Infusion of Presynthesized Iridium Nanocrystals into Mesoporous Silica for High Catalyst Activity
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- 作者：Gaurav Gupta ; Cynthia A. Stowell ; Mehul N. Patel ; Xiaoxia Gao ; Miguel J. Yacaman ; Brian A. Korgel ; Keith P. Johnston
- 刊名：Chemistry of Materials
- 出版年：2006
- 出版时间：December 26, 2006
- 年：2006
- 卷：18
- 期：26
- 页码：6239 - 6249
- 全文大小：415K
- 年卷期：v.18,no.26(December 26, 2006)
- ISSN：1520-5002
文摘
Traditionally, finely dispersed metal catalysts have been formed by reduction of precursors withinmesoporous supports. A new concept for designing catalysts with enhanced activities and selectivities isto infuse presynthesized nanocrystals with well-defined morphologies into ordered mesoporous materials.The decoupling of nanocrystal synthesis and infusion provides exquisite control of the nanocrystal size,morphology, and dispersibility within the pores. A dispersion of iridium nanocrystals was infused intomesoporous silica by expanding the solvent toluene with supercritical CO2. To achieve high nanocrystalloadings, up to 1.3 wt %, we tuned the solvent quality to strengthen the interactions of the nanocrystalswith the pore walls, but without precipitating the nanocrystals in the bulk solvent. Z-contrast STEMindicates conclusively that the iridium nanocrystals were located within the pores and not on the externalsilica surface. High catalytic activity was observed for 1-decene hydrogenation, which is consistent witha high degree of dispersion of the 4.5 nm nanocrystals throughout the pores, as observed by TEM. Amaximum turnover frequency (TOF) of 16 s-1 was measured, which was higher than the initial TOF forhomogeneous catalysis with the same nanocrystals in 1-decene. The iridium catalysts do not requirepretreatment to remove the tetraoctylammonium bromide ligands to achieve activation, as the ligandsbind weakly to the iridium surface. Consequently, the activity was not enhanced when calcined at 500
C in nitrogen or when annealed in supercritical CO2 at 275 bar. The ability to predesign nanocrystalmorphology and surface properties prior to infusion into the mesoporous silica support offers novelopportunities for enhanced catalyst activity, stability, and reaction selectivity.
C in nitrogen or when annealed in supercritical CO2 at 275 bar. The ability to predesign nanocrystalmorphology and surface properties prior to infusion into the mesoporous silica support offers novelopportunities for enhanced catalyst activity, stability, and reaction selectivity.