of noble metals. To achieve this goal, one may attempt to design better supported catalysts by decreasing the content of the noble metals while maintaining their high catalytic efficiency. Thus, the sustainability of related industries can be improved, and the costs of production can be minimized.
Liang et al. gave an example to illustrate the advantage of supported noble metal catalysts over bulk metals (Figure 6.1) [6]. Assuming that the price of pure gold is US$ 38.1 g−1, a gold brick with the dimensions of 20 × 10 × 5 cm3 is worth approximately US$ 736 000. For comparison, if a common brick (20 cents) with the same dimensions is coated with an atomic ultrathin layer of gold (1 cent), the total value of the brick is only 21 cents (if the cost of coating processing is not counted), much lower than that of a gold brick. Thus, dispersing noble metals as ultrafine nanoparticles (NPs), clusters, or single atoms (SAs) on solid supports is effective for the efficient use of noble metals and the minimization of catalysts' costs. Then what is the effect of downsizing metal NPs on the catalytic activity of supported metal catalysts?
Figure 6.1 Prices of different bricks with a size of 20 × 10 × 5 cm3. (a) A gold brick, (b) a common brick, and (c) a common brick coated with a single atomic layer of gold atoms.
Source: Liang et al. 2015 [6]. Reproduced with permission of John Wiley & Sons. (See online version for color figure).
Catalytically active sites (CASs) are the atoms where the chemical reaction actually occurs. This concept was introduced by Taylor [7]. Although the precise identification of CASs of supported metal catalysts is very difficult due to the quantum size effect [8–10] and the structure‐sensitive geometric effect [11, 12], CASs are generally the surface atoms in an unsaturated coordination environment [13–15]. When downsizing metal NPs, the number of surface atoms increases substantially, and the metal NPs expose more defects and active sites, thus leading to higher catalytic activity [13–15]. Forming SAs on the support is the most efficient approach to utilize metal atoms in a supported metal catalyst.
Single‐atom catalysts (SACs) are catalysts in which the active metal species either exist as isolated SAs stabilized by the support or exist by alloying with another metal [16–18]. Since the concept of SACs was proposed, research on SACs has progressed rapidly to obtain a better understanding of sample preparation and characterization, the role of support, the strong metal–support interactions, and the catalytic mechanisms. Herein, we review some of the recent research on SACs, focusing on various preparation methods. Future challenges and opportunities are also discussed.
6.2 Concept and Advantages of SACs
6.2.1 Concept of SACs
Thomas defined a new class of catalysts as uniform heterogeneous catalysts in 1988 [19]. His group synthesized a Ti‐based single‐site heterogeneous catalyst (SSHC) by grafting metallocene complexes onto mesoporous silica [20]. Heiz and coworkers loaded size‐selected Pdn clusters on MgO(100) films by mass‐selected soft‐landing techniques [21]. Interestingly, they found that a single Pd atom is enough for the production of benzene from acetylene cyclotrimerization. Thomas et al. renamed this class of catalysts as SSHCs [22]. Thomas also categorized SSHCs into four subclasses, one of which includes individual isolated atoms anchored to supports. Böhme and Schwarz proposed the concept of single‐site catalysis in gas‐phase experiments [23]. Qiao et al. observed single Pt atoms anchored on FeOx surfaces by using high‐resolution high‐angle annular dark‐field‐scanning transmission electron microscopy (HAADF‐STEM), and they coined a new concept of single‐atom catalysis in 2011 [16], thus provoking a hot debate on whether SAs alone can act as active sites in heterogeneous catalysis. Yang et al. generalized the concept and examples of “single‐atom catalysts” in 2013 [17]. Since then, the research on SACs has progressed rapidly. SACs have attracted much attention due to the following aspects.
6.2.2 Advantages of SACs
6.2.2.1 Maximum Atom Efficiency
Noble metals, widely used as catalyst components, are expensive and of limited supply. Thus, enormous efforts have been devoted to reducing the consumption of noble metals. In principle, the CASs of supported noble metal catalysts are either the perimeter atoms of metal NPs in contact with supports or exposed surface atoms of metal NPs [13–15], whereas the metal atoms inside NPs are not involved in catalysis. Thus, constructing SACs is effective for making full use of metal atoms.
6.2.2.2 Unique Catalytic Properties
SACs have been studied in catalytic oxidation, water–gas shift (WGS), hydrogenation, and electrocatalysis, showing superior catalytic performance vs. their counterparts (i.e. supported NPs) [15, 16, 24–26]. The high activity of SACs may be ascribed to the unique coordination of SAs with neighboring atoms of the support as well as metal–support interactions. For example, Pt1/FeOx SAC showed much higher activity for CO oxidation than Pt NPs supported on FeOx, owing to the partially vacant 5d orbitals of the positively charged, high‐valent Pt atoms [16]. Yang and coworkers reported the use of M1/TiO2 (M = Pt, Pd, Rh, or Ru) in photocatalytic hydrogen evolution and illustrated a 6‐ to 13‐fold increase in photocatalytic activity compared with the metal clusters loaded onto TiO2 [24]. Furthermore, the active single‐atom sites are well defined, and the identical geometric structure of each active site may result in excellent selectivity compared with the nanoscale counterparts that often have multiple types of active sites. Yan et al. reported that atomically dispersed Pd on graphene showed 100% selectivity to butenes in catalytic hydrogenation of 1,3‐butadiene. In particular, the selectivity to 1‐butene can reach ∼70% at 95% conversion at 50 °C, as explained by the change of 1,3‐butadiene's adsorption mode due to the geometric effect (Figure 6.2) [25]. Anderson and coworkers reported a study of oxygen reduction reaction catalyzed by size‐selected Ptn clusters deposited on indium tin oxide [26]. The materials showed increased H2O2 selectivity as the Ptn cluster size decreased, and a maximized H2O2 selectivity was observed with the smallest Pt1 species [26].
Figure 6.2 Schematic illustration of improvement of selectivity to butenes on single‐atom Pd1/graphene catalyst.
Source: Yan et al. 2015 [25]. Reproduced with permission of American Chemical Society.
(See online version for color figure).
6.2.2.3 Identification of Catalytically Active Sites
A thorough understanding of the nature of CASs is helpful for improving existing catalysts and designing superior new catalysts [27, 28]. However, the precise identification of CASs of supported metal NP catalysts is challenging. Fujitani and Nakamura found that the CASs of Au/TiO2 for CO oxidation are temperature dependent [29]. At low reaction temperatures the CASs are located at the perimeter interfaces of the Au NPs in contact with TiO2 support, whereas at high temperatures
