Monte Carlo Simulations on Rough and Porous Alumina

Abstract

Heterogeneous catalysts are of primary importance for many industrial processes. Catalysts are frequently obtained by including small amounts of active species on a porous carrier having a very high specific surface. For design and optimization purposes, electron probe micro analysis (EPMA) is routinely used in our laboratory. With EPMA, the sample is irradiated by a monoenergetic keV electron beam. X-ray Bremsstrahlung and characteristic photons are then selected by either an energy dispersive spectrometer (EDS) or a wavelength dispersive spectrometer (WDS). The characteristic line intensity (of relaxation of inner-shell ionisation photons) of the unknown sample is measured as well as the intensity of this same line emerging from a standard of a well known composition. Correction procedures are applied to achieve a quantification of the elements. Yet when traditional EPMA correction procedures are processed for our porous alumina samples, unrealistically low total composition are observed. The alumina porous samples (porosity ≈ 68%, specific surface ≈ 250m2/g, pore size ≈ 10 nm) emit about 10% less photons on Al Kα line than a massive sample of same composition.