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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 Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 National Science Review 5: 673–689, 2018 REVIEW doi: 10.1093/nsr/nwy056 Advance access publication 1 June 2018 MATERIALS SCIENCE Special Topic: Single-Atom Catalysts Recent advances in the precise control of isolated single-site catalysts by chemical methods 1,† 3,† 1,∗ 1,2,∗ Zhijun Li , Dehua Wang , Yuen Wu and Yadong Li 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 1 their potential use in the industrial area. In this regard, a novel top-down strategy has been recently Department of developed to fabricate SACs to address these practical issues. The metal loading can be increased to 5% and Chemistry, iChEM the coordination environments can also be precisely controlled. This review highlights approaches to the (Collaborative Innovation Center of chemical synthesis of SACs towards diverse chemical reactions, especially the recent advances in improving Chemistry for Energy the mass activity and well-defined local structures of SACs. Also, challenges and opportunities for the SACs Materials), University will be discussed in the later part. of Science and Technology of China, Keywords: single-atom catalysts, bottom-up, top-down, catalytic performance Hefei 230026, China; Department of in heterogeneous catalysis; however, to greatly in- INTRODUCTION Chemistry, Tsinghua crease the turnover frequency of surface active sites University, Beijing In the worldwide theme of exploring efficient and and to enhance the mass activity remain the primary 100084, China and low-cost technologies for energy conversion and goals in catalysis [9,10]. In most circumstances, it School of chemical transformations, substantial effort has been has been demonstrated that the surface atoms of the Pharmaceutical and devoted to the development of general, practical and Chemical Engineering, nanomaterials in an unsaturated coordination envi- simple chemical approaches for catalyst preparation Taizhou University, ronment generally act as the active sites to catalyse in past decades [1–6]. Studies have shown that ul- Taizhou 318000, China specific reactions [ 11]. Therefore, extensive stud- trasmall assemblies, compared to their macroscopic ies have been devoted to rationally controlling the counterparts [7], can exhibit essentially different shapes, structures, crystal phases and compositions Corresponding physical and chemical properties. These unique of nanocatalysts [3,8,12–18]. authors. E-mails: properties would drastically alter their practical ap- With decreasing the size of nanomaterials, the [email protected]; plications in a variety of areas, such as cataly- number of surface atoms is increased substantially, [email protected] sis, biomedical research, energy and environmental Equally contributed exposing more defects and active sites, tuning fields [ 6]. Therefore, metal nanoparticles represent to this work. geometric and electronic properties involved in a rich resource for a variety of chemical processes, chemical reactions [10]. Nanoclusters have shown employed both in industry and in academia [8]. intriguing properties because of the reduced size The maximized surface area of support, increased Received 21 March compared to nanoparticles, exposing more uncoor- 2018; Revised 10 number of catalytic active sites, minimized catalyst dinated active sites and changing molecular orbital May 2018; Accepted loading and strong catalyst-support interaction de- energy levels [19,20]. Heiz and co-workers found 31 May 2018 termine the nature of nanocatalysts [2,6]. The sup- a pronounced size effect for model catalysts of ported metal nanoparticles are frequently employed The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For permissions, plea se e-mail: [email protected] Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 674 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW size-selected Pd (n ≤ 30) clusters supported on tivity of allylic alcohols [31]. The key discovery was MgO(100) for the cyclotrimerization of acetylene that the employment of homogeneously dispersed to benzene [21]. Anderson et al. studied size- SACs generally confers a dramatic improvement in selected palladium clusters and deposited them on catalytic activity, selectivity and stability, or even the rutile phase of titanium dioxide (TiO )for CO considerably different catalytic properties than oxidation [22]. By employing X-ray photoemission the corresponding nanoparticles and nanoclusters spectroscopy and temperature-programmed reac- [32–35]. This is highly desirable and has attracted tion measurements, they found that the activity of extensive scientific attention, as they might poten- these catalysts was associated with Pd binding tially act as alternatives to circumvent the problems 3d energy. Li et al. utilized a double-solvent method of scarcity and high cost of the noble-metal catalysts combined with a photoreduction process to prepare used in large-scale catalysis applications [36]. active Pd nanoclusters encapsulated inside the cage Specifically, the active single-atom sites are well of NH -Uio-66 [23]. The resultant catalyst showed defined and atomically stabilized on the supports, exceptional performance for a Suzuki coupling re- and the identical geometric structure of each active action under visible-light irradiations. Nevertheless, site is similar to that of a homogeneous catalyst. although the sizes of the nanoclusters have been Recently, studies have clearly demonstrated that reduced, their multiple distributions of active metal the utility and uniqueness of these SACs have great sites alongside different geometric and electric potential to bridge the gap between homogeneous structures might not be ideal for specific catalytic and heterogeneous catalysis [37–42]. This would reactions [9,10]. The stability of nanoclusters would solve the problems of the difficulty in separating also be a problem for their application in hetero- the homogeneous catalysts from raw materials and geneous catalysis, especially at higher operational products, as well as combining the merits of both temperatures [24]. hetero- and homogeneous catalysts. The SACs Further downsizing nanoclusters to the atomic also provide a good avenue to identify the detailed level, namely single atoms (SAs), maximum atom structural features for the active sites and an ideal utilization and superior/distinguishing catalytic model to elucidate the structure–activity relation- performance are supposed to be obtained [9,10]. ship [43–45]. Such catalysts have shown intriguing Of note is the fact that unexpected superior cat- interests in the catalysis field [ 10,32,34,43,46–49]. alytic performances of the single-atom catalysts To meet the practical demand, the most important (SACs) are often observed as one of the key challenges for fabricating the SACs are to increase advances of these novel catalysts versus their the density of active sites and to improve their nanoscale counterparts. This can be ascribed to intrinsic activities [32,36,45]. The first challenge is the unsaturated environments of metal active the high propensity for aggregation of SAs once the sites, quantum size effects and metal–support size of the nanomaterials is greatly reduced [10]. interactions [25–28]. Therefore, the research on The second challenge is the rational control of the SACs has rapidly progressed from their funda- coordination environment of the single metal atoms. mental aspects to pursing practical applications Recently, several strategies for constructing in areas of nanotechnology and materials science. atomically dispersed metal sites on catalyst supports A study that has attracted considerable attention have been extensively studied [9,10,32,42]. These since its publication in 1995 is that of Thomas strategies include enhancing the metal–support et al., who reported that direct grafting of interactions, engineering vacancy defects and voids organometallic complexes onto the walls of on the supports, and modifying surface functional mesoporous silica gives a shape-selective high- groups [9,42]. In most cases, the supports for performance catalyst with well-separated, homoge- isolated SACs are chosen on purpose, as they can neously dispersed and high surface concentrations stabilize the isolated catalytic SAs or activate nearby of active sites for the epoxidation of cyclohexenes reactants to form intermediate species for the and their derivatives [29]. Later, in 2003, Flytzani- catalytic active sites [50–52]. For example, zeolites Stephanopoulos et al. discovered that the water–gas could provide effective voids to anchor individual shift reaction was not affected by the catalytic metal atoms to maintain the high dispersion of activity of metallic Au or Pt nanoparticles; instead, the isolated metal atoms and prevent them from nonmetallic Au or Pt species on the ceria surface sintering at high temperatures under oxidative or played a key role in this reaction [30]. In 2007, Lee reductive atmospheres during catalysis processes et al. successfully synthesized Pd-Al O catalyst and [53]. Nanoparticles and nanoclusters can also serve 2 3 validated that the extremely low metal loading leads as supports. Through elegant studies of support II to the formation of atomically isolated Pd species, materials, Sykes et al. showed that the isolated which greatly contribute to the excellent selox ac- Pd atoms can be supported on a Cu surface and Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 Top-down REVIEW Li et al. 675 because of their high propensity for aggregation and the difficulty in constructing homogeneous coor- dination environments for the reactive sites. These drawbacks lead to limited selectivity and stability of the SACs, greatly limiting their potential use in Co-precipitation various industrial fields. In this regard, Li and Wu proposed a top-down strategy to construct SACs Adsorption Galvanic by the pyrolysis of metal nodes in metal-organic SACs replacement frameworks (MOFs) for the first time [ 57]. In this case, the introduction of Zn atoms into MOFs is im- portant and can effectively prevent the formation of Co NPs (nanoparticles) during the high- temperature pyrolysis process. The resulting High-temp High-temp Co SAC has a high metal loading close to 5% and pyrolysis migration showed exceptional chemical and thermal stability. A distinguishing feature of this strategy is not only that the metal loading can be substantially increased Figure 1. Schematic representation of the bottom-up and from 1% to 5%, which is important from practical top-down strategies for the synthesis of SACs. perspectives, but it can control the coordination environments to construct high-performance SACs by exposing real active sites. This top-down significantly lower the energy barrier to hydrogen strategy overcome challenges in the fabrication of uptake on and subsequent desorption from the SACs with a traditional bottom-up strategy and nearby Cu atoms [48]. This facile hydrogen disso- has great potential to meet the requirement for ciation at the isolated Pd atoms and weak binding use in practical applications. In addition to the to the Cu surface together facilitate selective hydro- fabrication strategies, the use of these methods in genation of styrene and acetylene. Toshima et al. different chemical reactions will also be presented. described a crown-jewel concept for the construc- Finally, future challenges and opportunities will be tion of catalytically highly active top gold atoms discussed. on palladium nanoclusters [54]. Interestingly, the gold atoms can be controllably assembled at the top position on the cluster and exhibit high catalytic ac- tivity because of their high negative-charge density BOTTOM-UP SYNTHETIC and unique structure. Recent reports have begun METHODOLOGIES FOR THE to document that the defects in reducible oxides CONSTRUCTION OF SACS (e.g. TiO and CeO ), graphene or C N also help 2 2 3 4 to stabilize isolated metal atoms [32,42,55]. For The bottom-up strategy is the most common example, Du et al. investigated the favorable role of method to synthesize metal SACs, during which isolated palladium and platinum atoms supported the metal precursors are adsorbed, reduced and on graphitic carbon nitride (g-C N )toactas confined by the vacancies or defects of the supports 3 4 photocatalysts for CO reduction [56]. Overall, [9,10,32,52]. Nevertheless, how to effectively an important conclusion derived from these works increase the SACs loading with well-defined dis- is that the further development of this SACs field persion on the supports is still challenging. First, requires a more fundamental understanding of SA aggregation would occur during a chemical synthe- formation at the atomic scale. sis or catalytic process when high loading of SAs This review covers the preparation strategies is required. Second, the architectural structures of for SACs, which can be categorized according to anchor sites for confining and stabilizing the metal how their components are integrated, namely via SACs on the support remain elusive; therefore, the bottom-up and top-down approaches (Fig. 1). coordination environment for metal SAs might be Currently, a large majority of SACs are synthesized inhomogeneous and poorly defined [ 58]. Optimiza- via a bottom-up strategy by using oxides or carbon tion of the precursors and supports and controlling supports to construct N or O defects to enable the of the synthetic procedures play a key role in tuning deposition of metal precursors. This is followed by a the metal–support interaction and guaranteeing the chemical reduction process to generate SACs from homogeneous dispersion of SACs. high-oxidation-state ions to low oxidation state. For the wet-chemistry strategy, the precursor so- However, the following drawbacks are encountered lutions of mononuclear metal complexes are first frequently: difficulty in accessing high metal loading anchored to the supports by a coordination effect Bottom-up Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 676 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW Co-precipitation 3.0 PtO ( (b) b) (a) 2.5 0.08% Pt/FeOx R200 0.08% Pt/FeOx R250 0.75% Pt/FeOx R250 2.0 2.73% Pt/FeOx R250 4.30% Pt/FeOx R250 1.5 Pt foil 1.0 0.5 0.0 11.540 11.560 11.580 11.600 11.620 Energy (eV) (c) () Pt foil PtO Sample A × Sample B PtO Sample A (used) Sample A Sample B Pt foil 11.55 11.60 11.65 0 1 2 3 4 5 6 E (keV) R (A) Figure 2. (a) HAADF-STEM images of Pt1/FeO . Adapted with permission from [46]. (b) HAADF-STEM image of 0.08%Pt/FeO -R200. Adapted with x x permission from [60]. (c) HAADF-STEM images of Ir /FeO . Adapted with permission from [61]. 1 x between the metal complexes and the functional SAC showed extremely high atom efficiency, excel- groups of the support surfaces [32]. Then, the or- lent stability and superior activity for both CO oxi- ganic ligands of the metal complexes are removed by dation and preferential oxidation of CO in H .They a post-treatment to expose more active sites to meet found that these merits can be attributed to the par- the requirement of catalytic reactions. Particularly, tially vacant 5d orbitals of the positively charged the advantage of wet chemistry for preparing SACs high-valent Pt atoms, as they can effectively reduce is that this method does not require specialized CO-adsorption energy and activation barriers that equipment and can be routinely practiced in any are required for CO oxidation. This study demon- chemistry lab [59]. strated the feasibility of using the defects of oxide supports to serve as anchoring sites for metal clus- ters and single metal atoms. Subsequently, the fea- Co-precipitation approach sibility and efficiency of this approach were further Co-precipitation is one of the commonly employed demonstrated by the Zhang group showing that the approaches for preparing SACs, during which the high-performance Pt- and Ir-based SACs could also be obtained (Fig. 2b and c) for use in organic trans- substances that are normally soluble under the formation [60] and water–gas shift reactions [ 61]. conditions would be precipitated. A significant ad- In these examples, defects in the oxide supports and vantage of this method lies in its extreme simplicity, the amount of metal loading were found to be criti- as no additional complicated steps are involved. cal for accessing high-performance SACs that would For a classical example, Zhang et al. employed this normally lead to aggregation. method to fabricate single Pt atoms supported on iron oxide nanocrystallites (Pt /FeO )[46]. 