Ultrasmall Metal Nanoparticles Confined within Crystalline Nanoporous Materials: A Fascinating Class of Nanocatalysts

Abstract

Crystalline nanoporous materials with uniform porous structures, such as zeolites and metal–organic frameworks (MOFs), have proven to be ideal supports to encapsulate ultrasmall metal nanoparticles (MNPs) inside their void nanospaces to generate high‐efficiency nanocatalysts. The nanopore‐encaged metal catalysts exhibit superior catalytic performance as well as high stability and catalytic shape selectivity endowed by the nanoporous matrix. In addition, the synergistic effect of confined MNPs and nanoporous frameworks with active sites can further promote the catalytic activities of the composite catalysts. Herein, recent progress in nanopore‐encaged metal nanocatalysts is reviewed, with a special focus on advances in synthetic strategies for ultrasmall MNPs (<5 nm), clusters, and even single atoms confined within zeolites and MOFs for various heterogeneous catalytic reactions. In addition, some advanced characterization methods to elucidate the atomic‐scale structures of the nanocatalysts are presented, and the current limitations of and future opportunities for these fantastic nanocatalysts are also highlighted and discussed. The aim is to provide some guidance for the rational synthesis of nanopore‐encaged metal catalysts and to inspire their further applications to meet the emerging demands in catalytic fields.

Selective steam reforming of n‐dodecane over stable subnanometric NiPt clusters encapsulated in Silicalite‐1 zeolite

“…As shown in Figures S23 and S24, the prepared catalysts retain MFI structure with high crystallinity and microporous structure, indicating the support maintain stable under high temperature hydrothermal and thermal conditions. The particle size of 2Ni0.5Pt@S‐1 and 2Ni@S‐1 showed nearly no increase after harsh treatment (Figures S25 and S26), demonstrating the support around the metal inhibited particle migration, coalescence, and Ostwald Ripening . Encapsulate the NiPt clusters into the nonacidic Silicalite‐1 zeolite exhibited exceptional stability under high temperature hydrothermal condition.…”

“…The particle size of 2Ni0.5Pt@S-1 and 2Ni@S-1 showed nearly no increase after harsh treatment ( Figures S25 and S26), demonstrating the support around the metal inhibited particle migration, coalescence, and Ostwald Ripening. 52,53,58,78,79 Encapsulate the NiPt clusters into the nonacidic Silicalite-1 zeolite exhibited exceptional stability under high temperature hydrothermal condition.…”

Characterization of Metal‐zeolite Composite Catalysts: Determining the Environment of the Active Phase

“…The preparation methods of these catalysts can be divided into two groups: (1) direct synthesis, in which the metal species are encapsulated in situ, and (2) post‐synthesis, in which the metal particles are impregnated into a desilicated zeolite support . Moreover, metal‐zeolite composite catalysts are classified into different categories depending on the location (surface or bulk), type (ions or particles) and preparation method (heteroatom substitution or supported nanoparticles) of the metallic species . The integration of metal‐based nanoparticles within the shape‐selective micropores of zeolites results in bifunctional catalysts with improved activity, selectivity and stability for various applications in the chemical industry …”

“…[10] Moreover, metal-zeolite composite catalysts are classified into different categories depending on the location (surface or bulk), type (ions or particles) [11] and preparation method (heteroatom substitution or supported nanoparticles) of the metallic species. [12] The integration of metal-based nanoparticles within the shape-selective micropores of zeolites results in bifunctional catalysts with improved activity, selectivity and stability for various applications in the chemical industry. [13] For example, linear paraffin isomerization over Pt supported on dealuminated ZSM-5 has been extensively used for the production of high-quality liquid fuels and lubricating oils.…”

Mesoporous Silica‐Encaged Ultrafine Bimetallic Nanocatalysts for CO2 Hydrogenation to Formates

“…Mesoporous silica nanospheres (MSNs) is regarded as promising support for the confined synthesis of small MNPs due to their well‐defined channels, large surface area and pore volume, and high thermal and chemical stability . Conventionally, the metal species were introduced to the MSNs via an incipient wetness impregnation method, but it often suffers from the formation of large MNPs with non‐uniform dispersion .…”