The Atomic Local Ordering of SBA-15 Studied with Pair Distribution Function Analysis, and Its Relationship to Porous Structure and Thermal Stability

Abstract

The atomic-scale structure of amorphous SBA-15 materials with different pore sizes (6.7 - 12.7 nm) was investigated using pair distribution functions calculated from the X-ray total scattering data. The data was collected with a laboratory X-ray powder diffraction instrument instead of synchrotron often considered necessary for pair distribution function analysis. A 3D model of the atomic structure within the pore walls was determined based on the pair distribution function analysis, and the degree of ordering was evaluated using a method based on autocorrelation. Nitrogen gas adsorption was utilized to determine the pore characteristics of the samples. X-ray diffraction and the information on pore diameter was utilized to estimate the wall thicknesses. It was observed that thicker walls possessed more micropores, and the wall structure was more disordered on the samples with lots of micropores. The thermal stability of the samples was investigated with thermogravimetric analysis, and the results showed that samples with higher degree of ordering and less microporosity possessed better thermal stability.

Keywords: Mesoporous silica, Atomic ordering, X-ray diffraction, Pair distribution function, Thermodynamic stability