Experimental evidence for the nanocage effect in catalysis with hollow nanoparticles.

Five different hollow cubic nanoparticles with wall length of 75 nm were synthesized from platinum and/or palladium elements. The five nanocatalysts are pure platinum nanocages (PtNCs), pure palladium nanocages (PdNCs), Pt/Pd hollow shell-shell nanocages (NCs) (where Pd is defined as the inner shell around the cavity), Pd/Pt shell-shell NCs, and Pt-Pd alloy NCs. These are used to catalyze the reduction of 4-nitrophenol with sodium borohydride. The kinetic parameters (rate constants, activation energies, frequency factors, and entropies of activation) of each shell/shell NCs are found to be comparable to that of pure metal NCs made of the same metal coating the cavity in the shell-shell NCs. These results strongly suggest that the catalytic reaction takes place inside the cavity of the hollow nanoparticles. Because of the nanoreactor confinement effect of the hollow nanocatalysts, the frequency factors obtained from the Arrhenius plots are found to be the highest ever reported for this reduction reaction. This is the reason for enhanced rate of this reaction inside the cavity. The importance of mechanism of the homogeneous and the heterogeneous nanocatalytic reactions occurring on the external surface of a solid nanoparticle are contrasted with those occurring on the nanocavity surface.