Single‐Atom Catalysts Supported by Crystalline Porous Materials: Views from the Inside

Abstract

Reducing the size of nanoparticle catalysts is one desirable method that can improve their catalytic activity. Single-atom catalysts (SACs) represent the smallest possible size a catalyst can achieve with maximum atomic efficiency. [6] Moreover, recent studies have shown that the use of single-atom catalytic sites, rather than ensembles of atoms, is crucial in many catalytic reactions. [7] Without proper protection, however, single-atom sites in these catalysts are not stable since they tend to aggregate and form larger particles. [8-10] To solve this problem, many different protecting supports have been developed such as polymers and porous organic cages. [11,12] Crystalline porous materials, such as zeolites and metal-organic frameworks (MOFs), have been widely used to stabilize small metal species in catalytic reactions due to the controllable size, shape, and dimensions of their pores and channels. [13-15] Therefore, they are highly desirable in supporting SACs. [13] Despite the promising aspects of zeolite and MOF-supported SACs, their characterization remains challenging due to the single-atomic nature of their bonding environments; atomic-level resolution is needed for the structural characterization of these catalysts. X-ray absorption spectroscopy (XAS) is a powerful tool in studying the structure and properties of atoms in their coordination environments with subatomic resolution, [16,17] and thus is considered particularly suitable for the study of SACs. [16,17] In this contribution, a comprehensive review of representative works on zeolite-and MOF-protected SACs will be presented from the XAS perspective. The subatomic resolution (0.01 Å level) of synchrotron-based XAS technique allows for views from "inside" the SACs regarding their structure, electronic properties, and catalytic activities. The processed data from extended X-ray absorption fine structure (EXAFS) will be shown as a powerful tool to probe the atomic structure of SACs in zeolites and MOFs. Through the standard fitting analysis of EXAFS, together with the assistance of advanced analysis tools, detailed information concerning the local structure of SACs can be obtained. Additionally, it will be demonstrated that the X-ray absorption near-edge structure (XANES) can provide insight into the unique electronic properties of SACs. Furthermore, in situ/operando XAS techniques, coupled with state-of-the-art Single-atom catalysts (SACs) have recently emerged as an exciting system in heterogeneous catalysis showing outstanding performance in many catalytic reactions. Single-atom catalytic sites alone are not stable and thus require stabilization from substrates. Crystalline porous materials such as zeolites and metal-organic frameworks (MOFs) are excellent substrates for SACs, offering high stability with the potential to further enhance their performance due to synergistic effects. This review features recent work on the structure, electronic, and catalytic properties of zeolite and MOF-protected SACs, offering atomic-scale views from the "insid...

In situ X‐ray Absorption Spectroscopy of Platinum Electrocatalysts

“… Example of a specialized sample holder for in situ gas absorption studies of XAS, including varied perspective views of the sample holder. Adapted with permission [57] . Copyright 2020, John Wiley and Sons.…”

Single‐atom catalysts supported on ordered porous materials: Synthetic strategies and applications

“…Generally, the XAS spectrum originated from core-level electronic transitions can divide two main regions including X-ray absorption near-edge spectroscopy (XANES) and EXAFS in Figure 10B. 106 The XANES region is extremely sensitive to element type and oxidation state of the absorbing atom due to the unique absorption edge energy of each element, while the EXAFS region can confirm the coordination number, bond distance, and adjacent atomic species of the absorbing atom. For example, Li and co-workers 65 IR spectroscopy also can be used for distinguishing singleatom metals from metal nanoparticles by directly monitoring the interaction between the probe molecules and the surfaces of the catalysts.…”

Which is Better for Nanomedicines: Nanocatalysts or Single‐Atom Catalysts?

“…The preparation of SACs has achieved significant progress since the concept of SACs was proposed in 2011. [14,68] The superior catalytic performance of SACs motivates scientists to work on the synthesis and design of more ingenious and optimized SACs. Presently, based on the underlying principles of preventing atom aggregation, increasing metal loading, and strengthening metal carrier interaction, various synthetic methods, including defect engineering strategy, space constraint strategy, and collaborative design strategy, have been proposed.…”