Recent advances in the precise control of isolated single-site catalysts by chemical methods

Recent advances in the precise control of isolated single-site catalysts by chemical methods Abstract The search for constructing high-performance catalysts is an unfailing topic in chemical fields. Recently, we have witnessed many breakthroughs in the synthesis of single-atom catalysts (SACs) and their applications in catalytic systems. They have shown excellent activity, selectivity, stability, efficient atom utilization and can serve as an efficient bridge between homogeneous and heterogenous catalysis. Currently, most SACs are synthesized via a bottom-up strategy; however, drawbacks such as the difficulty in accessing high mass activity and controlling homogeneous coordination environments are inevitably encountered, restricting their potential use in the industrial area. In this regard, a novel top-down strategy has been recently developed to fabricate SACs to address these practical issues. The metal loading can be increased to 5% and the coordination environments can also be precisely controlled. This review highlights approaches to the chemical synthesis of SACs towards diverse chemical reactions, especially the recent advances in improving the mass activity and well-defined local structures of SACs. Also, challenges and opportunities for the SACs will be discussed in the later part. single-atom catalysts, bottom-up, top-down, catalytic performance INTRODUCTION In the worldwide theme of exploring efficient and low-cost technologies for energy conversion and chemical transformations, substantial effort has been devoted to the development of general, practical and simple chemical approaches for catalyst preparation in past decades [1–6]. Studies have shown that ultrasmall assemblies, compared to their macroscopic counterparts [7], can exhibit essentially different physical and chemical properties. These unique properties would drastically alter their practical applications in a variety of areas, such as catalysis, biomedical research, energy and environmental fields [6]. Therefore, metal nanoparticles represent a rich resource for a variety of chemical processes, employed both in industry and in academia [8]. The maximized surface area of support, increased number of catalytic active sites, minimized catalyst loading and strong catalyst-support interaction determine the nature of nanocatalysts [2,6]. The supported metal nanoparticles are frequently employed in heterogeneous catalysis; however, to greatly increase the turnover frequency of surface active sites and to enhance the mass activity remain the primary goals in catalysis [9,10]. In most circumstances, it has been demonstrated that the surface atoms of the nanomaterials in an unsaturated coordination environment generally act as the active sites to catalyse specific reactions [11]. Therefore, extensive studies have been devoted to rationally controlling the shapes, structures, crystal phases and compositions of nanocatalysts [3,8,12–18]. With decreasing the size of nanomaterials, the number of surface atoms is increased substantially, exposing more defects and active sites, tuning geometric and electronic properties involved in chemical reactions [10]. Nanoclusters have shown intriguing properties because of the reduced size compared to nanoparticles, exposing more uncoordinated active sites and changing molecular orbital energy levels [19,20]. Heiz and co-workers found a pronounced size effect for model catalysts of size-selected Pdn (n ≤ 30) clusters supported on MgO(100) for the cyclotrimerization of acetylene to benzene [21]. Anderson et al. studied size-selected palladium clusters and deposited them on the rutile phase of titanium dioxide (TiO2) for CO oxidation [22]. By employing X-ray photoemission spectroscopy and temperature-programmed reaction measurements, they found that the activity of these catalysts was associated with Pd3d binding energy. Li et al. utilized a double-solvent method combined with a photoreduction process to prepare active Pd nanoclusters encapsulated inside the cage of NH2-Uio-66 [23]. The resultant catalyst showed exceptional performance for a Suzuki coupling reaction under visible-light irradiations. Nevertheless, although the sizes of the nanoclusters have been reduced, their multiple distributions of active metal sites alongside different geometric and electric structures might not be ideal for specific catalytic reactions [9,10]. The stability of nanoclusters would also be a problem for their application in heterogeneous catalysis, especially at higher operational temperatures [24]. Further downsizing nanoclusters to the atomic level, namely single atoms (SAs), maximum atom utilization and superior/distinguishing catalytic performance are supposed to be obtained [9,10]. Of note is the fact that unexpected superior catalytic performances of the single-atom catalysts (SACs) are often observed as one of the key advances of these novel catalysts versus their nanoscale counterparts. This can be ascribed to the unsaturated environments of metal active sites, quantum size effects and metal–support interactions [25–28]. Therefore, the research on SACs has rapidly progressed from their fundamental aspects to pursing practical applications in areas of nanotechnology and materials science. A study that has attracted considerable attention since its publication in 1995 is that of Thomas et al., who reported that direct grafting of organometallic complexes onto the walls of mesoporous silica gives a shape-selective high-performance catalyst with well-separated, homogeneously dispersed and high surface concentrations of active sites for the epoxidation of cyclohexenes and their derivatives [29]. Later, in 2003, Flytzani-Stephanopoulos et al. discovered that the water–gas shift reaction was not affected by the catalytic activity of metallic Au or Pt nanoparticles; instead, nonmetallic Au or Pt species on the ceria surface played a key role in this reaction [30]. In 2007, Lee et al. successfully synthesized Pd-Al2O3 catalyst and validated that the extremely low metal loading leads to the formation of atomically isolated PdII species, which greatly contribute to the excellent selox activity of allylic alcohols [31]. The key discovery was that the employment of homogeneously dispersed SACs generally confers a dramatic improvement in catalytic activity, selectivity and stability, or even considerably different catalytic properties than the corresponding nanoparticles and nanoclusters [32–35]. This is highly desirable and has attracted extensive scientific attention, as they might potentially act as alternatives to circumvent the problems of scarcity and high cost of the noble-metal catalysts used in large-scale catalysis applications [36]. Specifically, the active single-atom sites are well defined and atomically stabilized on the supports, and the identical geometric structure of each active site is similar to that of a homogeneous catalyst. Recently, studies have clearly demonstrated that the utility and uniqueness of these SACs have great potential to bridge the gap between homogeneous and heterogeneous catalysis [37–42]. This would solve the problems of the difficulty in separating the homogeneous catalysts from raw materials and products, as well as combining the merits of both hetero- and homogeneous catalysts. The SACs also provide a good avenue to identify the detailed structural features for the active sites and an ideal model to elucidate the structure–activity relationship [43–45]. Such catalysts have shown intriguing interests in the catalysis field [10,32,34,43,46–49]. To meet the practical demand, the most important challenges for fabricating the SACs are to increase the density of active sites and to improve their intrinsic activities [32,36,45]. The first challenge is the high propensity for aggregation of SAs once the size of the nanomaterials is greatly reduced [10]. The second challenge is the rational control of the coordination environment of the single metal atoms. Recently, several strategies for constructing atomically dispersed metal sites on catalyst supports have been extensively studied [9,10,32,42]. These strategies include enhancing the metal–support interactions, engineering vacancy defects and voids on the supports, and modifying surface functional groups [9,42]. In most cases, the supports for isolated SACs are chosen on purpose, as they can stabilize the isolated catalytic SAs or activate nearby reactants to form intermediate species for the catalytic active sites [50–52]. For example, zeolites could provide effective voids to anchor individual metal atoms to maintain the high dispersion of the isolated metal atoms and prevent them from sintering at high temperatures under oxidative or reductive atmospheres during catalysis processes [53]. Nanoparticles and nanoclusters can also serve as supports. Through elegant studies of support materials, Sykes et al. showed that the isolated Pd atoms can be supported on a Cu surface and significantly lower the energy barrier to hydrogen uptake on and subsequent desorption from the nearby Cu atoms [48]. This facile hydrogen dissociation at the isolated Pd atoms and weak binding to the Cu surface together facilitate selective hydrogenation of styrene and acetylene. Toshima et al. described a crown-jewel concept for the construction of catalytically highly active top gold atoms on palladium nanoclusters [54]. Interestingly, the gold atoms can be controllably assembled at the top position on the cluster and exhibit high catalytic activity because of their high negative-charge density and unique structure. Recent reports have begun to document that the defects in reducible oxides (e.g. TiO2 and CeO2), graphene or C3N4 also help to stabilize isolated metal atoms [32,42,55]. For example, Du et al. investigated the favorable role of isolated palladium and platinum atoms supported on graphitic carbon nitride (g-C3N4) to act as photocatalysts for CO2 reduction [56]. Overall, an important conclusion derived from these works is that the further development of this SACs field requires a more fundamental understanding of SA formation at the atomic scale. This review covers the preparation strategies for SACs, which can be categorized according to how their components are integrated, namely via bottom-up and top-down approaches (Fig. 1). Currently, a large majority of SACs are synthesized via a bottom-up strategy by using oxides or carbon supports to construct N or O defects to enable the deposition of metal precursors. This is followed by a chemical reduction process to generate SACs from high-oxidation-state ions to low oxidation state. However, the following drawbacks are encountered frequently: difficulty in accessing high metal loading because of their high propensity for aggregation and the difficulty in constructing homogeneous coordination environments for the reactive sites. These drawbacks lead to limited selectivity and stability of the SACs, greatly limiting their potential use in various industrial fields. In this regard, Li and Wu proposed a top-down strategy to construct SACs by the pyrolysis of metal nodes in metal-organic frameworks (MOFs) for the first time [57]. In this case, the introduction of Zn atoms into MOFs is important and can effectively prevent the formation of Co NPs (nanoparticles) during the high-temperature pyrolysis process. The resulting Co SAC has a high metal loading close to 5% and showed exceptional chemical and thermal stability. A distinguishing feature of this strategy is not only that the metal loading can be substantially increased from 1% to 5%, which is important from practical perspectives, but it can control the coordination environments to construct high-performance SACs by exposing real active sites. This top-down strategy overcome challenges in the fabrication of SACs with a traditional bottom-up strategy and has great potential to meet the requirement for use in practical applications. In addition to the fabrication strategies, the use of these methods in different chemical reactions will also be presented. Finally, future challenges and opportunities will be discussed. Figure 1. View largeDownload slide Schematic representation of the bottom-up and top-down strategies for the synthesis of SACs. Figure 1. View largeDownload slide Schematic representation of the bottom-up and top-down strategies for the synthesis of SACs. BOTTOM-UP SYNTHETIC METHODOLOGIES FOR THE CONSTRUCTION OF SACS The bottom-up strategy is the most common method to synthesize metal SACs, during which the metal precursors are adsorbed, reduced and confined by the vacancies or defects of the supports [9,10,32,52]. Nevertheless, how to effectively increase the SACs loading with well-defined dispersion on the supports is still challenging. First, aggregation would occur during a chemical synthesis or catalytic process when high loading of SAs is required. Second, the architectural structures of anchor sites for confining and stabilizing the metal SACs on the support remain elusive; therefore, the coordination environment for metal SAs might be inhomogeneous and poorly defined [58]. Optimization of the precursors and supports and controlling of the synthetic procedures play a key role in tuning the metal–support interaction and guaranteeing the homogeneous dispersion of SACs. For the wet-chemistry strategy, the precursor solutions of mononuclear metal complexes are first anchored to the supports by a coordination effect between the metal complexes and the functional groups of the support surfaces [32]. Then, the organic ligands of the metal complexes are removed by a post-treatment to expose more active sites to meet the requirement of catalytic reactions. Particularly, the advantage of wet chemistry for preparing SACs is that this method does not require specialized equipment and can be routinely practiced in any chemistry lab [59]. Co-precipitation approach Co-precipitation is one of the commonly employed approaches for preparing SACs, during which the substances that are normally soluble under the conditions would be precipitated. A significant advantage of this method lies in its extreme simplicity, as no additional complicated steps are involved. For a classical example, Zhang et al. employed this method to fabricate single Pt atoms supported on iron oxide nanocrystallites (Pt1/FeOx) [46]. The metal precursor of H2PtCl6·H2O was mixed with Fe(NO3)3·9H2O in a proper molar ratio and pH. After recovery, the precipitate was dried and calcined, resulting in the formation of Pt1/FeOx. The aberration-corrected scanning transmission electron microscopy (AC-STEM) and extended X-ray absorption fine structure (EXAFS) spectra demonstrated the individual Pt atoms were uniformly dispersed on FeOx support, with a metal loading level of 0.17 wt% (Fig. 2a). This SAC showed extremely high atom efficiency, excellent stability and superior activity for both CO oxidation and preferential oxidation of CO in H2. They found that these merits can be attributed to the partially vacant 5d orbitals of the positively charged high-valent Pt atoms, as they can effectively reduce CO-adsorption energy and activation barriers that are required for CO oxidation. This study demonstrated the feasibility of using the defects of oxide supports to serve as anchoring sites for metal clusters and single metal atoms. Subsequently, the feasibility and efficiency of this approach were further demonstrated by the Zhang group showing that the high-performance Pt- and Ir-based SACs could also be obtained (Fig. 2b and c) for use in organic transformation [60] and water–gas shift reactions [61]. In these examples, defects in the oxide supports and the amount of metal loading were found to be critical for accessing high-performance SACs that would normally lead to aggregation. Figure 2. View largeDownload slide (a) HAADF-STEM images of Pt1/FeOx. Adapted with permission from [46]. (b) HAADF-STEM