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Introduction: Heterogeneous Single-Atom Catalysis
Chemical Reviews ( IF 51.4 ) Pub Date : 2020-11-11 , DOI: 10.1021/acs.chemrev.0c01097 Jun Li 1 , Maria Flytzani Stephanopoulos 2 , Younan Xia 3
Chemical Reviews ( IF 51.4 ) Pub Date : 2020-11-11 , DOI: 10.1021/acs.chemrev.0c01097 Jun Li 1 , Maria Flytzani Stephanopoulos 2 , Younan Xia 3
Affiliation
This article is part of the Heterogeneous Single-Atom Catalysis special issue. 
Jun Li received his Ph.D. degree in Physical Chemistry from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, in 1992. He did postdoctoral research at the University of Siegen (Germany) and The Ohio State University (USA) from 1994 to 1997. He worked as a Research Scientist at The Ohio State University as well as Senior Research Scientist and Staff Scientist at the Pacific Northwest National Laboratory from 1997 to 2009. He is now a full professor in the Department of Chemistry, Tsinghua University. He is an elected AAAS Fellow and Chinese Chemical Society Fellow. His research involves theoretical chemistry, heavy-element quantum chemistry, and computational catalysis science. More information can be found at http://www.junlilab.org 
Maria Flytzani Stephanopoulos (Miretta, deceased) studied at the National Technical University of Athens and received her B.S. degree in Chemical Engineering in 1973. She continued her graduate studies in the US and received her M.S. degree from the University of Florida (1975) and Ph.D. degree from the University of Minnesota (1978). She worked at the Caltech Jet Propulsion Laboratory as a Senior Engineer and MIT as a Principal Research Associate and Visiting Professor between 1985–1994. She joined the faculty of Chemical and Biological Engineering of Tufts University as an Associate Professor in 1994 and Full Professor in 1996. She was appointed to the Robert and Marcy Haber Professorship in Energy Sustainability in 2009 and named Tufts Distinguished Professor in 2015. She served on the editorial boards of several journals and as an Associate Editor of Science Advances and Editor of Applied Catalysis B. Miretta worked on fundamental and applied topics of heterogeneous catalysis, including hot gas desulfurization, water gas shift reaction, and oxidative conversion of methane to oxygenates. She is one of the pioneers of single-atom catalysis. For her teaching and research, Miretta received numerous awards, including Fellow of AAAS and AIChE and the Henry J. Albert Medal of the International Precious Metal Institute. In 2014, she was elected a member of the National Academy of Engineering. 
Younan Xia received his B.S. degree in Chemical Physics from the University of Science and Technology of China in 1987, M.S. degree in Inorganic Chemistry from University of Pennsylvania (with Alan G. MacDiarmid) in 1993, and Ph.D. degree in Physical Chemistry from Harvard University (with George M. Whitesides) in 1996. He started as an Assistant Professor of Chemistry at the University of Washington, Seattle in 1997 and then joined the Department of Biomedical Engineering at Washington University in St. Louis in 2007 as the James M. McKelvey Professor. Since 2012, he has held the position of Brock Family Chair and Georgia Research Alliance Eminent Scholar at the Georgia Institute of Technology. His group invented a myriad of nanomaterials with controlled properties, and these nanomaterials have found use in applications related to plasmonics, electronics, photonics, photovoltaics, display, catalysis, fuel cells, nanomedicine, and regenerative medicine. More information can be found at http://www.nanocages.com. As defined by Berzelius in 1835, a catalyst is a substance capable of accelerating the rate of a chemical reaction without being consumed. For many decades, catalysts have been a major contributor to the world’s economy. Currently, they are required for more than 90% of the chemical processes and there is an ever growing need for the development of advanced catalysts to secure a sustainable future for our society. Catalysts can be broadly divided into two groups: homogeneous catalysts that work in the same phase as the reactants (with soluble metal complexes and enzymes serving as typical examples) and heterogeneous catalysts that operate in a distinct phase (with notable examples including zeolites and metal nanoparticles deposited on solid supports). Because of its molecular nature, it is much easier to characterize and precisely describe a homogeneous catalyst, and even the reaction mechanism is amenable to rational manipulation. However, it is a daunting task to separate a homogeneous catalyst from the reaction product. The capabilities of homogeneous catalysts are also limited by their thermal instability, and most of them can only be