The anodic behaviour of lead amalgam electrodes in HCl solution

The anodic behaviour of the lead amalgam electrode has been investigated in aqueous hydrochloric acid solutions. Both voltammetric and potential pulse results are described. The mechanism of passivation is shown to be the nucleation and growth of three-dimensional nuclei of PbCl₂ which progressively block the electrode. The nuclei are considered to be right circular cones, distributed at random on the electrode surface. As in the case of solid lead electrodes, a simultaneous dissolution of PbCl_n^2?n complexes is observed, which diffuse away from the electrode under mass transport control. Evidence is also presented that the first stage in the growth of the anodic film is the two-dimensional nucleation and growth of a monolayer of PhCl₂. Unfortunately, this process is partially obscured by the dissolution reaction. A reaction scheme is proposed.     

Films of supersaturated Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>S solid solutions: Composition prognostication,chemical synthesis,microstructure

Working conditions and the concentration regions of the joint precipitation of PbS and CdS at which substitution solid solutions Cd _x Pb_1–x can be formed were determined by calculation of ionic equilibria in the citrate-ammonia reaction mixture at 298 and 353 K with consideration for the conversion fractions of lead and cadmium sulfides into the corresponding sulfides. The hydrochemical precipitation onto glass-ceramic substrates was used to obtain Cd _x Pb_1–x S (0 < x ≤ 0.149) solid solution films with thicknesses of 0.5 to 1.7 μm and high supersaturation with the substituting component. All the films crystallize to form the B1 structure. The phase and elemental compositions and morphological specific features of the films were studied. It was shown that the thickness of the deposited layers is most strongly affected by the process temperature, ammonium hydroxide concentration, and relative amounts of the metal salts in the reaction mixture. It was found that there are oxygen and chlorine in the Cd _x Pb_1–x S solid solutions, and the distribution of these elements across the layer thickness was determined, with the layer-by-layer distribution of chlorine having a pronounced oscillatory nature. It was shown that, as the chemical precipitation temperature is raised, the content of CdS in the substitution solid solutions grows exponentially. The activation energies Ea.ic of the lead and cadmium interchange in the PbS crystal lattice were found to be, depending on the initial concentration of the lead salt, 75.3, 42.8, and 22.2 kJ mol^–1.     

How does the extent of substitution of methane with chlorine influence the mechanism and kinetics of the reactions between chloromethanes and atomic chlorine

A theoretical investigation of the reaction mechanism and kinetics of the reaction between chloromethanes CH_4–xCl_x (x = 1–3) and chlorine atoms was performed. The height of the reaction barrier was found to decrease with the degree of substitution of chloromethanes with atomic chlorine. A direct dynamics method was employed to study the kinetic nature of these hydrogen-abstraction reactions. The sequence of calculated reaction rate coefficients is: k(CH₃Cl + Cl) < k(CH₂Cl₂ + Cl) < k(CHCl₃ + Cl).     

p-Type electron transport in La1−xSrxFeO3−δ at high temperatures

Electrical conductivity, thermopower and oxygen content were measured for La_1−xSr_xFeO_3−δ (x=0.2, 0.5, 0.9) within the oxygen partial pressure range 10⁻⁴-0.5 atm and at temperatures 750-950 °C. The dominating charge carriers under these experimental conditions are electron holes. The results of oxygen nonstoichiometry measurements are used to estimate the concentration of holes and to analyze data on conductivity and thermopower. The changes in thermopower are described by the model assuming that the number of sites accessible for migration of holes is independent on oxygen content. The mobility of electron holes is calculated, and it is found to be virtually independent on temperature in the compositions with x<0.5 while compositions with x>0.5 exhibit thermally activated mobility and mobility values about 0.1 cm² V⁻¹ s⁻¹ or smaller. It is suggested that the main contribution to the composition dependent variations in electron hole mobility are associated with Coulomb interactions at small x's, whereas at high degrees of doping the mobility of holes is most strongly affected by the increasing amount of oxygen vacancies.     