image of 0.08%Pt/FeOx-R200. Adapted with permission from [60]. (c) HAADF-STEM images of Ir1/FeOx. Adapted with permission from [61]. Figure 2. View largeDownload slide (a) HAADF-STEM images of Pt1/FeOx. Adapted with permission from [46]. (b) HAADF-STEM image of 0.08%Pt/FeOx-R200. Adapted with permission from [60]. (c) HAADF-STEM images of Ir1/FeOx. Adapted with permission from [61]. Adsorption approach The adsorption method is one of the most fundamental approaches for constructing isolated metal atoms on the supports [62,63]. It is simple, direct and has been widely used in the preparation of supported metal catalysts. Generally, after the metal precursors are adsorbed on the support, the residual solution is removed and then the catalysts are dried and calcined. To ensure the SAs could be stably anchored onto the supports with atomic dispersion, appropriate functional groups on the supports should be given consideration. Oxides are generally employed as an efficient support for preparing catalysts. In 2013, Narula et al. [64] reported single Pt atoms supported on θ-Al2O3(010) prepared by a wet-impregnation method using alumina powder and chloroplatinic acid. In this work, water was gradually evaporated before the resulting powder was transferred to an alumina crucible and subjected to a pyrolysis process. The resultant catalyst was catalytically active in its ability to oxidize CO to CO2. In addition, serials of Pt/θ-Al2O3 catalysts with different metal loadings were prepared, and the results reveal that they are highly active towards NO oxidation [65]. In the same year, Tao et al. [66] developed an impregnation-reduction method for preparing singly dispersed Rh atoms supported on Co3O4 nanorods. This method involves the impregnation of Rh3+ on Co3O4 nanorods followed by on-site reduction of Rh3+ using NaBH4. In situ characterizations reveal evidence of the active sites of isolated Rh atoms in the formation of RhCon on Co3O4 nanorods, which were generated through restructuring of Rh1/Co3O4 at 220°C in reactant gases. The resulting new catalytic phase exhibits a high selectivity to produce N2 in the reduction of NO with H2 between 180°C and 300°C. A report by Li et al. demonstrated that single Pt1 and Au1 atoms can be stabilized by lattice oxygen on ZnO{1010} surface via an adsorption method [67]. In detail, ZnO-nanowires (nws) were dispersed in de-ionized water followed by the addition of H2PtCl6·6H2O or HAuCl4 solution. After an aging process, the suspension was filtered, washed and dried to give Pt1/ZnO and Au1/ZnO catalysts. Similarly, Zhang et al. fabricated an Rh SAC supported on ZnO nws by introducing RhCl3 solution into ZnO nws that were dispersed in de-ionized water [40]. After stirring and aging processes, the resulting precipitate was filtered, washed, dried and reduced. As the weight loading of Rh reduced from 0.03% to 0.006%, the isolated Rh SACs can be clearly observed. During the synthetic process, the Rh atoms bond with proximal Zn atoms which lose one or more O atoms. Therefore, electrons transfer from metallic Zn to Rh atoms to generate near-metallic Rh species. The results show that the as-obtained Rh1/ZnO-nws SACs exhibited comparable efficiency in the hydroformylation of several olefins to the homogeneous Wilkinson's catalyst, along with superior catalytic activity to those of the most highly reported heterogeneous nanoparticle-based catalysts. In a more recent piece of work, Wang and co-workers described a convenient two-step synthesis of an atomically dispersed Pt catalyst supported on ceria (CeO2), with 1 wt.% metal loading, by wetness impregnation and steam treatment [68]. Chloroplatinic acid was added drop-wise to the CeO2 support while being ground in a mortar and pestle. The as-obtained powder was then dried, calcined, thermal aged and stream treated to give the catalyst. The authors demonstrated that the activation of SACs on CeO2 via high-temperature steam treatment can accomplish excellent low-temperature CO-oxidation activity and superior thermal stability. This is because the steam treatment can enable the formation of active surface lattice oxygen near isolated Pt atoms to considerably enhance catalytic performance. Further investigation of the nature of this active surface lattice oxygen on Pt/CeO2 was supported by density functional theory (DFT) calculations and reaction kinetic analyses. They found the oxygen vacancies from the CeO2 bulk can redistribute to the CeO2(111) surface when exposed to water at a high temperature. During the steam-treatment process, H2O molecules can fill out the oxygen vacancy over the atomically dispersed Pt/CeO2 surface, affording two neighboring active Olattice[H] sites around Pt. This provides the significantly improved reactivity and stability. Yan and co-workers developed a unique adsorption approach to construct Pt SACs, anchored in the internal surface of mesoporous Al2O3, by a modified sol-gel solvent vaporization self-assembly method [69], as shown in Fig. 3a. Triblock copolymers P123, C9H21AlO3 and H2PtCl6 were first mixed in ethanol. With continued evaporation of the solvent, the amphiphilic P123 macromolecules and C9H21AlO3 assembled into a highly ordered hexagonally arranged mesoporous structure, with Pt precursor encapsulated in the matrix. The as-obtained gel was then calcined in air to decompose the P123 template. Meanwhile, the C9H21AlO3 was transformed into a rigid, well-aligned mesoporous Al2O3 framework. This was followed by a reducing step in 5% H2/N2 to give the isolated Pt SAs stabilized by the unsaturated pentahedral Al3+ centers. The authors showed that the catalyst retained its structural integrity and exceptional catalytic performance in several reactions under harsh conditions, such as hydrogenation of 1,3-butadiene after exposure to a reductive atmosphere at 200°C for 24 h, n-hexane hydroreforming at 550°C for 48 h and CO oxidation after 60 cycles between 100°C and 400°C over 1 month. Figure 3. View largeDownload slide (a) Schematic illustration of the 0.2Pt/m-Al2O3-H2 synthesis process. Adapted with permission from [69]. (b) HAAD–STEM images of the 0.25Au-Na/[Si]MCM41 catalyst. Adapted with permission from [72]. (c) TEM and HAADF-STEM images of 0.35 wt% Pt/TiN. Adapted with permission from [73]. (d) Scheme of proposed formation mechanisms, TEM and HAADF-STEM images for Ru SAs/N–C. Adapted with permission from [76]. Figure 3. View largeDownload slide (a) Schematic illustration of the 0.2Pt/m-Al2O3-H2 synthesis process. Adapted with permission from [69]. (b) HAAD–STEM images of the 0.25Au-Na/[Si]MCM41 catalyst. Adapted with permission from [72]. (c) TEM and HAADF-STEM images of 0.35 wt% Pt/TiN. Adapted with permission from [73]. (d) Scheme of proposed formation mechanisms, TEM and HAADF-STEM images for Ru SAs/N–C. Adapted with permission from [76]. Zeolites are crystalline materials with well-defined structures and high surface area, along with more sites for robust bonding with catalytic species [24,70]. Specifically, zeolites could provide effective voids to anchor individual metal atoms to maintain the high dispersion and prevent them from sintering at high temperatures under oxidative or reductive atmospheres during the catalysis processes [53]. In 2012, Gates et al. reported that atomically dispersed gold atoms catalyse with a high degree of uniformity supported on zeolite NaY [71]. The site-isolated gold complexes retained after CO-oxidation catalysis, confirming the robust stabilization effect of the zeolite channels for gold species. The addition of alkali ions, such as sodium or potassium, on inert KLTL-zeolite and mesoporous MCM-41 silica materials could structurally stabilize the single gold sites in Au–O(OH)x– ensembles (Fig. 3b), as demonstrated by Flytzani-Stephanopoulos and co-workers [72]. They have shown evidence that the active catalyst was composed of alkali ions linked to the gold atom through –O ligands, not merely on the support, making the reducible oxide supports no longer an essential requirement. The validation tests show that the single-site gold atoms were homogeneously dispersed and highly active for the industrially important low-temperature water–gas shift reaction. In addition to metal oxides and zeolites, other supports such as nitrides and carbides have also been explored and shown promise for stabilizing SAs for use in catalysis. Lee et al. described a Pt SAC supported on titanium nitride (TiN) nanoparticles with the aid of chlorine ligands [73]. H2PtCl6·6H2O was dissolved in anhydrous ethanol and mixed with acid-treated TiN nanoparticles before the resulting sample was dried and reduced. Transmission electron microscopy (TEM) and HAADF-STEM images of the samples are shown in Fig. 3c. The results show that the 0.35 wt% Pt/TiN sample affords a high mass activity and a unique selectivity towards electrochemical oxygen reduction, formic acid oxidation and methanol oxidation. Carbon nitride (C3N4) has been proved as an alternative support material by virtue of their porosity and high surface area [55]. Li et al. used an impregnation method to access isolated Au atoms anchored on polymeric mesoporous graphitic C3N4 (mpg-C3N4) [74]. The catalytically active AuI atom was coordinated by three nitrogen or carbon atoms in tri-s-triazine repeating units. This coordination feature significantly prevents the Au atoms from aggregation and makes the AuI surface highly active. Moreover, they demonstrated this catalyst as highly active, selective and stable for silane oxidation with water. In 2017, Ma et al. developed a highly efficient catalyst consisting of isolated Pt atoms uniformly dispersed on an α-molybdenum carbide (α-MoC) support that can enable low-temperature, base-free hydrogen production through aqueous-phase reforming of methanol [75]. They found that the α-MoC displays stronger interactions with Pt than other oxide supports or β-Mo2C; therefore, atomically dispersed Pt atoms can be formed on an α-MoC support following a high-temperature activation process. This generates an exceptionally high-density electron-deficient surface to stabilize Pt sites for the adsorption/activation of methanol. This catalyst affords an excellent turnover frequency and the corresponding hydrogen production greatly exceeds those of previously reported catalysts for low-temperature aqueous-phase reforming of methanol. They deduce that the unique structure of α-MoC, which affects water dissociation, and the synergic effects between Pt and α-MoC together affect the activation of methanol and the subsequent reforming process. In 2017, Wu et al. reported a novel synthetic approach to construct isolated single Ru atoms on nitrogen-doped porous carbon (Ru SAs/N–C) by a coordination-assisted strategy using MOFs for the hydrogenation of quinolones [76]. It is noticed that the strong coordination effect between the lone pair of nitrogen and d-orbital of Ru atoms is crucial for the formation of stable Ru SAs (Fig. 3d). Without the dangling −NH2 groups, the Ru atoms tend to aggregate into nanoclusters, even confined in the pores of MOFs. The results demonstrate the Ru SAs serve as an effective semi-homogeneous catalyst to the chemoselective catalyse hydrogenation of quinolones. This method has been shown to potentially broaden the substrate scope for the synthesis of SACs with unique properties for use in various chemical reactions. Together, the ease of preparation for SACs using a wet-chemistry strategy envisages a promising future in the field. However, these methods have their own disadvantages. For example, some metal atoms might be buried either in the interfacial regions of the support agglomerates or within the bulk of the support when co-precipitation methods are applied [43]. In addition, when high metal loading is required for the construction of SACs, aggregation would inevitably occur [9]. This trade-off should be minimized by developing new synthesis methods. Other methodologies have also been explored to design and synthesize SACs with varies chemical and physical functionalities and future underpinned studies in these directions. The photochemical method becomes particularly appealing to assist the effective adsorption of SAs on the supports and has been proven to be effective for the synthesis of nanocrystals, such as gold, silver, platinum, palladium, etc. [77–80]. In this process, regulating the nucleation and growth processes of nanocrystals has been a major topic. Flytzani-Stephanopoulos et al. constructed isolated gold atoms supported on titania with a loading of approximately 1 wt% under ultraviolet (UV) irradiation [81]. They found that the addition of ethanol can serve as a charge scavenger to facilitate the donation of electrons from gold atoms to −OH groups on the titania support. The catalytic performance was examined and the results showed that this catalyst displayed excellent activity for the low-temperature water–gas shift reaction, as well as admirable stability in long-term cool-down and startup operations. An important study by Zheng et al. demonstrated a room-temperature photochemical strategy to construct atomically dispersed palladium atoms supported on ethylene glycolate (EG)-stabilized ultrathin TiO2 nanosheets (Pd1/TiO2 catalyst) with a Pd loading up to 1.5% [82]. Typically, two-atom-thick TiO2 nanosheets were prepared by reacting TiCl4 with EG and used as the support. H2PtCl6 was then added to the TiO2 dispersion for adsorption of Pd species followed by irradiation by UV to give the Pd1/TiO2 catalyst. TEM, STEM and EXAFS revealed that the isolated Pd atoms were evenly dispersed over the TiO2 support, without any observable evidence of NPs (Fig. 4a). The catalyst exhibited excellent catalytic performance in the hydrogenation of C = C bonds, outperforming those commercial Pd catalysts. In addition, there was no observable decay in the catalytic activity for 20 cycles, suggesting the robustness of the Pd1/TiO2 catalyst. Importantly, they found this catalyst can activate H2 in a heterolytic pathway to drastically enhance its catalytic activity in the hydrogenation of aldehydes. This mechanism has been commonly observed for homogeneous catalysts, such as Au, Pd and Ru complexes; however, there is no report for heterogeneous Pd catalysts. This study set a good example using atomically dispersed metal catalysts for bridging the gap between heterogeneous and homogeneous catalysis. Figure 4. View largeDownload slide (a) Structural characterizations of Pd1/TiO2 catalyst. Adapted with permission from [82]. (b) Schematic illustration of the iced-photochemical process compared with the conventional photochemical reduction of H2PtCl6 aqueous solution. Adapted with permission from [83]. (c) Structural features of 0.5% Fe©SiO2. Adapted with permission from [39]. (d) Schematic illustrations of the Pt ALD mechanism on graphene nanosheets. Adapted with permission from [86]. Figure 4. View largeDownload slide (a) Structural characterizations of Pd1/TiO2 catalyst. Adapted with permission from [82]. (b) Schematic illustration of the iced-photochemical process compared with the conventional photochemical reduction of H2PtCl6 aqueous solution. Adapted with permission from [83]. (c) Structural features of 0.5% Fe©SiO2. Adapted with permission from [39]. (d) Schematic illustrations of the Pt ALD mechanism on graphene nanosheets. Adapted with permission from [86]. Very recent work by Wu and co-workers showed a novel synthetic approach to accessing atomically dispersed platinum species on mesoporous carbon via iced-photochemical reduction of frozen chloroplatinic acid solution (Fig. 4b) [83]. In this report, H2PtCl6 solution was first frozen by liquid nitrogen followed by irradiation using a UV lamp. The H2PtCl6 ice was kept overnight in dark conditions at room temperature to give a clear aqueous Pt single-atom solution. Then mesoporous carbon solution and Pt single-atom solution were mixed, filtered, and dried at room temperature. Finally, the ice lattice naturally confines