Chemisorption of C3 hydrocarbons on cobalt silica supported Fischer–Tropsch catalysts

Chemisorption of propene and propane was studied in a pulse reactor over a series of cobalt silica-supported Fischer–Tropsch catalysts. It was shown that interaction of propene with cobalt metal particles resulted in its rapid autohydrogenation. The reaction consists in a part of the propene being dehydrogenated to surface carbon and CH_x chemisorbed species; hydrogen atoms released in the course of propene dehydrogenation are then involved in hydrogenation of remaining propene molecules to propane at 323–423 K or in propene hydrogenolysis to methane and ethane at temperatures higher than 423 K. The catalyst characterization suggests that propene chemisorption over cobalt catalysts is primarily a function of the density of cobalt surface metal sites. A correlation between propene chemisorption and Fischer–Tropsch reaction rate was observed over a series of cobalt silica-supported catalysts. No propane chemisorption was observed at 323–373 K over cobalt silica-supported catalysts. Propane autohydrogenolysis was found to proceed at higher temperatures, with methane being the major product of this reaction over cobalt catalysts. Hydrogen for propane autohydrogenolysis is probably provided by adsorbed CH_x species formed via propane dehydrogenation. Propene and propane chemisorption is dramatically reduced upon the catalyst exposure to synthesis gas (H₂/CO = 2) at 323–473 K. Our results suggest that cobalt metal particles are probably completely covered by carbon monoxide molecules under the conditions similar to Fischer–Tropsch synthesis and thus, most of cobalt surface sites are not available for propene and propane chemisorption.     

Dielectric dispersion in [N(C2H5)4]2ZnCl4 single crystal

Temperature dependence of the complex dielectric permittivity of [N(C₂H₅)₄]₂ZnCl₄ crystal was measured in the frequency range 300 Hz / MHz. Jump-wise changes in the real and imaginary components of ε* were observed for the three crystallographic directions at the phase transition temperature. The dielectric dispersion clearly apparent in the high-temperature phase, rapidly disappears at the phase transition. The anomalous changes in the dielectric permittivity are related to vibrations of the tetraethylammonium ion. The activation energy of these vibrations amounts to 40 kJ/mol.     

Bronchial asthma: do boys or girls have the highest incidence?

The aim of the study was to assess the gender related prevalence of asthma and asthma symptoms in schoolchildren. The survey was performed using standardized ISAAC questionnaire in two age groups: 6-7 yr. (n = 2281; girls 49.7%) and 13-14 yr. (n = 4849; girls 49.8%). It was revealed that in older group the prevalence of ever diagnosed asthma was lower in girls than in boys (2.0% versus 3.3%; odds ratio [OR] = 0.58; p = 0.004). But the prevalence of symptoms: wheeze ever (OR = 1.16; p = 0.056), current wheeze (last 12 mo) (OR = 1.26; p = 0.029), current exercise wheeze (OR = 1.40; p = 0.0008), current night cough (OR = 1.67; p = 0.0001) were higher in girls than in boys. However, in younger group of schoolchildren the prevalence of ever diagnosed asthma and asthma symptoms were higher in boys (5.1% male versus 3.5% female). The girls in comparison to boys revealed lower risk of ever asthma diagnosis (OR = 0.66; p = 0.056) and symptoms: wheeze ever (OR = 0.63; p = 0.0001), current wheeze (OR = 0.69; p = 0.003), current exercise wheeze (OR = 0.59; p = 0.008) and current night cough (OR = 0.70; p = 0.0003).     

Simulation of metal flow and heat transfer during hot rolling of bimetal plates

Coupled thermomechanical finite element model is applied to simulate metal flow and temperature distribution during hot rolling of bimetal plates. Calculations are carried out for various reductions and dimensions of the plates. Two possible versions of the boundary conditions are assumed. The first considers the case when special guides or tensions prevent bending of the sample before entering and after exit from the roll gap. The second allows free bending of the plate on both sides of the mill. Results of calculations include distributions of strain rates, strains, roll pressure, friction forces and temperatures during rolling. Calculated rolling force and torque are compared with the experimental data. Several conclusions regarding the metal flow and heat transfer in various rolling conditions are drawn.     

Modification of a Solution Calorimeter for Thermometric Titrations

ABSTRACT

Modification of a solution calorimeter for use in thermometric titrations is described. Results of typical titrations are used to establish precision and accuracy. The instrument, which can be put together at moderate cost, is readily converted back to a solution calorimeter.     

Kinetics of the NO + H2 reaction over supported noble metal based catalysts: Support effect on their adsorption properties

This paper reports a kinetic investigation of the global reduction of NO by H₂ which has been considered as a probe reaction for characterising the adsorption properties of supported palladium based catalysts. A particular attention has been paid towards the influence of the support on the catalytic properties of Pd, particularly towards the production of undesirable by-products such as nitrous oxide (N₂O) and ammonia (NH₃). It has been found that the kinetics of the overall NO + H₂ reaction on Pd/Al₂O₃ can be correctly depicted according to a Langmuir–Hinshelwood mechanism involving the dissociation of nitrosyl species assisted by chemisorbed hydrogen atoms. On the other hand, Pd/LaCoO₃ exhibits a different kinetic behaviour towards the adsorption of hydrogen depending on the pre-activation thermal treatment. In that case, different mechanisms may occur.