- Email: [email protected]
Reaction: Open Up the Era of Atomically Precise Catalysis
development of SACs innovates a series of synthetic methodologies of novel catalytic materials in both homogeneous and heterogeneous catalysis. Industrial application is undoubtedly the ultimate goal of catalysis research. Many scientific and technical problems ranging from micro to macro scales have to be tackled for the industrialization of any new catalysts and processes. This research-and-development process is highly demanding because it not only involves a complex chemical reaction itself but also involves the physical process of momentum, quality, and energy transport.4 From the perspective of practical industrial application of SACs, more attention should be paid to technical problems such as the stability of catalysts, space-time yield, and large-scale manufacturing and shaping. Long-period operation with high stability is an important goal of industrial catalysis. Two aspects should be addressed regarding the stability of SACs, namely, the structure stability of single atoms and the resistance to poisoning of the active center. An active single metal atom usually involves strong interaction with supports through covalent bonding to maintain high structure stability. Several ingenious strategies have been systematically explored for the synthesis of stable SACs, for example, the defectengineering strategy, spatial-confinement strategy, and coordinationdesign strategy.5 In industrial catalysis, some impurities such as sulfur- and nitrogen-containing compounds in the feeds of certain reactions can poison the SACs by decreasing the number of active sites or changing their nature. It is therefore essential to address these kinds of poisoning phenomena and mechanisms before industrialization. The space-time yield is a very important index in industrial catalysis. The reactivity of SACs is usually evaluated on
the basis of the turnover frequency (TOF) to emphasize the efficiency of every metal atom. It is, however, the space-time yield (product yield per unit volume of catalyst and per unit time) that is focused on assessing the size of the reactor, capacity of equipment, and economy of the reaction. In most cases, the space-time yield of SACs is still lower than that of nanocatalysis because of the low density of active single atoms on supports. Increasing the loading of metals could boost the density of a single atom but inevitably enhances the tendency of sintering or aggregation of active sites. In addition, two or more distinct active centers are sometimes required for cooperatively catalyzing complex reactions. Therefore, preparing SACs with high density and flexible distribution of single metal atoms is particularly important in industrial application to increase its space-time yield of a variety of reactions. The synthesized catalysts usually need to be shaped for industrial application, and the influence of such a manufacturing process on catalytic performance of SACs is yet to be understood. In addition, other mechanical effects such as catalyst strength, catalyst abrasion, and granularity need to be taken into account as well. To recap, the concept and practice of single-atom catalysis provide distinct strategy and direction for many industrially important reactions. With the progress of in-depth understanding of dynamic structure evolution and complex reaction network and the development of controlled synthesis of high stable SACs in large scale, the important merits of SACs in industrial catalysis are highly anticipated.
ACKNOWLEDGMENTS We gratefully acknowledge the support from the National Natural Science Foundation of China (grant no. 21673295).
