Evolution of Isolated Atoms and Clusters in Catalysis

Structural evolution of isolated metal atoms and metal clusters with low atomicity is commonly observed in both homogeneous and heterogeneous catalysis. This evolution is directly associated with the formation of the working active sites and deactivation mechanism of the catalyst.Understanding the evolutional behavior of subnanometric metal catalysts can help to stabilize the active sites under harsh reaction conditions. Regeneration of the deactivated metal catalyst also relies on atomic-level understanding of the dynamic structural transformation processes. Structural transformation and evolution of active metal sites can occur in metal-catalyzed reactions in both homogeneous and heterogeneous systems. Such structural changes have an important impact on the catalytic behavior, including activity, selectivity, and stability. Aiming to establish a link between homogeneous and heterogeneous catalytic systems, this review begins with a discussion on dynamic structural transformations of metal catalysts in homogeneous reactions and the corresponding implications. We then discuss the evolution of isolated metal atoms and clusters in heterogeneous catalysts during catalyst activation and under reaction conditions. Finally, strategies for stabilizing subnanometric metal species on solid supports are presented for potential industrial applications. Structural transformation and evolution of active metal sites can occur in metal-catalyzed reactions in both homogeneous and heterogeneous systems. Such structural changes have an important impact on the catalytic behavior, including activity, selectivity, and stability. Aiming to establish a link between homogeneous and heterogeneous catalytic systems, this review begins with a discussion on dynamic structural transformations of metal catalysts in homogeneous reactions and the corresponding implications. We then discuss the evolution of isolated metal atoms and clusters in heterogeneous catalysts during catalyst activation and under reaction conditions. Finally, strategies for stabilizing subnanometric metal species on solid supports are presented for potential industrial applications. Since the 1960s, surface reconstruction or structural transformation of solid materials under chemical reaction conditions has been well established by surface science studies after the introduction of ultrahigh vacuum systems and the related measurement techniques [1.Ertl G. Reactions at surfaces: from atoms to complexity (Nobel Lecture).Angew. Chem. Int. Ed. 2008; 47: 3524-3535Crossref PubMed Scopus (679) Google Scholar,2.Somorjai G.A. 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Based on the significant development of advanced characterization techniques (e.g., aberration-corrected electron microscopy, advanced in situ spectroscopy) and new methodologies for catalyst preparation, research on the nature of the active sites in heterogeneous metal catalysts has shifted in focus from nanoparticles to subnanometric metal catalysts; namely, isolated single atoms and metal clusters consisting of only a few atoms [8.Bergmann A. Roldan Cuenya B. Operando insights into nanoparticle transformations during catalysis.ACS Catal. 2019; 9: 10020-10043Crossref Scopus (1) Google Scholar, 9.Wang A. et al.Heterogeneous single-atom catalysis.Nat. Rev. Chem. 2018; 2: 65-81Crossref Google Scholar, 10.Liu L. Corma A. Metal catalysts for heterogeneous catalysis: from single atoms to nanoclusters and nanoparticles.Chem. Rev. 2018; 118: 4981-5079Crossref PubMed Scopus (539) Google Scholar]. 