Confining isolated atoms and clusters in crystalline porous materials for catalysis

Abstract

The recent works on supported isolated metal atoms and metal clusters with a few atoms have reformed our understanding on the structure-reactivity relationship established on conventional nanoparticulate catalysts. One critical issue related to supported subnanometric metal catalysts (including both isolated atoms and clusters) on conventional open-structure carriers is the stability of the metal entities, since they may sinter into large nanoparticles under harsh reaction conditions. By confining the isolated metal atoms or subnanometric metal clusters in crystalline porous materials (such as zeolites and metal-organic frameworks) can improve the stability of these tiny species against sintering into nanoparticles. More importantly, the interaction between the metal species and the porous framework may modulate the geometric and electronic structures of the subnanometric metal species, especially in the cases of metal clusters. Such confinement effect can induce shape-selective catalysis or unique chemoselectivity compared to the counterpart metal entities supported on conventional open-structure solid carriers. In this review, we will discuss the structural features, synthesis methodologies, characterization techniques and catalytic applications of subnanometric species confined in zeolites and metal-organic frameworks, aiming to make critical comparison between confined and un-confined isolated atoms and metal clusters and to give perspectives on the future developments.

Constructing the Support as a Microreactor and Regenerator for Highly Active and In Situ Regenerative Hydrogenation Catalyst

“…[7,8] Porous materials are proven superior support because its large surface area provides numerous sites for metal nanoparticle loading and reactions. [9] Further, oriented structure (Figure S1, Supporting Information). Transmission electron microscope (TEM) images confirm the high porosity, with a uniform pore size ≈10 nm (Figure 2B).…”

“…[ 7,8 ] Porous materials are proven superior support because its large surface area provides numerous sites for metal nanoparticle loading and reactions. [ 9 ] Further, oriented diffusion and accumulation of reactants in the micropores can be achieved through surface modification of porous support, thus creating “hot‐spot” for enhanced reaction kinetics. [ 10 ] Moreover, our recent study show that porous carbon support itself can act as catalyst for reactions such as peroxide‐activation.…”

Migration of Zeolite-Encapsulated Pt and Au Under Reducing Atmospheres

“…[12][13] A promising solution to both issues is encapsulation of the active species into inert frameworks such as zeolites or MOFs, [14][15][16][17][18][19][20][21][22] which hinder sintering by two mechanisms: physical restriction to growth by confinement in the zeolite pore, and pinning of single atoms and clusters to particular binding sites in the framework. [23][24][25] In the case of physical confinement, there are several recent synthetic methods which produce trapped particles of Pt or Pd inside zeolite pores, either by co-precipitation of precursors of both metal and zeolite, or by mechanical trapping via the 2D 3D condensation of the framework, as was recently demonstrated for MWW by Corma et al, [26][27] and IPC-2 and IPC-4 by Cejka et al [28][29] It is difficult to precisely maintain the size distribution and thermal stability of the produced clusters via chemical means. However, these methods have shown excellent promise in catalytic tests towards reactions such as nitroarene hydrogenation and dehydrogenation of propane to propylene.…”

Synergizing strong metal-support interactions and spatial confinement boosts dynamics of atomic nickel for hydrogenations

“…micropores) 15 are two widely adopted methodologies to achieve for higher stability. However, too strong MSI might over stabilize the active sites, hindering its activation to reactants/intermediates 14,16 ; con nement of the metal species in micropores might also limit mass transfers 15,17 . Consequently, a seesaw relation between stability and activity have been often observed, obscuring the tremendous efforts for optimizing the catalytic performance of SADCs [18][19][20] .…”

“…Increasing metal-support interactions (MSIs) 13,14 or con ning the active metal species in restrained spaces (eg. micropores) 15 are two widely adopted methodologies to achieve for higher stability. However, too strong MSI might over stabilize the active sites, hindering its activation to reactants/intermediates 14,16 ; con nement of the metal species in micropores might also limit mass transfers 15,17 .…”