Mixed‐Valence Single‐Atom Catalyst Derived from Functionalized Graphene

Pawan Kumar
2019
SACs) (see also reviews [11] [12] [13] ). SACs could offer ultimate atom economy and make every active site accessible, like homogeneous catalysts but being recyclable, which is a subject of paramount importance. [14] Major challenges in the field though encompass: i) the development of materials with precise functionalities for robust metal ion binding and ii) metal cooperativity in heterometallic and mixed-valence SACs, as identified in the recent topical perspective. [12] Meeting the first
more » ... Meeting the first challenge could facilitate higher metal contents avoiding clustering and leaching upon reaction and catalyst recycling. This is also a prerequisite for the second challenge (metal-metal cooperation), since low metal content translates into large intermetallic distances. [6] Cooperation between two metal ions linked by a single-frame ligand has shown enormous potential in homogeneous catalysis. [15] For example, biocatalysts (metalloenzymes) use binuclear [16] and mixed-valence metal centers [17] for effective catalysis. Therefore, the development of heterogeneous catalysts with cooperativity between metal centers, keeping all the salient features of SACs, could offer a platform for the development of the next generation of catalysts. Graphene-based 2D materials have contributed to the development of SACs, [10, [12] [13] [14] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] in which metal ions are tetracoordinated in porphyrinic-like vacancies. Although only low contents of metal atoms can be achieved (up to ≈1 wt%), [10, 12, 14, 18, [22] [23] [24] [25] [26] Single-atom catalysts (SACs) aim at bridging the gap between homogeneous and heterogeneous catalysis. The challenge is the development of materials with ligands enabling coordination of metal atoms in different valence states, and preventing leaching or nanoparticle formation. Graphene functionalized with nitrile groups (cyanographene) is herein employed for the robust coordination of Cu(II) ions, which are partially reduced to Cu(I) due to graphene-induced charge transfer. Inspired by nature's selection of Cu(I) in enzymes for oxygen activation, this 2D mixed-valence SAC performs flawlessly in two O 2 -mediated reactions: the oxidative coupling of amines and the oxidation of benzylic CH bonds toward high-value pharmaceutical synthons. High conversions (up to 98%), selectivities (up to 99%), and recyclability are attained with very low metal loadings in the reaction. The synergistic effect of Cu(II) and Cu(I) is the essential part in the reaction mechanism. The developed strategy opens the door to a broad portfolio of other SACs via their coordination to various functional groups of graphene, as demonstrated by successful entrapment of Fe III /Fe II single atoms to carboxy-graphene. Single-Atom Catalysis The ORCID identification number(s) for the author(s) of this article can be found under https://doi. Heterogeneous nanocatalysis based on metallic species has undoubtedly evolved into a central discipline because it offers solutions to meet the challenges in the field. [1] Restricting the dimensions of the catalytically active particles down to few nanometers [2] or even to subnanometer clusters, [3, 4] has proven pivotal for enhancing performance. Flytzani-Stephanopoulos and co-workers in 2003 [5] highlighted the activity of supported Au and Pt single metal ions in water gas shift catalysts, while following reports [6] [7] [8] [9] [10] shed more light on single-atom catalysts
doi:10.7939/r3-yk4n-nf24 fatcat:3tjzsao26nfv5b4kki3gjobzle