Effect of calcination on the thermal stability of SBA-15 studied by positron annihilation spectroscopy

Background: Since the 1990s, porous materials have been widely used in the fields of gas adsorption, electrochemistry, drug delivery, catalyst and catalyst carrier because of their excellent physical and chemical properties such as high specific surface area, high permeability, adsorption and assemblability. The application of porous materials largely depends on their macroscopic properties, especially the diversity of pore structure and the adjustability of pore size, especially, the thermal stability of SBA-15 is very important for its large-scale practical application. Purpose: This study aims to observe the thermal stability of SBA-15 by positron annihilation spectroscopy (PAS). Methods: First of all, ordered mesoporous silica SBA-15 were synthesized by using TEOS (tetraethyl orthosilicate) as the silicon source and P123 as the structure-directing agent. Part of the obtained SBA-15 was further calcined at 550 ℃. Then, the above two kinds of SBA-15 were heated at 100~1 000 ℃ to check the thermal stability of their pore structure. Small angle X-ray scattering (SAXS), scanning electron microscopy (SEM),high-resolution transmission electron microscopy (HRTEM), N2 adsorption/desorption and positron annihilation spectroscopy (PAS) measurements were used to study the ordered pore structure of SBA-15. Results: Positron lifetime measurements reveal that four life components exist in both SBA-15 whilst the two longer lifetimes τ3 and τ4 correspond to the annihilation of o-Ps in the micropores and large pores of the material, respectively. With the increase of heat treatment temperature, the longest lifetime τ4 of these two kinds of synthesized silica decrease gradually, as well as the corresponding intensity I4. However, the longer life and intensity of calcined SBA-15 decrease obviously after 600 ℃. Meanwhile, the S parameter of calcined SBA-15 is in good agreement with the results of o-Ps lifetime, and the S-W curve also shows that the pore types of calcined SBA-15 nearly are unchanged during in the heat treatment process at the temperature range of 100~900 ℃. Conclusions: All the results indicate that the synthesized SBA-15 calcined at 550 ℃ exhibits relatively better thermal stability, and positronium (Ps) is a very sensitive probe to study pore structure. © 2022, Science Press. All right reserved.