used in a solution phase at temperatures below 100 °C. In contrast, heterogeneous catalysts are well-known for their robustness and easiness of separation and recovery from reaction mixtures. As a result, heterogeneous catalysts account for about 80% of all the catalytic processes currently used in industry. Despite their widespread use, heterogeneous catalysts are intrinsically plagued by surface heterogeneity and structural complexity, two factors that make it extremely difficult to decipher the catalytic mechanisms. Taking the metal nanoparticle as an example, its surface is typically terminated in atoms with all different coordination environments, including those situated on different vertices, edges, and faces. The presence of defects such as atomic steps, kinks, vacancies, stacking faults, and twin boundaries also makes such a catalytic system even more complicated. In practice, it is almost impossible to fabricate two identical nanoparticles, a situation completely different from the homogeneous catalysts based on molecular or biomacromolecular systems. There are two parallel approaches to addressing the heterogeneity and complexity issues of a heterogeneous catalyst. In the context of surface science, these issues are circumvented by switching to model catalysts based on single-crystal substrates that can be fabricated with well-defined surface structures. The research along this line has established that many catalytic reactions are highly sensitive to the exact packing of atoms on the surface. For example, it was reported that ammonia could be produced on an Fe(111) surface at a rate almost 20- and 420-fold faster than those on Fe(100) and Fe(110) surfaces, respectively. Recently, such fundamental studies in surface science were fruitfully combined with colloidal metal nanocrystals to provide a rational route to the development of heterogeneous catalysts with the optimal activity and/or selectivity toward a chemical reaction. Colloidal nanocrystals made of many metals and their alloys can now be prepared as small as those in conventional catalysts to maintain the large specific surface area. More significantly, they can be synthesized with a well-defined shape to present only one specific type of facet on the surface, for example, an octahedral shape for the exposure of {111} facets that are equivalent to the (111) substrates used in surface science. By maneuvering the shape taken by the nanocrystals, it is also feasible to introduce well-defined defects to the surface, such as the twin boundaries associated with decahedral or icosahedral nanocrystals. This new frontier of research is expected to fill the so-called structure gap between the heterogeneous catalysts used in practice and the model systems optimized through experimental and computational studies involving single-crystal substrates. The other approach simply reduces the size of a metal nanoparticle down to the atomic level so the surface heterogeneity will naturally disappear. This approach is also well-aligned with the concept of active center that was introduced by Taylor in 1925 to account for the activity of a heterogeneous catalyst. According to his proposal, it is the monatomic steps, fissures, and/or troughs, rather than the entire surface of a catalytic particle or crystal, that catalyze the reaction. In the 1960s, Cossee and Arlman experimentally demonstrated the existence of well-defined active centers by identifying undercoordinated titanium ions associated with chloride vacancies as the active sites of the Ziegler–Natta catalyst based on α-TiCl3. Now the concept of active center is widely recognized by the catalysis community. It is accepted that the activity and/or selectivity of a heterogeneous catalyst can be tuned by engineering the active center. At the current stage of development, however, it remains a grand challenge to identify the active centers of many heterogeneous catalysts due to the lack of tools capable of probing catalytic reactions in real time and at atomic resolution. Alternatively, one can disperse a metal as atomic species on a solid support to obtain a single-atom-based catalyst and then examine its activity and selectivity toward a chemical reaction. By engineering the coordination environment around each atom, it is possible to tailor the active center for the establishment of structure–property relationship. As another advantage, the metal of interest would be utilized in the highest efficiency, enabling the development of cost-effective and sustainable products from low-abundance elements. The concept of dispersing a metal as atomic species has been pursued on and off for many decades. As early as in the 1960s, Boudart used chemisorption measurement to establish that the degree of dispersion of the metal could approach unity for some low-loading catalysts, suggesting the presence of atomically dispersed active sites.