L. Zanderighi

Positron annihilation lifetime (PAL) in poly[1‐(trimethyl‐silyl)propine] (PTMSP): Free volume determination and time dependence of permeability

PTMSP membranes were prepared and characterized. The mean molecular weight of the polymer was found to be 450,000 Da. Time dependence of the density, mechanical properties, IR spectra, DSC, PAL, and permeability data of the polymer and membranes are presented. A detailed analysis of PAL results for PTMSP samples under vacuum, in air, oxygen, and nitrogen atmosphere is presented with the aim to investigate the influence of the external atmosphere on the experimental PAL measurements and to determine correctly the size and the number of the free volume holes. From all the experimental data a model of a channel network is proposed for PTMSP with large holes (mean radium r = 0.75 nm) connected by channel-like holes (mean radium 0.45 nm). The number of large holes decreases by ageing, but not their size, whereas the number of small holes does not change but their size decreases. According to our model the decrease in the permeability of PTMSP with time could be caused by the decrease of the size of the channel-like holes.

Hydrogenation of carbon monoxide: evidence of a strong metal-support interaction in Rh/ZrO2 catalysts

Abstract Temperature-programmed desorption (TPD) studies are reported on a series of Rh catalysts dispersed on ZrO2 and γ-Al2O3 and activated under H2 flow at 250, 400, and 600°C. While the high temperature of activation did not show any significant difference in the Rh γ-Al 2 O 3 samples, in the case or Rh ZrO 2 a pronounced effect is quite evident. The experimental observations have been associated to a modification of the metal-support interface, for which a possible model is suggested. Parallel experiments in the hydrogenation of carbon monoxide at 220°C and P = 1 atm have shown a sharp decrease in the overall activity while the selectivity was virtually unaffected by the temperature of reduction.

Hydrogenation of CO over Rh/SiO2-CeO2 catalysts: kinetic evidences

Abstract The CO hydrogenation over Rh/SiO 2 -CeO 2 was investigated kinetically in order to find the rate equation as a function of CO and H 2 partial pressures. Relative to previous findings obtained with Rh/SiO 2 a fairly higher dependence on adsorbed CO is evident. Together with additional evidences found by TPD/TPR and IR spectroscopy, this higher dependence has been tentatively associated to a CO in which both the carbon and the oxygen ends are bonded to Rh and Ce 3+ , respectively. The influence of the support has been emphasized in order to ascertain the role of CeO 2 (promoter) relative to Rh/SiO 2 . In agreement with previous findings we have found that with Rh/SiO 2 -CeO 2 catalysts, the promoter inhibits the total activity while favoring the formation of EtOH.

Isotopic equilibration of carbon monoxide catalyzed by supported ruthenium

The rate of isotopic equilibration of CO has been studied at 373/sup 0/K on a ruthenium/Al/sub 2/O/sub 3/ catalyst. CO partial pressures from 5 x 10/sup 2/ to 15 x 10/sup 3/ Pa were used, and the effect of H/sub 2/ (0 to 10% vol) was studied. The analysis of kinetic data and the absence of CO/sub 2/ or CH/sub 4/ in the reaction products show that CO is present on the catalyst surface in an undissociated and reactive form. Hydrogen has a strong inhibiting effect on the exchange reaction, and does not react with CO under these conditions. It is argued that carbon monoxide can be adsorbed as a single carbonyl or in geminal dicarbonyl form, on metallic or partially oxidized ruthenium sites. This latter form displays a lower back-donation effect and with a contiguous similar group can form a square planar transition complex, by means of which the exchange reaction occurs. The inhibiting effect of H/sub 2/ is two-fold, namely, competitive adsorption against CO and stabilization of the carbonyl bond. Ab initio molecular calculations on a simplified model (H instead of Ru) seem to support this mechanism.

Support effects on the catalytic activity and selectivity of ruthenium in CO and N2 activation

CO and N2 isotopic equilibration reactions have been studied on ruthenium catalysts supported on Al2O3, SiO2 and MgO. The reactivity in CO equilibration was found to follow the sequence: Al2O3 > MgO > SiO2. However, the reactivity on Ru/MgO catalysts was found to be strongly dependent on the activation procedure: samples heated in vacuo at 673 K and then reduced at the same temperature showed the highest activity. The activation does not influence the reactivity of Ru/MgO catalysts towards N2 isotopic equilibration, where samples directly reduced in H2 were found to be slightly more active.The reactivity scale in NH3 synthesis is as follows: MgO SiO2 > Al2O3, with an apparent activation energy of ca. 100 kJ mol–1 for all the supports.The Fischer–Tropsch reaction carried out on samples supported on Al2O3 or MgO at ca. 50 atm and 530 K gave rise to a noticeable amount of oxygenated compounds such as alcohols.Results obtained on the various supports are discussed in terms of the presence of interactions between the metal species and the support itself, which were found to influence the activity and selectivity of the investigated reactions.

