Single-Atom Catalysis toward Efficient CO2 Conversion to CO and

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Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

Single-Atom Catalysis toward Efficient CO2 Conversion to CO and Formate Products Xiong Su,† Xiao-Feng Yang,† Yanqiang Huang,*,† Bin Liu,*,‡ and Tao Zhang*,†,§ †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore § University of Chinese Academy of Sciences, Beijing 100049 P. R. China

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CONSPECTUS: Simply yet powerfully, single-atom catalysts (SACs) with atomically dispersed metal active centers on supports have received a growing interest in a wide range of catalytic reactions. As a specific example, SACs have exhibited distinctive performances in CO2 chemical conversions. The unique structures of SACs are appealing for adsorptive activation of CO2 molecules, transfer of intermediates from support to active metal sites, and production of desirable products in CO2 conversion. In this Account, we have exemplified our recent endeavors in the development of SACs toward CO2 conversions in thermal catalysis and electrocatalysis. In terms of the support not only stabilizing but also working collaboratively with the single active sites, the proper choice of support is of great importance for its stability, activity, and selectivity in single-atom catalysis. Three distinctive strategies for SAC architectureslattice-matched oxide supported, heteroatom-doped carbon anchored, and mimetic ligand chelatedare intensively discussed from the perspective of support design for SACs in different reaction environments. To achieve a high-temperature thermal reduction of CO2 to CO, TiO2 (rutile), lattice-matched to the IrO2 active site, was chosen as a support to realize the thermal stability of Ir1/TiO2 SAC, and it shows great capability toward CO2 conversion and excellent selectivity to CO due to the effective block of the over-reduction of CO2 to methane over single Ir active sites. In the electrochemical reduction of CO2 at low temperature, sulfur co-doped N-graphene was developed to achieve unique d9-Ni single atoms on the conductive graphene support, by which not only were the atomic Ni active sites trapped into the matrix of graphene for its stabilization, but also the modulation of electronic configuration of mononuclear Ni centers promoted the CO2 activation through facile electron transfer with an improved electroreduction activity. Inspired by the Ir mononuclear homogeneous catalysts in CO2 hydrogenation to formate, porous organic polymers (POPs) functionalized with a reticular aminopyridine group were purposely fabricated to mimic the homogeneous ligand environment for chelating the Ir single-atom active center, and this quasi-homogeneous Ir1/POP catalyst manifests high efficiency for hydrogenation of CO2 to formate under mild conditions in the liquid phase. Such SACs are of paramount importance for the transformation of CO2, with their coordination environment helping in the activation of CO2. Since the energy barrier for the dissociation of the second C−O bond of CO2 on single-atom sites is very high, these catalysts can give high selectivities toward CO or formate products. Thanks to SACs, the conversion of CO2 has become much easier in various chemical environments.

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INTRODUCTION

Homogeneous catalysts often possess a uniform structure with a well-defined coordination geometry comprising active metal centers and organic ligands, which have high specific activity and product selectivity.3,5 The mechanism can be easily tracked at the molecular level by labeling experiments and computational modeling.6 Considering that CO2 may act as an electrophile or Lewis acid, it can be activated by an electron donor or base.7 By introducing homogeneous catalysts that contain electron-donating ligands surrounded active metal sites, a facile CO2 conversion could be realized. However, the catalytic performance of homogeneous catalysts is sensitive to the nature of the ligands, which can be expensive and leachable

The intrinsic stability of CO2, with an extremely high CO bond energy of 806 kJ mol−1, makes the transformation of CO2 to chemicals or fuels very challenging.1 Since the activation of CO2 molecule is the first step toward CO2 conversion, according to the activation methods, it can be classified as a high-temperature energy-driven process with adsorption of CO2 on oxygen vacancy sites,2 a moderately gentle process in the form of intermediates with assistance of solvents and catalyst substrates,3 and an electric/photoelectron participated disruption of the CO bond.4 While activated, adsorptive intermediates will be simultaneously reacted on those preconstructed active centers on catalysts, either homogeneously or heterogeneously. © XXXX American Chemical Society