(1) In 1999, Iwasawa and co-workers demonstrated that Pt atoms deposited on MgO using the impregnation method were as active as Pt nanoparticles in catalyzing propane combustion.(2) They also characterized their catalyst using extended X-ray absorption fine structure (EXAFS) analysis to confirm the absence of Pt–Pt bonding and thus exclude the existence of Pt clusters or nanoparticles. In 2000, Abbet and co-workers examined the cyclotrimerization of acetylene on model catalysts prepared by depositing size-selected Pdn (1 ≤ n ≤ 30) clusters on MgO(100) thin films using the soft-landing technique.(3) They not only observed the formation of benzene on single Pd atoms at 300 K but also confirmed their atomic nature using Fourier transform infrared (FT-IR) spectroscopy, with 13CO serving as a probe. In a follow-up study, it was further demonstrated that CO oxidation could be catalyzed by the same model catalyst involving single Pd atoms supported on MgO(100).(4) In 2003, Flytzani-Stephanopoulos and co-workers discovered that the catalytic activity of Au or Pd nanoparticles toward water–gas shift (WGS) reaction did not change when the metal was selectively etched away from CeO2 support using a cyanide solution.(5) The authors proposed that the residual, nonmetallic species were the actual catalytic sites while the original nanoparticles only served as spectators. A similar observation was also made about two years later when the same etching protocol was applied to the Au/ZrO2 catalyst used for a hydrogenation reaction.(6) With the development of advanced electron microscopy, Lee and co-workers(7) and Gates and co-workers(8) reported the direct observation of isolated Pd atoms on Al2O3 and Ir atoms on MgO in 2007 and 2009, respectively. EXAFS measurements were also conducted to confirm the isolated nature of the metal atoms. Significantly, Lee and co-workers further demonstrated that the single Pd atoms were much more active toward alcohol oxidation than Pd clusters. In 2011, Zhang, Li, and Liu and their co-workers reported the preparation, characterization, and catalytic mechanism of a novel catalyst (denoted Pt1/FeOx) that involved single Pt atoms in the absence of any organic ligands or linkers.(9) The support-anchored single Pt atoms were clearly resolved by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) while the nature of interaction between the stable single Pt atoms and the iron oxide surface, as well as their mechanisms in catalyzing CO oxidation and preferential oxidation (PROX) reactions were theoretically elucidated. Most significantly, these authors coined the term of “single-atom catalysis” (SAC) to highlight the important role played by surface-supported single atoms in heterogeneous catalysis. The unequivocal identification and high activity of surface atoms have triggered an explosive growth of research activity around the general concept of SAC. Remarkably, there have been about 800 publications per year on this topic over the past several years. The catalytic support has been extended from the traditionally used metal oxides to metals, alloys, carbon materials, zeolites, metal–organic framework materials (MOFs), polyoxometallates, and two-dimensional materials (e.g., graphene, graphdiyne, MoS2, and MXene). In terms of application, SAC has been explored for a wide variety of reactions, including CO oxidation, PROX reaction, WGS reaction, Fischer–Tropsch synthesis (FTS), ammonia synthesis, methane conversion to ethylene, aromatics and alcohols, olefin and alkyne hydrogenation, and hydrocarbon dehydrogenation, as well as electrocatalysis and photocatalysis. With a unique combination of the merits associated with homogeneous catalysis and heterogeneous catalysis, SAC is quickly emerging as a research frontier for chemists, chemical engineers, physicists, and materials scientists. This thematic issue includes contributions from some of the most active players in the field of SAC. Pérez-Ramírez and co-workers provide a comprehensive review in terms of compositional diversity by joining different areas across the periodic table. They also highlight the coordination structures and associated properties accessed through distinct single atom–host combinations for applications in thermo-, electro-, and photocatalysis. Zheng and co-workers review the recent progress in understanding the coordination chemistry of SAC, with the inclusion of various combinations of metals and supports. Wang, Li, and co-workers review the various synthetic strategies, with a focus on how to stabilize single metal atoms on supports for the suppression of migration and agglomeration. They also highlight how synthetic conditions determine the structure and catalytic