the dispersed ions and atoms to affect the photochemical reduction products and further prevent the aggregation of atoms. To test the generality of this concept, they also fabricated isolated Pt atoms deposited on different supports, including mesoporous carbon, graphene, carbon nanotubes, TiO2 nanoparticles and zinc oxide nanowires. Among them, the isolated Pt atoms supported on mesoporous carbon exhibited exceptional catalytic performance for hydrogen evolution reaction, as well as an excellent long-time durability, outperforming the commonly employed Pt/carbon catalyst. This iced-photochemical reduction approach provides a promising avenue for the green synthesis of SAs and sub-nanometer clusters, and opens up possibilities for fine-tuning the nucleation and growth of nanocrystals in wet chemistry. Recently, high-energy bottom-up ball-milling synthesis has been proved as a powerful method to break and reconstruct chemical bonds of materials with high efficiency. Such an approach was taken by Bao et al., who reported a lattice-confined single iron site catalyst embedded within a silica matrix by a solid fusion method. Briefly, commercial SiO2 and Fe2SiO4 were mixed and subjected to ball milling under argon and fused in the air [39]. As expected, the unsaturated single Fe sites served as active centers (Fig. 4c) to efficiently enable the direct, non-oxidative conversion of methane, exclusively to ethylene and aromatics. The presence of single Fe sites effectively prevented catalytic C-C coupling, oligomerization and coke deposition. In addition, this catalyst showed extremely stable performance, with no deactivation observed during long-term testing, and the selectivity for total carbon of the three products was retained. Subsequently, the group used the same method to construct single-atom iron sites by embedding highly dispersed FeN4 centers in graphene matrix via high-energy ball milling of iron phthalocyanine and graphene nanosheets [84]. In this system, the FeN4 center is highly dispersed and well stabilized by the graphene matrix. The formation of the Fe = O intermediate is important in promoting the conversion of benzene to phenol. Remarkably, this reaction can proceed efficiently at mild conditions such as room temperature or even as low as 0°C. DFT calculations confirm that the catalytic activity stems from the confined iron sites, along with moderate activation barriers for the reaction that proceeded at room temperature. Both studies clearly show the potential of the highly efficient ball-milling method for the fabrication of SACs for use in catalysis areas. The atomic layer deposition (ALD) technique is a gas-phase chemical process and commonly used to deposit a thin layer of film in a bottom-up fashion with near-atomic precision on the substrate by repeated exposure of separate precursors [85]. This technique offers the feasibility of precise control of the catalyst size from a single-atom, sub-nanometer cluster to the nanoparticle. It is expected that ALD would potentially provide a powerful approach for the construction of intriguing SACs. This approach was first demonstrated by Sun et al. in 2013, who reported a practical synthesis of isolated single Pt atoms on graphene nanosheets using the ALD technique (Fig. 4d) [86]. In this work, Pt was deposited on graphene supports by the ALD method using MeCpPtMe3 and oxygen as precursors and nitrogen as a purge gas. The resulting Pt SAC showed improved catalytic activity compared with the commercial Pt/C catalyst. X-ray absorption fine structure (XAFS) analyses show that the low-coordination and partially unoccupied 5d orbital of Pt atoms are responsible for the excellent catalytic performance. In 2015, Lu et al. described a single-atom Pd1/graphene catalyst prepared by the ALD method with excellent performance in the selective hydrogenation of 1,3-butadiene [87]. First, the anchor sites were created by an oxidation process on pristine graphene nanosheets, followed by a reduction process via thermal de-oxygenation to control the surface oxygen functional groups. After an annealing step, phenolic oxygen was observed to be the dominated oxygen species on the graphene support. ALD was then performed on the reduced graphene to give a single-atom Pd catalyst by alternately exposing Pd(hfac)2 and formalin. This catalyst showed superior catalytic performance in the selective hydrogenation of 1,3-butadiene, affording nearly 100% butenes selectivity, and ∼70% selectivity for 1-butene at a conversion ratio of 95% under mild conditions. They speculate that both the mono-π-adsorption mode of 1,3-butadiene and the enhanced steric effect induced by 1,3-butadiene adsorption on the isolated Pd atoms contribute to the improved selectivity of butenes. In addition, the Pd1/graphene showed remarkable durability against deactivation via either metal atom aggregation or coking during a 100-h reaction time on stream. Using the same strategy, Sun and co-workers described the preparation of isolated single Pt atoms and clusters on nitrogen-doped graphene nanosheets (NGNs) [88]. Here, Pt was first deposited on the NGNs by the ALD technique using MeCpPtMe3 and O2 as precursors and N2 as a purging gas and a carrier gas. The size, density and distribution of the Pt atoms on the NGNs or graphene nanosheets (GNs) can be precisely controlled by the ALD cycles. As expected, the isolated Pt atoms and clusters on the NGNs have been demonstrated to show superior catalytic activity and stability for the hydrogen evolution reaction (HER) compared with the conventional Pt NP catalysts. This can be explained by the small size and the special electronic structure of the adsorbed single Pt atoms on NGNs. Together, the use of the ALD technique has shown great promise for large-scale synthesis of highly active and stable single-atom and cluster catalysts. The galvanic-replacement method Galvanic replacement is a highly versatile and effective approach for the construction of a variety of nanostructures, with the ability to control the size and shape, composition, internal structure and morphology [24,57,89]. It is an electrochemical process that consists of oxidation of one metal, termed as a sacrificial template, by other metal ions that have a higher reduction potential. When they are exposed to each other in solution, the sacrificial metal template will be preferably oxidized and dissolved into the solution, while the ions of the second metal will be reduced and deposited onto the template surface. In 2015, Sykes et al. demonstrated that low concentrations of isolated Pt atoms in the Cu(111) surface (Fig. 5a) can be prepared by galvanic replacement on pre-reduced Cu NPs to catalyse the butadiene hydrogenation with remarkable activity and high selectivity to butenes [50]. In this case, Cu NPs were first prepared and supported on γ-Al2O3 followed by calcination in air. The galvanic-replacement reaction was then carried out in an aqueous solution under nitrogen protection with constant stirring and refluxing. A desired amount of Pt precursor was introduced to a suspension of Cu NPs in an aqueous solution containing HCl. The resulting material was filtered, washed and dried to yield the catalyst. They notice that, at low Pt loadings, the isolated Pt atoms can substitute into the Cu(111) surface to activate the dissociation and spillover of H to Cu. The weak binding between butadiene and Cu would facilitate the highly selective hydrogenation reaction to butenes, without decomposition or poisoning of the catalysts. This catalyst, with less than one Pt atom per 100 copper atoms, also binds CO more weakly than metallic Pt, which is particularly important for use in many Pt-catalysed chemical reactions. Figure 5. View largeDownload slide (a) Characterization of Pt/Cu SAA NPs. Adapted with permission from [50]. (b) Scanning tunneling microscope image of a 0.01 ML Pt/Cu(111) SAA surface. Adapted with permission from [90]. Figure 5. View largeDownload slide (a) Characterization of Pt/Cu SAA NPs. Adapted with permission from [50]. (b) Scanning tunneling microscope image of a 0.01 ML Pt/Cu(111) SAA surface. Adapted with permission from [90]. In a follow-up report, the Sykes group used the same approach to construct Pt/Cu single-atom alloys (SAAs) to examine C–H activation in different systems, including methyl groups, methane and butane [90]. They observed that the Pt atoms were distributed over the Cu surface and across both terraces and at regions near step edges (Fig. 5b). The results show the Pt/Cu SAAs activate C–H bonds more efficiently than Cu, along with superior stability under realistic operating conditions, effectively avoiding the coking problem that typically occurred with Pt. Both pieces of work from the Sykes group demonstrated how SAs can be deposited on alloys—an important future direction for this field. Though a variety of SACs have been developed by the bottom-up strategy, the downside of the methods described here is that it is still challenging to access SACs with high metal loading and a homogeneous coordination environment for the active sites used in the catalytic process. This would lead to limited selectivity and stability of the SACs for their practical use in various industrial fields. In addition, although ground-breaking, some of these methods do require specific/sophisticated preparation procedures that might not be compatible with all kinds of SACs and ideal from practical perspectives. TOP-DOWN SYNTHETIC METHODOLOGIES FOR THE CONSTRUCTION OF SACS The top-down strategy is based on the dissolution of ordered nanostructures into smaller pieces to give desired properties and intriguing performances [59,91]. Extensive research efforts have pursued this strategy with the overarching aim of synthesizing SACs with unprecedented chemical and physical properties and understanding the complex mechanisms for catalysis that occur at the atomic level. This strategy has proven particularly useful in the formation of SACs with accurate control over the micro- or nanostructures [92]. The precise structure (such as coordination number, dispersion tendencies and binding mode) of metal SAs synthesized by the top-down methods has shown great promise in industrially important applications [9,89,93,94]. Efforts to further understand the underlying features and mechanisms are required for the development of new methods for the construction of SACs and represent a fertile area for future studies. The high-temperature pyrolysis method High-temperature pyrolysis has become one of the fascinating methods for synthesizing nanomaterials on different supports. Particularly, the development of a template-sacrificial approach via acid leaching or oxidative calcination has offered an alternative way to generate SACs. Of note is that an appropriate pyrolysis temperature is critically important to give the desired properties. MOFs and zeolitic imidazolate frameworks (ZIFs) have interconnected 3D molecular-scale cages that make them highly accessible through small apertures. Importantly, they can serve as templates to obtain nitrogen-doped porous carbon with abundant active nitrogen sites. Very recently, Wu et al. took advantage of the MOFs and originally developed an effective strategy for accessing single Co atoms supported on nitrogen-doped porous carbon with a particularly high metal loading of over 4 wt% via the pyrolysis of bimetallic Zn/Co MOFs [57]. This is pioneering work in this field and the strategy is particularly applicable to access high-loading metal SACs that would otherwise be difficult to produce. It should be noted that the enhancement of metal loading for preparing SACs in the present study is a significant breakthrough in this area, highlighting the specific requirement of SACs for practical applications. Importantly, the introduction of Zn atoms into MOFs is critical and acts as an elegant approach to efficiently manipulate the adjacent spatial distance between Co atoms, thereby effectively preventing the formation of Co NPs (Fig. 6a). The Zn atoms, with a low boiling point of 907°C, can be evaporated in the high-temperature pyrolysis process, providing abundant N sites. The Co nodes can be reduced in situ by carbonization of the organic linkers in MOFs and anchored on the as-obtained N-doped porous carbon support. Assuming the MOF as an integrated system, using this high-temperature pyrolysis of MOF to access unsaturated SAs anchored on the N-doped porous carbon support can be categorized into the top-down approach. Control testing demonstrated that the aggregated Co atoms were formed for Co-containing MOF (ZIF-67) after a pyrolysis treatment. HAADF-STEM and EXAFS verified the presence of isolated Co atoms dispersed on the N-doped porous carbon support. The resulting Co SAC shows exceptional oxygen-reduction reaction (ORR) catalytic performance with a half-wave potential more positive than the commercial Pt/C and most of the reported non-precious metal catalysts. Robust chemical stability during electrocatalysis and thermal stability that resists sintering at a high temperature of 900°C have also been confirmed, as little evidence of catalyst degradation was observed during the catalytic cycles. This work has underlined the significant importance of employing MOFs as an ideal carbon support for stabilizing single metal atoms at the atomic scale. Figure 6. View largeDownload slide (a) Schematic illustration of the construction of Co SAs/N–C. Adapted with permission from [57]. (b) Schematic illustration of the construction of Ni SAs/N–C. Adapted with permission from [95]. (c) Schematic illustrations of the construction of Fe-ISAs/CN. Adapted with permission from [97]. (d) Schematic illustration of the construction of ISAS-Co/HNCS. Adapted with permission from [99]. (e) Schematic illustration of the construction of SA-Fe/CN. Adapted with permission from [103]. (f) Schematic illustration of the construction of (Fe, Co)/N–C. Adapted with permission from [104]. Figure 6. View largeDownload slide (a) Schematic illustration of the construction of Co SAs/N–C. Adapted with permission from [57]. (b) Schematic illustration of the construction of Ni SAs/N–C. Adapted with permission from [95]. (c) Schematic illustrations of the construction of Fe-ISAs/CN. Adapted with permission from [97]. (d) Schematic illustration of the construction of ISAS-Co/HNCS. Adapted with permission from [99]. (e) Schematic illustration of the construction of SA-Fe/CN. Adapted with permission from [103]. (f) Schematic illustration of the construction of (Fe, Co)/N–C. Adapted with permission from [104]. Subsequently, an ionic exchange strategy was developed by the Wu group to assist in the construction of a single Ni atom catalyst (Fig. 6b) between Zn nodes and adsorbed Ni ions within the cavities of the MOF [95]. In this case, ZIF-8 was first dispersed in n-hexane under ultrasound until a homogeneous solution was formed. Then a small amount of Ni(NO3)2 aqueous solution was introduced, and the mixed solution was vigorously stirred to cause the Ni ions to be absorbed completely. Then the sample was centrifuged and dried, followed by a high-temperature heating process in an argon atmosphere to yield Ni SAC. This Ni SAC, with a metal weight loading of 1.53 wt%, delivered an excellent turnover frequency for CO2 electroreduction of 5273 h−1, along with a maximum Faradaic efficiency for CO production of 71.9% and a high current density of 10.48 mA cm−2. This work, for the first time, demonstrates the great potential of using MOF-based materials to access SACs for use in CO2 electroreduction. To investigate the relationship between coordination numbers and CO2 electroreduction catalytic performance, the Wu group sequentially prepared a series of Co SACs with different N coordination environments treated at different temperatures [96]. Bimetallic Co/Zn ZIFs were treated by a pyrolysis process, during which the Zn was evaporated away and the Co was reduced by carbonized organic linkers, generating