A. Borówka

Vacuum removal of the template in MCM-41 silica studied by the positron annihilation method.

Positron annihilation lifetime spectra were measured for ordered silica before template (octadecyltrimethyl ammonium) removal. An intermediate lifetime component, well pronounced in the spectra, indicates the presence of structural defects in the template being the size of a surfactant molecule. At the temperature near 420 K a radical change of template structure occurs, consisting in appearance of disorder and the degradation of template; evaporation of the fragments begins. It seems that moderate heating (up to about 500 K only) in vacuum for several hours is an effective and simple method of template removal, leaving no pure carbon residue in the pore structure. The results are compared with the data from the liquid nitrogen adsorption/desorption method and elemental analysis, confirming the positron annihilation results.

Temperature changes of the template structure in MCM-41 type materials; positron annihilation studies

Abstract The lifetime spectra of positrons annihilating in MCM-41 ordered silica were measured as a function of temperature. It was found that in alkyltrimethylammonium templates degradation in vacuum begins at 380–400 K, that consists in breaking the long micelle cylinders, rise of gaps between the fragments and finally pore evacuation. The latter process is not completed and particularly in C12TAB the spectrum component belonging to fragmented template preserves its high intensity up to 520 K (the highest temperature in our experiment). The pore surface area over 1000 m 2 /g is achieved already at temperatures about 490 K. The lifetime of the longest-lived component indicates that practically all ortho -positronium atoms leave the pores before annihilation.

Modification of the porosity of commercial silica gels by dissolution in alkaline mixtures

Abstract The modification of the porosity of commercial silica gels in sodium silicate solutions was investigated. The influence of the presence of preadsorbed substances on the effect of modifications was tested. The pore size distributions, total pore volumes and specific surface areas were determined by using nitrogen method. The same silica samples were investigated also using SAXS technique.

Porosity of chemically modified silica gels by nitrogen adsorption, positron annihilation and small angle X-ray scattering

Abstract The progress in the determination of porosity of various types of materials has arisen over the past ten years from advances in application of new spectroscopy techniques. In the present paper the application of small angle X-ray scattering (SAXS), positronium annihilation lifetime spectroscopy (PALS) and low temperature nitrogen adsorption methods to the characterization of mesoporosity is reviewed using different types of silica gels with chemically modified surface. The results from the three methods are compared and discussed.

Positronium annihilation study of as-synthesized MCM-41 silica under pressure

The lifetime spectra of positron annihilation in the MCM-41 ordered silica before template removal were measured as a function of pressure. The samples were compressed with various hydrostatic pressures by argon. Moreover, the same samples were compressed mechanically. Several lifetime components, well pronounced in the spectra, indicate the presence of various types of defects, voids or pores in the template as well as silica network. When the hydrostatic pressure increases, one can observe evolution of defects in the micellar interior and high stability of micropores in the silica walls of cylindrical pores. In the case of higher mechanical pressure, the samples exhibit strong degradation of silica framework. Simultaneously structural changes of template are less pronounced.

Migration of siloxane polymer in ordered mesoporous MCM-41 silica channels

Penetration of mesopore structure of MCM-41 silica by the siloxane polymer 50%methyl-50%phenyl polysiloxane (OV-17) has been investigated by positronium annihilation lifetime spectroscopy (PALS) using a series of silica samples with different coating levels. The changes of parameters characterizing porosity of investigated materials with increase of the coating was also monitored using low temperature nitrogen adsorption experiments (LN). It is shown that the gradual changes in the specific surface area and the pore volume are observed in the studied coating range. This work shows the usefulness of the PALS to monitor the changes in the pore structure of coated MCM-41 samples. Positron Annihilation Lifetime Spectroscopy provides the information on free volume size and their concentration in porous solids, independently if they are open or closed - inaccessible for adsorptives.

Pore volumes of activated carbons by wetting in saturated liquid mixtures

Abstract Total pore volumes for carbon adsorbents with various porosity were estimated by wetting in saturated liquid mixture: methanol+n-heptane. The obtained results are compared with results derived from adsorption data of nitrogen and mercury intrusion data. Moreover, for the components forming binary mixtures of limited miscibility used in wetting experiments, the adsorption isotherms were measured. The correlation between adsorption and wetting data is discussed.