Heterogeneous Catalysts. Группа авторов
similar catalyst with a Pt loading of 2.5 wt%.
Source: Qiao et al. 2011 [16]. Reproduced with permission of Springer Nature.
(See online version for color figure).
Adsorption Method The adsorption method, including impregnation, electrostatic adsorption, and adsorption assisted by other techniques, can also be used to synthesize SACs [50]. Generally, after the metal precursors are adsorbed on the support, the residual solution is removed, and the samples are then dried and calcined. The final metal loading and dispersion depend strongly on the nature of the anchoring sites on the support surfaces. This point should be considered for sample preparation.
Oxide supports are commonly used for preparing catalysts. For example, single Pt atoms were supported on θ‐Al2O3(010) surface by a wet impregnation method using alumina powder and chloroplatinic acid [38]. An impregnation–reduction method involves the impregnation of metal cations on oxides followed by on‐site reduction of metal cations, as exemplified by the preparation of singly dispersed Rh atoms supported on Co3O4 [51]. Gu et al. reported that single Pt1 and Au1 atoms can be stabilized by lattice oxygen on ZnO{1010} surface via an adsorption method [52]. High‐energy bottom‐up ball‐milling synthesis, a powerful method to break and reconstruct chemical bonds of materials with high efficiency, was used to synthesize a lattice‐confined single iron site catalyst embedded within a silica matrix [41, 53]. Liu et al. developed a room‐temperature photochemical strategy to fabricate stable atomically dispersed palladium–titanium oxide catalyst (Pd1/TiO2) with a Pd loading up to 1.5% [54]. Generally, the support, two‐atom‐thick TiO2(B) nanosheets, was prepared by reacting TiCl4 with ethylene glycolate. H2PdCl4 was added into a water dispersion of TiO2(B) for the adsorption of Pd species. Then the mixture was irradiated by ultraviolet (UV) light to give a Pd1/TiO2 catalyst. However, metal oxide supports are not ideal for making supported catalysts used in electrocatalysis because metal oxides are generally insulators or semiconductors with low electron conductivities and they are often unstable under corrosive electrochemical conditions.
Zeolites appear to be promising supports for preparing SACs due to their well‐defined structures and high surface areas. Specifically, zeolites can provide effective voids to anchor individual metal atoms, thus maintaining the high dispersion of metals [42]. An atomically dispersed gold catalyst was prepared by the adsorption of an organogold precursor on zeolite NaY, and STEM images were used to determine the locations of isolated gold complexes in NaY [55]. However, this single‐atom Au(III)–O–NaY catalyst is unstable at 25 °C and 760 Torr in a flow reactor, losing ∼75% of their initial activity after 15 minutes in CO oxidation [55]. Flytzani‐Stephanopoulos and coworkers reported a successful approach to activate and stabilize atomic Au for the WGS reaction on zeolite and mesoporous silica ([Si]MCM‐41) materials with additional alkali ions [56].
Nitrides, carbons, and C3N4 have also been used to fabricate SACs. For example, single‐atom Pt deposited on TiN was prepared using an incipient wetness impregnation method, and this catalyst was used for electrochemical oxygen reduction, formic acid oxidation, and methanol oxidation [57]. Chen et al. prepared an Au SAC supported on polymeric mesoporous graphitic C3N4 by impregnation [58]. Wu and coworkers demonstrated an iced‐photochemical reduction strategy to construct atomically dispersed Pt species on mesoporous carbon [59]. Graphene can also be used as a support to obtain SACs. For instance, isolated Pt atoms were deposited on graphene nanosheets by atomic layer deposition (ALD), which involves graphite oxidation, thermal exfoliation, and chemical reduction [60]. As shown in Figure 6.6, firstly, the precursor MeCpPtMe3 was injected to react with the adsorbed oxygen on the surface of graphene nanosheets, forming CO2, H2O, and hydrocarbon fragments. The limited supply of surface oxygen provides the self‐limiting growth necessary for ALD, creating a Pt‐containing monolayer. The subsequent oxygen exposure forms a new adsorbed oxygen layer on the Pt surface. The two processes form a complete ALD cycle. The Pt deposition can be precisely controlled by tuning the number of ALD cycles [60].
Figure 6.6 Schematic illustrations of Pt ALD mechanism on graphene nanosheets (GNSs).
Source: Sun et al. 2013 [60]. Reproduced with permission of Springer Nature.
(See online version for color figure).
The Galvanic Replacement Method Galvanic replacement is an electrochemical process that involves the oxidation of one metal, termed as a sacrificial template, by the ions of another metal with a higher reduction potential. When they contact each other at a liquid/solid interface, the sacrificial template will be oxidized and dissolved into the solution, while the ions of the second metal will be reduced, and the second metal will be doped onto the template surface [61]. Galvanic replacement is highly effective and versatile for the generation of a wide variety of metal nanostructures, due to its abilities to control the size and shape and to tune the composition, internal structure, and morphology of the resultant nanostructures [50].
This method has been applied to synthesize SACs in recent years. Sykes and coworkers showed that low‐concentration isolated Pt atoms on Cu(111) surface can be prepared by galvanic replacement on pre‐reduced Cu NPs (Figure 6.7a) [62]. They found that the isolated Pt atoms on Cu(111) surface can activate the dissociation and spillover of H to Cu. Sykes and coworkers also showed that such single‐atom Pt/Cu alloys can be used for C–H activation in methyl groups, methane, and butane [63]. An SAC consisting of isolated Rh atoms uniformly dispersed on the surface of VO2 nanorods can also be synthesized by this method. Simply, Na3RhCl6 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