Confining alloy or core-shell Au-Pd bimetallic nanocrystals in silica nanorattles for enhanced catalytic performance

Alloy or core-shell Au-Pd bimetallic nanocrystals confined in silica nanorattles are prepared in order to enhancing their catalytic activity, selectivity and stability. The formation of the alloy or core-shell structure is controlled via tuning the reduction kinetics in a one pot hydrothermal solution. A kind of silane with alkylamino groups is chosen as the reducing agent and pre-encapsulated in the middle layer of hybrid solid silica nanospheres, which ensures that the Au-Pd bimetallic nanocrystals are prepared inside the silica nanorattles. The composition of the Au-Pd alloy core is easily tuned by changing the ratio of Au-Pd precursor concentration from 3 : 1, to 1 : 1, to 1 : 3. Suzuki cross-coupling reactions are chosen as model reactions to evaluate the catalytic ability of these nanocatalysts. The conversion and yield from highest to lowest for all catalyst are in the sequence: alloy Au3Pd1 > Au1Pd1 > Au1Pd3 > core-shell Au-Pd > Pd > Au. Notably, the alloy Au-Pd@SiO2 with the lowest Pd content shows the highest catalytic activity and selectivity both for iodobenzene and bromobenzene. The mesoporous walls of the silica nanorattles prompt mass transportation and protect the Au-Pd bimetallic nanocrystal cores avoiding aggregation. The stability of the bimetallic nanocrystals is enhanced and the leaching of Pd is inhibited by the protection of the mesoporous shell. The alloy Au-Pd@SiO2 even shows a higher selectivity and stability than a commercial Pd-C catalyst. Our synthesis strategy is also successfully expanded to fabricate Au-Pd alloy nanocrystals confined in TiO2 hollow spheres, which shows the potential for preparing other types of nanoreactors and improves their catalytic activity, selectivity and stability simultaneously.