Preparation Aspects of Ru-Supported Catalysts and their Influence on the Final Products

A wide investigation of the influence of some preparation aspects, such as the nature of the support, the acidity of the precursor solution and the activation temperature, was carried out on Ru catalysts supported on SiO 2 , Al 2 O 3 , SiO 2 -Al 2 O 3 and MgO. For this purpose several bulk and surface techniques, such as thermal analysis, chemisorption, transmission electron microscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy, were employed. The dependence of the metal dispersion on the homogeneity of the metal distribution is discussed, together with the connection between the latter and the precursor solution acidity. A bimodal distribution of Ru-supported particles, massive metal crystallites and bidimensional sheets, was found. Binding energy measurements demonstrated that only on silica is Ru in a metallic state, while on the other supports an oxidized ruthenium species is prevalent, suggesting an interaction of the bidimensional sheets with the support.

Positron annihilation study of free volume in cross-linked amorphous polyurethanes through the glass transition temperature

Abstract The dependence of free volume on temperature has been monitored in some cross-linked polyurethanes, with different chain lengths, by using positron annihilation lifetime spectroscopy (PALS). The spectra were analysed in terms of continuous distributions in order to obtain the distribution of the longest component, τ 3 . The relative number and the mean size of free volume holes have been evaluated as a function of temperature: for T T g both the number and the volume of the holes increase; for T > T g the number of the holes reaches a plateau while the hole volume increases significantly. The amplitude of the plateau widens with the cross-linking and the hydrogen bond interactions. At the end of the plateau the number of the holes increases more than the hole volume: the analysis of hole distribution indicates that the dispersion decreases with increasing temperature. The dispersion of the size of the nanovoids around their average values at the various temperatures has been estimated by a suitable computer code that uses a continuous sum of the decay curves with a log-normal distribution. For each sample a glass transition temperature has been estimated and compared with that obtained by d.s.c. The influence of cross-linking on the free volume is discussed.

Polyurethane membrane for gas fractionation

Abstract Polyurethanes (PU) elastomeric membranes were synthetized using TDI reacted with bifunctional and polyfunctional polyols. In the case of bifunctional polyols, polypropyleneglycols of different chain length were used. Permeability, diffusion coefficient ( D ), solubility ( S ) were experimentally measured and solubility ( S ) was calculated with different gases (CO, H 2 , CO 2 , CH 4 , C 2 H 4 , C 2 H 6 ). The results show some possible practical applications. The diffusion coefficient D was found to depend upon the diameter of the permeating gas molecule and on the structure of the polymers, nevertheless no simple relation could be found with the free volume calculated with the W.L.F. theory. The solubility constant of the different gases in all the synthesized polymers could be interpreted on the basis of the regular solution theory founded on the solubility parameters of the gases and of the polymers.

Hydrogenation of CO over ZrO2-supported Rh catalysts: role of experimental parameters in modifying the C2H5OH/CH4 product ratio

The hydrogenation of CO over ZrO2-supported rhodium catalyst has been extensively investigated in an attempt to increase the C2H5OHCH4 ratio while minimizing CH3OH. Only one scheme has been found to account satisfactorily for the experimental rate equation determined by using CO and H2 partial pressure dependence. This scheme assumes the formation of a CHx moiety on the catalyst surface as the rate-determining step. Furthermore, in agreement with earlier findings, it is also consistent with the intermediate formation of acetaldehyde. Finally, the role of the support has been emphasized, as it has been found that ZrO2 significantly affects ethanol selectivity.

The effect of iron on the activity and the selectivity of Rh/ZrO2 catalysts in the CO hydrogenation

Abstract Rh/ZrO 2 catalysts (1% w/w) with different amounts of iron as promoter (Fe/Rh atomic ratio in the range 0–2) were prepared from three precursors: carbonyl clusters, chloride salts, nitrate salts. The catalysts were characterized by H 2 and O 2 chemisorption, titration of chemisorbed oxygen with hydrogen, TPR, X-ray analysis and IR spectroscopy of chemisorbed CO in the range of 300–773 K. The activity has been tested in the CO hydrogenation reaction (H 2 /CO = 3/1) at 493 K under atmospheric pressure. The physico-chemical characterization of the catalysts evidenced that iron is present either as oxidized Fe δ+ or in the reduced Fe 0 form: IR analysis revealed that some reduced iron either decorates or is near the border of Rh particles. The metal precursors do not seem to influence significantly the behaviour of the catalysts. Two opposing effects of iron were evidenced: (i) a decrease of Rh sites available for carbon monoxide and hydrogen chemisorption; (ii) an increase of doubly bonded CO (RhCOFe) favoured by Fe x + sites, contiguous to Rh sites, that are reduced to Fe 0 sites in reaction conditions. Depending on the surface environment, the bridged CO may dissociate to RhC and FeO or insert a hydrogen or an alkyl group with formation of an acyl species.