Manipulating metal atoms in a controllable way for the synthesis of materials with the desired structure and properties is the holy grail of chemical synthesis. The recent emergence of single atomic site catalysts (SASC) demonstrates that we are moving toward this goal. Owing to the maximum efficiency of atom-utilization and unique structures and properties, SASC have attracted extensive research attention and interest. The prerequisite for the scientific research and practical applications of SASC is to fabricate highly reactive and stable metal single atoms on appropriate supports. In this review, various synthetic strategies for the synthesis of SASC are summarized with concrete examples highlighting the key issues of the synthesis methods to stabilize single metal atoms on supports and to suppress their migration and agglomeration. Next, we discuss how synthesis conditions affect the structure and catalytic properties of SASC before ending this review by highlighting the prospects and challenges for the synthesis as well as further scientific researches and practical applications of SASC.

Chemical Synthesis of Single Atomic Site Catalysts.

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Metal-Organic Framework-Based Catalysts with Single Metal Sites.

Emerging single atom catalysts (SACs), especially carbon-based SACs are appealing materials in environmental catalysis because of their ultrahigh performances, environmental friendliness, structural/chemical robustness, and the maximum utilization of active metal sites. The metal centres, carbon matrixes, and coordination characteristics collectively determine the electronic features of carbon-based SACs, and their behaviours in catalysing peroxide activation and efficiencies in advanced oxidation processes (AOPs). However, there is lack of a comprehensive and critical review reporting the successful marriage of carbon-based SACs in AOP-based remediation technologies. It is particularly necessary to systematically compare and reveal the catalytic sites and the associated mechanisms of carbon-based SACs in diverse AOP systems. In this review, we highlight the synthetic strategies, characterisation, and computation of carbon-based SACs, and for the first time, showcase their innovative applications in AOP technologies. We unveil the origins of versatile catalytic oxidation pathways in different AOP systems and the mechanisms of micropollutant degradation over carbon-based SACs, distinguished from the upsized counterparts (metals/oxides and carbon substrates). We also provide directions to the rational design of on-demand SACs for green chemistry and environmental sustainability. Also, we suggest a designated and integrated experimental/theoretical protocol for revealing the structure-catalysis relations of SACs in AOP applications, and propose the prospects for future opportunities and challenges.

Single-atom catalysis in advanced oxidation processes for environmental remediation.

As low-cost electrocatalysts for oxygen reduction reaction applied to fuel cells and metal-air batteries, atomic-dispersed transition metal-nitrogen-carbon materials are emerging, but the genuine mechanism thereof is still arguable. Herein, by rational design and synthesis of dual-metal atomically dispersed Fe,Mn/N-C catalyst as model object, we unravel that the O2 reduction preferentially takes place on FeIII in the FeN4 /C system with intermediate spin state which possesses one eg electron (t2g4eg1) readily penetrating the antibonding π-orbital of oxygen. Both magnetic measurements and theoretical calculation reveal that the adjacent atomically dispersed Mn-N moieties can effectively activate the FeIII sites by both spin-state transition and electronic modulation, rendering the excellent ORR performances of Fe,Mn/N-C in both alkaline and acidic media (halfwave positionals are 0.928 V in 0.1 M KOH, and 0.804 V in 0.1 M HClO4), and good durability, which outperforms and has almost the same activity of commercial Pt/C, respectively. In addition, it presents a superior power density of 160.8 mW cm−2 and long-term durability in reversible zinc–air batteries. The work brings new insight into the oxygen reduction reaction process on the metal-nitrogen-carbon active sites, undoubtedly leading the exploration towards high effective low-cost non-precious catalysts. The working mechanism of several low-cost electrocatalyst materials is still arguable. Here the authors show a model Fe,Mn/N-C catalyst where the oxygen reduction preferentially takes place on Fe(III) sites with the intermediate spin state (t2g4 eg1) caused by the adjacent Mn-N moieties.

/pdf/regulating-fe-spin-state-by-atomically-dispersed-mn-n-in-fe-20li5wdwva.pdf

Regulating Fe-spin state by atomically dispersed Mn-N in Fe-N-C catalysts with high oxygen reduction activity.

Single-atom catalysts (SACs) have emerged as one of the most promising alternatives to noble metal-based catalysts for highly efficient oxygen reduction reaction (ORR). While SACs can offer notable benefits in terms of lowering overall catalyst cost, there is still room for improvement regarding catalyst activity. To this end, we designed and successfully fabricated an ORR electrocatalyst in which atomic clusters are embedded in an atomically dispersed Fe-N-C matrix (FeAC@FeSA-N-C), as shown by comprehensive measurements using aberration-corrected scanning transmission electron microscopy (AC-STEM) and X-ray absorption spectroscopy (XAS). The half-wave potential of FeAC@FeSA-N-C is 0.912 V (versus reversible hydrogen electrode (RHE)), exceeding that of commercial Pt/C (0.897 V), FeSA-N-C (0.844 V), as well as the half-wave potentials of most reported non-platinum-group metal catalysts. The ORR activity of the designed catalyst stems from single-atom active centers but is markedly enhanced by the presence of Fe nanoclusters, as confirmed by both experimental measurements and theoretical calculations.

Markedly Enhanced Oxygen Reduction Activity of Single-Atom Fe Catalysts via Integration with Fe Nanoclusters

Nitrogen-coordinated single iron atom catalysts derived from metal organic frameworks for oxygen reduction reaction

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Nitrogen-coordinated single iron atom catalysts derived from metal organic frameworks for oxygen reduction reaction