via temperature-programmed-desorption (TPD) of oxygen, from polycrystalline Pd films deposited on 8 mol%Y₂O₃–stabilized–ZrO₂ (YSZ), an O²⁻ conductor, under high-vacuum conditions and temperatures between 50 and 250 °C. Oxygen was adsorbed both via the gas phase and electrochemically, as O²⁻, via electrical current application between the Pd catalyst film and a Au counter electrode. Gaseous oxygen adsorption gives two adsorbed atomic oxygen species desorbing at about 300 °C (state β₁) and 340–500 °C (state β₂). The creation of the low temperature peak is favored at high exposure times (exposure >1 kL) and low adsorption temperatures (T_ads < 200 °C). The decrease of the open circuit potential (or catalyst work function) during the adsorption at high exposure times, indicates the formation of subsurface oxygen species which desorbs at higher temperatures (above 450 °C). The desorption peak of this subsurface oxygen is not clear due to the wide peaks of the TPD spectra. The TPD spectra after electrochemical O²⁻ pumping to the Pd catalyst film show two peaks (at 350 and 430 °C) corresponding to spillover O_ads and according to the reaction:

Electrocatalytic activity of copper alloys for NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> reduction in a weakly alkaline solution

A comparative study has been made of the influence of an addition of Sn to Cu as the basic cathode material on the electrocatalytic activity of the resulting material for nitrate (NO₃⁻) reduction in a weakly alkaline medium. Potentiodynamic and chronoamperometric experiments were carried out in an electrolyte simulating the solution from regeneration of an ion-exchange column for NO₃⁻ removal in drinking water treatment. A rotating ring-disk electrode was used for these experiments. An enhancement of the electrocatalytic activity of Cu by alloying with Sn was observed only in the composition region up to 10 wt.% Sn. A further increase in Sn content results in a rapid decline of the electrocatalytic activity caused by changes in the phase structure of the alloy material. For the most active material potentiostatic batch electrolysis was carried out in a divided and an undivided cell. The reduction products were determined.     

Unexpected Support Effect in Hydrotreating: Evidence of a Metallic Character for ReS<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and ReS<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> Catalysts

Abstract  

Rhenium sulfide based catalysts were prepared by the incipient wetness impregnation method over alumina and silica supports and evaluated for 4,6-dimethyldibenzothiophene hydrodesulfurization in a high-pressure stirred-tank reactor. The catalyst prepared over silica was about six times more active for hydrodesulfurization than the corresponding catalyst prepared over alumina and a NiMo/Al₂O₃ industrial reference catalyst. This surprising and positive SiO₂ support effect was explained by a metallic character of the supported sulfide, which was demonstrated using a kinetic approach of competitive hydrogenations and by XPS characterization.