isolated Co atoms stabilized on nitrogen-doped carbon. By controlling the pyrolysis temperatures, three Co SACs with different Co–N coordination numbers were obtained, being Co–N4 (800°C), Co–N3 (900°C), and Co–N2 (1000°C), respectively. The catalytic performance of these samples was examined, and the results show that the isolated Co atom with two coordinated nitrogen atoms (prepared at 1000°C) can afford significantly higher selectivity and superior activity, resulting in a CO formation Faradaic efficiency of 94% and a current density of 18.1 mA cm−2 at an overpotential of 520 mV. Importantly, this catalyst achieved a turnover frequency for CO formation of 18 200 h−1, outperforming most of the reported metal-based catalysts under comparable conditions. DFT calculation reveals that the decreased N coordination environment leads to more unoccupied 3d orbitals for Co atoms, thereby facilitating adsorption of CO2•− and increasing CO2 electroreduction performance. This study demonstrates the significant effect of N coordination environments on SACs for catalytic performance. The above studies further confirm the great potential of high-temperature pyrolysis of MOFs as a promising strategy to access SACs for different demanding industrial applications. With these attractive features, Li and co-workers prepared a highly stable isolated Fe atom catalyst, with Fe loading up to 2.16 wt%, that showed excellent ORR reactivity via a cage-encapsulated precursor pyrolysis approach [97]. This method is highly effective to access SACs because the precursors can be encapsulated inside the ZIF pores, thereby preventing them from aggregating into nanoparticles (Fig. 6c). In this study, Fe(acac)3 was mixed with ZIF-8, and the molecular-scale cages were formed with the assembly of Zn2+ and 2-methylimidazole, with one Fe(acac)3 molecule trapped in one cage. After a pyrolysis step, the ZIF-8 was transformed into nitrogen-doped porous carbon, whereas the Fe(acac)3 within the cage was reduced by carbonization of the organic linker, resulting in the formation of isolated iron atoms anchored on nitrogen species. The catalyst has been demonstrated to show exceptional ORR catalytic activity, good methanol tolerance and impressive stability. Importantly, the ORR catalytic activity of this SAC outperforms those of recently reported Fe-bases materials and other non-precious metal materials. Experimental results and DFT calculations reveal the excellent ORR performance stems from the formation of atomically isolated iron atoms coordinated with four N atoms and one O2 molecule adsorbed end-on. Using a similar approach, Li et al. described the synthesis of atomically dispersed Ru3 clusters via a cage-separated precursor pre-selection and pyrolysis strategy [98]. Generally, two steps are involved: (i) encapsulation and separation of preselected metal cluster precursors followed by (ii) a pyrolysis treatment. The resulting catalyst was characterized by HAADF-STEM and XAFS, and the catalytic performance was tested for the oxidation of 2-amino-benzyl alcohol. The results show that this Ru3/nitrogen-doped carbon (CN) catalyst possesses 100% conversion, 100% selectivity and an unexpectedly high turnover frequency (TOF), outperforming those of Ru SACs and small-sized Ru particle catalysts. An alternative approach to the thermal treatment of MOFs for achieving SACs has been employed by Li et al., who used SiO2 as a template to access a hollow N-doped carbon sphere with isolated Co atomic sites (Fig. 6d) [99]. Briefly, the SiO2 template was dispersed in Co–TIPP/TIPP solution before introducing another monomer. The collected powder was thermally treated under a flowing H2/Ar and then etched with sodium hydroxide to remove the SiO2 template to yield the Co SAC. Its ORR performance was investigated and the results demonstrate that exceptional catalytic activity was originated from the single Co sites that can significantly facilitate the proton and charge transfer to the adsorbed *OH species. Using the same approach, a Mo SAC was prepared by the Li group using sodium molybdate and chitosan as precursors and showed excellent HER performance [100]. Further studies of the structure of the catalyst were supported by AC-STEM and XAFS, which confirmed that the Mo atom was anchored with one nitrogen atom and two carbon atoms (Mo1N1C2). In 2016, Zhang et al. described a similar template-sacrificial approach to create a self-supporting Co–N–C catalyst with single-atom dispersion and showed excellent catalytic activity for the chemoselective hydrogenation of nitroarenes to yield azo compounds under mild conditions [101]. In this study, the Co(phen)2(OAc)2 complex was supported on Mg(OH)2 and then subjected to a pyrolysis process. This was followed by the removal of the MgO support by an acid-leaching treatment. The merit of employing Mg(OH)2 is that it can essentially prevent the aggregation of cobalt atoms. This is because of the moderate interaction between Mg(OH)2 and the Co species, as well as its inertness towards the reaction with Co during the pyrolysis process. After the acid-leaching step, the support material was removed to give a self-supporting Co–N–C material. X-ray absorption spectroscopy was tested and the exact structure of the catalyst was confirmed to be CoN4C8–1-2O2. Specifically, the Co single atom was coordinated with four pyridinic nitrogen atoms on the graphitic layer, along with oxygen atoms weakly adsorbed on the Co atoms perpendicular to the Co–N4 plane. Using the same approach, Zhang et al. prepared an atomically dispersed Fe−N−C catalyst, which exhibited exceptional activity and excellent reusability for the selective oxidation of the C−H bond, along with tolerance for a wide scope of substrates [102]. Briefly, the Fe(phen)x complex supported on the nano-MgO template was pyrolysed at different temperatures under N2 atmosphere, followed by an acid-leaching step to remove the MgO template. They observed that the properties of the Fe species were dependent on the pyrolysis temperature, with more metallic Fe particles formed at higher temperatures. The critical role of the Fe−Nx sites in catalysis was further confirmed by potassium thiocyanate titration experiments and Mössbauer spectroscopy. An effective core–shell strategy has been introduced by the Li group using metal hydroxides or oxides coated with polymers followed by high-temperature pyrolysis and acid-leaching steps, to synthesize single metal atoms anchored on the inner wall of hollow CN materials [103]. By employing different metal precursors or polymers, they have successfully synthesized a series of metal SAs dispersed on CN materials (Fig. 6e). In detail, α-FeOOH nanorods were first prepared by a hydrothermal method, followed by self-polymerizing dopamine monomers to generate α-FeOOH@PDA. Then it was thermally treated under an inert atmosphere, during which the polydopamine (PDA) layers were converted into the CN shell and α-FeOOH was reduced to iron, giving rise to the strong interaction between the Fe atoms and the CN shell. Finally, acid leaching was carried out to generate Fe SAs on the inner wall of the CN materials. The obtained SA-Fe/CN catalyst showed a high conversion of 45% and an excellent selectivity of 94% for the hydroxylation of benzene to phenol, outperforming Fe nanoparticles/CN. Notably, in a most recent research, Wu et al. originally developed a host–guest strategy based on MOFs to construct a Fe–Co dual-sites catalyst embedded in N-doped porous carbon support [104]. It involves binding between Co nodes and adsorbed Fe ions within the confined space of MOFs (Fig. 6f). Specifically, Zn/Co bimetallic MOF was employed as a host to encapsulate FeCl3 within the cavities by a double-solvents method. The Fe3+ species were reduced by the as-generated carbon and bond with the neighboring Co atoms. Meanwhile, the adsorbed Fe3+ species can accelerate the decomposition of metal–imidazolate–metal linkages and generate voids inside the MOF. EXAFS and Mössbauer spectroscopic analyses were performed to investigate the coordination environment of the Fe–Co dual sites. The experimental results show that FeCoN6 is the active site for the (Fe, Co)/N–C catalyst and has been demonstrated to endow excellent ORR performance in an acidic electrolyte, along with comparable onset potential and half-wave potential to those of the commercial Pt/C. DFT calculation reveals that the activation of O–O is favored on the dual sites, which is important for the four-electron oxygen-reduction process. The fuel cell testing revealed that this catalyst outperforms most of the reported Pt-free catalysts in H2/O2 and H2/air conditions. In addition, this cathode catalyst is rather robust in long-term operation for electrode measurement and H2/air single cell testing. Of note is that, despite the fact that SACs generally confer greater activity than the corresponding nanoparticles, it is still important to be aware of the potential aggregation pathways available to them. This is especially crucial in cases where higher operational temperatures were applied. Therefore, the superior catalytic activity, selectivity, stability and the ease of fabrication of the dual-sites Fe–Co catalyst make this type of SAC truly remarkable. Importantly, the main advantages of this host–guest strategy include the ability to incorporate different metal atoms and to permit the catalyst to be operated in a wider dynamic range. This study is expected to provide avenues for the synthesis of high-performance dual-sites catalysts with unique properties for use in chemical transformations. Overall, these studies have shown that the high-temperature pyrolysis method is capable of producing SACs with precisely controlled structures and morphologies. Additionally, this unique approach has been seen as a significant opportunity to enable the efficient construction of high-performance SACs for use in various reactions. The high-temperature atomic-migration method High temperatures are generally detrimental to catalysts’ activities. Although the SAs are homogeneously dispersed on the support materials, they have a high propensity to move and aggregate into nanoparticles when heated at high temperatures. Datye and co-workers take advantage of the phenomenon that metal nanoparticles can emit mobile species to prepare atomically dispersed metal catalysts [105]. In this study, a Pt/La-Al2O3 catalyst was physically mixed with different types of ceria powders followed by a thermal treatment in flowing air. Because of the strong interaction between PtO2 and ceria powders, the Pt species emitted from the alumina were trapped on the CeO2, forming thermally stable Pt1/CeO2 SACs (Fig. 7). The performance of the resulting SAC was tested for CO oxidation, and the results suggest that it can serve as a highly effective sintering-resistant CO-oxidation catalyst at high temperature. They believe that this atom-trapping approach is potentially applicable and might provide exciting possibilities to access a variety of high-performance SACs. This work represents a novel strategy and has been demonstrated as being particularly effective in fabricating SACs and connecting the relationship between the nanoparticles and SAs. Figure 7. View largeDownload slide Schematic illustration of Pt nanoparticle sintering, showing how ceria can trap the mobile Pt to suppress sintering. Adapted with permission from [105]. Figure 7. View largeDownload slide Schematic illustration of Pt nanoparticle sintering, showing how ceria can trap the mobile Pt to suppress sintering. Adapted with permission from [105]. CONCLUSIONS AND PERSPECTIVE Over just a few years, there has been remarkable progress in the development of various methods for the synthesis of SACs. In this review, we summarize the progress, bring new insights from recent years and pointed the way to the synthesis of SACs. Currently, two general approaches have been employed for accessing SACs: bottom-up and top-down. Though still being developed, SACs have emerged as an exceptional advancement in the development of highly efficient heterogeneous catalysts. The researchers have shown evidence that the size of the nanomaterials does affect catalytic efficiency in the catalysis process. A noteworthy result is that, by reducing the size of nanostructures from the nano- to the sub-nano scale and finally to SAs in atomic dimensions, catalytic performance has been observed to change drastically. This results from the low-coordination environment, quantum size effect and enhanced metal–support interactions. Moreover, the homogeneously and isolated metal active sites can maximize metal utilization, giving rise to the impressively enhanced catalytic performance. Recent experimental and theoretical progress has unambiguously validated the strong evidence for the high activity, selectivity and stability of the high-performance SACs. These intriguing properties of SACs are believed to endow great potential for applications in heterogeneous catalysis. Importantly, SACs can act as an ideal platform to serve as a bridge to connect hetero- and homogeneous catalysis. Thus, SACs are thought to have the potential to overcome the difficulty encountered in homogeneous catalysis. As discussed previously, a major limiting factor in the development of SACs is the lack of general methods to directly and efficiently access high-performance SACs. The construction of SACs for use in catalysis represents an important challenge, highlighting the need for more fundamental research into detailed mechanisms. Along with the emergence of new characterization and computational modeling techniques, single-atom active sites can be investigated further. More advanced, direct and effective in situ spectroscopic and microscopic techniques become particularly important to offer new insights into the chemical reactions involved in SACs. Elucidating the important role of metal precursors, support materials and experimental conditions and understanding the prerequisites for catalytic activity of a given catalytic system are crucial for developing effective strategies for the synthesis of SACs. Several aspects should also be given enough attention: first, the development of novel, controllable and facile synthesis methods for access high-loading SACs with finely and densely dispersed single atoms; second, the construction of single metal atoms with robust stabilization on the support for use in practical conditions; third, detailed experimental and theoretical work should be done to comprehensively understand SACs-support effects. The top-down strategy has shown great promise and significantly contributed to the simplified synthesis routes for SACs with exceptional activity and stability. Moreover, the metal loading can be markedly increased from 1% to 5%, and the coordination environments can be elaborately controlled. This will definitely facilitate the development of general protocols for accessing SACs and underpin the exploration of other intriguing applications. Together, the field of SAs is expansive and rapidly developing towards different applied research fields. The continued development of SACs represents an important advancement in heterogeneous catalysis and will surely be the important focus of extensive research efforts and a thriving field for various applications for years to come. FUNDING This work was supported by the National Key R&D Program of China (2017YFA0208300) and the National Natural Science Foundation of China (21522107, 21671180, 21521091, 21390393, U1463202). REFERENCES 1. 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Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png National Science Review Oxford University Press

Recent advances in the precise control of isolated single-site catalysts by chemical methods

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Oxford University Press
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© The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.
