Walden product, ion-solvent interactions and solvent structure in water-rich mixtures ionic conductances in water-sulfolane, water-acetonitrile and water-dimethylsulfoxide

Conductance measurements are reported for several salts in binary aqueous mixtures containing up to 60 mole % sulfolane, 20 mole % acetonitrile and 20 mole % dimethylsulfoxide. The variations of R = (λ^±₀η₀)_s/(λ^±₀η₀)_w with solvent composition have been compared with those observed in other water-rich mixtures. Alkali cations show R values greater than one with maxima in all the solvent mixtures. This behaviour has been discussed in terms of “sorting”, “averaging” and “steric” effects. Contrary to what happens to alkali cations, halide ions show R values greater or lesser than one according to whether the organic solvent respectively increases or decreases water structure. On these bases we suggest that conductometric behaviour of the halide ions may be indicative of the effect of the cosolvent on the water structure in water-rich mixtures and that DMSO is a water structure breaker.     

EPOXIDATION OF FUNCTIONALIZED OLEFINS OVER SOLID CATALYSTS

Heterogeneous catalytic epoxidation of functionalized olefins in the liquid phase has been reviewed, focusing on catalyst performance and its interrelation with the crucial parameters of the catalytic systems. Efficient catalysts include supported and mixed oxides, framework-substituted (“redox”) molecular sieves, layered-type materials, heterogenized homogeneous catalysts, and some others. Among the various substrates, allylic and homoallylic alcohols, and unsaturated carbonyl compounds have received most attention so far. The great variety of available catalysts enables selective epoxidation of most substituted olefins. The mechanistic understanding of heterogeneous catalytic epoxidation is still underdeveloped, rendering catalyst design rather empirical. A considerable potential for future development lies in the area of “heterogenization” of successful homogeneous catalysts especially for asymmetric epoxidation. Crucial requirements in the development of heterogeneous catalytic epoxidation catalysts are, besides good catalytic performance and cheap oxidant, recyclability and resistance to leaching of the active component. Some of the examples shown in the literature do not fulfill the latter requirement.     

“HETEROGENIZING” HOMOGENEOUS CATALYSTS

For many years, it has been customary to classify catalysts as “homogeneous” or “heterogeneous.” The former commonly operate through the formation of “intermediate compounds,” and the latter, by adsorption of the reactants on the catalyst surface. The line between the two is a fine one, for the distinction between adsorption and compound formation is not at all clear, and seems to be becoming less and less clear as we learn more about adsorption. In recent years, several writers [l-7] have stressed the point that there is a good deal of overlap between homogeneous and heterogeneous catalysis. Experimental evidence supporting this point of view is accumulating, and while we are not prepared to say that there is no distinction, we can say with certainty that many homogeneous catalysts can be converted into heterogeneous ones, retaining the advantages of great activity and selectivity inherent in homogeneity and, at the same time, assuming the ready recovery which is the great advantage of heterogenity. In practice, of course, the matter is not quite that simple, for other factors must be considered. On the whole, however, many advantages have been found in the use of heterogenized homogeneous catalysts.     

The impact of the “memory effect” on the catalytic activity of Mg/Al; Mg,Zn/Al; Mg/Al,Ga hydrotalcite-like compounds used as catalysts for cycloxene epoxidation

The epoxidation of cyclohexene by hydrogene peroxide and benzonitrile, in methanol as solvent on solid base catalysts, hydrotalcites (HT) and corresponding mixed oxides (C-HT) was investigated. The employed catalysts were as-synthesized Mg/Al; Mg,Zn/Al; Mg/Ga,Al hydrotalcites, their derived mixed oxides and the corresponding reconstructed hydrotalcite forms (R-HT) obtained via the “memory effect” upon a rehydration treatment of C-HT. The catalysts were characterized by XRD, CO₂-TPD and N₂ adsorption–desorption measurements.

The reconstructed HT samples exhibit higher activity in comparison to the as-synthesized carbonated HT samples. A tentative correlation of the catalytic properties with the base strength of the active sites, the structural and textural characteristics of the catalysts, has been explored.     

The application of “superacidic” metal oxides and their platinum doped counterparts to methane combustion

A range of metal oxides have been compared as methane combustion catalysts. The effect of modification to generate “superacidic” behaviour on the activities of ZrO₂ and Fe₂O₃ systems has been studied. It has been shown that whilst sulphation lowers the activity of Fe₂O₃, sulphation and, particularly, molybdation enhance the performance of ZrO₂. Despite enhancing the activity of the unmodified base oxides, the addition of low levels of Pt has been demonstrated to poison the activity of “superacidic” zirconias.     

