Catalyst: Single-Atom Catalysis: Directing the Way toward the Nature of Catalysis

Xuning Li is a research fellow at Nanyang Technological University (NTU). He earned his PhD at Dalian Institute of Chemical Physics (DICP) in 2017. His current research interest focuses on in situ/operando characterization techniques in single-atom catalysis.Yanqiang Huang received his PhD from DICP in 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 PhD from the University of Minnesota (2011) and then moved to the University of California, Berkeley, as a postdoctoral researcher (2012). He is now an associate professor at NTU. His main research interests are electrocatalysis, photovoltaics, and photo-electrochemistry. Xuning Li is a research fellow at Nanyang Technological University (NTU). He earned his PhD at Dalian Institute of Chemical Physics (DICP) in 2017. His current research interest focuses on in situ/operando characterization techniques in single-atom catalysis. Yanqiang Huang received his PhD from DICP in 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 PhD from the University of Minnesota (2011) and then moved to the University of California, Berkeley, as a postdoctoral researcher (2012). He is now an associate professor at NTU. His main research interests are electrocatalysis, photovoltaics, and photo-electrochemistry. Catalyst, forming temporary intermediates to reduce the kinetic barrier of a chemical reaction, is involved in over 80% of the chemical production process. Typically, catalyst can be classified as homogeneous, heterogeneous, and biological (enzymes), among which heterogeneous catalysis is of paramount importance and has received Nobel Prizes in many areas of chemical and energy industry including Fritz Haber, Carl Bosch, Irving Langmuir, and Gerhard Ertl in 1918, 1931, 1932, and 2007, respectively. In heterogeneous catalysis, the active sites, and their coordination environment and electronic structure, are the most important factors that affect the overall performance of the catalysts. Hence, a high degree of metal dispersion (i.e., the fraction of surface atoms) is critical, especially for catalysts comprising platinum group metals, as it determines the available active sites. One of the most common approaches to increase metal dispersion is by using support, in which nanosized metal particles are dispersed on the support for efficient use of catalytically active components. Even so, only a small fraction of coordinatively unsaturated metal atoms located at apices, edges, steps, and corners are exposed to reactants, making the utilization efficiency of metal atoms far from satisfactory. To this end, catalysts with atomically dispersed metal atoms, referred to as single-atom catalysts (SACs), are highly desirable for maximizing the atom efficiency in catalytic reactions. The terminology of SAC (defined as a supported metal catalyst exclusively composed of isolated monometal active sites) was first introduced in 2011 by Zhang, Li, Liu, and co-workers,1Qiao B. Wang A. Yang X. Allard L.F. Jiang Z. Cui Y. Liu J. Li J. Zhang T. Single-atom catalysis of CO oxidation using Pt1/FeOx.Nat. Chem. 2011; 3: 634-641Crossref PubMed Scopus (3935) Google Scholar who reported the extremely high atom efficiency of a single-atom catalyst consisting of only isolated single Pt atoms dispersed on FeOx (Pt1/FeOx) for CO oxidation. This work not only presents the first practical fabrication of single-Pt-atom catalyst but also elucidates the strong binding of single Pt atoms coordinated by three oxygen atoms on the Fe-vacancy and at the same time resolves the catalytic mechanism of CO oxidation with a combination of density functional theory (DFT) studies. At this point, we may take a new look at the significance of single-atom catalysis besides the extremely high atom efficiency. The atomic dispersion of coordinatively unsaturated metal atoms with unique structural and electronic properties shall afford great opportunities for the rational design of desirable catalysts with high activity, stability, and selectivity. Moreover, being viewed as a conceptual bridge between homogeneous and heterogeneous catalyst,2Wang A. Li J. Zhang T. Heterogeneous single-atom catalysis.Nat. Rev. Chem. 2018; 2: 65-81Crossref Scopus (1917) Google Scholar SAC offers us room for deeper insights into both the stepwise elementary reaction mechanism and the electronic environment of the smallest catalytic blocks, which may further sharpen our comprehension of electronic structures over the catalytic sites and even provide electronic-level insight into the underlying catalytic mechanism (Figure 1). Nowadays, arguably, single-atom catalysis has been becoming the most active research frontier in heterogeneous catalysis. Numerous progresses have been achieved in synthetic strategies, characterization techniques, and computational modeling of SACs for a wide variety of chemical reactions including CO oxidation, water-gas shift reaction, chemoselective hydrogenation, photocatalysis, electrocatalysis, etc. However, the rapid developments of SACs over the past few years appear to highlight some indistinct issues, which deserve further discussion around the long-term development of single-atom catalysis. First of all, constructing SACs with a high density of accessible active sites and stability is always significant for the potential industrial applications. Over the past few years, a variety of synthetic strategies including co-precipitation, electrostatic adsorption, photochemical, ion-exchange, high-temperature