1. Qiao, B., Wang, A., Yang, X., Allard, L.F., Jiang, Z., Cui, Y., Liu, J., Li, J., and Zhang, T. (2011). Single-atom catalysis of CO oxidation using Pt1/FeOx. Nat. Chem. 3, 634–641. 2. Kyriakou, G., Boucher, M.B., Jewell, A.D., Lewis, E.A., Lawton, T.J., Baber, A.E., Tierney, H.L., Flytzani-Stephanopoulos, M., and Sykes, E.C.H. (2012). Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations. Science 335, 1209–1212. 3. Wang, A., Li, J., and Zhang, T. (2018). Heterogeneous single-atom catalysis. Nat. Rev. Chem. 2, 65–81. 4. Hagen, J. (2015). Industrial Catalysis: A Practical Approach (WILEY-VCH). 5. Chen, Y., Ji, S., Chen, C., Peng, Q., Wang, D., and Li, Y. (2018). Single-atom catalysts: synthetic strategies and electrochemical applications. Joule 2, 1242–1264. 1State
Key Laboratory of Green Chemical Engineering and Industrial Catalysis, Sinopec Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China *Correspondence: [email protected] (Y.W.), [email protected] (Z.X.) https://doi.org/10.1016/j.chempr.2019.10.006
CATALYSIS
Reaction: Open Up the Era of Atomically Precise Catalysis Wei Zhu1,2 and Chen Chen1,* Wei Zhu is an associate professor at Beijing University of Chemical Technology. His research focuses on the development of catalysts and devices for energy-related electrochemical applications. Chen Chen joined the Department of Chemistry at Tsinghua University as an associate professor in 2015. His research focuses on catalyst design and mechanism study at the atomic scale for catalytic activation reactions of small molecules. Just like when the use and manufacture of tools motivated the evolution of human cognition and thus made us different from other species, the development of probe technologies is
Chem 5, 2733–2739, November 14, 2019 ª2019 Elsevier Inc. 2737
currently triggering the grand discovery of the microscopic images of physical and chemical processes. During the past two decades, the relationship between microscopic structures and catalytic performances has been well established for a wealth of the reactions by careful analysis of the nanocatalysts with well-defined structures. These findings deepened our understanding of modern catalysis on the nanometer scale or even at the atomic level. Most recently, scientists have been enjoying a ‘‘gold rush’’ spree in the field of single-atom catalysis. By virtue of the characteristic 100% dispersity, highly unsaturated coordination environment, and uniform reaction centers, singleatom catalysts (SACs) embrace the bold philosophy of ‘‘smaller than smaller’’ in catalysis and push the design strategy of catalysts to the ultimate dimension. In other words, SACs can potentially make full use of the catalytically active components and are reaching for the crown jewel of catalysis. In their Catalysis piece, Li et al. have overviewed the developmental history of single-atom catalysis from the conventional nanocatalysis and anticipate that SACs are going to make critical breakthroughs in various aspects of modern catalysis as well as in the chemical industry.1 As stated in the article, numerous efforts have been devoted to exploring universal synthesis routes and practical catalytic reactions for SACs since 2011, ‘‘the first year of the SAC age,’’ and SACs do reward us with their superior catalytic performances. Given that SACs have already taken the atomic utilization efficiency to an extreme, here comes the question: what is the next favorite of modern catalysis after the ‘‘SAC age’’? Of course, investigations on SACs will never end, just like SACs do not impede the development of nanocatalysis. The SAC boom facilitates the developments on atomic-manipulation synthesis methods (e.g., host-guest en-
2738 Chem 5, 2733–2739, November 14, 2019
capsulation, atomic layer deposition, etc.), atomic-precision structural characterization techniques (e.g., aberration-corrected transmission electron microscopy, synchrotron-based X-ray absorption spectroscopy [XAS], etc.), and computer simulations (e.g., density functional theory [DFT], etc.), which will in turn cultivate the next-generation protagonist. Many researchers are concerned with how SACs could adsorb large reactants or intermediates and drive intermolecular reactions. Actually, it has been demonstrated that SACs are not omnipotent for all the catalytic reactions, and some other atoms are needed to complete the reactions through an interatomic synergy. To some extent, SACs are multi-atom catalysts as well because the active centers of SACs typically refer to the collective entities of the central single atom and the coordinating atoms from the support, e.g., the Fe1–N4 site for oxygen reduction reaction, the Pt1–Ox site for CO oxidation, etc. Hence, this would shed light on the future of modern catalysis by developing efficient atomically precise catalysts via the potential interatomic synergy. Meanwhile, SACs can be regarded as a pioneer, as well as a footstone, of atomically precise catalysis. Atomically precise catalysis particularly focuses on the study of interatomic interfaces and interaction between the active atoms and the surrounding environment by constructing well-defined model catalysts with precise atomic number and configuration. SACs are one of the simplest and smallest representatives for atomically precise catalysis. By introducing a second atom near the single-atom center to form a homo- or hetero-nuclear diatomic site, we can probably modulate the catalytic property of a single atomic site via various interatomic interactions (e.g., bifunctional effect, electronic structure reconstruction, and geometric configuration) on the basis of the principles of bimetallic catalysts. However, it re-