Received: September 21, 2018

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DOI: 10.1021/acs.accounts.8b00478 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research in reaction media. Furthermore, homogeneous catalysts are usually not robust enough to withstand harsh reaction environments, such as high temperature and pressure, and are not so easily separable for catalyst recycling. On the other hand, heterogeneous catalysts can easily avoid the abovementioned drawbacks, but the inhomogeneity of heterogeneous catalysts makes the intrinsic structure of an exact active center and the reaction pathway poorly defined.8 Recently, a new type of heterogeneous catalysts with atomically dispersed metal active centers, referred to as single-atom catalysts (SACs), has aroused interest worldwide and has been growing as arguably the most active research frontier in heterogeneous catalysis.9 In a recent review article,10 we have demonstrated the distinct characteristics of SACs, which are able to provide isolated mononuclear metal active sites with no cross-talk between two neighboring metal atoms but strong chemical bonding interaction to the support. This distinct feature of the coordination environment of SACs results in 100% metal atom center opening to the reaction atmosphere, thus leading to maximized atom utilization efficiency. The unique structure of SACs, to some extent, imitates a biological catalytic system, which shall offer great opportunities for tuning reaction rates and selectivities and may also provide a platform to understand the structure− activity relations at the atomic scale. Accordingly, an important motif of SACs is to finely tune the electronic property of metal centers, as derived from the modulation from supports which anchor solely individual metals. Since the terminology of SACs was first defined by Zhang, Li, Liu, and co-workers,9,11 multifarious SACs with metal atoms fixed at defective sites of reducible oxide supports have been developed for several heterogeneous reactions.10,12,13 Oxide-supported SACs have stood high-temperature tests,14 which is crucial toward developing SACs for large-scale industrial processes. However, the density of single-atom sites in SACs is still unsatisfactory, owing to the lack of anchoring sites. Researchers are now developing architectural designs of chelators on supports, for example, via doping nitrogen to carbon materials or grafting coordinative ligands, to create high-density SACs.15,16 The incorporation of soft chelators enables the material to host high-density and thermally stable active sites, which is important to promote the specific activity of SACs. More importantly, the exquisitely designed surrounding environment of metal centers in SACs is able to reduce the activation barriers of reactants. Keeping this idea in mind, we designed a new type of Schiff-base-modified gold nanocatalyst for direct catalytic hydrogenation of CO2 to formate.17 The Schiff-base functional groups grafted on SiO2 support assist CO2 activation via formation of a weakly bonded carbamate zwitterion intermediate, followed by selective hydrogenation to formate on gold nanoparticles. The unusual CO2 activation pathway via a metastable intermediate avoids the “thermodynamic sink” during the conventional bicarbonate pathway (Figure 1), showing the powerful function of ligands in CO2 transformation. In theory, SACs are more flexible to tune the types of ligands through grafting molecular mononuclear metal complexes on supports.18 The state-of-the-art architecture of SACs is exemplified as a bridge to link homogeneous and heterogeneous catalysis,9,10,19 which has already been demonstrated to be affordable in a few catalytic processes.20,21 Over the past few years, we have successfully established a library of SACs for CO2 conversion.22−24 By properly choosing

Figure 1. Schematic illustration showing the “thermodynamic sink” in the conventional bicarbonate pathway and an alternative intermediate-regulated pathway with lower activation energy.

different functional supports and anchoring single atoms in an intimate cooperation, we find that the SACs thus prepared are capable of efficiently catalyzing CO2 reduction with excellent activities and selectivities, as well as showing superior stability under their respective working conditions. In this Account, we are going to summarize the preparations, characterizations, and activity studies of these SACs, aiming at shedding light on how these SACs work for CO2 transformation.

