Study of the impedance of a platinum electrode in the system Br2/Br (HClO4, aq.). I. Influence of the surface state

Impedance measurements, over a wide range of frequencies, of a platinum electrode immersed in a solution of Br₂/Br⁻ (HClO₄, aq.), allow us to obtain the values of the exchange current j_o, it being also possible to study its variation with the experimental conditions. The results thus obtained depend on the surface state of the electrode. With activated electrodes, the Randles circuit explains well enough the electrochemical behaviour of this system. With poisoned electrodes the circuit is inadequate, but it can be shown that the exchange current is decreased by the poisoning effect. Decrease of the exchange current with increase of surface oxidation of the electrode has also been observed.

As a first approximation the poisoning effects can be explained by a diminution of the effective area concerned with electron transfer, but the inadequacy of the Randles circuit indicates that adsorption processes have to be taken into account.     

On the photooxidative behavior of TiO2 and PW12O40: OH radicals versus holes

Parallel experiments under similar conditions, using various substrates (atrazine, fenitrothion, 4-chlorophenol and 2,4-D) and OH radical scavengers (Br⁻, isopropyl alcohol, tertiary butyl alcohol and acetone), have shown that the photooxidizing mode of PW₁₂O₄₀³⁻ and TiO₂, i.e., OH radicals and/or holes (h⁺), depends on the nature of substrate and the mode of investigation. This provides an explanation for the controversial results reported in the literature. Atrazine shows that both PW₁₂O₄₀³⁻ and TiO₂ operate, mainly, via OH radicals and to a lesser extent with holes (h⁺), whereas, fenitrothion suggests that both systems operate almost exclusively, via OH radicals. Differences in the action of the catalysts are encountered in the photodegradation of 4-chlorophenol (4-ClPh) and 2,4-dichlorophenoxyacetic acid (2,4-D). PW₁₂O₄₀³⁻ appears to operate essentially via OH radicals, whereas, h⁺ appear to be the major oxidant with TiO₂. Overall, though, the action of OH radicals relative to h⁺ appears to be more pronounced with PW₁₂O₄₀³⁻ than TiO₂.     

Effect of pH, inorganic ions, organic matter and H2O2 on E. coli K12 photocatalytic inactivation by TiO2: Implications in solar water disinfection

The effect of different chemical parameters on photocatalytic inactivation of E. coli K12 is discussed. Illumination was produced by a solar lamp and suspended TiO₂ P-25 Degussa was used as catalyst. Modifications of initial pH between 4.0 and 9.0 do not affect the inactivation rate in the absence or presence of the catalyst. Addition of H₂O₂ affects positively the E. coli inactivation rate of both photolytic (only light) and photocatalytic (light plus TiO₂) disinfection processes. Addition of some inorganic ions (0.2 mmol/l) like HCO₃⁻, HPO₄²⁻, Cl⁻, NO₃⁻ and SO₄²⁻ to the suspension affects the sensitivity of bacteria to sunlight in the presence and in absence of TiO₂. Addition of HCO₃⁻ and HPO₄²⁻ resulted in a meaningful decrease in photocatalytic bactericidal effect while it was noted a weak influence of Cl⁻, SO₄²⁻ and NO₃⁻. The effect of counter ion (Na⁺ and K⁺) is not negligible and can modify the photocatalytic process as the anions. Bacteria inactivation was affected even at low concentrations (0.2 mmol/l) of SO₄²⁻ and HCO₃⁻, but the same concentration does not affect the resorcinol photodegradation, suggesting that disinfection is more sensitive to the presence of natural anions than photocatalytic degradation of organic compounds. The presence of organic substances naturally present in water like dihydroxybenzenes isomers shows a negative effect on photocatalytic disinfection. The effect of a mixture of chemical substances on photocatalytic disinfection was also studied by adding to the bacterial suspension nutrient broth, phosphate buffer and tap water.     

