Highly Dispersed Molybdenum Oxide Supported on HZSM-5 for Methane Dehydroaromatization

Mo/HZSM-5 catalyst with highly dispersed MoO _x species was prepared by adding ammonia solution to the ammonium heptamolybdate aqueous solution during the impregnation process. Compared with the large species which is predominantly presented in the conventional impregnation solution, the monomer formed in ammonia solution could efficiently diffuse into the micropores and/or channels of the HZSM-5, resulting in higher dispersion of Mo species as well as enhanced interaction with the surface –OH groups of HZSM-5. Consequently, the obtained Mo/HSZM-5 catalyst showed rather high catalytic stability and greatly enhanced selectivity towards benzene for methane dehydroaromatization reaction by effectively inhibiting the coke formation.     

Electrochemical redox reactions of chromium and iron ions in molten NaCl–2CsCl eutectic for pyro-reprocessing of nuclear fuels

Basic electrochemical and spectroscopic properties of Cr³⁺, Cr²⁺, Fe³⁺, and Fe²⁺ were studied to analyze the cyclic redox reactions of Cr and Fe, which may decrease the current efficiency of the electro-winning method using NaCl–2CsCl melts. The formal redox potentials of the and couples, and , in NaCl–2CsCl melts at 923 K were spectroelectrochemically determined to be −0.648 ± 0.005 V and , respectively. These values were determined by measuring electromotive force and UV–VIS absorption spectra at varying concentration ratios of trivalent and divalent ions. Cyclic voltammetry was also carried out to examine the characteristics of the voltammograms for the and couples in NaCl–2CsCl melts. The determined by the spectroelectrochemical method was close to that determined by cyclic voltammetry . The effect of temperature on the in NaCl–2CsCl melts was studied by cyclic voltammetry in the range from 823 to 1,023 K . Diffusion coefficients of Cr³⁺ and Cr²⁺, and , were determined between 823 and 1,023 K to be and , respectively. Molar absorptivities of Cr³⁺ and Cr²⁺ in NaCl–2CsCl melts at 923 K were determined to be 77.8 ± 2.4 M⁻¹ cm⁻¹ at 17,670 cm⁻¹ and 48.0 ± 1.4 M⁻¹ cm⁻¹ at 9,170 cm⁻¹, respectively. In addition, the effects of these ions on the cyclic redox reaction of the pyro-reprocessing process were discussed.     

Determination of equilibrium silver sulphate concentration in the system Cu-H2SO4-CuSO4-H2O-Ag2SO4-Ag as a function of composition

The value of the ratio \(\gamma _{{\text{Cu}}^{{\text{2 + }}} } /\gamma _{{\text{Ag}}^{\text{ + }} }^2 \) ( \(\gamma _{{\text{Cu}}^{{\text{2 + }}} } ,\gamma _{{\text{Ag}}^{\text{ + }} } \) -are the mean activity coefficients of copper and silver ions, respectively) was calculated from the measured emf of the cell $${\text{Cu(Hg)|H}}_{\text{2}} {\text{SO}}_{\text{4}} {\text{ (}}c_{\text{x}} {\text{)}} - {\text{CuSO}}_{\text{4}} {\text{ (}}c_{\text{y}} {\text{)|Hg}}_{\text{2}} {\text{SO}}_{\text{4}} {\text{, Hg}}$$ and the solubility of Ag₂SO₄ in H₂SO₄ (c _x) and CuSO₄ (c _y) solutions. The concentration of H₂SO₄ in the solution was varied from 0.5 to 2.1 mol dm^?3 that of CuSO₄ from 0.4 mol dm^?3 to saturation. The results were presented as a function: $$\frac{{\gamma _{{\text{Cu}}^{{\text{2 + }}} } }}{{\gamma _{{\text{Ag}}^{\text{ + }} }^2 }} = a_0 + a_1 c_{\text{x}} + a_2 c_{\text{y}} + a_3 c_{\text{x}}^{\text{2}} + a_4 c_{\text{x}} c_{\text{y}} + a_5 c_{\text{y}}^2 .$$ This function allows the estimation of the equilibrium silver ion concentration \(c_{{\text{Ag}}^{\text{ + }} }^{{\text{eq}}} \) in solutions containing both H₂SO₄ and CuSO₄ in the presence of metallic copper. The function is also very useful for the estimation of the \(c_{{\text{Ag}}^{\text{ + }} }^{{\text{eq}}} \) near a working copper electrode.     