atom trapping, plasma sputtering, pyrolysis, etc. have been developed for producing SACs with a high density of thermally stable single atoms. As synthetic knowledge grows, many effective strategies to suppress the mobility and aggregation of single metal atoms into nanoparticles at elevated catalytic temperatures have been developed, such as confining atoms on support with surface defects or vacancies, electronic interaction, or chemical bonding to nonmetallic atoms (including N, S, P, etc.), or through electronic interaction have been recognized as the most effective ways to suppress the mobility and aggregation of single metal atoms into nanoparticles at elevated temperatures. In addition, single atoms also could be stabilized through a strong, covalent metal-support interaction, which is not associated with surface defects, as the genesis of high concentrations of thermally stable single Pt atoms on reducible Fe2O3 support.3Lang R. Xi W. Liu J.-C. Cui Y.-T. Li T. Lee A.F. Chen F. Chen Y. Li L. Li L. et al.Non defect-stabilized thermally stable single-atom catalyst.Nat. Commun. 2019; 10: 234Crossref PubMed Scopus (336) Google Scholar Recently, the rapid development in atomic-resolution characterization techniques such as in situ environmental transmission electron microscopy has realized direct observation of the dynamic processes for transformation of noble metal nanoparticles to thermally stable single atoms.4Wei S. Li A. Liu J.-C. Li Z. Chen W. Gong Y. Zhang Q. Cheong W.-C. Wang Y. Zheng L. et al.Direct observation of noble metal nanoparticles transforming to thermally stable single atoms.Nat. Nanotechnol. 2018; 13: 856-861Crossref PubMed Scopus (538) Google Scholar These achievements have greatly contributed to the fast development of single-atom catalysis. However, more insights into the principle of anchoring single atoms with high density and stability are still urgently needed. Second, developing SACs with comparable activity and selectivity to those of the state-of-the-art catalysts in much broader catalytic systems is of great importance. Since only atomically distributed active sites are available, SACs often show unique selectivity distinct from nanoparticles due to the inhibition of multi-atoms involved the catalytic pathway. Though SACs have been proven capable of accelerating various chemical reactions, the catalytic activity is generally compared based on turnover frequency (TOF, moles of product formed per active metal atom per unit time). The reaction rate per overall catalyst mass of SACs in most cases is still much lower than that of supported nanoparticle (NP)-based catalysts due to the lower loading of active atoms for the former. Nevertheless, by changing the adsorption and activation modes of active sites with the reactants, intermediates, and products, the unique and uniform structure of SACs is beneficial to improving the catalytic activities and selectivities. Recently, studies applying SACs in the once-dominant homogeneous catalytic processes like hydrogenation, selective hydrocarbon oxidations, and C-C coupling reactions have attracted special attentions. For instance, in the work conducted by Wang et al.,5Wang L. Zhang W. Wang S. Gao Z. Luo Z. Wang X. Zeng R. Li A. Li H. Wang M. et al.Atomic-level insights in optimizing reaction paths for hydroformylation reaction over Rh/CoO single-atom catalyst.Nat. Commun. 2016; 7: 14036-14043Crossref PubMed Scopus (212) Google Scholar single-atom Rh catalysts supported on CoO were demonstrated to have remarkable activity and selectivity toward propene hydroformylation comparable to that of the Rh-based homogeneous catalysts. DFT calculations reveal the reconstruction of single-Rh-atom sites, while kinetic analysis determines the dominating linear products during the catalytic process. Chen et al. reported a single-atom Pd catalyst with catalytic performance surpassing that of the homogeneous catalysts and conventional heterogeneous catalysts for Suzuki coupling.6Chen Z. Vorobyeva E. Mitchell S. Fako E. Ortuño M.A. López N. Collins S.M. Midgley P.A. Richard S. Vilé G. Pérez-Ramírez J. A heterogeneous single-atom palladium catalyst surpassing homogeneous systems for Suzuki coupling.Nat. Nanotechnol. 2018; 13: 702-707Crossref PubMed Scopus (354) Google Scholar The enhanced properties were attributed to the adaptive coordination of Pd atoms anchored on exfoliated graphitic carbon nitride. In a recent study performed by Shao et al.,7Shao X. Yang X. Xu J. Liu S. Miao S. Liu X. Su X. Duan H. Huang Y. Zhang T. Iridium single-atom catalyst performing a quasi-homogeneous hydrogenation transformation of CO2 to formate.Chem. 2019; 5: 693-705Abstract Full Text Full Text PDF Scopus (127) Google Scholar an atomically dispersed Ir catalyst with quasi-homogeneous structure was fabricated by designing a porous organic polymer with aminopyridine functionalities. The chemical structure of the active single-atom site was demonstrated as similar to a homogeneous mononuclear Ir pincer complex catalyst, which results in the best catalytic performance for the liquid-phase hydrogenation of CO2 to formate. These findings highlight the potential prospects for the rational design of efficient SACs in much broader catalytic systems, especially the once-dominant homogeneous catalytic processes. Third, in a catalytic process, catalyst generally reacts with one or more reactants to form intermediates that subsequently give the final reaction products. However, the reaction intermediates for most