Tetraphenylphosphonium-trichloroplumbat(II), PPh4PbCl3 · CH3CN

Tetraphenylphosphonium Trichloroplumbate(II), PPh₄PbCl₃ · CH₃CN PPh₄PbCl₃ · CH₃CN was obtained by reaction of PbCl₂ and PPh₄Cl in acetonitrile. It was also formed along with (PPh₄)₂Se₂Cl₆ when PbSe was treated with chlorine in the presence of PPh₄Cl. Its crystal structure was determined by X-ray diffraction (R = 0.029 for 4186 reflections). The triclinic crystals contain PbCl₃^– ions that are associated to polymer chains. Each Pb atom has distorted square pyramidal coordination; the pyramids share two opposite basal edges. The chloro bridges are rather asymmetrical.     

Interaction of –CClx (x=1–3) with Ru2 and RuSn dimers: a density functional study

Density functional calculation were performed on the Ru₂ and RuSn metal dimers and the species formed from their interaction with –CCl_x (x=1–3) fragments. The importance of these fragments in the hydrodechlorination of carbon tetrachloride has been motivated this study aiming to contribute to understand the effect of the tin in the performance of the noble metals based catalysts. We have observed that the carbon tetrachloride does not form a precursor with the metal dimers. The CCl₄ readily dissociates forming adsorbed chlorine and –CCl₃ complexes. The chlorine atoms prefer to adsorb on the bridge sites and the RuSn–Cl binding energy is about 5 kcal mol⁻¹ larger than the Ru₂–Cl binding energy. The Ru₂–CCl_x (x=1–3) binding energies are larger than the respective RuSn–CCl_x (x=1–3) binding energies. However, the reaction energy of the dechlorination of CCl₄ leading to adsorbed –CCl_x (x=1–3) and chlorine is thermodynamically more favorable for RuSn than the Ru₂. The differences between Ru₂ and RuSn systems have been discussed based on the different interaction mechanism due to the presence of the tin center and its affinity for the chlorine atoms.     

Zur Chlorierung von Aluminium in Salzschmelzen durch Halogenaustausch mit Blei(II)-chlorid

On Chlorination of Aluminium in Molten Salts by Exchange of Halogen with Lead(II) Chloride AlCl₃ is formed by reaction of metal aluminium with PbCl₂ in a molten salt mixture. If at the same time chlorine gas is supplied, the halogen carrier used is recovered with a good yield. The combination of both partial processes in a continuous way is described.     

Das System TlClPbCl2

A reinvestigation of the x-T diagram was performed using DTA and X-ray techniques. Four compounds exist in the sytem of $\text{TlCl}\text{PbCl}{}_2$. The transformations of Tl₃PbCl₅ and TlPb₂Cl₅ are discussed. The compound Tl₂PbCl₄ exists only within a temperature interval of about 20°C. A compound of the formula Tl₇Pb₃Cl₁₃ is reported for the first time. Crystallographic data of TlPb₂Cl₅, high and low Tl₃PbCl₅, and Tl₇Pb₃Cl₁₃ are presented.     

Heats and kinetics of oxidation of small copper particles in copper-containing catalysts

The r.t. interaction of oxygen with finely-divided copper in Cu/ZnO catalysts has been studied by gas-solid microcalorimetry. The oxidation is accompanied by the presence of a pseudo-equilibrium pressure, which increases as the reaction proceeds. Surface and several subsurface layers are involved, but a metallic core is always left. The heat of reaction is the same for all samples and does not vary with the extent of reaction (ΔH=181 kJ mol (1/20₂)^?1). The rate of reaction depends upon the pressure, and decreases dramatically to a point where oxidation stps and molecular oxygen is weakly chemisorbed. The overall process is interpreted by assuming that the rate-determining step is the surface dissociation of a molecular precursor onto Cu atoms.     