1 x The metal precursor of H PtCl ·H Owas mixed 2 6 2 Adsorption approach with Fe(NO ) ·9H O in a proper molar ratio and 3 3 2 pH. After recovery, the precipitate was dried and The adsorption method is one of the most funda- calcined, resulting in the formation of Pt /FeO . mental approaches for constructing isolated metal 1 x The aberration-corrected scanning transmission atoms on the supports [62,63]. It is simple, direct electron microscopy (AC-STEM) and extended and has been widely used in the preparation of sup- X-ray absorption fine structure (EXAFS) spec- ported metal catalysts. Generally, after the metal pre- tra demonstrated the individual Pt atoms were cursors are adsorbed on the support, the residual so- uniformly dispersed on FeO support, with a lution is removed and then the catalysts are dried metal loading level of 0.17 wt% (Fig. 2a). This and calcined. To ensure the SAs could be stably Intensity (a.u.) Intensity (a.u.) Normalized absorption (a.u.) Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 677 anchored onto the supports with atomic disper- In a more recent piece of work, Wang and co- sion, appropriate functional groups on the supports workers described a convenient two-step synthesis should be given consideration. of an atomically dispersed Pt catalyst supported on Oxides are generally employed as an efficient ceria (CeO ), with 1 wt.% metal loading, by wetness support for preparing catalysts. In 2013, Narula impregnation and steam treatment [68]. Chloropla- et al. [64] reported single Pt atoms supported tinic acid was added drop-wise to the CeO support on θ-Al O (010) prepared by a wet-impregnation while being ground in a mortar and pestle. The as- 2 3 method using alumina powder and chloroplatinic obtained powder was then dried, calcined, thermal acid. In this work, water was gradually evapo- aged and stream treated to give the catalyst. The au- rated before the resulting powder was transferred thors demonstrated that the activation of SACs on to an alumina crucible and subjected to a pyrol- CeO via high-temperature steam treatment can ac- ysis process. The resultant catalyst was catalyti- complish excellent low-temperature CO-oxidation cally active in its ability to oxidize CO to CO .In activity and superior thermal stability. This is be- addition, serials of Pt/θ-Al O catalysts with dif- cause the steam treatment can enable the formation 2 3 ferent metal loadings were prepared, and the re- of active surface lattice oxygen near isolated Pt atoms sults reveal that they are highly active towards NO to considerably enhance catalytic performance. Fur- oxidation [65]. ther investigation of the nature of this active surface In the same year, Tao et al. [66] developed lattice oxygen on Pt/CeO was supported by den- an impregnation-reduction method for preparing sity functional theory (DFT) calculations and re- singly dispersed Rh atoms supported on Co O action kinetic analyses. They found the oxygen va- 3 4 nanorods. This method involves the impregnation cancies from the CeO bulk can redistribute to the 3+ of Rh on Co O nanorods followed by on-site CeO (111) surface when exposed to water at a high 3 4 2 3+ reduction of Rh using NaBH . In situ character- temperature. During the steam-treatment process, izations reveal evidence of the active sites of iso- H O molecules can fill out the oxygen vacancy over lated Rh atoms in the formation of RhCo on Co O the atomically dispersed Pt/CeO surface, affording n 3 4 2 nanorods, which were generated through restructur- two neighboring active O [H] sites around Pt. lattice ing of Rh /Co O at 220 C in reactant gases. The This provides the significantly improved reactivity 1 3 4 resulting new catalytic phase exhibits a high selectiv- and stability. ity to produce N in the reduction of NO with H Yan and co-workers developed a unique adsorp- 2 2 ◦ ◦ between 180 C and 300 C. tion approach to construct Pt SACs, anchored in the A report by Li et al. demonstrated that single Pt internal surface of mesoporous Al O ,byamodified 1 2 3 and Au atoms can be stabilized by lattice oxygen sol-gel solvent vaporization self-assembly method on ZnO{1010} surface via an adsorption method [69], as shown in Fig. 3a. Triblock copolymers [67]. In detail, ZnO-nanowires (nws) were dis- P123, C H AlO and H PtCl were first mixed 9 21 3 2 6 persed in de-ionized water followed by the addition in ethanol. With continued evaporation of the sol- of H PtCl ·6H OorHAuCl solution. After an ag- vent, the amphiphilic P123 macromolecules and 2 6 2 4 ing process, the suspension was filtered, washed and C H AlO assembled into a highly ordered hexag- 9 21 3 dried to give Pt /ZnO and Au /ZnO catalysts. Simi- onally arranged mesoporous structure, with Pt pre- 1 1 larly, Zhang et al. fabricated an Rh SAC supported on cursor encapsulated in the matrix. The as-obtained ZnO nws by introducing RhCl solution into ZnO gel was then calcined in air to decompose the P123 nws that were dispersed in de-ionized water [40]. template. Meanwhile, the C H AlO was trans- 9 21 3 After stirring and aging processes, the resulting pre- formed into a rigid, well-aligned mesoporous Al O 2 3 cipitate was filtered, washed, dried and reduced. framework. This was followed by a reducing step As the weight loading of Rh reduced from 0.03% in 5% H /N to give the isolated Pt SAs stabilized 2 2 3+ to 0.006%, the isolated Rh SACs can be clearly by the unsaturated pentahedral Al centers. The observed. During the synthetic process, the Rh authors showed that the catalyst retained its struc- atoms bond with proximal Zn atoms which lose tural integrity and exceptional catalytic performance one or more O atoms. Therefore, electrons trans- in several reactions under harsh conditions, such as fer from metallic Zn to Rh atoms to generate hydrogenation of 1,3-butadiene after exposure to a near-metallic Rh species. The results show that the reductive atmosphere at 200 C for 24 h, n-hexane as-obtained Rh /ZnO-nws SACs exhibited compa- hydroreforming at 550 C for 48 h and CO oxida- ◦ ◦ rable efficiency in the hydroformylation of several tion after 60 cycles between 100 C and 400 C over olefins to the homogeneous Wilkinson’s catalyst, 1 month. along with superior catalytic activity to those of the Zeolites are crystalline materials with well- most highly reported heterogeneous nanoparticle- defined structures and high surface area, along with based catalysts. more sites for robust bonding with catalytic species Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 678 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW Adsorption 2- (a) a) (d) PtCl Al(OCHCH CH ) 6 3 3 3 Pt C H O CI AI P123 Self-assembly Calcination Complexing Reduction Pyrolysis HF etching Gel 0.2Pt/m-Al O -H 2 3 2 C N ZrO Ru Single atoms (c) (b) Nano clusters 0.2-0.3 0.4-0.5 0.6-0.9 ~1.0 nm Figure 3. (a) Schematic illustration of the 0.2Pt/m-Al O -H synthesis process. Adapted with permission from [69]. (b) HAAD–STEM images of the 2 3 2 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]. The results show that the 0.35 wt% Pt/TiN sample [24,70]. Specifically, zeolites could provide effective affords a high mass activity and a unique selectivity voids to anchor individual metal atoms to maintain towards electrochemical oxygen reduction, formic the high dispersion and prevent them from sintering acid oxidation and methanol oxidation. at high temperatures under oxidative or reductive Carbon nitride (C N ) has been proved as an al- atmospheres during the catalysis processes [53]. In 3 4 ternative support material by virtue of their poros- 2012, Gates et al. reported that atomically dispersed ity and high surface area [55]. Li et al. used an im- gold atoms catalyse with a high degree of uniformity pregnation method to access isolated Au atoms an- supported on zeolite NaY [71]. The site-isolated chored on polymeric mesoporous graphitic C N gold complexes retained after CO-oxidation cataly- 3 4 sis, confirming the robust stabilization effect of the (mpg-C N )[74]. The catalytically active Au atom 3 4 zeolite channels for gold species. was coordinated by three nitrogen or carbon atoms The addition of alkali ions, such as sodium in tri-s-triazine repeating units. This coordination or potassium, on inert KLTL-zeolite and meso- feature significantly prevents the Au atoms from ag- porous MCM-41 silica materials could structurally gregation and makes the Au surface highly active. stabilize the single gold sites in Au–O(OH) en- Moreover, they demonstrated this catalyst as highly x– sembles (Fig. 3b), as demonstrated by Flytzani- active, selective and stable for silane oxidation with Stephanopoulos and co-workers [72]. They have water. shown evidence that the active catalyst was com- In 2017, Ma et al. developed a highly efficient catalyst consisting of isolated Pt atoms uniformly posed of alkali ions linked to the gold atom through dispersed on an α-molybdenum carbide (α-MoC) –O ligands, not merely on the support, making the support that can enable low-temperature, base-free reducible oxide supports no longer an essential re- hydrogen production through aqueous-phase re- quirement. The validation tests show that the single- forming of methanol [75]. They found that the site gold atoms were homogeneously dispersed and α-MoC displays stronger interactions with Pt than highly active for the industrially important low- other oxide supports or β-Mo C; therefore, atom- temperature water–gas shift reaction. ically dispersed Pt atoms can be formed on an In addition to metal oxides and zeolites, other α-MoC support following a high-temperature acti- supports such as nitrides and carbides have also vation process. This generates an exceptionally high- been explored and shown promise for stabilizing SAs density electron-deficient surface to stabilize Pt sites for use in catalysis. Lee et al. described a Pt SAC for the adsorption/activation of methanol. This cat- supported on titanium nitride (TiN) nanoparticles alyst affords an excellent turnover frequency and with the aid of chlorine ligands [73]. H PtCl ·6H O 2 6 2 was dissolved in anhydrous ethanol and mixed the corresponding hydrogen production greatly ex- with acid-treated TiN nanoparticles before the re- ceeds those of previously reported catalysts for low- sulting sample was dried and reduced. Transmis- temperature aqueous-phase reforming of methanol. sion electron microscopy (TEM) and HAADF- They deduce that the unique structure of α-MoC, STEM images of the samples are shown in Fig. 3c. which affects water dissociation, and the synergic Count percentage (%) Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 679 effects between Pt and α-MoC together affect the ac- supported on ethylene glycolate (EG)-stabilized tivation of methanol and the subsequent reforming ultrathin TiO nanosheets (Pd /TiO catalyst) 2 1 2 process. with a Pd loading up to 1.5% [82]. Typically, In 2017, Wu et al. reported a novel synthetic two-atom-thick TiO nanosheets were prepared by approach to construct isolated single Ru atoms on reacting TiCl with EG and used as the support. nitrogen-doped porous carbon (Ru SAs/N–C) by H PtCl was then added to the TiO dispersion for 2 6 2 a coordination-assisted strategy using MOFs for the adsorption of Pd species followed by irradiation by hydrogenation of quinolones [76]. It is noticed that UV to give the Pd /TiO catalyst. TEM, STEM and 1 2 the strong coordination effect between the lone EXAFS revealed that the isolated Pd atoms were pair of nitrogen and d-orbital of Ru atoms is cru- evenly dispersed over the TiO support, without any cial for the formation of stable Ru SAs (Fig. 3d). observable evidence of NPs (Fig. 4a). The catalyst Without the dangling −NH groups, the Ru atoms exhibited excellent catalytic performance in the hy- tend to aggregate into nanoclusters, even confined drogenation of C = C bonds, outperforming those in the pores of MOFs. The results demonstrate the commercial Pd catalysts. In addition, there was Ru SAs serve as an effective semi-homogeneous cat- no observable decay in the catalytic activity for alyst to the chemoselective catalyse hydrogenation 20 cycles, suggesting the robustness of the of quinolones. This method has been shown to po- Pd /TiO catalyst. Importantly, they found 1 2 tentially broaden the substrate scope for the synthe- this catalyst can activate H in a heterolytic pathway sis of SACs with unique properties for use in various to drastically enhance its catalytic activity in the chemical reactions. hydrogenation of aldehydes. This mechanism Together, the ease of preparation for SACs us- has been commonly observed for homogeneous ing a wet-chemistry strategy envisages a promising catalysts, such as Au, Pd and Ru complexes; future in the field. However, these methods have however, there is no report for heterogeneous their own disadvantages. For example, some metal Pd catalysts. This study set a good example using atoms might be buried either in the interfacial re- atomically dispersed metal catalysts for bridging gions of the support agglomerates or within the bulk the gap between heterogeneous and homogeneous of the support when co-precipitation methods are catalysis. applied [43]. In addition, when high metal loading Very recent work by Wu and co-workers showed is required for the construction of SACs, aggregation a novel synthetic approach to accessing atomically would inevitably occur [9]. This trade-off should be dispersed platinum species on mesoporous carbon minimized by developing new synthesis methods. via iced-photochemical reduction of frozen chloro- Other methodologies have also been ex- platinic acid solution (Fig. 4b) [83]. In this report, plored to design and synthesize SACs with varies H PtCl solution was first frozen by liquid nitro- 2 6 chemical and physical functionalities and future gen followed by irradiation using a UV lamp. The underpinned studies in these directions. The photo- H PtCl ice was kept overnight in dark conditions 2 6 chemical method becomes particularly appealing to at room temperature to give a clear aqueous Pt assist the effective adsorption of SAs on the supports single-atom solution. Then mesoporous carbon so- and has been proven to be effective for the synthesis lution and Pt single-atom solution were mixed, fil- of nanocrystals, such as gold, silver, platinum, tered, and dried at room temperature. Finally, the palladium, etc. [77–80]. In this process, regulating ice lattice naturally confines the dispersed ions and the nucleation and growth processes of nanocrystals atoms to affect the photochemical reduction prod- has been a major topic. Flytzani-Stephanopoulos ucts and further prevent the aggregation of atoms. et al. constructed isolated gold atoms supported To test the generality of this concept, they also fab- on titania with a loading of approximately 1 wt% ricated isolated Pt atoms deposited on different sup- under ultraviolet (UV) irradiation [81]. They found ports, including mesoporous carbon, graphene, car- that the addition of ethanol can serve as a charge bon nanotubes, TiO nanoparticles and zinc oxide scavenger to facilitate the donation of electrons from nanowires. Among them, the isolated Pt atoms sup- gold atoms to −OH groups on the titania support. ported on mesoporous carbon exhibited exceptional The catalytic performance was examined and the catalytic performance for hydrogen evolution reac- results showed that this catalyst displayed excellent tion, as well as an excellent long-time durability, out- activity for the low-temperature water–gas shift performing the commonly employed Pt/carbon cat- reaction, as well as admirable stability in long-term alyst. This iced-photochemical reduction approach cool-down and startup operations. provides a promising avenue for the green synthe- An important study by Zheng et al. demon- sis of SAs and sub-nanometer clusters, and opens strated a room-temperature photochemical strategy up possibilities for fine-tuning the nucleation and to construct atomically dispersed palladium atoms growth of nanocrystals in wet chemistry. Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 680 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW Other techniques-assisted adsorption Photochemical strategy Iced-photochemical reduction strategy (a) (b) UV light H PtCl solution Pt nanoparticles 2 6 Pd-Pd Pd /TiO 1 2 Fitting Pd-O Pd foil UV light x1/3 1 2 3 4 5 Freeze Pt single atoms R (A) ALD strategy (c) (d) Ball milling strategy CH Fe foil CH Pt CH CH H C 3 FeSi Fe-Fe CH 3 Pt Pt o o CH CH MeCpPtMe HCH C 3 Fe O 3 3 2 3 3 Fe-Si Graphene Graphene MeCpPtMe MeCpPtMe 3 3 Fe-O O ALD Pt on GNS Fe foil Fe-C FeSi Fe O 2 3 C (1) Fresh Fe SiO ×2 CH CH 3 3 (2) In-situ Fe SiO ×2 Pt Pt o o (1) Fresh Fe SiO C CH CH H C H C ×2 3 3 (3) In-situ Fe/SiO 3 3 (2) In-situ Fe SiO 2 Pt Pt Pt Pt (3) In-situ Fe/SiO MeCpPtMe 2 3 7100 7110 7120 7130 7140 7150 7160 7170 -1 0 1 2 3 4 Energy (eV) R (A) Graphene Graphene Figure 4. (a) Structural characterizations of Pd /TiO catalyst. Adapted with permission from [82]. (b) Schematic illustration of the iced-photochemical 1 2 process compared with the conventional photochemical reduction of H PtCl aqueous solution. Adapted with permission from [83]. (c) Structural features 2 6 of 0.5% Fe SiO . Adapted with permission from [39]. (d) Schematic illustrations of the Pt ALD mechanism on graphene nanosheets. Adapted with permission from [86]. of methane, exclusively to ethylene and aromat- Recently, high-energy bottom-up ball-milling ics. The presence of single Fe sites effectively pre- synthesis has been proved as a powerful method vented catalytic C-C coupling, oligomerization and to break and reconstruct chemical bonds of ma- coke deposition. In addition, this catalyst showed terials with high efficiency. Such an approach was extremely stable performance, with no deactivation taken by Bao et al., who reported a lattice-confined observed during long-term testing, and the selec- single iron site catalyst embedded within a sil- tivity for total carbon of the three products was ica matrix by a solid fusion method. Briefly, com- retained. Subsequently, the group used the same mercial SiO and Fe SiO were mixed and sub- 2 2 4 jected to ball milling under argon and fused in method to construct single-atom iron sites by em- the air [39]. As expected, the unsaturated single bedding highly dispersed FeN centers in graphene Fe sites served as active centers (Fig. 4c) to effi- matrix via high-energy ball milling of iron phthalo- ciently enable the direct, non-oxidative conversion cyanine and graphene nanosheets [84]. In this Normalized absorbance (a.u.) * 2 FT χ(k) k 3 -3 FT k χ(k) (A ) Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 681 In 2015, Lu et al. described a single-atom Galvanic replacement (a) Pd /graphene catalyst prepared by the ALD (b) 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 reduc- tion process via thermal de-oxygenation to control the surface oxygen functional groups. After an an- Pt foil nealing step, phenolic oxygen was observed to be the Pt Cu /Al O 0.1 14 2 3 dominated oxygen species on the graphene support. ALD was then performed on the reduced graphene Pt Cu /Al O 0.2 12 2 3 to give a single-atom Pd catalyst by alternately ex- Pt Cu /Al O 2 6 2 3 posing Pd(hfac) and formalin. This catalyst showed superior catalytic performance in the selective hy- 0 2 4 6 R (A) drogenation of 1,3-butadiene, affording nearly 100% butenes selectivity, and ∼70% selectivity for Figure 5. (a) Characterization of Pt/Cu SAA NPs. Adapted with permission from [50]. (b) 1-butene at a conversion ratio of 95% under mild Scanning tunneling microscope image of a 0.01 ML Pt/Cu(111) SAA surface. Adapted conditions. They speculate that both the mono- π- with permission from [90]. 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 system, the FeN center is highly dispersed and well selectivity of butenes. In addition, the Pd /graphene stabilized by the graphene matrix. The formation of showed remarkable durability against deactivation the Fe = O intermediate is important in promoting via either metal atom aggregation or coking during the conversion of benzene to phenol. Remarkably, a 100-h reaction time on stream. this reaction can proceed efficiently at mild condi- Using the same strategy, Sun and co-workers tions such as room temperature or even as low as described the preparation of isolated single Pt 0 C. DFT calculations confirm that the catalytic ac- atoms and clusters on nitrogen-doped graphene tivity stems from the confined iron sites, along with nanosheets (NGNs) [88]. Here, Pt was first de- moderate activation barriers for the reaction that posited on the NGNs by the ALD technique using proceeded at room temperature. Both studies clearly show the potential of the highly efficient ball-milling MeCpPtMe and O as precursors and N as a purg- 3 2 2 method for the fabrication of SACs for use in cataly- ing gas and a carrier gas. The size, density and dis- sis areas. tribution of the Pt atoms on the NGNs or graphene The atomic layer deposition (ALD) technique is nanosheets (GNs) can be precisely controlled by the a gas-phase chemical process and commonly used ALD cycles. As expected, the isolated Pt atoms and to deposit a thin layer of film in a bottom-up fash- clusters on the NGNs have been demonstrated to ion with near-atomic precision on the substrate show superior catalytic activity and stability for the by repeated exposure of separate precursors [85]. hydrogen evolution reaction (HER) compared with This technique offers the feasibility of precise con- the conventional Pt NP catalysts. This can be ex- plained by the small size and the special electronic trol of the catalyst size from a single-atom, sub- structure of the adsorbed single Pt atoms on NGNs. nanometer cluster to the nanoparticle. It is expected Together, the use of the ALD technique has shown that ALD would potentially provide a powerful ap- great promise for large-scale synthesis of highly ac- proach for the construction of intriguing SACs. This tive and stable single-atom and cluster catalysts. 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 The galvanic-replacement method was deposited on graphene supports by the ALD method using MeCpPtMe and oxygen as precur- Galvanic replacement is a highly versatile and effec- sors and nitrogen as a purge gas. The resulting Pt tive approach for the construction of a variety of SAC showed improved catalytic activity compared nanostructures, with the ability to control the size with the commercial Pt/C catalyst. X-ray absorption and shape, composition, internal structure and mor- fine structure (XAFS) analyses show that the low- phology [24,57,89]. It is an electrochemical process coordination and partially unoccupied 5d orbital of that consists of oxidation of one metal, termed as a Pt atoms are responsible for the excellent catalytic sacrificial template, by other metal ions that have a performance. higher reduction potential. When they are exposed -4 χ (A ) Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 682 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW to each other in solution, the sacrificial metal tem- TOP-DOWN SYNTHETIC plate will be preferably oxidized and dissolved into METHODOLOGIES FOR THE the solution, while the ions of the second metal CONSTRUCTION OF SACS will be reduced and deposited onto the template The top-down strategy is based on the dissolution surface. of ordered nanostructures into smaller pieces to In 2015, Sykes et al. demonstrated that low give desired properties and intriguing performances concentrations of isolated Pt atoms in the Cu(111) [59,91]. Extensive research efforts have pursued this surface (Fig. 5a) can be prepared by galvanic replace- strategy with the overarching aim of synthesizing ment on pre-reduced Cu NPs to catalyse the butadi- SACs with unprecedented chemical and physical ene hydrogenation with remarkable activity and high properties and understanding the complex mecha- selectivity to butenes [50]. In this case, Cu NPs were nisms for catalysis that occur at the atomic level. first prepared and supported on γ -Al O followed 2 3 This strategy has proven particularly useful in the by calcination in air. The galvanic-replacement re- formation of SACs with accurate control over the action was then carried out in an aqueous so- micro- or nanostructures [92]. The precise structure lution under nitrogen protection with constant (such as coordination number, dispersion tenden- stirring and refluxing. A desired amount of Pt pre- cies and binding mode) of metal SAs synthesized by cursor was introduced to a suspension of Cu NPs in the top-down methods has shown great promise in an aqueous solution containing HCl. The resulting industrially important applications [9,89,93,94]. Ef- material was filtered, washed and dried to yield the forts to further understand the underlying features catalyst. They notice that, at low Pt loadings, the iso- and mechanisms are required for the development of lated Pt atoms can substitute into the Cu(111) sur- new methods for the construction of SACs and rep- face to activate the dissociation and spillover of H resent a fertile area for future studies. to Cu. The weak binding between butadiene and Cu would facilitate the highly selective hydrogenation reaction to butenes, without decomposition or poi- The high-temperature pyrolysis method soning of the catalysts. This catalyst, with less than one Pt atom per 100 copper atoms, also binds CO High-temperature pyrolysis has become one of the more weakly than metallic Pt, which is particularly fascinating methods for synthesizing nanomaterials important for use in many Pt-catalysed chemical on different supports. Particularly, the development reactions. of a template-sacrificial approach via acid leaching or In a follow-up report, the Sykes group used the oxidative calcination has offered an alternative way same approach to construct Pt/Cu single-atom al- to generate SACs. Of note is that an appropriate py- loys (SAAs) to examine C–H activation in differ- rolysis temperature is critically important to give the ent systems, including methyl groups, methane and desired properties. butane [90]. They observed that the Pt atoms were MOFs and zeolitic imidazolate frameworks distributed over the Cu surface and across both ter- (ZIFs) have interconnected 3D molecular-scale races and at regions near step edges (Fig. 5b). The cages that make them highly accessible through results show the Pt/Cu SAAs activate C–H bonds small apertures. Importantly, they can serve as more efficiently than Cu, along with superior stabil- templates to obtain nitrogen-doped porous carbon ity under realistic operating conditions, effectively with abundant active nitrogen sites. Very recently, avoiding the coking problem that typically occurred Wu et al. took advantage of the MOFs and originally with Pt. Both pieces of work from the Sykes group developed an effective strategy for accessing single demonstrated how SAs can be deposited on alloys— Co atoms supported on nitrogen-doped porous an important future direction for this field. carbon with a particularly high metal loading of Though a variety of SACs have been developed over 4 wt% via the pyrolysis of bimetallic Zn/Co by the bottom-up strategy, the downside of the MOFs [57]. This is pioneering work in this field methods described here is that it is still challenging and the strategy is particularly applicable to access to access SACs with high metal loading and a ho- high-loading metal SACs that would otherwise be mogeneous coordination environment for the active difficult to produce. It should be noted that the sites used in the catalytic process. This would lead to enhancement of metal loading for preparing SACs limited selectivity and stability of the SACs for their in the present study is a significant breakthrough practical use in various industrial fields. In addition, in this area, highlighting the specific requirement although ground-breaking, some of these methods of SACs for practical applications. Importantly, the do require specific/sophisticated preparation proce- introduction of Zn atoms into MOFs is critical and dures that might not be compatible with all kinds of acts as an elegant approach to efficiently manipulate SACs and ideal from practical perspectives. the adjacent spatial distance between Co atoms, Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 683 High-temperature pyrolysis (b) (a) Co CNZn Carbonization Reduction Evaporation dCo-Co O Zn Ni 2+ Ni Ionic Absorption Exchange (d) (c) Fe(acac) Zn(NO ) 3 2 OH 2-Methylimidazole Pyrolysis NaOH Pyrolysis Fe 24 h, DMF H /Ar Leaching Fe(acac) @ZIF-8 Fe-ISAs/CN 3 H CC SiO N o Br Volatilization (f) (e) Fe Adsorption Dopamine Carbonization Acid leaching Graphitization Tris HCI PH 8.5 Pyrolysis α-FeOOH@PDA Fe/FeO@CN SA-Fe/CN α-FeOOH nanorod Kirkendall Effect C N Co Zn Fe Cl Figure 6. (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]. thereby effectively preventing the formation of Co catalysts. Robust chemical stability during electro- NPs (Fig. 6a). The Zn atoms, with a low boiling catalysis and thermal stability that resists sintering point of 907 C, can be evaporated in the high- at a high temperature of 900 C have also been con- temperature pyrolysis process, providing abundant firmed, as little evidence of catalyst degradation was observed during the catalytic cycles. This work has N sites. The Co nodes can be reduced in situ by underlined the significant importance of employing carbonization of the organic linkers in MOFs and MOFs as an ideal carbon support for stabilizing sin- anchored on the as-obtained N-doped porous car- gle metal atoms at the atomic scale. bon support. Assuming the MOF as an integrated Subsequently, an ionic exchange strategy was de- system, using this high-temperature pyrolysis of veloped by the Wu group to assist in the construc- MOF to access unsaturated SAs anchored on the tion of a single Ni atom catalyst (Fig. 6b) between N-doped porous carbon support can be catego- Zn nodes and adsorbed Ni ions within the cavities rized into the top-down approach. Control testing of the MOF [95]. In this case, ZIF-8 was first dis- demonstrated that the aggregated Co atoms were persed in n-hexane under ultrasound until a homo- formed for Co-containing MOF (ZIF-67) after a geneous solution was formed. Then a small amount pyrolysis treatment. HAADF-STEM and EXAFS of Ni(NO ) aqueous solution was introduced, and verified the presence of isolated Co atoms dispersed 3 2 the mixed solution was vigorously stirred to cause on the N-doped porous carbon support. The result- ing Co SAC shows exceptional oxygen-reduction the Ni ions to be absorbed completely. Then the reaction (ORR) catalytic performance with a half- sample was centrifuged and dried, followed by a wave potential more positive than the commercial high-temperature heating process in an argon at- Pt/C and most of the reported non-precious metal mosphere to yield Ni SAC. This Ni SAC, with a Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 684 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW metal weight loading of 1.53 wt%, delivered an ex- After a pyrolysis step, the ZIF-8 was transformed cellent turnover frequency for CO electroreduction into nitrogen-doped porous carbon, whereas the −1 of 5273 h , along with a maximum Faradaic effi- Fe(acac) within the cage was reduced by carboniza- ciency for CO production of 71.9% and a high cur- tion of the organic linker, resulting in the formation −2 rent density of 10.48 mA cm . This work, for the of isolated iron atoms anchored on nitrogen species. first time, demonstrates the great potential of using The catalyst has been demonstrated to show excep- MOF-based materials to access SACs for use in CO tional ORR catalytic activity, good methanol toler- electroreduction. ance and impressive stability. Importantly, the ORR To investigate the relationship between catalytic activity of this SAC outperforms those of coordination numbers and CO electroreduction