Influence of chromium doping on the electrochemical performance of the 5 V spinel cathode LiMn1.5Ni0.5O4

With an aim to improve the 5 V capacity and cyclability of the LiMn_1.5Ni_0.5O₄ spinel oxide, three series of Cr substitutions have been pursued with y ≤ 0.2: LiMn_1.5Ni_0.5−yCr_yO₄, LiMn_1.5−0.5yNi_0.5−0.5yCr_yO₄, and LiMn_1.5−0.33yLi_0.33yNi_0.5−yCr_yO₄. While the first series involves an increase in the Mn³⁺ content, the second and third series are designed to maintain charge neutrality (Mn⁴⁺, Ni²⁺, Cr³⁺, and Li⁺) without introducing Mn³⁺ ions. The LiMn_1.5Ni_0.5−yCr_yO₄ series experiences a widening of the 4 V plateau and a decrease in the 5 V capacity compared to LiMn_1.5Ni_0.5O₄ due to an increase in the Mn³⁺ content. On the other hand, the LiMn_1.5−0.5yNi_0.5−0.5yCr_yO₄ series shows a suppression of the 4 V plateau and an increase in the 5 V capacity due to the elimination of the Mn³⁺ions. The LiMn_1.5−0.33yLi_0.33yNi_0.5−yCr_yO₄ series shows a suppression of the 4 V plateau at low Cr contents, but an increase in the 4 V plateau as the Cr content increases above 0.1. Among the various compositions investigated, LiMn_1.45Ni_0.45Cr_0.1O₄ exhibits the best combination of high 5 V capacity (128 mAh/g at 5–4.2 V) and excellent capacity retention (98% in 50 cycles) compared to 118 mAh/g and 92% for LiMn_1.5Ni_0.5O₄.     

Nanosized perovskite-type oxides La1−xSrxMO3−δ (M = Co, Mn; x = 0, 0.4) for the catalytic removal of ethylacetate

Nanometer perovskite-type oxides La_1−xSr_xMO_3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m² g⁻¹, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn⁴⁺ and Mn³⁺ in La_1−xSr_xMnO_3−δ and Co³⁺ and Co²⁺ in La_1−xSr_xCoO_3−δ, Sr substitution induced the rises in Mn⁴⁺ and Co³⁺ concentrations; adsorbed oxygen species (O⁻, O₂⁻, or O₂²⁻) were detected on the catalyst surfaces. The O₂-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H₂-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h⁻¹, the catalytic activity (as reflected by the temperature (T_100%) for EA complete conversion) increased in the order of LaCoO_2.91 (T_100% = 230 °C) ≈ LaMnO_3.12 (T_100% = 235 °C) < La_0.6Sr_0.4MnO_3.02 (T_100% = 190 °C) < La_0.6Sr_0.4CoO_2.78 (T_100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ ratios), and rich lattice defects of the nanostructured La_1−xSr_xMO_3−δ materials.     

Synthesis and properties of Bi0.5(Na1−x−yKxAgy)0.5TiO3 lead-free piezoelectric ceramics

Bi_0.5(Na_1−x−yK_xAg_y)_0.5TiO₃ piezoelectric ceramics were prepared by conventional ceramic processes. X-ray diffraction patterns show a pure perovskite structure, indicating that the K⁺ and Ag⁺ ions substitute for the Na⁺ ions in Bi_0.5Na_0.5TiO₃. The temperature dependence of the dielectric constant and dissipation factor shows all ceramics to experience two phase transitions: from ferroelectric to anti-ferroelectric and from anti-ferroelectric to paraelectric. The transition temperature from ferroelectric to anti-ferroelectric and the temperature at which the dielectric constant reaches its maximum value decrease with the increase of K⁺ amount. At room temperature, the ceramics containing 17.5–20 mol% K⁺ and 2 mol% Ag⁺ exhibit high piezoelectric constant (d₃₃ = 180 pC/N) and high electromechanical coupling factor (k_p = 35%).     

Formation of singlet molecular oxygen associated with the formation of superoxide radicals in aqueous suspensions of TiO2 photocatalysts

The production and decay of singlet molecular oxygen (¹O₂) in TiO₂ photocatalysis were investigated by monitoring its phosphorescence under various reaction conditions. First, the effects of additives such as KBr, KSCN, KI, H₂O₂, and ethanol on the amount of ¹O₂ produced by photo excitation of P25 TiO₂ were measured. The same additives were employed to investigate the effect on the amount of O₂⁻ produced. Comparison between the effects on ¹O₂ and O₂⁻ suggested that ¹O₂ is formed by the electron transfer mechanism, the reduction of molecular oxygens to O₂⁻ by photogenerated electrons and the subsequent oxidation of O₂⁻ to ¹O₂ by photogenerated holes. The formation of ¹O₂ decreased at pH < 5 and pH > 11, indicating that the intermediate O₂⁻ is stabilized at the terminal OH site of the TiO₂ surface in the pH range of 5 < pH < 11. Eighteen commercially available TiO₂ photocatalysts were compared on the formation of ¹O₂ and O₂⁻ in an aqueous suspension system. The formation of ¹O₂ was increased with decreasing size of TiO₂ particles, indicating that a large specific surface area causes a higher possibility of reduction producing O₂⁻ and then a large amount of ¹O₂ is formed. The difference in the crystal phase (rutile and anatase) did not affect the formation of ¹O₂.     