catalytic processes have rarely been well identified, not to mention the structural and electronic evolution of the catalytic sites. Fortunately, SACs provide us with great opportunities for capturing the reaction intermediates and for real-time monitoring of the dynamic behaviors of both the geometric structure and electronic environment of the active sites during catalytic processes. The knowledge thus obtained is significant for revealing the molecular structure-activity/selectivity relationships and providing electronic-level insight into stepwise elementary reaction mechanism. However, due to lack of an effective synthetic method for model SACs with definite coordination environment and the practical operando spectroscopic techniques with high atomic resolution, up to now, little experimental evidence is available regarding the structural and electronic dynamics of SACs during catalysis. To this end, it is crucial to rationally design SACs with a well-controlled coordination environment, which may offer us ideal model systems for deeper insights into the structure-activity relationships and the underlying catalytic mechanisms. Recently, to quantitatively establish correlation between the atomistic structure and the catalytic performance of the metal centers, Fei et al. reported a general approach to synthesize a series of atomically dispersed transition-metal catalysts embedded on nitrogen-doped graphene with common MN4C4 moiety.8Fei H. Dong J. Feng Y. Allen C.S. Wan C. Volosskiy B. Li M. Zhao Z. Wang Y. Sun H. et al.General synthesis and definitive structural identification of MN4C4 single-atom catalysts with tunable electrocatalytic activities.Nat. Catal. 2018; 1: 63-72Crossref Scopus (1113) Google Scholar In a more recent study performed by Xu et al.,9Xu H. Cheng D. Cao D. Zeng X.C. A universal principle for a rational design of single-atom electrocatalysts.Nat. Catal. 2018; 1: 339-348Crossref Scopus (883) Google Scholar a universal design principle of SACs toward a highly efficient and cost-effective electrochemical reaction was developed based on DFT calculations. All these results pave the way for the rational design and construction of SACs with well-controlled local environments, which are highly desirable but still challenging for both fundamental research and potential practical application. Moreover, the combination of advanced operando techniques with well-designed SACs as model catalysts may direct a new way for understanding active-center electronic environments and providing electronic-level insight into the underlying catalytic mechanism. Most recently, an example aiming to gain insights into the adsorption and activation of CO2 on a single-atom Ni catalyst was conducted by Yang et al.10Yang H.B. Hung S.-F. Liu S. Yuan K. Miao S. Zhang L. Huang X. Wang H.-Y. Cai W. Chen R. et al.Atomically dispersed Ni(I) as the active site for electrochemical CO2 reduction.Nat. Energy. 2018; 3: 140-147Crossref Scopus (1161) Google Scholar Monitored with operando X-ray absorption spectroscopy measurements, the structural evolution of the NiN4 site and charge transfer from Ni to the C2p orbital in CO2 with formation of CO2δ- species were identified. Nevertheless, more experimental evidences of the dynamic behaviors of SACs during reaction are highly desired for our in-depth understanding of the underlying mechanism of catalysis. To conclude, although great achievements have been made over the past few years, the development of SACs is still in its infancy. More catalytic systems where SACs may function with high activity, stability, and selectivity are still waiting to be discovered. In regard to this, the rational design of single-cluster catalysts (SCCs), including dimers, trimers, or larger metal clusters, is also a promising research direction to flat the defects of SACs for certain chemical processes. Novel synthetic strategies to realize controllable synthesis of SACs with high site density and definite coordination environment, as well as the underlying principles for anchoring single atoms on inorganic supports like transition metal oxides, have yet to be explored. Moreover, more experimental evidences of the structural and electronic dynamic behaviors of SACs during the catalysis process, associated with novel operando spectroscopic techniques, as well as further theoretical insights into the underlying stepwise elementary reaction mechanisms are still waiting to be found. Despite the twists and turns appareny in the long-term development of single-atom catalysis, a bright future is on its way. Reaction: Industrial Perspective on Single-Atom CatalysisWang et al.ChemNovember 4, 2019In BriefSingle-atom catalysis emerges as a new frontier in heterogeneous catalysis. This reaction discusses several benefits of single-atom catalysis as well as some issues related to its application in industrial catalysis. Full-Text PDF Open ArchiveReaction: Open Up the Era of Atomically Precise CatalysisZhu et al.ChemNovember 4, 2019In BriefSingle-atom catalysts (SACs) push catalyst design to the ultimate dimension and have been demonstrated to show superior performance in numerous reactions. Thus, we are left with the question, what is the next favorite after the SAC age? In this reaction piece, we extract the concept of “atomically precise catalysis” from the cutting-edge studies concerning the catalytic center composed of multi-atomic sites and explore this research trend in catalysis with a special focus on interatomic interfaces. Full-Text PDF Open Archive