The Mechanical Behaviour of Polyimide/Copper Laminates Part 2: Peel Energy Measurements

The mechanical peel behaviour of laminates consisting of polyimide films adhered to copper foil using a modified acrylic adhesive has been studied over a wide range of test rates and temperatures. The laminates were prepared from polyimide films which had been subjected to either a “high-thermal history” or a “low-thermal history” treatment during the production of the film. The measured peel energies of the laminates could be superimposed to give a master curve of peel energy versus the reduced rate of peel test, Ra_T, where R is the rate of peel test and a_T is the time-temperature shift factor. The appropriate shift factors were a function of the test temperature and were mainly deduced from tensile tests conducted on the bulk adhesive. The “high-thermal history” laminates gave higher peel energies and the locus of failure of the laminates was mainly by cohesive fracture through the adhesive layer. At low values of log₁₀ Ra_T, i.e. Low rates of peel and high test temperatures, the “low-thermal history” laminates also failed in the adhesive layer and possessed similar peel energies to those measured for the “high-thermal history” laminates. However, at high log₁₀ Ra_T values, the peel energies measured for the “low-thermal history” laminates were lower and showed a wider scatter. These arose from a different locus of failure occurring in these “low-thermal history” laminates when tested under these conditions. Namely, it was found that most of these laminates failed in a weak boundary layer in the outer regions of the “low-thermal history” polyimide film.     

Racemization of (R)-(−)-10-methyl-Δ-octalin: Stereochemical requirements for double bond migration (isomerization) on metal catalysts

(R)-(−)-10-methyl-Δ¹⁽⁹⁾-octalin was synthesized in a mixture with cis- and trans-(R)-(−)-10-methyl-Δ¹⁽²⁾-octalin in a five step sequence. After removal of the cis- and trans-(R)-(−)-10-methyl-Δ¹⁽²⁾-octalins by hydrogenation over 5% Pt/C, 5% Rh/C, or 5% Pd/C, the remaining (R)-(−)-10-methyl-Δ¹⁽⁹⁾-octalin was monitored for racemization (double bond migration) during continued hydrogenation. No racemization occurs over these catalysts. However, slow racemization does occur in increasing amounts during hydrogenations over a series of 1% Pd/SiO₂ catalysts with dispersions increasing from 36%D to 84%D. The ratio of cis- to trans-9-methyldecalin changes from 1.8 over 5% Rh/C, 1.6 over 5% Pt/C, 1.1 over 5% Pd/C, to 1.0 over the 1% Pd/SiO₂ catalysts. Since double bond migration does not occur over the 5% catalysts, it cannot be used to account for differences in the ratio of cis- to trans- products. Congestion around the double bond inhibits addition, but double bond migration seems to require the allylic hydrogens to be nearly perpendicular to the planes of the double bond and the surface.     

Copper–platinum catalysts prepared and characterized by electrochemical methods for the reduction of nitrate and nitrite

Platinized platinum catalysts were modified “in situ” by copper (sub)monolayers deposited under controlled conditions by electrochemical methods. They were characterized “in situ” by linear potential sweep cyclic voltammetry. Various Cu_ads/Pt catalysts with different copper coverages (θ_Cu) were obtained and tested for the reduction of nitrate and nitrite in aqueous solutions.

The intrinsic catalytic activity (at t=0) and the selectivity in ammonia of Cu_ads/Pt depend on the platinum surface fraction occupied by copper adatoms (θ_Cu). The highest activities and selectivities in ammonia are obtained with θ_Cu≈0.4–0.6. Moreover, higher activities were obtained in the reduction of nitrate than in the reduction of nitrite.

The change of catalysts potential, measured during the reaction, arises from the change of the solution pH, which is due to the generation of OH⁻ according to the global reduction reactions. The spontaneously established catalyst potential is a mixed potential corresponding to the different oxidoreduction reactions involved.     

SINTERING KINETICS AND TRANSPORT PROPERTY EVOLUTION OF LARGE MULTI-PARTICLE AGGREGATES

Ultrafine (“nano”-) particles produced from highly supersaturated vapors or liquids are usually aggregated, often containing thousands of small 'primary' particles bound together in tenuous structures characterized by mass fractal dimensions less than 3. Such aggregates have large initial surface area but are metastable with respect to more compact configurations. Available restructuring mechanisms include surface energy driven coalescence, which, in the case of viscous flow at high gas temperatures, is ultimately able to obliterate all evidence of the original (“primary”) particles. We here exploit the notion that, provided an aggregate is sufficiently large, it can be treated like a spatially non-uniform porous medium, undergoing finite-rate surface energy driven viscous flow sintering leading to final collapse to a single dense sphere. For this purpose, after a D_ƒ ≌s const stage of sintering [associated with a corresponding increase in mean apparent primary particle ('grain') size], we use an extension of the sintering rate models of Mackenzie and Shuttleworth (1949) and Scherer (1977), treating the material of the restructuring aggregate to be a Newtonian viscous fluid. We predict and report here the time-dependent increase in fractal dimension, D_ƒ, and associated decreases in: aggregate outer (maximum) radius, mobility radius, and changes in accessible surface area with dimension-less time [real time in multiples of the characteristic sintering time, μ (R₁)_t=0/σ cr, where u is the material's viscosity (R_l)_t=0 is the effective initial grain radius and a the material surface tension]. In these units, we find that the total required coalescence time does not increase with N as sensitively as N^1/3 an important observation for processes involving very large aggregates. With validation and the indicated extensions, our pseudo-continuum methods are efficient enough to be used for estimating the morphological- and transport property-evolution of entire populations of restructuring aggregates, perhaps characterized by some non-separable probability density function pdf(N, D_ƒ,R₁,) locally, in non-isothermal combustion-synthesis reactors.     