Effects of SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles on the electrodeposition of Zn,Co and ZnCo. I. Electrodeposition in the absence of SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Electrodeposition of Zn, Co and ZnCo from acid sulfate solutions onto steel was investigated in this first part of a study of the effects of SiC or Al₂O₃ particles on these processes and the formation of ZnCo–SiC and ZnCo–Al₂O₃ electrocomposites. Zn electrodeposition shows a well-defined pre-bulk region, where the hydrogen evolution reaction (HER) and Zn underpotential deposition (upd) compete. Zn bulk electrodeposition begins with primary nucleation and diffusion-controlled growth, strongly dependent on conditions favoring previous Zn upd against HER. It is assumed that this first bulk process takes place over the upd Zn. Zn bulk electrodeposition is followed by secondary nucleation and growth. Co electrodeposition begins with a slow reduction in parallel with HER, followed by a faster reduction. strongly hinders the initial reduction. The ZnCo and Zn electrodeposition curves are initially similar, retaining features of pre-bulk and bulk Zn electrodeposition.     

The Effect of CO2 and H2O on the Kinetics of NO Reduction by CH4 Over Sr-Promoted La2O3

The influence of CO₂ and H₂O on the activity of 4% Sr-La₂O₃ mimics that observed with pure La₂O₃, and a reversible inhibition of the rate is observed. CO₂ causes a greater effect, with decreases in rate of about 65% with O₂ present and 90% in its absence, while with H₂O in the feed, the rate decreased around 35-40% with O₂ present or absent. The influence of these two reaction products on kinetic behavior can be described by assuming competitive adsorption on the surface, incorporating adsorbed CO₂ and H₂O in the site balance, and using rate expressions previously proposed for this reaction over Sr-promoted La₂O₃. In the absence of O₂, the rate expression is $$r_{N_2 } = \frac{{k'P_{{\text{NO}}} P_{{\text{CH}}_{\text{4}} } }}{{{\text{(1 + }}K_{{\text{NO}}} P_{{\text{NO}}} {\text{ + }}K_{{\text{CH}}_{\text{4}} } P_{{\text{CH}}_{\text{4}} } {\text{ + }}K_{{\text{CO}}_{\text{2}} } P_{{\text{CO}}_{\text{2}} } {\text{ + }}K_{{\text{H}}_{\text{2}} {\text{O}}} P_{{\text{H}}_{\text{2}} {\text{O}}} {\text{)}}^{\text{2}} }},$$ which yields a good fit to the experimental data and gives optimized equilibrium adsorption constants that demonstrate thermodynamic consistency. With O₂ in the feed, nondifferential changes in reactant concentrations through the reactor bed were accounted for by assuming integral reactor behavior and simultaneously considering both CH₄ combustion and CH₄ reduction of NO, which provided the following rate law for total CH₄ disappearance: $$(r_{{\text{CH}}_{\text{4}} } )_{\text{T}} = \frac{{k'_{{\text{com}}} P_{{\text{CH}}_{\text{4}} } P_{{\text{O}}_{\text{2}} }^{{\text{0}}{\text{.5}}} + k'_{{\text{NO}}} P_{{\text{NO}}} P_{{\text{CH}}_{\text{4}} } P_{{\text{O}}_{\text{2}} }^{{\text{0}}{\text{.5}}} }}{{{\text{(1 + }}K_{{\text{NO}}} P_{{\text{NO}}} {\text{ + }}K_{{\text{CH}}_{\text{4}} } P_{{\text{CH}}_{\text{4}} } {\text{ + }}K_{{\text{O}}_{\text{2}} }^{{\text{0}}{\text{.5}}} P_{{\text{O}}_{\text{2}} }^{{\text{0}}{\text{.5}}} {\text{ + }}K_{{\text{CO}}_{\text{2}} } P_{{\text{CO}}_{\text{2}} } {\text{ + }}K_{{\text{H}}_{\text{2}} {\text{O}}} P_{{\text{H}}_{\text{2}} {\text{O}}} {\text{)}}^{\text{2}} }}.$$ The second term of this expression represents N₂ formation, and it again fit the experimental data well. The fitting constants in the denominator, which correspond to equilibrium adsorption constants, were not only thermodynamically consistent but also provided entropies and enthalpies of adsorption that were similar to values obtained with other La₂O₃-based catalysts. Apparent activation energies typically ranged from 23 to 28 kcal/mol with O₂ absent and 31-36 kcal/mol with O₂ in the feed. With CO₂ in the feed, but no O₂, the activation energy for the formation of a methyl group via interaction of CH₄ with adsorbed NO was determined to be 35 kcal/mol.     