properties. Gates and co-workers focus on a unique system of structurally uniform catalysts fabricated by dispersion of metal atoms on crystalline supports such as zeolite-type materials, MOFs, and covalent organic frameworks. This system allows for precise characterization using atomic-resolution electron microscopy, X-ray absorption spectroscopy, and infrared spectroscopy, in addition to the modeling capability by density functional theory (DFT). Zhang and co-workers discuss oxide-based systems, including their synthetic procedures, characterizations, and reaction mechanism in an array of thermocatalysis, including WGS, selective oxidation/hydrogenation, and coupling reactions. Sykes and co-workers focus on a system known as single-atom alloys (SAAs), which are typically fabricated by atomically dispersing catalytically active elements such as Pt, Pd, Ni, Rh, and Ru in a more inert, but more selective, host metal. They also highlight the potential of SAAs in catalyzing a range of industrially important reactions. Xu and co-workers review the recent progress in the development of MOF-based systems, with an emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. Xiong and co-workers discuss SAC in the context of photocatalysis, with a focus on the metal–support interaction and how this interaction can be manipulated to optimize the photocatalytic performance. Shao, Chen, Wu, Zeng, Wang, and co-workers provide a comprehensive review on the recent developments of single-atom electrocatalysis for various energy-conversion reactions. Finally, Li and co-workers provide an overview of recent progress in the development of graphene-based SAC, with a focus on the stability of metal single atoms on different sites of a graphene support and the performance of such catalysts toward various reactions, including thermocatalysis and electrocatalysis. In organizing this thematic issue, we aim to bring some exciting, representative, and timely snapshots of research on SAC to the readers. Since this is a highly dynamic and fast-evolving field, it is impossible to cover all aspects of the research. With the interests and contributions from researchers in all disciplines of science and engineering, this field will surely continue to develop strongly. It is also hoped that the readers will enjoy the mix of topics presented in this issue and, most importantly, find the inspiration to push this field a step further toward greater success in terms of knowledge development and industrial applications. Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. This article references 9 other publications.
更新日期：2020-11-12
Jun Li received his Ph.D. degree in Physical Chemistry from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, in 1992. He did postdoctoral research at the University of Siegen (Germany) and The Ohio State University (USA) from 1994 to 1997. He worked as a Research Scientist at The Ohio State University as well as Senior Research Scientist and Staff Scientist at the Pacific Northwest National Laboratory from 1997 to 2009. He is now a full professor in the Department of Chemistry, Tsinghua University. He is an elected AAAS Fellow and Chinese Chemical Society Fellow. His research involves theoretical chemistry, heavy-element quantum chemistry, and computational catalysis science. More information can be found at http://www.junlilab.org Maria Flytzani Stephanopoulos (Miretta, deceased) studied at the National Technical University of Athens and received her B.S. degree in Chemical Engineering in 1973. She continued her graduate studies in the US and received her M.S. degree from the University of Florida (1975) and Ph.D. degree from the University of Minnesota (1978). She worked at the Caltech Jet Propulsion Laboratory as a Senior Engineer and MIT as a Principal Research Associate and Visiting Professor between 1985–1994. She joined the faculty of Chemical and Biological Engineering of Tufts University as an Associate Professor in 1994 and Full Professor in 1996. She was appointed to the Robert and Marcy Haber Professorship in Energy Sustainability in 2009 and named Tufts Distinguished Professor in 2015. She served on the editorial boards of several journals and as an Associate Editor of Science Advances and Editor of Applied Catalysis B. Miretta worked on fundamental and applied topics of heterogeneous catalysis, including hot gas desulfurization, water gas shift reaction, and oxidative conversion of methane to oxygenates. She is one of the pioneers of single-atom catalysis. For her teaching and research, Miretta received numerous awards, including Fellow of AAAS and AIChE and the Henry J. Albert Medal of the International Precious Metal Institute. In 2014, she was elected a member of the National Academy of Engineering. Younan Xia received his B.S. degree