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2095-5138
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2053-714X
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Abstract

Abstract The search for constructing high-performance catalysts is an unfailing topic in chemical fields. Recently, we have witnessed many breakthroughs in the synthesis of single-atom catalysts (SACs) and their applications in catalytic systems. They have shown excellent activity, selectivity, stability, efficient atom utilization and can serve as an efficient bridge between homogeneous and heterogenous catalysis. Currently, most SACs are synthesized via a bottom-up strategy; however, drawbacks such as the difficulty in accessing high mass activity and controlling homogeneous coordination environments are inevitably encountered, restricting their potential use in the industrial area. In this regard, a novel top-down strategy has been recently developed to fabricate SACs to address these practical issues. The metal loading can be increased to 5% and the coordination environments can also be precisely controlled. This review highlights approaches to the chemical synthesis of SACs towards diverse chemical reactions, especially the recent advances in improving the mass activity and well-defined local structures of SACs. Also, challenges and opportunities for the SACs will be discussed in the later part. single-atom catalysts, bottom-up, top-down, catalytic performance INTRODUCTION In the worldwide theme of exploring efficient and low-cost technologies for energy conversion and chemical transformations, substantial effort has been devoted to the development of general, practical and simple chemical approaches for catalyst preparation in past decades [1–6]. Studies have shown that ultrasmall assemblies, compared to their macroscopic counterparts [7], can exhibit essentially different physical and chemical properties. These unique properties would drastically alter their practical applications in a variety of areas, such as catalysis, biomedical research, energy and environmental fields [6]. Therefore, metal nanoparticles represent a rich resource for a variety of chemical processes, employed both in industry and in academia [8]. The maximized surface area of support, increased number of catalytic active sites, minimized catalyst loading and strong catalyst-support interaction determine the nature of nanocatalysts [2,6]. The supported metal nanoparticles are frequently employed in heterogeneous catalysis; however, to greatly increase the turnover frequency of surface active sites and to enhance the mass activity remain the primary goals in catalysis [9,10]. In most circumstances, it has been demonstrated that the surface atoms of the nanomaterials in an unsaturated coordination environment generally act as the active sites to catalyse specific reactions [11]. Therefore, extensive studies have been devoted to rationally controlling the shapes, structures, crystal phases and compositions of nanocatalysts [3,8,12–18]. With decreasing the size of nanomaterials, the number of surface atoms is increased substantially, exposing more defects and active sites, tuning geometric and electronic properties involved in chemical reactions [10]. Nanoclusters have shown intriguing properties because of the reduced size compared to nanoparticles, exposing more uncoordinated active sites and changing molecular orbital energy levels [19,20]. Heiz and co-workers found a pronounced size effect for model catalysts of size-selected Pdn (n ≤ 30) clusters supported on MgO(100) for the cyclotrimerization of acetylene to benzene [21]. Anderson et al. studied size-selected palladium clusters and deposited them on the rutile phase of titanium dioxide (TiO2) for CO oxidation [22]. By employing X-ray photoemission spectroscopy and temperature-programmed reaction measurements, they found that the activity of these catalysts was associated with Pd3d binding energy. Li et al. utilized a double-solvent method combined with a photoreduction process to prepare active Pd nanoclusters encapsulated inside the cage of NH2-Uio-66 [23]. The resultant catalyst showed exceptional performance for a Suzuki coupling reaction under visible-light irradiations. Nevertheless, although the sizes of the nanoclusters have been reduced, their multiple distributions of active metal sites alongside different geometric and electric structures might not be ideal for specific catalytic reactions [9,10]. The stability of nanoclusters would also be a problem for their application in heterogeneous catalysis, especially at higher operational temperatures [24]. Further downsizing nanoclusters to the atomic level, namely single atoms (SAs), maximum atom utilization and superior/distinguishing catalytic performance are supposed to be obtained [9,10]. Of note is the fact that unexpected superior catalytic performances of the single-atom catalysts (SACs) are often observed as one of the key advances of these novel catalysts versus their nanoscale counterparts. This can be ascribed to the unsaturated environments of metal active sites, quantum size effects and metal–support interactions [25–28]. Therefore, the research on SACs has rapidly progressed from their fundamental aspects to pursing practical applications in areas of nanotechnology and materials science. A study that has attracted considerable attention since its publication in 1995 is that of Thomas et al., who reported that direct grafting of organometallic complexes onto the walls of mesoporous silica gives a shape-selective high-performance catalyst with well-separated, homogeneously dispersed and high surface concentrations of active sites for the epoxidation of cyclohexenes and their derivatives [29]. Later, in 2003, Flytzani-Stephanopoulos et al. discovered that the water–gas shift reaction was not affected by the catalytic activity of metallic Au or Pt nanoparticles; instead, nonmetallic Au or Pt species on the ceria surface played a key role in this reaction [30]. In 2007, Lee et al. successfully synthesized Pd-Al2O3 catalyst and validated that the extremely low metal loading leads to the formation of atomically isolated PdII species, which greatly contribute to the excellent selox activity of allylic alcohols [31]. The key discovery was that the employment of homogeneously dispersed SACs generally confers a dramatic improvement in catalytic activity, selectivity and stability, or even considerably different catalytic properties than the corresponding nanoparticles and nanoclusters [32–35]. This is highly desirable and has attracted extensive scientific attention, as they might potentially act as alternatives to circumvent the problems of scarcity and high cost of the noble-metal catalysts used in large-scale catalysis applications [36]. Specifically, the active single-atom sites are well defined and atomically stabilized on the supports, and the identical geometric structure of each active site is similar to that of a homogeneous catalyst. Recently, studies have clearly demonstrated that the utility and uniqueness of these SACs have great potential to bridge the gap between homogeneous and heterogeneous catalysis [37–42]. This would solve the problems of the difficulty in separating the homogeneous catalysts from raw materials and products, as well as combining the merits of both hetero- and homogeneous catalysts. The SACs also provide a good avenue to identify the detailed structural features for the active sites and an ideal model to elucidate the structure–activity relationship [43–45]. Such catalysts have shown intriguing interests in the catalysis field [10,32,34,43,46–49]. To meet the practical demand, the most important challenges for fabricating the SACs are to increase the density of active sites and to improve their intrinsic activities [32,36,45]. The first challenge is the high propensity for aggregation of SAs once the size of the nanomaterials is greatly reduced [10]. The second challenge is the rational control of the coordination environment of the single metal atoms. Recently, several strategies for constructing atomically dispersed metal sites on catalyst supports have been extensively studied [9,10,32,42]. These strategies include enhancing the metal–support interactions, engineering vacancy defects and voids on the supports, and modifying surface functional groups [9,42]. In most cases, the supports for isolated SACs are chosen on purpose, as they can stabilize the isolated catalytic SAs or activate nearby reactants to form intermediate species for the catalytic active sites [50–52]. For example, zeolites could provide effective voids to anchor individual metal atoms to maintain the high dispersion of the isolated metal atoms and prevent them from sintering at high temperatures under oxidative or reductive atmospheres during catalysis processes [53]. Nanoparticles and nanoclusters can also serve as supports. Through elegant studies of support materials, Sykes et al. showed that the isolated Pd atoms can be supported on a Cu surface and significantly lower the energy barrier to hydrogen uptake on and subsequent desorption from the nearby Cu atoms [48]. This facile hydrogen dissociation at the isolated Pd atoms and weak binding to the Cu surface together facilitate selective hydrogenation of styrene and acetylene. Toshima et al. described a crown-jewel concept for the construction of catalytically highly active top gold atoms on palladium nanoclusters [54]. Interestingly, the gold atoms can be controllably assembled at the top position on the cluster and exhibit high catalytic activity because of their high negative-charge density and unique structure. Recent reports have begun to document that the defects in reducible oxides (e.g. TiO2 and CeO2), graphene or C3N4 also help to stabilize isolated metal atoms [32,42,55]. For example, Du et al. investigated the favorable role of isolated palladium and platinum atoms supported on graphitic carbon nitride (g-C3N4) to act as photocatalysts for CO2 reduction [56]. Overall, an important conclusion derived from these works is that the further development of this SACs field requires a more fundamental understanding of SA formation at the atomic scale. This review covers the preparation strategies for SACs, which can be categorized according to how their components are integrated, namely via bottom-up and top-down approaches (Fig. 1). Currently, a large majority of SACs are synthesized via a bottom-up strategy by using oxides or carbon supports to construct N or O defects to enable the deposition of metal precursors. This is followed by a chemical reduction process to generate SACs from high-oxidation-state ions to low oxidation state. However, the following drawbacks are encountered frequently: difficulty in accessing high metal loading because of their high propensity for aggregation and the difficulty in constructing homogeneous coordination environments for the reactive sites. These drawbacks lead to limited selectivity and stability of the SACs, greatly limiting their potential use in various industrial fields. In this regard, Li and Wu proposed a top-down strategy to construct SACs by the pyrolysis of metal nodes in metal-organic frameworks (MOFs) for the first time [57]. In this case, the introduction of Zn atoms into MOFs is important and can effectively prevent the formation of Co NPs (nanoparticles) during the high-temperature pyrolysis process. The resulting Co SAC has a high metal loading close to 5% and showed exceptional chemical and thermal stability. A distinguishing feature of this strategy is not only that the metal loading can be substantially increased from 1% to 5%, which is important from practical perspectives, but it can control the coordination environments to construct high-performance SACs by exposing real active sites. This top-down strategy overcome challenges in the fabrication of SACs with a traditional bottom-up strategy and has great potential to meet the requirement for use in practical applications. In addition to the fabrication strategies, the use of these methods in different chemical reactions will also be presented. Finally, future challenges and opportunities will be discussed. Figure 1. View largeDownload slide Schematic representation of the bottom-up and top-down strategies for the synthesis of SACs. Figure 1. View largeDownload slide Schematic representation of the bottom-up and top-down strategies for the synthesis of SACs. BOTTOM-UP SYNTHETIC METHODOLOGIES FOR THE CONSTRUCTION OF SACS The bottom-up strategy is the most common method to synthesize metal SACs, during which the metal precursors are adsorbed, reduced and confined by the vacancies or defects of the supports [9,10,32,52]. Nevertheless, how to effectively increase the SACs loading with well-defined dispersion on the supports is still challenging. First, aggregation would occur during a chemical synthesis or catalytic process when high loading of SAs is required. Second, the architectural structures of anchor sites for confining and stabilizing the metal SACs on the support remain elusive; therefore, the coordination environment for metal SAs might be inhomogeneous and poorly defined [58]. Optimization of the precursors and supports and controlling of the synthetic procedures play a key role in tuning the metal–support interaction and guaranteeing the homogeneous dispersion of SACs. For the wet-chemistry strategy, the precursor solutions of mononuclear metal complexes are first anchored to the supports by a coordination effect between the metal complexes and the functional groups of the support surfaces [32]. Then, the organic ligands of the metal complexes are removed by a post-treatment to expose more active sites to meet the requirement of catalytic reactions. Particularly, the advantage of wet chemistry for preparing SACs is that this method does not require specialized equipment and can be routinely practiced in any chemistry lab [59]. Co-precipitation approach Co-precipitation is one of the commonly employed approaches for preparing SACs, during which the substances that are normally soluble under the conditions would be precipitated. A significant advantage of this method lies in its extreme simplicity, as no additional complicated steps are involved. For a classical example, Zhang et al. employed this method to fabricate single Pt atoms supported on iron oxide nanocrystallites (Pt1/FeOx) [46]. The metal precursor of H2PtCl6·H2O was mixed with Fe(NO3)3·9H2O in a proper molar ratio and pH. After recovery, the precipitate was dried and calcined, resulting in the formation of Pt1/FeOx. The aberration-corrected scanning transmission electron microscopy (AC-STEM) and extended X-ray absorption fine structure (EXAFS) spectra demonstrated the individual Pt atoms were uniformly dispersed on FeOx support, with a metal loading level of 0.17 wt% (Fig. 2a). This SAC showed extremely high atom efficiency, excellent stability and superior activity for both CO oxidation and preferential oxidation of CO in H2. They found that these merits can be attributed to the partially vacant 5d orbitals of the positively charged high-valent Pt atoms, as they can effectively reduce CO-adsorption energy and activation barriers that are required for CO oxidation. This study demonstrated the feasibility of using the defects of oxide supports to serve as anchoring sites for metal clusters and single metal atoms. Subsequently, the feasibility and efficiency of this approach were further demonstrated by the Zhang group showing that the high-performance Pt- and Ir-based SACs could also be obtained (Fig. 2b and c) for use in organic transformation [60] and water–gas shift reactions [61]. In these examples, defects in the oxide supports and the amount of metal loading were found to be critical for accessing high-performance SACs that would normally lead to aggregation. Figure 2. View largeDownload slide (a) HAADF-STEM images of