Synthesis and Thermal Analysis of 2,2′-bipyridine Divalent Transition Metal Hexachloroplumbates

Several complex salts of the general formula [M(II) (bipy)_x(H₂O)_y]PbCl₆ (where x=2–3, y=0–2 and M=Mn(II), Fe(II), Ni(II), Co(II), Cu(II), Zn(II), Cd(II) and Hg(II)) were synthesized and investigated by DTA, TG and DTG. Some of the decomposition products were identified by IR spectroscopy and other methods. The compounds decompose with the liberation of water (in the case of hydrates), chlorine (sometimes causing chlorination of organic fragments), organic molecules (sometimes chlorinated) and sometimes hydrogen chloride. The residues comprise metal(II) chlorides and PbCl₂. This revised version was published online in August 2006 with corrections to the Cover Date.     

Preparation and characterization of the system BaBrxCl2−x: The structure of BaBrCl

The variations in the lattice parameters in the BaBr_xCl_2?x system have been determined for the composition range 2.0 > x > 0.0. All phases exhibit the PbCl₂-type orthorhombic structure and are assignable to space group Pnma. The crystallographic parameters of BaBrCl were determined by the Rietveld method from a line profile analysis of a digitized powder X-ray diffraction pattern. The anions are ordered with the bromide ion occupying the square pyramidal hole, and the chloride ion the smaller tetrahedral hole. Atomic coordinates are presented.     

Titanium reduction in a soluble ziegler—natta catalyst

The spontaneous reduction of Ti^IV to Ti^III in soluble Ziegler—Natta catalysts of type Cp₂RTiCl·R’AlCl₂ (Cp = h⁵ -cyclopentadienyl, R and R’ = methyl or ethyl) was studied both spectrally and chromatographically. Varied were R, R’, Al/Ti ratio, total concentration, solvent, and added olefin. Kinetic order in [Ti] could be varied from zero to second order by changing solvent. This can be explained by a mechanism in which a Cp₂ RTiCl—R’AlCl₂—olefin complex forms in the rate determining step and ligand R is expelled as half alkane half olefin. The expelled olefin may either polymerize or catalyze reduction by forming the rate-determining complex. Different apparent kinetic orders arise from differences in the olefin competitive reactions. The reaction products appear to form in a rapid bimolecular reaction following the rate-determining step. Evidence is presented that neither free radicals nor Cp₂RR’Ti are reduction intermediates. The intermediate is postulated to be a Ti^IV transient hydride formed by a reverse insertion step.     

Gas diffusion hindrances on Ni cermet anode in contact with Zr<Subscript>0.84</Subscript>Y<Subscript>0.16</Subscript>O<Subscript>1.92</Subscript> solid electrolyte

The electrochemical behavior of Ni cermet electrode with CeO_{2 ? x} additive in contact with YSZ electrode was studied by means of impedance spectroscopy in H₂, H₂O, CO₂, CO, He, and Ar gas media of various composition within the temperature range of 700 to 950°C. Near the equilibrium potential, the electrochemical impedance spectra of the studied electrodes indicate to three stages of electrode reaction. The polarization conductivity of the low-frequency stage of electrode reaction (σ_lf) is characterized with the following regularities: (a) temperature dependence of σ_lf has a positive slope in Arrhenius coordinates; (b) σ_lf increases upon replacement of gas mixture with lower mutual diffusion coefficient by mixture with higher mutual diffusion coefficient, while polarization conductivity values of other stages remain practically invariable; (c) concentration relationships of 1/σ_lfrecorded for constant activity of oxygen in the gas phase are linear in the 1/σ_lf vs. 1/P _{CO 2} (P _CO) coordinates; (d) no low-frequency stage of the electrode reaction is observed upon electrochemical inflow (outflow) of the gas reagents (reaction products) to (from) the test electrodes (current passing through closely pressed specimens and central specimen impedance measurement); and (e) no change in the gas flow rate affects σ_lf value. The observed regularities were explained by assuming the gas diffusion nature of the low-frequency stage of the electrode reaction. The gas diffusion layer thickness was estimated.     

