Nested Metal Catalysts: Metal Atoms and Clusters Stabilized by Confinement with Accessibility on Supports

Abstract

Supported catalysts that are important in technology prominently include atomically dispersed metals and metal clusters. When the metals are noble, they are typically unstablesusceptible to sinteringespecially under reducing conditions. Embedding the metals in supports such as organic polymers, metal oxides, and zeolites confers stability on the metals but at the cost of catalytic activity associated with the lack of accessibility of metal bonding sites to reactants. An approach to stabilizing noble metal catalysts while maintaining their accessibility involves anchoring them in molecular-scale nests that are in or on supports. The nests include zeolite pore mouths, zeolite surface cups (half-cages), raft-like islands of oxophilic metals bonded to metal oxide supports, clusters of non-noble metals (e.g., hosting noble metals as single-atom alloys), and nanoscale metal oxide islands that selectively bond to the catalytic metals, isolating them from the support. These examples illustrate a trend toward precision in the synthesis of solid catalysts, and the latter two classes of nested catalysts offer realistic prospects for economical large-scale application.

Nature of the Active Center for the Oxygen Reduction on Ag-Based Single-Atom Alloy Clusters

“…9 Consequently, the exploration and development of alternative nonnoble metal-based catalysts demonstrating higher activity than PGM for the ORR have emerged as significant and innovative fields within the realm of electrocatalysis. 10,11 Advancing the frontier of ORR investigations involves the engineering of nanostructures in multimetallic alloys and clusters 12 to enhance their activity and stability by fine-tuning the electronic and geometric structures, as well as leveraging synergistic effects. Silver (Ag) metal, which is abundant and cost-effective, is considered a potential alternative to expensive catalysts for the ORR in alkaline environments over Pt-based catalysts.…”

“…At the heart of numerous energy conversion and storage systems is the oxygen reduction reaction (ORR), which plays a vital role in the functionality and efficiency of these systems. , Currently, there has been a renewed focus on ORR catalysts in alkaline environments due to the lower adsorption energies of anions and the enhancement of ORR kinetics under such conditions. − Platinum group metals (PGMs) , have traditionally served as standard catalysts for the ORR in alkaline media; however, the high cost associated with PGM limits their practical application . Consequently, the exploration and development of alternative nonnoble metal-based catalysts demonstrating higher activity than PGM for the ORR have emerged as significant and innovative fields within the realm of electrocatalysis. , Advancing the frontier of ORR investigations involves the engineering of nanostructures in multimetallic alloys and clusters to enhance their activity and stability by fine-tuning the electronic and geometric structures, as well as leveraging synergistic effects.…”

Challenges and Opportunities for Exploiting the Role of Zeolite Confinements for the Selective Hydrogenation of Acetylene

“…1,20,35 Additional stabilization of catalytic species within confinement can be provided through the use of ligands which bind to the support and prevent sintering. 40,41 With so many avenues toward rate and selectivity enhancement, it is difficult to probe individual effects leading many to broadly attribute their findings to a generic combination of steric and electronic effects. The situation becomes even more complex when venturing into the realm of liquid-phase reactions where the additional interactions between solvent molecules, reacting substrate, and active sites must be considered in addition to any stabilization provided by zeolite confinements.…”

“…Some of the smallest pore zeolites, such as chabazite (CHA), have a pore size (3.8 × 3.8 Å) smaller than the kinetic diameter of ethane (4.4 Å) thereby theoretically preventing the formation of undesired products, whether ethane or the even larger oligomers. , In the case of larger pore zeolites, pairing with SACs, can still result in selectivity enhancement on the basis of the ethylene desorption energy being less than the barrier for further hydrogenation. Additionally, single atom sites will prevent oligomerization when the mechanism requires neighboring metal atoms, and they appear to reduce the formation of metal hydrides which are required for full hydrogenation. , While it has been noted that SACs can suffer from decreased activity and stability compared to nanoparticle catalysts, utilizing zeolite supports may overcome these shortcomings with their ability to enhance rates and stabilize the catalytic species. ,, Additional stabilization of catalytic species within confinement can be provided through the use of ligands which bind to the support and prevent sintering. , With so many avenues toward rate and selectivity enhancement, it is difficult to probe individual effects leading many to broadly attribute their findings to a generic combination of steric and electronic effects. The situation becomes even more complex when venturing into the realm of liquid-phase reactions where the additional interactions between solvent molecules, reacting substrate, and active sites must be considered in addition to any stabilization provided by zeolite confinements. − As such, it would be beneficial to first determine the impact of confinement for simple gas phase reactions (e.g., acetylene hydrogenation) prior to moving to more complex molecules and liquid phase reactions.…”