Gerard Pajonk

SURFACE CHARACTERIZATION OF PALLADIUM-COPPER BIMETALLIC CATALYSTS BY FTIR SPECTROSCOPY AND TEST REACTIONS

Different bi-metallic palladium-copper catalysts have been prepared according to different sequences of impregnation of a commercial alumina. CO adsorption followed by FTIR spectroscopy was performed in order to identify the nature of the superficial sites. Two test reactions were also used to characterize the nature of the active surface site: gas phase toluene hydrogenation was selected in order to show the hydrogenating activity of metallic palladium, whereas the reaction of decomposition of ethanol, which exhibits 100% selectivity in the formation of ethanal on pure copper catalysts, was also studied on this series of catalysts. The results obtained by both methods are complementary. By comparison with the behaviour of pure palladium or copper based catalysts, they allow to propose hypotheses on the surface composition: separated phases, alloys or a phase covered by the other one.

Small-angle X-ray scattering of a new series of organic aerogels

Abstract During the last decade, several new precursors have been proposed for the synthesis of organic aerogels. In a large majority of cases, the sol–gel reaction is base catalyzed and water is used as the solvent. Because of the poor solubility of liquid CO 2 in water, a time consuming two-step exchange is necessary (exchange of water by acetone followed by exchange of acetone by liquid CO 2 ). To eliminate this step, a new process with the solvent acetone was developed. Also in order to reduce gelation time, acid catalysis is used. The aim of the present work is to compare the solid and the porous texture of resorcinol–formaldehyde (RF) aerogels prepared by the conventional method (in water and base catalysis) to that of samples prepared by the new method (in acetone and acid catalysis), all other parameters (mass ratio, catalyst concentration R/C, supercritical extraction, pyrolysis conditions) remaining the same. Determination of pore size distribution (PSD) in different series of organic and pyrolyzed aerogels by thermoporometry and characterization of the solid structure by small-angle X-ray scattering (SAXS) measurements over a wide range of length are reported. The main difference between the two series of aerogels lies in the aggregation of the primary particles: the new series prepared in acetone and acid catalysis displays fractal scaling over more than a decade in length, while the conventional one does not. It is also shown that thermoporometry yields reliable information for both series of aerogels.

DLS and SAXS investigations of organic gels and aerogels

Abstract Recent investigations have shown that the structure of organic aerogels can be significantly modified by changing the precursors, the solvent and the nature of the catalyst involved in the sol–gel reaction. It is therefore highly desirable to investigate the sol–gel mechanism. For this purpose, dynamic light scattering (DLS) measurements have been performed at different stages of the reaction for base- or acid-catalyzed gelation of resorcinol–formaldehyde (RF) using water or acetone as solvents. The structure of aged gels was investigated by small-angle X-ray scattering (SAXS) and compared to that of the aerogels obtained after exchange of solvent by supercritical CO2 and drying of the aged gels. It is shown that acid-catalyzed gelation of RF in acetone can be described by percolation, which explains that this series of aerogels consists of mass fractal aggregates (Dm=2.5). The partial collapse of this polymeric gel yielding colloidal particles in the aerogel can be attributed to deswelling in supercritical CO2. DLS indicates that gelation of RF with a base catalyst yields a colloidal gel whose structure remains practically unchanged in the aerogel, as shown by SAXS.

Optical transmission properties of silica aerogels prepared from polyethoxidisiloxanes

Transparent, crack-free low density silica monolithic aerogels synthesized from a series of polyethoxidisiloxanes have been characterized in the UV-visible-NIR optical range (0.3–2.5 μm). Normal hemispherical transmittances in the visible range τvnh were measured as well as extinction coefficients at 550 nm. These quantities are valuable parameters to quantify the image quality seen through a piece of aerogel by an empirical index called the transparency ratio (TR). The transparency ratios depend upon catalysis, volumic percentage of solvent and chemical nature of precursors, at the sol–gel step, as well as the supercritical temperature of drying and the sample thickness. Results show that the transparency ratio of the best aerogel is relatively close to that of a simple window pane of comparable thicknesses (0.92 versus 0.99, respectively) while their respective extinction coefficients at 550 nm are similar (16.3 m−1 versus 16.4 m−1, respectively).

