The electrochemistry of tellurium in methylene chloride

It was found that TeCl₄ in methylene chloride (containing tetrabutylammonium perchlorate as a supporting electrolyte) is reduced to TeCl₂, Te⁰ and Te⁻² in potential applied. Two reduction waves are observed during the reduction of TeCl₄ at a platinum rde (Ep₁ = 0.08 ± 0.02 V, Ep₂ = −1.10 ± 0.02 V) due to the reduction of TeCl₄ to Te and Te⁻² respectively. A cathodic deposition of tellurium from TeCl₄ is followed by anodic stripping wave (Ep₁ = 0.42 ± 0.02 V), corresponding to the oxidation of Te to TeCl₂. It has been shown that reduction of TeCl₄, or TeCl⁻²₆ to Te causes coating of the electrode with metallic tellurium, on whose surface chloride ions are strongly bonded. It was found that when the electrolysis solution contains excess of chloride ion with respect to Te^IV the peak potentials of the cathodic waves shift to more cathodic values (Ep₁ = 0.30 ± 0.02 V, Ep₂ = −1.25 ± 0.02 V). After cathodic deposition of at least a monolayer of tellurium from such a solution two anodic waves appear (Ep₁ = 0.12 ± 0.03 V, Ep₂ = 0.24 ± 0.03 V) which are both due to the oxidation of Te to Te^II.     

The determination of the potential of zero charge on solid metals

An attempt has been made to evaluate several experimental methods by which the potentials of zero charge (pzc) of metals could be concordantly measured. The methods chosen involved measurements of the adsorption in the double layer of a neutral organic compound as a function of the concentration of the supporting electrolyte; double-layer capacitance as a function of potential; and static friction between surfaces in solution.

The pzc for Pt containing a negligible amount of H is E_q=0 = 0·56 (±0·07) − 0·067 pH (3 methods). For Au, E_q=0 = 0·15 ± 0·02 (2 methods). For Ag, E_q=0 = −0·44 ± 0·02 (2 methods). For Ni, E_q=0 = −0·26 ± 0·06 (3 methods). Introduction of H into Pt shifts the pzc in the negative direction. Anodic and cathodic pulsing of Au and Ag electrodes produced surfaces which gave pzc values 0·2 and 0·4 V respectively more positive than those for unpulsed electrodes.

Results reported earlier for Pt are for Pt containing dissolved H. The dissolved proton concentration, abnormally high near the surface, contributes significantly to the surface potential. The results obtained oil pulsing of silver and gold are for dermasorbed surfaces. The most satisfactory methods for the determination of the pzc are the capacitance, static friction, immersion, and tension vibration methods.     

Mercury-mercurous propionate electrode: standard potentials at different temperatures and related thermodynamic quantities

The standard molal potentials E°_m of the Hg/Hg₂(OPr)₂, OPr⁻ electrode at 15°, 20°, 25°, 30° and 35° C have been determined. The E°_m values obtained are 0.5114, 0.5072, 0.5031, 0.4988 and 0.4942 V respectively, which can be fitted to the equation Edg_m/V = 0.5031 −8.56 × 10⁻⁴ (itt/°C − 25)−3.0588 × 10⁻⁶ (t/ °C -25)². The changes in standard free energy, entropy and enthalpy for the cell reaction have been calculated.     

CoBr2-MnBr2 containing catalysts for catalytic oxidation of p-xylene to terephthalic acid

Catalytic oxidation of p-xylene (PX) to terephthalic acid (TA) was studied with catalysts containing cobalt acetate, manganese acetate, CoBr₂ and MnBr₂. The catalysts contain neither highly corrosive hydrogen bromide nor other metal ions, and have the advantage of easy catalyst recovery. The effects of Br/Co atomic ratio, reaction time and temperature, PX concentration, oxygen pressure, and catalyst concentration on PX conversion and product/intermediate yields were investigated. The catalyst system had a suitable reaction temperature of 100 °C, which was much lower than the commercial process temperature (175–225 °C). The maximum product (TA) yield was 93.5%, obtained at a Br/Co atomic ratio of three. Higher Br concentration resulted in the lower TA yield, which was ascribed to the benzylic bromide formation. The synthesis of TA could be adequately described as four reaction steps in series (PX → p-tolualdehyde → p-toluic acid → 4-carboxybenzaldehyde → TA), with a pseudo-first-order rate equation for each step, and the third step was rate-limiting. The rate constant ratios (k_j/k₃, j = 1 → 4) obtained at 100 °C were similar to the k_j/k₃ values reported earlier for cobalt acetate/manganese acetate/HBr catalysts in a range of 185–191 °C.     