in Chemical Physics from the University of Science and Technology of China in 1987, M.S. degree in Inorganic Chemistry from University of Pennsylvania (with Alan G. MacDiarmid) in 1993, and Ph.D. degree in Physical Chemistry from Harvard University (with George M. Whitesides) in 1996. He started as an Assistant Professor of Chemistry at the University of Washington, Seattle in 1997 and then joined the Department of Biomedical Engineering at Washington University in St. Louis in 2007 as the James M. McKelvey Professor. Since 2012, he has held the position of Brock Family Chair and Georgia Research Alliance Eminent Scholar at the Georgia Institute of Technology. His group invented a myriad of nanomaterials with controlled properties, and these nanomaterials have found use in applications related to plasmonics, electronics, photonics, photovoltaics, display, catalysis, fuel cells, nanomedicine, and regenerative medicine. More information can be found at http://www.nanocages.com. As defined by Berzelius in 1835, a catalyst is a substance capable of accelerating the rate of a chemical reaction without being consumed. For many decades, catalysts have been a major contributor to the world’s economy. Currently, they are required for more than 90% of the chemical processes and there is an ever growing need for the development of advanced catalysts to secure a sustainable future for our society. Catalysts can be broadly divided into two groups: homogeneous catalysts that work in the same phase as the reactants (with soluble metal complexes and enzymes serving as typical examples) and heterogeneous catalysts that operate in a distinct phase (with notable examples including zeolites and metal nanoparticles deposited on solid supports). Because of its molecular nature, it is much easier to characterize and precisely describe a homogeneous catalyst, and even the reaction mechanism is amenable to rational manipulation. However, it is a daunting task to separate a homogeneous catalyst from the reaction product. The capabilities of homogeneous catalysts are also limited by their thermal instability, and most of them can only be used in a solution phase at temperatures below 100 °C. In contrast, heterogeneous catalysts are well-known for their robustness and easiness of separation and recovery from reaction mixtures. As a result, heterogeneous catalysts account for about 80% of all the catalytic processes currently used in industry. Despite their widespread use, heterogeneous catalysts are intrinsically plagued by surface heterogeneity and structural complexity, two factors that make it extremely difficult to decipher the catalytic mechanisms. Taking the metal nanoparticle as an example, its surface is typically terminated in atoms with all different coordination environments, including those situated on different vertices, edges, and faces. The presence of defects such as atomic steps, kinks, vacancies, stacking faults, and twin boundaries also makes such a catalytic system even more complicated. In practice, it is almost impossible to fabricate two identical nanoparticles, a situation completely different from the homogeneous catalysts based on molecular or biomacromolecular systems. There are two parallel approaches to addressing the heterogeneity and complexity issues of a heterogeneous catalyst. In the context of surface science, these issues are circumvented by switching to model catalysts based on single-crystal substrates that can be fabricated with well-defined surface structures. The research along this line has established that many catalytic reactions are highly sensitive to the exact packing of atoms on the surface. For example, it was reported that ammonia could be produced on an Fe(111) surface at a rate almost 20- and 420-fold faster than those on Fe(100) and Fe(110) surfaces, respectively. Recently, such fundamental studies in surface science were fruitfully combined with colloidal metal nanocrystals to provide a rational route to the development of heterogeneous catalysts with the optimal activity and/or selectivity toward a chemical reaction. Colloidal nanocrystals made of many metals and their alloys can now be prepared as small as those in conventional catalysts to maintain the large specific surface area. More significantly, they can be synthesized with a well-defined shape to present only one specific type of facet on the surface, for example, an octahedral shape for the exposure of {111} facets that are equivalent to the (111) substrates used in surface science. By maneuvering the shape taken by the nanocrystals, it is also feasible to introduce well-defined defects to the surface, such as the twin boundaries associated with decahedral or icosahedral nanocrystals. This new frontier of research is expected to fill the so-called