'Naked' Magnetically Recyclable Mesoporous Au-γ-Fe2 O3 Nanocrystal Clusters: A Highly Integrated Catalyst System

Naked magnetically recyclable mesoporous Au–γ-Fe2O3 clusters, combining the inherent magnetic properties of γ-Fe2O3 and the high catalytic activity of Au nanoparticles (NPs), are successfully synthesized. Hydrophobic Au–Fe3O4 dimers are first self-assembled to form sub-micrometer-sized Au–Fe3O4 clusters. The Au–Fe3O4 clusters are then coated with silica, calcined at 550 °C, and finally alkali treated to dissolve the silica shell, yielding naked-Au–γ-Fe2O3 clusters containing Au NPs of size 5–8 nm. The silica protection strategy serves to preserve the mesoporous structure of the clusters, inhibit the phase transformation from γ-Fe2O3 to α-Fe2O3, and prevent cluster aggregation during the synthesis. For the reduction of p-nitrophenol by NaBH4, the activity of the naked-Au–γ-Fe2O3 clusters is ≈22 times higher than that of self-assembled Au–Fe3O4 clusters. Moreover, the naked-Au–γ-Fe2O3 clusters display vastly superior activity for CO oxidation compared with carbon-supported Au–γ-Fe2O3 dimers, due to the intimate interfacial contact between Au and γ-Fe2O3 in the clusters. Following reaction, the naked-Au–γ-Fe2O3 clusters can easily be recovered magnetically and reused in different applications, adding to their versatility. Results suggest that naked-Au–γ-Fe2O3 clusters are a very promising catalytic platform affording high activity. The strategy developed here can easily be adapted to other metal NP–iron oxide systems.