recently reported Fe-bases materials and other non- catalytic performance, the Wu group sequentially precious metal materials. Experimental results and prepared a series of Co SACs with different N DFT calculations reveal the excellent ORR perfor- coordination environments treated at different mance stems from the formation of atomically iso- temperatures [96]. Bimetallic Co/Zn ZIFs were lated iron atoms coordinated with four N atoms and treated by a pyrolysis process, during which the one O molecule adsorbed end-on. Zn was evaporated away and the Co was reduced Using a similar approach, Li et al. described by carbonized organic linkers, generating isolated the synthesis of atomically dispersed Ru clus- Co atoms stabilized on nitrogen-doped carbon. By ters via a cage-separated precursor pre-selection controlling the pyrolysis temperatures, three Co and pyrolysis strategy [98]. Generally, two steps SACs with different Co–N coordination numbers are involved: (i) encapsulation and separation of were obtained, being Co–N (800 C), Co–N preselected metal cluster precursors followed by 4 3 ◦ ◦ (900 C), and Co–N (1000 C), respectively. (ii) a pyrolysis treatment. The resulting catalyst was The catalytic performance of these samples was characterized by HAADF-STEM and XAFS, and examined, and the results show that the isolated the catalytic performance was tested for the oxida- Co atom with two coordinated nitrogen atoms tion of 2-amino-benzyl alcohol. The results show (prepared at 1000 C) can afford significantly that this Ru /nitrogen-doped carbon (CN) catalyst higher selectivity and superior activity, resulting in possesses 100% conversion, 100% selectivity and a CO formation Faradaic efficiency of 94% and a an unexpectedly high turnover frequency (TOF), −2 current density of 18.1 mA cm at an overpotential outperforming those of Ru SACs and small-sized Ru of 520 mV. Importantly, this catalyst achieved a particle catalysts. −1 turnover frequency for CO formation of 18 200 h , An alternative approach to the thermal treatment outperforming most of the reported metal-based of MOFs for achieving SACs has been employed by catalysts under comparable conditions. DFT cal- Li et al., who used SiO as a template to access a hol- culation reveals that the decreased N coordination low N-doped carbon sphere with isolated Co atomic environment leads to more unoccupied 3d orbitals sites (Fig. 6d) [99]. Briefly, the SiO template was for Co atoms, thereby facilitating adsorption of dispersed in Co–TIPP/TIPP solution before intro- CO and increasing CO electroreduction per- ducing another monomer. The collected powder 2 2 formance. This study demonstrates the significant was thermally treated under a flowing H /Ar and effect of N coordination environments on SACs for then etched with sodium hydroxide to remove the catalytic performance. SiO template to yield the Co SAC. Its ORR per- The above studies further confirm the great po- formance was investigated and the results demon- tential of high-temperature pyrolysis of MOFs as a strate that exceptional catalytic activity was origi- promising strategy to access SACs for different de- nated from the single Co sites that can significantly manding industrial applications. facilitate the proton and charge transfer to the ad- With these attractive features, Li and co-workers sorbed OH species. Using the same approach, a Mo prepared a highly stable isolated Fe atom catalyst, SAC was prepared by the Li group using sodium with Fe loading up to 2.16 wt%, that showed excel- molybdate and chitosan as precursors and showed lent ORR reactivity via a cage-encapsulated precur- excellent HER performance [100]. Further studies sor pyrolysis approach [97]. This method is highly of the structure of the catalyst were supported by effective to access SACs because the precursors can AC-STEM and XAFS, which confirmed that the Mo be encapsulated inside the ZIF pores, thereby pre- atom was anchored with one nitrogen atom and two venting them from aggregating into nanoparticles carbon atoms (Mo N C ). 1 1 2 (Fig. 6c). In this study, Fe(acac) was mixed with In 2016, Zhang et al. described a similar template- ZIF-8, and the molecular-scale cages were formed sacrificial approach to create a self-supporting 2+ with the assembly of Zn and 2-methylimidazole, Co–N–C catalyst with single-atom dispersion with one Fe(acac) molecule trapped in one cage. and showed excellent catalytic activity for the 3 Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 685 chemoselective hydrogenation of nitroarenes to showed a high conversion of 45% and an excellent yield azo compounds under mild conditions [101]. selectivity of 94% for the hydroxylation of benzene In this study, the Co(phen) (OAc) complex was to phenol, outperforming Fe nanoparticles/CN. 2 2 supported on Mg(OH) and then subjected to a Notably, in a most recent research, Wu et al. pyrolysis process. This was followed by the removal originally developed a host–guest strategy based of the MgO support by an acid-leaching treatment. on MOFs to construct a Fe–Co dual-sites cata- The merit of employing Mg(OH) is that it can lyst embedded in N-doped porous carbon support essentially prevent the aggregation of cobalt atoms. [104]. It involves binding between Co nodes and ad- This is because of the moderate interaction between sorbed Fe ions within the confined space of MOFs Mg(OH) and the Co species, as well as its inertness (Fig. 6f). Specifically, Zn/Co bimetallic MOF was towards the reaction with Co during the pyrolysis employed as a host to encapsulate FeCl within 3+ process. After the acid-leaching step, the support the cavities by a double-solvents method. The Fe material was removed to give a self-supporting species were reduced by the as-generated carbon and Co–N–C material. X-ray absorption spectroscopy bond with the neighboring Co atoms. Meanwhile, 3+ was tested and the exact structure of the catalyst was the adsorbed Fe species can accelerate the de- confirmed to be CoN C –1-2O .Specifically,the composition of metal–imidazolate–metal linkages 4 8 2 Co single atom was coordinated with four pyridinic and generate voids inside the MOF. EXAFS and nitrogen atoms on the graphitic layer, along with Mossbauer ¨ spectroscopic analyses were performed oxygen atoms weakly adsorbed on the Co atoms to investigate the coordination environment of the perpendicular to the Co–N plane. Fe–Co dual sites. The experimental results show that Using the same approach, Zhang et al. prepared FeCoN is the active site for the (Fe, Co)/N–C cat- an atomically dispersed Fe−N−C catalyst, which alyst and has been demonstrated to endow excel- exhibited exceptional activity and excellent reusabil- lent ORR performance in an acidic electrolyte, along ity for the selective oxidation of the C−Hbond, with comparable onset potential and half-wave po- along with tolerance for a wide scope of substrates tential to those of the commercial Pt/C. DFT calcu- [102]. Briefly, the Fe(phen) complex supported lation reveals that the activation of O–O is favored on the nano-MgO template was pyrolysed at differ- on the dual sites, which is important for the four- ent temperatures under N atmosphere, followed by electron oxygen-reduction process. The fuel cell test- an acid-leaching step to remove the MgO template. ing revealed that this catalyst outperforms most of They observed that the properties of the Fe species the reported Pt-free catalysts in H /O and H /air 2 2 2 were dependent on the pyrolysis temperature, with conditions. In addition, this cathode catalyst is rather more metallic Fe particles formed at higher tem- robust in long-term operation for electrode mea- peratures. The critical role of the Fe −N sites in surement and H /air single cell testing. Of note x 2 catalysis was further confirmed by potassium thio- is that, despite the fact that SACs generally confer cyanate titration experiments and Mossbauer ¨ spec- greater activity than the corresponding nanoparti- troscopy. cles, it is still important to be aware of the poten- An effective core–shell strategy has been in- tial aggregation pathways available to them. This is troduced by the Li group using metal hydroxides especially crucial in cases where higher operational or oxides coated with polymers followed by high- temperatures were applied. Therefore, the superior temperature pyrolysis and acid-leaching steps, catalytic activity, selectivity, stability and the ease of to synthesize single metal atoms anchored on fabrication of the dual-sites Fe–Co catalyst make this the inner wall of hollow CN materials [103]. By type of SAC truly remarkable. Importantly, the main employing different metal precursors or polymers, advantages of this host–guest strategy include the they have successfully synthesized a series of ability to incorporate different metal atoms and to metal SAs dispersed on CN materials (Fig. 6e). permit the catalyst to be operated in a wider dynamic In detail, α-FeOOH nanorods were first pre- range. This study is expected to provide avenues pared by a hydrothermal method, followed by for the synthesis of high-performance dual-sites cat- self-polymerizing dopamine monomers to generate alysts with unique properties for use in chemical α-FeOOH@PDA. Then it was thermally treated transformations. under an inert atmosphere, during which the Overall, these studies have shown that the high- polydopamine (PDA) layers were converted into temperature pyrolysis method is capable of produc- the CN shell and α-FeOOH was reduced to iron, ing SACs with precisely controlled structures and giving rise to the strong interaction between the Fe morphologies. Additionally, this unique approach atoms and the CN shell. Finally, acid leaching was has been seen as a significant opportunity to enable carried out to generate Fe SAs on the inner wall of the efficient construction of high-performance SACs the CN materials. The obtained SA-Fe/CN catalyst for use in various reactions. Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 686 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW the synthesis of SACs. In this review, we summarize High-temperature atomic migration the progress, bring new insights from recent years Fresh Aged 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 de- velopment of highly efficient heterogeneous cata- lysts. The researchers have shown evidence that the size of the nanomaterials does affect catalytic effi- ciency 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. More- over, the homogeneously and isolated metal active sites can maximize metal utilization, giving rise to the impressively enhanced catalytic performance. Figure 7. Schematic illustration of Pt nanoparticle sintering, showing how ceria can Recent experimental and theoretical progress has trap the mobile Pt to suppress sintering. Adapted with permission from [105]. unambiguously validated the strong evidence for the high activity, selectivity and stability of the high- The high-temperature atomic-migration performance SACs. These intriguing properties of method SACs are believed to endow great potential for ap- High temperatures are generally detrimental to cat- plications in heterogeneous catalysis. Importantly, alysts’ activities. Although the SAs are homoge- SACs can act as an ideal platform to serve as a neously dispersed on the support materials, they bridge to connect hetero- and homogeneous catal- have a high propensity to move and aggregate ysis. Thus, SACs are thought to have the potential into nanoparticles when heated at high tempera- to overcome the difficulty encountered in homoge- tures. Datye and co-workers take advantage of the neous catalysis. phenomenon that metal nanoparticles can emit mo- As discussed previously, a major limiting fac- bile species to prepare atomically dispersed metal tor in the development of SACs is the lack of gen- catalysts [105]. In this study, a Pt/La-Al O cat- 2 3 eral methods to directly and efficiently access high- alyst was physically mixed with different types of performance SACs. The construction of SACs for ceria powders followed by a thermal treatment in use in catalysis represents an important challenge, flowing air. Because of the strong interaction be- highlighting the need for more fundamental re- tween PtO and ceria powders, the Pt species emit- search into detailed mechanisms. Along with the ted from the alumina were trapped on the CeO , emergence of new characterization and computa- forming thermally stable Pt /CeO SACs (Fig. 7). 1 2 tional modeling techniques, single-atom active sites The performance of the resulting SAC was tested can be investigated further. More advanced, direct for CO oxidation, and the results suggest that it can and effective in situ spectroscopic and microscopic serve as a highly effective sintering-resistant CO- techniques become particularly important to offer oxidation catalyst at high temperature. They believe new insights into the chemical reactions involved that this atom-trapping approach is potentially appli- in SACs. Elucidating the important role of metal cable and might provide exciting possibilities to ac- precursors, support materials and experimental con- cess a variety of high-performance SACs. This work ditions and understanding the prerequisites for represents a novel strategy and has been demon- catalytic activity of a given catalytic system are cru- strated as being particularly effective in fabricating cial for developing effective strategies for the syn- SACs and connecting the relationship between the thesis of SACs. Several aspects should also be given nanoparticles and SAs. enough attention: first, the development of novel, controllable and facile synthesis methods for ac- cess high-loading SACs with finely and densely dis- CONCLUSIONS AND PERSPECTIVE persed single atoms; second, the construction of Over just a few years, there has been remarkable single metal atoms with robust stabilization on progress in the development of various methods for the support for use in practical conditions; third, Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 687 detailed experimental and theoretical work should 11. Boles MA, Ling D and Hyeon T et al. The surface science of be done to comprehensively understand SACs- nanocrystals. Nat Mater 2016; 15: 141–53. support effects. The top-down strategy has shown 12. Xia BY, Wu HB and Wang X et al. One-pot synthesis of cubic great promise and significantly contributed to the PtCu nanocages with enhanced electrocatalytic activity for simplified synthesis routes for SACs with excep- the methanol oxidation reaction. J Am Chem Soc 2012; 134: tional activity and stability. Moreover, the metal 13934–7. loading can be markedly increased from 1% to 5%, 13. Christopher P, Xin H and Linic S. Visible-light-enhanced cat- and the coordination environments can be elabo- alytic oxidation reactions on plasmonic silver nanostructures. rately controlled. This will definitely facilitate the de- Nat Chem 2011; 3: 467–72. velopment of general protocols for accessing SACs 14. Wu Y, Wang D and Li Y. Nanocrystals from solutions: catalysts. and underpin the exploration of other intriguing ap- Chem Soc Rev 2014; 43: 2112–24. plications. 15. Lu Q, Wang AL and Gong Y et al. Crystal phase-based epitax- Together, the field of SAs is expansive and rapidly ial growth of hybrid noble metal nanostructures on 4H/fcc Au developing towards different applied research fields. nanowires. Nat Chem 2018; 10: 456–61. The continued development of SACs represents an 16. Fan Z and Zhang H. Crystal phase-controlled synthesis, proper- important advancement in heterogeneous catalysis ties and applications of noble metal nanomaterials. Chem Soc and will surely be the important focus of extensive Rev 2016; 45: 63–82. research efforts and a thriving field for various appli- 17. Chen Y, Fan Z and Luo Z et al. High-yield synthesis of crystal- cations for years to come. phase-heterostructured 4H/fcc Au@Pd core-shell nanorods for electrocatalytic ethanol oxidation. Adv Mater 2017; 29: 1701331–5. FUNDING 18. Fan Z and Zhang H. 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Recent advances in the precise control of isolated single-site catalysts by chemical methods

