Heterogeneous Catalysts. Группа авторов

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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.

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).

      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.

      6.2.1 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

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

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