Pitting corrosion of Al and Al–Cu alloys by ClO4 ions in neutral sulphate solutions

The influence of various concentrations of NaClO₄, as a pitting corrosion agent, on the corrosion behaviour of pure Al, and two Al–Cu alloys, namely (Al + 2.5 wt% Cu) and (Al + 7 wt% Cu) alloys in 1.0 M Na₂SO₄ solution was investigated by potentiodynamic polarization and potentiostatic techniques at 25 °C. Measurements were conducted under the influence of various experimental conditions, complemented by ex situ energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) examinations of the electrode surface. In free perchlorate sulphate solutions, for the three Al samples, the anodic polarization exhibits an active/passive transition. The active dissolution region involves an anodic peak (peak A) which is assigned to the formation of Al₂O₃ passive film on the electrode surface. The passive region extends up to 1500 mV with almost constant current density (j_pass) without exhibiting a critical breakdown potential or showing any evidence of pitting attack. For the three Al samples, addition of ClO₄⁻ ions to the sulphate solution stimulates their active anodic dissolution and tends to induce pitting corrosion within the oxide passive region. Pitting corrosion was confirmed by SEM examination of the electrode surface. The pitting potential decreases with increasing ClO₄⁻ ion concentration indicating a decrease in pitting corrosion resistance. The susceptibility of the three Al samples towards pitting corrosion decreases in the order: Al > (Al + 2.5 wt% Cu) alloy > (Al + 7 wt% Cu) alloy. Potentiostatic measurements showed that the rate of pitting initiation increases with increasing ClO₄⁻ ion concentration and applied step anodic potential, while it decreases with increasing %Cu in the Al samples. The inhibitive effect of SO₄²⁻ ions was also discussed.     

Electrochemical properties of non-stoichiometric V6O13

Non-stoichiometric V₆O_13±0.2 (VO_y) prepared by thermal decomposition of NH₄VO₃ in N₂ atmosphere is investigated cathode material for Li batteries. The maximum lithium composition is found to be Li_2y-3VO_y. The emf vs composition relationship of Li/Li_xVO_2.144 is determined in the interval 0 < x < 1.3, yielding a theoretical energy density of 890 Wh kg⁻¹. The kinetics and reversibility of the VO_y electrode is investigated by cyclic voltammetry and cycling of test cells. The lithium diffusion coefficient and the electronic conductivity of the oxide is found to decrease with increasing lithium content. The cyclability of very thin electrodes is found to be excellent, but inferior results are obtained with practical cells, presumably due to degradation of electrode structure.     

The effect of oxygen concentration on the reduction of NO with propylene over CuO/γ-Al2O3

The effect of oxygen concentration on the pulse and steady-state selective catalytic reduction (SCR) of NO with C₃H₆ over CuO/γ-Al₂O₃ has been studied by infrared spectroscopy (IR) coupled with mass spectroscopy studies. IR studies revealed that the pulse SCR occurred via (i) the oxidation of Cu⁰/Cu⁺ to Cu²⁺ by NO and O₂, (ii) the co-adsorption of NO/NO₂/O₂ to produce Cu²⁺(NO₃⁻)₂, and (iii) the reaction of Cu²⁺(NO₃⁻)₂ with C₃H₆ to produce N₂, CO₂, and H₂O. Increasing the O₂/NO ratio from 25.0 to 83.4 promotes the formation of NO₂ from gas phase oxidation of NO, resulting in a reactant mixture of NO/NO₂/O₂. This reactant mixture allows the formation of Cu²⁺(NO₃⁻)₂ and its reaction with the C₃H₆ to occur at a higher rate with a higher selectivity toward N₂ than the low O₂/NO flow. Both the high and low O₂/NO steady-state SCR reactions follow the same pathway, proceeding via adsorbed C₃H₇---NO₂, C₃H₇---ONO, CH₃COO⁻, Cu⁰---CN, and Cu⁺---NCO intermediates toward N₂, CO₂, and H₂O products. High O₂ concentration in the high O₂/NO SCR accelerates both the formation and destruction of adsorbates, resulting in their intensities similar to the low O₂/NO SCR at 523–698 K. High O₂ concentration in the reactant mixture resulted in a higher rate of destruction of the intermediates than low O₂ concentration at temperatures above 723 K.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Fabrication and thermoelectric properties of heavily rare-earth metal-doped SrO(SrTiO3)n (n = 1, 2) ceramics