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To achieve that unified mechanistic understanding, the dynamic structural transformation of the metal species will play a critical role, since such dynamic behavior exists commonly in both homogeneous and heterogeneous systems (Box 1).Box 1The Structures of Typical Homogeneous and Heterogeneous Metal CatalystsAccording to the size, chemical composition, and coordination environment, typical metal catalysts used in homogeneous and heterogeneous systems can be categorized into mononuclear metal complexes (supported isolated atoms), metal clusters, and nanoparticles. In Figure I, the structures of the metal catalysts involved in this review are briefly illustrated. It should be noted that the atomic structure of a metal particle (either a cluster or a nanoparticle) will be quite complicated and highly dynamic under reaction conditions. Nevertheless, in many cases the ligands or coordination atoms to the metal sites should also be considered when discussing the structure of the active sites and the reaction mechanism. According to the size, chemical composition, and coordination environment, typical metal catalysts used in homogeneous and heterogeneous systems can be categorized into mononuclear metal complexes (supported isolated atoms), metal clusters, and nanoparticles. In Figure I, the structures of the metal catalysts involved in this review are briefly illustrated. It should be noted that the atomic structure of a metal particle (either a cluster or a nanoparticle) will be quite complicated and highly dynamic under reaction conditions. Nevertheless, in many cases the ligands or coordination atoms to the metal sites should also be considered when discussing the structure of the active sites and the reaction mechanism. In this review, we begin with a discussion of how molecular metal compounds or complexes may transform into metal clusters or nanoparticles in homogeneous catalytic systems. Implicated by the evolutional behavior of molecular metal catalysts, we proceed to heterogeneous systems and discuss how the atomicity of the supported metal catalysts may change under reaction conditions, as well as discussing the corresponding implications for reactivity. Finally, perspectives on strategies for stabilization of the supported subnanometric metal catalysts, methods for catalyst regeneration, and relevant future research trends are presented. Conventionally, active sites in homogeneous catalytic systems are usually associated with mononuclear metal complexes with ligands or reactants coordinating with the metal centers, which originate from metal complexes added in the reaction mixture. In some cases, binuclear or multinuclear metal species are also proposed to be the working active sites [20.Farley C.M. Uyeda C. Organic reactions enabled by catalytically active metal–metal bonds.Trends Chem. 2019; 1: 497-509Abstract Full Text Full Text PDF Scopus (0) Google Scholar]; however, the detailed structural transformation, changes of the atomicity of the metal species in homogeneous catalysis, and the final active species formed are rarely studied. In 2012, Corma and colleagues reported the in situ generation of subnanometric Au clusters during the ester-assisted hydration of alkynes and bromination of p-dimethoxybenzene when using HAuCl4 and AuCl compounds as the initial Au catalyst [21.Oliver-Meseguer J. et al.Small gold clusters formed in solution give reaction turnover numbers of 107 at room temperature.Science. 2012; 338: 1452-1455Crossref PubMed Scopus (277) Google Scholar]. As shown in Figure 1A,B , it has been observed in both reactions that the initially introduced Au salts were not the active species, but rather the metal clusters formed during the reactions. Moreover, each reaction was better catalyzed by in situ-formed subnanometric Au clusters of different atomicity. Those observations are not exclusive for Au-catalyzed reactions but have also been observed in Pd-catalyzed C–C coupling reactions, Cu-catalyzed C–N, C–O, and C–N coupling reactions, and Pt-catalyzed hydrosilylation of alkynes (Figure 1C–E) [22.Leyva-Perez A. et al.Water-stabilized three- and four-atom palladium clusters as highly active catalytic species in ligand-free C–C cross-coupling reactions.Angew. Chem. Int. Ed. 2013; 52: 11554-11559Crossref PubMed Scopus (69) Google Scholar, 23.Oliver-Meseguer J. et al.Stabilized naked sub-nanometric Cu clusters within a polymeric film catalyze C–N, C–C, C–O, C–S, and C–P bond-forming reactions.J. Am. Chem. 