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THERMALLY STABLE SACS FOR SELECTIVE CO2-TO-CO CONVERSION It has been a long-term goal to develop efficient CO2 conversion processes to produce energy-rich oxygenated compounds and hydrocarbon fuels. The transitioning of CO2 to CO occupies a pivotal stage, since CO often serves as a critical intermediate in these processes. A key to realizing this goal is the introduction of effective catalysts with defective sites on supports that can adsorb CO2 and weaken one of its CO bonds. The adsorbed intermediates, such as carbonate, bicarbonate, formate, or carbonyl, are ready to be transferred onto metal sites for further hydrogenation. Thus, the activity and selectivity of CO2 hydrogenation have been demonstrated to be very sensitive to the structure of the catalyst, especially for the well-defined structure of metal sites.25−28 For gas-phase CO2 hydrogenation, methanation and reverse water−gas shift reactions are the two competing reaction pathways over supported group VIII transition metal (TM) catalysts at atmospheric pressure. For adsorption of CO2, reducible oxides, such as TiO2, CeO2, In2O3, and ZrO2, are among the most preferred choices of catalyst support.2,29 The role of surface oxygen vacancies in CO2 activation and hydrogenation has been predicted theoretically30,31 and confirmed experimentally.32 To increase the amount of adsorption sites on the “inert support”, alkali promoters are intentionally introduced.33 Besides these beneficial approaches, we are more concerned about the specific details of the catalyst microenvironment, which governs the reaction pathway and eventually the product selectivity. To study the influence of catalyst microenvironment on gasphase CO2 hydrogenation, we used phase-pure rutile TiO2 (denoted as r-TiO2) as support to prepare a series of iridium catalysts.22 Because IrO2 and r-TiO2 have the same rutile crystal structure with similar lattice parameters, Ir species were B


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ATOMICALLY DISPERSED Ni(I) SACs FOR HIGHLY EFFICIENT ELECTROCHEMICAL CO2 REDUCTION REACTION Electrochemical CO2 reduction offers a promising solution for the storage of low-density renewable electricity as well as an intelligent way of coping with global carbon emission, yet it is still challenging because of the low reaction activity and poor product selectivity.34−36 Owing to the extremely stable CO bond in CO2 molecules, high overpotential is required to overcome the activation barrier, making electrochemical conversion of CO2 rather inefficient. To promote a more practical electrochemical CO2 reduction progress, development of efficient CO2 reduction reaction (CO2RR) catalysts with high activity, selectivity, and stability but low cost is still a long-term goal. Recently, construction of single-atom TM catalysts (such as Fe,37 Co,38 and Ni23,39,40) has been successfully achieved in the electrochemical CO2RR. By virtue of their coordinatively unsaturated environment with bonding to substrate and, more importantly, the unique electronic properties of metal centers, the SACs show impressive electrocatalytic performance for converting CO2 to CO. One of the most important aspects of the electrochemical CO2 reduction study is to make clear the exact structure of the active site(s). SACs provide an ideal platform to probe active site(s) and the reaction pathway in the CO2RR. Toward this end, we prepared single-Ni-atom catalysts by pyrolyzing a mixture of melamine, L-alanine or L-cysteine, and nickel acetate tetrahydrate. The structural details of the prepared single-Niatom catalysts are shown in Figure 3a−d. High-density Ni atoms with a mean size of ∼0.2 nm are clearly visible in highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. Further determination of chemical composition and elementary states were made by high-resolution X-ray photoelectron spectroscopy (XPS, Figure 3e,f). The binding energy of nickel is between those of Ni2+ and metallic Ni0, showing a low-valent state. As an important feature for electrocatalysis, the electronic states of single Ni atoms were carefully probed by X-ray absorption spectroscopy (XAS). The Ni K-edge X-ray absorption near-edge structure (XANES) shows an elaborate picture of oxidation states and charge transfer in single Ni atoms. From the position and intensity of the rising edge of the white line, Ni atoms are determined to be in the +1 oxidation state with a d9 electronic configuration. The features of fingerprints of different intensities also indicate an enhanced dipole-allowed electron transition from 1s to 4p with distorted D4h symmetry of Ni atoms. In the complementary phaseuncorrected extended X-ray absorption fine structure (EXAFS) spectra, a primary peak at approximately 1.45 Å could be observed, which represents the first layer Ni−N coordination in the single-Ni-atom catalysts. These unique structural characteristics of single-Ni-atom catalysts endow them with high performance in electrochemical CO2 reduction to CO. For instance, the A-Ni-NSG electrode shows a large cathodic current density of 115 mA cm−2 at an applied potential of −1.0 V (vs reversible hydrogen electrode (RHE)), as displayed in Figure 4. With a secondary heteroatom doping by S, the onset potential of CO2 reduction over A-Ni-NSG can be improved by at least 100 mV as compared to the one without S-doping (A-Ni-NG). The A-NiNSG electrode also exhibits a maximum Faradaic efficiency