Pt1/FeOx. Adapted with permission from [46]. (b) HAADF-STEM image of 0.08%Pt/FeOx-R200. Adapted with permission from [60]. (c) HAADF-STEM images of Ir1/FeOx. Adapted with permission from [61]. Figure 2. View largeDownload slide (a) HAADF-STEM images of Pt1/FeOx. Adapted with permission from [46]. (b) HAADF-STEM image of 0.08%Pt/FeOx-R200. Adapted with permission from [60]. (c) HAADF-STEM images of Ir1/FeOx. Adapted with permission from [61]. Adsorption approach The adsorption method is one of the most fundamental approaches for constructing isolated metal atoms on the supports [62,63]. It is simple, direct and has been widely used in the preparation of supported metal catalysts. Generally, after the metal precursors are adsorbed on the support, the residual solution is removed and then the catalysts are dried and calcined. To ensure the SAs could be stably anchored onto the supports with atomic dispersion, appropriate functional groups on the supports should be given consideration. Oxides are generally employed as an efficient support for preparing catalysts. In 2013, Narula et al. [64] reported single Pt atoms supported on θ-Al2O3(010) prepared by a wet-impregnation method using alumina powder and chloroplatinic acid. In this work, water was gradually evaporated before the resulting powder was transferred to an alumina crucible and subjected to a pyrolysis process. The resultant catalyst was catalytically active in its ability to oxidize CO to CO2. In addition, serials of Pt/θ-Al2O3 catalysts with different metal loadings were prepared, and the results reveal that they are highly active towards NO oxidation [65]. In the same year, Tao et al. [66] developed an impregnation-reduction method for preparing singly dispersed Rh atoms supported on Co3O4 nanorods. This method involves the impregnation of Rh3+ on Co3O4 nanorods followed by on-site reduction of Rh3+ using NaBH4. In situ characterizations reveal evidence of the active sites of isolated Rh atoms in the formation of RhCon on Co3O4 nanorods, which were generated through restructuring of Rh1/Co3O4 at 220°C in reactant gases. The resulting new catalytic phase exhibits a high selectivity to produce N2 in the reduction of NO with H2 between 180°C and 300°C. A report by Li et al. demonstrated that single Pt1 and Au1 atoms can be stabilized by lattice oxygen on ZnO{1010} surface via an adsorption method [67]. In detail, ZnO-nanowires (nws) were dispersed in de-ionized water followed by the addition of H2PtCl6·6H2O or HAuCl4 solution. After an aging process, the suspension was filtered, washed and dried to give Pt1/ZnO and Au1/ZnO catalysts. Similarly, Zhang et al. fabricated an Rh SAC supported on ZnO nws by introducing RhCl3 solution into ZnO nws that were dispersed in de-ionized water [40]. After stirring and aging processes, the resulting precipitate was filtered, washed, dried and reduced. As the weight loading of Rh reduced from 0.03% to 0.006%, the isolated Rh SACs can be clearly observed. During the synthetic process, the Rh atoms bond with proximal Zn atoms which lose one or more O atoms. Therefore, electrons transfer from metallic Zn to Rh atoms to generate near-metallic Rh species. The results show that the as-obtained Rh1/ZnO-nws SACs exhibited comparable efficiency in the hydroformylation of several olefins to the homogeneous Wilkinson's catalyst, along with superior catalytic activity to those of the most highly reported heterogeneous nanoparticle-based catalysts. In a more recent piece of work, Wang and co-workers described a convenient two-step synthesis of an atomically dispersed Pt catalyst supported on ceria (CeO2), with 1 wt.% metal loading, by wetness impregnation and steam treatment [68]. Chloroplatinic acid was added drop-wise to the CeO2 support while being ground in a mortar and pestle. The as-obtained powder was then dried, calcined, thermal aged and stream treated to give the catalyst. The authors demonstrated that the activation of SACs on CeO2 via high-temperature steam treatment can accomplish excellent low-temperature CO-oxidation activity and superior thermal stability. This is because the steam treatment can enable the formation of active surface lattice oxygen near isolated Pt atoms to considerably enhance catalytic performance. Further investigation of the nature of this active surface lattice oxygen on Pt/CeO2 was supported by density functional theory (DFT) calculations and reaction kinetic analyses. They found the oxygen vacancies from the CeO2 bulk can redistribute to the CeO2(111) surface when exposed to water at a high temperature. During the steam-treatment process, H2O molecules can fill out the oxygen vacancy over the atomically dispersed Pt/CeO2 surface, affording two neighboring active Olattice[H] sites around Pt. This provides the significantly improved reactivity and stability. Yan and co-workers developed a unique adsorption approach to construct Pt SACs, anchored in the internal surface of mesoporous Al2O3, by a modified sol-gel solvent vaporization self-assembly method [69], as shown in Fig. 3a. Triblock copolymers P123, C9H21AlO3 and H2PtCl6 were first mixed in ethanol. With continued evaporation of the solvent, the amphiphilic P123 macromolecules and C9H21AlO3 assembled into a highly ordered hexagonally arranged mesoporous structure, with Pt precursor encapsulated in the matrix. The as-obtained gel was then calcined in air to decompose the P123 template. Meanwhile, the C9H21AlO3 was transformed into a rigid, well-aligned mesoporous Al2O3 framework. This was followed by a reducing step in 5% H2/N2 to give the isolated Pt SAs stabilized by the unsaturated pentahedral Al3+ centers. The authors showed that the catalyst retained its structural integrity and exceptional catalytic performance in several reactions under harsh conditions, such as hydrogenation of 1,3-butadiene after exposure to a reductive atmosphere at 200°C for 24 h, n-hexane hydroreforming at 550°C for 48 h and CO oxidation after 60 cycles between 100°C and 400°C over 1 month. Figure 3. View largeDownload slide (a) Schematic illustration of the 0.2Pt/m-Al2O3-H2 synthesis process. Adapted with permission from [69]. (b) HAAD–STEM images of the 0.25Au-Na/[Si]MCM41 catalyst. Adapted with permission from [72]. (c) TEM and HAADF-STEM images of 0.35 wt% Pt/TiN. Adapted with permission from [73]. (d) Scheme of proposed formation mechanisms, TEM and HAADF-STEM images for Ru SAs/N–C. Adapted with permission from [76]. Figure 3. View largeDownload slide (a) Schematic illustration of the 0.2Pt/m-Al2O3-H2 synthesis process. Adapted with permission from [69]. (b) HAAD–STEM images of the 0.25Au-Na/[Si]MCM41 catalyst. Adapted with permission from [72]. (c) TEM and HAADF-STEM images of 0.35 wt% Pt/TiN. Adapted with permission from [73]. (d) Scheme of proposed formation mechanisms, TEM and HAADF-STEM images for Ru SAs/N–C. Adapted with permission from [76]. Zeolites are crystalline materials with well-defined structures and high surface area, along with more sites for robust bonding with catalytic species [24,70]. Specifically, zeolites could provide effective voids to anchor individual metal atoms to maintain the high dispersion and prevent them from sintering at high temperatures under oxidative or reductive atmospheres during the catalysis processes [53]. In 2012, Gates et al. reported that atomically dispersed gold atoms catalyse with a high degree of uniformity supported on zeolite NaY [71]. The site-isolated gold complexes retained after CO-oxidation catalysis, confirming the robust stabilization effect of the zeolite channels for gold species. The addition of alkali ions, such as sodium or potassium, on inert KLTL-zeolite and mesoporous MCM-41 silica materials could structurally stabilize the single gold sites in Au–O(OH)x– ensembles (Fig. 3b), as demonstrated by Flytzani-Stephanopoulos and co-workers [72]. They have shown evidence that the active catalyst was composed of alkali ions linked to the gold atom through –O ligands, not merely on the support, making the reducible oxide supports no longer an essential requirement. The validation tests show that the single-site gold atoms were homogeneously dispersed and highly active for the industrially important low-temperature water–gas shift reaction. In addition to metal oxides and zeolites, other supports such as nitrides and carbides have also been explored and shown promise for stabilizing SAs for use in catalysis. Lee et al. described a Pt SAC supported on titanium nitride (TiN) nanoparticles with the aid of chlorine ligands [73]. H2PtCl6·6H2O was dissolved in anhydrous ethanol and mixed with acid-treated TiN nanoparticles before the resulting sample was dried and reduced. Transmission electron microscopy (TEM) and HAADF-STEM images of the samples are shown in Fig. 3c. The results show that the 0.35 wt% Pt/TiN sample affords a high mass activity and a unique selectivity towards electrochemical oxygen reduction, formic acid oxidation and methanol oxidation. Carbon nitride (C3N4) has been proved as an alternative support material by virtue of their porosity and high surface area [55]. Li et al. used an impregnation method to access isolated Au atoms anchored on polymeric mesoporous graphitic C3N4 (mpg-C3N4) [74]. The catalytically active AuI atom was coordinated by three nitrogen or carbon atoms in tri-s-triazine repeating units. This coordination feature significantly prevents the Au atoms from aggregation and makes the AuI surface highly active. Moreover, they demonstrated this catalyst as highly active, selective and stable for silane oxidation with water. In 2017, Ma et al. developed a highly efficient catalyst consisting of isolated Pt atoms uniformly dispersed on an α-molybdenum carbide (α-MoC) support that can enable low-temperature, base-free hydrogen production through aqueous-phase reforming of methanol [75]. They found that the α-MoC displays stronger interactions with Pt than other oxide supports or β-Mo2C; therefore, atomically dispersed Pt atoms can be formed on an α-MoC support following a high-temperature activation process. This generates an exceptionally high-density electron-deficient surface to stabilize Pt sites for the adsorption/activation of methanol. This catalyst affords an excellent turnover frequency and the corresponding hydrogen production greatly exceeds those of previously reported catalysts for low-temperature aqueous-phase reforming of methanol. They deduce that the unique structure of α-MoC, which affects water dissociation, and the synergic effects between Pt and α-MoC together affect the activation of methanol and the subsequent reforming process. In 2017, Wu et al. reported a novel synthetic approach to construct isolated single Ru atoms on nitrogen-doped porous carbon (Ru SAs/N–C) by a coordination-assisted strategy using MOFs for the hydrogenation of quinolones [76]. It is noticed that the strong coordination effect between the lone pair of nitrogen and d-orbital of Ru atoms is crucial for the formation of stable Ru SAs (Fig. 3d). Without the dangling −NH2 groups, the Ru atoms tend to aggregate into nanoclusters, even confined in the pores of MOFs. The results demonstrate the Ru SAs serve as an effective semi-homogeneous catalyst to the chemoselective catalyse hydrogenation of quinolones. This method has been shown to potentially broaden the substrate scope for the synthesis of SACs with unique properties for use in various chemical reactions. Together, the ease of preparation for SACs using a wet-chemistry strategy envisages a promising future in the field. However, these methods have their own disadvantages. For example, some metal atoms might be buried either in the interfacial regions of the support agglomerates or within the bulk of the support when co-precipitation methods are applied [43]. In addition, when high metal loading is required for the construction of SACs, aggregation would inevitably occur [9]. This trade-off should be minimized by developing new synthesis methods. Other methodologies have also been explored to design and synthesize SACs with varies chemical and physical functionalities and future underpinned studies in these directions. The photochemical method becomes particularly appealing to assist the effective adsorption of SAs on the supports and has been proven to be effective for the synthesis of nanocrystals, such as gold, silver, platinum, palladium, etc. [77–80]. In this process, regulating the nucleation and growth processes of nanocrystals has been a major topic. Flytzani-Stephanopoulos et al. constructed isolated gold atoms supported on titania with a loading of approximately 1 wt% under ultraviolet (UV) irradiation [81]. They found that the addition of ethanol can serve as a charge scavenger to facilitate the donation of electrons from gold atoms to −OH groups on the titania support. The catalytic performance was examined and the results showed that this catalyst displayed excellent activity for the low-temperature water–gas shift reaction, as well as admirable stability in long-term cool-down and startup operations. An important study by Zheng et al. demonstrated a room-temperature photochemical strategy to construct atomically dispersed palladium atoms supported on ethylene glycolate (EG)-stabilized ultrathin TiO2 nanosheets (Pd1/TiO2 catalyst) with a Pd loading up to 1.5% [82]. Typically, two-atom-thick TiO2 nanosheets were prepared by reacting TiCl4 with EG and used as the support. H2PtCl6 was then added to the TiO2 dispersion for adsorption of Pd species followed by irradiation by UV to give the Pd1/TiO2 catalyst. TEM, STEM and EXAFS revealed that the isolated Pd atoms were evenly dispersed over the TiO2 support, without any observable evidence of NPs (Fig. 4a). The catalyst exhibited excellent catalytic performance in the hydrogenation of C = C bonds, outperforming those commercial Pd catalysts. In addition, there was no observable decay in the catalytic activity for 20 cycles, suggesting the robustness of the Pd1/TiO2 catalyst. Importantly, they found this catalyst can activate H2 in a heterolytic pathway to drastically enhance its catalytic activity in the hydrogenation of aldehydes. This mechanism has been commonly observed for homogeneous catalysts, such as Au, Pd and Ru complexes; however, there is no report for heterogeneous Pd catalysts. This study set a good example using atomically dispersed metal catalysts for bridging the gap between heterogeneous and homogeneous catalysis. Figure 4. View largeDownload slide (a) Structural characterizations of Pd1/TiO2 catalyst. Adapted with permission from [82]. (b) Schematic illustration of the iced-photochemical process compared with the conventional photochemical reduction of H2PtCl6 aqueous solution. Adapted with permission from [83]. (c) Structural features of 0.5% Fe©SiO2. Adapted with permission from [39]. (d) Schematic illustrations of the Pt ALD mechanism on graphene nanosheets. Adapted with permission from [86]. Figure 4. View largeDownload slide (a) Structural characterizations of Pd1/TiO2 catalyst. Adapted with permission from [82]. (b) Schematic illustration of the iced-photochemical process compared with the conventional photochemical reduction of H2PtCl6 aqueous solution. Adapted with permission from [83]. (c) Structural features of 0.5% Fe©SiO2. Adapted with permission from [39]. (d) Schematic illustrations of the Pt ALD mechanism on graphene nanosheets. Adapted with permission from [86]. Very recent work by Wu and co-workers showed a novel synthetic approach to accessing atomically dispersed platinum species on mesoporous carbon via iced-photochemical reduction of frozen chloroplatinic acid solution (Fig. 4b) [83]. In this report, H2PtCl6 solution was first frozen by liquid nitrogen followed by irradiation using a UV lamp. The H2PtCl6 ice was kept overnight in dark conditions at room temperature to give a clear aqueous Pt single-atom solution. Then mesoporous carbon solution and Pt single-atom