The mechanism of manganese electrodeposition

The mechanism of manganese electrodeposition from a sulphate bath on to a stainless-steel substrate has been studied by using current efficiency data to resolve the totali-E curves. A simple, two-step electron transfer mechanism: $${\text{Mn}}^{{\text{ + + }}} + {\text{e}}\xrightarrow{{{\text{r}}{\text{.d}}{\text{.s}}}}{\text{Mn}}^{\text{ + }} $$ $${\text{Mn}}^{\text{ + }} + {\text{e}} \to {\text{Mn}}$$ is proposed to explain the following experimentally obtained parameters: cathodic and anodic transfer coefficients, reaction order and stoichiometric number. The mechanism also explains the effect of pH oni _o,Mn and on the corrosion currents.     

Analysis of the kinetics for the H2 + - \frac{1}{2}O2 ⇌ H2O reaction on a hot Pt surface in the pressure range 0.10–10 Torr

The H₂ + O₂ ⇌ H₂O reaction on platinum at 700 and 1300 K has been studied. A stagnation flow geometry was used with a gas mixture of H₂ and O₂ at pressures between 0.10 and 10 Torr. Comparing SHG results with simulations using different reaction parameters, it was concluded that , and . LIF measurements showed an ambiguity in the choice of main water-producing channel. Both hydrogen addition with low sticking coefficients and hydroxyl disproportionation with high sticking coefficients are plausible. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of temperature on the electro-dissolution of tin in NaCl aqueous solution at pH4

The influence of temperature in the range 25 to 80°C on the dissolution of tin was investigated in an acidic solution at pH4 containing 0.1 to 1m NaCl. The corrosion current is slightly dependent on both the temperature and the Cl^– ion concentration. The main dissolution characteristics of tin are

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:

Solid state ionics—the electrochemical analog memory cell with solid electrolyte

A new type analog memory cell with variable output voltage has been proposed and its performance examined. The cell construction is $$\begin{gathered} {\text{Ag|RbAg}}_{\text{4}} {\text{I}}_{\text{5}} {\text{|(Ag}}_{\text{2}} {\text{Se)}}_{{\text{0}} \cdot {\text{925}}} {\text{(Ag}}_{\text{3}} {\text{PO}}_{\text{4}} {\text{)}}_{{\text{0}} \cdot {\text{075}}} {\text{|RbAg}}_{\text{4}} {\text{I}}_{\text{5}} {\text{|Ag}} \hfill \\ {\text{ }} \uparrow \hfill \\ {\text{ Pt}} \hfill \\ \end{gathered} $$ in which (Ag₂Se)_0.925(Ag₃PO₄)_0.075 is a mixed conductor exhibiting high ionic and electronic conductivity at room temperature. The potential difference between the silver electrode and the platinum electrode depends on the silver activity in the mixed conductor, and it is changed by passing the current between one silver electrode and the platinum electrode. The output voltage of the cell is changed in the range of 150 to 0 mV. At open circuit, the memorized cell voltage decreased by only 1% over several hours.     

Electrochemical behavior of AZ91D magnesium alloy in phosphate medium: Part II. Induced passivation

Induced passivation of AZ91D magnesium alloy in phosphate solution was carried out both chemically, using various inorganic oxidants, namely, molybdate, vanadate and iodate, as well as electrochemically by anodizing the alloy under various controlled overpotentials within the range 0.1–3.4 V. In acidic phosphate (pH 4.5), molybdate and vanadate anions exhibit similar behavior, as they show a dissolution effect at lower concentrations and passivation at higher concentrations. On the other hand, iodate anions shows critical behavior with a passivation effect up to 0.1 mM and depassivation for higher concentrations. Generally, over the concentration domain (0.01–1.0 mM) the results reveal small inhibitive effects with maximum values of 19.7% for and 24–25% for and manifesting weak propensities for these inorganic species to enhance the corrosion resistance of AZ91D alloy in acidic phosphate medium. The effect of anodic potential on the characteristics of surface films formed on the alloy in alkaline phosphate solution (pH 11.9) indicates that higher forming overpotential induces better passivation due to the formation of rather thicker and more resistive anodic films. The stability of the films is greater in alkaline as compared to acidic phosphate solutions.     