structure gap between the heterogeneous catalysts used in practice and the model systems optimized through experimental and computational studies involving single-crystal substrates. The other approach simply reduces the size of a metal nanoparticle down to the atomic level so the surface heterogeneity will naturally disappear. This approach is also well-aligned with the concept of active center that was introduced by Taylor in 1925 to account for the activity of a heterogeneous catalyst. According to his proposal, it is the monatomic steps, fissures, and/or troughs, rather than the entire surface of a catalytic particle or crystal, that catalyze the reaction. In the 1960s, Cossee and Arlman experimentally demonstrated the existence of well-defined active centers by identifying undercoordinated titanium ions associated with chloride vacancies as the active sites of the Ziegler–Natta catalyst based on α-TiCl3. Now the concept of active center is widely recognized by the catalysis community. It is accepted that the activity and/or selectivity of a heterogeneous catalyst can be tuned by engineering the active center. At the current stage of development, however, it remains a grand challenge to identify the active centers of many heterogeneous catalysts due to the lack of tools capable of probing catalytic reactions in real time and at atomic resolution. Alternatively, one can disperse a metal as atomic species on a solid support to obtain a single-atom-based catalyst and then examine its activity and selectivity toward a chemical reaction. By engineering the coordination environment around each atom, it is possible to tailor the active center for the establishment of structure–property relationship. As another advantage, the metal of interest would be utilized in the highest efficiency, enabling the development of cost-effective and sustainable products from low-abundance elements. The concept of dispersing a metal as atomic species has been pursued on and off for many decades. As early as in the 1960s, Boudart used chemisorption measurement to establish that the degree of dispersion of the metal could approach unity for some low-loading catalysts, suggesting the presence of atomically dispersed active sites.(1) In 1999, Iwasawa and co-workers demonstrated that Pt atoms deposited on MgO using the impregnation method were as active as Pt nanoparticles in catalyzing propane combustion.(2) They also characterized their catalyst using extended X-ray absorption fine structure (EXAFS) analysis to confirm the absence of Pt–Pt bonding and thus exclude the existence of Pt clusters or nanoparticles. In 2000, Abbet and co-workers examined the cyclotrimerization of acetylene on model catalysts prepared by depositing size-selected Pdn (1 ≤ n ≤ 30) clusters on MgO(100) thin films using the soft-landing technique.(3) They not only observed the formation of benzene on single Pd atoms at 300 K but also confirmed their atomic nature using Fourier transform infrared (FT-IR) spectroscopy, with 13CO serving as a probe. In a follow-up study, it was further demonstrated that CO oxidation could be catalyzed by the same model catalyst involving single Pd atoms supported on MgO(100).(4) In 2003, Flytzani-Stephanopoulos and co-workers discovered that the catalytic activity of Au or Pd nanoparticles toward water–gas shift (WGS) reaction did not change when the metal was selectively etched away from CeO2 support using a cyanide solution.(5) The authors proposed that the residual, nonmetallic species were the actual catalytic sites while the original nanoparticles only served as spectators. A similar observation was also made about two years later when the same etching protocol was applied to the Au/ZrO2 catalyst used for a hydrogenation reaction.(6) With the development of advanced electron microscopy, Lee and co-workers(7) and Gates and co-workers(8) reported the direct observation of isolated Pd atoms on Al2O3 and Ir atoms on MgO in 2007 and 2009, respectively. EXAFS measurements were also conducted to confirm the isolated nature of the metal atoms. Significantly, Lee and co-workers further demonstrated that the single Pd atoms were much more active toward alcohol oxidation than Pd clusters. In 2011, Zhang, Li, and Liu and their co-workers reported the preparation, characterization, and catalytic mechanism of a novel catalyst (denoted Pt1/FeOx) that involved single Pt atoms in the absence of any organic ligands or linkers.