Ligand–Metal Charge Transfer Induced via Adjustment of Textural Properties Controls the Performance of Single-Atom Catalysts during Photocatalytic Degradation

Abstract

Because of their peculiar nitrogen-rich structure, carbon nitrides are convenient polydentate ligands for designing single atom-dispersed photocatalysts. However, the relation between catalysts’ textural properties and their photophysical–photocatalytic properties is rarely elaborated. Herein, we report the preparation and characterization of a series of single-atom heterogeneous catalysts featuring highly dispersed Ag and Cu species on mesoporous graphitic C 3 N 4 . We show that adjustment of materials textural properties and therefore metal single-atom coordination mode enables ligand-to-metal charge transfer (LMCT) or ligand-to-metal-to-ligand charge transfer (LMLCT), properties that were long speculated in single-atom catalysis but never observed. We employ the developed materials in the degradation of organic pollutants under irradiation with visible light. Kinetic investigations under flow conditions show that single atoms of Ag and Cu decrease the number of toxic organic fragmentation products while leading to a higher selectivity toward full degradation. The results correlate with the selected mode of charge transfer in the designed photocatalysts and provide a new understanding of how the local environment of a single-atom catalyst affects the surface structure and reactivity. The concepts can be exploited further to rationally design and optimize other single-atom materials.

Single‐Atom Catalysts‐Enabled Reductive Upgrading of CO2

“…Unlike MÀ NÀ Cs, the SACs of carbon materials and metal oxides as carriers lack electronegative N, which results in ligand-to-metal charge transfer (LMCT) or ligand-to-metal-to-ligand charge transfer (LMLCT) being inferior to MÀ NÀ Cs SACs containing N. The catalytic activity (especially the electrocatalytic activity) and selectivity of N-free SACs in the carrier are not as good as MÀ NÀ Cs SACs because the charge transfer between the single atomic metal and the carrier drives the catalysis. [27] Although impregnation and co-precipitation methods have many advantages, they are easy to cause aggregation and thus lead to clusters in the preparation of SACs. To avoid clusters, the Antonietti group developed co-polymerization methods to enable the incorporation of the metal during the synthesis of the support.…”

“…Finally, the co-polymerization significantly increased the BET surface area of SACs, which was conducive to the promotion of catalytic activity and reaction selectivity. Using the copolymerization method, Liu et al [27] designed and prepared a series of Ag and Cu species to disperse on mesoporous graphite C 3 N 4 based on the unique nitrogen-rich structure of carbon nitrides, and ligand-to-metal charge transfer (LMCT) or ligand-to-metal-to-Ligand charge transfer (LMLCT) can be achieved by adjusting the structural properties of the material and the coordination mode of the metal single-atom. By using the developed SACs to degrade organic pollutants under visible light irradiation, the single-atom Ag and Cu can reduce the number of toxic organic fragments and lead to higher selectivity (Figure 3b).…”

“…Liu et al [27] focused on the relationship between SACs structure and its catalytic activity and selectivity. XPS tests show that metal single-atom selectively binds to electron-rich sites in the carrier and induces the redistribution of electron density in the material.…”

“…XPS can not only provide information on molecular structure and valence state for chemical research but also provide information on element composition, chemical state, and molecular structure of various compounds. Liu et al [27] used XPS to characterize the electronic structure of the prepared SACs, and Lorentz-Gaussian curve fitting analysis was performed. The quantitative analysis of the XPS spectrum showed that the carbon and nitrogen atomic ratio of the samples was consistent with the total carbon and nitrogen ratio.…”

“…[20] The existence of monatomic structure is usually proved by the fact that EXAFS does not detect the MÀ M peak of metal bond and only the MÀ O (M-support) peak is observed. However, EXAFS is not sensitive to polydispersity, [27] which means that it is difficult for EXAFS to recognize the presence of mixed materials of different sizes at the same time. EXAFS reflects the average signal of each component and cannot clearly show the characteristics of each component.…”