Sintering-stability is a key feature for nanoparticle catalysts. Two promising protocols have been developed.
Catalysis is widely used by industry to convert raw materials into chemicals and fuels in an economical way with low environmental impact. Nanoparticles are attractive as catalysts, since the nanoparticles have much higher surface-to-volume ratio than bulk materials. However, nanoparticles can easily agglomerate. This phenomenon, known as sintering, may reduce and eventually block the catalytic activity. The project demonstrates the ability of two different synthesis methods to create sintering-stable nanoparticle catalysts.
Nanoparticles of noble metals are often effective catalysts. However, noble metals like gold, palladium and platinum are expensive raw materials, and it is highly desirable to preserve the catalytic capability of the nanoparticles for as long as possible. The project investigates two experimental protocols for encapsulation of metal nanoparticles inside porous materials.
The first method applied was the pressure assisted impregnation and reduction method (PAIR). Here, a porous material is impregnated with a solution of a metal precursor, and its reduction is performed under elevated pressure. The procedure was successfully used to synthesize 2-3 nm gold nanoparticles inside porous silicas. The synthesized samples showed increased stability towards sintering.
As the second method, in situ incorporation was chosen. This is a one-pot procedure based on simultaneous growth of a porous silica and entrapment of a metal precursor in the form of an ethylenedia-mine complex inside the forming crystalline network, leading to encapsulated metal nanoparticles after reduction in hydrogen. The in situ incorporation method produced single metal palladium and platinum nanoparticles, 2-3 nm in size, and bi-metallic palladium/platinum nanoparticles inside silica.
The synthesized catalysts were able to decompose formic acid with 85 % selectivity towards hydrogen at temperatures around 100 °C. Encapsulated palladium catalysts were shown to be very active in Suzuki cross-coupling reaction.
In conclusion, both the PAIR method and in situ incorporation were shown to be feasible
and easy protocols for synthesis of metal nanoparticles inside a zeolite matrix. Small size, high catalytic activity and stability towards sintering make both protocols promising for further research.
Illustration:
STEM image of the Pd-Pt nanoparticles encapsulated in porous silica.