Single-atom catalysts (SACs) offer many advantages, such as atom economy and high chemoselectivity; however, their practical application in liquid-phase heterogeneous catalysis is hampered by the productivity bottleneck as well as catalyst leaching. Flow chemistry is a well-established method to increase the conversion rate of catalytic processes, however, SAC-catalysed flow chemistry in packed-bed type flow reactor is disadvantaged by low turnover number and poor stability. In this study, we demonstrate the use of fuel cell-type flow stacks enabled exceptionally high quantitative conversion in single atom-catalyzed reactions, as exemplified by the use of Pt SAC-on-MoS2/graphite felt catalysts incorporated in flow cell. A turnover frequency of approximately 8000 h−1 that corresponds to an aniline productivity of 5.8 g h−1 is achieved with a bench-top flow module (nominal reservoir volume of 1 cm3), with a Pt1-MoS2 catalyst loading of 1.5 g (3.2 mg of Pt). X-ray absorption fine structure spectroscopy combined with density functional theory calculations provide insights into stability and reactivity of single atom Pt supported in a pyramidal fashion on MoS2. Our study highlights the quantitative conversion bottleneck in SAC-mediated fine chemicals production can be overcome using flow chemistry.

However, downsizing to single-atom level is not always beneficial to the catalytic process because the cooperative interaction between adjacent singleatom sites reduces with increasing distance from each other. [3,4] In the case of a reaction that requires activation of two (or more) reactants, a dinuclear type catalytic mechanism involving two metal atoms is often more efficient than the mononuclear mechanism. For low-density SACs, the majority of metal sites are isolated from each other and only a single-site mechanism is possible (Figure 1). [5][6][7][8][9][10] The use of few-atom clusters offers multisite pathways, however, this requires specific metal precursors like dimer or trimer complexes to avoid uncontrolled aggregation during heat treatment. [3,[11][12][13] The use of terms like low-density or high-density SACs (or low-loading and high-loading) is not sufficiently quantitative. A more quantitative and fundamental criterion involves the inter-atom distance between SAC that allows a switch from a single-site-catalyzed mechanism to a dinuclear-type mechanism. A systematic study of this problem would require a variation of the inter-atom distance to see if a two-reactant reaction, usually promoted mainly by metal-cluster-type catalytic system, works well for SAC that observes a minimum inter-atom distance. According to previous reports, high loading SACs showed higher activity than low loading counterparts, which was usually explained by the higher density of active sites and/or its distinctive electronic structure. [14][15][16][17][18] Only a handful of studies considered the relationship between performance and inter-atom distance. [14] Overall, there is a lack of understanding on how inter-atom distance influences bimolecular or more complex reactions.Herein, we investigate the efficiency of nitrile-azide cycloaddition as a function of the inter-atom distance of Cu 1 SACs supported on C 3 N 4 by tuning the loading of the SAC. At low loading of the SAC, poor activity was obtained as expected of the singlesite mechanism. At high loading level, Cu 1 -C 3 N 4 SAC enjoys an average inter-atom distance of 0.74 ± 0.13 nm. This is analogous to dinuclear copper intermediate and promotes the 1,3-dipolar cycloaddition. [19,20] The dinuclear-like pathway at high loading Cu 1 -C 3 N 4 SACs was evidenced by operando X-ray absorption (XAS) investigations, which include individual steps such as activation of sodium azide (NaN 3 ), dynamic ligand exchange between NaN 3 and benzylnitrile (PhCN), cycloaddition and formation of tetrazole compound on two nearby Cu sites.

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