|
Abstract:
|
Traditionally , supported metal catalysts have been synthesized by reduction of precursors directly over the support . In these techniques , it is challenging to control the metal cluster size , composition and crystal structure . Herein , we have developed a novel approach to design catalysts with controlled morphologies by infusing presynthesized nanocrystals into the supports . High surface area mesoporous materials , including graphitic carbons , have been utilized for obtaining a high degree of metal dispersion to enhance catalyst stabilities and activities . Gold and iridium nanocrystals have been infused in mesoporous silica with loadings up to 2 wt % using supercritical CO₂ as an antisolvent in toluene to enhance the van der Waals interactions between nanocrystals and the silica . The iridium catalysts show high catalytic activity and do not require high temperature annealing for ligand removal , as ligands bind weakly to the iridium surface . To further enhance metal loadings to >10 % in the catalysts , short -ranged interactions between the metal nanocrystals and the support are further strengthened with weakly binding ligands to expose more of the metal surface to the support . For pre -synthesized FePt nanocrystals , coated with oleic acid and oleylamine ligands , high loadings >10 wt % in mesoporous silica are achieved , without using CO₂ . The strong metal -support interactions favor FePt adsorption on the support and also enhance stability against sintering at high temperatures . High resistance to sintering favors formation of the FePt intermetallic crystal structure with <4 nm size upon thermal annealing at 700 °C . The fundamental understanding of the metal -support interactions gained from these studies is then utilized in the design of highly stable Pt and Pt -Cu electrocatalysts with controlled size , composition and alloy structure supported on graphitized mesoporous carbons for oxygen reduction . The resistance of the graphitic carbons to oxidation coupled with strong metal -support interactions mitigate nanoparticle isolation from the support , nanoparticle coalescence , Pt dissolution and subsequent Ostwald ripening and thus enhance catalyst stability . The control of the Pt nanocrystal morphology with high concentrations of highly active (111 ) surface leads to 25 % higher activities than commercial Pt catalysts . Furthermore , the catalyst activities obtained for Pt -Cu catalysts are 4 -fold higher than Pt catalysts due to strained Pt shell generated from electrochemical dealloying of copper from the nanoparticle surface . |