Three-Dimensional Analytical Surface Quantification of Heterogeneous Silica-Alumina Catalyst Supports

Abstract : The ability of energy-filtered transmission electron microscopy (EFTEM) tomography to provide 3D chemical maps at the nanoscale opens a new way to analyse heterogeneous materials quantitatively. In association with other techniques, EFTEM tomography has been employed in the study of amorphous silica-alumina catalyst supports. Two types of samples prepared either by mechanical mixing (MM) or by the precipitation of silica on boehmite (PSB) that have similar proportions of silica and alumina were analysed. The sample synthesised by the PSB method shows a smaller degree of heterogeneity than the sample obtained by MM. For both types of samples, a higher concentration of alumina was found at the surface, whereas silica mostly constituted the core of the sample. A thermal treatment in a humid atmosphere was shown to redistribute the silica inside the sample as well as on its surface, which decreased the specific surface area at the same time. The acid sites localisation was defined as a specific curve at the interface between the two components upon reaching the surface of the support. The length of this curve, the “alumina–silica boundary line”, was estimated by using EFTEM tomography and discussed qualitatively with the chemical inter-mixing information deduced from additional techniques such as FTIR and NMR spectroscopy.