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.     

A comparison of the attachment of the decay products of radon-220 and radon-222 to monodispersed aerosols

The adsorption of radioactive aerosols is of prime importance when assessing the hazard of inhalation of radioactive materials, especially in the uranium mining industry. In previous studies of the attachment rate, contradicting results on the dependence on particle size have been found. The attachment rate for the attachment of the decay products of Rn-222 to monodispersed polystyrene aerosols (of a diameter from 1.2 to 5.3 μm) in the concentration range 1–67 particles cm⁻³ was measured directly. This is compared with the attachment rate of radon-220 decay products which was measured in a previous experiment. As before, the attachment between the radioactive atoms and the aerosols was found to be directly proportional to the particle concentration N, as well as to the square of the particle radius R.

The attachment coefficient of 4.1 × 10⁵ R² cm³ min⁻¹ and the sticking probability of 0.13 for Rn-222 decay products were found to be larger than that for the decay products of Rn-220 (2.4 × 10⁵ R² cm³ min⁻¹ and 0.08, respectively). The mechanism for the attachment, however, is the same.     

CoMo/MgO–Al2O3 supported catalysts: An alternative approach to prepare HDS catalysts

Three different supports were prepared with distinct magnesia–alumina ratio x = MgO/(MgO + Al₂O₃) = 0.01, 0.1 and 0.5. Synthesized supports were impregnated with Co and Mo salts by the incipient wetness method along with 1,2-cyclohexanediamine-N,N,N′,N′-tetraacetic acid (CyDTA) as chelating agent. Catalysts were characterized by BET surface area, Raman spectroscopy, SEM-EDX and HRTEM (STEM) spectroscopy techniques. The catalysts were evaluated for the thiophene hydrodesulfurization reaction and its activity results are discussed in terms of using chelating agent during the preparation of catalyst. A comparison of the activity between uncalcined and calcined catalysts was made and a higher activity was obtained with calcined MgO–Al₂O₃ supported catalysts. Two different MgO containing calcined catalysts were tested at micro-plant with industrial feedstocks of heavy Maya crude oil. The effect of support composition was observed for hydrodesulfurization (HDS), hydrodemetallization (HDM), hydrodeasphaltenization (HDAs) and hydrodenitrogenation (HDN) reactions, which were reported at temperature of 380 °C, pressure of 7 MPa and space-velocity of 1.0 h⁻¹ during 204 h of time-on-stream (TOS).     

Thermal diffusivity measurements by “instantaneous phase portrai” method

A new method of “instantaneous phase portrait” was developed for measurements of thermal diffusivity χ of thin films with high values of χ, such as diamond films. This method is based on the registration of the air layer refractive index spatial distribution by means of double exposure holographic interferometry and the derivation of the thermal diffusivity value χ from the spatial distribution. The values of thermal diffusivity for copper and steel foils measured by this technique coincide with the tabular ones. The method was also applied to measure thermal diffusivity of thin diamond films. The measured values were in 2–3 cm² s⁻¹ range.     

O.r.d.-c.d. and protonation of amide-containing polymers in strong acid media: 1. The case of poly(−)1,2-diaminopropane sebacamide, an optically active nylon-type polyamide

Optical rotatory dispersion (o.r.d.) and circular dichroism (c.d.) of poly(−)1,2-diaminopropane sebacamide I and of its model compound, N-N′-didecanoyl(−)1,2-diaminopropane II, are investigated in trifluoroethanol-formic acid, -trifluoroacetic, -methanesulphonic and -sulphuric acid mixtures, with emphasis on the two latter solvent systems. It is shown that the optical activity of both compounds depends on the nature and the concentration of the acid. The stronger the acid, the larger the chiroptical changes observed for a given value of the acid/amide molar ratio τ. The drastic changes caused by small amounts of methanesulphonic and sulphuric acids are assigned to protonation of amide chromophores solely.     

