LAUSR

Single atoms that act as catalysts are widely found in nature and play important roles in vital molecules, e.g., Mo in nitrogenase [1], Fe in heme [2], and Mg in chlorophylls [3]. In vivo, single atoms that act as catalysts are coordinated with biomacromolecules and work cooperatively. Similarly, synthetic chemical ligands can provide single atoms in biomimetic functions and other applications. For example, Wilkinson’s catalyst (RhCl[P(C6H6)3]3), the first successful homogeneous catalyst, is commonly used for hydrogenation, hydroformylation, hydrosilylation, and hydroboration of alkenes and alkynes. Such homogeneous catalysts usually have excellent catalytic turnover numbers because they take full advantage of each metal atom, but the instability of these organic molecular ligands and the difficulty of catalyst recovery are major drawbacks. Inorganic solid materials can also serve as specific ligands for single metal atoms [4], and the concept of single-site [5,6] or single-atom catalysts (SACs) [7,8] is being increasingly developed [8–12]. Among heterogeneous supports, traditional bulk metal oxides, e.g., MgO, Al2O3, SiO2, TiO2, FeOx, ZnO, and CeO2, zeolites, and metal-organic frameworks are generally used to stabilize various single metal atoms [9]. The excellent catalytic performance of a SAC arises from both the coordinatively unsaturated state of the active center and its stable coordination with the support. Two-dimensional (2D) materials with special physical and chemical properties could therefore provide a new class of promising and ideal supports for single atoms, but this has not been widely studied. The successful exfoliation of graphene [13] has led to an increase in the use of 2D materials, and the use of analogous materials such as h-NB, C3N4, and MoS2 is expanding [14–16]. These 2D materials are not only excellent catalysts in themselves [17] but also have may specific advantages in constructing SACs. First, 2D materials with unique geometric and electronic structures [17] can modulate the catalytic behavior of single atoms in unusual ways. Secondly, 2D materials always have large specific surface areas, which can create more active sites by anchoring single atoms on the surface. Thirdly, strictly single-atom-layer 2D materials can facilitate the adsorption and diffusion of reactive molecules on confined single atoms from two sides. Fourthly, such composites provide model catalysts for which uniformly active sites can be well identified and catalytic performances can be predicted using theoretical chemical methods [18]. Finally, the confined single atoms can promote or activate the intrinsic catalytic activities of 2D materials. In this perspective, we summarize recent progress in research on catalysts based on the confinement of single atoms in 2D materials, mainly graphene, C3N4, and MoS2. We focus on how 2D materials and confined single atoms work cooperatively in catalysis, the identity of the real active sites, and the potential practical applications of such novel catalysts.