solution was injected into an aqueous solution containing VO2 nanorods, and the mixture was stirred at 300 rpm at room temperature for one hour. In this process, Rh3+ ions were reduced to Rh+ by V4+ in the VO2 nanorods. Isolated Rh atoms were then uniformly dispersed on the surface of the VO2 nanorods, as seen by STEM (Figure 6.7b) [36].
Figure 6.7 (a) HAADF‐STEM and (b) magnified HAADF‐STEM images of Rh1/VO2.
Source: Wang et al. 2017 [36]. Reproduced with permission of John Wiley & Sons.
6.3.2.2 Top‐Down Synthetic Methods
The top‐down strategy is based on turning ordered nanostructured into smaller pieces to give desirable properties and intriguing performance [50]. This strategy can also be used to synthesize SACs. A high‐temperature atomic migration method is a typical top‐down synthetic method to obtain SACs.
High temperatures are usually detrimental to catalysts' activities because SAs tend to move and agglomerate due to its high surface free energy. Ostwald ripening is one of the mechanisms to explain this process. The elemental steps of that process typically include the detachment of the metal atoms from smaller particles to form monomers, the diffusion of monomers on supports, and the attachment toward larger particles [64]. Tang and coworkers prepared single‐Ag‐atom catalysts, starting from Ag NPs supported on HMO [65]. Upon heating at 400 °C, the Ag NPs on HMO can be dispersed to form single Ag atoms, as evidenced by in situ transmission electron microscopy (TEM), in situ X‐ray diffraction (XRD), and ex situ X‐ray absorption near edge spectroscopy (XANES) (Figure 6.8) [15, 65–67]. Datye and coworkers showed that at 800 °C in oxidizing ambient, Pt can transfer to ceria and be trapped there to form a sinter‐resistant, atomically dispersed catalyst [68]. Li and coworkers reported the successful reversion of sintering effects and the conversion of noble metal NPs into thermally stable and highly active SAs (Pd, Pt, Au) above 900 °C in an inert atmosphere by using nitrogen‐doped carbon derived from a metal–organic framework [69]. Such a high‐temperature atomic migration method requires a supply of mobile atoms and a support that can bind the mobile species. This method is potentially applicable for synthesizing high‐performance thermally stable SACs and reactivating sintered noble metal nanocatalysts.
Figure 6.8 (A) HAADF‐STEM image of Ag1/HMO. (B) Three‐dimensional projected image of the dash rectangle in (A). (C) The shrinking process of the supported Ag NPs heated by an electron beam. (D) Three‐dimensional and contour maps of in situ differential XRD patterns of AgNP/HMO. (E) Ag K‐edge χ(R) k3‐weighted FT EXAFS spectra of the samples.
Source: Adapted from Chen et al. 2015 [66] and Chen et al. 2016 [15].
(See online version for color figure).
6.4 Challenges and Perspective
SACs, as a kind of frontier catalysts, have attracted widespread attention and been extensively studied in recent years. Various methods have been developed to synthesize SACs, as summarized above. Specially, chemical methods are the most commonly used. Chemical methods can be categorized into two general approaches, i.e. bottom‐up and top‐down strategies. Although the top‐down strategy has provided a possibility to construct thermally stable SACs, it is still challenging to find practical methods for synthesizing stable SACs with high metal loadings.
Liu et al. proposed that, in principle, there are three kinds of stability of SACs: intrinsic thermodynamic stability, kinetic stability, and dynamic stability. They suggested that the stability of SACs may depend not only on the support but also on the reaction conditions that are difficult to ascertain and vary from one system to another [70]. Strong metal–metal bonds or coordination bonds with O, N, S, or other atoms on supports may be the key for the stability of SACs. However, it is still challenging to regulate and control such metal–support interaction directly and efficiently. So far, successful examples on SACs are still limited in number [17, 18, 50, 71–73]. This situation calls for more fundamental research on detailed mechanisms.
For future research, it is desirable to develop novel, controllable, and facile synthesis methods for obtaining high‐loading SACs with excellent stability for use in practical conditions. More advanced in situ techniques and theoretical calculations should be used to understand the nature of metal–support interactions comprehensively. In addition, more examples on the high efficiencies of these SACs in various catalytic reactions should be actively sought.
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