Microstructural development and catalytic performance of Au-Pd nanoparticles on Al2O3 supports:: The effect of heat treatment temperature and atmosphere

Au-Pd nanoparticles supported on a variety of oxides are known to be highly active catalysts for a number of interesting reactions, including the direct synthesis of H2O2 from H-2 and O-2. A systematic series of Au-Pd/Al2O3 catalysts, subjected to a variety of heat treatments, have been examined by STEM-XEDS and XPS in an effort to track the microstructural development of the bimetallic particles. It was observed that the nature and size distribution of the AuPd particles was highly dependent upon both the heat treatment temperature and the atmosphere. As synthesized particles were homogeneous alloys, but subsequent calcination in air produced metal particles which showed a progressive enrichment of Pd at their surface. The change from homogeneous alloy to Pd-rich shell/Au-rich core morphology and increase in average particle size induced by the calcination treatment was accompanied by a significant decrease in the activity of the catalyst. However, the calcination step is deemed to be necessary to produce a stable reusable catalyst that resists leaching of the metal from the oxide surface into solution. Furthermore, it is shown that the subsequent reduction of a previously calcined catalyst is highly deleterious to its performance and stimulates many of the AuPd particles to "invert" and form a Pd-rich core/Au-rich shell morphology.