(9) The support-anchored single Pt atoms were clearly resolved by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) while the nature of interaction between the stable single Pt atoms and the iron oxide surface, as well as their mechanisms in catalyzing CO oxidation and preferential oxidation (PROX) reactions were theoretically elucidated. Most significantly, these authors coined the term of “single-atom catalysis” (SAC) to highlight the important role played by surface-supported single atoms in heterogeneous catalysis. The unequivocal identification and high activity of surface atoms have triggered an explosive growth of research activity around the general concept of SAC. Remarkably, there have been about 800 publications per year on this topic over the past several years. The catalytic support has been extended from the traditionally used metal oxides to metals, alloys, carbon materials, zeolites, metal–organic framework materials (MOFs), polyoxometallates, and two-dimensional materials (e.g., graphene, graphdiyne, MoS2, and MXene). In terms of application, SAC has been explored for a wide variety of reactions, including CO oxidation, PROX reaction, WGS reaction, Fischer–Tropsch synthesis (FTS), ammonia synthesis, methane conversion to ethylene, aromatics and alcohols, olefin and alkyne hydrogenation, and hydrocarbon dehydrogenation, as well as electrocatalysis and photocatalysis. With a unique combination of the merits associated with homogeneous catalysis and heterogeneous catalysis, SAC is quickly emerging as a research frontier for chemists, chemical engineers, physicists, and materials scientists. This thematic issue includes contributions from some of the most active players in the field of SAC. Pérez-Ramírez and co-workers provide a comprehensive review in terms of compositional diversity by joining different areas across the periodic table. They also highlight the coordination structures and associated properties accessed through distinct single atom–host combinations for applications in thermo-, electro-, and photocatalysis. Zheng and co-workers review the recent progress in understanding the coordination chemistry of SAC, with the inclusion of various combinations of metals and supports. Wang, Li, and co-workers review the various synthetic strategies, with a focus on how to stabilize single metal atoms on supports for the suppression of migration and agglomeration. They also highlight how synthetic conditions determine the structure and catalytic properties. Gates and co-workers focus on a unique system of structurally uniform catalysts fabricated by dispersion of metal atoms on crystalline supports such as zeolite-type materials, MOFs, and covalent organic frameworks. This system allows for precise characterization using atomic-resolution electron microscopy, X-ray absorption spectroscopy, and infrared spectroscopy, in addition to the modeling capability by density functional theory (DFT). Zhang and co-workers discuss oxide-based systems, including their synthetic procedures, characterizations, and reaction mechanism in an array of thermocatalysis, including WGS, selective oxidation/hydrogenation, and coupling reactions. Sykes and co-workers focus on a system known as single-atom alloys (SAAs), which are typically fabricated by atomically dispersing catalytically active elements such as Pt, Pd, Ni, Rh, and Ru in a more inert, but more selective, host metal. They also highlight the potential of SAAs in catalyzing a range of industrially important reactions. Xu and co-workers review the recent progress in the development of MOF-based systems, with an emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. Xiong and co-workers discuss SAC in the context of photocatalysis, with a focus on the metal–support interaction and how this interaction can be manipulated to optimize the photocatalytic performance. Shao, Chen, Wu, Zeng, Wang, and co-workers provide a comprehensive review on the recent developments of single-atom electrocatalysis for various energy-conversion reactions. Finally, Li and co-workers provide an overview of recent progress in the development of graphene-based SAC, with a focus on the stability of metal single atoms on different sites of a graphene support and the performance of such catalysts toward various reactions, including thermocatalysis and electrocatalysis. In organizing this thematic issue, we aim to bring some exciting, representative, and timely snapshots of research on SAC to the readers. Since this is a highly dynamic and fast-evolving field, it is impossible to cover all aspects of the research. With the interests and contributions from researchers in all disciplines of science and engineering, this field will surely continue to develop strongly. It is also hoped that the readers will enjoy the mix of topics presented in this issue and, most importantly, find the inspiration to push this field a step further toward greater success in terms of knowledge development and industrial applications. Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. This article references 9 other publications.
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