Positron-Annihilation-Lifetime-Spectroscopy (PALS) for the characterization of bimodal silica-gel synthesized by pseudomorphic transformation

Abstract

In this study the results of the characterization of bimodal silica-gel samples with nitrogen adsorption and Positron-Annihilation-Lifetime-Spectroscopy (PALS) are compared. The bimodal silica-gel samples were synthesized by pseudomorphic transformation of a mesoporous silica gel. A set of eight samples was generated using different volumes of 0.07 M cetyltrimethylammonium hydroxide (5–42 ml/g) to obtain bimodal pore systems with controlled proportions of 15 nm pores of the original silica-gel and 4 nm pores formed by the MCM-41 phase. The aim was to proof if it is possible to determine the pore volumes directly from the PALS experiment. The results of the PALS measurements clearly indicate the bimodal pore structure in the materials. Two individual pore systems can be detected with a good resolution. The peak area of the signals can be calculated and compared with the pore volumes determined by nitrogen adsorption. The results show a correlation between the pore volume determined by nitrogen adsorption and the peak area calculated from PALS. Some differences between the measured pore volumes of the starting material can be an indication for the formation of a closed porosity in the starting material and after the processes of pseudomorphic transformation. The PALS measurements enable a complementary characterisation of pore systems in the micro- and mesopore range. Bimodal structured porous materials can be studied and the individual pore systems are well separated. The observed correlation between intensities (respectively peak areas) from PALS and pore volumes determined by nitrogen adsorption for the different pore classes allows the direct estimation of the pore volumes by PALS via calibration and the quantification of a potential closed porosity.