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In recent work, Rh5 clusters have been found to be generated from organometallic mononuclear Rh complexes in the presence of H2, and the Rh5 clusters serve as the working catalysts for homogeneous hydrogenation of N-heteroarenes [25.Kim S. et al.Hydrogenation of N-heteroarenes using rhodium precatalysts: reductive elimination leads to formation of multimetallic clusters.J. Am. Chem. Soc. 2019; 141: 17900-17908Crossref PubMed Scopus (0) Google Scholar]. By contrast, the transformation of organometallic complexes under reaction conditions can cause catalyst deactivation. This is shown with the evolution of a mononuclear Ir complex into Ir6 clusters during the glycerol dehydrogenation reaction [26.Campos J. et al.A carbene-rich but carbonyl-poor [Ir6(IMe)8(CO)2H14]2+ polyhydride cluster as a deactivation product from catalytic glycerol dehydrogenation.Angew. Chem. Int. Ed. 2014; 53: 12808-12811Crossref PubMed Scopus (24) Google Scholar], which could be caused by the reduction of the mononuclear Ir complex by the product (H2) or the reactant (glycerol). In homogeneous systems, the solvent can also play a role in the structural transformation of metal compounds. For example, when heating a mixture of CuCl and dimethyl sulfoxide (DMSO) in the presence of O2, CuCl can transform into CuOx clusters or nanoparticles [27.Liu L. et al.Facile synthesis of surface-clean monodispersed CuOx nanoparticles and their catalytic properties for oxidative coupling of alkynes.ACS Catal. 2016; 6: 2211-2221Crossref Scopus (18) Google Scholar]. Such a transformation has also been observed under the reaction conditions for CuCl-catalyzed oxidative coupling of terminal alkynes. Mechanistic and isotopic studies have shown that the in situ-formed CuOx nanoparticles (~2 nm in diameter) are the active species for oxidative coupling of terminal alkynes, which is also confirmed with heterogeneous CuOx/TiO2 catalysts. In the earlier case, the role of DMSO could be to promote the decomposition of CuCl and stabilize the resultant CuOx nanoparticles, while in some other cases solvents with weak reducibility [e.g., dimethylformamide (DMF)] can partially reduce the metal complex and stabilize the resultant metal clusters [28.Kawasaki H. et al.Surfactant-free solution synthesis of fluorescent platinum subnanoclusters.Chem. Commun. 2010; 46: 3759-3761Crossref PubMed Scopus (87) Google Scholar]. The growth of metal species from molecular precursors into metal clusters or nanoparticles during catalytic reactions can be treated as nucleation-growth processes, which have been intensively studied in the synthesis of colloid metal nanocrystals [29.Xia Y. et al.Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics?.Angew. Chem. Int. Ed. 2009; 48: 60-103Crossref PubMed Scopus (3712) Google Scholar]. The solvent and reactants are the reducing agents that drive the transformation of the molecular metal catalysts into clusters or nanoparticles (Figure 2A ) [30.Polte J. Fundamental growth principles of colloidal metal nanoparticles – a new perspective.CrystEngComm. 2015; 17: 6809-6830Crossref Google Scholar]. However, the concentration of the molecular metal precursor in the catalytic reactions is usually much lower than that for the wet-chemistry synthesis of metal nanoparticles, which makes the transformation between molecular metal species, clusters, and nanoparticles in liquid-phase reactions a dynamic process that depends strongly on the reaction conditions. As mentioned before, it has been reported that metal nanoparticles can disintegrate into smaller clusters or even single atoms in homogeneous systems [5.Corma A. Heterogeneous catalysis: understanding for designing, and designing for applications.Angew. Chem. Int. Ed. 2016; 55: 6112-6113Crossref PubMed Scopus (61) Google Scholar,10.Liu L. Corma A. Metal catalysts for heterogeneous catalysis: from single atoms to nanoclusters and nanoparticles.Chem. Rev. 2018; 118: 4981-5079Crossref PubMed Scopus (539) Google Scholar]. A typical example is the leaching of Pd nanoparticles into Pd atoms and clusters during a C–C coupling reaction (Figure 2B). The oxidative addition of halide compounds to Pd nanoparticles is a critical step that causes the leaching of Pd species into solution [31.de Vries J.G. A unifying mechanism for all high-temperature Heck reactions. The role of palladium colloids and anionic species.Dalton Trans. 