solution were mixed, filtered, and dried at room temperature. Finally, the ice lattice naturally confines the dispersed ions and atoms to affect the photochemical reduction products and further prevent the aggregation of atoms. To test the generality of this concept, they also fabricated isolated Pt atoms deposited on different supports, including mesoporous carbon, graphene, carbon nanotubes, TiO2 nanoparticles and zinc oxide nanowires. Among them, the isolated Pt atoms supported on mesoporous carbon exhibited exceptional catalytic performance for hydrogen evolution reaction, as well as an excellent long-time durability, outperforming the commonly employed Pt/carbon catalyst. This iced-photochemical reduction approach provides a promising avenue for the green synthesis of SAs and sub-nanometer clusters, and opens up possibilities for fine-tuning the nucleation and growth of nanocrystals in wet chemistry. Recently, high-energy bottom-up ball-milling synthesis has been proved as a powerful method to break and reconstruct chemical bonds of materials with high efficiency. Such an approach was taken by Bao et al., who reported a lattice-confined single iron site catalyst embedded within a silica matrix by a solid fusion method. Briefly, commercial SiO2 and Fe2SiO4 were mixed and subjected to ball milling under argon and fused in the air [39]. As expected, the unsaturated single Fe sites served as active centers (Fig. 4c) to efficiently enable the direct, non-oxidative conversion of methane, exclusively to ethylene and aromatics. The presence of single Fe sites effectively prevented catalytic C-C coupling, oligomerization and coke deposition. In addition, this catalyst showed extremely stable performance, with no deactivation observed during long-term testing, and the selectivity for total carbon of the three products was retained. Subsequently, the group used the same method to construct single-atom iron sites by embedding highly dispersed FeN4 centers in graphene matrix via high-energy ball milling of iron phthalocyanine and graphene nanosheets [84]. In this system, the FeN4 center is highly dispersed and well stabilized by the graphene matrix. The formation of the Fe = O intermediate is important in promoting the conversion of benzene to phenol. Remarkably, this reaction can proceed efficiently at mild conditions such as room temperature or even as low as 0°C. DFT calculations confirm that the catalytic activity stems from the confined iron sites, along with moderate activation barriers for the reaction that proceeded at room temperature. Both studies clearly show the potential of the highly efficient ball-milling method for the fabrication of SACs for use in catalysis areas. The atomic layer deposition (ALD) technique is a gas-phase chemical process and commonly used to deposit a thin layer of film in a bottom-up fashion with near-atomic precision on the substrate by repeated exposure of separate precursors [85]. This technique offers the feasibility of precise control of the catalyst size from a single-atom, sub-nanometer cluster to the nanoparticle. It is expected that ALD would potentially provide a powerful approach for the construction of intriguing SACs. This approach was first demonstrated by Sun et al. in 2013, who reported a practical synthesis of isolated single Pt atoms on graphene nanosheets using the ALD technique (Fig. 4d) [86]. In this work, Pt was deposited on graphene supports by the ALD method using MeCpPtMe3 and oxygen as precursors and nitrogen as a purge gas. The resulting Pt SAC showed improved catalytic activity compared with the commercial Pt/C catalyst. X-ray absorption fine structure (XAFS) analyses show that the low-coordination and partially unoccupied 5d orbital of Pt atoms are responsible for the excellent catalytic performance. In 2015, Lu et al. described a single-atom Pd1/graphene catalyst prepared by the ALD method with excellent performance in the selective hydrogenation of 1,3-butadiene [87]. First, the anchor sites were created by an oxidation process on pristine graphene nanosheets, followed by a reduction process via thermal de-oxygenation to control the surface oxygen functional groups. After an annealing step, phenolic oxygen was observed to be the dominated oxygen species on the graphene support. ALD was then performed on the reduced graphene to give a single-atom Pd catalyst by alternately exposing Pd(hfac)2 and formalin. This catalyst showed superior catalytic performance in the selective hydrogenation of 1,3-butadiene, affording nearly 100% butenes selectivity, and ∼70% selectivity for 1-butene at a conversion ratio of 95% under mild conditions. They speculate that both the mono-π-adsorption mode of 1,3-butadiene and the enhanced steric effect induced by 1,3-butadiene adsorption on the isolated Pd atoms contribute to the improved selectivity of butenes. In addition, the Pd1/graphene showed remarkable durability against deactivation via either metal atom aggregation or coking during a 100-h reaction time on stream. Using the same strategy, Sun and co-workers described the preparation of isolated single Pt atoms and clusters on nitrogen-doped graphene nanosheets (NGNs) [88]. Here, Pt was first deposited on the NGNs by the ALD technique using MeCpPtMe3 and O2 as precursors and N2 as a purging gas and a carrier gas. The size, density and distribution of the Pt atoms on the NGNs or graphene nanosheets (GNs) can be precisely controlled by the ALD cycles. As expected, the isolated Pt atoms and clusters on the NGNs have been demonstrated to show superior catalytic activity and stability for the hydrogen evolution reaction (HER) compared with the conventional Pt NP catalysts. This can be explained by the small size and the special electronic structure of the adsorbed single Pt atoms on NGNs. Together, the use of the ALD technique has shown great promise for large-scale synthesis of highly active and stable single-atom and cluster catalysts. The galvanic-replacement method Galvanic replacement is a highly versatile and effective approach for the construction of a variety of nanostructures, with the ability to control the size and shape, composition, internal structure and morphology [24,57,89]. It is an electrochemical process that consists of oxidation of one metal, termed as a sacrificial template, by other metal ions that have a higher reduction potential. When they are exposed to each other in solution, the sacrificial metal template will be preferably oxidized and dissolved into the solution, while the ions of the second metal will be reduced and deposited onto the template surface. In 2015, Sykes et al. demonstrated that low concentrations of isolated Pt atoms in the Cu(111) surface (Fig. 5a) can be prepared by galvanic replacement on pre-reduced Cu NPs to catalyse the butadiene hydrogenation with remarkable activity and high selectivity to butenes [50]. In this case, Cu NPs were first prepared and supported on γ-Al2O3 followed by calcination in air. The galvanic-replacement reaction was then carried out in an aqueous solution under nitrogen protection with constant stirring and refluxing. A desired amount of Pt precursor was introduced to a suspension of Cu NPs in an aqueous solution containing HCl. The resulting material was filtered, washed and dried to yield the catalyst. They notice that, at low Pt loadings, the isolated Pt atoms can substitute into the Cu(111) surface to activate the dissociation and spillover of H to Cu. The weak binding between butadiene and Cu would facilitate the highly selective hydrogenation reaction to butenes, without decomposition or poisoning of the catalysts. This catalyst, with less than one Pt atom per 100 copper atoms, also binds CO more weakly than metallic Pt, which is particularly important for use in many Pt-catalysed chemical reactions. Figure 5. View largeDownload slide (a) Characterization of Pt/Cu SAA NPs. Adapted with permission from [50]. (b) Scanning tunneling microscope image of a 0.01 ML Pt/Cu(111) SAA surface. Adapted with permission from [90]. Figure 5. View largeDownload slide (a) Characterization of Pt/Cu SAA NPs. Adapted with permission from [50]. (b) Scanning tunneling microscope image of a 0.01 ML Pt/Cu(111) SAA surface. Adapted with permission from [90]. In a follow-up report, the Sykes group used the same approach to construct Pt/Cu single-atom alloys (SAAs) to examine C–H activation in different systems, including methyl groups, methane and butane [90]. They observed that the Pt atoms were distributed over the Cu surface and across both terraces and at regions near step edges (Fig. 5b). The results show the Pt/Cu SAAs activate C–H bonds more efficiently than Cu, along with superior stability under realistic operating conditions, effectively avoiding the coking problem that typically occurred with Pt. Both pieces of work from the Sykes group demonstrated how SAs can be deposited on alloys—an important future direction for this field. Though a variety of SACs have been developed by the bottom-up strategy, the downside of the methods described here is that it is still challenging to access SACs with high metal loading and a homogeneous coordination environment for the active sites used in the catalytic process. This would lead to limited selectivity and stability of the SACs for their practical use in various industrial fields. In addition, although ground-breaking, some of these methods do require specific/sophisticated preparation procedures that might not be compatible with all kinds of SACs and ideal from practical perspectives. TOP-DOWN SYNTHETIC METHODOLOGIES FOR THE CONSTRUCTION OF SACS The top-down strategy is based on the dissolution of ordered nanostructures into smaller pieces to give desired properties and intriguing performances [59,91]. Extensive research efforts have pursued this strategy with the overarching aim of synthesizing SACs with unprecedented chemical and physical properties and understanding the complex mechanisms for catalysis that occur at the atomic level. This strategy has proven particularly useful in the formation of SACs with accurate control over the micro- or nanostructures [92]. The precise structure (such as coordination number, dispersion tendencies and binding mode) of metal SAs synthesized by the top-down methods has shown great promise in industrially important applications [9,89,93,94]. Efforts to further understand the underlying features and mechanisms are required for the development of new methods for the construction of SACs and represent a fertile area for future studies. The high-temperature pyrolysis method High-temperature pyrolysis has become one of the fascinating methods for synthesizing nanomaterials on different supports. Particularly, the development of a template-sacrificial approach via acid leaching or oxidative calcination has offered an alternative way to generate SACs. Of note is that an appropriate pyrolysis temperature is critically important to give the desired properties. MOFs and zeolitic imidazolate frameworks (ZIFs) have interconnected 3D molecular-scale cages that make them highly accessible through small apertures. Importantly, they can serve as templates to obtain nitrogen-doped porous carbon with abundant active nitrogen sites. Very recently, Wu et al. took advantage of the MOFs and originally developed an effective strategy for accessing single Co atoms supported on nitrogen-doped porous carbon with a particularly high metal loading of over 4 wt% via the pyrolysis of bimetallic Zn/Co MOFs [57]. This is pioneering work in this field and the strategy is particularly applicable to access high-loading metal SACs that would otherwise be difficult to produce. It should be noted that the enhancement of metal loading for preparing SACs in the present study is a significant breakthrough in this area, highlighting the specific requirement of SACs for practical applications. Importantly, the introduction of Zn atoms into MOFs is critical and acts as an elegant approach to efficiently manipulate the adjacent spatial distance between Co atoms, thereby effectively preventing the formation of Co NPs (Fig. 6a). The Zn atoms, with a low boiling point of 907°C, can be evaporated in the high-temperature pyrolysis process, providing abundant N sites. The Co nodes can be reduced in situ by carbonization of the organic linkers in MOFs and anchored on the as-obtained N-doped porous carbon support. Assuming the MOF as an integrated system, using this high-temperature pyrolysis of MOF to access unsaturated SAs anchored on the N-doped porous carbon support can be categorized into the top-down approach. Control testing demonstrated that the aggregated Co atoms were formed for Co-containing MOF (ZIF-67) after a pyrolysis treatment. HAADF-STEM and EXAFS verified the presence of isolated Co atoms dispersed on the N-doped porous carbon support. The resulting Co SAC shows exceptional oxygen-reduction reaction (ORR) catalytic performance with a half-wave potential more positive than the commercial Pt/C and most of the reported non-precious metal catalysts. Robust chemical stability during electrocatalysis and thermal stability that resists sintering at a high temperature of 900°C have also been confirmed, as little evidence of catalyst degradation was observed during the catalytic cycles. This work has underlined the significant importance of employing MOFs as an ideal carbon support for stabilizing single metal atoms at the atomic scale. Figure 6. View largeDownload slide (a) Schematic illustration of the construction of Co SAs/N–C. Adapted with permission from [57]. (b) Schematic illustration of the construction of Ni SAs/N–C. Adapted with permission from [95]. (c) Schematic illustrations of the construction of Fe-ISAs/CN. Adapted with permission from [97]. (d) Schematic illustration of the construction of ISAS-Co/HNCS. Adapted with permission from [99]. (e) Schematic illustration of the construction of SA-Fe/CN. Adapted with permission from [103]. (f) Schematic illustration of the construction of (Fe, Co)/N–C. Adapted with permission from [104]. Figure 6. View largeDownload slide (a) Schematic illustration of the construction of Co SAs/N–C. Adapted with permission from [57]. (b) Schematic illustration of the construction of Ni SAs/N–C. Adapted with permission from [95]. (c) Schematic illustrations of the construction of Fe-ISAs/CN. Adapted with permission from [97]. (d) Schematic illustration of the construction of ISAS-Co/HNCS. Adapted with permission from [99]. (e) Schematic illustration of the construction of SA-Fe/CN. Adapted with permission from [103]. (f) Schematic illustration of the construction of (Fe, Co)/N–C. Adapted with permission from [104]. Subsequently, an ionic exchange strategy was developed by the Wu group to assist in the construction of a single Ni atom catalyst (Fig. 6b) between Zn nodes and adsorbed Ni ions within the cavities of the MOF [95]. In this case, ZIF-8 was first dispersed in n-hexane under ultrasound until a homogeneous solution was formed. Then a small amount of Ni(NO3)2 aqueous solution was introduced, and the mixed solution was vigorously stirred to cause the Ni ions to be absorbed completely. Then the sample was centrifuged and dried, followed by a high-temperature heating process in an argon atmosphere to yield Ni SAC. This Ni SAC, with a metal weight loading of 1.53 wt%, delivered an excellent turnover frequency for CO2 electroreduction of 5273 h−1, along with a maximum Faradaic efficiency for CO production of 71.9% and a high current density of 10.48 mA cm−2. This work, for the first time, demonstrates the great potential of using MOF-based materials to access SACs for use in CO2 electroreduction. To investigate the relationship between coordination numbers and CO2 electroreduction catalytic performance, the Wu group sequentially prepared a series of Co SACs with different N coordination environments treated at different temperatures [96]. Bimetallic Co/Zn