Investigation of the multi-scale structure of silica aerogels by SAXS

An investigation is described into polyethoxydisiloxane (PEDS-Px) silica precursors of aerogels prepared in ethylacetoacetate (etac) under HF conditions and dried under CO2 supercritical conditions. The influence upon the internal nanostructure of aerogels of the number of water molecules (n*) used to synthesize the precursors is studied, as well as that of their volume fraction (xPrec) in solution during the second catalytic step. Correlation between structure and optical transmission (%TR) is necessary to improve optical behavior of monolithic silica aerogels for double-window applications. For this reason, small angle X-ray scattering (SAXS) was performed to characterize the nanostructural silica skeleton, and especially to investigate particle and cluster properties. Optical transmission was also measured to estimate visible optical quality of the samples (%TR). Among other results, it is shown in this study that increasing n* and xPrec contributes to the decrease of the particle and cluster sizes, which indirectly improves %TR in the visible range. These observations confirm and help to explain previously published results.

Fluorescent dye doped aerogels for the enhancement of Čerenkov light detection

An aerogel Cerenkov threshold detector for the velocity identification of charged particles is under construction at the elementary particle physics experiment SAPHIR (spectrometer arrangement for photon induced reactions). Aerogels are the only possible radiator material because of their optical properties. To enhance the efficiency a fluorescent dye in the aerogel radiator allows spectral matching between the near-UV part of the Cerenkov spectrum and the light collection system. Fluorescent dye doped aerogels were prepared and their optical properties were investigated.

Synthesis and properties of some monolithic silica carbogels produced from polyethoxydisiloxanes dissolved in ethylacetoacetate (etac) and acid catalysis

Abstract The aim of the present work described in this paper was to synthesize transparent and superinsulating monolithic carbogels in order to use them as transparent double window insulation spacers at the laboratory scale. The monolithic silica carbogels were made by sol–gel technique using mixtures of a new family of precursors noted PEDS-Px (or polyethoxydisiloxanes which are prepolymers of silica differing by their substoichiometric hydrolysis degrees) [G.M. Pajonk et al., J. Non-Cryst. Solids 186 (1995) 1] with organic solvents in the presence of hydrofluoric acid with no excess of water added, and subsequent drying in the supercritical conditions with respect to CO2 [P.H. Tewari et al., in: L.L. Hench, D.R. Ulrich (Ed.), Science of Ceramic Chemical Processing, Wiley, New York, 1986, p. 123]. Our research was focused at finding the best conditions of preparation of the monolithic silica carbogels in terms of optical and thermal insulation properties. A new solvent ethylacetoacetate (etac) was found to be much more soluble in liquid CO2 than common alcohols and acetone, and the so obtained carbogels, with etac, present very interesting optical visible transmission and thermal properties respectively. The best samples were prepared from mixtures composed of 60% of a prepolymer prepared with 1.8xa0moles of water per TEOS mole (noted P900) and 40% of etac (on a volumetric basis), and presented very good transparency close to the one shown by a flat glass pane together with a very small thermal conductivity (unequalled, to the best of our knowledge, by any other solid material at the present time).

Improved monolithic aerogel for transparent glass spacer in innovative windows

A collaborative research program is engaged to produce `super window elements with monolithic silica aerogel used as a spacer between two glass panes. The first improvement is a new precursor called polyethoxysiloxane, for an easier and shorter process. This modification is followed by optical measurements, introducing a new concept: the `transparency ratio. And we show how the diffuse part of the transmitted light may be corrected by the chemical composition and process. With the two further improvements (`in situ production of aerogel and CO2 substitution method) we anticipate progress toward a high transparency of the final product.