Heterogeneous Catalysts. Группа авторов
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 all the surface Au atoms can act as CASs. Ertl's group reported that active metal 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