mains a great challenge to unveil the catalytic process over multi-atom sites because of the difficulties in atomically precise preparations and characterizations. Quite a few examples of multiatom catalysis have been reported. As early as 2005, Goodman and his colleagues demonstrated the promotional effect between two noncontiguous Pd monomers on Au singlecrystal surfaces on the acetoxylation reaction of ethylene to produce vinyl acetate.2 A proper distance between the two Pd atoms, which separately adsorbed the reactants of acetic acid and ethylene, was revealed to be the critical factor for the promoted specific activity by the ingenious design of single-crystal catalysts via surface-engineering techniques. Benefiting from the rapid development of characterization techniques and controllable synthesis methods, Zeng and his collaborators prepared MoS2-nanosheet-supported neighboring Pt monomers with high activity for CO2 hydrogenation by increasing the loading of single-atom Pt sites.3 They demonstrated that the synergetic interaction between neighboring Pt monomers lowered the reaction barrier by altering the mechanism via a formic acid reaction pathway. Not only the spatial distance but also the electronic structure could be manipulated to tune the overall performance of the severalatom catalysts. Jiao et al. found that nanowires with the optimized loading ratio of Cu (0.10%) exhibited higher current density and faradic efficiency to CO than the nanowires with the other loading ratios of Cu in the electrochemical reduction of CO2.4 XAS results and DFT simulations verified that the homonuclear Cu10–Cu1x+ pair with different valences on the surface of nanowires probably activated the reactants of H2O and CO2 individually and thus remarkably reduced the activation barrier of CO2. Furthermore, atomically dispersed hetero-nuclear Pt–Cu sites were synthesized on the surface of Pd nanorings via a surface-confining
reduction strategy, and the Pt–Cu sites showed an extremely high hydrogen evolution reaction (HER) activity as a result of the optimized interaction between the H intermediate and Pt by Cu.5 Therefore, these studies verify that there is plenty of room at the bottom for catalysis by expanding singleatom sites to homo- or hetero-nuclear multi-atom ensembles. For now, a mass of trial-and-error screening works on SACs are still required because SACs act differently from the nano-sized counterparts for a variety of reactions, and it is the screening works that form the foundation of the huge library of atomically precise catalysis. Thorough investigations on single-atom sites as well as the electronic and geometric structures of the interfacial surroundings can push the concept of SACs into a more generalized multi-atom ensemble. It is also necessary to establish an intimate collaboration between
the catalytic studies and the advanced supporting techniques on atomicresolution structural characterization, operando spectroscopy, and computational simulation for the rapid development of atomically precise catalysis. The advent of this era would probably elucidate the black box of catalysis and shed light on the rational design of atom-efficient catalysts for industrial applications.
ACKNOWLEDGMENTS This work was supported by the Beijing Natural Science Foundation (JQ18007) and the Fundamental Research Funds for the Central Universities (buctrc201823). The authors also thank Dr. Chao Zhang for his linguistic assistance during the manuscript preparation.
2. Chen, M., Kumar, D., Yi, C.-W., and Goodman, D.W. (2005). The promotional effect of gold in catalysis by palladium-gold. Science 310, 291–293. 3. Li, H., Wang, L., Dai, Y., Pu, Z., Lao, Z., Chen, Y., Wang, M., Zheng, X., Zhu, J., Zhang, W., et al. (2018). Synergetic interaction between neighbouring platinum monomers in CO2 hydrogenation. Nat. Nanotechnol. 13, 411–417. 4. Jiao, J., Lin, R., Liu, S., Cheong, W.C., Zhang, C., Chen, Z., Pan, Y., Tang, J., Wu, K., Hung, S.F., et al. (2019). Copper atom-pair catalyst anchored on alloy nanowires for selective and efficient electrochemical reduction of CO2. Nat. Chem. 11, 222–228. 5. Chao, T., Luo, X., Chen, W., Jiang, B., Ge, J., Lin, Y., Wu, G., Wang, X., Hu, Y., Zhuang, Z., et al. (2017). Atomically dispersed copper–platinum dual sites alloyed with palladium nanorings catalyze the hydrogen evolution reaction. Angew. Chem. Int. Ed. Engl. 56, 16047– 16051. 1Department
of Chemistry, Tsinghua University, Beijing, China
2State
1. Li, X.N., Huang, Y.Q., and Liu, B. (2019). Single-atom catalysis: directing the way toward the nature of catalysis. Chem 5, this issue, 2733–2735.