Kinetics of Fischer–Tropsch synthesis on titania-supported cobalt

Rate data have been obtained for CO hydrogenation on a well-characterized 11.7% Co/TiO₂ catalyst in a differential fixed bed reactor at 20 atm, 180–240°C, and 5% conversion over a range of reactant partial pressures. The resulting kinetic parameters can be used to model precisely and accurately the kinetics of this reaction within this range of conditions. Turnover frequencies and rate constants determined from this study are in very good to excellent agreement with those obtained in previous studies of other cobalt catalysts, when the data are normalized to the same conditions of temperature and partial pressures of the reactants. Based on this comparison CO conversion and the partial pressure of product water apparently have little effect on specific rate per catalytic site. The data of this study are fitted fairly well by a simple power law expression of the form −r_CO=kP_H2^0.74P_CO^−0.24, where k=5.1×10⁻³ s⁻¹ at 200°C, P=10 atm, and H₂/CO=2/1; however, they are best fitted by a simple Langmuir–Hinshelwood (LH) rate form −r_CO=aP_H2^0.74P_CO/(1+bP_CO)² similar to that proposed by Yates and Satterfield.     

Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered -titanate materials, Na_xM_x/2Ti_1−x/2O₂ (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na_0.4Ni_0.2Ti_0.8O₂ were ca. 7×10³ S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm⁻¹ K⁻¹ at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na_0.4Ni_0.2Ti_0.8O₂ and Na_0.4Co_0.2Ti_0.8O₂ was about 1.9×10⁻⁴ and 1.2×10⁻⁴ K⁻¹ around 700 °C, respectively.     

On the formation of pyronitrate in molten salts

N₂O₅ reacts with O²⁻ ion in LiCl---KCl eutectic at 450° to give NO₃⁻. By analogy to the salts of the other oxides of Group V, NO₃⁻ can be considered as metanitrate and is expected to give—under appropriate conditions—the corresponding pyro-salt. Experiments are described in which the O²⁻ ion in LiCl---KCl melt is potentiometrically titrated with KNO₃. The titration curves show an inflexion at the composition corresponding to pyronitrate, N₂O₇⁴⁻.

The formation of pyronitrate in KNO₃ melts is also established. Strong oxide-ion donors, eg Na₂O₂ or NaOH, or electrolytically generated O²⁻ ion, react slowly with the melt to produce a compound of less basic character. The reaction is zero-order with respect to O²⁻ and has an activation energy of ca 6·17 Kcal/mole.

Pyronitrate in molten KNO₃ possesses a basicity comparable to that of the carbonate ion in the same melt. It readily lends its oxide ion to strong acids eg, Cr₂O₇²⁻ and PO₃⁻. X-ray diffraction patterns of NO₃⁻-N₂O₇⁴⁻ mixtures show peaks that can be correlated to the new anion.     

Decolorization of organic dyes on Pilkington Activ™ photocatalytic glass

The air–solid photocatalytic degradation of organic dye films Acid Blue 9 (AB9) and Reactive Black 5 (RBk5) is studied on Pilkington Activ™ glass. The Activ™ glass comprises of a colorless TiO₂ layer deposited on clear glass. The Activ™ glass is characterized using atomic force microscopy (AFM) and X-ray diffraction (XRD). Using AFM, the TiO₂ average agglomerate particle size is 95 nm, with an apparent TiO₂ thickness of 12 nm. The XRD results indicate the anatase phase of TiO₂, with a calculated crystallite size of 18 nm.

Dyes AB9 and RBk5 are deposited in a liquid film and dried on the Activ™ glass to test for photodecolorization in air, using eight UVA blacklight-blue fluorescent lamps with an average UVA irradiance of 1.4 mW/cm². A novel horizontal coat method is used for dye deposition, minimizing the amount of solution used while forming a fairly uniform dye layer. About 35–75 monolayers of dye are placed on the Activ™ glass, with a covered area of 7–10 cm². Dye degradation is observed visually and via UV–vis spectroscopy.