2006; : 421-429Crossref PubMed Google Scholar, 32.Phan N.T.S. et al.On the nature of the active species in palladium catalyzed Mizoroki–Heck and Suzuki–Miyaura couplings – homogeneous or heterogeneous catalysis, a critical review.Adv. Synth. Catal. 2006; 348: 609-679Crossref Scopus (1589) Google Scholar, 33.Kashin A.S. Ananikov V.P. Catalytic C–C and C–heteroatom bond formation reactions: in situ generated or preformed catalysts? Complicated mechanistic picture behind well-known experimental procedures.J. Org. Chem. 2013; 78: 11117-11125Crossref PubMed Scopus (93) Google Scholar]. This process is similar to oxidative etching in the synthesis and shape control of colloid metal nanoparticles [34.Long R. et al.Oxidative etching for controlled synthesis of metal nanocrystals: atomic addition and subtraction.Chem. Soc. Rev. 2014; 43: 6288-6310Crossref PubMed Google Scholar]. It has been directly observed via transmission electron microscopy (TEM) that metal nanoparticles can be dissolved in solution under certain conditions as a consequence of oxidative etching [35.Wu J. et al.Dissolution kinetics of oxidative etching of cubic and icosahedral platinum nanoparticles revealed by in situ liquid transmission electron microscopy.ACS Nano. 2017; 11: 1696-1703Crossref PubMed Scopus (30) Google Scholar,36.Jiang Y. et al.In situ study of oxidative etching of palladium nanocrystals by liquid cell electron microscopy.Nano Lett. 2014; 14: 3761-3765Crossref PubMed Scopus (66) Google Scholar]. The presence of nucleophilic molecules (e.g., water, amines) can promote the disintegration of Pd nanoparticles into Pd clusters under the reaction conditions for C–C cross-coupling reactions [21.Oliver-Meseguer J. et al.Small gold clusters formed in solution give reaction turnover numbers of 107 at room temperature.Science. 2012; 338: 1452-1455Crossref PubMed Scopus (277) Google Scholar]. Due to the small size of the in situ-formed metal clusters and their flexible geometric structures, it is extremely difficult to resolve their atomic structures. The presence of metal clusters is usually confirmed by spectroscopy or mass spectrometry techniques. For example, Jiang and colleagues have isolated Pd3 clusters coordinated with ligands that formed during Pd-catalyzed cleavage of both CS2 carbon–sulfur bonds in the presence of HNO3 [37.Jiang X.F. et al.Hydrolytic cleavage of both CS2 carbon–sulfur bonds by multinuclear Pd(II) complexes at room temperature.Nat. Chem. 2017; 9: 188-193Crossref PubMed Google Scholar]. The crystalline structure of the complex indicates that two S atoms are coordinated to Pd atoms in a bridged conformation, suggesting that the structure of the in situ-formed metal clusters interacts with the reactants and/or intermediates. Although the metal-catalyzed reaction usually occurs with the functional group in the substrate molecule, the interaction between the reactant and in situ-formed metal clusters can be related to the whole substrate molecule. Through experimental and theoretical studies, Cordon and colleagues have shown that medium to long alkyl/aryl side chains can stabilize smaller Aun-alkyne intermediates than phenylacetylene for the hydration reaction of alkyne. Higher reactivity can be achieved due to the optimized Au···H–C and Au···π interactions [38.Cordon J. et al.The key role of Au–substrate interactions in catalytic gold subnanoclusters.Nat. Commun. 2017; 81657Crossref PubMed Scopus (0) Google Scholar]. Decomposition of metal agglomerates has also been observed in liquid-phase oxidation reactions. For instance, Au25 clusters protected by thiol groups have been disrupted into Au(I) thiolates during the oxidation of styrene with t-butylhydrogenperoxide [39.Dreier T.A. et al.Oxidative decomposition of Au25(SR)18 clusters in a catalytic context.Chem. Commun. 