Key Lab of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2019.10.005
Chem 5, 2733–2739, November 14, 2019 2739
the basis of the turnover frequency (TOF) to emphasize the efficiency of every metal atom. It is, however, the space-time yield (product yield per unit volume of catalyst and per unit time) that is focused on assessing the size of the reactor, capacity of equipment, and economy of the reaction. In most cases, the space-time yield of SACs is still lower than that of nanocatalysis because of the low density of active single atoms on supports. Increasing the loading of metals could boost the density of a single atom but inevitably enhances the tendency of sintering or aggregation of active sites. In addition, two or more distinct active centers are sometimes required for cooperatively catalyzing complex reactions. Therefore, preparing SACs with high density and flexible distribution of single metal atoms is particularly important in industrial application to increase its space-time yield of a variety of reactions. The synthesized catalysts usually need to be shaped for industrial application, and the influence of such a manufacturing process on catalytic performance of SACs is yet to be understood. In addition, other mechanical effects such as catalyst strength, catalyst abrasion, and granularity need to be taken into account as well. To recap, the concept and practice of single-atom catalysis provide distinct strategy and direction for many industrially important reactions. With the progress of in-depth understanding of dynamic structure evolution and complex reaction network and the development of controlled synthesis of high stable SACs in large scale, the important merits of SACs in industrial catalysis are highly anticipated.
ACKNOWLEDGMENTS We gratefully acknowledge the support from the National Natural Science Foundation of China (grant no. 21673295).
1. Qiao, B., Wang, A., Yang, X., Allard, L.F., Jiang, Z., Cui, Y., Liu, J., Li, J., and Zhang, T. (2011). Single-atom catalysis of CO oxidation using Pt1/FeOx. Nat. Chem. 3, 634–641. 2. Kyriakou, G., Boucher, M.B., Jewell, A.D., Lewis, E.A., Lawton, T.J., Baber, A.E., Tierney, H.L., Flytzani-Stephanopoulos, M., and Sykes, E.C.H. (2012). Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations. Science 335, 1209–1212. 3. Wang, A., Li, J., and Zhang, T. (2018). Heterogeneous single-atom catalysis. Nat. Rev. Chem. 2, 65–81. 4. Hagen, J. (2015). Industrial Catalysis: A Practical Approach (WILEY-VCH). 5. Chen, Y., Ji, S., Chen, C., Peng, Q., Wang, D., and Li, Y. (2018). Single-atom catalysts: synthetic strategies and electrochemical applications. Joule 2, 1242–1264. 1State
Key Laboratory of Green Chemical Engineering and Industrial Catalysis, Sinopec Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China *Correspondence: [email protected] (Y.W.), [email protected] (Z.X.) https://doi.org/10.1016/j.chempr.2019.10.006
CATALYSIS
Reaction: Open Up the Era of Atomically Precise Catalysis Wei Zhu1,2 and Chen Chen1,* Wei Zhu is an associate professor at Beijing University of Chemical Technology. His research focuses on the development of catalysts and devices for energy-related electrochemical applications. Chen Chen joined the Department of Chemistry at Tsinghua University as an associate professor in 2015. His research focuses on catalyst design and mechanism study at the atomic scale for catalytic activation reactions of small molecules. Just like when the use and manufacture of tools motivated the evolution of human cognition and thus made us different from other species, the development of probe technologies is
Chem 5, 2733–2739, November 14, 2019 ª2019 Elsevier Inc. 2737