The kinetics of photodecolorization satisfactorily fit a two-step series reaction model, indicating that the dye degrades to a single colored intermediate compound before reaching its final colorless product(s). Each reaction step follows a simple irreversible first-order reaction rate form. The average k₁ is 0.017 and 0.021 min⁻¹ for AB9 and RBk5, respectively, and the corresponding average k₂ is 2.0 × 10⁻³ and 1.5 × 10⁻³ min⁻¹. Variable light intensity experiments reveal a p = 0.44 ± 0.02 exponent dependency of initial decolorization rate on the UV irradiance. Solar experiments are conducted outdoors with an average temperature, water vapor density, and UVA irradiance of 30.8 °C, 6.4 g water/m³ dry air, and 1.5 mW/cm², respectively. For AB9, the average solar k₁ is 0.041 min⁻¹ and k₂ is 5.7 × 10⁻³ min⁻¹.     

Degradation of PZT-4D hard piezoceramics under moderate heating

The changes produced in the dielectric permittivity, the transverse piezoelectric coefficient, the k₃₁ coupling factor, the s₁₁^E elastic compliance and the mechanical quality factor of PZT-4D hard piezoceramics by heating have been investigated. The ceramics were then repoled, and the reversible and irreversible components of the changes quantified. The results showed that depolarisation began at 150°C. A high level of poling was retained even after heating at 300°C (d₃₁=−83×10⁻¹² C N⁻¹ and k₃₁=0.225), only 20°C below the transition temperature. However, a significant irreversible degradation of the mechanical quality factor, Q_m, occurred at a temperature as low as 100°C. Experiments on thinned specimens showed that the degradation of Q_m took place in the Ag doped layer produced by the electrodes. Indentation surface cracks were also introduced into the ceramics to investigate the behaviour of cracks during the thermal treatments.     

The anodic oxidation of thiocyanate ion dissolved as KSCN in dimethylsulphoxide

The anodic oxidation of SCN⁻ ion dissolved as KSCN in dimethylsulphoxide, on platinum electrodes, was investigated at ca 25, 60 and 160°C by means of various non-stationary electrochemical techniques. At low temperatures one anodic and two main cathodic processes were found. The anodic oxidation of SCN⁻ ion yields as primary product the SCN radical, which readily produces (SCN)₂. The latter can be in part reduced back to SCN⁻ ion, because it reacts in part yielding solvated hydrogen ions which cause the second cathodic reaction. In the region of 60°C, no (SCN)₂ cathodic current is observed. In the region of 160°C the only reaction is SCN⁻ ion oxidation and the primary product polymerizes to (SCN)_x, which forms a film on the electrode, causing passivation.

On the basis of kinetic data obtained for the different reactions, a mechanism for the anodic oxidation of the SCN⁻ ion and for the passivating film formation is suggested. The second process only partly fits the Müller model for the electrochemical growth of insoluble layers.     

Theoretical investigation on cyclic reciprocal derivative chronopotentiometry: Kinetic study of totally irreversible electrode processes

Theoretical expressions and mathematical analysis in cyclic reciprocal derivative chronopotentiometry (CRDCP) are presented for totally irreversible electrode processes corresponding to the application of symmetrical and unsymmetrical programmed currents. For two successive unsymmetrical programmed currents, the effect of the currents ratio b (b = |I₂(t)/I₁(t)|) on the (dt/dE)–E curves is discussed. The electrochemical behavior of totally irreversible electrode processes has been studied corresponding to the application of the unique unsymmetrical programmed Φ^m(I₀) proposed recently. CRDCP characteristic parameters obtained for totally irreversible electrode processes are different from those of reversible electrode processes. Therefore, a comparison of CRDCP between both mechanisms is presented. Based on the mathematical derivation, alternative methods for kinetic measuring are described. It is prospected that CRDCP is convenient and applicable for studying the reversibility of the electrode processes in form of CRDCP characteristic parameters.     

The solubility of ettringite at 25°C

Available solubility constants indicate that ettringite should be the stable form of calcium aluminate sulfate hydrate with respect to monosulfate in cement porewater. However, monosulfate is generally present in mature cement pastes, usually in the absence of ettringite. The objectives of this study were to determine the solubility product of ettringite under equilibrium conditions and to examine the solubility data used in predictive thermodynamic models. Solubility products were calculated for ettringite prepared from both supersaturated and undersaturated solutions with a pH range between 10.4 and 13.7. The mean solubility product for ettringite dissolution: Ca₆Al₂O₆(SO₄)₃ · 32H₂O → 6Ca²⁺ + 2Al(OH)₄⁻ + 3SO₄²⁻ + 4OH⁻ + 26H₂O was 10^−44.91, i.e. Log K_sp = -44.91 ± 1.06 (2 S.D.). Activity coefficients were calculated using the specific ion interaction approach. The mean solubility product was close to other values calculated from concentrations reported elsewhere for the solubility of ettringite. As is the case for all solubility products, this value cannot be inserted directly into the databases of other thermodynamic models because of differences in the methods used to calculate activity coefficients and the manner in which ion-pairing is handled by different models. However, raw solubility data are provided for recalculation of the solubility product for use in other models.     

