Pd on boron-doped hollow carbon spheres – PdO particle size and support effects

Boron-doped hollow carbon spheres (B-HCSs) were synthesized using a CVD injection method (BCl3 in toluene; Ar; 900 °C; 4 h) using Stöber silica spheres as template. A Pd complex was loaded onto the B-HCSs using deposition and impregnation methods, and the materials were studied by XRD, SEM, TEM, Raman spectroscopy, TGA, ICP-OES, XPS, and 11B solid-state NMR spectroscopy. The boron-doped carbon-supported Pd catalysts (Pd/B-HCS) were compared with a Pd-loaded boron-doped carbon nanotube (Pd/B-CNT) catalyst (both have a hollow interior) in the solvent-free oxidation of alcohols using oxygen as an oxidant at 125 °C under base-free conditions. The Pd particle size was varied (2.5–12 nm) by changing the Pd/B-HCS calcination temperature (200–550 °C), and this affected the activity but not the selectivity of the benzyl alcohol to benzaldehyde reaction. The data revealed the key role of the Pd particle size on the reaction that was influenced by the B–Pd–C interaction. The reaction rate depended on the mean size of Pd particles and showed a maximum when catalysts were calcined at 300 °C, revealing that the aerobic oxidation of benzyl alcohol catalyzed by the supported PdO (dPd > 2.5 nm) nanoparticles was not a structure sensitive reaction.