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Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 National Science Review 5: 673–689, 2018 REVIEW doi: 10.1093/nsr/nwy056 Advance access publication 1 June 2018 MATERIALS SCIENCE Special Topic: Single-Atom Catalysts Recent advances in the precise control of isolated single-site catalysts by chemical methods 1,† 3,† 1,∗ 1,2,∗ Zhijun Li , Dehua Wang , Yuen Wu and Yadong Li 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 1 their potential use in the industrial area. In this regard, a novel top-down strategy has been recently Department of developed to fabricate SACs to address these practical issues. The metal loading can be increased to 5% and Chemistry, iChEM the coordination environments can also be precisely controlled. This review highlights approaches to the (Collaborative Innovation Center of chemical synthesis of SACs towards diverse chemical reactions, especially the recent advances in improving Chemistry for Energy the mass activity and well-defined local structures of SACs. Also, challenges and opportunities for the SACs Materials), University will be discussed in the later part. of Science and Technology of China, Keywords: single-atom catalysts, bottom-up, top-down, catalytic performance Hefei 230026, China; Department of in heterogeneous catalysis; however, to greatly in- INTRODUCTION Chemistry, Tsinghua crease the turnover frequency of surface active sites University, Beijing In the worldwide theme of exploring efficient and and to enhance the mass activity remain the primary 100084, China and low-cost technologies for energy conversion and goals in catalysis [9,10]. In most circumstances, it School of chemical transformations, substantial effort has been has been demonstrated that the surface atoms of the Pharmaceutical and devoted to the development of general, practical and Chemical Engineering, nanomaterials in an unsaturated coordination envi- simple chemical approaches for catalyst preparation Taizhou University, ronment generally act as the active sites to catalyse in past decades [1–6]. Studies have shown that ul- Taizhou 318000, China specific reactions [ 11]. Therefore, extensive stud- trasmall assemblies, compared to their macroscopic ies have been devoted to rationally controlling the counterparts [7], can exhibit essentially different shapes, structures, crystal phases and compositions Corresponding physical and chemical properties. These unique of nanocatalysts [3,8,12–18]. authors. E-mails: properties would drastically alter their practical ap- With decreasing the size of nanomaterials, the [email protected]; plications in a variety of areas, such as cataly- number of surface atoms is increased substantially, [email protected] sis, biomedical research, energy and environmental Equally contributed exposing more defects and active sites, tuning fields [ 6]. Therefore, metal nanoparticles represent to this work. geometric and electronic properties involved in a rich resource for a variety of chemical processes, chemical reactions [10]. Nanoclusters have shown employed both in industry and in academia [8]. intriguing properties because of the reduced size The maximized surface area of support, increased Received 21 March compared to nanoparticles, exposing more uncoor- 2018; Revised 10 number of catalytic active sites, minimized catalyst dinated active sites and changing molecular orbital May 2018; Accepted loading and strong catalyst-support interaction de- energy levels [19,20]. Heiz and co-workers found 31 May 2018 termine the nature of nanocatalysts [2,6]. The sup- a pronounced size effect for model catalysts of ported metal nanoparticles are frequently employed The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For permissions, plea se e-mail: [email protected] Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 674 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW size-selected Pd (n ≤ 30) clusters supported on tivity of allylic alcohols [31]. The key discovery was MgO(100) for the cyclotrimerization of acetylene that the employment of homogeneously dispersed to benzene [21]. Anderson et al. studied size- SACs generally confers a dramatic improvement in selected palladium clusters and deposited them on catalytic activity, selectivity and stability, or even the rutile phase of titanium dioxide (TiO )for CO considerably different catalytic properties than oxidation [22]. By employing X-ray photoemission the corresponding nanoparticles and nanoclusters spectroscopy and temperature-programmed reac- [32–35]. This is highly desirable and has attracted tion measurements, they found that the activity of extensive scientific attention, as they might poten- these catalysts was associated with Pd binding tially act as alternatives to circumvent the problems 3d energy. Li et al. utilized a double-solvent method of scarcity and high cost of the noble-metal catalysts combined with a photoreduction process to prepare used in large-scale catalysis applications [36]. active Pd nanoclusters encapsulated inside the cage Specifically, the active single-atom sites are well of NH -Uio-66 [23]. The resultant catalyst showed defined and atomically stabilized on the supports, exceptional performance for a Suzuki coupling re- and the identical geometric structure of each active action under visible-light irradiations. Nevertheless, site is similar to that of a homogeneous catalyst. although the sizes of the nanoclusters have been Recently, studies have clearly demonstrated that reduced, their multiple distributions of active metal the utility and uniqueness of these SACs have great sites alongside different geometric and electric potential to bridge the gap between homogeneous structures might not be ideal for specific catalytic and heterogeneous catalysis [37–42]. This would reactions [9,10]. The stability of nanoclusters would solve the problems of the difficulty in separating also be a problem for their application in hetero- the homogeneous catalysts from raw materials and geneous catalysis, especially at higher operational products, as well as combining the merits of both temperatures [24]. hetero- and homogeneous catalysts. The SACs Further downsizing nanoclusters to the atomic also provide a good avenue to identify the detailed level, namely single atoms (SAs), maximum atom structural features for the active sites and an ideal utilization and superior/distinguishing catalytic model to elucidate the structure–activity relation- performance are supposed to be obtained [9,10]. ship [43–45]. Such catalysts have shown intriguing Of note is the fact that unexpected superior cat- interests in the catalysis field [ 10,32,34,43,46–49]. alytic performances of the single-atom catalysts To meet the practical demand, the most important (SACs) are often observed as one of the key challenges for fabricating the SACs are to increase advances of these novel catalysts versus their the density of active sites and to improve their nanoscale counterparts. This can be ascribed to intrinsic activities [32,36,45]. The first challenge is the unsaturated environments of metal active the high propensity for aggregation of SAs once the sites, quantum size effects and metal–support size of the nanomaterials is greatly reduced [10]. interactions [25–28]. Therefore, the research on The second challenge is the rational control of the SACs has rapidly progressed from their funda- coordination environment of the single metal atoms. mental aspects to pursing practical applications Recently, several strategies for constructing in areas of nanotechnology and materials science. atomically dispersed metal sites on catalyst supports A study that has attracted considerable attention have been extensively studied [9,10,32,42]. These since its publication in 1995 is that of Thomas strategies include enhancing the metal–support et al., who reported that direct grafting of interactions, engineering vacancy defects and voids organometallic complexes onto the walls of on the supports, and modifying surface functional mesoporous silica gives a shape-selective high- groups [9,42]. In most cases, the supports for performance catalyst with well-separated, homoge- isolated SACs are chosen on purpose, as they can neously dispersed and high surface concentrations stabilize the isolated catalytic SAs or activate nearby of active sites for the epoxidation of cyclohexenes reactants to form intermediate species for the and their derivatives [29]. Later, in 2003, Flytzani- catalytic active sites [50–52]. For example, zeolites Stephanopoulos et al. discovered that the water–gas could provide effective voids to anchor individual shift reaction was not affected by the catalytic metal atoms to maintain the high dispersion of activity of metallic Au or Pt nanoparticles; instead, the isolated metal atoms and prevent them from nonmetallic Au or Pt species on the ceria surface sintering at high temperatures under oxidative or played a key role in this reaction [30]. In 2007, Lee reductive atmospheres during catalysis processes et al. successfully synthesized Pd-Al O catalyst and [53]. Nanoparticles and nanoclusters can also serve 2 3 validated that the extremely low metal loading leads as supports. Through elegant studies of support II to the formation of atomically isolated Pd species, materials, Sykes et al. showed that the isolated which greatly contribute to the excellent selox ac- Pd atoms can be supported on a Cu surface and Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 Top-down REVIEW Li et al. 675 because of their high propensity for aggregation and the difficulty in constructing homogeneous coor- dination environments for the reactive sites. These drawbacks lead to limited selectivity and stability of the SACs, greatly limiting their potential use in Co-precipitation various industrial fields. In this regard, Li and Wu proposed a top-down strategy to construct SACs Adsorption Galvanic by the pyrolysis of metal nodes in metal-organic SACs replacement frameworks (MOFs) for the first time [ 57]. In this case, the introduction of Zn atoms into MOFs is im- portant and can effectively prevent the formation of Co NPs (nanoparticles) during the high- temperature pyrolysis process. The resulting High-temp High-temp Co SAC has a high metal loading close to 5% and pyrolysis migration showed exceptional chemical and thermal stability. A distinguishing feature of this strategy is not only that the metal loading can be substantially increased Figure 1. Schematic representation of the bottom-up and from 1% to 5%, which is important from practical top-down strategies for the synthesis of SACs. perspectives, but it can control the coordination environments to construct high-performance SACs by exposing real active sites. This top-down significantly lower the energy barrier to hydrogen strategy overcome challenges in the fabrication of uptake on and subsequent desorption from the SACs with a traditional bottom-up strategy and nearby Cu atoms [48]. This facile hydrogen disso- has great potential to meet the requirement for ciation at the isolated Pd atoms and weak binding use in practical applications. In addition to the to the Cu surface together facilitate selective hydro- fabrication strategies, the use of these methods in genation of styrene and acetylene. Toshima et al. different chemical reactions will also be presented. described a crown-jewel concept for the construc- Finally, future challenges and opportunities will be tion of catalytically highly active top gold atoms discussed. on palladium nanoclusters [54]. Interestingly, the gold atoms can be controllably assembled at the top position on the cluster and exhibit high catalytic ac- tivity because of their high negative-charge density BOTTOM-UP SYNTHETIC and unique structure. Recent reports have begun METHODOLOGIES FOR THE to document that the defects in reducible oxides CONSTRUCTION OF SACS (e.g. TiO and CeO ), graphene or C N also help 2 2 3 4 to stabilize isolated metal atoms [32,42,55]. For The bottom-up strategy is the most common example, Du et al. investigated the favorable role of method to synthesize metal SACs, during which isolated palladium and platinum atoms supported the metal precursors are adsorbed, reduced and on graphitic carbon nitride (g-C N )toactas confined by the vacancies or defects of the supports 3 4 photocatalysts for CO reduction [56]. Overall, [9,10,32,52]. Nevertheless, how to effectively an important conclusion derived from these works increase the SACs loading with well-defined dis- is that the further development of this SACs field persion on the supports is still challenging. First, requires a more fundamental understanding of SA aggregation would occur during a chemical synthe- formation at the atomic scale. sis or catalytic process when high loading of SAs This review covers the preparation strategies is required. Second, the architectural structures of for SACs, which can be categorized according to anchor sites for confining and stabilizing the metal how their components are integrated, namely via SACs on the support remain elusive; therefore, the bottom-up and top-down approaches (Fig. 1). coordination environment for metal SAs might be Currently, a large majority of SACs are synthesized inhomogeneous and poorly defined [ 58]. Optimiza- via a bottom-up strategy by using oxides or carbon tion of the precursors and supports and controlling supports to construct N or O defects to enable the of the synthetic procedures play a key role in tuning deposition of metal precursors. This is followed by a the metal–support interaction and guaranteeing the chemical reduction process to generate SACs from homogeneous dispersion of SACs. high-oxidation-state ions to low oxidation state. For the wet-chemistry strategy, the precursor so- However, the following drawbacks are encountered lutions of mononuclear metal complexes are first frequently: difficulty in accessing high metal loading anchored to the supports by a coordination effect Bottom-up Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 676 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW Co-precipitation 3.0 PtO ( (b) b) (a) 2.5 0.08% Pt/FeOx R200 0.08% Pt/FeOx R250 0.75% Pt/FeOx R250 2.0 2.73% Pt/FeOx R250 4.30% Pt/FeOx R250 1.5 Pt foil 1.0 0.5 0.0 11.540 11.560 11.580 11.600 11.620 Energy (eV) (c) () Pt foil PtO Sample A × Sample B PtO Sample A (used) Sample A Sample B Pt foil 11.55 11.60 11.65 0 1 2 3 4 5 6 E (keV) R (A) Figure 2. (a) HAADF-STEM images of Pt1/FeO . Adapted with permission from [46]. (b) HAADF-STEM image of 0.08%Pt/FeO -R200. Adapted with x x permission from [60]. (c) HAADF-STEM images of Ir /FeO . Adapted with permission from [61]. 1 x between the metal complexes and the functional SAC showed extremely high atom efficiency, excel- groups of the support surfaces [32]. Then, the or- lent stability and superior activity for both CO oxi- ganic ligands of the metal complexes are removed by dation and preferential oxidation of CO in H .They a post-treatment to expose more active sites to meet found that these merits can be attributed to the par- the requirement of catalytic reactions. Particularly, tially vacant 5d orbitals of the positively charged the advantage of wet chemistry for preparing SACs high-valent Pt atoms, as they can effectively reduce is that this method does not require specialized CO-adsorption energy and activation barriers that equipment and can be routinely practiced in any are required for CO oxidation. This study demon- chemistry lab [59]. strated the feasibility of using the defects of oxide supports to serve as anchoring sites for metal clus- ters and single metal atoms. Subsequently, the fea- Co-precipitation approach sibility and efficiency of this approach were further Co-precipitation is one of the commonly employed demonstrated by the Zhang group showing that the approaches for preparing SACs, during which the high-performance Pt- and Ir-based SACs could also be obtained (Fig. 2b and c) for use in organic trans- substances that are normally soluble under the formation [60] and water–gas shift reactions [ 61]. conditions would be precipitated. A significant ad- In these examples, defects in the oxide supports and vantage of this method lies in its extreme simplicity, the amount of metal loading were found to be criti- as no additional complicated steps are involved. cal for accessing high-performance SACs that would For a classical example, Zhang et al. employed this normally lead to aggregation. method to fabricate single Pt atoms supported on iron oxide nanocrystallites (Pt /FeO )[46]. 