lattice-matched and epitaxially bonded to the surface of r-TiO2. The distribution of supported Ir particles was finely tuned, ranging from single-atom to nanosize of ∼2 nm. Furthermore, the lattice-match interaction could ensure extreme stability of these single Ir atoms and nanoparticles at high reaction temperatures. Based on the two catalyst models, the calculated activation energy barriers for CO2 hydrogenation to CO and H2 dissociation are significantly lower than that required for CO dissociation. Thus, we mainly considered the elementary reaction step of CO methanation to make a comparison. The free energy diagrams of CO methanation on both sites (Figure 2a) show that CO dissociation is the slowest among all

Figure 2. (a) Free energy diagrams for CO methanation on B5 sites of stepped Ir (black line) and PR1-Ir1/TiO2 (110) surface (red line). Insets show the TS structures for direct CO dissociation. (b) Structures of B5 step sites (left) and PR1-Ir1/TiO2 (110) (right). Blue, dark yellow, and red balls represent Ir, Ti, and O atoms, respectively. Metal atoms at the B5 step edge are highlighted, and the B5 sites are indicated by a black pentagon. Reproduced with permission from ref 22. Copyright 2017 American Chemical Society.

elementary steps. CO dissociation is found to be sensitive to the coordination number (Nc) of metal sites. A single-atom Ir site is the least effective. The binding energy of dissociated fragments on a single-atom site is increased as compared to that on the Ir5 sites. The energy difference between CO dissociation and desorption is greatly enhanced on single-atom Ir, which shall significantly impede the methanation process. Density functional theory (DFT) calculation results were validated by conducting CO2 hydrogenation experiments on single-atom Ir (Ir/TiO2-0.1, 0.1 wt% of Ir) and nanoparticle (Ir/TiO2-5, 5 wt% of Ir) catalysts prepared in our laboratory. During catalytic hydrogenation of CO2, CO selectivity reached almost 100% on single-atom Ir but decreased to only 21.8% on Ir nanoparticles if conversion of CO2 was controlled at the same level, signifying the powerful function of single atoms in governing the product selectivity. These results, both theoretically and experimentally, demonstrate that the capability of C−O bond scission can be regarded as a descriptor in governing the CO2 hydrogenation product selectivity. At the atomic scale, single atoms may serve as the active sites for efficient conversion of CO2 to CO product. C


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Figure 3. Structural characterization of single nickel atoms dispersed on nitrogenated graphene. (a) SEM and AFM (inset) images. Scale bars: 100 and 500 nm for the SEM and AFM images, respectively. (b) Bright-field TEM and HRTEM (inset) images. Scale bars: 200 and 10 nm for the TEM and HRTEM images, respectively. (c) X-ray diffraction patterns. The JCPDS profiles of graphite (black) and metallic nickel (dark green) are displayed for reference. (d) HAADF-STEM image of A-Ni-NG. Scale bar: 5 nm. (e) Elemental content of A-Ni-NG, obtained from XPS and ICPAES measurements. Inset shows the high-resolution XPS N 1s spectrum, which is deconvoluted into five Voigt-type line-shape peaks using the Shirley background. (f) High-resolution XPS Ni 2p spectra. The binding energies of Ni 2p in A-Ni-NG and A-Ni-NSG are in a valence state lower than +2 but higher than 0, highlighted by the dashed line. Reproduced with permission from ref 23. Copyright 2018 Springer Nature.

