Tailoring High-Performance Pd Catalysts for Chemoselective Hydrogenation Reactions via Optimizing the Parameters of the Double-Flame Spray Pyrolysis

Tuning the chemical composition during the synthesis is a widely used method to control the activity of catalysts. Here, we reported an alternative synthesis strategy to tune the catalytic properties of nanocatalysts without changing their precursors and compositions. We synthesized a series of Pd catalysts on the most popular SiO₂-, Al₂O₃-, and silica-alumina supports using the double-flame spray pyrolysis (FSP) technique. It was observed that various flow rates used for the synthesis of catalysts with the same composition affected the formation of the catalyst particles and their structures to further tune the surface acidity due to the correlation between acidity and structure, but the flow rates did not influence the electronic properties of Pd particles. It was observed that surface OH groups could associate Pd for the hydrogenation, but Lewis acid sites could not, as Pd/SA-30 and Pd/SiO₂ showed much higher activity than Pd/Al₂O₃ for the same Pd size and surface properties. For Pd catalysts with Brønsted acid sites (silica-alumina) or weak/nonacidic SiOH groups (SiO₂), their catalytic performance for the chemoselective hydrogenation of acetophenone was obviously enhanced by tuning the surface OH groups via changing the flow rates for the same precursor solution during this ultrafast synthesis.