1 x The metal precursor of H PtCl ·H Owas mixed 2 6 2 Adsorption approach with Fe(NO ) ·9H O in a proper molar ratio and 3 3 2 pH. After recovery, the precipitate was dried and The adsorption method is one of the most funda- calcined, resulting in the formation of Pt /FeO . mental approaches for constructing isolated metal 1 x The aberration-corrected scanning transmission atoms on the supports [62,63]. It is simple, direct electron microscopy (AC-STEM) and extended and has been widely used in the preparation of sup- X-ray absorption fine structure (EXAFS) spec- ported metal catalysts. Generally, after the metal pre- tra demonstrated the individual Pt atoms were cursors are adsorbed on the support, the residual so- uniformly dispersed on FeO support, with a lution is removed and then the catalysts are dried metal loading level of 0.17 wt% (Fig. 2a). This and calcined. To ensure the SAs could be stably Intensity (a.u.) Intensity (a.u.) Normalized absorption (a.u.) Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 677 anchored onto the supports with atomic disper- In a more recent piece of work, Wang and co- sion, appropriate functional groups on the supports workers described a convenient two-step synthesis should be given consideration. of an atomically dispersed Pt catalyst supported on Oxides are generally employed as an efficient ceria (CeO ), with 1 wt.% metal loading, by wetness support for preparing catalysts. In 2013, Narula impregnation and steam treatment [68]. Chloropla- et al. [64] reported single Pt atoms supported tinic acid was added drop-wise to the CeO support on θ-Al O (010) prepared by a wet-impregnation while being ground in a mortar and pestle. The as- 2 3 method using alumina powder and chloroplatinic obtained powder was then dried, calcined, thermal acid. In this work, water was gradually evapo- aged and stream treated to give the catalyst. The au- rated before the resulting powder was transferred thors demonstrated that the activation of SACs on to an alumina crucible and subjected to a pyrol- CeO via high-temperature steam treatment can ac- ysis process. The resultant catalyst was catalyti- complish excellent low-temperature CO-oxidation cally active in its ability to oxidize CO to CO .In activity and superior thermal stability. This is be- addition, serials of Pt/θ-Al O catalysts with dif- cause the steam treatment can enable the formation 2 3 ferent metal loadings were prepared, and the re- of active surface lattice oxygen near isolated Pt atoms sults reveal that they are highly active towards NO to considerably enhance catalytic performance. Fur- oxidation [65]. ther investigation of the nature of this active surface In the same year, Tao et al. [66] developed lattice oxygen on Pt/CeO was supported by den- an impregnation-reduction method for preparing sity functional theory (DFT) calculations and re- singly dispersed Rh atoms supported on Co O action kinetic analyses. They found the oxygen va- 3 4 nanorods. This method involves the impregnation cancies from the CeO bulk can redistribute to the 3+ of Rh on Co O nanorods followed by on-site CeO (111) surface when exposed to water at a high 3 4 2 3+ reduction of Rh using NaBH . In situ character- temperature. During the steam-treatment process, izations reveal evidence of the active sites of iso- H O molecules can fill out the oxygen vacancy over lated Rh atoms in the formation of RhCo on Co O the atomically dispersed Pt/CeO surface, affording n 3 4 2 nanorods, which were generated through restructur- two neighboring active O [H] sites around Pt. lattice ing of Rh /Co O at 220 C in reactant gases. The This provides the significantly improved reactivity 1 3 4 resulting new catalytic phase exhibits a high selectiv- and stability. ity to produce N in the reduction of NO with H Yan and co-workers developed a unique adsorp- 2 2 ◦ ◦ between 180 C and 300 C. tion approach to construct Pt SACs, anchored in the A report by Li et al. demonstrated that single Pt internal surface of mesoporous Al O ,byamodified 1 2 3 and Au atoms can be stabilized by lattice oxygen sol-gel solvent vaporization self-assembly method on ZnO{1010} surface via an adsorption method [69], as shown in Fig. 3a. Triblock copolymers [67]. In detail, ZnO-nanowires (nws) were dis- P123, C H AlO and H PtCl were first mixed 9 21 3 2 6 persed in de-ionized water followed by the addition in ethanol. With continued evaporation of the sol- of H PtCl ·6H OorHAuCl solution. After an ag- vent, the amphiphilic P123 macromolecules and 2 6 2 4 ing process, the suspension was filtered, washed and C H AlO assembled into a highly ordered hexag- 9 21 3 dried to give Pt /ZnO and Au /ZnO catalysts. Simi- onally arranged mesoporous structure, with Pt pre- 1 1 larly, Zhang et al. fabricated an Rh SAC supported on cursor encapsulated in the matrix. The as-obtained ZnO nws by introducing RhCl solution into ZnO gel was then calcined in air to decompose the P123 nws that were dispersed in de-ionized water [40]. template. Meanwhile, the C H AlO was trans- 9 21 3 After stirring and aging processes, the resulting pre- formed into a rigid, well-aligned mesoporous Al O 2 3 cipitate was filtered, washed, dried and reduced. framework. This was followed by a reducing step As the weight loading of Rh reduced from 0.03% in 5% H /N to give the isolated Pt SAs stabilized 2 2 3+ to 0.006%, the isolated Rh SACs can be clearly by the unsaturated pentahedral Al centers. The observed. During the synthetic process, the Rh authors showed that the catalyst retained its struc- atoms bond with proximal Zn atoms which lose tural integrity and exceptional catalytic performance one or more O atoms. Therefore, electrons trans- in several reactions under harsh conditions, such as fer from metallic Zn to Rh atoms to generate hydrogenation of 1,3-butadiene after exposure to a near-metallic Rh species. The results show that the reductive atmosphere at 200 C for 24 h, n-hexane as-obtained Rh /ZnO-nws SACs exhibited compa- hydroreforming at 550 C for 48 h and CO oxida- ◦ ◦ rable efficiency in the hydroformylation of several tion after 60 cycles between 100 C and 400 C over olefins to the homogeneous Wilkinson’s catalyst, 1 month. along with superior catalytic activity to those of the Zeolites are crystalline materials with well- most highly reported heterogeneous nanoparticle- defined structures and high surface area, along with based catalysts. more sites for robust bonding with catalytic species Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 678 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW Adsorption 2- (a) a) (d) PtCl Al(OCHCH CH ) 6 3 3 3 Pt C H O CI AI P123 Self-assembly Calcination Complexing Reduction Pyrolysis HF etching Gel 0.2Pt/m-Al O -H 2 3 2 C N ZrO Ru Single atoms (c) (b) Nano clusters 0.2-0.3 0.4-0.5 0.6-0.9 ~1.0 nm Figure 3. (a) Schematic illustration of the 0.2Pt/m-Al O -H synthesis process. Adapted with permission from [69]. (b) HAAD–STEM images of the 2 3 2 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]. The results show that the 0.35 wt% Pt/TiN sample [24,70]. Specifically, zeolites could provide effective affords a high mass activity and a unique selectivity voids to anchor individual metal atoms to maintain towards electrochemical oxygen reduction, formic the high dispersion and prevent them from sintering acid oxidation and methanol oxidation. at high temperatures under oxidative or reductive Carbon nitride (C N ) has been proved as an al- atmospheres during the catalysis processes [53]. In 3 4 ternative support material by virtue of their poros- 2012, Gates et al. reported that atomically dispersed ity and high surface area [55]. Li et al. used an im- gold atoms catalyse with a high degree of uniformity pregnation method to access isolated Au atoms an- supported on zeolite NaY [71]. The site-isolated chored on polymeric mesoporous graphitic C N gold complexes retained after CO-oxidation cataly- 3 4 sis, confirming the robust stabilization effect of the (mpg-C N )[74]. The catalytically active Au atom 3 4 zeolite channels for gold species. was coordinated by three nitrogen or carbon atoms The addition of alkali ions, such as sodium in tri-s-triazine repeating units. This coordination or potassium, on inert KLTL-zeolite and meso- feature significantly prevents the Au atoms from ag- porous MCM-41 silica materials could structurally gregation and makes the Au surface highly active. stabilize the single gold sites in Au–O(OH) en- Moreover, they demonstrated this catalyst as highly x– sembles (Fig. 3b), as demonstrated by Flytzani- active, selective and stable for silane oxidation with Stephanopoulos and co-workers [72]. They have water. shown evidence that the active catalyst was com- In 2017, Ma et al. developed a highly efficient catalyst consisting of isolated Pt atoms uniformly posed of alkali ions linked to the gold atom through dispersed on an α-molybdenum carbide (α-MoC) –O ligands, not merely on the support, making the support that can enable low-temperature, base-free reducible oxide supports no longer an essential re- hydrogen production through aqueous-phase re- quirement. The validation tests show that the single- forming of methanol [75]. They found that the site gold atoms were homogeneously dispersed and α-MoC displays stronger interactions with Pt than highly active for the industrially important low- other oxide supports or β-Mo C; therefore, atom- temperature water–gas shift reaction. ically dispersed Pt atoms can be formed on an In addition to metal oxides and zeolites, other α-MoC support following a high-temperature acti- supports such as nitrides and carbides have also vation process. This generates an exceptionally high- been explored and shown promise for stabilizing SAs density electron-deficient surface to stabilize Pt sites for use in catalysis. Lee et al. described a Pt SAC for the adsorption/activation of methanol. This cat- supported on titanium nitride (TiN) nanoparticles alyst affords an excellent turnover frequency and with the aid of chlorine ligands [73]. H PtCl ·6H O 2 6 2 was dissolved in anhydrous ethanol and mixed the corresponding hydrogen production greatly ex- with acid-treated TiN nanoparticles before the re- ceeds those of previously reported catalysts for low- sulting sample was dried and reduced. Transmis- temperature aqueous-phase reforming of methanol. sion electron microscopy (TEM) and HAADF- They deduce that the unique structure of α-MoC, STEM images of the samples are shown in Fig. 3c. which affects water dissociation, and the synergic Count percentage (%) Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 679 effects between Pt and α-MoC together affect the ac- supported on ethylene glycolate (EG)-stabilized tivation of methanol and the subsequent reforming ultrathin TiO nanosheets (Pd /TiO catalyst) 2 1 2 process. with a Pd loading up to 1.5% [82]. Typically, In 2017, Wu et al. reported a novel synthetic two-atom-thick TiO nanosheets were prepared by approach to construct isolated single Ru atoms on reacting TiCl with EG and used as the support. nitrogen-doped porous carbon (Ru SAs/N–C) by H PtCl was then added to the TiO dispersion for 2 6 2 a coordination-assisted strategy using MOFs for the adsorption of Pd species followed by irradiation by hydrogenation of quinolones [76]. It is noticed that UV to give the Pd /TiO catalyst. TEM, STEM and 1 2 the strong coordination effect between the lone EXAFS revealed that the isolated Pd atoms were pair of nitrogen and d-orbital of Ru atoms is cru- evenly dispersed over the TiO support, without any cial for the formation of stable Ru SAs (Fig. 3d). observable evidence of NPs (Fig. 4a). The catalyst Without the dangling −NH groups, the Ru atoms exhibited excellent catalytic performance in the hy- tend to aggregate into nanoclusters, even confined drogenation of C = C bonds, outperforming those in the pores of MOFs. The results demonstrate the commercial Pd catalysts. In addition, there was Ru SAs serve as an effective semi-homogeneous cat- no observable decay in the catalytic activity for alyst to the chemoselective catalyse hydrogenation 20 cycles, suggesting the robustness of the of quinolones. This method has been shown to po- Pd /TiO catalyst. Importantly, they found 1 2 tentially broaden the substrate scope for the synthe- this catalyst can activate H in a heterolytic pathway sis of SACs with unique properties for use in various to drastically enhance its catalytic activity in the chemical reactions. hydrogenation of aldehydes. This mechanism Together, the ease of preparation for SACs us- has been commonly observed for homogeneous ing a wet-chemistry strategy envisages a promising catalysts, such as Au, Pd and Ru complexes; future in the field. However, these methods have however, there is no report for heterogeneous their own disadvantages. For example, some metal Pd catalysts. This study set a good example using atoms might be buried either in the interfacial re- atomically dispersed metal catalysts for bridging gions of the support agglomerates or within the bulk the gap between heterogeneous and homogeneous of the support when co-precipitation methods are catalysis. applied [43]. In addition, when high metal loading Very recent work by Wu and co-workers showed is required for the construction of SACs, aggregation a novel synthetic approach to accessing atomically would inevitably occur [9]. This trade-off should be dispersed platinum species on mesoporous carbon minimized by developing new synthesis methods. via iced-photochemical reduction of frozen chloro- Other methodologies have also been ex- platinic acid solution (Fig. 4b) [83]. In this report, plored to design and synthesize SACs with varies H PtCl solution was first frozen by liquid nitro- 2 6 chemical and physical functionalities and future gen followed by irradiation using a UV lamp. The underpinned studies in these directions. The photo- H PtCl ice was kept overnight in dark conditions 2 6 chemical method becomes particularly appealing to at room temperature to give a clear aqueous Pt assist the effective adsorption of SAs on the supports single-atom solution. Then mesoporous carbon so- and has been proven to be effective for the synthesis lution and Pt single-atom solution were mixed, fil- of nanocrystals, such as gold, silver, platinum, tered, and dried at room temperature. Finally, the palladium, etc. [77–80]. In this process, regulating ice lattice naturally confines the dispersed ions and the nucleation and growth processes of nanocrystals atoms to affect the photochemical reduction prod- has been a major topic. Flytzani-Stephanopoulos ucts and further prevent the aggregation of atoms. et al. constructed isolated gold atoms supported To test the generality of this concept, they also fab- on titania with a loading of approximately 1 wt% ricated isolated Pt atoms deposited on different sup- under ultraviolet (UV) irradiation [81]. They found ports, including mesoporous carbon, graphene, car- that the addition of ethanol can serve as a charge bon nanotubes, TiO nanoparticles and zinc oxide scavenger to facilitate the donation of electrons from nanowires. Among them, the isolated Pt atoms sup- gold atoms to −OH groups on the titania support. ported on mesoporous carbon exhibited exceptional The catalytic performance was examined and the catalytic performance for hydrogen evolution reac- results showed that this catalyst displayed excellent tion, as well as an excellent long-time durability, out- activity for the low-temperature water–gas shift performing the commonly employed Pt/carbon cat- reaction, as well as admirable stability in long-term alyst. This iced-photochemical reduction approach cool-down and startup operations. provides a promising avenue for the green synthe- An important study by Zheng et al. demon- sis of SAs and sub-nanometer clusters, and opens strated a room-temperature photochemical strategy up possibilities for fine-tuning the nucleation and to construct atomically dispersed palladium atoms growth of nanocrystals in wet chemistry. Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 680 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW Other techniques-assisted adsorption Photochemical strategy Iced-photochemical reduction strategy (a) (b) UV light H PtCl solution Pt nanoparticles 2 6 Pd-Pd Pd /TiO 1 2 Fitting Pd-O Pd foil UV light x1/3 1 2 3 4 5 Freeze Pt single atoms R (A) ALD strategy (c) (d) Ball milling strategy CH Fe foil CH Pt CH CH H C 3 FeSi Fe-Fe CH 3 Pt Pt o o CH CH MeCpPtMe HCH C 3 Fe O 3 3 2 3 3 Fe-Si Graphene Graphene MeCpPtMe MeCpPtMe 3 3 Fe-O O ALD Pt on GNS Fe foil Fe-C FeSi Fe O 2 3 C (1) Fresh Fe SiO ×2 CH CH 3 3 (2) In-situ Fe SiO ×2 Pt Pt o o (1) Fresh Fe SiO C CH CH H C H C ×2 3 3 (3) In-situ Fe/SiO 3 3 (2) In-situ Fe SiO 2 Pt Pt Pt Pt (3) In-situ Fe/SiO MeCpPtMe 2 3 7100 7110 7120 7130 7140 7150 7160 7170 -1 0 1 2 3 4 Energy (eV) R (A) Graphene Graphene Figure 4. (a) Structural characterizations of Pd /TiO catalyst. Adapted with permission from [82]. (b) Schematic illustration of the iced-photochemical 1 2 process compared with the conventional photochemical reduction of H PtCl aqueous solution. Adapted with permission from [83]. (c) Structural features 2 6 of 0.5% Fe SiO . Adapted with permission from [39]. (d) Schematic illustrations of the Pt ALD mechanism on graphene nanosheets. Adapted with permission from [86]. of methane, exclusively to ethylene and aromat- Recently, high-energy bottom-up ball-milling ics. The presence of single Fe sites effectively pre- synthesis has been proved as a powerful method vented catalytic C-C coupling, oligomerization and to break and reconstruct chemical bonds of ma- coke deposition. In addition, this catalyst showed terials with high efficiency. Such an approach was extremely stable performance, with no deactivation taken by Bao et al., who reported a lattice-confined observed during long-term testing, and the selec- single iron site catalyst embedded within a sil- tivity for total carbon of the three products was ica matrix by a solid fusion method. Briefly, com- retained. Subsequently, the group used the same mercial SiO and Fe SiO were mixed and sub- 2 2 4 jected to ball milling under argon and fused in method to construct single-atom iron sites by em- the air [39]. As expected, the unsaturated single bedding highly dispersed FeN centers in graphene Fe sites served as active centers (Fig. 4c) to effi- matrix via high-energy ball milling of iron phthalo- ciently enable the direct, non-oxidative conversion cyanine and graphene nanosheets [84]. In this Normalized absorbance (a.u.) * 2 FT χ(k) k 3 -3 FT k χ(k) (A ) Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 681 In 2015, Lu et al. described a single-atom Galvanic replacement (a) Pd /graphene catalyst prepared by the ALD (b) 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 reduc- tion process via thermal de-oxygenation to control the surface oxygen functional groups. After an an- Pt foil nealing step, phenolic oxygen was observed to be the Pt Cu /Al O 0.1 14 2 3 dominated oxygen species on the graphene support. ALD was then performed on the reduced graphene Pt Cu /Al O 0.2 12 2 3 to give a single-atom Pd catalyst by alternately ex- Pt Cu /Al O 2 6 2 3 posing Pd(hfac) and formalin. This catalyst showed superior catalytic performance in the selective hy- 0 2 4 6 R (A) drogenation of 1,3-butadiene, affording nearly 100% butenes selectivity, and ∼70% selectivity for Figure 5. (a) Characterization of Pt/Cu SAA NPs. Adapted with permission from [50]. (b) 1-butene at a conversion ratio of 95% under mild Scanning tunneling microscope image of a 0.01 ML Pt/Cu(111) SAA surface. Adapted conditions. They speculate that both the mono- π- with permission from [90]. 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 system, the FeN center is highly dispersed and well selectivity of butenes. In addition, the Pd /graphene stabilized by the graphene matrix. The formation of showed remarkable durability against deactivation the Fe = O intermediate is important in promoting via either metal atom aggregation or coking during the conversion of benzene to phenol. Remarkably, a 100-h reaction time on stream. this reaction can proceed efficiently at mild condi- Using the same strategy, Sun and co-workers tions such as room temperature or even as low as described the preparation of isolated single Pt 0 C. DFT calculations confirm that the catalytic ac- atoms and clusters on nitrogen-doped graphene tivity stems from the confined iron sites, along with nanosheets (NGNs) [88]. Here, Pt was first de- moderate activation barriers for the reaction that posited on the NGNs by the ALD technique using proceeded at room temperature. Both studies clearly show the potential of the highly efficient ball-milling MeCpPtMe and O as precursors and N as a purg- 3 2 2 method for the fabrication of SACs for use in cataly- ing gas and a carrier gas. The size, density and dis- sis areas. tribution of the Pt atoms on the NGNs or graphene The atomic layer deposition (ALD) technique is nanosheets (GNs) can be precisely controlled by the a gas-phase chemical process and commonly used ALD cycles. As expected, the isolated Pt atoms and to deposit a thin layer of film in a bottom-up fash- clusters on the NGNs have been demonstrated to ion with near-atomic precision on the substrate show superior catalytic activity and stability for the by repeated exposure of separate precursors [85]. hydrogen evolution reaction (HER) compared with This technique offers the feasibility of precise con- the conventional Pt NP catalysts. This can be ex- plained by the small size and the special electronic trol of the catalyst size from a single-atom, sub- structure of the adsorbed single Pt atoms on NGNs. nanometer cluster to the nanoparticle. It is expected Together, the use of the ALD technique has shown that ALD would potentially provide a powerful ap- great promise for large-scale synthesis of highly ac- proach for the construction of intriguing SACs. This tive and stable single-atom and cluster catalysts. 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 The galvanic-replacement method was deposited on graphene supports by the ALD method using MeCpPtMe and oxygen as precur- Galvanic replacement is a highly versatile and effec- sors and nitrogen as a purge gas. The resulting Pt tive approach for the construction of a variety of SAC showed improved catalytic activity compared nanostructures, with the ability to control the size with the commercial Pt/C catalyst. X-ray absorption and shape, composition, internal structure and mor- fine structure (XAFS) analyses show that the low- phology [24,57,89]. It is an electrochemical process coordination and partially unoccupied 5d orbital of that consists of oxidation of one metal, termed as a Pt atoms are responsible for the excellent catalytic sacrificial template, by other metal ions that have a performance. higher reduction potential. When they are exposed -4 χ (A ) Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 682 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW to each other in solution, the sacrificial metal tem- TOP-DOWN SYNTHETIC plate will be preferably oxidized and dissolved into METHODOLOGIES FOR THE the solution, while the ions of the second metal CONSTRUCTION OF SACS will be reduced and deposited onto the template The top-down strategy is based on the dissolution surface. of ordered nanostructures into smaller pieces to In 2015, Sykes et al. demonstrated that low give desired properties and intriguing performances concentrations of isolated Pt atoms in the Cu(111) [59,91]. Extensive research efforts have pursued this surface (Fig. 5a) can be prepared by galvanic replace- strategy with the overarching aim of synthesizing ment on pre-reduced Cu NPs to catalyse the butadi- SACs with unprecedented chemical and physical ene hydrogenation with remarkable activity and high properties and understanding the complex mecha- selectivity to butenes [50]. In this case, Cu NPs were nisms for catalysis that occur at the atomic level. first prepared and supported on γ -Al O followed 2 3 This strategy has proven particularly useful in the by calcination in air. The galvanic-replacement re- formation of SACs with accurate control over the action was then carried out in an aqueous so- micro- or nanostructures [92]. The precise structure lution under nitrogen protection with constant (such as coordination number, dispersion tenden- stirring and refluxing. A desired amount of Pt pre- cies and binding mode) of metal SAs synthesized by cursor was introduced to a suspension of Cu NPs in the top-down methods has shown great promise in an aqueous solution containing HCl. The resulting industrially important applications [9,89,93,94]. Ef- material was filtered, washed and dried to yield the forts to further understand the underlying features catalyst. They notice that, at low Pt loadings, the iso- and mechanisms are required for the development of lated Pt atoms can substitute into the Cu(111) sur- new methods for the construction of SACs and rep- face to activate the dissociation and spillover of H resent a fertile area for future studies. to Cu. The weak binding between butadiene and Cu would facilitate the highly selective hydrogenation reaction to butenes, without decomposition or poi- The high-temperature pyrolysis method soning of the catalysts. This catalyst, with less than one Pt atom per 100 copper atoms, also binds CO High-temperature pyrolysis has become one of the more weakly than metallic Pt, which is particularly fascinating methods for synthesizing nanomaterials important for use in many Pt-catalysed chemical on different supports. Particularly, the development reactions. of a template-sacrificial approach via acid leaching or In a follow-up report, the Sykes group used the oxidative calcination has offered an alternative way same approach to construct Pt/Cu single-atom al- to generate SACs. Of note is that an appropriate py- loys (SAAs) to examine C–H activation in differ- rolysis temperature is critically important to give the ent systems, including methyl groups, methane and desired properties. butane [90]. They observed that the Pt atoms were MOFs and zeolitic imidazolate frameworks distributed over the Cu surface and across both ter- (ZIFs) have interconnected 3D molecular-scale races and at regions near step edges (Fig. 5b). The cages that make them highly accessible through results show the Pt/Cu SAAs activate C–H bonds small apertures. Importantly, they can serve as more efficiently than Cu, along with superior stabil- templates to obtain nitrogen-doped porous carbon ity under realistic operating conditions, effectively with abundant active nitrogen sites. Very recently, avoiding the coking problem that typically occurred Wu et al. took advantage of the MOFs and originally with Pt. Both pieces of work from the Sykes group developed an effective strategy for accessing single demonstrated how SAs can be deposited on alloys— Co atoms supported on nitrogen-doped porous an important future direction for this field. carbon with a particularly high metal loading of Though a variety of SACs have been developed over 4 wt% via the pyrolysis of bimetallic Zn/Co by the bottom-up strategy, the downside of the MOFs [57]. This is pioneering work in this field methods described here is that it is still challenging and the strategy is particularly applicable to access to access SACs with high metal loading and a ho- high-loading metal SACs that would otherwise be mogeneous coordination environment for the active difficult to produce. It should be noted that the sites used in the catalytic process. This would lead to enhancement of metal loading for preparing SACs limited selectivity and stability of the SACs for their in the present study is a significant breakthrough practical use in various industrial fields. In addition, in this area, highlighting the specific requirement although ground-breaking, some of these methods of SACs for practical applications. Importantly, the do require specific/sophisticated preparation proce- introduction of Zn atoms into MOFs is critical and dures that might not be compatible with all kinds of acts as an elegant approach to efficiently manipulate SACs and ideal from practical perspectives. the adjacent spatial distance between Co atoms, Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 683 High-temperature pyrolysis (b) (a) Co CNZn Carbonization Reduction Evaporation dCo-Co O Zn Ni 2+ Ni Ionic Absorption Exchange (d) (c) Fe(acac) Zn(NO ) 3 2 OH 2-Methylimidazole Pyrolysis NaOH Pyrolysis Fe 24 h, DMF H /Ar Leaching Fe(acac) @ZIF-8 Fe-ISAs/CN 3 H CC SiO N o Br Volatilization (f) (e) Fe Adsorption Dopamine Carbonization Acid leaching Graphitization Tris HCI PH 8.5 Pyrolysis α-FeOOH@PDA Fe/FeO@CN SA-Fe/CN α-FeOOH nanorod Kirkendall Effect C N Co Zn Fe Cl Figure 6. (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]. thereby effectively preventing the formation of Co catalysts. Robust chemical stability during electro- NPs (Fig. 6a). The Zn atoms, with a low boiling catalysis and thermal stability that resists sintering point of 907 C, can be evaporated in the high- at a high temperature of 900 C have also been con- temperature pyrolysis process, providing abundant firmed, as little evidence of catalyst degradation was observed during the catalytic cycles. This work has N sites. The Co nodes can be reduced in situ by underlined the significant importance of employing carbonization of the organic linkers in MOFs and MOFs as an ideal carbon support for stabilizing sin- anchored on the as-obtained N-doped porous car- gle metal atoms at the atomic scale. bon support. Assuming the MOF as an integrated Subsequently, an ionic exchange strategy was de- system, using this high-temperature pyrolysis of veloped by the Wu group to assist in the construc- MOF to access unsaturated SAs anchored on the tion of a single Ni atom catalyst (Fig. 6b) between N-doped porous carbon support can be catego- Zn nodes and adsorbed Ni ions within the cavities rized into the top-down approach. Control testing of the MOF [95]. In this case, ZIF-8 was first dis- demonstrated that the aggregated Co atoms were persed in n-hexane under ultrasound until a homo- formed for Co-containing MOF (ZIF-67) after a geneous solution was formed. Then a small amount pyrolysis treatment. HAADF-STEM and EXAFS of Ni(NO ) aqueous solution was introduced, and verified the presence of isolated Co atoms dispersed 3 2 the mixed solution was vigorously stirred to cause on the N-doped porous carbon support. The result- ing Co SAC shows exceptional oxygen-reduction the Ni ions to be absorbed completely. Then the reaction (ORR) catalytic performance with a half- sample was centrifuged and dried, followed by a wave potential more positive than the commercial high-temperature heating process in an argon at- Pt/C and most of the reported non-precious metal mosphere to yield Ni SAC. This Ni SAC, with a Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 684 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW metal weight loading of 1.53 wt%, delivered an ex- After a pyrolysis step, the ZIF-8 was transformed cellent turnover frequency for CO electroreduction into nitrogen-doped porous carbon, whereas the −1 of 5273 h , along with a maximum Faradaic effi- Fe(acac) within the cage was reduced by carboniza- ciency for CO production of 71.9% and a high cur- tion of the organic linker, resulting in the formation −2 rent density of 10.48 mA cm . This work, for the of isolated iron atoms anchored on nitrogen species. first time, demonstrates the great potential of using The catalyst has been demonstrated to show excep- MOF-based materials to access SACs for use in CO tional ORR catalytic activity, good methanol toler- electroreduction. ance and impressive stability. Importantly, the ORR To investigate the relationship between catalytic activity of this SAC outperforms those of coordination numbers and CO electroreduction recently reported Fe-bases materials and other non- catalytic