ment and oxidation state of Ni(I) as well as the CO2 to CO reduction performance can be well maintained after 100 h of continuous operation. Based on a series of operando XAS, XPS, and 13CO2 labeling experiments together with DFT calculations, we clarified the adsorption of CO2, the dynamic electron transfer in CO2 activation, and the formation of intermediates on the Ni−N4 sites during CO2RR. The oxidation state of Ni increases under open-circuit voltage, which is probably caused by the delocalization of an unpaired electron and a spontaneous formation of CO2δ− species with a charge transfer from Ni atom to the 2p orbital (3dx2−y2 → 2p) of carbon in CO2.41 After completing the CO2 reduction cycle, Ni atoms return back to a lower oxidation state to recover the active sites. Operando EXAFS results provide information about the geometric changes in the local environment of Ni, which shows an elongation and distortion of the Ni−N bond during CO2 adsorption. As probed by the 13CO2 labeling experiment, a surface-bonded *COOH species is suggested to be the crucial intermediate, and this stage is regarded as the rate-limiting step for CO evolution. On the basis of the above characterizations, we are now able to make a clear picture (Figure 5) for the evolution of CO on the well-defined Ni(i) single-atom active site during electrochemical CO2RR.

Figure 4. CO2 reduction in aqueous solution. (a) LSV curves acquired in CO2-saturated 0.5 M KHCO3 solution on a rotating disc electrode at a rotation speed of 1600 rpm and a scan rate of 5 mV s−1. Catalyst loading: 0.1 mg cm−2. (b) CO Faradaic efficiency at various applied potentials. (c) Turnover frequency of A-Ni-NSG compared with those of other state-of-the-art CO2-to-CO reduction catalysts. (d) Current−time response of A-Ni-NSG on carbon fiber paper for CO2 reduction at an overpotential of 0.61 V. All measurements were conducted under the same conditions: 0.5 M KHCO3 (pH = 7.3), 1 atm CO2, room temperature. Reproduced with permission from ref 23. Copyright 2018 Springer Nature.

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A QUASI-HOMOGENEOUS SAC FOR AQUEOUS-PHASE CO2 HYDROGENATION UNDER MILD CONDITIONS A homogeneous catalyst with a molecular-level active center is expected to promote activity and specific product selectivity in CO2RR.42−44 However, molecular homogeneous catalysts are not sufficient to meet the requirements of recycling for largescale industrial applications. Herein, our primary goal is to make heterogeneous catalysts as homogeneous as possible. To

(CO) of 97% at −0.5 V (vs RHE). The measured exchange current density of around 10−3 mA/cm2geometry and turnover frequency (TOF) of 14 800 h−1 at an overpotential of 0.61 V on A-Ni-NSG are at least 1 order of magnitude larger than the best reported results.21 Moreover, the coordination environD


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Figure 6. Schematic illustration showing the synthesis of Ir/AP-POP single-atom catalyst. 2,6-Diaminopyridine (DAP) and 1,3,5-benzenetricarbonyl chloride (TMC) were copolymerized in a dichloromethane solution of triethylamine to produce AP-POP substrate, which was then developed into Ir/AP-POP catalyst via a wetchemistry impregnation (H2IrCl6) and reduction (NaBH4) procedure. Reproduced with permission from ref 24. Copyright 2018 Elsevier.

and the optimized geometric structure with the aid of computational modeling. The local environment of Ir in Ir/ AP-POP is identified as a singly dispersed Ir atom with strong chemical interactions with −OH and −Cl groups. This coordination geometry is very similar to that of an Ir molecular complex, which has been demonstrated to be powerful in the CO2 hydrogenation reaction.47 The electronic structure of individual Ir atoms on AP-POP shows a partially reduced state. From normalized Ir-L3 XANES and XPS characterizations, the rising edge on the white line and the binding energy clearly demonstrate that single Ir atoms have partially occupied dstates, situated between Ir3+ and Ir0. The Ir/AP-POP SACs exhibit remarkable catalytic activity for CO2 hydrogenation to formate, with turnover number (TON) as high as 25 135 and formic acid-to-amine ratio (AAR) close to 0.9 at 120 °C and 8.0 MPa, significantly surpassing the values for all reported heterogeneous catalytic systems. After the first run of the reaction, all Cl ligands were leached out of the fresh Ir/AP-POP catalyst. The catalyst thereafter showed a stable single-atom structure with an Ir− O(N) coordination number of 4 and almost without degradation in catalytic activity after the reaction was run four times. The activation barrier for CO2 hydrogenation to formate was calculated to be 86.3 kJ mol−1, comparable to the reported value on a homogeneous Ir complex. A molecular-level understanding on the activation and transformation mechanism of CO2 to formate was gained by computational modeling. Starting with a stable and wellorganized catalyst structure, CO2 is adsorbed on the Ir-hydride active sites, accompanied with electron transfer from the negatively charged Ir-hydride to the π orbital of carbon in CO2, which induces a bending of the linear OCO bond to 135° out-of-plane, with a stretching of the CO bond. After rotation of the CO bond to a more stable configuration, the adsorbed species is transferred into a HCOO− intermediate. The active amide-H species nearby is ready to connect with this HCOO− moiety and then be transferred to an adsorbed formic acid intermediate. Only after another H2 molecule is adsorbed and dissociated by the Ir metal site and a nearby amide group will the final formic acid product be released and the initial state of the active site in Ir/AP-POP reset. Figure 8 displays the complete catalytic transformation cycle, which is very similar to that in homogeneous catalysis. Therefore, we call the Ir/AP-POP a quasi-homogeneous catalyst regarding its