Estimation of particle size distribution parameters in animal lungs

A constant of specific solubility of 2·5 × 10⁻⁸ g cm⁻²day⁻¹ was determined for fused aluminosilicate particles, by observing in vivo retention kinetics after intravenous injection into rats. Studies over the past years in this laboratory, in which dogs and rats have inhaled labeled aerosols of these particles, have shown retention half-lives in the lung of 460 and 285 days, respectively. By applying these values for solubility and half-life to Mercer's theory of dissolution from the deep lung, the initial distribution of particles deposited in the pulmonary regions of dogs and rats following inhalation was calculated. From an inhaled aerosol with a mass median diameter, D_m, of 1·0 μm and σ_o = 1·7, a distribution described by D_m = 0·51 μm and σ_o's ranging from 1·16–1·48 was estimated to have been deposited in the Beagle dog lung. and a D_m =< 0·32 μm and σ_o's ranging from 1·18–1·29 was similarly calculated for rats.     

Conductivity of Na5+xYAlxSi4-xO12 ceramics

A series of ceramics samples, Na_5+xYAl_xSi_4-xO₁₂, has been prepared by a solid state reaction with the starting materials of SiO₂, Y₂O₃, Al₂O₃ and Na₂CO₃. Their crystalline structure and morphology have been studied by the determination of XRD, IR, TG, DTA and SEM. Their conductivity has been measured by means of the complex impedance method. The dependence of the conductivity and density of the samples on the amount of the added Al₂O₃ and the reaction between the conductivity and the temperature have been discussed. When x = 0, the density of the sintering sample is 90% T.D., and the conductivity is 1·48 x 10⁻¹ (ωcm)⁻¹ at 300°C; when x = 0·1, the density is up to 97% T.D., and the conductivity up to 1·74 x 10⁻¹ (ω cm)⁻¹ at 300°C.     

Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC

Supported LaCoO₃ perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce_1−xZr_xO₂ (x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N₂ adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T₅₀ than the corresponding Ce_1−xZr_xO₂ supports. Twenty weight percent LaCoO₃ catalysts presented lower T₅₀ than bulk LaCoO₃. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO₃ by the surface area and the oxygen mobility.     

Polarographic behaviour of methyl 3-mercapto-propionate

The polarographic behaviour of methyl-3-mercaptopropionate, H₃COOC CH₂CH₂SH has been studied in 25% ethanol, 0·1 M NaClO₄ and 0·002% Triton X-100 in the pH range 4·60−11·36 at a dme. The electrode process is irreversible and the plateau current is diffusion-controlled. The half wave potentials of the waves shift towards more negative potentials with increase in pH and the dissociation constant of the sulphydryl group is found to be 9·95. The values of the kinetic parameters, transfer coefficient () and formal rate constant (K°_b,h), have been calculated by Koutecky's method at pH 9·37 and are 0·48 and 1·13 × 10⁻⁴ cm/s respectively.     

Electrocatalytic oxidation of p-nitrophenol from aqueous solutions at Pb/PbO2 anodes

In this work, the elimination of p-nitrophenol (p-NPh) from aqueous solutions by electrochemical oxidation at Pb/PbO₂ anodes was investigated. The process was studied under galvanostatic polarization mode in acidic and alkaline media, as a function of the temperature (20, 40 and 60 °C) and of the anodic current density (J = 10, 20 and 30 mA cm⁻²). In acidic media (0.5 M H₂SO₄), the oxidation process allowed a 94% p-NPh conversion in 7 h, at 20 °C and with J = 20 mA cm⁻², with a wide distribution of degradation products (in particular: 39% p-benzoquinone and 26% hydroquinone, as given by a mass balance at the above electrolysis time). Under these conditions, the current efficiency for the substrate oxidation was 15.4% ([Ah L⁻¹]_exp = 7 versus [Ah L⁻¹]_theo = 1.08 Ah L⁻¹). In alkaline media (0.1 M NaOH, pH 8.5), the most effective p-NPh elimination (97%) was obtained at 60 °C, 20 mA cm⁻² and 420 min of electrolysis time, again with the production of p-benzoquinone and hydroquinone (52.7 and 15.1%, respectively). Under the latter conditions, an almost complete chemical oxygen demand (COD) abatement was attained, with a high level of p-NPh mineralization (>80%), a yield of p-NPh conversion greater than 95% and a scarce formation of aliphatic acids (most probably maleic acid). From the degradation curves ([p-NPh] versus t), in both acidic and alkaline media, the UV analyses and/or COD measurements, a complete oxidation of aliphatic acids to form CO₂ could be predicted for electrolysis time >420 min, according to a suggested oxidation pathway.     