performance, the Wu group sequentially precious metal materials. Experimental results and prepared a series of Co SACs with different N DFT calculations reveal the excellent ORR perfor- coordination environments treated at different mance stems from the formation of atomically iso- temperatures [96]. Bimetallic Co/Zn ZIFs were lated iron atoms coordinated with four N atoms and treated by a pyrolysis process, during which the one O molecule adsorbed end-on. Zn was evaporated away and the Co was reduced Using a similar approach, Li et al. described by carbonized organic linkers, generating isolated the synthesis of atomically dispersed Ru clus- Co atoms stabilized on nitrogen-doped carbon. By ters via a cage-separated precursor pre-selection controlling the pyrolysis temperatures, three Co and pyrolysis strategy [98]. Generally, two steps SACs with different Co–N coordination numbers are involved: (i) encapsulation and separation of were obtained, being Co–N (800 C), Co–N preselected metal cluster precursors followed by 4 3 ◦ ◦ (900 C), and Co–N (1000 C), respectively. (ii) a pyrolysis treatment. The resulting catalyst was The catalytic performance of these samples was characterized by HAADF-STEM and XAFS, and examined, and the results show that the isolated the catalytic performance was tested for the oxida- Co atom with two coordinated nitrogen atoms tion of 2-amino-benzyl alcohol. The results show (prepared at 1000 C) can afford significantly that this Ru /nitrogen-doped carbon (CN) catalyst higher selectivity and superior activity, resulting in possesses 100% conversion, 100% selectivity and a CO formation Faradaic efficiency of 94% and a an unexpectedly high turnover frequency (TOF), −2 current density of 18.1 mA cm at an overpotential outperforming those of Ru SACs and small-sized Ru of 520 mV. Importantly, this catalyst achieved a particle catalysts. −1 turnover frequency for CO formation of 18 200 h , An alternative approach to the thermal treatment outperforming most of the reported metal-based of MOFs for achieving SACs has been employed by catalysts under comparable conditions. DFT cal- Li et al., who used SiO as a template to access a hol- culation reveals that the decreased N coordination low N-doped carbon sphere with isolated Co atomic environment leads to more unoccupied 3d orbitals sites (Fig. 6d) [99]. Briefly, the SiO template was for Co atoms, thereby facilitating adsorption of dispersed in Co–TIPP/TIPP solution before intro- CO and increasing CO electroreduction per- ducing another monomer. The collected powder 2 2 formance. This study demonstrates the significant was thermally treated under a flowing H /Ar and effect of N coordination environments on SACs for then etched with sodium hydroxide to remove the catalytic performance. SiO template to yield the Co SAC. Its ORR per- The above studies further confirm the great po- formance was investigated and the results demon- tential of high-temperature pyrolysis of MOFs as a strate that exceptional catalytic activity was origi- promising strategy to access SACs for different de- nated from the single Co sites that can significantly manding industrial applications. facilitate the proton and charge transfer to the ad- With these attractive features, Li and co-workers sorbed OH species. Using the same approach, a Mo prepared a highly stable isolated Fe atom catalyst, SAC was prepared by the Li group using sodium with Fe loading up to 2.16 wt%, that showed excel- molybdate and chitosan as precursors and showed lent ORR reactivity via a cage-encapsulated precur- excellent HER performance [100]. Further studies sor pyrolysis approach [97]. This method is highly of the structure of the catalyst were supported by effective to access SACs because the precursors can AC-STEM and XAFS, which confirmed that the Mo be encapsulated inside the ZIF pores, thereby pre- atom was anchored with one nitrogen atom and two venting them from aggregating into nanoparticles carbon atoms (Mo N C ). 1 1 2 (Fig. 6c). In this study, Fe(acac) was mixed with In 2016, Zhang et al. described a similar template- ZIF-8, and the molecular-scale cages were formed sacrificial approach to create a self-supporting 2+ with the assembly of Zn and 2-methylimidazole, Co–N–C catalyst with single-atom dispersion with one Fe(acac) molecule trapped in one cage. and showed excellent catalytic activity for the 3 Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 685 chemoselective hydrogenation of nitroarenes to showed a high conversion of 45% and an excellent yield azo compounds under mild conditions [101]. selectivity of 94% for the hydroxylation of benzene In this study, the Co(phen) (OAc) complex was to phenol, outperforming Fe nanoparticles/CN. 2 2 supported on Mg(OH) and then subjected to a Notably, in a most recent research, Wu et al. pyrolysis process. This was followed by the removal originally developed a host–guest strategy based of the MgO support by an acid-leaching treatment. on MOFs to construct a Fe–Co dual-sites cata- The merit of employing Mg(OH) is that it can lyst embedded in N-doped porous carbon support essentially prevent the aggregation of cobalt atoms. [104]. It involves binding between Co nodes and ad- This is because of the moderate interaction between sorbed Fe ions within the confined space of MOFs Mg(OH) and the Co species, as well as its inertness (Fig. 6f). Specifically, Zn/Co bimetallic MOF was towards the reaction with Co during the pyrolysis employed as a host to encapsulate FeCl within 3+ process. After the acid-leaching step, the support the cavities by a double-solvents method. The Fe material was removed to give a self-supporting species were reduced by the as-generated carbon and Co–N–C material. X-ray absorption spectroscopy bond with the neighboring Co atoms. Meanwhile, 3+ was tested and the exact structure of the catalyst was the adsorbed Fe species can accelerate the de- confirmed to be CoN C –1-2O .Specifically,the composition of metal–imidazolate–metal linkages 4 8 2 Co single atom was coordinated with four pyridinic and generate voids inside the MOF. EXAFS and nitrogen atoms on the graphitic layer, along with Mossbauer ¨ spectroscopic analyses were performed oxygen atoms weakly adsorbed on the Co atoms to investigate the coordination environment of the perpendicular to the Co–N plane. Fe–Co dual sites. The experimental results show that Using the same approach, Zhang et al. prepared FeCoN is the active site for the (Fe, Co)/N–C cat- an atomically dispersed Fe−N−C catalyst, which alyst and has been demonstrated to endow excel- exhibited exceptional activity and excellent reusabil- lent ORR performance in an acidic electrolyte, along ity for the selective oxidation of the C−Hbond, with comparable onset potential and half-wave po- along with tolerance for a wide scope of substrates tential to those of the commercial Pt/C. DFT calcu- [102]. Briefly, the Fe(phen) complex supported lation reveals that the activation of O–O is favored on the nano-MgO template was pyrolysed at differ- on the dual sites, which is important for the four- ent temperatures under N atmosphere, followed by electron oxygen-reduction process. The fuel cell test- an acid-leaching step to remove the MgO template. ing revealed that this catalyst outperforms most of They observed that the properties of the Fe species the reported Pt-free catalysts in H /O and H /air 2 2 2 were dependent on the pyrolysis temperature, with conditions. In addition, this cathode catalyst is rather more metallic Fe particles formed at higher tem- robust in long-term operation for electrode mea- peratures. The critical role of the Fe −N sites in surement and H /air single cell testing. Of note x 2 catalysis was further confirmed by potassium thio- is that, despite the fact that SACs generally confer cyanate titration experiments and Mossbauer ¨ spec- greater activity than the corresponding nanoparti- troscopy. cles, it is still important to be aware of the poten- An effective core–shell strategy has been in- tial aggregation pathways available to them. This is troduced by the Li group using metal hydroxides especially crucial in cases where higher operational or oxides coated with polymers followed by high- temperatures were applied. Therefore, the superior temperature pyrolysis and acid-leaching steps, catalytic activity, selectivity, stability and the ease of to synthesize single metal atoms anchored on fabrication of the dual-sites Fe–Co catalyst make this the inner wall of hollow CN materials [103]. By type of SAC truly remarkable. Importantly, the main employing different metal precursors or polymers, advantages of this host–guest strategy include the they have successfully synthesized a series of ability to incorporate different metal atoms and to metal SAs dispersed on CN materials (Fig. 6e). permit the catalyst to be operated in a wider dynamic In detail, α-FeOOH nanorods were first pre- range. This study is expected to provide avenues pared by a hydrothermal method, followed by for the synthesis of high-performance dual-sites cat- self-polymerizing dopamine monomers to generate alysts with unique properties for use in chemical α-FeOOH@PDA. Then it was thermally treated transformations. under an inert atmosphere, during which the Overall, these studies have shown that the high- polydopamine (PDA) layers were converted into temperature pyrolysis method is capable of produc- the CN shell and α-FeOOH was reduced to iron, ing SACs with precisely controlled structures and giving rise to the strong interaction between the Fe morphologies. Additionally, this unique approach atoms and the CN shell. Finally, acid leaching was has been seen as a significant opportunity to enable carried out to generate Fe SAs on the inner wall of the efficient construction of high-performance SACs the CN materials. The obtained SA-Fe/CN catalyst for use in various reactions. Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 686 Natl Sci Rev, 2018, Vol. 5, No. 5 REVIEW the synthesis of SACs. In this review, we summarize High-temperature atomic migration the progress, bring new insights from recent years Fresh Aged 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 de- velopment of highly efficient heterogeneous cata- lysts. The researchers have shown evidence that the size of the nanomaterials does affect catalytic effi- ciency 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. More- over, the homogeneously and isolated metal active sites can maximize metal utilization, giving rise to the impressively enhanced catalytic performance. Figure 7. Schematic illustration of Pt nanoparticle sintering, showing how ceria can Recent experimental and theoretical progress has trap the mobile Pt to suppress sintering. Adapted with permission from [105]. unambiguously validated the strong evidence for the high activity, selectivity and stability of the high- The high-temperature atomic-migration performance SACs. These intriguing properties of method SACs are believed to endow great potential for ap- High temperatures are generally detrimental to cat- plications in heterogeneous catalysis. Importantly, alysts’ activities. Although the SAs are homoge- SACs can act as an ideal platform to serve as a neously dispersed on the support materials, they bridge to connect hetero- and homogeneous catal- have a high propensity to move and aggregate ysis. Thus, SACs are thought to have the potential into nanoparticles when heated at high tempera- to overcome the difficulty encountered in homoge- tures. Datye and co-workers take advantage of the neous catalysis. phenomenon that metal nanoparticles can emit mo- As discussed previously, a major limiting fac- bile species to prepare atomically dispersed metal tor in the development of SACs is the lack of gen- catalysts [105]. In this study, a Pt/La-Al O cat- 2 3 eral methods to directly and efficiently access high- alyst was physically mixed with different types of performance SACs. The construction of SACs for ceria powders followed by a thermal treatment in use in catalysis represents an important challenge, flowing air. Because of the strong interaction be- highlighting the need for more fundamental re- tween PtO and ceria powders, the Pt species emit- search into detailed mechanisms. Along with the ted from the alumina were trapped on the CeO , emergence of new characterization and computa- forming thermally stable Pt /CeO SACs (Fig. 7). 1 2 tional modeling techniques, single-atom active sites The performance of the resulting SAC was tested can be investigated further. More advanced, direct for CO oxidation, and the results suggest that it can and effective in situ spectroscopic and microscopic serve as a highly effective sintering-resistant CO- techniques become particularly important to offer oxidation catalyst at high temperature. They believe new insights into the chemical reactions involved that this atom-trapping approach is potentially appli- in SACs. Elucidating the important role of metal cable and might provide exciting possibilities to ac- precursors, support materials and experimental con- cess a variety of high-performance SACs. This work ditions and understanding the prerequisites for represents a novel strategy and has been demon- catalytic activity of a given catalytic system are cru- strated as being particularly effective in fabricating cial for developing effective strategies for the syn- SACs and connecting the relationship between the thesis of SACs. Several aspects should also be given nanoparticles and SAs. enough attention: first, the development of novel, controllable and facile synthesis methods for ac- cess high-loading SACs with finely and densely dis- CONCLUSIONS AND PERSPECTIVE persed single atoms; second, the construction of Over just a few years, there has been remarkable single metal atoms with robust stabilization on progress in the development of various methods for the support for use in practical conditions; third, Downloaded from https://academic.oup.com/nsr/article/5/5/673/5026635 by DeepDyve user on 15 July 2022 REVIEW Li et al. 687 detailed experimental and theoretical work should 11. Boles MA, Ling D and Hyeon T et al. The surface science of be done to comprehensively understand SACs- nanocrystals. Nat Mater 2016; 15: 141–53. support effects. The top-down strategy has shown 12. Xia BY, Wu HB and Wang X et al. One-pot synthesis of cubic great promise and significantly contributed to the PtCu nanocages with enhanced electrocatalytic activity for simplified synthesis routes for SACs with excep- the methanol oxidation reaction. J Am Chem Soc 2012; 134: tional activity and stability. Moreover, the metal 13934–7. loading can be markedly increased from 1% to 5%, 13. Christopher P, Xin H and Linic S. Visible-light-enhanced cat- and the coordination environments can be elabo- alytic oxidation reactions on plasmonic silver nanostructures. rately controlled. This will definitely facilitate the de- Nat Chem 2011; 3: 467–72. velopment of general protocols for accessing SACs 14. Wu Y, Wang D and Li Y. Nanocrystals from solutions: catalysts. and underpin the exploration of other intriguing ap- Chem Soc Rev 2014; 43: 2112–24. plications. 15. Lu Q, Wang AL and Gong Y et al. Crystal phase-based epitax- Together, the field of SAs is expansive and rapidly ial growth of hybrid noble metal nanostructures on 4H/fcc Au developing towards different applied research fields. nanowires. Nat Chem 2018; 10: 456–61. The continued development of SACs represents an 16. Fan Z and Zhang H. Crystal phase-controlled synthesis, proper- important advancement in heterogeneous catalysis ties and applications of noble metal nanomaterials. Chem Soc and will surely be the important focus of extensive Rev 2016; 45: 63–82. research efforts and a thriving field for various appli- 17. Chen Y, Fan Z and Luo Z et al. High-yield synthesis of crystal- cations for years to come. phase-heterostructured 4H/fcc Au@Pd core-shell nanorods for electrocatalytic ethanol oxidation. Adv Mater 2017; 29: 1701331–5. FUNDING 18. Fan Z and Zhang H. 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National Science ReviewOxford University Press

Published: Sep 1, 2018

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