Electrochemical oxidation of alcoholamines at gold

Gold is a good electrocatalyst for alcoholamine oxidation in basic media. In this work the effect of alcoholamine concentration, electrolyte pH and potential scan rate on electrooxidation was studied. The adsorption of alcoholamines on a layer plays a significant role in the oxidation mechanism. The rate determining step of the process was found to be heterogeneous dehydrogenation of the alcoholamine molecule, involving electron transfer to the gold electrode and the formation of water molecule. The catalytic effect of the gold electrode on alcoholamine oxidation is higher than that observed both for aliphatic alcohol and amines.     

Feasibility of determining corrosion rates by means of stray current-induced polarisation

To measure metal corrosion it is necessary to make direct physical contact with the specimen or component acting as the working electrode. The most widely used method for determining the corrosion current is electrochemical, the so-called polarisation resistance method. The feasibility of a new method to polarise the metal without the need for direct physical contact was investigated in the present study. The required potential shift is obtained by induction of a current using an external electrical field. The model used for calculation of the corrosion rate assumes that the applied current runs in parallel through the electrolyte and the metal, and electrostatically polarises the metal. The overall electrical resistance of the system can then be expressed as , where R _e+M is the total resistance measured, R _e is electrolyte resistance and R _M is resistance due to the metal. This latter resistance to electrostatic polarisation is related to the faradaic reaction, which was verified by comparison with the gravimetric losses. This resistance has been called inductive polarisation resistance R _pi to distinguish it from the traditional polarisation resistance R _p.     

Reaction in the silver zinc cell

An adiabatic calorimeter was used to measure the thermodynamics of the silver zinc cell. The charge and discharge reactions were shown to take place in two stages involving the production of argentous oxide and argentic oxide respectively. No thermal evidence was found to suggest the existence of a higher oxide of silver. The cell reactions were (1) $$2{\text{Ag + ZnO}} \leftrightharpoons {\text{Ag}}_{\text{2}} {\text{O + Zn, }}\Delta {\text{H = 158}} \cdot {\text{7 kJF}}^{ - {\text{1}}}$$ (2) $${\text{Ag}}_{\text{2}} {\text{O + ZnO}} \leftrightharpoons {\text{Ag}}_{\text{2}} {\text{O}}_{\text{2}} {\text{ + Zn, }}\Delta {\text{H = 176}} \cdot 1{\text{ kJF}}^{ - {\text{1}}}$$ If the cell was left on open circuit for a long period, or the positive electrodes heated, reaction (2) was suppressed and the discharge took place via reaction (1), without any reduction in capacity.     

Study of macroinitiator efficiency and microstructure–thermal properties in the atom transfer radical polymerization of methyl methacrylate

The atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) with poly vinylacetate macroinitiator (PVAc-CCl₃) and CuCl/PMDETA as catalyst was successfully carried out in bulk and solution. The apparent propagation rate constant () and concentration of active species ([P°]) were higher in the bulk. In solution they increased with polarity of solvent. Two different molecular weights of macroinitiators were used in ATRP of MMA. The linear relation of Ln[M]₀/[M] versus time was only confirmed for the low molecular weight macroinitiator. The ratio of was calculated in the bulk reaction with the low molecular weight macroinitiator, this ratio was 1.77 × 10¹⁴ M⁻¹ s⁻¹ for larger macroinitiator in solution. The MWD of block copolymers were sharper with lower molecular weight macroinitiator in the solution, but it appeared broader in the bulk polymerization. Our results indicated that smaller molecular weight macroinitiator was more efficient and formed a block copolymer with lower PDI. Thermal analysis and microstructure of the block copolymers are investigated by ¹H NMR, FT-IR, TGA and DSC. The chain tacticity of the MMA units is found not to be sensitive to the kinetic of the reactions with two different molecular weights of macroinitiator. DSC measurement shows two different transitions at 39 and 108 °C assigned to PVAc and PMMA blocks. The TGA profile shows a three-step degradation. The initial small weight loss that occurs around 220 °C and two large weight loss around 238 and 310 °C are attributed to dechlorination step and decomposition of the PMMA and PVAc blocks.     

