surfaces are often oscillating during catalytic oxidation [30], meaning that CASs can be changeable.
On the contrary, it is straightforward to identify the CASs of SACs because the single metal atoms in contact with their immediate neighboring atoms of the support surfaces are usually CASs. For example, Yang et al. [31] reported that at higher gold loadings, both gold atoms and NPs existed on titania. After leaching gold NPs by a sodium cyanide solution, the atomically dispersed gold still bound on titania and the catalytic activity in the WGS reaction was intact. Hence, the atomically dispersed gold species with surrounding surface −OH groups should be the CASs in this case. Similar results were reported in other reactions [32, 33]. Therefore, it is easier to study the nature of the CASs by comparing different SACs. We synthesized two thermally stable SACs by two methods, and single Ag atoms were found to be anchored at the {001} top facets of hollandite‐type manganese oxide (HMO) (Figure 6.3a–d). One SAC denoted as AgAOR‐HMO was prepared, starting from a supported Ag (particle) sample, by thermal diffusion [34]. The other SAC denoted as AgIMP‐HMO was prepared by wet impregnation with AgNO3 as a precursor. Thus we can establish the correlation between the electronic structure of CASs and activity by studying the structure and catalytic performance of two single‐atom silver catalysts. Our results showed that the higher depletion of the 4d electronic states of the Ag atoms caused stronger electronic metal–support interactions, leading to easier reducibility of the sample and higher catalytic activity for formaldehyde (HCHO) oxidation [34]. In another work, single‐atom sodium and silver catalysts were also used to differentiate the function of alkalis from that of noble metals under identical conditions [35]. Wang et al. also established a quantitative correlation between the catalytic performance (in NH3BH3 hydrolysis to generate H2) and metal–support interactions by using Rh1/VO2 catalysts [36].
Figure 6.3 High‐resolution TEM and energy dispersive X‐ray (EDX) line scans along the yellow lines of AgAOR‐HMO (a, c) and AgIMP‐HMO (b, d). In the structural models, an oxygen atom is represented by a pink ball (c, d), and a silver atom is represented by a yellow ball (c) or a gray ball (d). Scale bars: 1 nm (a, b), 40 nm (c, d).
Source: Hu et al. 2014 [34]. Reproduced with permission of John Wiley & Sons.
(See online version for color figure).
6.2.2.4 Establishment of Intrinsic Reaction Mechanisms
For supported metal NPs, it is challenging to establish an intrinsic reaction mechanism even for a simple model reaction such as CO oxidation. Density functional theory (DFT) calculations can help us understand catalytic reaction mechanisms [16, 37–42]. Because single metal atoms act as the CASs of SACs, establishing an intrinsic reaction mechanism involving single metal atoms thus becomes simplified significantly. For example, the mechanisms of CO oxidation on Ir1/FeOx or Pt1/FeOx were proposed based on DFT calculations and experimental results [16, 39]. The differences in the reaction rates between Ir1/FeOx and Pt1/FeOx for CO oxidation were understood with the help of theoretical investigation. The mechanisms of other reactions such as oxygen reduction reaction [41] and benzene oxidation [42] over SACs were also studied by DFT calculations.
6.3 Synthesis of SACs
SACs have attracted much attention in the catalysis community due to the maximum atom use efficiency and unique catalytic properties. However, due to the excess surface free energy of SAs, SAs tend to aggregate into larger particles at elevated temperatures or during catalytic reactions [38, 43–45], resulting in a decrease or even complete loss of catalytic activity. Thus, it is challenging to fabricate SACs with high‐loading SAs dispersed finely and densely. Significant progress has been made in recent years to develop various methods for the synthesis of SACs. Here we summarize a few common methods that can be divided into two categories, i.e. physical and chemical methods. The chemical method can also be categorized into two types: bottom‐up and top‐down methods.
6.3.1 Physical Methods
Physical synthesis of SACs can be realized through high vacuum physical deposition. For example, mass‐selected soft landing involves the use of a gas‐phase cluster as an ion source and a downstream mass spectrometer to mass‐select nanoclusters prior to their deposition onto a substrate (Figure 6.4) [18, 47]. In this case, metal clusters with precisely defined number of atoms can be produced and “soft‐landed” onto the surface of a desired substrate. This approach allows for independent control of cluster size and coverage and, in principle, can be used for any combination of cluster and flat support. Thus, this method can provide excellent model catalysts for fundamental research of active sites, metal–support interactions, and cluster size effects [21, 46–49]. Anderson and coworkers deposited Pd clusters (Pdn, n = 1, 2, 4, 7, 10, 16, 20, and 25) on clean, vacuum‐annealed rutile TiO2(110) to find out the correlation between the catalytic activity and the electronic structure of Pd clusters that varied with cluster size [48]. Supported size‐selected Pdn clusters were also tested in acetylene cyclotrimerization, and a single Pd atom adsorbed on MgO was found to be enough for the production of benzene at 300 K [21]. However, the disadvantage of this method is that the cost is high and the yield to desired catalysts is low, which makes it unsuitable for practical industrial applications. Moreover, this method is not suitable for making catalysts using high surface area or mesoporous supports, and it is difficult to achieve high metal loadings using this method. In all, this method is merely useful for making model catalysts for fundamental research.
Figure 6.4 Schematic drawing of size‐selected cluster deposition apparatus at Brookhaven National Laboratory.
Source: Vajda and White 2015 [46]. Reproduced with permission of American Chemical Society.
6.3.2 Chemical Methods
Chemical methods are more common and can be routinely practiced. Chemical methods can be categorized according to how their components are integrated, namely, via bottom‐up or top‐down approaches. For bottom‐up strategy, single metal atom species (the metal precursors) are directly anchored to the support by a coordination effect between the metal complexes and the anchoring sites on the support surfaces [50]. For top‐down strategy, the metal NPs are directly introduced onto the support surface and then dispersed into SAs to form SACs.
6.3.2.1 Bottom‐Up Synthetic Methods
The bottom‐up strategy, including coprecipitation, adsorption, and galvanic replacement methods, is the most common strategy to synthesize SACs. Firstly, mononuclear metal precursors are introduced onto the support surface. Then the product was dried and calcinated to remove organic ligands of the metal complexes.