currently triggering the grand discovery of the microscopic images of physical and chemical processes. During the past two decades, the relationship between microscopic structures and catalytic performances has been well established for a wealth of the reactions by careful analysis of the nanocatalysts with well-defined structures. These findings deepened our understanding of modern catalysis on the nanometer scale or even at the atomic level. Most recently, scientists have been enjoying a ‘‘gold rush’’ spree in the field of single-atom catalysis. By virtue of the characteristic 100% dispersity, highly unsaturated coordination environment, and uniform reaction centers, singleatom catalysts (SACs) embrace the bold philosophy of ‘‘smaller than smaller’’ in catalysis and push the design strategy of catalysts to the ultimate dimension. In other words, SACs can potentially make full use of the catalytically active components and are reaching for the crown jewel of catalysis. In their Catalysis piece, Li et al. have overviewed the developmental history of single-atom catalysis from the conventional nanocatalysis and anticipate that SACs are going to make critical breakthroughs in various aspects of modern catalysis as well as in the chemical industry.1 As stated in the article, numerous efforts have been devoted to exploring universal synthesis routes and practical catalytic reactions for SACs since 2011, ‘‘the first year of the SAC age,’’ and SACs do reward us with their superior catalytic performances. Given that SACs have already taken the atomic utilization efficiency to an extreme, here comes the question: what is the next favorite of modern catalysis after the ‘‘SAC age’’? Of course, investigations on SACs will never end, just like SACs do not impede the development of nanocatalysis. The SAC boom facilitates the developments on atomic-manipulation synthesis methods (e.g., host-guest en-
2738 Chem 5, 2733–2739, November 14, 2019
capsulation, atomic layer deposition, etc.), atomic-precision structural characterization techniques (e.g., aberration-corrected transmission electron microscopy, synchrotron-based X-ray absorption spectroscopy [XAS], etc.), and computer simulations (e.g., density functional theory [DFT], etc.), which will in turn cultivate the next-generation protagonist. Many researchers are concerned with how SACs could adsorb large reactants or intermediates and drive intermolecular reactions. Actually, it has been demonstrated that SACs are not omnipotent for all the catalytic reactions, and some other atoms are needed to complete the reactions through an interatomic synergy. To some extent, SACs are multi-atom catalysts as well because the active centers of SACs typically refer to the collective entities of the central single atom and the coordinating atoms from the support, e.g., the Fe1–N4 site for oxygen reduction reaction, the Pt1–Ox site for CO oxidation, etc. Hence, this would shed light on the future of modern catalysis by developing efficient atomically precise catalysts via the potential interatomic synergy. Meanwhile, SACs can be regarded as a pioneer, as well as a footstone, of atomically precise catalysis. Atomically precise catalysis particularly focuses on the study of interatomic interfaces and interaction between the active atoms and the surrounding environment by constructing well-defined model catalysts with precise atomic number and configuration. SACs are one of the simplest and smallest representatives for atomically precise catalysis. By introducing a second atom near the single-atom center to form a homo- or hetero-nuclear diatomic site, we can probably modulate the catalytic property of a single atomic site via various interatomic interactions (e.g., bifunctional effect, electronic structure reconstruction, and geometric configuration) on the basis of the principles of bimetallic catalysts. However, it re-
mains a great challenge to unveil the catalytic process over multi-atom sites because of the difficulties in atomically precise preparations and characterizations. Quite a few examples of multiatom catalysis have been reported. As early as 2005, Goodman and his colleagues demonstrated the promotional effect between two noncontiguous Pd monomers on Au singlecrystal surfaces on the acetoxylation reaction of ethylene to produce vinyl acetate.2 A proper distance between the two Pd atoms, which separately adsorbed the reactants of acetic acid and ethylene, was revealed to be the critical factor for the promoted specific activity by the