Silica nanosphere-supported shaped Pd nanoparticles encapsulated with nanoporous silica shell: Efficient and recyclable nanocatalysts

A synthetic method to a highly efficient heterogeneous nanocatalyst, consisting of silica nanosphere (SiO₂) decorated with palladium nanoparticles (Pd-NP) that are further encapsulated within a nanoporous silica shell (Porous-SiO₂), is reported. First, monodisperse, 5 nm and 20 nm Pd nanoparticles are synthesized and anchored onto silica nanospheres of ∼250 nm in diameter. These core-shell nanospheres are then coated with a secondary silica shell by the sol-gel process. This is followed by controlled etching of the outer silica shell to yield nanoporous silica shell around the Pd-NP. The thickness of the nanoporous silica shell and its pore structures are controlled by changing the synthetic conditions. The resulting nanoporous silica shell is proved to permit reactants to reach the Pd-NP while at the same time protect them from aggregation. These new nanomaterials, dubbed SiO ₂/Pd Nanoparticles/Nanoporous SiO₂ (or SiO ₂/Pd-NP/Porous-SiO₂) core-shell-shell nanospheres behave as heterogeneous nanocatalyst exhibiting high catalytic activity and turn-over-numbers (TONs) in hydrogenation reaction of various substrates at room temperature and 20 bar hydrogen pressure. The core-shell-shell nanospheres are proven to be versatile catalysts as they are also able to catalyze C-C coupling reactions effectively. Moreover, these heterogeneous nanocatalysts are stable showing negligible Pd leaching and aggregation, and can be recycled multiple times without loss of catalytic activity.