2015; 51: 1240-1243Crossref PubMed Google Scholar]. In this case, t-butylhydrogenperoxide should be the oxidant that drives the decomposition of the Au25(SCH2CH2Ph)18 clusters. The disintegration of Au particles has also been observed in Au/SiO2 catalysts during the direct epoxidation of cis-cyclooctene with O2. The in situ-formed subnanometric Au clusters from the leaching of Au nanoparticles are shown to be the working catalyst [40.Qian L. et al.Stable and solubilized active Au atom clusters for selective epoxidation of cis-cyclooctene with molecular oxygen.Nat. Commun. 2017; 814881Crossref PubMed Scopus (21) Google Scholar]. It should be mentioned that one issue associated with the leaching of metal nanoparticles is the phantom reactivity observed in some organic catalytic reactions, which is caused by contamination with metal particles in magnetic stir bars [41.Pentsak E.O. et al.Phantom reactivity in organic and catalytic reactions as a consequence of microscale destruction and contamination-trapping effects of magnetic stir bars.ACS Catal. 2019; 9: 3070-3081Crossref Scopus (11) Google Scholar]. Therefore, when using colloid metal clusters or nanoparticles for liquid-phase reactions, the leaching of metal species into solution is a critical issue to be addressed before determining whether a reaction is classified as heterogeneous or homogeneous. When isolated metal atoms are supported on solid carriers, the bonding interaction between the metal atoms and the support may not be strong enough to prevent sintering of the metal atoms (Box 2). One typical example is the structural evolution of supported isolated Pt atoms during the CO oxidation reaction. On the basis of an in situ X-ray absorption spectroscopy (XAS) study, the sintering of Pt atoms into Pt aggregates is associated with the reduction of positively charged Pt atoms by CO and further agglomeration caused by the higher mobility of the reduced Pt species [16.Liu L. et al.Determination of the evolution of heterogeneous single metal atoms and nanoclusters under reaction conditions: which are the working catalytic sites?.ACS Catal. 2019; 9: 10626-10639Crossref PubMed Scopus (2) Google Scholar,42.Dessal C. et al.Dynamics of single Pt atoms on alumina during CO oxidation monitored by operando X-ray and infrared spectroscopies.ACS Catal. 2019; 9: 5752-5759Crossref Scopus (6) Google Scholar]. However, when the support is changed from Al2O3 to CeO2, the stability of isolated Pt atoms is greatly enhanced due to the stronger bonding interaction between Pt and CeO2, resulting in the preservation of the dispersed Pt atoms [43.Daelman N. et al.Dynamic charge and oxidation state of Pt/CeO2 single-atom catalysts.Nat. Mater. 2019; 18: 1215-1221Crossref PubMed Scopus (4) Google Scholar,44.Nie L. et al.Activation of surface lattice oxygen in single-atom Pt/CeO2 for low-temperature CO oxidation.Science. 2017; 358: 1419-1423Crossref PubMed Scopus (307) Google Scholar]. Such CO-induced sintering of isolated Pt atoms has also been recently reported with Pt atoms supported on an MCM-22 zeolite during CO oxidation, the water–gas shift reaction, and a low-temperature CO+NO reaction [45.Fernández E. et al.Low-temperature catalytic NO reduction with CO by subnanometric Pt clusters.ACS Catal. 2019; 9: 11530-11541Crossref PubMed Scopus (0) Google Scholar,46.Liu L. et al.Evolution and stabilization of subnanometric metal species in confined space by in situ TEM.Nat. Commun. 2018; 9574Crossref PubMed Scopus (36) Google Scholar]. As shown in Figure 3, the evolution of subnanometric Pt species depends strongly on the reaction conditions, exhibiting more flexible transformation than their nanoparticle counterparts under the same conditions. Despite the stability of Pt atoms supported on CeO2 for the CO oxidation reaction, Pt atoms can transform into Pt nanoparticles of ~2 nm at >550°C when catalyzing the propane dehydrogenation to propylene [47.Xiong H. et al.Thermally stable and regenerable platinum–tin clusters for propane dehydrogenation prepared by atom trapping on ceria.Angew. Chem. Int. Ed. 2017; 56: 8986-8991Crossref PubMed Scopus (0) Google Scholar]. These results indicate that the isolated metal atoms present in the as-prepared catalysts may not be the working active sites; rather, it is the in situ-formed metal clusters or nanoparticles.Box 2Evolution of