Figure 5. Structural evolution of the active site in electrochemical CO2 reduction. Reproduced with permission from ref 23. Copyright 2018 Springer Nature.

a large extent, we have achieved stable single-atom active sites on the surface of oxide- and heteroatom-doped carbon supports. Besides, there are great opportunities for the fabrication of single-atom inner surfaces of the porous substrates to maximize the loading amount toward individual metal centers. A key step to realize this goal is to graft sufficient chelator sites inside the pocket of solid substrates, such as porous organopolymers (POPs) or hybrid organic−inorganic materials.18,21,45 As a promising and well-studied process in homogeneous chemistry, catalytic transformation of CO2 to formic acid is considered a viable method to reduce CO2 emission. Ir-based mononuclear pincer complexes are currently among the most effective catalysts in aqueous media.42,46 In these molecular complexes, the ligands, such as pyridyl or phosphine, are indispensable for promoting conversion efficiency. Influenced by the nature of ligands and their substitutions, the CO2 bonded to metal complexes is well activated. With the intention to improve the efficiency of catalytic conversion of CO2 to formate, 2,6-diaminopyridine and 1,3,5-benzenetricarbonyl chloride were employed as the basic building blocks for covalent coupling, to fabricate a reticular aminopyridine group functionalized solid polymer (referred as APPOP, see Figure 6). The condensed AP-POP is spherical-like but porous and amorphous in nature (Figure 7). The successful incorporation of electron-donating functional groups was monitored by group characterization techniques. H2IrCl6 was impregnated and reduced onto AP-POP by NaBH4, achieving an Ir content as high as 1.25 wt% in the catalyst. As observed directly by HAADF-STEM, all Ir species are present as individual bright spots. The EXAFS spectrum at the L3-edge gives information regarding the local environment of Ir in the Ir/AP-POP catalyst. We further examined the exact anchoring ligands for Ir E


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Figure 7. Characterizations of the AP-POP and the atomic dispersion of Ir on the support. (A) Solid-state 13C/CP-MAS NMR spectrum of the APPOP, suggesting the presence of CO, CN, and CAr groups. (B) N 1s core-level XPS spectrum of the AP-POP, which distinguishes N in the pyridinic and amide groups in the framework. (C) UV−vis absorbance spectra of samples with and without Ir. (D) SEM image of the AP-POP. (E,F) HAADF-STEM images of fresh and used 1.25% Ir/AP-POP. Adapted from ref 24. Copyright 2018 Elsevier.

Figure 8. Proposed reaction mechanism for CO2 hydrogenation over the oppositely H-bonded Ir single-atom active sites. Adapted from ref 24. Copyright 2018 Elsevier.

the design and structural characterizations of SACs and summarized the advantages of SACs on both geometric and electronic features for adsorption and activation of CO2, as well as their catalytic performances in CO2 conversion. Looking forward, there still remain many grand challenges in studying single-atom catalysis for CO2 reduction. First, having greater uniformity of active sites than traditional supported

heterogeneous nature, but it still can bridge the gap between heterogeneous and homogeneous catalysis.