A mechanism for the photochemical transformation of nitrate in snow

Photochemical reactions of trace compounds in snow have important implications for the composition of the atmospheric boundary layer in snow-covered regions and for the interpretation of concentration profiles in snow and ice regarding the composition of the past atmosphere. One of the prominent reactions is the photolysis of nitrate, which leads to the formation of OH radicals in the snow and to the release of reactive nitrogen compounds, like nitrogen oxides (NO and NO₂) and nitrous acid (HONO) to the atmosphere. We performed photolysis experiments using artificial snow, containing variable initial concentrations of nitrate and nitrite, to investigate the reaction mechanism responsible for the formation of the reactive nitrogen compounds. Increasing the initial nitrite concentrations resulted in the formation of significant amounts of nitrate in the snow. A possible precursor of nitrate is NO₂, which can be transformed into nitrate either by the attack of a hydroxy radical or the hydrolysis of the dimer (N₂O₄). A mechanism for the transformation of the nitrogen-containing compounds in snow was developed, assuming that all reactions took place in a quasi-liquid layer (QLL) at the surface of the ice crystals. The unknown photolysis rates of nitrate and nitrite and the rates of NO and NO₂ transfer from the snow to the gas phase, respectively, were adjusted to give an optimum fit of the calculated time series of nitrate, nitrite, and gas phase NO_x with respect to the experimental data. Best agreement was obtained with a ∼25 times faster photolysis rate of nitrite compared to nitrate. The formation of NO₂ is probably the dominant channel for the nitrate photolysis. We used the reaction mechanism further to investigate the release of NO_x and HONO under natural conditions. We found that NO_x emissions are by far dominated by the release of NO₂. The release of HONO to the gas phase depends on the pH of the snow and the HONO transfer rate to the gas phase. However, due to the small amounts of nitrite produced under natural conditions, the formation of HONO in the QLL is probably negligible. We suggest that observed emissions of HONO from the surface snow are dominated by the heterogeneous formation of HONO in the firn air. The reaction of NO₂ on the surfaces of the ice crystals is the most likely HONO source to the gas phase.     

Titanium Anodes with Active Coatings Based on Iridium Oxides: The Corrosion Resistance and Electrochemical Behavior of Anodes Coated by Mixed Iridium,Ruthenium, and Titanium Oxides

A study of the corrosion resistance and electrochemical behavior of titanium anodes with active coatings prepared from mixed oxides iridium, ruthenium, and titanium (OIRTA) is continued. The dependence of the catalytic activity, selectivity, and corrosion resistance of these anodes with x mol % RuO₂ + (30 ? x ) mol % IrO₂ + 70 mol % TiO₂ is studied in conditions of chlorine electrolysis on the ratio of concentrations of IrO₂ and RuO₂ in them at a constant loading of iridium in the coatings. It is established that the maximum corrosion resistance and selectivity is inherent in OIRTA with the RuO₂ concentration close to 4 mol %. Partial curves, which describe the dependence of the rates of dissolution of iridium out of OIRTA and the evolution of chlorine and oxygen in them on the electrode potential, are obtained. The dependence of the rates of these processes on the solution pH, the concentration of NaCl in it, and the thickness of the active layer is studied. It is shown that the rate of dissolution of iridium out of OIRTA and the concentration of oxygen in chlorine at a constant potential increase approximately proportionally to the coating thickness, from whence it follows that the said processes proceed over the entire depth of the coating. An assumption is put forth that the chlorine evolution on OIRTA of the optimum composition, with a loading of iridium equal to 2.5 g m^?2, at high anodic currents occurs in an outer-kinetics regime in the presence of diffusion limitations on the removal of chlorine out of the coating's depth.     