Supported Metal Catalysts under Reaction ConditionsHeterogeneous catalysts can show dynamic structural transformations during catalytic processes. The structural transformations will be reflected in the change of geometric and electronic structures. Several typical structural transformations are illustrated here in Figure I. It should be noted that, for a given catalyst in a given reaction, the structure of the initial metal species may present several types of evolution behavior, such as surface migration, sintering, and redispersion, depending on the reaction conditions, the metal–support interaction, and other factors.Figure 3Comparison of the Evolution of Pt Species under Different Reaction Conditions.Show full captionThe evolution of subnanometric Pt species is related to the reactants and reaction temperature. As summarized in this figure, the evolution is related to the chemical properties of the atmosphere. In a more reductive atmosphere, the sintering of the metal particles can be more favorable than that in an oxidative atmosphere. Nevertheless, the reaction temperature can also influence the state of the metal species. Redispersion of metal particles can occur when driven by thermal treatment. Such redispersion behavior could be related to the high mobility of metal atoms at high temperature. Adapted, with permission, from [46.Liu L. et al.Evolution and stabilization of subnanometric metal species in confined space by in situ TEM.Nat. Commun. 2018; 9574Crossref PubMed Scopus (36) Google Scholar].View Large Image Figure ViewerDownload (PPT) Heterogeneous catalysts can show dynamic structural transformations during catalytic processes. The structural transformations will be reflected in the change of geometric and electronic structures. Several typical structural transformations are illustrated here in Figure I. It should be noted that, for a given catalyst in a given reaction, the structure of the initial metal species may present several types of evolution behavior, such as surface migration, sintering, and redispersion, depending on the reaction conditions, the metal–support interaction, and other factors. The evolution of subnanometric Pt species is related to the reactants and reaction temperature. As summarized in this figure, the evolution is related to the chemical properties of the atmosphere. In a more reductive atmosphere, the sintering of the metal particles can be more favorable than that in an oxidative atmosphere. Nevertheless, the reaction temperature can also influence the state of the metal species. Redispersion of metal particles can occur when driven by thermal treatment. Such redispersion behavior could be related to the high mobility of metal atoms at high temperature. Adapted, with permission, from [46.Liu L. et al.Evolution and stabilization of subnanometric metal species in confined space by in situ TEM.Nat. Commun. 2018; 9574Crossref PubMed Scopus (36) Google Scholar]. The sintering of singly dispersed metal atoms into clusters and nanoparticles has also been observed with other metals. Followed by in situ extended X-ray absorption fine structure (EXAFS) spectroscopy, the atomically dispersed Au species in a fresh Au/CeO2 sample transformed into Au nanoclusters under water–gas shift reaction conditions, even at 100–200°C [48.Deng W. et al.Reaction-relevant gold structures in the low temperature water–gas shift reaction on Au–CeO2.J. Phys. Chem. C. 2008; 112: 12834-12840Crossref Scopus (116) Google Scholar]. The sintering of Au catalysts during the water–gas shift reaction has also been studied by TEM and the change in particle size can be associated with deactivation of the Au catalyst [49.Carter J.H. et al.Activation and deactivation of gold/ceria–zirconia in the low-temperature water–gas shift reaction.Angew. Chem. Int. Ed. 2017; 56: 16037-16041Crossref PubMed Scopus (19) Google Scholar]. The sintering of Au atoms should be related to the reduction of cationic Au species into metallic Au by CO or H2 produced in the water–gas shift reaction, since the sintering behavior is related to the CO/H2O ratio in the feed gas. Although the sintering of isolated atom