To clarify the effect of substitutional electron doping on the thermoelectric figure of merit (ZT = S²σTκ⁻¹) of Ruddlesden–Popper phase SrO(SrTiO₃)_n (or Sr_n+1Ti_nO_3n+1), measurements were conducted for several thermoelectric parameters, e.g. electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (κ), of (Sr_1−xRE_x)_n+1Ti_nO_3n+1 (n = 1 or 2, RE (rare earth): La or Nd, x = 0.05 and 0.1) dense ceramics prepared by a conventional solid-state reaction and hot-pressing technique. Crystal structures of the resultant ceramics were represented as (Sr_1−xRE_x)_n+1 Ti_nO_3n+1 evaluated by powder X-ray diffraction followed by the Rietveld analysis. All the ceramics exhibited electrical conductivity and the σ values simply depended on the dopant concentration, indicating that both La³⁺ and Nd³⁺ ions act as electron donors. The |S| values increased with temperature due to decrease in the chemical potential. Significant reduction of the κ values was observed as compared to cubic-perovskite SrTiO₃. The ZT value increased with temperature and reached 0.15 at 1000 K for (Sr_0.95La_0.05)₃Ti₂O₇.     

Catalytic behavior of La–Sr–Ce–Fe–O mixed oxidic/perovskitic systems for the NO+CO and NO+CH4+O2 (lean-NOx) reactions

Mixed oxides of the general formula La_0.5Sr_xCe_yFeO_z were prepared by using the nitrate method and characterized by XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO₃ and SrFeO_3−x and oxides -Fe₂O₃ and CeO₂ depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH₄+O₂ (“lean-NO_x”) reactions in the temperature range 250–550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La³⁺ cation at A-site with divalent Sr⁺² and tetravalent Ce⁺⁴ cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La_0.5Sr_xCe_yFeO_z materials to reduce NO by CO or by CH₄ under “lean-NO_x” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s⁻¹) at 300°C (based on NO chemisorption) revealed values comparable to Rh/-Al₂O₃ catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La_0.5Sr_xCe_yFeO_z solids and their catalytic activity. O₂ TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O₂ TPD studies was observed for the La_0.5Sr_0.2Ce_0.3FeO_z and La_0.5Sr_0.5FeO_z solids.     

The homogeneity range and the microwave dielectric properties of the BaZn2Ti4O11 ceramics

Ceramics with a composition close to BaZn₂Ti₄O₁₁ were synthesized according to various substitutional mechanisms in order to verify an existence of a homogeneity range in the vicinity of this composition. Structural and microstructural investigations showed that the crystal structure of BaZn₂Ti₄O₁₁ was formed in the homogeneity range corresponding to the formula BaZn_2 − xTi₄O_11 − x (0 < x < 0.1). Densely sintered BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) ceramics exhibited a dielectric constant around 30, τ_f = −30 ppm/K and high Q × f values, which increased from 68,000 GHz at x = 0 to 83,000 GHz at x = 0.05. Structurally, the deficiency of Zn in BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) resulted in a slight decrease in the unit-cell volume. The influence of secondary phases in the BaZn₂Ti₄O₁₁-based materials on the microwave dielectric properties was also investigated. A presence of small amounts of ZnO, BaTiO₃, hollandite-type solid solutions (Ba_xZn_xTi_8 − xO₁₆) and BaTi₄O₉ caused a decrease in Q × f values.     

Mass spectra of negative ions in air-like gas mixtures at atmospheric pressure

We have obtained mass spectra of negative ions produced by rays in artificial air at atmospheric pressure (N₂: 80%, O₂: 20%, H₂O: 20–1500 ppm, CO₂: 0.2–300 ppm, NO, NO₂ 0.02 ppm). We observed two main categories: hydrates built on simple ions (O₂⁻, O₃⁻, OH⁻, CO₃⁻, CO₄⁻, HCO₃⁻, NO₂⁻, NO₃⁻), hydrates built on complex ions (NO_x⁻, HNO_γ, HCO₃⁻HNO_x, x = 2,3; Y = 2, 3). For high values of hygrometry, CO₂ content and ageing time (5 msec) we observe the disappearance of O₂⁻, O₃⁻, OH⁻ hydrates whereas the major part of the spectrum consists of complex ions.     