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CONCLUSIONS AND PERSPECTIVES Because of their exquisite architectures, SACs have emerged as a new research focus for catalytic CO2 transformation. In this Account, we have reviewed our recent research progresses on F


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Accounts of Chemical Research catalysts, the microenvironment (geometric and electronic structure) of SACs, which is still quite complicated, deserves more efforts to investigate. Developing operando characterization techniques is crucial to determine the real active sites and track the specific reaction mechanism. Second, the existing environment of SACs features the coordination of single metal atoms onto a support with a variety of interactions. The key point we emphasize here is the fine-tuning of the interactions between the single atoms and catalyst support, which is most important but still remains elusive to the promotion of reactivity and selectivity in CO2 transformation. Third, we have achieved transformation of CO2 to CO or formate on a number of single-atom catalysts with high efficiency. Urgently, we need to develop new SAC systems to catalyze CO2 in a way to give industrially more desirable commodities, such as methanol, ethanol, and long carbon chain chemicals, which is important to the pursuit of the “liquid sunshine” strategy to achieve a sustainable energy and environment.48 Although there are many challenges, we are on the way.

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He is now an associate professor at NTU. His main research interests are electrocatalysis, photovoltaics, and photoelectrochemistry. Tao Zhang received his Ph.D. in physical chemistry from DICP in 1989 under the supervision of Prof. Liwu Lin. After graduation, he worked as a postdoc with Prof. Frank Berry at Birmingham University for one year. He founded his own group at DICP in 1995. He was appointed as an associate professor in 1993 and then promoted to full professor in 1995. He was the director of DICP from 2007 to 2017 and was selected as an Academician by the CAS in 2013. His main research interests are single-atom catalysis and biomass conversion.

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ACKNOWLEDGMENTS The authors acknowledge the National Key R&D Program of China (2016YFB0600902), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020400), the National Natural Science Foundation of China (Nos. 21506204, 21476226, 21776269), Dalian Science Foundation for Distinguished Young Scholars (2016RJ04), the Youth Innovation Promotion Association CAS, Singapore Ministry of Education Academic Research Fund (AcRF) Tier 1: RG9/17 and RG115/17, and Tier 2: MOE2016-T2-2-004 for financial support.

AUTHOR INFORMATION

Corresponding Authors

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*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

DEDICATION This paper is dedicated to the 70th anniversary of Dalian Institute of Chemical Physics, Chinese Academy of Sciences.

ORCID

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Yanqiang Huang: 0000-0002-7556-317X Bin Liu: 0000-0002-4685-2052 Tao Zhang: 0000-0001-9470-7215

REFERENCES

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Notes

The authors declare no competing financial interest. Biographies Xiong Su received his B.S. degree from Dalian University of Technology and Ph.D. degree in industry catalysis from Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS). He continued his academic career in DICP as an assistant professor and was promoted to associate professor in 2017. His research interests include CO2 and syngas conversion and synthesis and application of molecular sieves and nanostructured materials. Xiao-Feng Yang received his B.S. degree from Nanjing University (2003) and Ph.D. degree in 2010 from DICP under the supervision of Prof. Tao Zhang. After 2 years as a postdoctoral fellow in Prof. Jun Li’s group at Tsinghua University for theoretical and computational catalysis, he joined DICP again and was promoted to associate professor in 2012. His research interests involve theoretical and computational catalysis, nanocatalysis, and single-atom catalysis. Yanqiang Huang received his B.S. degree from Dalian University of Technology (2002) and Ph.D. degree from DICP (2008). Afterward, he joined DICP as a staff scientist and was promoted to full professor in 2016. His research interests include propellant catalytic decomposition, CO2 capture and utilization, and C1 chemistry. Bin Liu received his B.Eng. (1st Class Honors) and M.Eng. degrees in Chemical Engineering from the National University of Singapore and Ph.D. degree in Chemical Engineering from the University of Minnesota in 2011. Thereafter, he moved to the University of California, Berkeley, and worked as a postdoctoral researcher in the Department of Chemistry during 2011−2012 before joining the School of Chemical and Biomedical Engineering at Nanyang Technological University (NTU) as an assistant professor in 2012. G


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Single-Atom Catalysis toward Efficient CO2 Conversion to CO and

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