Studying the metal-support interaction in Pd/γ-Al₂O₃ catalysts by atomic-resolution electron energy-loss spectroscopy

A 2 wt% Pd/γ-Al₂O₃ catalyst, reduced in hydrogen at different temperatures, has been studied by combined atomic resolution Z-contrast imaging and electron energy loss spectroscopy (EELS) techniques. Z-contrast images have been used primarily to evaluate the particle-size distribution and morphology as a function of the catalyst reduction temperature. Energy-loss spectra obtained from specific locations in and around the metal particles show a shift in the Pd M_2,3 edge, which is an indication of the Pd nanoparticles getting negatively charged. This acquired charge on the Pd particles is the result of electron transfer from the alumina support and appears to be dependent on the size of the Pd nanoparticles. No aluminum signal is observed on the surface of the metallic nanoparticles, and the observed oxygen signal on the Pd nanoparticles is probably due to oxygen that gets adsorbed on the reduced catalyst.