Comparative study of lithium ion conductors in the system Li1+xAlxA2−x (PO4)3 with A=Ti or Ge and 0≤x≤0·7 for use as Li sensitive membranes

Preparations and physico-chemical characterizations of NASICON-type compounds in the system Li_1+xAl_xA_2−x^IV(PO₄)₃ (A^IV=Ti or Ge) are described. Ceramics have been fabricated by sol-gel and co-grinding processes for use as ionosensitive membrane for Li⁺ selective electrodes. The structural and electrical characteristics of the pellets have been examined. Solid solutions are obtained with Al/Ti and Al/Ge substitutions in the range 0≤x≤0·6. A minimum of the rhombohedral c parameter appears for x about 0·1 for both solutions. The grain ionic conductivity has been characterized only in the case of Ge-based compounds. It is related to the carrier concentration and the structural properties of the NASICON covalent skeleton. The results confirm that the Ti-based framework is more calibrated to Li⁺ migration than the Ge-based one. A grain conductivity of 10⁻³ S cm⁻¹ is obtained at 25°C in the case of Li_1·3Al_0·3Ti_1·7(PO₄)₃. A total conductivity of about 6×10⁻⁵ S cm⁻¹ is measured on sintered pellets because of grain boundary effects. The use of such ceramics in ISE devices has shown that the most confined unit cell (i.e. in Ge-based materials) is more appropriate for selectivity effect, although it is less conductive.©     

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 and equilibrium swelling of gelatine gels

The swelling features of gelatine gels in water (good solvent) were studied as a function of thermodynamic conditions of sol—gel transition and ripening. It is shown that the degree of equilibrium swelling Q_e varies with the volume fraction of the polymer in a casting solution φ_o in accordance with the prediction of the classic theory: Q_e φ_o^−0.4. Q_c, as a function of the gelation temperature T_g, the ripening time t_r and φ_o, can be rescaled and described by the single empirical equation: Q_e T_g^−x t_r^−yφ_o^−0.4, where x = 0.1, y = 0.15 for wet gels and X = −0.5, y = 0.04 for dried gels. The kinetics of macroscopic swelling is described by the equation of Peters and Candau, with values of collective diffusion coefficients being in good agreement with values obtained by other workers via photon correlation spectroscopy.     

Microwave dielectric properties of rutile (Zn1/3Nb2/3)0.40(Ti1−xSnx)0.60O2 (0.15 ≤ x ≤ 0.30) ceramics

Microwave dielectric properties of (Zn_1/3Nb_2/3)_0.40(Ti_1−xSn_x)_0.60O₂ ceramics were investigated as a function of SnO₂ content (0.15 ≤ x ≤ 0.30). A single phase with tetragonal rutile structure was obtained through the entire composition. The unit-cell volume of the specimens was increased with SnO₂ content, due to the larger ionic radius of Sn⁴⁺ (0.69 Å) than that of Ti⁴⁺ (0.605 Å) for octahedral site. Dielectric constant (K) of the sintered specimens was affected by the dielectric polarizability. Quality factor (Qf) was dependent on the degree of reduction of Ti⁴⁺ ion. With an increase of SnO₂ content, the temperature coefficient of resonant frequency (TCF) of the specimens decreased due to the decrease of the octahedral distortion of rutile structure.     

Methane combustion and CO oxidation on LaAl1−xMnxO3 perovskite-type oxide solid solutions

Structural, redox and catalytic deep oxidation properties of LaAl_1−xMn_xO₃ (x=0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl_1−xMn_xO₃ samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 Å and 4–33 m² g⁻¹ ranges, respectively. Redox properties and the content of Mn⁴⁺ were derived from temperature programmed reduction (TPR) with H₂. Two reduction steps are observed by TPR for pure LaMnO₃, the first attributed to the reduction of Mn⁴⁺ to Mn³⁺ and the second due to complete reduction of Mn³⁺ to Mn²⁺. The presence of Al in the LaAl_1−xMn_xO₃ solid solutions produces a strong promoting effect on the Mn⁴⁺→Mn³⁺ reducibility and inhibits the further reduction to Mn²⁺. Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO₃, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion.