Heterogeneous Catalysts. Группа авторов
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. Finally, SACs are produced by reduction or activation [19].
Coprecipitation Method Coprecipitation seems to be the simplest method to prepare SACs. The precursors of the metal and the support should be soluble and could be coprecipitated by the precipitant at a certain pH value. The characteristics of the final catalysts, however, depend on many parameters including the order and the speed of the addition of the component solutions, the size of the droplets, efficient mixing, the temperature of the base solution, the pH value, and the aging time. Zhang and coworkers employed this method to fabricate single Pt atoms supported in iron oxide nanocrystallites (Pt1/FeOx), as demonstrated by aberration‐corrected scanning transmission electron microscopy (AC‐STEM) and extended X‐ray absorption fine structure (EXAFS) spectra (Figure 6.5) [16]. A series of SACs, M/TiO2 (M = Pt, Pd, Rh, or Ru), were also synthesized by this method [24]. The SACs exhibit high photocatalytic hydrogen evolution performance compared with metal NPs. However, the main disadvantage of this approach is that some metal atoms can be buried at the interfacial regions of the support agglomerates and within the bulk of the support crystallites, thus compromising the effectiveness and efficiency of the SACs.
Figure 6.5 (a, b) HAADF‐STEM images of Pt1/FeOx. (c) The k3‐weighted Fourier transform spectra from EXAFS. (d) The normalized XANES spectra at the Pt L3 edge of samples. Sample A refers to Pt1/FeOx