ingenious design of single-crystal catalysts via surface-engineering techniques. Benefiting from the rapid development of characterization techniques and controllable synthesis methods, Zeng and his collaborators prepared MoS2-nanosheet-supported neighboring Pt monomers with high activity for CO2 hydrogenation by increasing the loading of single-atom Pt sites.3 They demonstrated that the synergetic interaction between neighboring Pt monomers lowered the reaction barrier by altering the mechanism via a formic acid reaction pathway. Not only the spatial distance but also the electronic structure could be manipulated to tune the overall performance of the severalatom catalysts. Jiao et al. found that nanowires with the optimized loading ratio of Cu (0.10%) exhibited higher current density and faradic efficiency to CO than the nanowires with the other loading ratios of Cu in the electrochemical reduction of CO2.4 XAS results and DFT simulations verified that the homonuclear Cu10–Cu1x+ pair with different valences on the surface of nanowires probably activated the reactants of H2O and CO2 individually and thus remarkably reduced the activation barrier of CO2. Furthermore, atomically dispersed hetero-nuclear Pt–Cu sites were synthesized on the surface of Pd nanorings via a surface-confining
reduction strategy, and the Pt–Cu sites showed an extremely high hydrogen evolution reaction (HER) activity as a result of the optimized interaction between the H intermediate and Pt by Cu.5 Therefore, these studies verify that there is plenty of room at the bottom for catalysis by expanding singleatom sites to homo- or hetero-nuclear multi-atom ensembles. For now, a mass of trial-and-error screening works on SACs are still required because SACs act differently from the nano-sized counterparts for a variety of reactions, and it is the screening works that form the foundation of the huge library of atomically precise catalysis. Thorough investigations on single-atom sites as well as the electronic and geometric structures of the interfacial surroundings can push the concept of SACs into a more generalized multi-atom ensemble. It is also necessary to establish an intimate collaboration between
the catalytic studies and the advanced supporting techniques on atomicresolution structural characterization, operando spectroscopy, and computational simulation for the rapid development of atomically precise catalysis. The advent of this era would probably elucidate the black box of catalysis and shed light on the rational design of atom-efficient catalysts for industrial applications.
ACKNOWLEDGMENTS This work was supported by the Beijing Natural Science Foundation (JQ18007) and the Fundamental Research Funds for the Central Universities (buctrc201823). The authors also thank Dr. Chao Zhang for his linguistic assistance during the manuscript preparation.
2. Chen, M., Kumar, D., Yi, C.-W., and Goodman, D.W. (2005). The promotional effect of gold in catalysis by palladium-gold. Science 310, 291–293. 3. Li, H., Wang, L., Dai, Y., Pu, Z., Lao, Z., Chen, Y., Wang, M., Zheng, X., Zhu, J., Zhang, W., et al. (2018). Synergetic interaction between neighbouring platinum monomers in CO2 hydrogenation. Nat. Nanotechnol. 13, 411–417. 4. Jiao, J., Lin, R., Liu, S., Cheong, W.C., Zhang, C., Chen, Z., Pan, Y., Tang, J., Wu, K., Hung, S.F., et al. (2019). Copper atom-pair catalyst anchored on alloy nanowires for selective and efficient electrochemical reduction of CO2. Nat. Chem. 11, 222–228. 5. Chao, T., Luo, X., Chen, W., Jiang, B., Ge, J., Lin, Y., Wu, G., Wang, X., Hu, Y., Zhuang, Z., et al. (2017). Atomically dispersed copper–platinum dual sites alloyed with palladium nanorings catalyze the hydrogen evolution reaction. Angew. Chem. Int. Ed. Engl. 56, 16047– 16051. 1Department
of Chemistry, Tsinghua University, Beijing, China
2State
1. Li, X.N., Huang, Y.Q., and Liu, B. (2019). Single-atom catalysis: directing the way toward the nature of catalysis. Chem 5, this issue, 2733–2735.
Key Lab of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2019.10.005
Chem 5, 2733–2739, November 14, 2019 2739