ZIFs were treated by a pyrolysis process, during which the Zn was evaporated away and the Co was reduced by carbonized organic linkers, generating isolated Co atoms stabilized on nitrogen-doped carbon. By controlling the pyrolysis temperatures, three Co SACs with different Co–N coordination numbers were obtained, being Co–N4 (800°C), Co–N3 (900°C), and Co–N2 (1000°C), respectively. The catalytic performance of these samples was examined, and the results show that the isolated Co atom with two coordinated nitrogen atoms (prepared at 1000°C) can afford significantly higher selectivity and superior activity, resulting in a CO formation Faradaic efficiency of 94% and a current density of 18.1 mA cm−2 at an overpotential of 520 mV. Importantly, this catalyst achieved a turnover frequency for CO formation of 18 200 h−1, outperforming most of the reported metal-based catalysts under comparable conditions. DFT calculation reveals that the decreased N coordination environment leads to more unoccupied 3d orbitals for Co atoms, thereby facilitating adsorption of CO2•− and increasing CO2 electroreduction performance. This study demonstrates the significant effect of N coordination environments on SACs for catalytic performance. The above studies further confirm the great potential of high-temperature pyrolysis of MOFs as a promising strategy to access SACs for different demanding industrial applications. With these attractive features, Li and co-workers prepared a highly stable isolated Fe atom catalyst, with Fe loading up to 2.16 wt%, that showed excellent ORR reactivity via a cage-encapsulated precursor pyrolysis approach [97]. This method is highly effective to access SACs because the precursors can be encapsulated inside the ZIF pores, thereby preventing them from aggregating into nanoparticles (Fig. 6c). In this study, Fe(acac)3 was mixed with ZIF-8, and the molecular-scale cages were formed with the assembly of Zn2+ and 2-methylimidazole, with one Fe(acac)3 molecule trapped in one cage. After a pyrolysis step, the ZIF-8 was transformed into nitrogen-doped porous carbon, whereas the Fe(acac)3 within the cage was reduced by carbonization of the organic linker, resulting in the formation of isolated iron atoms anchored on nitrogen species. The catalyst has been demonstrated to show exceptional ORR catalytic activity, good methanol tolerance and impressive stability. Importantly, the ORR catalytic activity of this SAC outperforms those of recently reported Fe-bases materials and other non-precious metal materials. Experimental results and DFT calculations reveal the excellent ORR performance stems from the formation of atomically isolated iron atoms coordinated with four N atoms and one O2 molecule adsorbed end-on. Using a similar approach, Li et al. described the synthesis of atomically dispersed Ru3 clusters via a cage-separated precursor pre-selection and pyrolysis strategy [98]. Generally, two steps are involved: (i) encapsulation and separation of preselected metal cluster precursors followed by (ii) a pyrolysis treatment. The resulting catalyst was characterized by HAADF-STEM and XAFS, and the catalytic performance was tested for the oxidation of 2-amino-benzyl alcohol. The results show that this Ru3/nitrogen-doped carbon (CN) catalyst possesses 100% conversion, 100% selectivity and an unexpectedly high turnover frequency (TOF), outperforming those of Ru SACs and small-sized Ru particle catalysts. An alternative approach to the thermal treatment of MOFs for achieving SACs has been employed by Li et al., who used SiO2 as a template to access a hollow N-doped carbon sphere with isolated Co atomic sites (Fig. 6d) [99]. Briefly, the SiO2 template was dispersed in Co–TIPP/TIPP solution before introducing another monomer. The collected powder was thermally treated under a flowing H2/Ar and then etched with sodium hydroxide to remove the SiO2 template to yield the Co SAC. Its ORR performance was investigated and the results demonstrate that exceptional catalytic activity was originated from the single Co sites that can significantly facilitate the proton and charge transfer to the adsorbed *OH species. Using the same approach, a Mo SAC was prepared by the Li group using sodium molybdate and chitosan as precursors and showed excellent HER performance [100]. Further studies of the structure of the catalyst were supported by AC-STEM and XAFS, which confirmed that the Mo atom was anchored with one nitrogen atom and two carbon atoms (Mo1N1C2). In 2016, Zhang et al. described a similar template-sacrificial approach to create a self-supporting Co–N–C catalyst with single-atom dispersion and showed excellent catalytic activity for the chemoselective hydrogenation of nitroarenes to yield azo compounds under mild conditions [101]. In this study, the Co(phen)2(OAc)2 complex was supported on Mg(OH)2 and then subjected to a pyrolysis process. This was followed by the removal of the MgO support by an acid-leaching treatment. The merit of employing Mg(OH)2 is that it can essentially prevent the aggregation of cobalt atoms. This is because of the moderate interaction between Mg(OH)2 and the Co species, as well as its inertness towards the reaction with Co during the pyrolysis process. After the acid-leaching step, the support material was removed to give a self-supporting Co–N–C material. X-ray absorption spectroscopy was tested and the exact structure of the catalyst was confirmed to be CoN4C8–1-2O2. Specifically, the Co single atom was coordinated with four pyridinic nitrogen atoms on the graphitic layer, along with oxygen atoms weakly adsorbed on the Co atoms perpendicular to the Co–N4 plane. Using the same approach, Zhang et al. prepared an atomically dispersed Fe−N−C catalyst, which exhibited exceptional activity and excellent reusability for the selective oxidation of the C−H bond, along with tolerance for a wide scope of substrates [102]. Briefly, the Fe(phen)x complex supported on the nano-MgO template was pyrolysed at different temperatures under N2 atmosphere, followed by an acid-leaching step to remove the MgO template. They observed that the properties of the Fe species were dependent on the pyrolysis temperature, with more metallic Fe particles formed at higher temperatures. The critical role of the Fe−Nx sites in catalysis was further confirmed by potassium thiocyanate titration experiments and Mössbauer spectroscopy. An effective core–shell strategy has been introduced by the Li group using metal hydroxides or oxides coated with polymers followed by high-temperature pyrolysis and acid-leaching steps, to synthesize single metal atoms anchored on the inner wall of hollow CN materials [103]. By employing different metal precursors or polymers, they have successfully synthesized a series of metal SAs dispersed on CN materials (Fig. 6e). In detail, α-FeOOH nanorods were first prepared by a hydrothermal method, followed by self-polymerizing dopamine monomers to generate α-FeOOH@PDA. Then it was thermally treated under an inert atmosphere, during which the polydopamine (PDA) layers were converted into the CN shell and α-FeOOH was reduced to iron, giving rise to the strong interaction between the Fe atoms and the CN shell. Finally, acid leaching was carried out to generate Fe SAs on the inner wall of the CN materials. The obtained SA-Fe/CN catalyst showed a high conversion of 45% and an excellent selectivity of 94% for the hydroxylation of benzene to phenol, outperforming Fe nanoparticles/CN. Notably, in a most recent research, Wu et al. originally developed a host–guest strategy based on MOFs to construct a Fe–Co dual-sites catalyst embedded in N-doped porous carbon support [104]. It involves binding between Co nodes and adsorbed Fe ions within the confined space of MOFs (Fig. 6f). Specifically, Zn/Co bimetallic MOF was employed as a host to encapsulate FeCl3 within the cavities by a double-solvents method. The Fe3+ species were reduced by the as-generated carbon and bond with the neighboring Co atoms. Meanwhile, the adsorbed Fe3+ species can accelerate the decomposition of metal–imidazolate–metal linkages and generate voids inside the MOF. EXAFS and Mössbauer spectroscopic analyses were performed to investigate the coordination environment of the Fe–Co dual sites. The experimental results show that FeCoN6 is the active site for the (Fe, Co)/N–C catalyst and has been demonstrated to endow excellent ORR performance in an acidic electrolyte, along with comparable onset potential and half-wave potential to those of the commercial Pt/C. DFT calculation reveals that the activation of O–O is favored on the dual sites, which is important for the four-electron oxygen-reduction process. The fuel cell testing revealed that this catalyst outperforms most of the reported Pt-free catalysts in H2/O2 and H2/air conditions. In addition, this cathode catalyst is rather robust in long-term operation for electrode measurement and H2/air single cell testing. Of note is that, despite the fact that SACs generally confer greater activity than the corresponding nanoparticles, it is still important to be aware of the potential aggregation pathways available to them. This is especially crucial in cases where higher operational temperatures were applied. Therefore, the superior catalytic activity, selectivity, stability and the ease of fabrication of the dual-sites Fe–Co catalyst make this type of SAC truly remarkable. Importantly, the main advantages of this host–guest strategy include the ability to incorporate different metal atoms and to permit the catalyst to be operated in a wider dynamic range. This study is expected to provide avenues for the synthesis of high-performance dual-sites catalysts with unique properties for use in chemical transformations. Overall, these studies have shown that the high-temperature pyrolysis method is capable of producing SACs with precisely controlled structures and morphologies. Additionally, this unique approach has been seen as a significant opportunity to enable the efficient construction of high-performance SACs for use in various reactions. The high-temperature atomic-migration method High temperatures are generally detrimental to catalysts’ activities. Although the SAs are homogeneously dispersed on the support materials, they have a high propensity to move and aggregate into nanoparticles when heated at high temperatures. Datye and co-workers take advantage of the phenomenon that metal nanoparticles can emit mobile species to prepare atomically dispersed metal catalysts [105]. In this study, a Pt/La-Al2O3 catalyst was physically mixed with different types of ceria powders followed by a thermal treatment in flowing air. Because of the strong interaction between PtO2 and ceria powders, the Pt species emitted from the alumina were trapped on the CeO2, forming thermally stable Pt1/CeO2 SACs (Fig. 7). The performance of the resulting SAC was tested for CO oxidation, and the results suggest that it can serve as a highly effective sintering-resistant CO-oxidation catalyst at high temperature. They believe that this atom-trapping approach is potentially applicable and might provide exciting possibilities to access a variety of high-performance SACs. This work represents a novel strategy and has been demonstrated as being particularly effective in fabricating SACs and connecting the relationship between the nanoparticles and SAs. Figure 7. View largeDownload slide Schematic illustration of Pt nanoparticle sintering, showing how ceria can trap the mobile Pt to suppress sintering. Adapted with permission from [105]. Figure 7. View largeDownload slide Schematic illustration of Pt nanoparticle sintering, showing how ceria can trap the mobile Pt to suppress sintering. Adapted with permission from [105]. CONCLUSIONS AND PERSPECTIVE Over just a few years, there has been remarkable progress in the development of various methods for the synthesis of SACs. In this review, we summarize the progress, bring new insights from recent years and pointed the way to the synthesis of SACs. Currently, two general approaches have been employed for accessing SACs: bottom-up and top-down. Though still being developed, SACs have emerged as an exceptional advancement in the development of highly efficient heterogeneous catalysts. The researchers have shown evidence that the size of the nanomaterials does affect catalytic efficiency in the catalysis process. A noteworthy result is that, by reducing the size of nanostructures from the nano- to the sub-nano scale and finally to SAs in atomic dimensions, catalytic performance has been observed to change drastically. This results from the low-coordination environment, quantum size effect and enhanced metal–support interactions. Moreover, the homogeneously and isolated metal active sites can maximize metal utilization, giving rise to the impressively enhanced catalytic performance. Recent experimental and theoretical progress has unambiguously validated the strong evidence for the high activity, selectivity and stability of the high-performance SACs. These intriguing properties of SACs are believed to endow great potential for applications in heterogeneous catalysis. Importantly, SACs can act as an ideal platform to serve as a bridge to connect hetero- and homogeneous catalysis. Thus, SACs are thought to have the potential to overcome the difficulty encountered in homogeneous catalysis. As discussed previously, a major limiting factor in the development of SACs is the lack of general methods to directly and efficiently access high-performance SACs. The construction of SACs for use in catalysis represents an important challenge, highlighting the need for more fundamental research into detailed mechanisms. Along with the emergence of new characterization and computational modeling techniques, single-atom active sites can be investigated further. More advanced, direct and effective in situ spectroscopic and microscopic techniques become particularly important to offer new insights into the chemical reactions involved in SACs. Elucidating the important role of metal precursors, support materials and experimental conditions and understanding the prerequisites for catalytic activity of a given catalytic system are crucial for developing effective strategies for the synthesis of SACs. Several aspects should also be given enough attention: first, the development of novel, controllable and facile synthesis methods for access high-loading SACs with finely and densely dispersed single atoms; second, the construction of single metal atoms with robust stabilization on the support for use in practical conditions; third, detailed experimental and theoretical work should be done to comprehensively understand SACs-support effects. The top-down strategy has shown great promise and significantly contributed to the simplified synthesis routes for SACs with exceptional activity and stability. Moreover, the metal loading can be markedly increased from 1% to 5%, and the coordination environments can be elaborately controlled. This will definitely facilitate the development of general protocols for accessing SACs and underpin the exploration of other intriguing applications. Together, the field of SAs is expansive and rapidly developing towards different applied research fields. The continued development of SACs represents an important advancement in heterogeneous catalysis and will surely be the important focus of extensive research efforts and a thriving field for various applications for years to come. FUNDING This work was supported by the National Key R&D Program of China (2017YFA0208300) and the National Natural Science Foundation of China (21522107, 21671180, 21521091, 21390393, U1463202). REFERENCES 1. 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Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

Journal

National Science ReviewOxford University Press

Published: Sep 1, 2018

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