Characterization of Pd-based FCC CO/NOx control additives by in situ FTIR and extended X-ray absorption fine structure spectroscopies

Pdⁿ⁺/Ceⁿ⁺/Na⁺/γ-Al₂O₃-type materials used as FCC additives for CO/NO_x control were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and in situ FTIR. The EXAFS data indicate that in freshly prepared samples palladium is present in the form of highly dispersed PdO species. Reduction with H₂ at 500 °C leads to the formation of small Pd clusters incorporating on average approximately six to eight metal atoms at a Pd−Pd bond distance of 2.76 Å. All components of these materials can interact with NO and promote the formation of nitrate/nitrite species, essentially “trapping” NO_x species on the catalyst surface. However, the Na⁺ species dominate the surface chemistry and readily form sodium nitrates with a characteristic IR band at 1370–1385 cm⁻¹. Finally, hydroxyls from the support are also actively participating in the formation of HNO_x type compounds with characteristic stretching vibrations in the 3500–3572 cm⁻¹ region.     

CO oxidation over CuOx-CeO2-ZrO2 catalysts: Transient behaviour and role of copper clusters in contact with ceria

The effects of ZrO₂ content on the CO oxidation activity in a series of CuO_x/Ce_xZr_1−xO₂ (x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H₂, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures.     

Structural characterization of Bi–Zn–Nb–O cubic pyrochlores

Structure and dielectric properties of oxide dielectrics with the composition of Bi_1.5Zn_NNb_2.5−NO_8.5−1.5N (with N=0.73 to 1.20) have been studied. These samples were treated at 1050 °C for 4 h. The cubic pyrochlore phase was found to be predominant as from X-ray diffraction and Raman spectra analysis. Lattice constant of the cubic pyrochlore and dielectric constant of the sample have been found to increase with the increase of Zn content. A model of the structural defects has been proposed to explain the stabilization of the pyrochlore structure. The limitation of composition for the formation of single cubic pyrochlore phase has been attributed to the distribution of oxygen defects.     

Processing and characterization of sol–gel fabricated mixed metal aluminates

Nanocrystalline lanthanum aluminate (LaAlO₃, LAP) and Sr-substituted LAP ceramics were synthesized by sol–gel processes using mixtures of inorganic salts of the respective elements. The metal ions, generated by dissolving metal nitrates or acetates in acetic acid and/or water were complexed by 1,2-ethanediol to obtain the precursor for LAP. The XRD patterns of the LaAlO₃ and La_1−xSr_xAlO_3−δ (x ≤ 0.50) ceramic sintered at 1000 °C were almost identical with the perovskite LAP composition. The phase transformations, composition and microstructural features in the gels and polycrystalline samples were studied by thermal analysis (TG/DTA), powder X-ray diffraction analysis (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and inductively coupled argon plasma emission spectroscopy (ICP). The quality of the resulting products (homogeneity, crystallisation temperature, grain size and grain size distribution, etc.) of the “chimie douce” synthetic route is discussed.     

Methane combustion on Mg-doped LaMnO3 perovskite catalysts

LaMn_1−xMg_xO₃ perovskite catalysts (x=0–0.5) were synthesised by the so-called “citrates method”, characterised (chemical analysis, TEM, BET, XRD, temperature-programmed desorption of oxygen) and tested for their activity towards the catalytic combustion of methane. The role of MgO as a textural promoter, which hinders the sintering of the catalyst crystals by geometrical interposition, has also been assessed. Finally, a kinetics study was performed on the most promising catalysts prepared (LaMnO₃ and LaMn_0.8Mg_0.2O₃). The major results obtained are: (i) Mg substitution in the basic LaMnO₃ perovskite has a positive effect on the catalytic activity only at low x values (x≤0.2); (ii) as opposed to the results of previous studies on the LaCr_1−xMg_xO₃ system, the role of MgO as a textural promoter is not always significant and depends strongly on the calcination temperature of the samples (800–1200°C) and on the value of x; (iii) an Eley–Rideal mechanism could satisfactorily fit the experimental kinetics results for both catalysts, even though, as opposed to LaMnO₃, the catalytic combustion over LaMn_0.8Mg_0.2O₃ seems to involve two different types of adsorbed oxygen species, depending on the operating temperature.     

TayNb1−yVO5 (0<y<1) mixed oxides synthesized by sol–gel method: electrochemical Li-insertion

Mixed oxides of general formulae Ta_yNb_1−yO₅ (0<y<1) have been synthesized by a modified sol–gel method. Characterization of the samples has been carried out by x-ray diffraction, SEM/EDX, FTIR spectroscopy and thermal analysis. The electrochemical properties have been studied in a lithium cell. The first discharge capacity decrease from 206 (y=0.25) to 136 mA h g⁻¹ (y=0.75). Ta_yNb_1−yO₅ samples undergo an irreversible structural changes induced by electrochemical Li⁺-insertion. For all compositions, the new compounds formed after the first discharge has a very high cyclability, as shown the low capacity loss <1% per cycle.