Heat capacity measurements of MnxFe3−xO4

Heat capacities of Mn_xFe_3?xO₄ with the composition x = 1.0, 1.5, and 2.0 were measured from 200 to 740 K. λ-type heat capacity anomalies due to the ferri-paramagnetic transition were observed for all the compositions. The transition temperatures were 577, 471, and 385 K for the composition x = 1.0, 1.5 and 2.0, respectively, which are in good agreement with the results of magnetic measurements. The difference in heat capacities between the different samples was small except for the temperature range of the transition. The magnetic contribution to the observed heat capacity was obtained by assuming that the heat capacity can be expressed by the sum of the lattice heat capacity C_v (l), the dilation contribution C(d), and the magnetic contribution C(m). Entropy changes due to the transition were obtained from C(m) as 55.5, 50.7 and 49.2 J K^?1 mole^?1 for the composition x = 1.0, 1.5, and 2.0, respectively. The entropy changes were also calculated by assuming the randomization of unpaired electron spins on each ion, but they were from 6 to 10 J K^?1 mole^?1 smaller than the observed ones. The difference between the experimental and the calculated values is roughly explained by taking into account the cation exchange reaction between the tetrahedral and the octahedral sites in the spinel structure.     

X-ray study of the PbCl2−xIx and PbBr2−xIx systems

Compounds from the systems PbCl₂/PbI₂ and PbBr₂/PbI₂ were examined by x-ray diffraction. The lattice parameters of these phases are presented and the refined crystal structures of the intermediate compounds PbClI and PbBr_1.2I_0.8 are reported. Both structures have Pbnm symmetry, are isostructural with PbCl₂, and have the different halogens ordered in the two Cl sites. Phase studies showed that PbCl₂ and PbClI have practically no mutual solubility, while PbBr₂ and PbBr_1.2I_0.8 have appreciable solubility ranges, particularly for PbBr₂-rich concentrations. At least 17% Br is present in the I site of PbBr_1.2I_0.8. Nevertheless, it is a distinct phase with miscibility gaps toward PbBr₂ and PbI₂. This behavior is explained by the size disparity between the halogens. The intermediate phases do not form solid solutions with hexagonal PbI₂.     

Hydrogen evolution and dissolution on smooth ruthenium in sulfuric acid solution

Hydrogen evolution and dissolution were studied under steady-state conditions on smooth ruthenium electrodes in 0.5 M H₂SO₄ at room temperature. Both the anodic current-potential curves, measred under H₂ stirring, and the cathodic curves, measured under N₂ stirring, suggest that the Tafel reaction and convective diffusion are the rate-determining steps at small current densities. At larger cathodic current densities the Volmer reaction becomes predominant. Hydrogen evolution occurs by the Volmer-Tafel mechanism. The coverage with adsorbed hydrogen atoms remains small.     

Spectroscopic properties and luminescence concentration quenching of the Pr3+ ion in La1−xPrxOCl

Luminescence spectra ofLa_1−xPr_xOCl(0.01 ≤ x ≤ 0.3) were recorded at liquid helium and room temperature. From these spectra, the energy level diagram of Pr³⁺ in LaOCl was deduced. The decay times of the Pr³⁺ emission in La_1−xPr_xOCl were measured using a high-resolution dye laser system. The linear concentration quenching and exponential decay times are explained assuming cross relaxation and energy transfer between the Pr³⁺ ions.     

Circumventing the OCl versus OOH scaling relation in the chlorine evolution reaction: Beyond dimensionally stable anodes

The development of selective electrocatalysts for the chlorine evolution reaction (CER) is majorly restrained by a scaling relation between the OCl and OOH adsorbates, rendering that active CER catalysts are also reasonably active in the competing oxygen evolution reaction (OER). While theory predicts that the OCl versus OOH scaling relation can be circumvented as soon as the elementary reaction steps in the CER comprise the Cl rather than the OCl adsorbate, it was demonstrated recently that PtN₄ sites embedded in a carbon nanotube follow this theoretical prediction. Advanced experimental analyses illustrate that the PtN₄ sites also reveal a different reaction kinetics compared to the industrial benchmark of dimensionally stable anodes (DSA). A reverse Volmer–Heyrovsky mechanism was identified, in which the rate-determining Volmer step for small overpotentials is followed by the kinetically limiting Heyrovsky step for larger overpotentials. Since the PtN₄ sites excel DSA in terms of activity and chlorine selectivity, we suggest the Cl intermediate as well as the reverse Volmer–Heyrovsky mechanism as the design criteria for the development of next-generation electrode materials beyond DSA.