Single-Atom Catalysis in Chemistry World

The recent interest on the heterogeneous single-atom catalysts (SACs) were composed of atomically dispersed active metal centers in catalyst research field, because of the increased atom utilization and unique catalytic properties of such materials, which differ greatly from those of conventional nano or subnano counter parts. In this case, the fabrication of SACs are challenging, especially in the case of noble metal based catalysts and many researches are ongoing in this field for the development of improved catalysts.

Many challenges have faced for the hybridization of controlling of single atoms in suitable host materials, but it has also equally opened with unique opportunities for catalyst design. SACs with atomically dispersed active metal centers on supports represent an intermediary between heterogeneous and homogeneous catalysis. Therefore, understanding the homogeneous catalysis prototype creates a great opportunity for designing SACs and developing related applications. 

Origin and evolution of single-atom catalysis

Huang Yanqiang and his colleagues of Prof. ZHANG Tao's research group from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences have recently developed a strategy to rationally design a single-atom catalyst for applications. This method involves by creating a single-atom active sites with the support based upon the homologous homogeneous prototypes. This process ensures the stability of the active sites during the corresponding homogeneous processes and also preserves a catalytic capability.

Figure 1. A porous organic polymer with aminopyridine functionalities with Ir single atom active sites analogous to the mononuclear Ir pincer complexes was demonstrated [1].  

X. Shao et al. have developed a porous organic polymer with aminopyridine functionalities was designed to fabricate a stable, atomically dispersed Ir catalyst. This Ir based SACs have exhibited with excellent catalytic activity during the liquid phase hydrogenation of CO₂ to formate, with a turnover number as high as 25, 135. This represents the best performance yet, for a heterogeneous conversion of CO₂ to formate. The chemical structure of the Ir SACs are analogous to that of a homogeneous mononuclear complex catalysts, representing an intermediary between heterogeneous and homogeneous catalysis [1].

The isolation of elements as atoms in chemically-distinct substances has played a fundamental catalytic role, and open frame work structures and this concept was extended to widely apply heterogeneous catalysts based on supported metals, for e.g.,in metallo enzymes and organometallic complexes.

Flytzani-Stephanopoulos et al., have provided a convincing evidence that the ionic gold or platinum species were strongly associated with the surface of ceria, and not the metal nanoparticles, which are responsible for the activity observed in the water–gas shift reaction [2]. Subsequently, Bashyam and Zelenay have also proposed that the oxidized cobalt and iron species were coordinated to nitrogen and oxygen in functionalized carbons which can deliver high performance in the electrochemical oxygen reduction reaction [3]. After years of speculation about the catalytic role of single atoms of this group of elements, advances in experimental techniques made it possible to confirm the exclusive presence of isolated centers.

Qiao et al. have been reported on high efficiency of platinum atoms supported by iron oxide for CO oxidation (Figure 2) using heterogeneous catalysis [4]. The first catalytic applications of single-atom alloys were based on metal hosts [5].The scope of this method is the electrochemical conversions growing in emphasis and promising findings emerging in photocatalysis [6]. In just a decade, the topic of single-atom catalysis has become a highly transversal field of contemporary chemical research. Hence, they have highlighted some of the main directions developing from the intense efforts of the scientific community as well as frontiers in the design of SACs.

Breakthroughs for sustainable chemistry 

The potential of single-atom catalysts has been explored in diverse of thermo, electro, and photochemical applications ranging from small-molecule activation to the construction of fine chemicals. Research on single-atom photocatalysts is at an earlier stage than electrocatalysts, but also shows promising potential contribution to solving the energy crisis. The well-defined geometric and electronic properties created by the specific interaction of isolated atoms with host materials resemble those defined by ligands in molecular catalysts. The structural parallels present new opportunities to develop heterogeneous catalysts displaying competitive performance to state-of-the-art homogeneous analogs [7,8]. 

Figure 2. Progress in single-atom catalysis. Timeline showing landmarks (L = 1–15) leading up to and during the last decade of research on the synthesis (purple), characterization (green), and application (blue) of single-atom metal catalysts [9]. 

Frontiers in the design
Catalysts based on single atoms have existed long in different forms. However, the ability to visualize their presence in previously unknown systems has sparked an enormous renewed interest in the past decade. The creativity of researchers, for example, towards the atomic design of host materials and the exploration of nuclearity trends, shows no boundaries. Concerning the synthesis of SACs, areas for improvement include the development of strategies to achieve ultra-high loadings and to precisely control the speciation and nuclearity of atomically-dispersed species, as well as a greater focus on scalable routes to accelerate commercialization [10]. The present strategy provides a promising basis for the design of efficient SACs for use in present-day homogeneous chemical conversions and serves to illustrate potential bridging between homogeneous and heterogeneous catalysis.

References

1. X. Shao et al., Chem. 5, 693 (2019).
2. Q. Fu et al., Science 301, 935 (2003).
3. R. Bashyam et al., Nature 443, 63 (2006).
4. B. Qiao et al., Nat.Chem. 3, 634 (2011).
5. G. Kyriakou et al., Science 335, 1209 (2012).
6. X. Li et al., ACS Catal. 9, 2521 (2019).
7. R. Lang et al., Angew. Chem. Int. Ed. 55, 16054(2016).
8. Z. Chen et al., Nat. Nanotechnol. 13, 702 (2018).
9. S. Mitchell & J. Pérez-Ramírez, Nat. Commun. 11, 4302 (2020).
10. H. Yang et al., Nat. Commun. 10, 4585 (2019).

Blog Written By

Dr. Y. SASIKUMAR

School of Materials Science & Engineering,

Tianjin University of Technology,

Tianjin, China

Editors

Dr. A. S. Ganeshraja

Dr. K. Rajkumar

Dr. S. Chandrasekar

Reviewer

Dr. S. Thirumurugan