Reaction kinetics of silicon etching in HF-K2Cr2O7-H2O solution

The reaction kinetics of silicon etching in HF-K₂Cr₂O₇-H₂O solution was studied experimentally. The etch rates were measured with varying HF and K₂Cr₂O₇ concentrations, agitation speed reaction temperature and time. The etch rates of n- and p-Si (100) were both similar. The etchec surfaces consisted mainly of silicon and showed a relatively smooth and planar morphology. At suffi ciently high HF concentration, the etch rate was increased with increasing K₂Cr₂O₇ concentratior due to the increase of hole formation on the silicon surface. However, at low HF concentration the etch rate maintains low value and increases very slowly because of insufficient hole concentratior for etching reaction. The apparent activation energy was about 7.8 kcal/g-mole, and the rate equatior for the silicon etching reaction in HF-K₂Cr₂O₇-H₂O solution was obtained as-r_Si = 600 exp(-3900/T) $$ - r_{Si} = 600{\text{ exp( - 3900/T) C}}_{{\text{K}}_{\text{2}} {\text{Cr}}_{\text{2}} {\text{O}}_{\text{7}} } ^{05} {\text{ C}}_{{\text{HF}}} ^3 $$ C_hk ³ at HF concentrations greater than 8 M.     

Measurement and regulation of oxygen content in selected gases using solid electrolyte cells. IV. Accurate preparation of CO2-CO and H2O-H2 mixtures

Mixtures of CO₂-CO, H₂O-H₂ and Ar-H₂O-H₂ of precise composition were prepared using a zirconia pump and analysed with a zirconia gauge. The ratio was varied from 5×10^–2 to 10⁴ and the ratio from 3×10^–4 to 10^–2. A Faraday's Law test proved to be a simple and reliable procedure for checking the conditions of utilization of these gaseous mixtures and for verifying that no significant disproportionation of CO or leakage along the gas circuit altered the prepared composition. From a practical point of view the best methods of preparing mixtures with low oxygen activity are reduction of carbon dioxide in the range 5×10^–11 to 10^–17 atm and oxidation of inert gas-H₂ in the range 10^–19 to 10^–27 atm at 800°C.     

Hydrogen half-cells in molten salts. A potentiometric investigation of the systems (Pt or Au) H2O/H2, OH− in fused alkali nitrates

The potentiometric behaviour of the hydrogen electrodes (Pt or Au) H₂O-H₂, OH_–has been investigated in molten (Na_0·5, K_0·5)NO₃ at 503 K. In both cases the potential of the indifferent electrode could be expressed by the general equation [H₂O]/[H₂] [OH^–] which is different from the one expected on the basis of a Nernstian behaviour of the theoretical overall system 2H₂O+2e=H₂+2OH^–.The experimental findings are discussed in terms of mechanistic models involving the actual electrode surface and the standard potential for the theoretical (reversible) hydrogen electrode is calculated: =–·0V(versus Ag/Ag⁺ 0·07 M).     

Behaviour of nitrate impurity in nickelcadmium cells

The oxidation-reduction behaviour of NO₃ ^?, NO₂ ^?, N₂O₂ ^2?, NH₂OH and NH₃ at a platinum electrode in alkaline solution has been investigated using cyclic voltammetry. The results have been compared with the corresponding behaviour of these species at charged, porous cadmium and nickel hydroxide electrodes in order to understand the likely behaviour of NO₃ ^? impurities in nickelcadmium cells. The reactions are shown to be irreversible processes and strongly dependent on the nature of the electrode surfaces. The reactions which are likely to be involved in a charged cell can be represented by the overall scheme: $${\text{NO}}_{\text{3}} ^\_ {\text{NO}}_{\text{2}} ^\_ {\text{NH}}_{\text{3}} \xrightarrow{{{\text{slow}}}}{\text{N}}_2 $$ It is suggested that the self-discharge of cells containing NO₃ ^? is limited by slow kinetic effects rather than by diffusion as previously supposed.     

Theoretical maximum dye-uptake of acrylic fiber to basic dye

The theoretical maximum dye-uptake of the fiber to dye is considered to be related to the fraction of the polymer segment which is mobile. This is studied by dyeing the acrylic fiber at temperatures within a 40 °C range while treating the samples with different carriers. The relation of the equilibrium dye-uptake (dyeability) of the acrylic fiber to the structural parameter, , with the presence or absence of carrier in solution, is linear, confirming the validity of the hypotheses, where (tanδ)_c is the loss tangent correlated at a temperature of 20 °C. Moreover, The effect of the carrier in swelling the amorphous region of the acrylic fiber was less effective as temperature rose beyond T _g(mechanical).