Noble metal nanocrystals with different shapes and compositions are embedded in hollow mesoporous metal oxide microspheres through an ultrasonic aerosol spray. Polystyrene (PS) nanospheres are employed simultaneously as a hard template to create hollow interiors inside the oxide microspheres and as the carrier to bring pregrown metal nanocrystals, including Pd nanocubes, Au nanorods, and Au core/Pd shell nanorods, into the oxide microspheres. Calcination removes the PS template and causes the metal nanocrystals to adsorb on the inner surface of the hollow oxide microspheres. The catalytic performances of the Pd nanocube-embedded TiO2 and ZrO2 microspheres are investigated using the reduction of 4-nitrophenol as a model reaction. The presence of the mesopores in the oxide microspheres allows the reactant molecules to diffuse into the hollow interiors and subsequently interact with the Pd nanocubes. The embedding of the metal nanocrystals in the hollow oxide microspheres prevents the aggregation of the metal nanocrystals and reduces the loss of the catalyst during recycling. The Pd nanocube-embedded ZrO2 microspheres are found to exhibit a much higher catalytic activity, a much larger catalytic reaction rate, and a superior recyclability in comparison with a commercial Pd/C catalyst. This preparation approach could potentially be utilized to incorporate various types of mono- and multimetallic nanocrystals with different sizes, shapes, and compositions into hollow mesoporous oxide microspheres. Such a capability can facilitate the studies of the catalytic properties of various combinations of metal nanocrystals and metal oxide supports and therefore guide the design and creation of high-performance catalysts.
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