Structure of a cyclopropane sorption complex of dehydrated fully Mn-exchanged zeolite X

The structure of a cyclopropane sorption complex of dehydrated fully Mn²⁺-exchanged zeolite X, Mn₄₆Si₁₀₀Al₉₂O₃₈₄ · 30C₃H₆ (a=24.690(4) Å), has been determined by single-crystal X-ray diffraction techniques in the cubic space group at 21(1)°C. The crystal was prepared by ion exchange in a flowing stream of 0.05 M aqueous Mn(NO₃)₂ for three days, followed by dehydration at 460°C and 2×10⁻⁶ Torr for two days, and exposure to 100 Torr of cyclopropane gas at 21(1)°C. The structure was determined in this atmosphere and was refined to the final error indices R₁=0.065 and R₂=0.071 with 509 reflections for which I>3σ (I). In this structure, Mn²⁺ ions are located at two crystallographic sites. Sixteen Mn²⁺ ions fill the octahedral site I at the centers of the hexagonal prisms (Mn–O=2.290(9) Å). The remaining 30 Mn²⁺ ions are at site II; each extends 0.41 Å into the supercage (an increase of 0.27 Å upon C₃H₆ sorption as compared to fully dehydrated Mn₄₆Si₁₀₀Al₉₂O₃₈₄) where it coordinates to three trigonally arranged framework oxygens at 2.148(8) Å and complexes weakly and facially to a cyclopropane molecule by a primarily quadrupolar interaction. The carbon atoms of each cyclopropane molecule are equivalent and equidistant from its Mn²⁺ ion (Mn–C=2.95(9) Å). Because of high thermal motion, the C–C bond length is inaccurately determined; the value found, 1.21(8) Å, is too small.     

Electrosorption and inhibition properties of p-norborn2-yl phenolate ions at the mercury/solution interface

The electrosorption properties of p-norborn-2-yl phenolate ions in alkaline solutions were investigated by ac polarographic and electrocapillary measurements.

Two adsorption regions were found. At low bulk surfactant concentrations the adsorption at the positively charged electrode (−0.2 E −0.6 V) is predominant while at higher surfactant concentrations the adsorption at the negatively charged electrode (−0.6 E −1.0 V) is more pronounced. At E = −0.40 V the adsorption parameters were determined (a ≈ 2; ΔG°_A = −32.5 ± 1 kJ mol⁻¹. Between −0.6 E −1.0 V one potential of maximum adsorption for all concentrations does not exist and therefore the adsorption parameters could not be calculated.

At E = −0.40 V progressive two-dimensional nucleation with a nucleation order of 3 was observed which corresponds well with the high attraction constant.

The electrode reaction S₂O²⁻₈ + 2e⁻ → 2 SO²⁻₄ is inhibited by norborn-2-yl phenolate ions in the potential range −0.2 E −0.6 V. In the second potential range of capacity decrease the electrode process is much less retarded. At E = −0.40 V, in a similar manner as described for neutral molecules, a linear dependence of the log k_s (k_s apparent rate constant) on ln c_A and π (π = surface film pressure), respectively, has been found.     

Use of amalgam concentration cells for determination of transference numbers in solid electrolytes—I. Electronic transference numbers of β-aluminas

The determination of transference numbers for different doped as well as undoped sodium-β-aluminas has been carried out by means of a method based on emf measurements of amalgam concentration cells of the type Na_x2Hg_1−x2/sodium-β-alumina/Na_x1Hg_1−x1 at 25°C. The details of this method, whose validity extends also to cationic conducting solids other than β-aluminas, are described and discussed for comparison with the results of previous investigations.